The Preliminary Front-End Engineering and Design of Manganese Nodules Mining Vessel

Dae-Hoon Kang, Hyo-Dong Kang, Kwang-Min Lee, Min-Cheol Ryu, Sung-Geun Lee

Subsea R&D Group, DSME R&D Institute, Deawoo Shipbuilding & Marine Engineering Co., Ltd Nonhyun-Dong, Gangnam-Gu, Seoul, 135-010, Korea

Se-Hun Park Korea Ocean Research & Development Institute

1270 Sadong, Ansan, 426-744 Korea

The topic of this paper is preliminary front-end engineering and design (pre-FEED) of mining vessel (MV). The vessel will be used exclusively for mining manganese nodules located at around 5,000 meter water depth of northern east Pacific Ocean, called Clarion-Clipperton fracture zone. In this engineering process, the scope of MV’s pre-FEED is confined to design basis, general arrangement (G/A), trim and stability calculation, motion analysis, down time analysis, DP capability analysis and electric load analysis (ELA).

Keywords-component; mining vessel; manganese; pre-FEED; design basis; general arrangement; down time analysis; electric load analysis.

I. INTRODUCTION

Fig. 1 Clarion-Clipperton fracture zone (Courtesy of International Seabed Authority)

Target site for MV’s pre-FEED is Clarion-Clipperton fracture zone located in the northeastern tropical Pacific Ocean (Fig. 1). Water depth in this region varies between about 4,000 and 5,500 meters.

Systems for developing manganese nodules consist of the

following three components: a mining system to collect manganese nodules on the seabed; a nodule lifting system for transferring manganese nodules to the surface; and a surface system to store and unload the nodules and maintain the above

two systems. It is highly necessary to establish well-prepared integration and maintenance strategy for efficient development of the systems. Among the above-mentioned components, the subject of this paper is design of the surface system. Pre-FEED, a preliminary front-end engineering and design, has been carried out to develop a vessel which is called Mining Vessel (MV). Generally pre-FEED means a concept design in offshore oil & gas field. The scope of pre-FEED in this paper is defined as follows:

• definition of design philosophy to clarify a design basis and concept;

• general arrangement (GA) to arrange main compartments by considering rules and design philosophy;

• trim and stability calculation to verify the hydrostatic and structural stability of MSV;

• motion and wave load analysis for topside equipment design and structural design of hull;

• downtime analysis based on the motion response to estimate working days;

• dynamic positioning system capability analysis to estimate required thruster’s specification;

• electric load analysis (ELA) to calculate minimum power consumption.

II. DESIGN BASIS Main functions of MV, unlike other offshore vessels,

include storing manganese nodules transferred through the nodule lifting system, unloading them to a barge or a bulk carrier and supporting both the nodule lifting system and the mining system. When capacity of cargo tank is estimated, the following factors are should be considered:

• nodule production a year;

• target operating days a year

• offloading frequency;

• specific gravity (SG) of nodule;

978-1-4577-2091-8/12/$26.00 ©2011 IEEE

Annual production and workable days for pre-FEED are set as follows; (1) annual production is 1,500,000 tons; (2) workable days a year are 250. Specific gravity of nodule is 2.0.

Special considerations should be needed at the design stage to satisfy workable day requirement because MV operates for a long period in a specific region with harsh environmental conditions and downtime is likely to be occurred due to possible malfunction, faulty maintenance of equipment or excessive motion response.

MV should be equipped with riser handling machines for nodule lifting system that are composed of several lifting pumps, rigid risers, butter tank and jumper. The handling equipments of MV and conventional handling machines of drilling rig are quite similar. Space and payloads for nodule lifting risers should be considered in pre-FEED.

Manganese nodules transferred by the lifting system are stored in cargo tanks until amount of nodules reaches a specific filling rate. Afterwards, they are offloaded through an unloading system to a barge or a bulk carrier. In selecting the unloading system, unloading rate and environmental limitation such as wind speed and wave height are significant parameters.

Once MV arrives at operation site, the vessel lowers a feeder, buffering chamber linked to the miner by flexible riser, in order to be connected with nodule lifting riser. Second, the feeder is connected to the riser and then it sinks to operation water depth. Third, nodule lifting risers are connected each other continuously in the derrick and lowering through the moonpool. Finally, normal and offloading operations begin sequentially.

MV has several operation conditions that can be occurred during mining operation. Among them, five (5) cases were selected as primary operating conditions summarized in TABLE 1. These conditions are generally used for trim & stability calculation and motion analysis.

TABLE 1 DEFINITION OF OPERATION CONDITIONS No Operation Condition Description

1 Lowering feeder(buffer) through splash zone

Feeder connected to lifting riser is lowered through splash zone

2 Connecting flexible riser to feeder(buffer)

Flexible riser linked to miner is connected to feeder by ROV

3 Lifting riser handling/ connection Riser is lifted, fed and connected by the riser handling system

4 Normal operation Miner is in operation and produced nodules are lifted through the riser

5 Offloading operation Nodules in storage tank are unloaded to a transport barge or bulk carrier by

the unloading system

MV has to experience a lot of loading conditions for life time. Designer should select critical loading conditions, taking

into account vessel and operation characteristics. In this design stage, five (5) loadings conditions were defined and each condition has both 10% and 100% of fuel oil ratio. In TABLE 2, loading conditions are summarized for each operation condition. For example, lowering feeder and connection riser condition have only light and heavy ballast loading condition because those two operation conditions are not producing nodules. But for three operation conditions, there are tank-filling conditions. It should be performed that trim & stability calculation, motion analysis as well as wave load analysis for all loading conditions.

TABLE 2 LOADING CONDITIONS FOR OPERATION CONDITIONS Operation Condition Loading Condition

Lowering feeder through splash zone - Light Ballast Condition - Heavy Ballast Condition Connecting riser to feeder

Lifting riser handling/ connection - One-Tank Full Load Condition - Two-Tank Full Load Condition - Three-Tank Full Load Condition

Normal operation

Offloading operation

Specific gravities of water and oil for the trim and stability calculation are specified in TABLE 3.

TABLE 3 SPECIFIC GARVITIES

Contents Specific Gravity Specific Heat (kcal/kg℃) Viscosity

Sea Water 1.025 0.94 -

Fresh Water 1.000 1.00 -

Heavy Fuel Oil 0.980 0.45 180 sCt at 50℃

Marine Diesel Oil 0.850 0.45 6 sCt at 40℃

Lubricant Oil 0.900 0.45 70 sCt at 50℃

Design speed of MV at transit draft on even keel and given moonpool effect is set to be approximately 14.0 knots with the six (6) azimuth thrusters running in full without sea margin. Hull shape development for design speed will be carried out at the next design stage of FEED. Specification of propulsion and generator engine was selected by referring to several offshore projects. ELA (electric load analysis) and DP capability analysis should be performed at this design stage to verify the following specifications.

• Thruster - Type: Azimuth, FPP (flexible pitch), 6EA

- Capacity: Electric Motor 5,500 kW • Main Diesel Generator Engine

- Type: 4 stroke, trunk piston, V-type x 8 sets - Rated output: 7,000 kW x 720 rpm

MV should have dynamic position system, and the system should be implemented for automatic control of all thrusters with the aim of providing station and course keeping control. The system should efficiently control the horizontal motions of MV. MV’s station keeping capabilities are defined as TABLE 4.

TABLE 4 DEFINITION OF STATION KEEPING CAPABILITY Station Keeping

Capability Description

Position keeping for normal operation

Keep position within the offset range acceptable for normal operations

Position keeping for standby operation

Keep position within the offset range acceptable for standby mode operations

MV’s positioning system enables MV to conduct normal

operations in normal environmental condition. Under the environmental conditions defined as standby, all operations should be suspended and MV should be at operation draft with the lifting riser connected.

Under the environmental conditions exceeding the standby limit, the riser should be disconnected and pulled or hung off. It should be made sure that environmental forces act concurrently from the direction which maximizes total force or create the strictest operating constraint. Environmental conditions for operation conditions are given in TABLE 5.

TABLE 5 ENVIRONMENTAL CONDITIONS

Normal Operation

Max. Operation (Standby)

Wind Speed (1min.) (m/s) 22.0 25.0

Current Speed (Surface, m/s) 0.5 0.7

Significant Wave Height (m) 5.5 8.5

Wave Period (s) 10.5 13.0

Wave Spectrum PM(Pierson-Moskowitz)

MV has quite strict limitations of motion responses in operation because equipments located on the deck of MV have design values. As result of combining equipment’s design value, acceptable motion for each operation condition can be estimated in TABLE 6. Those value are based on the previous offshore projects and applied in the downtime analysis

TABLE 6 MAXIMUM ACCEPTABLE MOTIONS

Type of Operation Max. Accepted Motion

Pitch (deg)

Roll (deg)

Heave (deg)

Lowering feeder through splash zone 2.0 2.5 2.0

Connecting riser to feeder 2.0 2.5 2.0 Lifting Riser Handling 3.5 4.0 3.5

Normal Operation 5.0 5.5 5.0 Offloading Operation 2.0 2.5 2.0

III. GENERAL ARRANGEMENT In this design process, arrangement of main compartment

and main dimension of the vessel and main compartment should be decided by considering regulations and codes, main functions of vessel and correlations between each sections.

Main compartments of MV’s hull can be divided into manganese storage tanks, ballast water tanks, fuel oil tanks and machinery space. Additionally, nodule unloading system, riser handling system and racking area are installed or secured on the upper deck. Relations between tanks and equipments should be considered to arrange main compartments.

Manganese storage tanks are located at forward part of the vessel because this operation is very frequent and can be monitored with the unaided eye from the deck house. For the riser handling operation, a moonpool is arranged at amid ship as nearly as possible to minimize the effect of vessel’s motion responses and downtime. Fuel oil and cargo ballast tanks are arranged behind the moonpool in order to adjust trim of the vessel.

Fig. 2 shows preliminary general arrangement and principle dimensions of MV were estimated by considering design basis, which are summarized in TABLE 7.

Fig. 2 General Arrangement

TABLE 7 MAIN DIMENSIONS

Length Overall Approx. 265.1 m

Length between perpendicular 257.1 m

Breadth 42.0 m

Depth 19.0 m

Transit draft 8.0 m

Operating draft 10.0 m

Designed draft 12.6 m

Scantling draft 13.0 m

Accommodation and helideck are located in forward part and life boats are installed in both forward and afterward parts. The unloading system is located at fore part, and self-unloading system was selected as an offloading measure in reflection of the design basis. Derrick is installed above the moonpool while riser rack and handling system are arranged in after part.

Breadth of wing ballast tank is much larger than that of other vessels, almost the same size with cargo tank, because of operation requirement. When manganese nodules are unloaded, draft and trim of MV can change significantly without ballast system. If relative motion between heave compensator and flexjoint increases, motion absorption capacity decreases. Consequently, probability of downtime is likely to be high. High water ballast capacity is needed to minimize the change of draft during unloading operation as well as to meet transit draft requirement for guaranteeing design speed.

Fig. 3Midship section at frame no. 65

Capacity of storage tanks was calculated by considering

annual production rate and offloading method. It was assumed that transport route between Clarion-Clipperton fracture zone and Mexico combined with round trip would take 24 days including 20% of buffer. Required tank capacity was calculated by taking daily production rate and transportation frequency into account. Required number of tank is three, size of each tank is 6,400m3 and total capacity is 19,200m3. Capacities of other tanks are summarized in TABLE 8.

TABLE 8 TANKS CAPACITY Cargo tanks (3EA) approx. 19,200 m3

WB tanks including peak tanks (21EA) approx. 100,000 m3 HFO tanks including settling and service tanks (10EA) approx. 10,000 m3

FW tanks (4EA) approx. 1,200 m3

LO tank (4EA) approx. 200 m3

With the storage capacity of the tanks, manganese nodules should be offloaded every 5 days and 1-day-buffer was considered for contingency. Handymax bulk carrier was selected as a transportation vessel and a total of 5 vessels were needed.

Special equipments should be installed on MV for mining, lifting, storage and offloading operation. Self-unloading system was adopted to offload manganese from storage tanks to a transport barge or a bulk carrier. Self-unloading system can discharge dry bulk cargo without assistance from any shore-side equipment or personnel. The system is capable of transporting and unloading almost any free-flowing, dry bulk commodity, including iron ore, coal, limestone, sand, gypsum, and grain. The system can self-discharge cargo at rates up to 10,000 tons per hour. The fast discharge ratio was a determining factor behind the selection of the system given that downtime of offloading is most likely to be occurred when motion response exceeds limitation due to harsh environmental condition.

Manganese nodules are unloaded by a conveyor system embedded in MV. The storage tanks are hopper-sloped, or slanted on their sides, so that nodules will flow down through gates located at the bottom of the tanks. Nodules drop onto a tunnel conveyor belt, which carries nodules to one end of MSV and transfers it onto a loop or incline conveyor belt system.

This system carries nodules up to the main deck of the vessel where it is then transferred onto the boom conveyor belt. The boom conveyor can be lifted and swung hydraulically left or right to position nodules on the dock or into a receiving hopper. The length of the boom conveyor is approximately 60 meters. Fig. 4 shows a conventional self-unloading vessel.

Fig. 4 Self-unloading vessel (Courtesy of DSME)

A hatch cover installed on the top of storage tank was

designed and its size is 15.0 m (L) x 17.0m (B).

IV. TRIM AND STABILITY CALCULATION The objective of this design process is to validate the design

of a vessel by using international rules and to calculate hydrostatic stability and loads acting on a vessel. The results of loads, vertical shear forces and bending moments, are input of structural design. In the calculation, a trim, defined as difference between forward draft and afterward draft, should be zero in order to reduce the possibility of downtime.

Loading conditions for the calculation are: (1) lightship condition; (2) transit condition; and (3) operation condition. NAPA program was used for this calculation, and Fig. 5 shows calculation result of three-tank full load condition (LC11).

Fig. 5 Trim and Stability calculation result for full loading condition

According to the calculation, all loading conditions were

satisfied with the design rules. Main results such as displacement, draft, trim, GM (transverse metacentric height), KG (vertical height of centre of gravity) and maximum bending moment are summarized in TABLE 9. For example, the maximum bending moment and shear forces were -208,044 ton-m and -5556.1 ton respectively for three-tank full with

100% of fuel oil condition. Forces and weight along the hull are represented in Fig. 6.

-250000

-200000

-150000

-100000

-50000

0

50000

100000

150000

200000

250000

-8000

-6000

-4000

-2000

0

2000

4000

6000

8000

-10 10 30 50 70 90 110 130 150 170

Bending Moment (ton-m)

Weight, Shear Force (ton)

Weight

Shear Force

Bending Moment

Fig. 6 Longitudinal forces for loading condition 11

TABLE 9 RESULTS OF TRIM AND STABILITY CALCULATION

No Loading Condition

DIS (ton)

Draft (m)

TRIM (m)

GM (m)

KG (m)

Max.B.M (ton-m)

LC1 Lightship 42,108 4.45 -1.69

12.65 22.00 123,695

LC2 Light Ballast 10% of Fuel

77,274 8.00 0.00

4.97 14.28 67,565

LC3 Light Ballast 100% of Fuel

72,360 7.51 0.00

6.26 13.71 55,548

LC4 Heavy Ballast 10% of Fuel

97,318 10.00 0.00

5.82 12.53 -74,097

LC5 Heavy Ballast 100% of Fuel

88,782 9.15 0.00

7.38 12.11 -146,474

LC6 One-Tank Full 10% of Fuel

95,159 9.79 0.00

4.15 13.58 -161,894

LC7 One-Tank Full 100% of Fuel

91,157 9.39 0.00

4.93 12.96 -173,599

LC8 Two-Tank Full 10% of Fuel

98,966 10.16 0.00

3.37 14.54 175,251

LC9 Two-Tank Full 100% of Fuel

94,968 9.77 0.00

4.08 13.99 186,292

LC10 Three-Tank Full 10% of Fuel

123,439 12.58 0.00

4.27 13.36 -184,736

LC11 Three-Tank Full 100% of Fuel

111,163 11.37 0.00

4.82 13.70 -208,044

V. MOTION AND WAVE LOAD ANALYSIS In FEED stage, motion analysis should be carried out to

find out motion characteristics of floating structures and calculate motion responses as well as dynamic wave loads. Motion responses are used in design of topside equipments and major supports. For structure design of a hull, not only

hydrostatic forces, i.e. still water shear forces and bending moments but also dynamic wave loads should be taken into account.

For these analyses, recognized software, WADAM of DNV, was used. The analysis was performed for several selected loading conditions and detail data are given in TABLE 10, TABLE 11 and TABLE 12. Several significant wave heights and spectrum peak periods were used in order to carry out sensitivity study with respect to wave data. Spreading functions was not considered for conservative designs. Motion responses, especially sway and roll, of LC5 are most likely to be larger than under other conditions because high GM value means that roll natural period of the vessels tends to approach the peak period of wave spectrum.

TABLE 10 ANALYSIS CONDITIONS Case Case I Case II Case III

Water Depth 5,000 m

Wave Spectrum Pierson-Moskowitz

Hs(m) 8.0 8.5 9.0

Zero crossing period, Tz (sec) 9.5-11.5

Spreading Function none

Loading condition LC 5, LC 8, LC11

TABLE 11 MAIN PARTICULARS FOR THE ANALYSIS

Loading Condition Unit LC5 LC8 LC11

Equivalent Draft m 9.15 10.16 11.37

Draft at A.P. m 9.15 10.16 11.37

Draft at F.P. m 9.15 10.16 11.37

Displacement ton 88,782 98,966 111,163

Longitudinal Center of gravity m -3.882 -3.915 -3.953

Transverse Center of gravity m 0.000 0.000 0.000

Vertical Center of gravity m 12.140 23.336 13.780

Roll Radius of Gyration m 17.94 17.98 18.50

Pitch Radius of Gyration m 68.26 68.27 68.52

Yaw Radius of Gyration m 70.86 70.86 70.97

TABLE 12 POINTS OF EQUIPMENT FOR MOTION ANALYSIS

No Point X(m) Y(m) Z(m)

1 Riser Handling Crane 25.69 19.80 65.38

2 Derrick Top 128.70 0.00 119.23

3 Derrick Floor 128.70 0.00 43.69

4 Conveyor Boom Tip 211.90 0.00 38.44

2

3

4

1

Fig. 7 Equipments points for motion analysis

Specific points of topside equipment used for motion analysis can be found in both and Fig. 7 and TABLE 12.

Fig. 8 shows panel model for hydrodynamic analysis. Moonpool should be included in the panel model to assess its effects. Flow in moonpool affects vertical responses, heave and pitch mode. Natural frequency is defined as 2π(h/g)0.5, and the effect of moonpool can be found in heave and pitch RAO at natural frequency of moonpool.

Fig. 8 Panel model for calculating hydrodynamic coefficients

Motion and acceleration results are summarized in TABLE

13 and TABLE 14 respectively. In motion results, it was found that roll responses were much higher than pitch, and acceleration results showed the same tendency. The results were higher than those of other ship-shaped offshore structures by as much as 20 to 30%. Motion characteristics need to be improved since these results are used for topside equipment design. For example, bilge keel is a viable solution, GM should be lower than the present design to reduce roll response and transverse acceleration and viscous roll damping from model tests may be considered.

TABLE 13 MOTION RESPONSES FOR LC5 & CASE III

Point

Case3 Loading Condition 05

Surge (m)

Sway (m)

Heave (m)

Roll (deg)

Pitch(deg)

CoG 3.18 6.22 8.61

15.63 4.55 1 2.76 14.97 9.29 2 6.61 28.76 8.61 3 1.93 9.86 8.61 4 1.97 9.56 8.63

TABLE 14 MOTION RESPONSES FOR LC8 & CASE III

Point

Case 3 Loading Condition 08

Surge (m)

Sway (m)

Heave (m)

Roll (deg)

Pitch (deg)

CoG 3.19 6.20 8.78

4.43 4.85 1 3.13 8.93 10.09 2 7.27 12.52 8.78 3 1.92 7.63 8.78 4 2.05 7.38 9.65

TABLE 15 ACCELERATION RESULTS FOR CASE III

Point

Case3 Loading Condition 05 Loading Condition 08

Longi.Acc(g)

Trans.Acc(g)

Ver. Acc(g)

Longi. Acc(g)

Trans.Acc(g)

Ver. Acc(g)

CoG 0.05 0.56 0.29 0.05 0.22 0.29 1 0.19 0.33 0.34 0.20 0.19 0.36 2 0.32 0.82 0.29 0.35 0.25 0.29 3 0.12 0.46 0.29 0.13 0.20 0.29 4 0.11 0.46 0.29 0.11 0.20 0.33

Wave load analysis was carried out for structural design of

MV and hull was divided into 22 sections for the analysis. Long crested wave was used for conservative design, and wave directions are from head sea (180 degrees) to following sea (0 degrees) with 15 degrees interval. Fig. 9 represents vertical shear force, and maximum shear force is about 77,808 tons.

Fig. 9 Vertical shear forces for LC11

Fig. 10 Vertical bending moments for LC11

Results of wave bending moment are shown in Fig. 10 and maximum bending moment is 513,761.5 ton-m which was occurred around mid ship. Maximum value is about 250 % larger than still water bending moment due to waves. It is highly expected that wave bending moment will decrease if motion characteristics of MV is improved.

VI. DOWNTIME ANALYSIS Downtime analysis was carried out to evaluate workability

and economics of MV. In-house program based on frequency domain analysis was applied for the estimation. Scatter diagram for this analysis was derived from BMT data.

Motion criteria were used for normal operation, and wave heading probability was assumed to be 70%, 20% and 10% for 180, 165 and 150 degrees respectively because DP system controls MV heading. Long crested wave was also applied to the analysis for conservative design.

Results for monthly and yearly analysis are summarized in Fig. 11. According to the results, downtime could happen with less than 1% of probability in both June and July. The probability was over 3% in January and February. Annual downtime probability was about 2.0% which is equivalent to 7.3 days a year. The result is similar with standard ship-shaped structures although MV doesn’t have better motion characteristics. This is because motion limitation used for MV’s downtime analysis is less strict.

0.0%

0.5%

1.0%

1.5%

2.0%

2.5%

3.0%

3.5%

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year

January to December and Yearly Down-Time Results

Fig. 11 Downtime analysis results

VII. PYNAMIC POSITIONING SYSTEM CAPABILITY ANALYSIS DPs capability analysis should be carried out in order to

assess arrangement of thrusters and those specifications for given operational and environmental conditions. Six azimuth type thrusters were adopted and installed on the bottom of the hull. Locations of thrusters are given in TABLE 16.

TABLE 16 LOCATIONSOF THRUSTERS Thruster 1 2 3 4 5 6

X(m) -119.5 -89.5 -89.5 124.8 115.2 115.2 Y(m) 0.0 -3.9 3.9 0.0 -10.8 10.8

Capability analysis was performed for two operation conditions as defined in TABLE 5. For this analysis, wind and

current coefficient of the offshore structure are necessary. Because of these data were not available at Pre-FEED stage, data from other similar projects conducted by DSME were used in the analysis and loads coefficients of wind and current are presented in Fig. 12 and Fig. 13. The waves, wind and current were assumed to unidirectional for conservative analysis.

Fig. 12 Current loads coefficients

Fig. 13 Wind loads coefficients

A design criterion of this analysis is endurable angle and owner of the vessel shall decide the value by considering equipments specifications and operational know-how. MV shall be able to select its optimum heading to the environment either minimize motion response or to minimize the power requirement. In this analysis, it was assumed the vessel heading is maintained within +/- 25 degrees from the bow. Results of the analysis are plotted in Fig. 14 and Fig. 15.

Fig. 14 DP capability plot for operating condition

Fig. 15 DP capability plot for standby condition

According to the results, it was concluded that DP system

could maintain MV heading within +/- 38 degrees and +/-27.5 degrees for operating and standby condition respectively.

VIII. ELECTRIC LOADS ANALYSIS The purpose of electric loads analysis is to evaluate

required power for various operation conditions and to validate the specifications of generator. The analysis was performed four operation conditions as follow:

• Normal production condition;

• Normal production with offloading condition;

• Standby condition;

• One thruster failure condition.

Major consumptions of equipment used in this analysis are summarized in TABLE 17.

TABLE 17 Power consumptions of main equipment

Equipment Power Consumption (MW) Units Total

(MW)

Thruster 5.5 6 33.0

Offloading & Nodule Handing 3.0 1 3.0

Nodule Lifting Pump 5.0 5 35.0

According to the results of the analysis, given number of power generators was satisfied with required power consumptions for given operation conditions. Condition of

normal operation with nodules offloading required the highest electric power as much as 44.2 MW and 85% of maximum power. As for ratio of power consumption, nodule lifting pump occupied 57% of required power. Total eight (8) generator were necessary for the normal operation and normal operation with offloading condition and five (5) generators were need to be operating for single failure condition. Detail results are presented in Fig. 16

Fig. 16 Results of electric load analysis

ACKNOWLEDGMENT The authors gratefully acknowledge the support of the

Korea Ocean Research & Development Institute and Ministry of Land, Transport and Maritime Affairs of Korea.

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