on the assessment of thruster assisted mooring

8/6/2019 On the Assessment of Thruster Assisted Mooring

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On the Assessment of Thruster Assisted Mooring

Sue Wang

American Bureau of Shipping

Houston, Texas, USA

ABSTRACT

Increasingly newly built semi-submersibles are being fitted with advanced mooring systems and

some level of thrust capability for improved station keeping. The operation modes range from

mooring only in relatively shallow water, thruster assisted mooring in deep water and Dynamic

Positioning (DP) mode in ultra deep water conditions. In support of classification services for

station keeping with thrust assisted mooring, a comparative study on the performance of mooring

systems with and without thruster assistance has been carried out. Time-domain numerical

simulations have been employed to assess the mooring load and station keeping capability. This

paper presents the findings from the study in the interests of sharing information with industry.

INTRODUCTION

The station keeping system is one of the major components of a floating offshore unit. For asemi-submersible unit, the station keeping system can be a mooring system only, a DP with

thrusters, or a DP assisted mooring system. A mooring system normally includes 8 or 12

mooring lines of chain or chain and wire, while a typical DP system uses 8 or 6 thrusters.

DP assisted mooring has been used in the design of station keeping systems for MODUs and for

FPSOs. The thrusters (or DP) are used for heading control and for reducing the mooring load.

One of the principal reasons for using thruster assisted mooring is it can reduce overall project

costs by reducing the mooring system in terms of either fewer mooring lines or a lighter overallsystem, especially for deepwater projects. Figure 1 depicts the station keeping cost for mooring

and DP system as a function of water depth [1].


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Figure 1 Cost Curve of Station Keeping

For a DP assisted mooring system, the thrusters may only be active under relatively severe

weather conditions such that therefore the operation and maintenance cost for the DP system

could be relatively lower than that of a DP only operation. With exploration and production

moving into increasingly greater water depths, thruster assisted mooring has become one of the

alternative concepts for station keeping.

However, thruster assisted mooring is a very complex station keeping system. The scope of the

assessment of both the DP (thruster) system and the mooring system needs to be expanded to

include the new failure modes for the DP system and the effectiveness of the thruster capacity on

the overall mooring system. The load sharing between the thrusters and the mooring system may

only be accounted for through model testing or time domain numerical simulation.

This paper presents a simulation study of thruster assisted mooring for a generic semi-

submersible unit. The semi-submersible is a drilling unit and is designed to operate in mooring

mode in relatively shallow water of 300 meters, thruster assisted mooring in deepwater of less

than 1000 meters, and DP in ultra deepwater of more than 1000 meters.

The unit has two pontoons and four columns. The length of pontoons is 114.5 meters and the

height of the pontoons is 10 meters. The displacement of the unit is 53718 tons at the

operational draft of 20.5 meters. The unit is equipped with a typical DP-2 class system [2] that

has eight azimuth thrusters powered by four generators. Two mooring arrangements are used in

the analysis: 8-mooring lines and 12 mooring lines. Each line includes three components: chain,

rope and chain. Figure 2 illustrates the layout of the mooring lines and the thrusters.


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Figure 2 Layout of Mooring Lines and Thrusters

The station keeping analysis covers the mooring system only, the DP system only and the DP

assisted mooring system. The analysis was carried out using the time domain simulation

program aNySIM developed by MARIN [3]. The results in terms of mooring load and DP

capability are presented. The comparative evaluation of these station keeping systems is also

presented.

METHODOLOGY OF DP ASSISTED MOORING

A DP system is a feedback control system. Figure 3 depicts a typical DP control loop. The

inputs to the system are the measured and required positions. The system determines the

required thruster power to keep the unit at the position required. The motions induced by the

first order wave load are beyond the scope of the DP system, and Extended Kalman Filter (EKF)

[6] is applied to filter out the first order component. The PID (Proportional-Integral-Derivative)

control algorithm is to determine the needed power for the station keeping. The allocation

algorithm makes the assignment for each thruster normally based on minimizing total required

power.

When DP is used in combination with the mooring system, mooring load effect on the vessel and

the mooring stiffness for the selection of PID coefficients need to be considered.

ENVIRONMENTAL LOAD

The maximum design environmental conditions for mooring system for a drilling unit are 5-year

and 10-year environments for operation away from other structures and operation in the vicinity


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of other structures, respectively [4]. The environment for drilling operation is normally

determined by owners and, in general, a 1-year environment is applied.

VESSEL THRUST

EKF PID

Requiredposition

Measured

Position

ALLOC

Wind, Waves, Current

Wind Feed Forward

Mooring load

VESSEL THRUST

EKF PID

Requiredposition

Measured

Position

ALLOC

Wind, Waves, Current

Wind Feed Forward

Mooring load

Figure 3 DP control Loop

For a DP system, there is no regulatory requirement for the maximum design environmental

condition. The operation environmental conditions are normally specified by owners. In general,

similar environmental conditions to that of mooring system are applied in practice.

Table 1 lists environmental conditions for a specific site.

Table 1 Environmental Conditions1-year return period 5-year return period 10-year return period

Hs (m) 6.00 9.80 10.8

Tp(s) 11.2 12.5 13.5

1 min wind (kts) 42.7 69.0 74.0

Current (kts) 1.02 1.27 1.43

Wave loads of first order and second order drift are calculated from a three-dimensional

seakeeping program. Wind and current forces are calculated using the formula below [5]:

2CVF

Where C is the load coefficient that can be obtained from model test, V is wind or current

velocity.


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ANALYSIS RESULTS

The main particulars of the semi-submersible and mooring system are given in Tables 2 and 3.

The total length of the mooring line changes with the water depth to keep the pretension at

2000kN while the length of the chain part is kept the same as 1650m. There are eight thrusters

for the DP system and each has maximum thrust of 750 kN. The analysis has been carried out

for the mooring system, the DP system performance and for the DP assisted mooring system.

Table 2 Main Particulars of Semi-submersible

Displacement 53718 tonne

Pontoon Length 114.5 m

Draft 20.5 m

KG 24.5 m

Kxx 35.1 m

Kyy 35.1 mWind frond area 3500 m2

Wind side area 3500 m2

Table 3 Mooring Line Characteristics

Unit mooring line

Pre-tension kN 2000

Chain segments

Chain diameter mm 84

Chain length m 1500+150

Mass in air kg 155

Weight in water kg 134

Stiffness kN 633000

Breaking strength kN 8152

Rope segment

Rope diameter mm 1600

Mass in air kg 4.2

Weight in water kg 4.1

Stiffness kN 235440

Breaking strength kN 8280

Mooring system

For a drilling unit, the following cases, cited in Table 4, need to be covered in an analysis for

assessing mooring line strength. The damaged condition is defined, in general, as the maximum

loaded mooring line is broken.


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Table 4 Analysis Cases for Mooring System

Operation(1-year return period)

Survival(5 or 10-year return period)

Mooring SystemIntact condition Intact condition

Damaged condition Damaged condition

For the mooring arrangement given in Figure 2, the environment load from 45-deg, 135-deg,

225-deg and 315-deg could be among the worst case scenarios for mooring line load. This study

focuses on a wind, wave and current co-linear environmental condition and the heading is 45

degrees. Two water depths of 300m and 1000m are included in the analysis. Tables 5 to 8 list

maximum mooring line load, and the maximum offset for 12-line and for 8-line mooring systems,

respectively.

The 12-line mooring system, in general, meets the design requirement for mooring line strength

and for the targeted offset limits. However, the 8-line mooring system does not meet the over all

design requirements.

Table 5 Maximum Mooring Line Load-12 Mooring Lines (kN)

Water

Depth (m)Condition

1-year

Environment

5-year

Environment

300Intact 3085 4395

Damaged 3820 5772

1000 Intact 2903 3787Damaged 3474 4807

Table 6 Maximum Offset-12 Mooring Lines (m)

WaterDepth (m)

Condition1-year

Environment5-year

Environment

300Intact 8.70 17.90

Damaged 14.30 26.14

1000Intact 20.91 42.60

Damaged 36.71 64.47


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Table 7 Maximum Mooring Line Load-8 Mooring Lines (kN)

WaterDepth (m)

Condition1-year

Environment5-year

Environment

300Intact 3427 5384

Damaged 5136 Failed

1000Intact 3287 4631

Damaged 4599 Failed


WaterDepth (m)

Condition1-year

Environment5-year

Environment

300Intact 11.84 24.56

Damaged 23.16 Failed

1000Intact 29.43 60.22

Damaged 60.48 Failed

DP Performance

Similar to the mooring analysis, Table 9 lists the analysis cases for the selected DP system. For

the semi-submersible used in this study, the DP capability is comparable among all headings (see

Figure 4) when the thruster interaction effects are neglected. In reality, the DP capability is

smaller in beam sea and close to beam sea conditions due to thruster-thruster interactions [7].

However, this study does not include the interaction effects and the simulation analysis focuses

on 45-degree case. The water depth is 1000m.

Table 9 Analysis Cases for DP System

Operation(1-year return period)

Survival(5 or 10-year return period)

DP systemIntact condition Intact condition

Worst case failure Worst case failure

The worst case failure for this design is one generator down, which could result in two thrustersbeing inoperable. For the 45 degree deading condition, damage to thrusters 1 and 6 is most

critical condition. Thrusters 1 and 6 are diagonally located to each other.


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Thrust usage 80%

Current speed=1.03 m/s

Thrust usage 80%

Current speed=1.03 m/s

Figure 4 DP Capability Plot

Table 10 provides the results for DP performance analysis. Figure 5 shows the time history of

the offset and Figure 6 plots the time history for total used thrusts.

Table 10 DP Performance

Performance Condition1-year

Environment

5-year

Environment

Offset (m)Intact 39.47 79.45

Damaged 39.12 Drifted

Max Thrust

Intact(kN)

ST1 473 750

ST2 473 750ST3 478 750

ST4 479 750

ST5 467 750

ST6 466 750

ST7 469 750

ST8 470 750

Max Thrust

Damaged(kN)

ST1 0 0

ST2 584 750

ST3 579 750

ST4 579 750

ST5 599 750ST6 0 0

ST7 618 750

ST8 623 750


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10

15

20

25

30

35

40

10000 11000 12000 13000 14000 15000 16000 17000 18000 19000 20000

time (s)

Offset(m)

Figure 5 Time History of the Offset

2.E+03

3.E+03

3.E+03

4.E+03

4.E+03

5.E+03

10000 11000 12000 13000 14000 15000 16000 17000 18000 19000 20000

Time (s)

Total

Thrust(kN)

Figure 6 Time History of Used Total Thrust

From Table 10, it indicates that the selected thrusters could not keep the position of the semi-

submersible for the 5-year wave environmental condition. For the 1-year environmental

condition, thrusters 7 and 8 have used more than 80% of their maximum thrust at the damaged

condition.

DP Assisted Mooring

For DP assisted mooring, API recommends using the following definitions for intact and

damaged conditions and hence to use factors of safety accordingly.

Table 11 DP Assisted Mooring Case Definition

DP Assisted mooring Mooring system Thruster systemIntact Intact Intact

Damaged (T) Intact Damaged

Damaged (M) Damaged Intact


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The DP assisted mooring analysis focuses on the 8-line mooring system that does not meet the

design requirements. Similar to the mooring analysis, two environmental conditions are

considered. Table 11 summarizes the maximum offsets for the analyzed cases.


Water Depth

(m)Condition

1-year

Environment

5-year

Environment

300

Intact 9.03 17.28

Damaged (T) 9.02 17.01

Damaged (M) 16.33 27.08

1000

Intact 17.12 33.69

Damaged (T) 16.99 33.25

Damaged (M) 31.54 47.43

Mooring line load and utilized thrust are plotted in Figures 7 to 14. In the figures, ML stands for

mooring line and TH stands for thruster.

0%

5%

10%

15%

20%

25%

30%35%

40%

45%

50%

ML1 ML2 ML3 ML4 ML5 ML6 ML7 ML8

Mooring Line

PercentageofBreaking

Strength Intact

Damage (Thrust)

Damage (M Line)

Figure 7 Mooring Line Load (300m, 1-year environment)


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0%

5%

10%

15%

20%

25%

30%

35%

40%

TH1 TH2 TH3 TH4 TH5 TH6 TH7 TH8

Thruster

PercentageofMaximumT

hrust

Intact Damage (Thrust) Damage (M Line)

Figure 8 Utilized Thrust (300m, 1-year environment)

0%

10%

20%

30%

40%

50%

60%

70%


Mooring Line

PercentageofBreakingStrength

Intact

Damage (Thrust)

Damage (M Line)



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0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%


Thruster


hrust



0%

5%

10%

15%

20%

25%

30%

35%

40%

45%


Mooring Line

PercentageofBreakingStrength

Intact

Damage (Thrust)

Damage (M Line)



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0%

10%

20%

30%

40%

50%

60%


Thruster


hrust



0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

50%


Mooring Line

PercentageofBreaking

Strength

Intact

Damage (Thrust)

Damage (M Line)



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0%

20%

40%

60%

80%

100%

120%


Thruster

PercentageofMaximum

Thrust



Table 11 and Figures 7 to 14, it indicates that by using of a combination of the 8-line mooring

system with the DP system, the performance meets the over all requirements. However, the

thruster and thruster interaction, thruster and hull interaction, and other factors are not included

in the analysis, which could reduce the performance of the DP system.

In general, the mooring line loads for water depths of 300 meters and 1000 meters are

comparable. For 1000-meter water depth, an extra 1000-meter rope has been added to each

mooring line that was used for the 300-meter water depth mooring.

The usage of the thrust is related to the PID coefficients of the DP system. Higher thruster is

required to keep a relatively small offset.

SUMMARY AND DISCUSSION

Time domain simulations have been carried out for station keeping systems of three types:

mooring system, DP system and DP assisted mooring system. Two water depths of 300m and

1000m are considered in the analysis. Two mooring arrangements: 12 mooring lines and 8

mooring lines are used for comparative study. The findings are summarized as following.

Time Domain simulation provides transparency between the mooring load and utilized thrust.


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For DP assisted mooring, the DP control algorithm needs to include the mooring effect on the

motions of the units. The PID coefficients need to include the effect of mooring stiffness.

With the assistance of the thrusters, the twelve-line mooring system may be reduced to a

lighter eight-line mooring system.

Time domain DP simulation includes slowly varying motion component. The results show

that the dynamic component could be higher than 20% of the total steady component, which

is normally considered as the margin in the steady state DP capability analysis. Therefore, a

20% margin for dynamic effect may not be conservative for certain conditions.

Although there are regulatory requirement for the factors of safety for mooring line load,

there is no regulatory requirement for the maximum utilization of the thrust

Thruster efficiency due to interactions between thrusters, hull effect, current and others need

to be further included in the analysis.

REFERENCE

[1] Ryu, S., Hull/Mooring/Riser Coupled Motion Simulations of Thruster-Assisted Mooring

Platforms, PhD Dissertation, Texas A&M University, 2003.

[2] ABS, Vessel System and machinery, Rules for Building and Classing Steel Vessels, 2010.

[3] MARIN, aNySIM-Pro, 2008.

[4] API, Design and Analysis of Stationkeeping Systems for Floating Structures, RP 2SK, 2008.

[5] ABS, Rules for Building and Classing Mobile Offshore Drilling Units, 2008.

[6] Lewis, F.L., Applied Optimal Control & Estimation, Prentice Hall & Texas Instruments

Digital Signal Processing Series, 1992.

[7] Serraris, J.W., Validation of the DP module of aNySim, M.Sc. thesis, T.U. Delft, 2007

on the assessment of thruster assisted mooring

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