World Academy of Science, Engineering and Technology

International Journal of Geological and Environmental Engineering

Vol:10, No:5, 2016

Fatigue Analysis of Spread Mooring Line

Chanhoe Kang, Changhyun Lee, Seock-Hee Jun, Yeong-Tae Oh

 invisible risk levels.

Abstract—Offshore floating structure under the various Offshore floating structures require mooring systems to
environmental conditions maintains a fixed position by mooring maintain the station keeping under surrounding environment
system. Environmental conditions, vessel motions and mooring loads actions such as current, wind, and wave. Mooring systems are
are applied to mooring lines as the dynamic tension. Because global
composed of specially designed devices for the purpose and
responses of mooring system in deep water are specified as wave
frequency and low frequency response, they should be calculated from widely applied to the most floaters. Therefore, the mooring
the time-domain analysis due to non-linear dynamic characteristics. systems have to provide such station keeping capability and
high global performance to ensure allowable excursions against
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To take into account all mooring loads, environmental conditions,

added mass and damping terms at each time step, a lot of computation environmental loads. The performance characteristics of
time and capacities are required. Thus, under the premise that reliable mooring systems is typically a function of the type and size of
fatigue damage could be derived through reasonable analysis method,
floater, the operational water depth, environmental loads,
it is necessary to reduce the analysis cases through the sensitivity
studies and appropriate assumptions. In this paper, effects in fatigue seabed condition, and the arrangement and weight of mooring
are studied for spread mooring system connected with oil FPSO which components.
is positioned in deep water of West Africa offshore. The target FPSO Besides, unlike general trading ships, offshore floaters stay
with two Mbbls storage has 16 spread mooring lines (4 bundles x 4 at a fixed position for their whole life without regular dry
lines). The various sensitivity studies are performed for environmental docking for inspection and repair. The mooring lines have to be
loads, type of responses, vessel offsets, mooring position, loading
designed to withstand severe weather conditions since they
conditions and riser behavior. Each parameter applied to the sensitivity
studies is investigated from the effects of fatigue damage through shall be in place without any failure of mooring lines during
fatigue analysis. Based on the sensitivity studies, the following results life-time. Especially the various environmental loads during
are presented: Wave loads are more dominant in terms of fatigue than operation of floaters lead to increasing fatigue damage in their
other environment conditions. Wave frequency response causes the mooring lines.
higher fatigue damage than low frequency response. The larger vessel This paper deals with fatigue analysis of permanent mooring
offset increases the mean tension and so it results in the increased line under the various environmental conditions. As permanent
fatigue damage. The external line of each bundle shows the highest
fatigue damage by the governed vessel pitch motion due to swell wave moorings are normally applied for floating production systems
conditions. Among three kinds of loading conditions, ballast condition such as FPSO with design lives of over 20 years, mooring
has the highest fatigue damage due to higher tension. The riser system fatigue is an important design factor. And since it has
damping occurred by riser behavior tends to reduce the fatigue non-linear dynamic characteristics, fatigue analysis should be
damage. The various analysis results obtained from these sensitivity performed through time domain analysis [1].
studies can be used for a simplified fatigue analysis of spread mooring
The spread mooring system of deep water FPSO installed in
line as the reference.
West Africa offshore is chosen in this study. It has the
Keywords—Mooring system, fatigue analysis, time domain, chain-wire-chain structure. Using the mooring design data of
non-linear dynamic characteristics. West Africa offshore, various sensitivity studies were
performed in accordance with the effect of fatigue damage for
I. INTRODUCTION spread mooring lines. The studied parameters for sensitivity are
environmental loads, type of responses, vessel offsets, mooring
C ONSISTENTLY the demand for offshore energy
development with regard to oil and gas resources has
increased and the field has required the state of the art
position, loading conditions and riser behavior.
To perform fatigue analysis, tension time series of mooring
lines were calculated through dynamic analysis with OrcaFlex
technologies and concepts for better engineering productivity.
[2]. The rain-flow counting method proposed by Matsuishi and
In particular, for ultra-deep water above 1000m depth the
Endo [3] was applied to calculate number of cycles of mooring
sustained drive to improve the harvests from offshore oil
line tension. And then fatigue damages of mooring lines were
exploration, production and transportation has led to the
obtained using the T-N curve and the Miner linear cumulative
specific needs on the various floating structures like FPSO,
law model which is the commonly used calculation method of
SPAR, etc. Also, most of the offshore floaters have been
fatigue damage.
required to meet the rigorous design requirements in term of the
structural strength and fatigue in order to reduce the potential
II. FPSO PLATFORM MODEL AND ENVIRONMENTAL
Chanhoe Kang is with the Daewoo Shipbuilding & Marine Engineering Co.,
CONDITIONS
Ltd (DSME), Seoul, South Korea (phone: 82-10-4933-6114; fax:
A. FPSO Platform
82-2-2129-3700; e-mail: kangchanhoe@ dsme.co.kr).
Changhyun Lee, Seock-Hee Jun, and Yeong-Tae Oh are with Daewoo In this paper, the FPSO installed in West Africa offshore was
Shipbuilding & Marine Engineering Co., Ltd (DSME), Seoul, South Korea considered. The installation field has approximately 1200m
(e-mail: changhyunlee@dsme.co.kr, sheejun@dsme.co.kr, ytoh@dsme.co.kr).

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Vol:10, No:5, 2016

water depth and seabed conditions with a regular slope in

global south-west direction of 2%. The main characteristic
parameters of the FPSO platform model are listed in Table I.
Three kind of loading conditions have been studied in fatigue
analysis.
TABLE I
PARAMETERS OF FPSO PLATFORM
Parameters Value
Length (m) 305
Breath (m) 61
Depth (m) 32
Loading conditions Ballast Intermediate Full
Draft (m) 11.69 17.46 23.04
Displacement (metric ton) 205573 312441 417645
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COG from the stern (m) 162.35 161.85 158.34

COG from the keel (m) 22.85 20.59 18.47
Roll gyration radius (m) 25.55 21.81 22.04
Pitch gyration radius (m) 85.67 80.46 80.69
Yaw gyration radius (m) 85.9 80.47 81.24
Fig. 1 Arrangement of mooring system
B. Mooring System
The heading of FPSO is 22.5deg from true north in east
direction as shown in Fig. 1. The FPSO is installed by spread
mooring system. Mooring system consists of 16 mooring lines
around FPSO. The 4 mooring lines are composed as a bundle.
The mooring lines are arranged with 1600m pattern radius. As
shown in Fig. 2, mooring lines of the FPSO are made up of
three components which are top chain, wire rope and bottom
chain. The range of mooring line is from FPSO fairlead point to
TAP (so-called Theoretical Anchor Point). Top chain is
connected at the fairleads of the FPSO with chain stopper and
bottom chain is linked to the suction anchor on seabed. The
buried parts of bottom chains to suction anchoring point are not
considered in mooring analysis. The mooring line properties Fig. 2 Configuration of mooring line
are listed in Table II.
TABLE II
C. Environmental Conditions MOORING LINE CHARACTERISTICS
Global responses of mooring line connected with FPSO were Component Characteristics Value
calculated under sea state conditions such as wind, current, and Property of material R3 Studless Chain
wave loads. The sea state condition was selected from the Nominal diameter 147 mm
measured data of West Africa offshore. The long-term sea state Top chain
Line length 27 ~ 62 m
usually consists of a number of short-term sea states. The Breaking strength 15536 kN
chosen sea states are listed in Tables III-V. The Ochi-Hubble Line axial stiffness 1319.26 MN
wave spectrum and KAIMAL wind spectrum was used for each Line weight in water 380.0 kg/m
Property of material Spiral Strand Wire Rope
sea state.
Nominal diameter 111 mm
D.Riser Effect Line length 1850 m
Wire rope
FPSO platform has been connected with a lot of risers which Breaking strength 12500 kN
are necessary to consider the mooring analysis. In this paper, Line axial stiffness 1200.48 MN
Line weight in water 50.68 kg/m
the effect of risers is compared in view of the calculated fatigue
Property of material R3 Studless Chain
damage of mooring lines.
Nominal diameter 132 mm
Line length 190 m
Bottom chain
Breaking strength 14508 kN
Line axial stiffness 1189.06 MN
Line weight in water 306.0 kg/m

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Vol:10, No:5, 2016

TABLE III
WIND SCATTER DIAGRAM
Direction from North (Deg)
Vw a (m/s)
0 22.5 45 67.5 90 112.5 135 157.5 180 202.5 225 247.5 270 292.5 315 337.5
0.5 0.01 0.01 0.00 0.01 0.01 0.00 0.02 0.02 0.03 0.03 0.01 0.02 0.01 0.01 0.01 0.00
1.5 0.05 0.03 0.06 0.05 0.09 0.15 0.23 0.38 0.49 0.49 0.47 0.34 0.17 0.09 0.05 0.06
2.5 0.04 0.03 0.06 0.07 0.14 0.35 0.70 1.62 2.54 2.86 2.03 1.08 0.50 0.25 0.11 0.07
3.5 0.02 0.01 0.01 0.01 0.06 0.20 0.81 2.93 6.65 7.07 3.96 1.43 0.45 0.12 0.06 0.01
4.5 0.01 0.01 0.01 0.06 0.39 2.90 9.42 10.35 4.88 1.28 0.27 0.06 0.01 0.01
5.5 0.00 0.00 0.01 0.12 1.27 6.09 8.45 3.97 0.86 0.10 0.01 0.01 0.01
6.5 0.00 0.01 0.26 1.83 3.77 2.08 0.28 0.01
7.5 0.02 0.24 0.69 0.42 0.07 0.01
8.5 0.01 0.01 0.07 0.04 0.01
9.5 0.00 0.00
a
Vw = averaged wind velocity for one hour
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TABLE IV
CURRENT SCATTER DIAGRAM
Direction from North (Deg) Direction from North (Deg)
Vcr a (m/s) Vcr a (m/s)
0 45 90 135 180 225 270 315 0 45 90 135 180 225 270 315
0.01 0.11 0.12 0.09 0.10 0.10 0.08 0.09 0.07 0.37 0.25 0.30 0.30 0.19 0.05 0.06 0.20 0.44
0.03 0.33 0.29 0.32 0.29 0.27 0.28 0.28 0.32 0.39 0.26 0.21 0.32 0.17 0.02 0.05 0.18 0.37
0.05 0.50 0.58 0.57 0.49 0.40 0.38 0.46 0.47 0.41 0.24 0.13 0.18 0.10 0.01 0.03 0.11 0.33
0.07 0.69 0.84 0.78 0.64 0.57 0.56 0.58 0.57 0.43 0.15 0.08 0.11 0.03 0.01 0.02 0.11 0.25
0.09 0.84 1.11 1.01 0.78 0.74 0.63 0.71 0.60 0.45 0.16 0.08 0.10 0.04 0.01 0.09 0.18
0.11 0.88 1.18 1.21 0.84 0.73 0.74 0.73 0.71 0.47 0.11 0.05 0.07 0.02 0.01 0.07 0.16
0.13 0.96 1.31 1.33 0.88 0.80 0.69 0.75 0.81 0.49 0.11 0.04 0.05 0.01 0.00 0.07 0.15
0.15 0.99 1.54 1.27 1.01 0.93 0.62 0.72 0.80 0.51 0.10 0.04 0.03 0.01 0.06 0.13
0.17 0.97 1.41 1.39 0.98 0.85 0.67 0.74 0.75 0.53 0.08 0.01 0.03 0.01 0.01 0.04 0.08
0.19 0.98 1.28 1.37 0.90 0.75 0.54 0.66 0.81 0.55 0.07 0.01 0.04 0.00 0.03 0.07
0.21 0.84 1.32 1.22 0.94 0.60 0.42 0.56 0.78 0.57 0.03 0.01 0.03 0.02 0.05
0.23 0.73 1.00 1.16 0.93 0.58 0.39 0.48 0.67 0.59 0.03 0.05 0.00 0.02 0.05
0.25 0.61 0.97 1.10 0.71 0.47 0.32 0.44 0.61 0.61 0.01 0.05 0.01 0.04
0.27 0.54 0.81 0.99 0.67 0.39 0.26 0.39 0.64 0.63 0.01 0.02 0.01 0.06
0.29 0.51 0.77 0.97 0.55 0.28 0.22 0.42 0.58 0.65 0.00 0.02 0.05
0.31 0.46 0.56 0.76 0.50 0.19 0.15 0.35 0.57 0.67 0.01 0.00 0.01
0.33 0.42 0.50 0.55 0.41 0.13 0.12 0.25 0.55 0.69 0.01 0.01
0.35 0.35 0.35 0.41 0.33 0.07 0.12 0.20 0.53 0.71 0.00 0.00
a
Vcr = current velocity
n

III. FATIGUE ANALYSIS D  

i
Di
1
(2)

A. S-N Curve
According to API RP 2SK [4], S-N curve presents the The fatigue damage Di in the i-th short-term sea state is
number of cycles to failure for a specific mooring component as calculated from (3):
a function of a constant normalized tension range, based on the
results of experiments. For mooring lines, T-N approach which
N n
only considers the tension fatigue and ignores the bending
D i  
j N
j

1
(3)
j
fatigue is normally used. Equation (1) presents the T-N curve:
where nj is number of cycles within the j-th tension range, Nj is
NR M
K (1) allowable number cycles at the j-th normalized tension range
given by T-N curve.
where N is the number of cycles, R is the ratio of tension range For fatigue analysis, the values of M = 3.0 and K = 316 for
to reference breaking strength, and M and K are material chain, and M = 5.05 and K = 10(3.25 - 3.42 Lm) for wire, were
parameters in the T-N curve. chosen from [1]. Lm is the ratio of mean load to reference
According to Miner’s linear cumulative damage rule, the breaking strength for wire rope.
annual cumulative fatigue damage D can be summed up from
the fatigue damage Di arising in a set of short-term sea states as B. Numerical Simulation
shown in (2): For fatigue analysis, cyclic loading of mooring line needs to
be obtained firstly. To calculate dynamic tension of mooring

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line, dynamic analysis was performed to generate a time where M(p,a) is the system inertia load, C(p,v) is the system
simulation of 6 DOF motions of model using equation of damping load, K(p) is the system stiffness load and F(p,v,t) is
motion. The equation of motion applied is shown in (4): the external load. Also p, v, a and t are the position vectors,
velocity vectors, acceleration vectors and simulation time,
M (p ,a )  C (p ,v )  K (p )  F (p ,v ,t ) (4) respectively.

TABLE V
WAVE SCATTER DIAGRAM
No. Inc1 Hs1 Tp1 Inc2 Hs2 Tp2 Inc3 Hs3 Tp3 Occ No. Inc1 Hs1 Tp1 Inc2 Hs2 Tp2 Inc3 Hs3 Tp3 Occ
1 225 1.3 13 203 0.8 9 180 0.3 7 230 43 225 0.3 11 203 0.8 9 203 0.3 5 46
2 203 1.3 13 203 0.8 9 203 0.3 5 228 44 225 1.3 15 203 1.3 9 203 0.8 7 46
3 203 0.8 11 203 0.8 7 180 0.3 5 218 45 203 1.3 15 203 1.3 11 180 0.8 9 44
4 203 1.3 11 0 0.0 0 0 0.0 0 198 46 203 0.8 9 203 0.8 7 203 0.8 5 43
5 225 0.8 13 203 0.3 9 180 0.3 5 184 47 203 0.3 11 203 0.3 9 180 0.3 5 42
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6 225 0.8 13 203 0.8 9 180 0.3 5 184 48 225 1.8 15 203 0.8 9 203 0.3 9 42
7 225 1.3 13 203 0.3 7 158 0.8 5 179 49 203 0.3 17 203 0.8 11 180 0.3 9 40
8 203 0.8 11 203 0.3 7 338 0.8 5 146 50 225 0.3 17 225 0.8 11 180 0.3 5 40
9 225 0.3 15 203 0.8 11 203 0.3 5 146 51 203 1.3 13 180 0.3 7 180 0.3 7 39
10 203 1.3 9 0 0.0 0 0 0.0 0 141 52 203 2.3 15 0 0.0 0 0 0.0 0 39
11 203 0.8 13 203 0.8 9 180 0.3 7 131 53 225 0.8 15 225 0.8 11 203 0.8 7 39
12 203 1.3 13 0 0.0 0 0 0.0 0 126 54 203 0.3 13 203 0.3 9 203 0.3 5 38
13 203 1.3 11 203 0.8 7 180 0.3 5 102 55 225 0.3 15 225 0.3 11 203 0.3 7 38
14 225 0.3 13 203 0.8 9 203 0.3 5 102 56 203 1.3 13 180 0.8 9 180 0.3 5 36
15 203 1.3 13 203 0.3 7 180 0.3 5 91 57 225 0.3 13 203 1.3 9 180 0.3 7 36
16 225 0.8 11 203 0.8 7 135 0.8 5 90 58 225 1.8 13 0 0.0 0 0 0.0 0 36
17 203 0.8 13 203 0.3 9 203 0.3 5 88 59 203 1.8 13 203 0.8 9 203 0.3 7 35
18 203 1.3 11 203 0.3 5 225 0.3 5 88 60 225 1.3 15 203 0.3 9 180 0.3 9 35
19 203 1.3 13 203 1.3 9 180 0.3 9 85 61 203 0.8 11 180 0.3 7 180 0.3 5 34
20 225 0.8 15 203 0.8 9 180 0.3 9 85 62 203 1.3 11 225 0.3 5 203 0.3 5 34
21 225 1.3 15 203 0.8 9 203 0.3 9 85 63 225 0.8 13 203 1.3 9 225 0.3 5 34
22 203 1.8 13 0 0.0 0 0 0.0 0 84 64 225 1.3 13 180 0.8 7 203 0.8 5 34
23 225 0.3 15 225 0.8 11 203 0.3 5 83 65 225 2.3 15 0 0.0 0 0 0.0 0 34
24 225 0.3 15 203 1.3 11 203 1.3 9 79 66 203 2.3 13 0 0.0 0 0 0.0 0 33
25 203 0.8 9 203 0.3 7 203 0.3 3 77 67 225 1.3 13 180 0.3 7 203 0.3 5 33
26 225 1.3 13 203 1.3 9 90 0.8 5 75 68 203 0.8 9 0 0.0 0 0 0.0 0 31
27 225 0.8 11 203 0.3 7 180 0.3 5 74 69 203 0.8 11 0 0.0 0 0 0.0 0 31
28 225 1.3 13 0 0.0 0 0 0.0 0 73 70 203 1.3 11 180 0.3 7 225 0.3 5 30
29 203 0.8 13 203 1.3 9 203 0.3 7 70 71 203 0.8 11 225 0.3 7 180 0.3 7 29
30 203 0.3 15 203 0.8 11 203 0.3 7 67 72 203 1.8 15 0 0.0 0 0 0.0 0 29
31 203 1.3 15 203 0.8 9 203 0.8 9 58 73 203 0.3 15 203 0.3 11 203 0.3 7 28
32 225 0.3 17 203 0.8 11 180 0.3 7 58 74 225 0.8 15 203 1.3 9 180 0.8 7 27
33 225 0.3 13 203 0.3 9 203 0.3 5 56 75 225 1.3 11 203 0.3 7 90 0.8 5 27
34 225 1.8 15 0 0.0 0 0 0.0 0 56 76 225 1.8 13 203 0.8 7 225 0.3 7 27
35 225 0.8 15 203 0.3 9 203 0.3 7 53 77 203 0.8 15 203 1.3 9 180 0.8 9 26
36 203 0.8 15 203 0.8 11 203 0.8 7 51 78 203 2.3 11 0 0.0 0 0 0.0 0 26
37 203 0.3 13 203 1.3 9 203 0.3 7 48 79 203 1.8 15 203 1.3 9 0 0.0 0 25
38 203 0.3 15 203 1.3 11 203 0.3 9 48 80 225 0.3 17 203 1.3 11 203 0.8 9 25
39 203 0.3 11 203 0.8 7 180 0.3 5 47 81 225 1.3 17 225 0.8 13 180 0.8 9 24
40 203 0.3 13 203 0.8 9 180 0.3 7 47 82 203 1.8 15 203 0.8 7 203 0.8 9 23
41 203 1.8 11 0 0.0 0 0 0.0 0 47 83 225 0.8 15 225 0.3 11 203 0.3 7 23
42 225 0.3 15 203 0.3 11 203 0.3 7 47 84 203 0.3 15 225 0.8 11 203 0.3 7 22
Inc = wave heading, Hs = significant wave height, Tp = peak period, Occ = occurrence.

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The dynamic behavior of mooring lines can be split into two the effects of fatigue damage. In fatigue analysis, because of the
modes as shown in Fig. 3; wave frequency (WF) from the six most critical point, first chain link connected with fairleads of
degrees of freedom of vessel motion, and low frequency (LF) FPSO was selected as the target segment. In this study, the only
due to second order drift forces [5]. representative intermediate condition was mainly taken into
account for sensitivity study.
A. Environmental Loads
Fatigue analysis of mooring system should consider the
environmental conditions of wind, current, and wave.
Reflecting the sea states of West Africa offshore, fatigue
damages at the first chain link position from fairleads of FPSO
were calculated as shown in Fig. 6. The fatigue damage in wave
is much higher than others. And fatigue damage in current is
significantly smaller than that of wind. In terms of mooring line
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fatigue, the contribution of environmental forces from wind and

current acting on the mooring lines is relatively small.
Fig. 3 Typical dynamic behavior of wave frequency and low frequency

To perform dynamic analysis, OrcaFlex was applied in this

study. The model for dynamic analysis of the FPSO platform
and its mooring lines is shown in Fig. 4.

Fig. 6 Fatigue damage of mooring lines under each environmental

loads

Fig. 4 Dynamic analysis model

Fig. 5 shows a tension time series of the representative P1

mooring line calculated from dynamic analysis. Line tension
loads were captured at the first chain link position from
fairleads of FPSO under wave scatter diagram No. 183. The
rain-flow counting method is employed to count the calculated
mooring line tension. The time domain cycle counting is
generally considered to be the most accurate method for fatigue
damage calculation, and then resultant fatigue damages of
mooring lines were calculated for each condition.

Fig. 7 Fatigue damage of mooring lines for mode of dynamic behavior

Fig. 5 Tension time series of P1 mooring line for wave scatter No. 183

IV. SENSITIVITY STUDY

Spread mooring system of FPSO installed in West Africa
offshore was considered in this sensitivity studies. Based on the
sea states of target installation field, several governing Fig. 8 Fatigue damage of P1 mooring line for vessel offsets
parameters applied to the sensitivity studies were investigated

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C. Vessel Offsets
The primary purpose of mooring system is to maintain an
FPSO on station within a specified tolerance, typically based on
an offset limit determined from the configuration of the risers.
The mooring system provides a restoring force that acts against
the surrounding environmental loads as wind, current and wave.
The horizontal components of the mooring line tension give
such restoring force. Until horizontal restoring forces by
mooring lines are balanced from environmental loadings, the
FPSO will be offset as shown in Fig. 12.

Fig. 9 Fatigue damage of each mooring positions for wave loads

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Fig. 12 Vessel offsets and tension effect

The fatigue damages for P1 mooring line were calculated at a

fixed offset position according to the level of vessel offset in the
most tensioned direction of mooring line. According to Fig. 8
which shows the results for 1%, 2%, and 8% offsets of water
Fig. 10 Fatigue damage of mooring lines for loading conditions
depth, the longer offset is horizontally moved due to the
environmental loadings, the higher mean tension of the
mooring line is occurred. The higher mean tension of the
mooring line causes the larger fatigue damages.
D. Mooring Positions
The fatigue damages of mooring lines for wave loads were
obtained from fatigue analysis. As shown in Fig. 9, external
lines of P1, P8, S1 and S8 were the more damaged ones in each
mooring bundles. According to the sea state of West Africa
offshore, swell component which mainly has the direction in
longitudinal axis of FPSO is predominant. These wave loads
give an effect on the pitch motion of FPSO. External mooring
Fig. 11 Fatigue damage of mooring lines for riser behavior lines positioned at the bow and stern side would be more
damaged by the larger heave motion of fairleads due to vessel
B. Type of Responses
pitch motion.
Global responses of moored vessel have WF motions and LF
Motions. The dynamic behavior of mooring lines would be also E. Loading Conditions
occurred into combination of WF and LF modes. In this study, Floating production systems such as FPSO installed at field
the effect of fatigue damage calculated from two modes of has been positioned during a long life-time. FPSO platform has
response was compared respectively. To find out wave various loading conditions for operation. To compare the effect
frequency effect in term of fatigue, two cases were studied. of loading conditions, it is assumed that each loading condition
Firstly, fatigue analysis by WF and LF case was carried out and has 100% of the life-time for fatigue calculations. The
secondly only WF motion case was dynamically simulated in calculated fatigue damages of external mooring lines were
the state which the vessel position was fixed at static presented in Fig. 10. The highest fatigue damage occurred at
equilibrium position and the resultant fatigue damages were the ballast condition with lower draft which has higher mean
calculated. As shown in Fig. 7, fatigue damages by the only WF tension.
motion were not much different with the fatigue damages by F. Riser Behavior
WF and LF motions. The fatigue damage of WF mode is more
significant than that of LF mode. Therefore, fatigue damage of FPSO platform is connected with risers for production and
mooring lines is highly impacted by wave frequency motions. injection in addition to mooring lines. The dynamic effects for

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the risers were considered through a coupled analysis that all

interactions among mooring lines, risers and vessel are
modelled directly [6]. As shown in Fig. 11, from the results of
fatigue analysis, it is found that the dynamic effects of the risers
give lower fatigue damages than without risers. In case of
mooring analysis model including risers, mean drift loads on
risers and mooring lines are sufficiently accounted. And also,
for weakly damped systems such as moored vessels, the
damping effects from the mooring and riser systems give the
influence in accordance with response of low frequency
motion.

V. CONCLUSION
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This paper deals with sensitivity study for the various

parameters which affect the fatigue damage of the mooring
lines. Mooring lines connected with spread moored FPSO in
the West Africa field have been studied through this research
and the following results from fatigue analysis of mooring line
are summarized,
1) A wave load in view of fatigue is a governing parameter
among environmental loads such as wind, current, and
wave.
2) Fatigue damage of mooring lines is highly impacted by
WF motions of moored vessel.
3) Wave frequency fatigue analysis at larger vessel offset
position leads to the higher fatigue damage when it is
applied with the equivalent environmental loads.
4) In the West Africa offshore, swell component which
mainly orientated into the direction in longitudinal axis of
spread moored FPSO gives the higher fatigue damage of
external mooring lines by the larger heave motion of
fairleads due to vessel pitch motion.
5) Regarded with loading conditions, the lower draft results
in the higher mean tension of mooring line. These high line
tensions produce the more fatigue damage.
6) Regarding riser behavior, mean drift loads and damping
effect of riser gives the good vessel LF motions and good
fatigue performance.
Based on these sensitivity studies, mooring fatigue analysis
can be made as a simplified approach to reduce a lot of load
cases and computation time regarded with the effects of design
parameters. However, simplified fatigue analysis method of
spread mooring line should only be applied for initial
scantlings.

REFERENCES
[1] DNV, Offshore standard—position mooring, DNV-OS-E301; 2001.
[2] Orcina Ltd., OrcaFlex Manual version 9.6C. Orcina Ltd., Daltongate,
Ulverston, Cumbria. UK, 2013.
[3] M. Matsuishi and T. Endo, Fatigue of metals subjected to varying stress,
Presented to the Japan Society of Mechanical Engineers, Fukuoka, Japan.
[4] API, Recommended practice for design and analysis of stationkeeping
systems for floating structures, API RP 2SK; 1997.
[5] Pinkster, J.A., “Low-frequency phenomena associated with vessels
moored at sea”, Soc. Petroleum Engineers Journal, Dec. 1975, pp.
487-94.
[6] H. Ormberg, N. Sødahl, and O. Steinkjer, “Efficient analysis of mooring
systems using de-coupled and coupled analysis”, OMAE98-0351, 1998.

International Scholarly and Scientific Research & Innovation 10(5) 2016 510 scholar.waset.org/1307-6892/10004227