BY
S.Nallayarasu, S.Goswami, J.S.Manral, R.M.Kotresh
Presenter: S.K. Bhattacharyya
Dept. of Ocean Engineering
IIT Madras

Mumbai high field location
 Historically, Bombay High Field of ONGC has several offshore platforms in the shallow water region of 50 to 80m water depth.
 Most of these platforms are fixed template type structures with either main or skirt piles.
 Many of these structures are as old as 20 to 30 years & have been designed as per API
RP 2A guidelines.
 These structures mostly produce oil & Gas and have both process & well head platforms.
 These platforms have been designed against fatigue from cyclic wave loads.

DIRECTIONAL DISTRIBUTION OF WAVES
1.
2.
The field is located on the west coast of India and the wave approach is from south to northwest directions and the other directions are shielded from land.
Generally waves are approaching the platforms only from
South, South-West,
West and North-West.
The directional distribution of waves used in the deterministic and spectral methods is shown in Figure

FATIGUE RESPONSE ANALYSIS
Deterministic method of analysis



Seastate is discretised in discrete (deterministic) waves the scatter data based sea state specific information is used.
Structural response to these discrete waves is then calculated either with or without dynamic effects depending on natural period.
Spectral method of analysis




Seastate is characterised by the spectral energy.
Further, the scatter data for different directions and wave heights are used to simulate the seastate.
The structural response is then calculated using stochastic method of structural analysis.
Dynamic analysis is performed to generate the dynamic characteristics such as mode shapes and mass characteristics.

WAVE SCATTER DATA
 Wave scatter data and exceedance information used for the deterministic fatigue analysis is shown in Table 1 and 2.
 The exceedance data has been converted to occurrence cyclic data with intermediate data range by interpolation
 It has been summarised in Table 3.

WAVE SCATTER DATA – Deterministic Table - 1
WAVE HEIGHT
(M)
0.0-1.524
1.524 - 3.047
3.048 – 4.571
4.572 – 6.095
6.096 – 7.619
7.620 - 9.143
9.144 – 10.667
10.668 – 12.192
S
8.7
9.2
9.5
9.7
9.9
10.5
10.6
10.8
11.0
PERIOD (SEC)
SW W
9.6
8.3
10.1
10.3
10.4
8.7
9.2
9.6
10.0
10.3
10.6
10.9
8.9
--
--
--
NW
6.6
7.4
7.9
8.4

WAVE SCATTER DATA – Deterministic Table - 2
0
1.524
3.048
4.572
6.096
7.620
9.144
10.668
12.192
Wave
Height (m)
S DIR
-
-
11
0
-
1276045
61704
3132
167
Number of Waves Exceeding Specified Height
In One Year
SW DIR W DIR NW DIR
770535 1015713 1220511
CUMULATIV
E
4282804
219347 220985 69788 571824
37929
5878
31902
4073
3764
177
76727
10295
2
0
869
126
18
493
59
7
1
0
8
0
-
-
-
1381
185
25
3
0

WAVE SCATTER DATA – Deterministic Table - 3
Wave Height
(m)
0.381
1.143
1.905
2.667
3.429
4.191
4.953
5.715
6.447
7.239
8.001
8.763
924
322
112
39
13
541944
252784
137022
52061
20503
7326
2656
W SW
1391
538
205
78
30
359421
191767
128135
53283
22998
9053
3618
0
0
32
11
0
995444
218897
47802
10770
2409
556
124
S NW
0
0
30
8
0
928660
222063
53581
12443
2948
639
139



WAVE SCATTER DATA – Spectral
The wave scatter data for spectral analysis obtained from National
Institute of Oceanography is summarized in Tables 4 to 8 for south, south-west, west and north-west directions respectively.
The percentage distribution for each combination of wave period and height will be used for the spectral representation of the seastate using JONSWAP spectra.
Table-4 ( South)
Hs
(m)
3-4
0.0 -
0.5
0.5 -
1.0
1.0 -
1.5
1.5 -
2.0
0.38
0.00
0.00
0.00
Total 0.38
4-5
0.77
5.00
2.69
5-6
0.00
17.31
10.77
Mean wave period (s)
6-7
0.00
18.85
15.00
7-8
0.00
11.54
1.92
0.00
2.31
2.31
2.31
8.46 30.38 36.15 15.77
8-9
0.00
1.15
2.31
3.85
7.31
9-10
0.00
0.00
0.00
0.77
0.77
10-11
0.00
0.00
0.00
0.77
0.77
Total
1.15
53.85
32.69
12.31
100.00

WAVE SCATTER DATA – Spectral
Hs
(m)
3-4 4-5 5-6
0.0 - 0.5
0.5 - 1.0
0.00
0.21
0.00
2.92
0.00
5.22
6-7
Mean wave period (s)
7-8 8-9
0.00
1.67
0.00
0.84
0.00
0.00
1.0 - 1.5
1.5 - 2.0
2.0 - 2.5
2.5 - 3.0
3.0 - 3.5
3.5 - 4.0
4.0 - 4.5
4.5 - 5.0
5.0 - 5.5
5.5 - 6.0
Total
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.21
0.00
0.00
0.00
0.00
0.84
0.00
0.00
0.00
0.00
0.00
3.76
11.90
4.59
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
21.71
9.81
16.08
3.97
3.55
1.88
0.63
0.00
0.00
0.00
0.00
37.58
2.51
3.34
0.42
1.25
2.71
9.60
5.22
2.51
0.63
0.00
29.02
0.00
0.00
0.00
0.84
0.21
2.09
2.30
0.42
1.67
0.21
7.72
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
9-10
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
10-11
0.00
0.00
0.00
3.97
0.42
2.09
2.30
0.21
100.00
Total
0.00
10.86
25.47
32.36
11.48
6.47
4.38

WAVE SCATTER DATA – Spectral
Hs
(m)
3-4 4-5 5-6 6-7
Mean wave period (s)
7-8 8-9
0.0 - 0.5 0.00
0.00
0.00
0.00
0.00
0.00
0.5 - 1.0
1.0 - 1.5
1.5 - 2.0
2.0 - 2.5
2.5 - 3.0
3.0 - 3.5
3.5 - 4.0
4.0 - 4.5
4.5 - 5.0
5.0 - 5.5
5.5 - 6.0
6.0 - 6.5
Total
9.85
6.61
2.53
0.14
0.00
30.52
0.00
0.00
0.00
0.56
0.84
3.66
6.33
0.14
0.00
0.00
0.00
0.00
34.60
0.00
0.70
2.81
5.63
12.80
9.00
3.52
0.00
0.00
0.00
0.00
0.00
23.63
1.83
4.22
9.00
6.05
2.39
0.14
0.00
0.00
0.00
0.00
0.00
0.00
3.94
1.83
1.69
0.42
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.28
0.28
0.00
0.00
0.00
0.00
0.00
0.00
0.00
1.69
3.38
1.27
0.42
7.03
0.00
0.00
0.00
0.00
0.14
0.00
0.14
0.00
0.00
0.00
0.00
0.00
0.00
10-11
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
9-10
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Total
0.00
3.94
6.61
12.24
12.24
16.17
12.80
9.99
9.99
8.30
5.91
1.41
0.42
100.00

WAVE SCATTER DATA – Spectral
Hs
(m)
0.0 - 0.5
0.5 - 1.0
1.0 - 1.5
Total
3-4
0.00
4.35
0.00
4.35
4-5
0.00
34.78
17.39
52.17
5-6
0.00
19.57
19.57
39.13
6-7
Mean wave period (s)
7-8 8-9
0.00
0.00
0.00
0.00
0.00
0.00
2.17
2.17
0.00
0.00
0.00
0.00
9-10
0.00
0.00
0.00
0.00
10-11
0.00
0.00
2.17
2.17
Total
0.00
58.70
41.30
100.00

SELECTED STRUCTURES







4 LEGGED PRODUCTION CUM DRILLING PLATFORM
WATER DEPTH- 76.2 M
0 MAIN & 8 SKIRT PILES
16 WELL SLOTS & CONDUCTORS
MODULAR DRILLING RIG HAVING RIG MAST, RIG
SUPPORT & LQ MODULE
TOPSIDE WEIGHT- 6000 MT
JACKET WEIGHT-3300 MT (GROSS)

SELECTED STRUCTURE
MNP PROCESS PLATFORM









EIGHT LEGGED 4 LEVEL TOPSIDES
WATER DEPTH-72 M
16 SKIRT PILES
20 PRE-INSTALLED RISERS
LAUNCH JACKET WEIGHT-7200 MT
PROCESS HUB-
TOTAL TOPSIDE WEIGHT-20000 MT
3 PROCESS GAS COMPRESSORS, 1
BOOSTER GAS COMPRESSOR.
SUBSTRUCTURE SUPPORTS FOR 3
BRIDGES

DETERMINISTIC ANALYSIS
The calculation of cyclic stresses on the tubular joints shall include dynamic amplification. The effects of dynamic amplification can be ignored when the natural period of the structure is below 3 seconds as stated in API RP 2 A. This is due to the fact that most of the wave period inducing cyclic loads will be in range of 4 to 12 seconds.
The dynamic amplification factor (DAF) can be calculated using the following formula assuming a single degree of freedom system for the fixed type jacket structures.
DAF

1

T
T
N
2
2
 
T
(1 ) (2 )
T n
2 where T n is the natural period of the structure, T is the wave period and  is the damping ratio( 2%). It can be shown that the the response and cyclic stress ranges can be linearly multiplied by the DAF and hence the total response can be calculated without going into the full fledged dynamic response of the structure against waves.
However, the accuracy of the analysis depends highly on the descretization of the seastate and any simplification will lead to erroneous estimation of response and fatigue damage.

}
Where [K] is the stiffness matrix, {X} and {F} are the displacement and force vectors respectively. The above approach indicates a simplified method and is very easy to implement for practice. This method has been in use for several years for the prediction response of offshore structures.

SPECTRAL ANALYSIS
Alternatively, the response and the cyclic stresses can be calculated using dynamic wave response including dynamic effects due to the above. This method of calculation involves procedures involving dynamic characteristics of the structure and performing the analysis in close intervals of frequency / wave period. However, the method of calculation involved several approximations and the discussion on these issues is outside the scope of this paper and can be found elsewhere.

M

(3)
Solution to the following equation will lead to Eigen modes and vectors. The dynamic analysis is performed to obtain the dynamic characteristics such as mode shapes and frequencies.

Where X” is the Eigen frequencies and X is the displacements. The mode shapes and frequencies are then used in the subsequent wave response calculation in which the following equation is solved including the dynamic response of the system.

X

M

F
( 4)
The response is calculated as a transfer function to facilitate the computation of the fatigue damage for various waves in different directions. Typical wave response stress transfer function for base shear and overturning moment is shown in Figure 1 and 2 respectively

SPECTRAL ANALYSIS FIG-1
TRANSFER FUNCTION FOR BASE SHEAR

SPECTRAL ANALYSIS FIG-2
TRANSFER FUNCTION FOR OVERTUNING MOMENT

SPECTRAL ANALYSIS
 Selection of frequencies for the generation of transfer function is an important task such that the peaks and valleys of the response is not missed. Following the guidelines given API RP 2A, the frequencies near the natural period of the structure and its multiples shall be selected. The transfer function has been generated for various frequencies from 0.1 Hz to 0.5Hz (Typically from wave periods in the range of 2 to 10 seconds). The frequency interval is selected such that more number of points is generated near the natural period,

 The transfer function and the response are generated for both maximum base shear and maximum overturning moment cases and the worst case is used for the calculation of fatigue damage.

 A wave steepness of 1/20 is used for the all the waves as recommended by API RP
2A for the calculation of wave height for each frequency. This has been used for the generation of the transfer function.

 It can be observed from Figure 1 and 2 that the maximum values of transfer function occurs near the frequency of 0.4 which corresponds to a period of 2.5 sec.
The natural period of the structures for MNP and RS14 is noted to be between 2.5 sec and 3 sec.

ESTIMATION OF FATIGUE DAMAGE
Fatigue damage has been calculated for all the tubular connections using Miner’s rule using cumulative fatigue damage model stated as below.

RMS i   
0
H i
2
( ) h
( )
(5)
T z

 
0

RMS i
2 2 f H ( ) h
( )
(6)

ESTIMATION OF FATIGUE DAMAGE – (Contd.) where σ is the RMS (Root mean square value) of the stress calculated from the transfer function for a given Seastate, H is the transfer function and S is the spectral density of the seastate.
(7) n ( s )
 mL
T z where n(s) is the number of applied cycles, L is the design life and Tz is the spectral mean period calculated above.
Fatigue damage
D

 n ( s )
2
RMS i
 
0
N s
( s ) exp(
 s
2
2
RMS i
) ds (8) where N(s) is the allowable cycles from the S-N curve and S is the stress range.
Stress concentration factor (SCF) for the tubular joints has been calculated as per
Effthimiou formulas as recommended by API RP 2A for tubular joints and the S-N curve has been adopted as per API RP 2A for tubular joints.

FACTOR OF SAFETY
FAILURE
CRITICAL
NO
YES
API RP 2A
INSPECTABLE
2
5
NON-INSPECTABLE
5
10
ONGC
2
4
ONGC USE A FOS OF 4.0 FOR JOINTS BELOW TOW LEVELS OF JACKET FRAMING TO COVER FOR
FATIGUE DUE TO WAVE LOADS

RESULTS AND DISCUSSIONS
 MHN (Mumbai High North) field has been presented in
Table 8 and 9 respectively. The fatigue life of major tubular joints along the jacket legs and X braces is presented. Fatigue life greater than 1000 years is marked as * since it is very high compared to the required design fatigue life of 50 years.



The fatigue life predicted by deterministic analysis for RS
14 well platforms seems to be on a higher side compared to the spectral fatigue analysis. In the case of MNP
Process platform deterministic results are lower than spectral for lower three levels and reverse is the case for
4 th and 5 th level.
This is due to the fact that the Seastate has been condensed to discrete waves and the DAF has been treated approximately.

Table 8. RS-14 Well platform
Comparison of results of deterministic & spectral fatigue on selected joints
JOINT NO.
FATIGUE LIFE
DETERMINISTIC SPECTRAL
397L
301X
302X
303X
303
304
305
403L
203L
217L
283L
297L
201X
303L
317L
383L
417L
483L
497L
401X
402X
74.71
31.17
968.58
*
*
224.9
1039.15
*
456.34
*
287.44
416.70
*
*
*
307.89
1287.7
369.62
118.87
23.18
4.11
17.84
31.79
473.76
627.151
*
83.01
215.84
245.01
187.75
*
172.09
241.13
*
*
*
844.62
*
90.76
32.14
5.32
0.87
DIFFERENCE
(D-S)
0
-
0
0
269
0
115
175
56.87
0
494
400
0
142
824
750
0
278.86
86.73
17.86
3.24

503X
504X
603L
617L
683L
697L
601X
602X
403X
404X
503L
517L
583L
597L
501X
502X
603X
604X
703L
717L
783L
797L
Table 8. Continued
JOINT NO.
FATIGUE LIFE
DETERMINISTIC SPECTRAL
*
*
255.38
541.82
78.56
67.82
49.13
18.42
*
*
145.32
273.35
160.99
28.88
*
*
*
*
1344.463
*
*
*
*
*
252.72
432.30
14.58
25.80
3.86
1.91
655.58
*
141.13
12.08
19.34
7.21
399.95
398.85
23.92
24.27
6.60
6.055
6.099
5.54
DIFFERENCE
(D-S)
0
0
2.66
109.52
63.98
42.02
45.27
16.51
345
0
131.19
261.27
141.65
21.67
600
600
976
976
994
994
994
994

JOINT
NO.
504X
603L
617L
683L
697L
601X
602X
603X
404X
503L
517L
583L
597L
501X
502X
503X
604X
703L
717L
783L
797L
Table 8. Continued
FATIGUE LIFE
DETERMINISTIC SPECTRAL
*
255.38
541.82
78.56
67.82
49.13
18.42
*
*
145.32
273.35
160.99
28.88
*
*
*
*
1344.463
*
*
*
*
252.72
432.30
14.58
25.80
3.86
1.91
655.58
*
141.13
12.08
19.34
7.21
399.95
398.85
23.92
24.27
6.60
6.055
6.099
5.54
DIFFERENCE
(D-S)
0
2.66
109.52
63.98
42.02
45.27
16.51
345
0
131.19
261.27
141.65
21.67
600
600
976
976
994
994
994
994

Table 9. MNP Process platform
Comparison of results of deterministic & spectral fatigue on selected joints
JOINT NO.
FATIGUE LIFE
DETERMINISTIC SPECTRAL
DIFFERENCE
(D-S)
204X
205X
206X
207X
208X
209X
210X
211X
203L
207L
213L
217L
283L
287L
293L
297L
212X
213X
203L
207L
213L
217L
*
*
*
*
*
*
*
*
52.41
9.47
9.26
78.80
52.93
11.55
11.14
43.65
*
*
52.41
9.47
9.26
78.80
*
*
*
*
*
*
*
*
108.38
34.14
21.43
127.34
129.05
81.14
69.06
88.38
*
*
108.38
34.14
21.43
127.34
0
0
0
0
0
0
0
0
-77
-70
-58
-45
-56
-24
-12
-49
0
0
-56
-24
-12
-49

JOINT
NO.
303L
307L
305X
306X
307X
308X
309X
310X
311X
312X
313X
313L
317L
383L
387L
393L
397L
304X
403L
407L
Table 9. Continued
FATIGUE LIFE
DETERMINISTIC SPECTRAL
20.89
70.45
*
*
*
*
*
*
*
*
*
69.36
18.49
19.56
197.60
253.92
18.97
*
149.07
20.62
202.21
508.03
*
*
*
*
*
*
*
*
*
806.51
302.99
267.49
485.68
783.73
358.24
*
*
200.44
DIFFERENCE
(D-S)
-182
-438
-737
-284
-248
-288
-530
-339
-851
-180

JOINT
NO.
406X
407X
408X
409X
410X
411X
412X
413X
417L
483L
487L
493L
497L
404X
405X
503L
507L
513L
517L
583L
Table 9. Continued
DETERMINISTIC SPECTRAL
156.09
185.24
168.31
140.70
135.05
*
*
*
*
*
*
*
*
*
429.95
104.71
24.17
23.69
153.88
301.03
72.32
163.23
147.37
132.46
118.97
*
*
*
*
*
*
96.67
513.86
125.03
21.49
0.88
0.06
1.07
0.85
0.87
DIFFERENCE
(D-S)
84
22
21
8
16
903
486
875
409
104
24
22
153
300

JOINT
NO.
505X
506X
507X
508X
509X
510X
603L
607L
587L
593L
597L
501X
502X
503X
504X
613L
617L
683L
687L
693L
Table 9. Continued
DETERMINISTIC SPECTRAL
73.39
156.72
181.49
*
*
*
*
*
*
*
*
*
*
151.58
99.52
184.74
370.08
206.42
105.74
180.44
0.89
1.01
0.69
127.26
136.83
234.38
115.94
108.49
233.61
1.42
1.57
1.42
0.21
0.57
1.18
1.21
0.56
0.70
1.20
1.13
DIFFERENCE
(D-S)
892
767
999
999
999
999
151
98
183
369
205
104
179
72
155
180
873
864
760
884

CONCLUSIONS
 Based on the results obtained from the fatigue analysis of platforms in
Mumbai High North and South platforms, following observations are made.

 Generally both methods predict fatigue life reasonably well for most of the joints except for some joints at the bottom of the jacket, the deterministic method predicts the fatigue life lower than the spectral methods. This is due to the fact that the dynamic response of the structure over-predicted by deterministic method by approximate calculations of DAF due to course discretisation of wave periods.

 However, the joints near the top of the jacket, the predicted fatigue life using deterministic methods seems to be higher than the spectral methods. This is due to the fact that the wave load and associated cyclic stresses are only due to the local wave loads rather than the dynamic response.

 It is recommended that spectral fatigue analysis be used for large platforms to assess the fatigue life since the inaccuracy introduced due to the treatment of dynamic amplification factor.

References
 API RP 2A Recommended Practice for the Design and
Construction of fixed offshore platforms, working stress design.
 Fatigue User Manual, SACS Software, EDI
 Identification of wave spectra for Mumbai offshore region,
National Institute of Oceanography, December 2007.