Anchor Handling Stability

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Formulating Stability Requirements for Anchor Handling Tug and Supply (AHTS) Vessels

Dephne Chea Wei Peng, Arun Kr Dev, Ivan Tam, (Newcastle University)

Kenneth Hanks, Kalyan Chatterjea (EMAS Training Academy & Simulation Centre)

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BACKGROUND No specific internationally applicable stability requirements for

AHTS vessels.

After AHTS “Bourbon Dolphin” tragedy in April 2007, initiatives were taken for improving Design

Operational safety & specifically

Stability and the performance of anchor handling winches.

Probably the most important initiative is from the Norwegian Maritime Directorate (NMD):

NMD Circular - Series V, RSV 04-2008, 14 July 2008.

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IMO Guidelines…BV Presentations, 2010 @ KL

Refer to Intact Stability Code, 2008, IMO Res. MSC.267(85). Code covers

intact stability criteria &

addresses offshore supply ships & special purpose ships,

requirement for a minimum freeboard at the stern of at least 0.005L to be maintained in all operating conditions.

Code does not include any specific stability criteria for towing and anchor handling operations

Minimum required GM0 = 0.15 m should NOT be considered as sufficient stability margin for towing and anchor handling.

.

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IMO Criteria for OSV

Max GZ of an OSV is allowed to occur at much smaller angle of heel than normal, providing that the positive stability up to this angle of heel (area A) is greater than for a cargo ship

This requires a large upright GM

Minimum freeboard criteria for OSV is inadequate for anchor-handling operation

Clark & Hancox (2012)

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Recommendation from Marine Safety Forum (MSF)

Quoting from Anchor-handling Manual template by Marine Safety Forum (MSF), incorporating recommendations from NMD. Prior to sailing, a document must be displayed on the

bridge, where it is visible to be navigator on duty, to show the acceptable vertical and horizontal transverse force/tensions to which the vessel can be exposed.

This should show a sketch of the GZ curve and a table of the tension/forces which give the maximum acceptable heeling moment.

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Further quoting from the Anchor-handling Manual template by MSF incorporating recommendations from NMD. Calculations must show the maximum acceptable tension in

wire/chain, including transverse force, that can be accepted in order for the vessel’s maximum heeling to be limited by one of the following angles, whichever occurs first:-o Heeling angle equivalent to a GZ value equal to 50% of GZ max.

o The angle of flooding of the work deck – i.e. the angle which results in water on working deck when the deck is flat.

o 15 degrees.

Recommendation from Marine Safety Forum (MSF)

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NMD Criteria

1. Heeling angle equivalent to a GZ value equal to 50% of GZ max.

2. The angle of flooding of the work deck – i.e. the angle which results in water on working deck when the deck is flat.

3. 15 degrees.

Vessel’s Maximum Heeling to be limited by one of the following angles, whichever occurs first:

Heeling moment must be calculated as the total effect of the horizontal & vertical transverse components of the force /tension in the wire/chain

The torque arm of the horizontal components shall be calculated as the distance from height of the work deck at the guide pins to the centre of the main propulsion propeller or to the centre of stern side thruster if it projects deeper

The torque arm of the vertical components shall be calculated from the centre of the outer edge of the stern roller & with a vertical straining point on the upper edge of the stern roller

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External Acting Forces & Healing Moments…NMD Proposal to IMO

α – angle between towline & ship’s centre line

β – angle between towline & the waterplane

y & v – refers to application point of the of the line tension

Ft – towline tension

Unified Stability Criteria & Operational Guidance for AH Vessels

Application Point & Direction of a Towline in the Stern of a multi-operational mode vessel

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External Acting Forces & Healing Moments…NMD Proposal to IMO

Heeling Moment MH = (Ft .sin α.cos β x v)+(Ft .sin β x y)

Application Point & Direction of a Towline in the Stern of a multi-operational mode vessel

TransverseComponent

VerticalComponen

t


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Tension Directions vs Healing Moments MH

…NMD Proposal to IMO

A typical exampleof Heeling Momentswith some specific‘y’ & ‘v’ levers

β

α

So, in general, As ‘α’ increases

MH is increasing As ‘β’ increases

MH is decreasing


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Influence of ‘α’ & ‘β’ on MH

…NMD Proposal to IMOα

As ‘α’ is increased MH could reach to an unacceptable level!

β

MH


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Concept of Constant Heeling Moment…NMD Proposal to IMO

The Norwegian proposal is based on a concept of α constant heeling moment MH , providing limits

to the line tension only dependent on sideways angle ‘α’

From earlier,Heeling Moment MH = (Ft .sin α.cos β x v)+(Ft .sin β x y)

Rearranging,

‘β’ can be defined as a function of ‘α’

VerticalComponen

t

TransverseComponent


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Fixed Moment vs Tension…NMD Proposal to IMO

Tension distribution with a constant Heeling Moment


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Operating Tension vs Angle Alpha…NMD Proposal to IMO

Finally, a simplified Angle Alpha vs Tension could be created

For use by the operators

15 30 60 90


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Graphical Presentation…NMD Proposal to IMO

A simplified graphical presentation on bridge

T1 – 0 to 150 T2 – 15 to 300 T3 - 30 to 600 T4 - 60 to 900


DF – 1.4 to 1.6 times the static load is common

Sector-1

Sector-2

Sector-3

Sector-4

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Stability Limiting Curves…NMD Proposal to IMO


𝑯𝒆𝒆𝒍𝒊𝒏𝒈 𝑳𝒆𝒗𝒆𝒓 𝑯𝑳∅=𝑴𝑯

𝒈 .∆.𝒄𝒐𝒔∅

For AH-Operations, the stability limiting curves should be developed as a function of draught/displacement against initial KG or GM & applied tension

Covering lightest anticipated draught to Summer Load Line & trim range

Heeling Angle

Flooding Angle

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Stability Limiting Curves…NMD Proposal to IMO


𝑯𝒆𝒆𝒍𝒊𝒏𝒈 𝑳𝒆𝒗𝒆𝒓 𝑯𝑳∅=𝑴𝑯

𝒈 .∆.𝒄𝒐𝒔∅

The minimal residual area between the righting lever curve and the heeling lever curve ≥ 0.055 m-rad [θe to θf or θc

whichever is less]

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IMO Amendment - Part B 2008 IS Code

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Ultra Deep Water Multifunctional AHTS Vessel

LOA – 93.4mLBP – 82.0m B – 22.0m D – 9.5m (main deck)Design Draught – 6.5mBollard Pull – 300 tonnes

AH/Towing Winch – 500 tonnes pull [600 tonnes brake]Propulsion - 23,467 BHPDP IIDeadweight Approx. 4,700 TGRT Approx. 6,000 T

Lewek Fulmar

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Lewek Fulmar uses AUTOLOAD 6AUTOLOAD 6 ASUMPTIONS:

Tension from chain is placed at the full breath of aft roller, & the chain angle (tension) is calculated from 0o(vertical) to 90o(horizontal)

The vertical moment arm from main thrusters (or aft side thrusters if existing) and up to aft roller is kept constant, independent of vessels heel angle.

The horizontal moment arm from end of aft roller to centerline is kept constant, independent of vessels heel angle.

Both the effect of the horizontal forces and the tensions offset from CL is converted to an external moment acting on the vessel.

To keep correct displacement during all calculations, the vertical tension component (the weight) is placed at centerline on top of aft roller.

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Lewek Fulmar uses AUTOLOAD 6

Tank Sensors in AUTOLOAD 6:

Autoload installations can be fitted with a Tank Sensor program that interfaces directly with Autoload

Thus providing immediate tank loadings for an accurate and up-to-the-minute analysis of the vessel’s stability.

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NMD stability criteria for anchor handling is incorporated in AUTOLOAD 6

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NMD stability criteria for anchor handling is incorporated in AUTOLOAD 6

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General Interface in AUTOLOAD 6Hydrostatics Anchor-Handling Margins

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IMO 469 Intact Stability in AUTOLOAD 6Righting Arms vs. Heel - IMO 469, INTACT STABILITY

Heel angle (Degrees)

Arms in

m

0.0 10.0p 20.0p 30.0p 40.0p 50.0p 60.0p

0.0

0.5

1.0Righting Arm

R. AreaFlood Pt

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NMD Criteria in AUTOLOAD 6Righting Arms vs. Heel - NMD ANCHOR HANDLING CRITERIA


Arms in

m

0.0 10.0s 20.0s 30.0s 40.0s 50.0s 60.0s 70.0s

-0.5

0.0

0.5

1.0Righting Arm

Heeling ArmEquilibrium

Crit. Pt

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Arms in

m

10.0s 20.0s 30.0s 40.0s 50.0s 60.0s 70.0s

-1.0

-0.5

0.0

0.5

Righting Arm


Crit. Pt

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Arms in

m

10.0s 20.0s 30.0s 40.0s 50.0s 60.0s 70.0s

-1.5

-1.0

-0.5

0.0

0.5

Righting Arm


Crit. Pt

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Conclusions

Recommendation for Stability Requirements during Towing & Anchor-handling Operations are likely to be covered by the Par ‘B’ of the Intact Stability Code, 2008, IMO Res. MSC.267(85) in 2014

We should be prepared with a suitable course to support the professionals at the operational level

We hope there will be continuous support from the academia

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Thank You!!

Technology

Anchor Handling Stability