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
Table 8 Maximum Offset-8 Mooring Lines (m)
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.
Table 11 Maximum Offset-8 Mooring Lines (m)
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%
ML1 ML2 ML3 ML4 ML5 ML6 ML7 ML8
Mooring Line
PercentageofBreakingStrength
Intact
Damage (Thrust)
Damage (M Line)
Figure 9 Mooring Line Load (300m, 5-year environment)
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0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
TH1 TH2 TH3 TH4 TH5 TH6 TH7 TH8
Thruster
PercentageofMaximumT
hrust
Intact Damage (Thrust) Damage (M Line)
Figure 10 Utilized Thrust (300m, 5-year environment)
0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
ML1 ML2 ML3 ML4 ML5 ML6 ML7 ML8
Mooring Line
PercentageofBreakingStrength
Intact
Damage (Thrust)
Damage (M Line)
Figure 11 Mooring Line Load (1000m, 1-year environment)
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0%
10%
20%
30%
40%
50%
60%
TH1 TH2 TH3 TH4 TH5 TH6 TH7 TH8
Thruster
PercentageofMaximumT
hrust
Intact Damage (Thrust) Damage (M Line)
Figure 12 Utilized Thrust (1000m, 1-year environment)
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 13 Mooring Line Load (1000m, 5-year environment)
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0%
20%
40%
60%
80%
100%
120%
TH1 TH2 TH3 TH4 TH5 TH6 TH7 TH8
Thruster
PercentageofMaximum
Thrust
Intact Damage (Thrust) Damage (M Line)
Figure 14 Utilized Thrust (1000m, 5-year environment)
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
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