Transocean offshore operation 2

OFFSHORE OPERATIONS COURSE SECTION 2 MOORING AND ANCHOR HANDLING TABLE OF CONTENTS

Training to be FIRST

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SECTION 2 MOORING AND ANCHOR HANDLING

TABLE OF CONTENTS MOORING

CHAPTER 1 INTRODUCTION 4

1.1 TYPES OF DRILLING RIGS 4 1.2 CRITERIA FOR THE DESIGN OF AN ANCHOR MOORING SYSTEM 5

CHAPTER 2 SOIL 9

2.1 SOIL CLASSIFICATION AND SOIL MECHANICS 9 2.1.1 SOIL TYPE 9 2.1.2 SOIL STRENGTH 10

CHAPTER 3 ANCHORS 11

3.1 NOMENCLATURE OF ANCHORS 11 3.2 TYPES OF ANCHORS 13 3.3 CRITERIA FOR A GOOD ANCHOR DESIGN 15 3.4 FLUKE ANGLE 19 3.5 PROOF LOAD AND STRENGTH OF ANCHORS 20 3.6 MOORING SYSTEM ANALYSIS WITH THE SEAMOOR SYSTEM. 20

CHAPTER 4 MOORING CHAIN AND WIRE 22

4.1 CHAIN OR WIRE 22 4.2 ADVANTAGES OF CHAIN COMPARED TO WIRE 22 4.3 CHAIN AND WIRE CONSTRUCTION 23 4.3.1 CHAIN CONSTRUCTION 23 4.3.2 CHAIN GRADES AND STANDARDS BY CLASS SOCIETIES 25 4.3.3 CHAIN SIZES 25 4.3.4 CHAIN INSPECTION 26 4.3.5 ANCHOR WIRE 27 4.3.6 MAXIMUM SAFE WORKING LOAD AND DIAMETER . 27 4.3.7 CONSTRUCTION , LAY , GRADE OF STEEL , COATING 27

CHAPTER 5 THE MOORING SYSTEM AND ATTACHMENTS 32

5.1 LAY-OUT DIAGRAM OF MOORING SYSTEM 32 5.2 ANCHOR PATTERNS 32 5.3 THE CATENARY SYSTEM 35 5.4 ATTACHMENTS AND CONNECTIONS 36



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CHAPTER 6 ANCHOR RUNNING AND RETRIEVING PROCEDURES 48

6.1 SELECTION OF ANCHOR HANDLING VESSELS 48 6.2 PRE-MOVE PREPARATION AND PLANNING 51 6.3 PRE-MOVE MEETING 52 6.4 INFORMATION FOR AHT CAPTAINS 54 6.5 ANCHOR HANDLING CHECK L IST 54 6.6 POSITIONING SYSTEMS 57 6.7 NOTES ON APPROACHING AND LEAVING THE LOCATION 57 6.8 WEATHER CRITERIA 59 6.9 APPROACHING THE LOCATION AND RUNNING ANCHORS 60 6.10 RETRIEVING ANCHORS AND DEPARTING LOCATION 69 6.11 SOAKING AND PENETRATION OF THE ANCHOR 74 6.12 TEST TENSION AND OPERATION TENSION PROCEDURES 75 6.13 THE HOLDING POWER OF THE CHAIN OR WIRE. 78 6.14 STORM CONDITION 79 6.15 TEST TESNION PROCEDURES 79 6.16 ANCHOR HOLDING PROBLEMS 79 6.17 RUNNING PIGGY -BACK ANCHORS 82 6.18 FISHING AND GRAPPLING OPERATIONS 84 6.19 ANCHOR WINCHES. 86

CHAPTER 7 DEEPWATER MOORING SYSTEM DEVELOPMENTS 91

7.1 INTRODUCTION 91 7.2 THE PRELAID MOORING SYSTEM . 92 7.3 ANCHORS FOR VERTICAL LOAD SYSTEM (FIG 7.7) 97 7.4 DEAWEIGTH ANCHOR. 97 7.5 PILE ANCHOR. 98 7.6 SUCTION EMBEDDED ANCHORS (SEAS) 98 7.7 VERTICAL LOADED EMBEDDED ANCHORS (VLA S) 100 7.8 SUCTION EMBEDDED PLATE ANCHORS .(SEPLAS) 105 7.9 SYNTHETIC ROPES FOR DEEPWATER MOORING. 105

CHAPTER 8 MOORING SYSTEM CERTIFICATION AND INSPECTION 108

8.1 CERTIFICATION OF ANCHOR CHAIN 108 8.2 TYPE AND SCHEDULE OF INSPECTION 108 8.3 INSPECTION LOCATION 109 8.4 ANCHOR CHAIN INSPECTION (FIG .8.3) 111 8.5 ANCHOR WIRE INSPECTION 112

CHAPTER 9 APPENDIX SECTION II –MOORING 115

9.1 ANCHOR CHAIN PROOF AND BREAKING LOAD TABLES SI UNITS. 115



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9.2 ANCHOR CHAIN PROOF AND BREAKING LOAD TABLES INCH/POUND

UNITS. 115 9.3 STEVPRIS MK5 DRAWINGS, WEIGHT, AND DIMENSION. 115 9.4 VRYHOF STEVPRIS MK5ANCHOR UHC CHART 115 9.5 VRYHOF STEVPRIS MK5 ANCHOR DRAWINGS, WEIGHT AND DIMENSIONS 115 9.6 VRYHOF STEVMANTA ANCHOR UPC 115 9.7 BRUCE FFTS MK4 ANCHOR DRAWINGS, WEIGHT, AND DIMENSIONS 115 9.8 BRUCE FFTS MK4 ANCHOR HOLDING CAPACITY CHART. 115

OFFSHORE OPERATIONS COURSE SECTION 2 MOORING AND ANCHOR HANDLING CHAPTER 1 INRODUCTION


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CHAPTER 1 INTRODUCTION

1.1 TYPES OF DRILLING RIGS The development to maintain Mobile Offshore Drilling (Units MODU’s) in a fixed position started with the use of drilling tenders, which were moored with a spread anchor mooring system alongside a fixed drilling platform. The need and desire to be able to drill in deeper water and to move between drilling locations generated various designs of Mobile Offshore Drilling Units (MODU`s):

a) Submersibles. Columns with flotation hulls support the deck with the drilling equipment and

accommodation. The submersible maintains it position by ballasting down until the lower hulls rest on the seabed.

To maintain position these units do not need a spread anchor mooring system. Maximum of water depth 100 Ft.

b) The Independent Leg and Mat-Supported Jack Ups.

A watertight floating barge type hull fitted with cylindrical or truss type legs with a jacking arrangement. The legs are connected to spud cans or a mat type support. In the drilling operation mode the hull is jacked up to a save distance above the water level. Some JU's are self propelled or have propulsion assistance.

As for the submersibles the JU's do not need a spread anchor mooring system. Maximum water depth: 450 Ft

c) The Semi-Submersible. (Column Stabilised Drilling Units)

These units consist of lower displacement hulls (pontoons) with columns to support the upper deck with the drilling equipment and accommodation. The Semi-Submersible can ballast up or down from operating draft to towing /moving draft and visa versa. A few designs can be used as Submersibles. Some Semi-Submersibles are self-propelled with a DP System or have propulsion assistance.

To maintain position in the floating drilling mode the Semi-Submersible needs a spread anchor mooring or a Dynamic Positioning (DP) system.

Maximum water depth: Anchored: 8000 ft. DP system: No limits

d) Drilling vessels

Drilling vessels are mono hull ship- or barge shaped drilling units. Drilling vessels always operate in the floating condition. Swamp barges are not classed as drilling vessels.

Drilling vessels maintain position with a spread mooring system or with a Dynamic Position (DP) system.

Maximum water depth: Anchored: 8000 ft DP system: No limits



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Fig 1.1 shows a drawing from Aker Marine the maximum water depth for various types of mooring systems in operation and developments for future taut leg systems. The section Mooring and Anchor Handling discusses the concept of the anchor mooring systems for Semi-Submersibles and Drilling Vessels

1.2 CRITERIA FOR THE DESIGN OF AN ANCHOR MOORING SYSTEM a) Forces Wind, wave, and current produce loads on the rig. These loads are transferred to the mooring system. To perform a mooring analysis it is necessary to know the maximum combined forces caused by the environmental loads. In the mooring analysis all forces work in the same direction at the same time. The combined total mooring force generated by wind, wave, and current depends on:

1) The size and weight of the drilling unit. 2) The maximum wind speed 3) The maximum combined wave height and period 4) The maximum current force 5) The water depth 6) The height above the water level

The forces on the anchor system generate two types of loads:

1) The Quasi-Static Load (QSL). The combined load of wind, swell, current and the frequency of the system. The system moves with a low frequency of 140 to 200 seconds. The analysis for the QSL is used most often to evaluate the mooring for a specific location. This analysis assesses the capabilities of the system when acted on upon by environmental loads.

2) The Total Dynamic Load (TDL). In addition to the QSL, a high frequency load occurs with a period of 10 to 14 seconds caused by the individual action of the wave and swell forces. The roll, pitch, and heave of the drilling unit and the movement of the anchor chain through the water are responsible for the shock loads in the mooring system. The TDL analysis contains line dynamics completed in the frequency domain. This analysis is complicated and therefore subcontracted

To calculate the required Ultimate Holding Capacity (UHC) of an anchor the designer adds a safety factor of 1.5 to 2 to the QSL and TDL. A major factor contributing to the UHC of an anchor is the soil mobilised by the anchor or with other words the penetration. To design an efficient modern anchor, the manufacturer needs to know the principles of soil mechanics (See Fig 1.2)



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b) Selection of Equipment With the results from the calculation of the forces, the designer selects the mooring equipment for a specific rig and the operations limits such as:

1) To use anchor chain or anchor wire or a combination of both 2) The size and type of anchor 3) The amount of anchors to that will be installed or deployed 4) The type and capacity of the anchor winches

Apart from the weather criteria and the size and type of the rig, the selection of the equipment depends too on:

1) The maximum operation water depth 2) The type of soil

c) The Efficiency and Holding Power of an Anchor Because there are many types of anchors available, each with their own characteristics, we have to know the efficiency and holding power of an anchor. One way of defining the efficiency of an anchor is: The efficiency as quoted by the manufacturer "the holding power" in good holding soil should be at least 12. Modern designed anchors claim much higher values of 20 and more. Holding power depends on:

1) The type and weight of the anchor 2) The type of soil 3) The friction of the soil along the fraction lines and the fluke area 4) The final penetration including the friction of the sub-soil part of the

anchor line Because the soil can have a detrimental effect on the holding power, we should always be cautious with the manufacturer's specification. On rock or coral the holding power of an anchor will be close to the anchor weight.

weightAnchor power Holding

efficiencyAnchor ====



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Fig. 1.1.Maximum water depth for various type of mooring system (With courtesy to Aker Marine)



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d) Rig Motions The rig motions for floating MODU’s are the surge, sway, heave, roll, pitch, and jaw (Fig. 1.3) The motion characteristics for each rig are different and all motions i.e. surge, sway, heave, roll, pitch and jaw induce forces on the anchor system.

Fig. 1.2 Penetration and soil (Courtesy of Vryhof Anchors)

Fig. 1.3 Rig motions

heave

pitch

roll

sway

surge

yaw

OFFSHORE OPERATIONS COURSE SECTION 2 MOORING AND ANCHOR HANDLING CHAPTER 2 SOIL


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CHAPTER 2 SOIL

2.1 SOIL CLASSIFICATION AND SOIL MECHANICS The theory on the strength and characteristics of soils discussed in the JU section applies too to the behaviour of anchors. To predict the penetration of an anchor we need to know the type of soil. The penetration of an anchor determines the Ultimate Holding Power. 2.1.1 Soil Type The soil type is classified by particle size. The Table in Fig 2.1 classifies the soil by its particle size

Grain Size Soil Classification

>2 µm Clay 2 - 6 µm Fine Silt 6 - 20 µm Medium Silt 20 - 60 µm Coarse Silt 60 - 200 µm Fine Sand 200 - 600 µm Medium Sand 0.6 - 2 mm Coarse Sand 2 - 6 mm Fine Gravel 6 - 20 mm Coarse Gravel 60 - 200 mm Cobbles - 200 mm Boulders

Fig. 2.1 Soil Classification The holding capacity of the seabed can be categorised in three groups:

1) Good Holding Soil. Normal clay and dense sand or silt. 2) Poor Holding Soil. Soft clay/mud, fine gravel with coarse sand. 3) Extremely Poor Holding Soil Rock, coral, boulders and cobbles

The designer of an anchor shows the maximum performance for a specific type of soil. Some anchors perform better in soft muddy soil than in hard sand.

OFFSHORE OPERATIONS COURSE SECTION 2 MOORING AND ANCHOR HANDLING CHAPTER 2 SOIL


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2.1.2 Soil Strength For soil strength we refer to section in the Jack-up seminar. It is not common practice to carry out soil surveys for the anchor locations. However, soil condition is important to decide what type and size of anchor is required to obtain the maximum holding effect on the mooring system. If it is not possible to obtain a soil test for each anchor location, we should use any information that is available from the client from the spud location or from previous operations in the same area. The soil test depth depends on the type of soil. The test depth for sand is twice the length of the anchor flukes and for soft clay for 8 times the fluke length. Generally, a depth of 8 to 10 meter will be sufficient. The most common locations soil types are sand and clay, or a combination of both The table in Fig. 2.2 gives a description of the density (mechanical resistance) for fine to medium sand in relation to the angle of internal friction, the Standard Penetration test (SPT) and Cone Penetrometer Test (CPT).

Descriptive Term Relative Angle SPT CPT Sand Density ø N MPa Very Loose < 0.15 < 30 0 - 0.4 0 - 5 Loose 0.15 - 0.35 30 - 32 4 - 10 5 - 10 Medium Dense 0.5 - 0.65 32 - 35 10 - 30 10 - 15 Dense 0.65 0.85 35 - 38 30 - 50 5 - 20 Very Dense > 0.85 > 38 >50 > 20

Fig. 2.2 Soil characteristics for sand To describe clay we use the undrained shear test (SU) in relation to the STP and the CPT (Fig. 2.3)

Descriptive Term SU SPT CPT Clay kPa N Mpa Very Soft 0 - 13 0 - 2 0.0 - 0.2 Soft 13 - 25 2 - 4 0.2 - 0.4 Firm 25 - 50 4 - 8 0.4 - 0.7 Stiff 50 - 100 8 - 15 0.7 - 1.5 Very Stiff 100 - 200 15 - 30 1,5 - 3.0 Hard/Very Hard >200 > 30 > 3.0

Fig 2.3 Soil characteristics for clay

OFFSHORE OPERATIONS COURSE SECTION 2 MOORING AND ANCHOR HANDLING CHAPTER 3 ANCHORS


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CHAPTER 3 ANCHORS

3.1 NOMENCLATURE OF ANCHORS The manufacturer fabricates the anchors from high quality cast steel or high quality steel The anchor various parts are shown in. Fig 3.1 B and C. The flukes are hinged or fixed. On most anchors with hinged flukes, the angle allows adjustment for sand or mud.

Fig 3.1.A Anchor systems

Anchor Systems

Dead weight anchor Pile anchor Suction anchor

2Stevmanta VLA

Drag embedment anchors.1) Conventional Anchors

2) Vertical Loaded Anchors

1Bruce FFTS MK4 Anchor



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Fig. 3.1.B. Nomenclature conventional anchor

Fig. 3.1. C Nomenclature of Anchor (Flipper Delta)



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3.2 TYPES OF ANCHORS The very first anchors dated from 2000 years BC. Primitive anchors in the form of heavy stones, baskets filled with bags of sand and any type of weight attached to a rope held the vessel in position. The invention of iron produced the first anchor that resembles the modern anchor. During the last decennia only a relative small amount of anchor design- and construction companies took the challenge to improve the traditional ship anchors to develop anchors for the offshore industry with light weight, deep penetration and high holding power. At present most common the anchors are the drag embedment anchors, designed to penetrate the soil. Recent developments are Suction Anchors, Vertical Load Anchors (VLA) and Suction Embedded Plate Anchors (SEPLA). Fig. 3.1.A For very deep-water anchors systems these new generation mooring system deploys a taut line system instead of the catenary system in combination with synthetic fibre ropes and vertical loaded anchors. To identify the drag embedment types of anchor of anchor we can categorise the anchors in seven groups. (As per Vryhof definition and figures) (See fig 3.2) CLASS A Anchors with ultra-penetration in which holding power extends to the third power of penetration (Stevpris, Stevshark, Delta, Kite) CLASS B Anchors with "elbowed" shank, allowing deep penetration. (Bruce SS, Bruce T.S. Hook, AC12) CLASS C Anchors with open crown hinge near the centre of gravity and relative

short shank and stabilisers. (Stevin, Stevfix, Stevmud, Flipper Delta) CLASS D Anchors with hinge and relative long stabilisers at the rear and relative long shanks . (Danforth, LWT, Moorfast-Stato-Offdril, Boss) CLASS E Anchors with extremely short, thick stabilizers, hinge at the rear and a

relative short square shaped shank (AC-14, Stokes, Snugstow, Weldhold) CLASS F Anchors with square shank, no stock stabilizers, but built in stabilizing

effect in the fluke design. (US Navy Stockless, Beyers, Union, Spek) CLASS G Stock anchors with small fluke area and stabilizers at the front of the

shank. (Single Fluke Stock, Stock, Dredger, Mooring Anchor) The anchors listed by Vryhof Anchors shows the mixture of conventional and modern anchors.



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Fig. 3.2.Classes of Anchors (Courtesy Vryhof Anchors)

Fig. 3.3 A. Comparison of anchors with equal scale and weight



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3.3 CRITERIA FOR A GOOD ANCHOR DESIGN Soil conditions differentiate from very soft or very loose to very hard or very dense. Until now, it has been impossible to design and fabricate a single type of anchor for the offshore industry that will perform with maximum holding power in all types of soil. The anchor for the offshore industry should meet the following qualification:

1) Fast Engagement and Penetration To commence penetration the anchor should orientate itself in the correct position when the tension is applied. This will minimise the amount of drag. The shank design cuts through the soil. Tripping-palms force the flukes into the angled position. Cutting edges on the flukes and a minimum of obstructions enable free movement through the soil.

2) Stability Throughout the penetration process and until the maximum embedment, the anchor should maintain good stability. Only well designed stabilizers guarantee stability of an anchor during penetration

3) High Holding power (HHP) The fluke area determines the holding power. Any increase in fluke area improves the holding power. The fluke area and the size of the anchor determine the structural strength of the anchor. The size of the anchor should be must be manageable for running, retrieving and decking of the anchor. Fig. 3.3.A and Fig 3.3.B shows clearly the amount of difference in fluke area for various types of anchors with equal weight at the same scale, including the comparison the modern design anchors with like Stevpris MK5 ,Bruce FFTS MK4 and the older conventional type of anchor.

4) Variable Fluke Angle. The anchor should be capable to give HHP in a range of soils from loose sand to stiff clay as per tables in Fig. 2.2 and Fig.2.3. For this purpose anchors are equipped with hinged flukes and a system to change the fluke angle.

5) Approved by Class Society The construction and strength of the anchor must be in accordance with the Class Societies such as ABS, Bureau Veritas. It is obvious important not to buy an anchor without the proper certification.

6) Easy to handle and to retrieve A good designed anchor penetrates deep. The design of the anchor should give minimum resistance to pull the anchor free. The new designs have a minimum area at the back and cutting edges on the flukes..

An explanation from Flipper Delta anchors shows the effect of well designed tripping palms in Fig. 3.4. Independent studies and tests with conventional anchors and modern anchors confirm the difference of the holding power in sand and clay as shown in Fig 3.5 A and 3.5 B.



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Fig. 3.3.B. Fluke area difference between Stevpris MK5 anchor and Moorfast anchor compared to fluke areas of modern design anchors like Stevpris MK5 and Bruce FFTS

MK4.

Fig. 3.5 The effect of tripping palms



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Fig. 3.5.A. Test results anchor holding capacity for various anchors in sand.



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Fig. 3.5.B. Test results anchor holding capacity for various anchors in soft clay



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3.4 FLUKE ANGLE The fluke angle is a significant factor to determine the final penetration depth of the anchor. From extensive tests and experience from operations, the best fluke angle for hard and stiff soil is 32 º and for soft soil 50º. Only anchors equipped with hinged flukes have the possibility to change the angle. The fluke angle should be set before running the anchor. An incorrect angle may cause the anchor to fall over and drag without penetration or restrict penetration as clarified by Fig. 3.6 and Fig.3.7.

Fig. 3.6 Fluke angles for sand and mud as per Vryhof Anchors Stevpris anchors.

Fig. 3.7 Correct and wrong angles. Courtesy Vryhof and Bruce anchors.

Correct angle. Good penetration

Angle for sand 35° Angle for mud 50°

Wrong angle. No penetration



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3.5 PROOF LOAD AND STRENGTH OF ANCHORS After the completion of the fabrication, all anchors are subject to a proof load test in accordance with the Class Societies Rules. Special rules are applicable for the Mobile Offshore Units and Permanently Moored Units. Depending on area and Classification Society for Mobile Offshore Units the rules may require an anchor proof load of 50% of the breaking load of the chain. For the older type of anchors like Danforth, this causes a problem. The reinforcements required to comply with the rules decrease the holding power of the anchors. The construction of modern anchors like Stevpris and Bruce anchors is in accordance with the latest rules and no additional reinforcements are required. As an indication, for an anchor of 12 ton with 84 mm chain (breaking load 720 ton) the proof load will be 360 ton. Compared to the old rules the proof load is only 133 tons. In operations, numerous loads work on the anchor such as:

1) Loads caused by the soil during penetration, tensioning and retrieving. In sand and soft soils, the loads are less than in hard soil. In hard soil point loads on the flukes tips may damage the anchor.

2) In soft sticky soil and deep penetration, the anchor handling vessel may need to apply excessive load to break out the anchor. These forces are transferred to the anchor shaft and can cause damage

3) An anchor that is embedded with deep penetration and orientated under an angle may be subject to excessive side loads on the shank under high tension or when in the process of retrieving the anchor.

4) While running and retrieving anchors, damage may occur because the anchor is wedged or jammed behind an obstacle. Racking an anchor with high-powered winches and/or using a chain chaser requires careful attention to prevent excessive loads on the anchor or the anchor rack. The wrong type of chain chaser will damage the anchor.

3.6 MOORING SYSTEM ANALYSIS WITH THE SEAMOOR SYSTEM. The SEAMOOR system is a mooring simulation system developed by Noble Denton. The simulation program meets the needs of those involved in operational management of spread moorings. It enables all aspects to assess the mooring and station keeping performances. SEAMOOR uses well-proven algorithms from the Noble Denton MECA program, which has NMD approval. All regulatory authorities, classification societies, and mooring system designers accept and use the quasi static analysis method.



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There are two ways to input user data to SEAMOOR:

1. Pre-prepared data files 2. Interactive data input

Data files are plain text (ASCII) files, which must be prepared prior to running the program. Any text editor or word processor with a text file mode accepts the system to prepare the files. Interactive data input can be made directly to the program whilst it is running, either in response to prompts or by editing accessible fields on the displays. All SEAMOOR input data can be specified using the 4 separate data files listed below:

1. SEAMOOR control file 2. Vessel data file 3. System data file 4. Field data file

Vessel data file are in accordance to the data from MOM by R & E therefore it is a constant for every type of rig. Once it is developed, the program uses it all the time System data file is where the mooring parameter likes water depth, length of pre-laid mooring line for each system. It is different for every location, therefore the parameter change according to the particular location Field data file is necessary with the existence of pipeline or obstacle within the location of mooring. It is useful for simulating the clearance of the mooring lines from the obstacles. Important criteria and information to perform a anchor mooring analysis are:

1) Water depth 2) Rig draft. 3) If not already available at R & E, the general description of the rig 4) Type, size, weight, and amount of anchors and anchor chain. 5) Prevailing weather conditions. 6) Survival weather conditions. 7) Any Government Regulations and Requirements concerning mooring systems. 8) Maximum allowable offset limits for operations. 9) Type of soil 10) Required anchor pattern and heading. 11) Operations-, Stand By- and Survival Conditions if different from company policy

and MOM. 12) Thruster assistance for special conditions. 13) Any other information that may effect the mooring system and is not common

knowledge for the Engineering Department.

OFFSHORE OPERATIONS COURSE SECTION 2 MOORING AND ANCHOR HANDLING CHAPTER 4 MOORING CHAIN AND WIRE


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CHAPTER 4 MOORING CHAIN AND WIRE

4.1 CHAIN OR WIRE Most mooring systems for the Mobile Offshore Drilling Units (MODU's) are equipped with chain for water depth from 1000 to 2000 ft. For deeper water depth a combination chain-wire is used. Some of the latest generation of Semi-Submersibles are equipped with a combination chain-wire system with the capacity to drill in water depth of 3000 ft. The decision to use chain or wire or a combination depends on:

1) The maximum water depth. 2) The size and shape of the rig. 3) The maximum winch capacity of the AHT. 4) The winch capacity of the anchor winch on the rig. 5) The capacity of the storage area on board of the rig and on the AHT. 6) The type of operation 7) The maximum weather criteria for the operations area.

4.2 ADVANTAGES OF CHAIN COMPARED TO WIRE The advantages of anchor chain compared to the same diameter of wire are:

1) Higher breaking load strength 2) Up to a certain water depth the additional weight provides a better catenary

system 3) Less shock loads and more spring effect 4) Less wear an tear and therefore longer life 5) Additional friction and holding power in soil (See the Appendix for the anchor

chain coefficients). 6) Storage in the bottom of the columns will give more stability than wire stored on

the anchor winch on deck The advantage of anchor wire is:

1) Faster deployment and retrieval 2) Cheaper per unit of length

Offshore units, which move a lot such as pipe lay barges, work barges, support vessels and crane barges prefer to use wire.



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4.3 CHAIN AND WIRE CONSTRUCTION 4.3.1 Chain Construction Most offshore mooring systems use stud-link chain, but in deep water operations the offshore industry started to deploy studless chain. The advantages of studless chain are

• Lower weight with same safety factor • Higher safety fatigue failure • More corrosion allowance

Studless chain is preferable for long-term mooring systems. Any mentioning of chain in the discussion of this chapter means stud-link chain The studs add strength to the chain and prevent fouling in the chain locker or twisting on the seabed. Anchor chain qualifications is expressed by grade of steel and by type of construction The construction depends on the manufacturer’s procedure. The Baldt-DiLOk chain link is composed of two members. A forged and heat treated serrated member and a forged upset member with a stud that is impossible to dislodge. The stem member mates at ambient temperature with the socket member at forging temperature. (Fig 4.1) The chain factory manufactures the flash weld chain from heated rolled steel bars formed into a link shape. A flash welding machine welds the ends together. A hydraulic press presses the hot drop forged studs in the open link (Fig.4.1). Because of the rugged use of offshore chain, the studs need to be welded.(Fig. 4.3) Some Classification Societies do not accept welding of the studlinks. Long-term anchor systems often use the studless chain. Fig 4.2 shows with some photographs of the manufacturer’s process.

Fig 4.1. Baldt DiLOk Chain- Studded link and Studless link



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Fig. 4.2.Manufacturer process anchor chain. (Photos courtesy of Zhengmao Group) After the six processes the chain undergoes heat treatment and tensile testing.

1) Cutting

3) Bending

2) Heating

6) Stud setting

5) Trimming

4) Welding



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Fig. 4.3. Example of stud welding. 4.3.2 Chain Grades and Standards by Class Societies The chain manufacturers offer chains in standard classifications from grade 1 through grade 4.and the Oil Rig Quality (ORQ) A higher grade means higher steel quality and higher breaking load strength. The offshore industry mainly uses ORQ chain. Grade 4 is specifically for areas with extreme cold weather such as drilling near the arctic area. The graphs of Fig.4.4 show the various grades and breaking load/size To obtain certification, proof and breaking loads of anchor chain has to be in accordance with The Class Societies specifications. More information with tables of Proof Load and Breaking Load Tables are in the Appendix. 4.3.3 Chain sizes Most MODU’s use 3" or 76-mm. chain but other sizes from 2½" (64 mm) up to 4" (102 mm) are utilised. Chain sizes are in inches or mm. When ordering new chain it is important to verify with the manufacturer that the size will fit the wildcat. In case you order the size in inches and the manufacturer’s process is in mm. The manufacturer will round off the mm size to the higher decimal. This may cause problems if the original chain already had a tight fit on the wildcat.



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4.3.4 Chain Inspection The anchor chains are subject to the Class Societies periodic inspection schedules. In addition the barge engineer should visual inspect at regular intervals the anchor chain on any deformation and loose or missing studs. Details on inspection will follow in another section.

Fig 4.4 Example with comparison of chain grades (From Ramnäs)



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4.3.5 Anchor wire To select a wire we have to specify:

1) The type of utilisation 2) The maximum safe working load 3) Diameter 4) Characteristics of the construction. 5) Lay 6) Grade of steel and coating 7) Length 8) Certification

MODU’s use anchor wire mainly in combination with chain for deep-water operations. Once moored the anchor wire on MODU's will remain in almost the same position for prolonged periods. Construction units run and retrieve anchors at a much higher interval. For wires with the same diameter, the wire with the largest amount of wires will be the more flexible, but the diameter of the individual wires will be smaller. This means that with excessive use individual small wires will wear faster. In addition, the breaking load and safe working load will be less than a wire construction with thicker wires For anchor wire on drilling-rigs, we need a strong wire with good resistance to abrasion and wear and tear. The flexibility is less important than for example the use of anchor wire on construction units. 4.3.6 Maximum Safe Working Load and Diameter. There is a direct relation between the diameter and the safe working load. The safe working load for the anchor chain or wire will be the maximum calculated tension for storm conditions. 4.3.7 Construction, Lay, Grade of Steel, Coating This section explains briefly the wire rope construction of steel wires used for slings, pennant wires, and anchor wire. ((Fig 4.5, 4.6, and 4.7)

• Strands and Wires. Wire rope consists of strands. Each strand consists of wires. (Fig. 4.5)

• Ordinary Lay and Lang’s Lay. The wire rope construction is designed in ordinary

lay or Lang’s lay. In ordinary lay the strand and wire run in opposite direction, in Lang’s lay the strands and ropes run in the same direction. For general purpose only use ordinary lay

• Right Hand or Left Hand. This indicates the direction the wire spirals. The lay

runs left-handed or right-handed In general, only order right hand ordinary lay wire. Always install the wire on the drum in the same direction as the lay.(Fig. 4.8)



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• Core. The strands lay around a central core. The core is made of steel wire or soft

rope. Except for special purpose always use steel core wires. • Construction. Most wire ropes have six strands with a steel core. Specify wire by

the amount of strands, the amount of wires and the type of core. The steel grade (steel tensile) is Improved Plow Steel (IPS) equivalent to 180 kgf/mm³ or Extra Improved Plow Steel (EIPS) equivalent to 190 kgf/mm³. Because of the higher breaking strength, the EIPS wire is the better choice. Typical anchor wire construction is 6 x 19 IWRC or 6 x 37 IWRC. (Fig. 4.6.and 4.7))

• Coating. Wire rope comes with a corrosion protecting lubricant that will wear off

in operation. The maximum effective coating is a permanent galvanised coating. • Pre-forming. In a pre-formed wire rope, the strand and the wires have been given

the twist they take up in the completed wire • Length. The correct length of wire is important because it is not possible to make a

splice on location to connect one of more length with a shackle like done with anchor chain. The storage and transport require special care. Use lifting beams.

• Certification. Like anchor chain, wire needs proper certification in accordance

with the local and Class Society regulations.

When ordering wire specify:

a) The number of strands in the rope b) The number and arrangements of wires in the strand c) The tensile strength d) The type of core e) Any special processing, pre-formed etc f) Zinc coating and lubrication

Fig. 4.5. Regular and Lang’s lay



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Fig. 4.6 Wire rope construction

Centre wire

Strand

Wire rope



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Fig. 4.7. Various types of wire rope.



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Fig.4.8Thumb rule to reel wire in same direction as lay.

OFFSHORE OPERATIONS COURSE SECTION 2 MOORING AND ANCHOR HANDLING CHAPTER 5 MOORING ATTACHMENTS


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CHAPTER 5 THE MOORING SYSTEM AND ATTACHMENTS

5.1 LAY-OUT DIAGRAM OF MOORING SYSTEM Fig. 5.1 shows the various sections and components of the mooring system configuration with an anchor buoy, piggyback anchor, and main anchor with a permanent chain chaser.

5.2 ANCHOR PATTERNS The purpose of a spread mooring system is:

1) In the Operating Condition: To maintain the drilling unit position within certain offset limits under normal environmental conditions.

2) In Maximum Operating Condition : Not to exceed the maximum allowable offset from the centre of the drilling hole, which depends on the maximum allowable angle between the riser and BOP before disconnection is required, which is between 8°and 10°. The radius of the circle for the Maximum Operating Condition is expressed in percentage of water depth.

3) In the Survival Condition : To maintain a safe position in severe weather and current condition. The mooring pattern should give protection for severe weather/current conditions from any direction. In the Survival Condition the riser system is disconnected which allows much larger offset values than during operating conditions.

4) For all conditions To give enough reserve and strength to prevent the mooring system to break or to go beyond the holding power of the anchor(s).

The mooring analysis calculates the optimum tension to incorporate the corresponding various conditions as mentioned above. The standard mooring assessment determines the following:

An anchor pattern defined for the existing bathymetric and geological layout of the location.

Pretension and initial line tension. Chain and/or wire payout. The rig heading. Ranges and bearing of the anchor positions. Mooring line configuration such as chain and wire size..



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Pennant

200ft to 600ft 2.5 pennant wires (2.5” or

75t to 85t

Piggyback anchor

Pigtail chain

Chain-wire pigtail

Primary anchor

600ft pennant wires (3”)

Chain chaser

Rig anchor chain (3”)

Fig 5.1. Mooring components of buoyed and piggyback configuration. If we expect the most severe weather and current condition from a certain direction, a mooring analysis also calculates the best heading and anchor pattern. To moor the rig with the bow pointing into the most severe weather has some advantages and disadvantages:

1) The control room in most cases is located on the bow. When the rig arrives on location the view from the control room looks into the direction of the location, which it easier for the barge engineer to monitor the progress.

2) In case of a blowout or H2S situation the wind will blow the gasses away from the accommodation.

3) For most Semi-Submersibles the forces from wind, current and seas are less in the fore ward-aft direction than in the port-starboard direction

4) For the helicopter pilots the bow heading into the wind makes landing more difficult because with the landing approach into the wind the helicopter has to pass by the derrick.

5) With seas and wind from straight-ahead, no real lee-side is available for the crane to load/discharge cargo to the supply vessel. With item 4 and 5 in mind often the rigs heading is offset 15 ° from the prevailing wind and weather direction.

Most drilling rigs are fitted with 8 anchors but there are rigs with 9 or 10 anchors. The ideal anchor pattern is a symmetric configuration with all anchors at the same water depth, deployed with the same length, in the same type of soil and same depth of anchor penetration, which is a hypothetical case.



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Fig 5.2 shows a few examples of anchor patterns of which the 45º/45º and 30º/60º patterns are the most common. Asymmetric patterns are sometimes necessary to avoid contact with obstructions or pipelines. When alongside platforms work-over rigs, construction- and accommodation units often have to use asymmetric patterns.

Fig.5.1 Examples of anchor patterns

Vessel Excursion

Mean Offset : 73.0m Motion : 0.9 m Max Offset : 73.7 m

Wind : 15.1 m/s Wave : 2.7m & 6.2 Tz Current Dir : 45 deg Wind Dir : 90 deg

Current : 1.25 m/s

Fairlead : C1 Payout : 1436.6 m WD : 1524.0 m A Ten : 115.2 MT F Ten : 155.7 MT

Current

Wind

Environmental








Well Location

6.5% of 5000’ WD

Case 1 : Worst Weather Scenario without Line Adjustment

Line Breaking Strength : 333.71 MT Anchor Holding Power : 360 MT



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5.3 THE CATENARY SYSTEM a) Definition and Configuration By definition, a catenary is the curve formed by a perfectly flexible chain or cord hanging freely between two fixed points at the same level. To discuss the catenary we use anchor chain. The configuration of the catenary curve depends on:

1) The horizontal distance between the two points 2) The length of the line 3) The weight of the line

In respect of the anchor chain catenary, we always discuss the half catenary, i.e. the distance from the fairleader to the touch down point on the seabed. If we leave the anchor chain without tension, the anchor chain will hang straight up and down. By heaving on the anchor chain we start to apply tension and the anchor chain will follow an increasingly shallow curve until it approaches a straight line. In operation, the half catenary will be a gentle curve, which will partly straighten as the drilling unit oscillates by the environmental forces. The catenary functions as a dampening spring. This flexibility is the strength of an anchor system since the rig can return to its original position and does not move beyond a certain distance away from point zero because of the restraining forces. Less tension on the system means more movement; a high tension restricts the movement but reduces the flexibility (spring effect). Fig. 5.3 (Detailed calculations of the catenary system are discussed in Stability II course.)

Fig. 5.3. The catenary system. High tensions restrict movement – reduces spring effect.



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5.4 ATTACHMENTS AND CONNECTIONS Each length of chain needs a connection link or shackle to attach the next length of chain. Often these connections are patented and named after the manufacturer. 5.4.a. The Open-End Link (Fig. 5.4. E) For easy transport and installation, chain comes in lengths as ordered. To give room for the installation of connecting links or shackles a chain length has at each end an open-end link. To maintain the same strength as the stud link the end link is larger in diameter. To select the size of connecting link or shackle, use the larger diameter. 5.4.b. Connecting Links. Connecting links are designed to inter connect the anchor chain and to give smooth passage over the anchor winch. These links are patented and named after the manufacturer such as: 1) The Kenter and Ramfor Links (Fig 5.A.)

These connection links come in three asymmetric parts, which make them more difficult to assemble. Fix the assembly with a tapered pin. Hammer the lead plug in the larger hole size. On the other side the hole is left open to punch the pin out if we have to replace the link. One punch is used to place the tapered pin and a smaller punch is used to remove the pin. The link has to pass horizontal over the wildcat. The links have tight fit. Damage to the rims may cause problems with assembly and disassembly

2) The Baldt Connection Links (Fig 5.4.B)

These links are easy to put together. Use the Baldt Pear Shaped to connect different diameters such as the anchor shackle to the chain or chain to wire.

3) The “D” Type Anchor Shackle (Fig. 5.4.C and 5.4.D)

This shackle connects the chain to the anchor shank. Do not confuse this shackle with the D -Type joining shackle, which is an alternative for a chain connection link.

4) Swivels, Links and various possible combinations of Couplings to the

Anchor.(Fig 5.4.D and 5.4.E) Many operators do not favour the use of swivels at or close to the anchors because they have the tendency to break on side loads and do not work under load. Swivels used in anchor wires may cause damage in case of a quick load. One theory is that the outer layers will open and the wire core carries the load for a very short moment. This can cause a “bird cage” or break the wire rope core. In some cases, the extra length of the swivel may cause a problem with the available distance between the anchor shackle and the fairleader. The swivel can be a bow and eye type or a jaw and swivel type. The latter one fits direct to the anchor shackle.



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Fig. 5.4.A .Pictures of Kenter and Ramfor Links (courtesy Rämnas)

Fig. 5.4 B. Baldt Connecting links for chain and anchor connection.



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Fig 5.4.C. D Type anchor- and joining shackle

Fig. 5.4.D. Swivels and links



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Fig.5.4.E.Various possible coupling combinations to the anchor

Fig.5.4.F.Bow or ‘D “type Safety shackle. Safety shackle with large bow. D type safety shackles as connection between pennant wires may cause damage to the pennants stored on the drums because of the obstacles on this type of shackles. Always check the safety pins before transferring the pennant to the anchor handling vessel. D shackles with a larger and faired bow facilitate the eyes better and reduces the wear and tear. (Fig. 5.4.F)



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5.4.c. Crown Chain The crown chain is a short piece of stud link chain (30-60 m) between the anchor crown and the first pennant wire. The crown chain purpose is:

1) To provide a strong protection against wear-and-tear at the seabed. Without he crown chain the circular and up and down movement of the buoy will wear the bottom part of a pennant wire within a short period.

2) In case of replacement of the first pennant wire the anchor-handling vessel does not have pull the anchor on deck to disconnect the pennant wire.

3) When the anchor is decked, the greatest strain on the pennant or crown chain will be just behind the crown when the anchor passes the stern roller. Fig 5.5 See Fig. 5.6.B for a typical pennant wire damage

4) If the anchor needs double securing on deck of the AHT, the crown chain provides easy securing points.

It is good practice to have an open link at each end of the crown chain because it makes it easier to pass the connecting links or shackles. 5.4.d. Pennant wires If the anchor system is not equipped with a permanent-chain chaser-system we need to deploy the anchors with a pennant-and-buoy-system. The pennant wire function is:

1) To run and lower the anchor to the sea bed 2) To pull the anchors free and to retrieve the anchors 3) To be able to reposition the anchors if they drag 4) To be used as the connection between the first and second anchor if we

piggyback anchors.

Generally shackles connect the ends between the pennant wires, Baldt hinged links (Fig. 5.6 A.) or similar types of connections between pennant wires are a better way of connection but more expensive. Normal shackles or D type safety shackles may damage the pennant wire when reeled on the storage winch on the AHT. See shackle in Fig. 5.4 F. On each end of the pennant wires, the recommended termination is an eye with a high quality heavy-duty thimble with gusset and tapered pressed ferrule or with an aluminium alloy ferrule. For heavy-duty work sometimes closed spelter sockets are used. Fig. 5.7 shows various wire terminations. Only use heavy-duty thimbles with gusset plates or the closed spelter sockets.



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Fig 5.5. Pig tail chain prevents damage when anchor passes over the stern roller

Fig.5.6 A. Hinged links

High bending forces on the pennant wire and eye

High bending forces on the chain.

Wire

Chain



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The pennant wire construction is usually of the Warrington-Seale 6 x 19 or 6 x 37 EIPS-IWRC Classification.. Pennant wires can be ordered in any length form 50 ft, 100 ft, 150 ft. etc. The length of pigtails varies from 30ft to 120 ft. Store pennant wires in coils and mark the ferule with a colour code to identify the length. To hang off the pennant wires use a funnel shaped design of catcher or other safe designs. Never use pieces of rope to hang off the pennant wire, this is not a safe procedure. Fig 5.7.A shows two systems to hang off pennant wires.

Fig.5.6.B. Damage to pennant wire thimble by high bending forces.

Fig. 5.7Pennant wire terminations and pennant wire hang off system



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5.4.e Pigtail Pennant wires The function of the pigtail pennant wire is:

1) To have a short and easy way to handle the connection between the anchor buoy and the rest of the pennant wires.

2) The section under the buoy sustains more wear and tear. It saves time and money to replace a short section.

3) Short pigtails from 50 ft to 100 ft are useful to complement the last section to reach the total required length. It may prevent to install another long length.

5.4.f. Inspections Each time the anchor hangs under the stern roller or comes on deck of the AHT, ask the crew to inspect the anchor and attachments. After retrieving the barge engineer should inspect the pennant wires on "kinks"", "birdcages", wear patterns and broken wires and loose or damaged thimbles. 5.4.g. The Permanent Chain Chaser System (PCC) The PCC system enables to run and set the anchors without the use of a buoy/pennant system The PCC system is faster to run but the strain on the equipment is much higher. To operate the system heavier PCC pennants and connections are required. Various type and shapes of chain chaser are available. Fig. 5.8 A and 5.8.B

• The J Chaser The AHT deploys the J chaser from the stern. Hanging at approximately 1/3 of the water depth the AHT tows the chaser across the mooring line until it catches the chain. The AHT tows the chaser until contact with the anchor shank/fluke for anchor break out and retrieval.

• The Permanent Chaser

The permanent chaser is the alternative to the buoy-and pennant system. • The Detachable Chain Chaser.

The detachable chaser makes it possible to install or replace the chaser without removing the anchor.

• The Permanent Wire Chaser.

The permanent wire chaser came into operation when the rigs moved to deeper waters and a composite wire/chain mooring system became necessary. The chaser incorporates a “rocker”, which is centrally mounted on a hinged bolt. The rocker has two opposing wire grooves, which enables the wire to slide through with minimum friction. The material of the groove is less hard than the wire. This means the rocker takes the wear. The rocker is easy removable. The permanent wire chaser is easily detachable by withdrawal and re-assembly of the hinged bolt and rocker.



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• The Permanent and J Chain Lock Chasers. Increased rig dimensions and anchor forces lead to higher requirements to break out the anchors. The chain lock chasers provide a design to break out an anchor without having to contend the force in the mooring and the break out force. By locking on the chain ahead of the anchor shackle, the AHT only deals with the weight of the anchor and its resistance to break out.

The Bruce permanent chain chaser and roller chasers serve the same purpose as the chasers described above. Apart from the right chaser for Bruce anchors, Bruce chain chaser mate with other anchor types. Regardless of the system used to retrieve the anchor, large forces are required to breakout a heavy anchor with deep penetration. If not done with skill and patience the AHT tug easily exceeds the breaking strength of the recovery system

Fig. 5.8 A. Various types of chain chasers



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Fig. 5.8.B Pictures of a grapnel, chain chasers and fishing hook. It is important to install the proper type and shape of the chain chasers on the anchor system. If in doubt, contact the anchor manufacturer. Damage and serious running problems will occur with the wrong type of chain chaser. To reduce the amount of connections on the winch drum of the AHT use the longest available length of pennants as work wire. Long lengths of pennant wires reduce the possibility to have a connection at the stern roller when setting or pulling the anchor. Always use a short length of chain connected to the chain chaser for the same reason as we use a crown chain on the anchor.



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5.4.h. Anchor Buoys The function of the anchor buoy is:

1) To uphold the weight of the pennant wire 2) To be able to retrieve the anchor 3) To mark off the anchor pattern and the position of the anchor

Anchor buoys come in various sizes, shapes, and material of construction. In the early days of the offshore drilling and construction industry only used steel buoys. In many cases the buoys disappeared to the seabed because of damage during handling or due to corrosion. The design of the buoys improved with the construction of watertight compartments and with the use of foam filled buoys. Because steel buoys are a hazard to smaller vessels, resilient buoys were developed. A core of Expanded Polyurethane surrounded by solid Polyethylene Foam covered with an elastic skin assembles around a core of steel fitting. The steel fitting includes a cross bar to snatch the buoy. These buoys are unsinkable. Some manufacturers use inter-changeable sections, which in case of damage are easy to replace. When the drilling unit moves to deeper water, additional sections increase the buoyancy. Compared with steel buoys the resilient type of buoy is more expensive .In certain areas local regulations only allow the use of resilient type of buoys. Fig. 5.9.. One type is a sliding or suitcase buoy. The buoy slides through the pennant wire. The advantage is that the buoy remains attached to the system.

Fig. 5.9 Resilient polyurethane pennant wire buoys

OFFSHORE OPERATIONS COURSE SECTION 2 MOORING AND ANCHOR HANDLING CHAPTER 6 RUNNING AND RETRIEVING


47

Summarising the modern buoy design should have the following features:

1) Unsinkable. Use of foam filled compartments or built from unsinkable

material 2) Hull and shape should be collision safe. Preferable constructed from

resilient material 3) Easy to handle for the anchor handling vessel with a cross bar at the top

and an easy accessible eye on the bottom 4) Strong to withstand rough handling 5) Easy to repair on board or made out of replaceable sections 6) Painted in highly visible colour like orange or yellow with reflecting

strips. 7) Buoys should be painted with number and rig name 8) Provided with a radar reflector and automatic activated light

In deep water, it takes a very large buoy to support the pennant wires. The use of a spring buoy system limits the size of the surface buoy and reduces the wear and tear on the crown chain and bottom section of the pennant wire system. The spring buoy should be strong enough to withstand the compression of the water depth.



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CHAPTER 6 ANCHOR RUNNING AND RETRIEVING PROCEDURES

6.1 SELECTION OF ANCHOR HANDLING VESSELS Depending on the contract, the Company or the Drilling Contractor selects the anchor handling vessels. In most cases, the Company has the AHTS and Supply Vessels under contract and the Drilling Contractor can only advise the Drilling Contractor what type of AHT is required to perform a good and safe mooring operation. Important factors to select the right type of anchor handling vessel are:

1) The availability on the market. In a tight market the required vessel may not be available.

2) The area. As an example, the type and design of anchor handling vessels used in the North Sea is different from the vessels used in West Africa. The average weather, wave height and current conditions are important factors to decide on the type of anchor handling vessel

3) The type and size of the rig 4) The manoeuvrability. For confined areas a vessel with high

manoeuvrability is required 5) The water depth.(see also item 7) 6) The size type and weight of the anchor. The size of the stern gate and

stern roller should accommodate the size of the anchor. 7) The amount and size of chain to be deployed. The vessel must have

enough power and bollard pull to deploy the length of chain for the water depth on the location. The capacity of the winch and other deck equipment must be adequate to handle the weight of the chain and attachments

8) The deployment of the system with buoys or with a permanent chaser. Modern deck equipment with the right size of pins and stoppers are necessary to work fast and safe. Check the storage capacity of the pennant storage winches on the anchor handling vessel

9) The deck space and chain locker space. If we need to store chain and/or have to run piggyback anchors the vessel should have enough deck space and chain lockers.

The Towing Section explains in detail the selection of Anchor Handling Vessels and Ocean Going Tugs. To tow and mooring a large Semi Submersible the choice will be between an Anchor Handling Tug (AHT) or an Anchor Handling Tug Supply Vessel (AHTS) all with a bollard pull of at least 125 Tonnes. Although the difference between AHT and AHTS is disappearing, there are still vessels in the filed from both classes. In general, the anchor handling tug is easier to handle as it is build for the job and the AHTS has the advantage of more storage capacity. Both types of vessels are suitable for the job.



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To determine which AHT(S) is the best equipped to operate with the drilling unit check the following details:

1) The required minimum power: 2) Brake horse power 3) Bollard pull 4) The capacity and details of the deck equipment 5) Amount and capacity of the storage reels 6) Towing winch specification, size and length of the wire 7) Spare towing wire 8) Specification of the work wire winch, wire size and length 9) Size and capacity of the hydraulic operated wire/chain stopper 10) Hydraulic operated guide pins 11) Availability of a towing bar 12) Deck space and storage capacity for bulk, fuel, and water. 13) Chain locker capacity 14) Stern roller and stern gate size 15) Jewellery, such as shackles and grapnels 16) Manoeuvrability.

Below is a list, as a guidance to compare the characteristics of AHT or AHTS. Limited = L Good = G Excellent = E Characteristics

AHT AHTS

1 Effective Bollard Pull in rough seas L G 2 Effective Bollard Pull in normal conditions G E 3 Manoeuvrability and sea keeping ability in rough seas E E 4 Towing facilities, double drum and anchor handling winches E E 5 Chain locker capacity L G/E 6 Deck space for storage of buoys and anchors L E 7 Pennant wire storage reels capacity L/G G/E 8 Barite, Cement and Bentonite storage capacity G G/E 9 Fuel and water storage capacity G E 10 Ability for anchor handling, fishing, buoy retrieval E G 11 Deep water anchor handling capacity L E 12 Salvage capabilities G E 13 Work in confined areas E L/G



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It is important too know how well the AHT is equipped with modern navigation and manoeuvring equipment. Radar, Global Position System (GPS), computer steered positioning systems (Joystick-Poscon and DP systems), three or four thrusters Check the power of the thrusters. DP assisted manoeuvring systems are now common for an AHT. For tows, it may be necessary to use a towing gog and stern gate. Modern equipment on deck such as remote controlled guide pins and wire/chain stoppers are common standards.(Fig. 6.1).

Fig. 6.1. Pictures with guide pins and chain/pennant wire stoppers. Note: Dual set retracted position flush with deck and combined use of wire and chain

stopper



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6.2 PRE-MOVE PREPARATION AND PLANNING The discussion on the preparation and procedures for moving is for guidance only. There may be different procedures depending on the area, operator, local regulations and even from rig to rig. Always consult the M.O.M. for rig-specific operations procedures. The staff onshore and the OIM on board should start preparations well ahead before the intended date of the move. It will make some difference if the rig will move within the field or embarks on a long ocean tow. Pre-move actions are:

1) Prepare the site survey in time. This is normally the responsibility of the operator. The divers should perform the survey only a few days before the date of arrival. The survey includes confirmation of the existence of any sub-sea pipelines, templates or other major obstructions. Request an accurate large-scale survey chart of the area for the barge engineer and AHT to plot positions while running the anchors.

2) If applicable, advise the Warranty Surveyor about the estimated date of the move. If a Warranty Surveyors is required to attend the move (JU), needs to schedule his arrival in time to survey the rig and to prepare the move approval certificate. If no surveyor is required to be on board a move approval certificate is prepared ashore. The Certificate of Approval gives detailed guidelines and instructions concerning the stability, securing, weather limits and AHT vessel requirements.

3) Advise the local authorities. The Government Authorities have to approve any movements within the territorial waters or Continental Shelf. This can be the Mining Agent, the Department of Energy, Marine Directorate, Coast Guard, or Port Authorities.

4) Establish the chain of command. With the drilling superintendent, barge engineer or captain, the company man, the warranty surveyor and other supervisors on board only one can be the O.I.M. Confirm with the operator if any additional company persons need to attend the move.

5) Confirm with the client that on the estimated move date the area will be free to arrive or depart. If possible, avoid any move to coincide with other rig move operations near the rig location.

6) Establish what position system will be used and where the system will be installed



52

7) Confirm with the operator what maximum allowable offset limits are acceptable for the drilling unit in respect of heading and distance from the hole position.

8) For prolonged moves, arrange adequate storage of additional water, fuel and other supplies. Sometimes the crew-change dates need to be changed.

9) Set up a long- and short-term professional weather forecast service. Normally the weather forecast service company is arranged by the Company

10) Obtain soil information for the jack up leg position and for the anchors. It may be necessary to find additional piggy-back anchors what will take time.

11) Decide if placement of a marker buoy system on the new location is

required

12) Confirm pretension procedures and tension values.

13) Establish anchor pattern and heading. Contact the Engineering Department to carry out a Mooring Analysis with the SEAMOOR program for non-conventional anchor patterns.

14) Start the selecting procedure for the AHT’s. If the client provides the

AHT’s verify if the bollard pull and winch capacities are in accordance with the move approval requirement.

6.3 PRE-MOVE MEETING 6.3.a. Persons to attend Arrange a pre-move meeting on board of the drilling unit. In addition to the rig manager the following persons should attend the meeting:

1) Rig or Drilling Superintendent 2) Master or Barge Engineer 3) Company Representative 4) Warranty Surveyor (If required to be on board) 5) Positioning Surveyor 6) Safety |Supervisor 7) Chief Mechanic 8) Chief Electrician 9) Night Pusher



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6.3.b. Pre-move Meeting Topics Subjects and information to discuss in the pre-move meeting are:

1) General sequence of operations and the time schedule to complete the well

2) The sequence of operations and time schedule to prepare the rig for the move.

3) The ballast and de-ballast procedure and corresponding draft for the mooring operation.

4) The responsibilities and tasks for each supervisor. 5) The chain of command. 6) The role of the Warranty Surveyor and move approval requirements. 7) Communication system and channels to be used on VHF/UHF 8) The water depth and anchor pattern on the new location 9) The soil condition and fluke angle. 10) Availability of piggy-back anchors, additional pennant wires and

connections. 11) The exact position, heading and maximum offset limits for the new

location 12) The type of positioning system. Requirements for the installation on

board. 13) The weather criteria. Determine the prevailing weather and current

condition. Agree on the best possible heading. 14) The information on the amount and type of anchor handling AHT's. 15) Any special operations that may be required because of pipelines and

obstruction. 16) Authorities that need to be informed. Assign the person(s) to advise the

authorities. (Department of Energy, Port Authorities, Navigational Warning Notification, Coast Guard).

17) The marker buoys system. 18) Last but not least the safety on job. Initiate Safety Meetings before each

rig move to discuss and explain the specific dangers related to the anchor handling procedures.



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6.4 INFORMATION FOR AHT CAPTAINS After the pre-move meeting the barge engineer writes up a move procedure and planning. Each AHT captain receives a copy of the move procedure. This information plays an important role while running anchors because if there are questions the persons involved can refer to the information without going into a long discussion. Essential information is:

1) The name and position of the man in charge (OIM) on board of the rig. 2) The geographic and grid co-ordinates of the new location. 3) A drawing of the rig with the indication Forward/Aft and SB/Port. The

American system sometimes uses Left/Right. The horizontal and vertical plan should indicate the anchor rack , the location of the thrusters and any under water hazards of the rig.

4) The draft of the rig during anchor handling and under tow. 5) The type, weight and numbering system, of the anchors. 6) The proposed routing between locations 7) Drawing and details of the towing bridle arrangement and attachments 8) Crown chain size and connection 9) The type and pay-out speed of the anchor winches 10) The anchor pattern chart including water depth, the heading and

maximum allowed offset 11) Communication and channels on VHF and SSB 12) The survey chart of the area with the position of pipelines and

obstructions 13) Weather limitations 14) Soil information 15) The make up of the pennant and connection for a buoy/pennant system

and colour code system. 16) If a chain chaser system is installed give details of the make up of the

system including size/length of the pennant to be used to lower the anchor 17) The maximum crane reach and position of the cranes 18) If applicable special running procedures for passing over pipe lines 19) Procedures to lower and to set the anchors on the seabed. 20) Approaching plan, the marker buoy system and designation of the lead tug 21) Any other special procedures discussed in the pre-move meeting. 22) Discuss and if appropriate add the comments from the AHT captains.

It is a good practice, if time allows, for the barge engineer to pay a short visit to the AHT's to discuss the plan. It is for both parties a good opportunity to check each other’s experience, which is a very important factor in the entire operation.

6.5 ANCHOR HANDLING CHECK LIST The Barge Engineer and/or OIM use their own checklists to prepare and follow up on the moving procedure. The contents of the checklists are based on the experience. Alternations are added as required because of changes in conditions or circumstances.



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The use of a checklist may be expressly prescribed as a company policy or serves as a reminder for an individual. Whatever the reason, the use of a check list is a good practice The following list serves as a guideline line to create a model for a check list for running or retrieving anchors. List of Items to create a planning check list for Anchor Running and Anchor Retrieving

Anchor Retrieving

Anchor Running

1. Completed pre-move meeting X X

2. Confirmed Longitude and Latitude of the new position X

3. Verified water depth with tide tables X

4. Heading and anchor pattern, for prevailing weather-and current condition

X

5. Passed written information on anchor handling procedure and rig plan to AHT’s captains

X X

6. Obtained approval from local authorities to move X X

7. Inspected and install tow wire and emergency tow wire X X

8. AHT’s. Captain names, HP, bollard pull, storage capacity, winches, tow wire, equipment etc.

X X

9. Confirmed anchor running/retrieving sequence with AHT’s captains

X X

10. Confirmed minimum required amount of AHT’s and bollard pull is in accordance with move approval.

X X

11. Obtained information of soil condition X

12. Set fluke angle for soil condition X

13. Plotting charts, navigating charts and grid charts on board X X

14. Carry out a safety meeting for each shift X X

15. Arranged weather forecast service X X

16. Established maximum weather acceptable weather condition for mooring operations

X X

17. Confirmed weather forecast for next 24- 12 – and 6 hours before move date

X X

18. Installation of positioning system completed. Equipment operational

X

19. Established communication channels X X



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20. Verified position of sub-sea pipelines and obstructions X X

21. Prepared materials (slings, shackles, buoys, pennant wires, split pins. lead shots etc.)

X X

22. Checked availability of piggy-back anchors, additional pennant wires and enough connections

X

23. Advised the Warranty Surveyor. Confirm date to be on board X X

24. Ballast and de-ballast procedure. Complete ballasting/de-ballasting

X X

25. Note draft at departure and arrival X X

26. Checked condition of pennant wires and shackles. X

27. Checked buoys. No leaks. Pigtail pennant condition and shackles

X

28. Function tested anchor winches X X

29. Loaded AHT’s with anchor handling equipment, buoys pennants, slings, shackles, piggy-back anchor(s) etc.

X

30. Confirmed pre-tension and operation tension values X

31. Completed stability calculation. Check results with actual draft.

X X

32. Loaded enough fuel, water and supplies X X

33. Discharged equipment and cargo not required to remain on board

X X

34. Received warranty surveyor’s move approval X X

35. Secured all loose equipment and deck cargo. X X

36. Function tested all cranes. Snatch slings and safety slings ready

X X

37. Verified watertight integrity, all doors and hatches closed. X X

38. Confirmed allowable offset for final position X

39. J Chaser and/or grapnel ready for use X X

40. Submitted anchor break out procedure X

41. Submitted chain chaser procedure for running and retrieving X X



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6.6 POSITIONING SYSTEMS To position the drilling rig within a few meters of the spud location requires a high accuracy positioning system. A service company installs the equipment on board of the rig. The most used positioning systems are:

1) Short range systems. These systems use laser beams and microwaves. The accuracy is within two meters. The installation of the equipment must be within a few kilometres of the drilling rig location

2) Medium range systems. These systems (Pulse 8) use three radio wave stations on the principle of Decca Systems but with a higher accuracy between 5 and 10 meters.

3) Long range satellite positioning systems. The most popular system nowadays is the Global Position System (GPS). This system makes use of a Geostationary Satellite Network. The accuracy is 5 meters or less

4) Transponder systems. Sometimes Hydrophones are placed on the sea bed or a well head template When the rig arrives on location the hydrophone is switched on and the location is calculated and displayed the same way as on a DP drilling vessel

To locate the position of the anchors AHT plots his position with radar and the GPS system. There are some limitations to each system:

1) Satellite systems can loose accuracy because of loss of differential correction data 2) Laser systems and micro wave system do not like heavy rain, dense fog and may

loose the signal by obstructions. 3) Hydrophones signals can be masked by disturbed and aerated water 4) Radio waves from the Pulse 8 system are sensitive to atmospheric conditions

All over the GPS system is now the most accurate and easy to use system.

6.7 NOTES ON APPROACHING AND LEAVING THE LOCATION Each approach to a location or leaving a location requires a specific procedure. The following section explains some basic principles without going into details of a specific move. Some important factors effecting the approaching and leaving procedures are.

1) If the location is free or if we approach a congested area with other rigs or platforms close by

2) The existing weather/current condition 3) The amount and type of AHT’s 4) The existence of sub sea obstructions within the anchor pattern area

The final decision to start to run or retrieve the anchors depends very much on the weather condition. Ensure that the weather window is long enough to deploy at least four anchors.



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With four anchors deployed, the rig will maintain position even in a storm condition. Of course, the best situation is to have a weather window that allows enough time to run all anchors. If any doubt about the weather window i.e. marginal conditions it is better to wait than to suffer. Always calculate the risk that the anchor(s) may not hold. As a guide, do not continue anchor handling in worsening weather conditions once four anchors are set on the seabed. Only retrieve anchors when there is enough time to retrieve all anchors and safely leave the area before the weather deteriorates. The average time to run or retrieve one anchor is between 1½ and 2 hours. When using the PCC system, the average time may be less. The margin to start or continue depends for a great deal too on the experience of the barge engineer and the AHT captains. Another significant factor is the capacity of the AHT’s. If the AHT are under powered the weather margin is much lower. In good weather and water depth up to 120 ft at least two AHT’s are required to arrive or depart from the location and to be able to complete a safe anchor handling operation. For the large Semi- Submersibles and/or deep water mooring operations it is advisable to use three AHT’s. The ideal situation is to approach the location with the bow (heading) into the wind and current. The wind and current act as a break. For the lead-tug it is easier to maintain the rig position into the wind/current direction. Assign the most powerful AHT to the windward corner of the rig. If the first anchor is a “drop” anchor from the AHT do not worry about the exact position. Reset the anchor after completion of deployment of the other anchors. Run and retrieve the anchors in de-ballast condition with the anchor racks just in or above the water level. This has the advantage that it is possible:

To see if the pennant wire is fouled around the anchor To see fair leaders are lined up and function properly If within crane reach to grease the fairleader before starting the operation To see the anchor to engage the anchor rack in the correct position To see direction of the anchor chain

When running anchors first deploy the anchors without additional work (Piggy-back., special procedure, running over pipe lines). Run four anchors, one at each corner and follow up with the anchors that need special procedures. Complete fishing and grappling work before retrieving the other anchors. With four anchor still deployed the rig will maintain position even in bad weather.



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With other rigs or platforms in the vicinity, the last anchors to retrieve should move the rig away from the danger zone. Again, these guidelines are not hard rules. Judge each situation together with the captains of the AHT. Use common sense and experience.

6.8 Weather Criteria As mentioned before it is extremely important to have a good weather forecast service set up in time. Do not wait until the last moment. In parts of the world with very constant and predictable weather such as tropical areas, the weather forecast service may not be available or will not be necessary. For harsh environment areas and locations with unpredictable weather, the weather forecast from the radio or TV is not accurate enough to predict the weather on the mooring location. Information over the internet is nowadays another source. From the operators economic point of view, towing- and anchor handling is a non-productive operation. The drilling contractor may be on reduced day rate or receive no day rate at all during the move. The OIM/Barge engineer is therefore always under pressure to finish the anchor handling work within the shortest possible time. To take the decision to abandon the move is sometimes difficult. These are some basic suggestions to use to make the decision easier:

To abandon or not to start the anchor handling work is not a one-man decision. Although the OIM is the responsible man on board he takes advice from other experts to make a decision:

In the pre-move meeting the participants agreed on the weather limitations. Do not change the limits except if there is a change in circumstances or conditions

Do not start the operations if the weather is marginal and deteriorates In marginal conditions, consult the AHT’s captains. If there is doubt, call the weather forecast service direct and discuss the situation. A good indicator is to look at the deck of the AHT’s. If the deck takes a lot of water do not risk the life of the AHT’s crew. With a chain chaser system the limits of operations are higher

The experience of the barge engineer and the captains of the AHT’s are very important. The limits are lower for anchor handling work with crews without experience. Be more patient and do not question time and again the captains about their progress. If there are problems let the captains of the AHT sort them out and discuss any dissatisfaction about the anchor work after the job is done.

If the AHT’s can not hold their position against the weather and/or current, it is time to stop or not start the operation. In this respect consider too that the vessels coming to the windward side of the rig to pick up pennants and anchors, run high risks to collide with the rig

If different types of vessels are used the shorter AHT may be able to continue anchor handling while the longer AHTS has to stop operations because of heavy pitching. If two out of three AHT have higher HP the operation may continue with only two vessel.

Allow for an eight-hour weather window to run 4 anchors. In a congested area with other rigs, platforms or construction barges in the vicinity do not take any risk.



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It is not possible to give exact figures for work limitations of each AHT. As an indication, the work limit for anchor handling with the AHT or AHTS in the range between 9000 BHP to 14000 BHP is cross winds of 30 knots and about 15 ft significant wave height. With 2 knots crosscurrents these vessels start to have problems to maintain position. A combination of wind current and waves requires judgement on a case by case situation.

6.9 APPROACHING THE LOCATION AND RUNNING ANCHORS The example we use in this discussion is just one of the many possible scenarios. But in general running and retrieving anchors follow the same procedures. We assume to run eight anchors with three AHT on a location without sub-sea obstructions and no other platforms or rigs in the vicinity. The procedure to run or retrieve anchors with only two AHT's is not very much different. This easiest scenario is to approach the location into the weather and current. To approach a location in a congested area and in the same direction of the weather and current requires different procedures and more skill in boat handling and teamwork. 6.9.a Towing arrangement at arrival The towing arrangement at arrival can be:

1) One Ocean Going Tug on towing the bridle. Three AHT standing by to connect. This is typical for the arrival after a long ocean tow. The towing vessel is not equipped for anchor handling. At a safe distance from the anchor location, the AHT’s make the connection to the primary pennants. Release the ocean going tug. In most cases, two AHT’s take the forward inside anchor pennants and one AHT connects to an aft inside anchor.

2) One high powered AHT on the towing bridle. The modern high-powered AHT have 16000 HP and 160 ton bollard pull. Although the towing characteristics may not be as good as an ocean going tug the modern AHT are capable to perform field tows and ocean tows. The advantage is that at arrival the towing vessel can take part in the mooring operation. At a safe distance from the anchor location, one AHT connects to the forward inside anchor and the second AHT connects to one aft inside anchor. The towing vessel releases the towing bridle and connects to the other inside forward anchor. Fig 6.2.A.

3) Two AHT’s connected to the bow (forward) anchors. The AHT’s tow from the anchor chain or pennant wire. This is more typical for a short tow or field move. At a safe distance from the anchor location, the third AHT connects to the aft inside anchor and the two forward AHT’s re-arrange the towing set up. When towing from the anchor chain the AHT needs to reconnect the anchor and pennant wire system.

The section Towing explains and shows the various towing arrangements.



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Fig. 6.2.A. Initial set up to run anchors with three anchor handling vessels.



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Fig. 6.2.B Sequence of running anchors with three anchor handling vessels 6.9.b Moving in and Running anchors Fig. 6.2.B Shows the sequence after the initial set up with three AHT’s at about 3 miles away from the anchor location, followed by the approach and final position to run eight anchors. One AHT towed the rig from the towing bridle. Two AHT are waiting for orders area. During the approach with the three AHT’s the line distance to the vessels is approximately 400 ft.

Arrange the AHT’s to run anchors at a distance between 3 to 5 nautical miles away from the anchor location. (1) and (2)

In this case, the two AHT’s connect to the forward SB inside anchor and the aft

Port inside anchor. The AHT at the stern assists to maintain the heading while moving in. Her function is to stop the rig moving forward as requested. The towing AHT releases the bridle after the two other AHT’s are safely connected. (3) It is possible to leave the towing AHT attached to the bridle until arrival at the anchor location. The procedure will be slightly different.

Proceed slowly toward the anchor location.(4)



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Prepare the Port aft anchor. (5). Lower Port aft anchor on the seabed and set the

anchor as close as possible to the required position. (6) If necessary, reposition the anchor the anchor at the final stage of the mooring procedure. After the first anchor is set on the seabed, the AHT aft moves to the other stern inside anchor and makes the connection. (7). Continue to move into the direction of the anchor location. Slack the stern anchor chain but maintain a low tension while moving towards the anchor location.

At arrival on the anchor location, apply the brake to the stern anchor winch.

Watch the anchor tension and slack off if needed. Turn the rig heading into the correct direction, with the windward anchor fairleader pointing into the direction of the anchor position. (8) Run the windward forward anchor. Lower the anchor to the seabed. (9)

While the two AHT’s maintain the rig in position the third vessel moves to the

next outside bow anchor. First run the inside anchors and then the breast anchors. Normally run anchors in opposite pairs.

If necessary, reset the drop stern anchor.

Reconfirm that all parties agree on the final position.

Start the pretension procedure.

Self propelled drilling units with four or more thrusters may not use a drop anchor but proceed to the anchor location. The rig maintains position with assistance of the thrusters. The AHT tugs start running the four inside bow and stern anchors followed by the breast anchors. The MOM explains the procedure to follow. For a work-over location alongside a platform the rig will move in and run the anchors at a safe distance from the platform. Complete the anchor tension procedure and move to the final position alongside the platform. 6.9.c Running and Setting the Anchors with the Permanent Chain Chaser System (PCC System) As mentioned before it is important to verify that the chain chaser of the PCC system fits the anchor. Bruce Anchor Limited and other manufacturers designed the ring chaser and a roller chaser. The roller chaser reduces the friction and gives a smoother chasing process. Both chasers accommodate chain and wire and allow easy passing of over the anchor. When ready the AHT moves within crane reach. The example is the course of action from one the MOM as per Fig. 6.3, 6.4. Generally, this procedure applies to running anchors with minor differences specific for each anchor design. The pictures in Fig. 6.5 show three different anchors and chaser arrangements.



64

Lower the pennant wire and ensure that the chaser engages over the anchor shaft.

To transfer the pennant wire to the AHT use two slings. One sling (double or single) is the connection between the pennant wire and the crane hook. This so called the messenger of safety sling has a s purpose:

In event of fouling, it functions as a weak link that will break, prior to damaging the crane.

It keeps the crane hook and ball at a safe distance from the crew on the AHT. The other sling is a loose hanging sling (Lazy sling)

� The other sling is a loose hanging sling (lazy sling) which enables the crew on deck on the AHT to make a safe connection to the work wire. (Fig. 6. 6)

When AHT captain confirms he is ready to go, start to slack off on the anchor winch while the AHT starts to move slowly ahead. Maintain enough tension on the system to prevent that the anchor slips through the chaser. (Fig 6.7) Do not use thrusters at the corner when working with pennant wires or running anchors.

After securing the anchor against the stern roller of the AHT, slack off just enough chain to touch the seabed. The crew on the AHT will check if the correct engagement of the chaser and verify all connections are in good shape. The rig confirms with the AHT captain when they are ready.

The AHT slowly increases to medium or full power and haul out the chain. The rig maintains tension on the system to ensure to stretch out the chain and that the anchor will not slip through the chaser during the running procedure. For a winch with a dynamic pay out system, ensure that the spline clutch is engaged. Do not exceed the maximum allowable pay out speed. If the chain catenary starts to drag over the seabed reduce slowly the tension on the anchor winch, enough to keep the chain stretched.

Pay out chain to the required distance. Verify distance with the positioning system. With reduced power, the AHT continues to go ahead to stretch the chain.

The AHT lowers the anchor by slacking off the work wire. To orientate the anchor in the correct position on the seabed the AHT continues to move ahead with a work wire length of about 1.5 to 2 times the water depth. At this stage, the anchor hangs still about 30 feet above the seabed.

After confirmation from the AHT captain that the anchor touches bottom the rig starts to haul in the anchor chain and the AHT releases the tension on the work wire/pennant. The AHT should not pull on the pennant wire. (Fig 6.8)

The rig continues to heave on the anchor winch until the tension meter shows about 150 Kips. This figure is arguable and may be different. The purpose is to let the anchor flukes set firmly in the soil.

The AHT must come astern and heave on the work wire. The AHT continues to move astern in the direction of the chain and the rig to pull the chaser back over the anchor without lifting the anchor. Experience of the captain of the AHT plays an important role. Only after the chaser has cleared the anchor the AHT starts to turn around with the bow in the direction of the rig

The AHT returns with the bow or stern in the direction of the rig with a work wire length between 1.5 to 2 times the water depth The AHT knows that the chaser is running free over each chain link because of the low tension and the jumping action of the wire



65

On approaching the rig, the AHT shortens the work wire to negotiate the catenary

of the system close to the rig. The AHT turn around to a safe position for the crane to pick up the pennant chaser system. (Fig 6.9)

Avoid a bight in the chain because the heavy weight of the chain bight may cause shock loads on the crane wire

Fig. 6.3 Example running anchor from a MOM.



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Fig. 6.4. Example from MOM showing the importance to maintain the proper tension on

the anchor line to prevent chaser slipping from the anchor.

Fig. 6.5 Chain chaser arrangements for three different anchors. Courtesy Vryhof and Bruce anchors



67

Fig. 6.6. Transfer of chain chaser pennant

Fig. 6.7 Pictures from Bruce and Vryhof anchors anchor manuals showing the importance to maintain tension when passing or returning anchors.



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Fig. 6.8 Wrong and right procedures to set an anchor. As per Vryhof’s anchor manual. (Courtesy Vryhof anchors)

Fig 6.9 Returning the chain chaser. (Courtesy Bruce Anchors)

Wrong procedures setting an anchor

Right procedures setting an anchor



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6.9.d Running Anchors with Buoy – Pennant System For the components of the pennant wire system see Fig 5.1 The course of action for running the buoy and pennant wire system:

Before starting the mooring operation, the barge engineer organises distribution and loading of pennant wires, buoys, shackles, and split pins to the AHT’s. The AHT crew spools the pennant wires on the storage wire drums. To gain time use the maximum storage capacity of the drums and long pennant wires.

On the AHT the crew prepares the deck with one or more buoys.

When ready the AHT is called to take position within crane reach

The rig transfers the primary pennant (attached to the anchor) similar as the

procedure for the PCC system.

The running and anchor setting procedure is identical as for the PCC system. The only difference is that there is no chaser that can slip back from the anchor

After the anchor is set on bottom, the rig applies a tension of about150 kips. The

AHT attaches the buoy to the top pennant wire and launches the buoy.

6.10 RETRIEVING ANCHORS AND DEPARTING LOCATION 6.10 a Sequence of Action to Retrieve Anchors and to Leave the Location To retrieve the anchors and leave the location the course of action is (Fig. 6.11):

Start to retrieve all four breast anchors. Keep the AHT’s as far apart as possible. Avoid using thrusters during retrieving operations.

The recovery sequence depends on the weather and current condition. The

upwind anchor is normally the last anchor to recover of the four inside anchors

Transfer the towing bridle to the towing vessel or AHT assigned for the tow. Lengthen tow wire to about 500 ft.

Retrieve the last four anchors with the two remaining AHT. In case of a rig field

tow with two AHT’s towing from the bow anchor pennant wires, retrieve the forward anchors first. The two AHT’s keep the rig heading into the wind while the third AHT retrieves both stern anchors.(Fig. 6.11)

Turn the rig into the heading required to leave the area. Start the tow.



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Fig. 6.10. Example of anchor retrieving sequence.

1

2

3

4 4) Retrieve last fwd sb anchor

3) Retrieve aft inside anchors

Wind direction 1) Retrieve breast anchors

2) Attach one AHT to bridle. Retrieve fwd port anchor

FWD



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6.9 b. Retrieving Anchors with PCC System (Fig 6.11 Step 1 to 9 as per Bruce Anchor

Handling Manual for MK4 Anchor)) The following procedure from Bruce Anchor handling manual generally applies to other drag embedment anchors. Always verify the on board anchor procedures based on the specific anchor design in use.

Start up and function test the anchor winch.

Call the AHT to take position within crane reach. Transfer the primary pennant as per PCC procedure

Secure pennant wire in shark jaw or pelican hook. Disconnect crane hook.

Connect the pennant wire to the work wire. The AHT lines up into the direction

of the anchor bearing. As the vessel moves away, maintain an anchor tension of about 50% of the test tension (approximate 150.000 lbs.). The AHT moves ahead and slacks the work wire 1.5 to 2 times the water depth. Without enough tension on the anchor chain, the chaser may pick up a bight and not engage over the anchor shaft.

The twitching action of the work wire indicates that the chain chaser is moving

free along the chain. The disturbed soil around the chain on the seabed reduces the friction when retrieving the anchor.

Monitor the AHT position between rig and AHT captain.

The chasing load increases when the chaser arrives at the part of the deep buried

chain and anchor. The captain of the AHT takes every effort to seat the chaser onto the anchor shank to avoid damage to the anchor connection and anchor

Reduce the tension on the anchor chain. The AHT increases the power to about

50% with a heading lined up with the chain direction away from the rig. When the chaser engages over the shaft of the anchor the tension on the chaser pennant increases rapidly. This moment is critical because with rough weather and heavy movement of the AHT the pennant wire can break. Avoid a vertical pull.

The normal break out procedure is to move ahead with the AHT lined up in the

direction of the chain away from the rig, to pull the anchor out along its original penetration path. The AHT maintains a work wire length of 1.5 to 2 times the water depth (Fig. 6.12). Another method is to almost stop the AHT but with some tension on the work wire. The rig hauls in on the anchor winch. Once the anchor drags the AHT starts to heave on the work wire.



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Under normal conditions, the combined forces of the tension on the PCC pennant

and the movement of the AHT should break out the anchor. The tension meters on the rig and the AHT show a drop in tension when the anchor the anchor lifts off the seabed. When the anchor drags from its buried position the rig notices a gradual decrease in tension. The break out forces can be very high. Lack of experience of the crew, rough boat handling or impatience result often in an expensive fishing job.

Stop the anchor winch when the anchor clears the seabed. The AHT now hauls in

on the work wire until the anchor firmly contacts the stern roller. The AHT maintains the heading and enough power to keep some tension on the system. This prevents the chaser sliding off the anchor. Use the opportunity to request the crew on the AHT to inspect the anchor and the connections.

The operator of the anchor winch on the rig hauls in the anchor chain. The

combination of the AHT under low power and the weight of the system prevent the chaser to slide away from the anchor.

The AHT remains on a heading away from the rig during the entire operation to

retrieve the anchor to the rig.

Inform the AHT captain when there is 500 feet left to rack the anchor. Reduce the haul in rate.

The AHT starts to pay out the pennant connected to the work wire. The anchor

winch on the rig continues to haul in until the anchor is about 40 feet from the racking position. Secure the pennant in the shark jaw. The work wire remains attached to the pennant wire eye

Keep the system under tension until the flukes are in the correct position just

under the anchor rack. Engage the anchor against the rack. Check the correct position of the chaser. This position depends on the type of anchor and anchor rack design (Fig 6.13). To prevent damage to the hull, do not let the anchor swing during the anchor racking.

Connect the crane to the eye of the PCC pennant. Release the tension on the

pennant wire eye with a pull on the work wire. Disengage the shark jaw. Transfer the pennant to the rig.

Take out the slack, and double secure the pennant wire. Do not secure the pennant

wire on the handrail. Use the pennant hang-off system

Rack the anchors with a tension of approximate 30,000 lbs. above the weight of the anchor.



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Fig. 6.11 Anchor retrieving procedure as per Bruce Anchor Manual (Courtesy Bruce Anchors)

Step 3

Step 1 and 2. Transfer chaser and running chaser to anchor. Step 3. Run to anchor with distance stern roller to anchor between 1.5 to2 times

Step 5 Step 6

Break out procedure. Step 4.Approach anchor with chaser. Step 5. Rig maintains tension. AHT with chaser at the anchor AHT increases thrust. Step 6.The motion of the

Step7 Step 8

Step 9

Step 1 Step 2

Step 7. Return anchor to the rig. Step 8. Start racking. Maintain tension to prevent anchor slips off the chaser and may hit

Step 4

Maintain tension

Step 9. Rack anchor and return pennant wire to the rig



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As an indication, the break out forces for different types of soil, in relation to the test tension is: • Sandy soil approximate 16% of test tension • Clay soil approximate 60% of test tension • Soft soil approximate 100% of test tension

6.10 c) Retrieving Anchors with Buoy – Pennant System The rig crew prepares the deck to receive the buoys. Some rigs have the buoys stored in special built racks. The AHT proceeds to the anchor buoy.

The AHT approaches with the stern towards the buoy. The crew catches the buoy with the lasso sling and pulls the buoy on deck.

Secure the pennant in the shark jaws. Disconnect and secure the buoy.

Connect the work wire to the pennant eye

Often the AHT works with a shorter wire. The procedure to break out the anchor

is similar to the PCC method.

The AHT returns the anchor and pennant wire with the same procedure as for the PCC system.

6.11 Soaking and Penetration of the Anchor When the anchor lands on the seabed the anchor disturbs the soil. The same theory discussed in the soil theory for disturbed shear strength applies to the disturbed soil around the anchor. If we soak the anchor with tension for a short period, the holding power of the anchor increases significant. Fig. 6.12 shows a picture of a scaled module test from Vryhof Anchors and a drawing with a cross section through a Stevpris anchor surrounded by soil. Each time the anchor enters a new surrounding of soil negative pore pressure is induced and the shear strength increases, until a rupture tears off a layer. This scenario repeats itself layer after layer.

Fig. 6.12. Soaking and penetration of the anchor (Courtesy Vryhof Anchors)



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6.12 Test Tension and Operation Tension Procedures The test tension often is referred to on board as “pretension”. All anchors are subject to a test tension procedure to ensure that the anchor mooring system is capable to maintain the rig within the predetermined off set limits under various environmental conditions.

1 Survival Condition Tension The mooring system capacity must be able to maintain the rig safely moored in severe weather and current conditions with the riser system disconnected from the seabed. The maximum tension on the mooring system for the Survival Condition (Maximum Design Criteria) should not exceed 50% of the breaking load test of the anchor chain. For 3” ORQ chain this value is rounded off to 500.000 lbs. The length of the deployed catenary system should lift the anchor at the Survival Condition value. At the survival tension, value the chain or wire is stretched to a straight line towards the anchor. The anchor shank starts to lift and the anchor will tend to slip instead of holding up the system to tension higher than 500.000lb. The criterion for the survival tension is the 100kt wind.

Fig. 6.13. Survival Tension



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2 The Maximum Operating Test Tension (Fig.6.14)

To comply with certification requirements the manufacturer conducts break tests and proof load tests on each chain. The Maximum Operating Test Tension value represents about 33% of the break load test. For 3” ORQ chain the Maximum Operating Test Tension is 350.000 lbs. For 3" wire this value is 240.000lbs.

Fig.6.14. The Maximum Operating Tension



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3 The Optimum Tension This is the tension applied to each anchor after completion of the test tension to give the optimum working circle before disconnection of the riser and BOP assembly.(Fig.6.15.). With the optimum tension the anchor line tension will reach the test tension value when the rig offset equals the maximum allowable offset for the riser system or maximum angle for BOP system.

Fig.6.15. The Optimum Tension

4 The Minimum Test Tension The Minimum Test Tension should not exceed the Maximum Operating Test, i.e. in case of the 3”ORQ chain < 350.000 lbs. Formula’s and graphs in the MOM provides information to calculate the Minimum Test Tension. The Stability 2 course explains the details how to calculate the Minimum Test Tension from graphs and formula in the MOM

In the Stability 2 course the details of the various tension values are discussed in more detail.



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6.13 THE HOLDING POWER OF THE CHAIN OR WIRE. The holding power of the chain or wire depends on:

1) The length of the anchor cable on the seabed. 2) The weight of the anchor cable. 3) The friction coefficient of the anchor cable. 4) The soil condition.

In formula the approximate holding power of the anchor cable is: The following tables display the friction coefficients for chain and wire (Fig, 6.16)

Fig 6.16 Friction coefficients (f) for anchor chain and wire

lbs/ft 15.7 wire3" for and lbs/ft 78.2 is chain3" for Wcexample For

seawater. in chain the of weightWc

seabed. the on cable anchor the of LengthL

t.coefficien frictionf

.erholdingpowPc

WLfPc

c

cc

========

========

∗∗∗∗∗∗∗∗====



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6.14 STORM CONDITION If in storm condition the peak tensions in the wind ward chains approach the maximum operation tension a few options are available:

If available use thrusters to reduce the chain tensions in the wind ward quadrant Equalise the windward chain tensions as much as possible. Avoid one chain to take most of the load. Adjust the chain tensions before the peak line tension reaches the windlass stall load.

Reduce the down wind chain tension to a minimum. Continuously observe any change of direction of the storm and adjust the tension of the anchor lines accordingly.

Note: Disconnect the marine riser system in time. Do not wait until the last moment.

6.15 TEST TESNION PROCEDURES After the completion of the deployment of the last anchor, built up the tension of each anchor. Increase the tension slowly and in stages. Depending on the rig and conditions, the tension of all anchors should be between 150.000 lbs. and 175.000 lbs. Re-confirm that the rig position is within the allowable offset limits. Ballast the rig down to the operating draft and start test tension procedure.

To prevent the rig moving off target, test two opposite anchors at the same time. Testing anchors takes time. Increase the tension slowly and in stages. Poor holding soil needs more time than firm and good holding soil. Record the amount of chain hauled in. Maintain the final test load for 15 minutes. Local regulations or specific requirements from the client may ask for longer periods.

6.16 Anchor Holding Problems In case tension fails to build up to the requested test tension and the anchor drags within the minimum required distance, reset the anchor. The main reasons for an anchor to drag are:

1) The soil condition is different as expected. The soil problem may be a small patch or it may concern the entire area. Reset the fluke angle

2) The anchor is unstable because the stabilizers are to short. If possible, enlarge the stabilizers or use a different type of anchor.

3) The buoy-pennant wire fouled around the anchor. Pull the anchor on deck of the AHT and untangle the pennant wire. (Fig.6.17)

4) The anchor is not heavy enough for the soil condition. The only solution is to change the type of anchor or use a heavier anchor.



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5) The anchor is unstable because the anchor remains upside down and does not rollover. This self-righting problem occurs sometimes with drop anchors or with old-fashioned anchors on a hard seabed. Reposition the anchor or change the anchor type. (Fig 6.18)The soil is very hard. Only specialised anchor designs may give better results.

6) The solution is to add chain within the allowable maximum length or to deploy piggy-back anchors.

Before taking any drastic action, retrieve the anchor up to the stern roller of the AHT to inspect the anchor on a fouled pennant wire and to verify the fluke angle

Fig.6.17. Pennant wire fouled around anchor. (Picture courtesy Flipper Delta Anchors)



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Fig. 6.18. An unstable Anchor drags in the upside down position (Courtesy Flipper Delta

Anchors)



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6.17 Running Piggy-back Anchors 6.17.a. Piggy-back as Solution for Dragging Anchor If all efforts to improve the performance of an anchor that drags are without result, there are three possible solutions:

1) Add more chain. This is only possible with a chain-out length within the maximum allowable limit.

2) Change the anchor for an anchor with a better holding performance. This is only possible if a better type of anchor is available within a short period.

3) Run a piggy-back anchor. This is the most used solution.(Fig. 6.19) The anchor with the superior holding power should be the second (piggy-back) anchor. The first (primary) anchor is the closest to the rig. (Fig. 6.20) The object of the piggy-back anchor is to increase the total holding power. The maximum chain length to the first anchor should not exceed the maximum allowable chain length calculated for the survival condition. The additional length of the anchor system after the primary anchor can exceed the maximum allowed length. The total holding power of the two anchors adds up to only 1.6 times of the total holding power of the two anchors because:

If both anchors have hinged flukes, the tension in the pennant wire of the secondary anchor will reduce the fluke angle of the primary anchor. This reduces the holding power of the primary anchor.

With use of the PCC system, the chaser may obstruct the hinge function of the flukes of the primary anchor. This too reduces the holding power of the primary anchor.

If the primary anchor is an anchor with a cranked shaft, the tension in the pennant to the secondary anchor reduces the stability of the primary anchor.

The equipment required to deploy a piggy-back anchor is:

A second anchor behind the first anchor A wire connection and suitable shackles between the two anchors A wire size with the same breaking strength (if available) as the anchor chain. For 3” anchor chain we need wire with ∅ of 86 mm. (breaking strength 1044 lbs.) As an example, the MOM for Sedco 706 requires a breaking strength of 600,000 lbs. Only use wire in very good condition.

The length of the wire should be 150 ft longer than the water depth or water depth plus 10% for deep water.

In addition to the equipment, prepare good planning with drawings and details of the additional wire, anchor, connections, and running procedures. Send a copy the captain of the AHT.



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Fig. 6.19. Piggyback anchor system.

Fig. 6.20. Piggy-back with secondary anchor behind anchor with better holding power and picture of two same anchors with primary anchor with chain chaser (courtesy Vryhof

Anchors)

Primary Secondary anchor



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6.17.b. Preparation and Running Piggy-back Anchor The preparation and procedure to run a piggy-back is:

1) The barge engineer prepares the anchor, the shackles to fit the system, crown chain, the additional pennant wire, and split pins.

2) Transfer all equipment with instructions and procedures to the AHT. 3) The AHT proceeds with the piggy-back system to the buoy. 4) Pull the buoy on deck. If the pennant wire connected to the primary anchor is long

enough and in perfect condition, no additional wire is required. If the condition of the pennant wire is doubtful, pull the primary anchor on deck of the AHT and replace the pennant.

5) With the primary anchor on the sea bed, connect the anchor shackle of the piggy-back anchor to the additional wire.

6) Run and set the piggy-back anchor. Stretch the system on the seabed. Attach and release the buoy.

7) With the PCC system, use the PCC pennant with additional pennants as required to run the piggy-back anchor.

8) Tension the new placed piggy-back system as required.

6.18 FISHING AND GRAPPLING OPERATIONS Lost or broken anchor recovery systems require fishing operations. Carry out fishing operations only in de-ballast condition. The possible reasons to loose the anchor recovery system are:

• On the PCC system; the chaser wire breaks • On the AHT; the work wire breaks • On the buoy-pennant system; the buoy sinks or the pennant breaks • On the anchor system; the anchor chain or wire breaks

The equipment available to salvage the lost anchor with its chain or wire is (See Fig.5.8):

• The shepherd hook or J chaser • The spare permanent chain chaser • The grapnel • The chasing block. This design is for wire recovery only

One or two types of chasers are stored on the rig.



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Various options are available to retrieve the chain or wire with the lost anchor:

• Use the J Chaser. Pass the J chaser to the AHT. The crew on the AHT rigs up the J chaser. The winch operator on the rig hauls in until the anchor chain is clear above the water. The AHT crosses the anchor chain with the J chaser hanging below the stern. With luck, the chaser catches the chain on the first pass. The AHT strips the chain to the anchor. With caution the AHT decks the anchor and renews PCC chaser wire. With the buoy pennant system, replace the broken or lost pennant wire. Return the anchor to the rig and rack the anchor. It is possible to replace the PCC wire or pennant wire within crane reach of the rig or close to the anchor location.

• Retrieve the Anchor with the Rig. Retrieve the anchor with the lost recovery system as the last anchor. Mover the rig with the assistance of one or two AHT’s into the direction of the anchor position and simultaneously heave in on the anchor. If available use thrusters. Recover and rack the anchor. Replace or rearrange the primary pennant. This procedure is only possible with one lost system.

• Replace PCC on the AHT. Transfer a spare chaser and pennant to the AHT. Pay

out the anchor chain until a connection link passes the windlass. With assistance of the rig crane and crew, the AHT transfers a rope around the anchor chain. The AHT connects a heavy sling or pig tail pennant to the rope. Connect both eyes from the sling on the work wire. The AHT takes the chain on deck and disconnects the connecting link to pass the spare chaser over the chain. Connect both ends of the chain again recover the anchor. This procedure requires a lot of handling with the AHT close to the rig.

• Use Grapnel. Grapple for the chain with a grapnel. Pull a bight of chain on the deck of the AHT and install a detachable chain chaser or use a short piece of chain as an “oil field chaser” around the chain. Recover the anchor.

All fishing jobs require a lot of skill and experience from the AHT captain. Large forces act on the recovery system, particularly when the anchor starts to pass the stern roller.



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6.19 ANCHOR WINCHES. In addition to the information from the MOM, the barge engineer and person in charge to run the anchor winches should use the technical dates related to on board the equipment. Only experienced operators should control the anchor winches. 6.19.a. Basic Operating and Instruction Principles When running anchors, pay out at the same rate as the AHT headway. Do not exceed the recommended maximum windlass speed. Leave at least 200 ft in the anchor chain locker for adjustment. Only engage the chain stopper to hang off the chain when working on the anchor winch. Local regulations may dictate a different procedure. On winches with a controlled pay out system, do not exceed the recommended maximum speed and amperes as per instruction manual. Reduce headway speed of the AHT accordingly. Maintain good communication with the captain of the AHT. Advise in time when the pay out speed approaches the maximum allowable value. Be prepared to assist the controlled pay out system with the band brake in case of excessive speed or power failure Return to band brake operation when the AHT arrives at the anchor location. Check the band brake pads and wear pattern on the brake rim at regular intervals. Adjust the band brake as required. Change pads out in time. Use of the controlled pay out system produces a sustained load on the gear box and bearings. The winch must be in good condition and serviced at regular intervals. Before operating an anchor winch, always test the functions of all systems and indicators. Check the lubrication before operating the winch. 6.19.b. General Arrangement and Function Indicators Anchor Winch. (Fig.6.21) Some common control functions on anchor winches are:

Band Brake � The band brake operates under hydraulic power. A fail safe spring

loaded design. Engage or disengage position.

Ratchet Pawl � Disengage before operating the winch. Do not engage/disengage while

the winch is running. The anchor winch can not slack off with the pawl engaged. Engage or disengage position.



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Back Stop Brake

� This is the disc brake on the motor drive shaft. The back stop brake comes on automatic in stop or off power position.

Clutch Engage

� The clutch engages the drive shaft with the wild cat gear. Engage disengage position. Do never operate when the winch wildcat is running.

Torque Converters/ Controlled Pay Out.

� Apply and adjust as per instruction manual

Indicators � Tension meters in Kips or tonnes � Air pressure. A minimum amount of air pressure is required before

starting the anchor winch. � Amount of chain out. In feet or meters. � Motor Amps. The amperes are an indication too about the amount of

tension applied to the system. Light indicator for low pressure. � Band brake. Light indicator with green and red light. Some times with

indication super brake. � Cooling water temperatures. Warning light indicator � Gear box selector. High, Low and Neutral � Haul In – Pay Out. Different from rig to rig. � Start – Stop button.

Fig. 6.21.Electric anchor winch lay out

Pivot Rising prevente Load



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6.19.c Combined chain-wire anchor winches The requirement to drill in water depth over 3000ft capacity requires a combined chain wire system as the weight of chain for deep water will not be manageable anymore (Fig.6.25). For very deep water a pre-laid system in combination with a chain-wire system on the rig enables to go to 8000ft. The anchor winch manufacturers developed various chain-wire systems. Generally, a transfer from chain to wire is achieved by special chain wire connections. The anchor winches are installed in the pontoons or on deck. (Fig 6.24 and 6.25) .Another combination can be the chain winch on deck and the wire winch in the pontoon. The chain part is deployed first followed with by the wire part. (Fig.6.23) The special chain-wire connection link provides for the transfer from chain to wire. One (Fig. 6.22)

Fig.6.22. Chain-wire connetions



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Fig. 6.23 Pictures of running sequence of a combined wire chain system.

Fig. 6.24 Pictures of wire-chain connection and combined wire chain anchor winch.



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Fig. 6.25. Various types of chain-wire mooring systems

OFFSHORE OPERATIONS COURSE SECTION 2 MOORING AND ANCHOR HANDLING CHAPTER 7 DEEPWATER DEVELOPMENTS


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CHAPTER 7 DEEPWATER MOORING SYSTEM DEVELOPMENTS

7.1 INTRODUCTION Floating drilling units historically have been moored by catenary systems. The limit of the mooring systems of the second-generation rig is about 1500ft. The third and fourth generation extended the mooring systems capabilities with combined chain wire systems to about 3000ft. A limited amount of semi submersibles of the fourth and fifth generation is able to operate up to 5000ft of water depth. Beyond the 5000ft the catenary systems becomes very problematic because:

1. The horizontal restoring efficiency decreases because the weight of the system consumes a large part of efficiency i.e. the anchor chain hangs almost vertical downwards.

2. The enormous weight of the mooring lines and winches consumes part of the variable deck load.

3. The winch capacity is restricted to lift the chain/wire from the seabed. 4. To handle the load and equipment the capacity and deck area of the AHT is critical.

To allow using mooring systems in very deep water the solutions are:

Use of a pre-laid anchor system. Use vertical loaded systems. This reduces the foot print of the mooring system

(Fig 7.1) and consequently the amount of weight Use of Vertical Loaded Anchors (VLA) Use of suction anchors or plate embedded anchors. To reduce the weight by installation of synthetic mooring lines. This allows

vertical loading and weight reduction.

Fig.7.1. Foot print catenary versus taut line mooring system.



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7.2 THE PRELAID MOORING SYSTEM. The mooring system on many rigs is limited between 1500ft and 2000ft. To utilize these rigs for greater water depth the only solution is to deploy pre-laid mooring systems. Before the rig arrives at location, the AHT deploys the anchors and chain and/or wire on the seabed. The pre-laid system is buoyed off. At arrival of the rig the AHT runs the rig mooring system towards the buoy. The buoy is retrieved on deck followed by lifting the bottom section to the AHT deck. With the utilization of the deck stoppers on the AHT the rig chain/wire end and the bottom section are connected and released with a buoy and pennant wire to enable future retrieval. The following pictures are examples of a pre-laid mooring system procedure used by the Sedco 600in Indonesia. Fig 7.2 Step 1 to Sep 4 shows the self-explaining pre-lay procedure.

Fig. 7.2. STEP 1

Fig 7.2. STEP 2

500 ft. 3" Chain

500 ft. 3" Chain

2 3/4" Wire

3000' WATER DEPTH PRELAID MOORING SYSTEM Pre-laying Anchor Procedures

Step1: Connecting Chains and Anchor Wire to the Anchor

500 ft. 3" Chain

500 ft. 3" Chain

3000 ft x 2 3/4" Wire


Step 2 : Setting the Anchor on Bottom



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Fig 7.2. STEP 3

Fig.7.2. STEP 4

500 ft. 3" Chain 500 ft. 3" Chain

2 x 3000 ft x 2 3/4" Wire


Step3: Lowering The Chain Hawser and fish plate

Fish Plate

2 1/4" Wire

2 x 500 ft. 3" Chain

2 x 3000 ft x 2 3/4" Wire

3000' WATER DEPTH PRELAID ANCHORING SYSTEM Pre-laying Anchor Procedures

Step 4: Releasing The Buoy In Water

Fish Plate

Buoy

3000 ft x 2 1/4" Wire



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Fig.7.3 Step 1 to Step 4 shows the sequence of the connection procedure between the rig mooring system and the pre-laid mooring system.

Fig. 7.3 STEP 1

Fig 7.3. STEP 2

Step1: Taking Rig Anchor Wire to Buoy Drop Point

3000' WATER DEPTH PRELAID MOORING SYSTEMHawser Connection Procedures

AHV "B"

3000' WATER DEPTH PRELAID MOORING SYSTEM Fish Plate Connection Procedures

Step2: Retreiving the Fish Plate

AHV "A"



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Fig. 7.3 STEP 3

Fig 7.3.STEP 4

Step3: Passing Rig Anchor Wire From AHV "A" To AHV "B"

3000' WATER DEPTH PRELAID MOORING SYSTEM Hawser Connection Procedures

AHV "B" AHV "A"

3000' WATER DEPTH PRELAID MOORING SYSTEM

Fish Plate Connection Procedures

AHV "B" AHV "A"

Step 4 Finishing connection on AHV “A”



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Fig 7.3 STEP 5 Fig. 7.4 Shows the design of the pre-laid system and connections to the rig system spread on the seabed for 5000ft of water depth used for the same location by the Sedco 600 in Indonesia

Fig. 7.4 Pre-laid mooring system and rig system connected and spread on the sea bed

PROPOSED 5000 FT WATER DEPTH PRELAID MOORING

than 5000', a 1000' wire will be added to the prelaid system

SYSTEM FOR 600 SERIESBuoy 49 T

55 T Shackle

30' x 2 1/4" Wire

Connecting Link

3000' x 2 1/4" Wire

Pear Link

12 ' Chain

55 T Shackle

40' x 1.5" Wire

Pear Link

Pear Link

Pear Link

Connecting

Link

Connecting

Link

Conn Link

Sw

ivel

Fishplate

3000' x 2 3/4"W

ire

3000' x 2 3/4"W

ire

500 ' Chain

500 ' Chain

Anchor

Note

- A 500' pendant will be added for water depths greater

- In water depth between 6000' and 6500', 250' chain willbe added to the system

Rig A

nchorLine

Pear Link

Sw

ivel

Connecting Link

Swivel

Connecting

Link

Conn Link

85T S

hackle

85T S

hackleC

onn Link

Revision 123 Mar. 98

Step5: Releasinging The Buoy Into Water

3000' WATER DEPTH PRELAID MOORING SYSTEM

Hawser Connection Procedures

AHV "A"

Hawser Swivel



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7.3 ANCHORS FOR VERTICAL LOAD SYSTEM (FIG 7.7) The vertical loaded mooring system utilises design anchors in combination with a full taut leg (vertical loaded) synthetic mooring rope. Various types of design anchors are available to deploy a vertical loaded mooring system such as:

Dead weight anchors. Pile anchors. Suction embedded anchors. Vertical loaded drag embedment anchors (VLAs). Suction embedded plate anchors.(SEPLAs)

Compared to the catenary mooring system the taut line system used elastic cycles to maintain the unit in position (Fig 7.5)

Fig 7.5. Catenary versus taut leg system.

7.4 DEAWEIGTH ANCHOR. The dead weight anchor is a clump weight made from a steel tank filled with heavy material. The weight can be over 80t. The dead weight anchor is mainly used for long-term single leg taut line mooring systems. The weight of the anchor in combination with the mooring line holds the floating object in position. This anchor is not common for mooring systems of semi submersibles.



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7.5 PILE ANCHOR. Pile anchors are mainly for permanent mooring system. The piles filled with grout are drilled or driven by a hammer. Consequently, the water depth is restricted to the capacity of the sub sea equipment, which is around 4000ft to place the piles. The cost is very high.

7.6 SUCTION EMBEDDED ANCHORS (SEAS) The suction embedded anchors developed from 1996 model test into an anchor for very deep water. The suction embedded anchor is a pile with a large diameter closed at the top and open at the bottom. Vent and fill lines allows to pump seawater out of the pile. The principle of the SEA is to lower the pile to self penetrate the open end by the hydrostatic pressure. Water is then pumped out from the space beneath the top until the SAE reaches the required depth into the soil. It is possible to retrieve the anchor and as such, this mooring system is applicable for permanent and temporarily deep-water mooring systems. Fig.7.6 shows a drawing from Delmar Anchors with the principles of the suction pile anchor. The SEA is expensive and heavy with weights in excess of 100t. Handling of the SEA’s

Fig. 7.6 Example of a suction embedded anchor (with courtesy of Delmar systems).



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Fig. 7.7 Picturesof anchors used for the taut line system.

Dead weight anchor Pile anchor Suction anchor

Stevmanta VLA anchor Bruce Denla VLA anchor

Suction embedded plate anchor Load out of suction anchors



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7.7 VERTICAL LOADED EMBEDDED ANCHORS (VLA) The concept of the Vertical Loaded Drag Embedded Anchors (VLA’s) developed from the observation that conventional drag embedment anchors under circumstances are difficult to retrieve. Vryhof and Bruce anchors used the principle of high retrieval forces to develop the VLA anchors. Vryhof’s Stevmanta and Bruce’s Denla anchors are the result of such VLA anchors. VLA anchors are plate anchors with relative very low weight and excellent efficiency. Both mentioned anchors have a pull-in (installation mode) and a normal (vertical) load. Depending on the manufacturers design the VLA embed deeply in a direction about 20° to 50°. After the embedment, the load position changes to an angle perpendicular to the fluke by a trigger system. The large normal load angle makes these anchors ideal for taut leg mooring systems. A wire-chain follower or so called drogue installed to the rear fluke provides for the correct orientation n of the anchor before embedment The VLA resist a normal tension load of 500t to 600t before they break out and start to drag. The installation and retrieval procedures are specific for each anchor but use the same principles as shown in running and retrieval procedures from Bruce and Vryhof anchors in Fig 7.8 to7.9. with some additional photos (Fig. 7.10 A-D) running Stevmanta and Bruce DENLA anchors and a load out picture of pile suction anchors. (Fig. 7.11)

Fig7.8 A Bruce DENLA anchor showing the installation low fluke angle versus the perpendicular triggered fluke angle for normal load position.



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Fig7.8 B. Bruce DENLA anchor embedment. Note the drogue at the fluke rear side.

Trigger the anchor fluke angle for load position Connection of pre-laid system with rig

Recover anchor-change fluke angle Recover anchor with small fluke angle Fig. 7.8 C. Bruce DENLA Anchor procedures for installing and recovering as per Bruce

documentation.



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1 2 3 Fig. 7.9.A Coloured pictures from Vryhof’s Stevmanta anchors installation sequence

From left to right – 1) Embedding the anchor – 2) Shear pin.- 3) Load position Fig.7.9 B. Drawings of Vryhof’s Stevmanta with same principle of installation as Fig.7.9A.

Fig. 7.9 C. Drawing with one of the retrieval procedures of Vryhof’s Stevmanta anchor.



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Fig. 7.10 A Stevmanta and Bruce DENLA anchor on deck. .

Fig. 7.10 B. Picture of synthetic rope on reel



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Fig. 7.10 C. Running deepwater Stevmanta anchors with two AHT’s

Fig.7.10 D. Retrieving Stevmanta anchor



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Fig. 7.11 Load out of pile suction anchors.

7.8 SUCTION EMBEDDED PLATE ANCHORS.(SEPLAS) The patented Suction Embedded Plate Anchor (SEPLA) is a combination of a suction pile and embedded plate anchor. The suction pile is reusable and functions to install the plate embedded anchor. The pile or so-called follower lifts to the surface after the plate embedded anchor reached the position depth.

7.9 SYNTHETIC ROPES FOR DEEPWATER MOORING. Compared to the chain/wire catenary system, the use of synthetic ropes dramatically increased because of the many advantages for deep water mooring systems:

Low submerged weight. Restoring forces provided by elastic elongation (for taut leg system under 45°) Smaller foot print. Less load to carry. Better station keeping. Very low leeward tension. Reduces possible damage to pipelines an sub sea structures.

The disadvantages are:

Large diameter. Requires long length to provide the best performance (minimum amount of inter

connections).



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As an example, the weight of average steel rope for mooring systems with a diameter of 96mm (3”) is about 22½ lbs/ft compared to 3½ lbs/ft for polyester. Like with wire rope, various designs are available. However, the synthetic rope construction is much more complicated (Fig. 7.12). The most popular materials for the offshore industry synthetic mooring systems are:

1. Aramid fibres with high strength and low extension. 2. Polyester with synthetic multifilament fibre. 3. Polypropylene with floating synthetic fibre 4. HMPE which stands for High Modulus Polyethylene

Further design improvement could probably allow to use synthetic mooring line system up to 15,00ft in the future



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Fig. 7.12. Examples of synthetic rope construction



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CHAPTER 8 MOORING SYSTEM CERTIFICATION AND INSPECTION

8.1 CERTIFICATION OF ANCHOR CHAIN The Anchor chain manufacturer supplies the chain with the Class Society Certificates and Quality Assurance (QA) Standards such as API, ISO 9001 and ISO 9002. When ordering chain or chain connections always specify the certification and QA requirements.

8.2 TYPE AND SCHEDULE OF INSPECTION The anchor mooring system endures rough handling, high loads, corrosion, and wear and tear. Scheduled inspections are essential to prevent failure. Most authorities and companies use the API RP21 “Recommended Practice for In-Service Inspection of Mooring Hardware for Floating Drilling Units” which provides guidelines for the anchor chain and wire inspection The inspections are part of the PMS system or by classification societies requirements. The persons and /or authorities to carry out the inspections are:

The staff on board When retrieving the anchor chain the barge engineer it is good practise to assign one person to visual check the chain

The official inspectors

The Classification Societies. Flag Government Regulations. In co-ordination with Classification Societies.

The API recommended inspection schedule for chain and wirer is :

No of Years in Service Chain

Max. Intervals Between Major Inspections Chain

No of years in Service Wire

Max. Intervals between Inspection Wire

0 – 3 36 months 0-2 18 months 4 – 10 24 months 3-5 12 months

Over 10 18 months Over 5 9 months



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8.3 INSPECTION LOCATION The inspection location is:

Offshore The advantage is that no dry dock is required. The disadvantage is that it is quite difficult to inspect the chain. Pay out all chain with the assistance of an AHT. The last section of the chain is the most difficult part. Depending on the situation, bring the last section on deck or transfer all to the AHT. (Fig. 8.1)

In Dry Dock

The advantage is that the chain is laid out and is easy accessible for cleaning and inspection. The disadvantage is that a dry dock is required. (Fig. 8.2). Semi Submersibles normally do not go in dry dock. Only for major modification such as installation of additional buoyancy, a dry dock is required to carry out the work. For major inspections the Semi Submersible often lays alongside a quay. The quay may give room to lay out the chain for inspection. Because the modern AHT have enough chain locker space to store up to 10.000ft of chain, some rigs have one spare chain to rotate for inspection. Between locations one chain at a time is transferred ashore for inspection. This system reduces the down time considerable.

Fig 8.1 Chain inspection offshore.(As par API picture



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Fig. 8.2 Chain inspection onshore



111

8.4 ANCHOR CHAIN INSPECTION (FIG.8.3) a) By Staff on Board When the chain passes over the anchor winch during retrieving operations, check the chain on:

Any deformation on the links. Missing or loose studs. Wear or excessive friction on the wildcat. Lead plugs on the connection links. Check trouble free passing of chain over the wildcat (worn wildcat)

b) By Class Society Inspectors (Fig.8.4 API-RP 21Recommended Practice)

Missing studs. Possible cause, high bending stresses and low fatigue life. Loose studs. Possible cause, abusive handling or excessive corrosion. Bent links. Possible cause, rough handling, excessive torque and worn wildcat. Corrosion. Sharp gouges. Possible cause, contact with obstructions and sharp edges Cracks. Possible cause, high cyclic loading. Heavy wear on wildcat and links. Possible cause wear and tear because of the age

of the system Elongation Possible cause, wear in the grip area of the chain and excessive work

loads.

Fig 8.3. Examples of chain damage

Loose stud

Missing

Bend link

Excessive



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Fig. 8.4. Criteria and inspection methods as per API RP21 strandards As an indication, an MPI is performed on 5 links in every 100 meters and the diameter of one link in every 100 meters is checked

8.5 ANCHOR WIRE INSPECTION The anchor wire is subject to the same certification and inspection procedures. The inspectors look at:

Broken wires Change in rope diameter Wear and wear patterns Loss of lubrication Deformation Termination damage

Figure 8.5 and 8.6 shows the method to measure wire and various wear and damage conditions of wire. The figures are useful for any wire inspection (pennants, slings, crane wires).

Fig. 8.5. Wire diameter measurement



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Fig. 8 6 Typical wire damage and deterioration examples (Courtesy of S.M.R.E.).



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The wire rope termination is prone to damage. The first signs of the wire rope termination deterioration is splits in the sockets, socket draw back from original position, an broken wires at the termination.( Fig. 8.7)

Fig. 8.7 Typical wire rope termination damage and deterioration.

OFFSHORE OPERATIONS COURSE SECTION 2 MOORING AND ANCHOR HANDLING CHAPTER 7 APPENDIX


115

CHAPTER 9 APPENDIX SECTION II –MOORING

9.1 ANCHOR CHAIN PROOF AND BREAKING LOAD TABLES SI UNIT S.

9.2 ANCHOR CHAIN PROOF AND BREAKING LOAD TABLES INCH/PO UND UNITS.

9.3 STEVPRIS MK5 DRAWINGS, WEIGHT, AND DIMENSION.

9.4 VRYHOF STEVPRIS MK5ANCHOR UHC CHART

9.5 VRYHOF STEVPRIS MK5 ANCHOR DRAWINGS, WEIGHT AND DIMENSIONS

9.6 VRYHOF STEVMANTA ANCHOR UPC

9.7 BRUCE FFTS MK4 ANCHOR DRAWINGS, WEIGHT, AND DIMENSI ONS

9.8 BRUCE FFTS MK4 ANCHOR HOLDING CAPACITY CHART.



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9.1 – ANCHOR CHAIN PROOF AND BREAKING LOADS SI UNIT S



117

9.2 ANCHOR CHAIN PROOF AND BREAKING LOADS IMPERIAL UNITS



118

9.3 VRYHOF STEVPRIS MK5 ANCHOR (WITH COURTESY VRYHO F ANCHORS)



119

9.4 VRYHOF STEVPRIS MK5 ANCHOR UHC (WITH COURTESY VRYHOF ANCHORS)



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9.5 VRYHOF STEVPRIS MK5 ANCHOR (WITH COURTESY VRYHO F ANCHORS)



121

9.6 VRYHOF STEVMANTA ANCHOR UPC (WITH COURTESY VRYHOF ANCHORS)



122

9.7 BRUCE MK4 ANCHOR (WITH COURTESY BRUCE ANCHORS)



123

9.8 BRUCE FFTS MK4 ANCHOR HOLDING CAPACITY CHART.