DOT 2004 New Orleans 1/22

Understanding Fatigue for Deepwater Mooring Systems

- The Footprint of Fatigue

FMC SOFEC Floating Systems14741 Yorktown Plaza DriveHouston, TX 77040

Cindy Bowline & Arun Duggal

DOT 2004 New Orleans 2/22

Objective: 7 Case Studies

• Show high Th on flowlines influences fatigue life of chain

• Increasing the EPS improves fatigue life• Compare fatigue life computations from 3

guidelines• Need to account for flowline tensions in

FEED studies

DOT 2004 New Orleans 3/22

Assumptions for 7 Cases:

• Intact Mooring Analyses only• Water Depth 1000 meters• Maximum offset is 5% of W.D.• Assume meets API RP 2SK Safety Factors • Studless Chain, Sheathed Spiral Strand Wire• All mooring legs use same size components

DOT 2004 New Orleans 4/22

Additional Assumptions for 7 Cases:

• All systems have 12 legs• 55, 60, and 65 degree plan angles• Vessel heading is “South”

– middle of swell sector

• Buoy to the “North”• All analyses use Ballast Condition• 20 year design life• 0.4 mm/yr corrosion and wear allowance

North

DOT 2004 New Orleans 5/22

Environmental Conditions

• 100-Year Winds– Swell from 3 directions

(S, SSE, and SSW)– Wind and Current

together, from 16 Directions (every 22.5 degrees)

– Total of 48 Cases analyzed

• Fatigue Seastates– Swell from the South

70% occurrence– Swells from SSE, SSW

15% occurrence each– 33 cases of Hs, Tp,

wind and current directions, each swell direction

– Total of 99 Fatigue cases

DOT 2004 New Orleans 6/22

Case 1 (Base Case)

• FPSO, no buoy or flowlines

• EPS = WD for all mooring legs

DOT 2004 New Orleans 7/22

Case 1 Results• Maximum Line Load

537 MT– 90 mm wire– 103 mm R4 chain

• Chain Fatigue Life (current API RP 2SK edition):– 2200+ yrs for “South”– 15,000 yrs for “North”

Midships Offsets in Squalls

-60

-40

-20

0

20

40

60

-60 -40 -20 0 20 40 60

Offset in meters EastO

ffse

t in

met

ers

No

rth

Extreme Midships Offsets5% of Water Depth Envelope

DOT 2004 New Orleans 8/22

Case 2 (Base Case with Flowlines)

• An FPSO with an export buoy to the North

• EPS = WD for all mooring legs

DOT 2004 New Orleans 9/22

Case 2 Results• Maximum Line Load

655 MT– 100 mm wire (1.69)– 114 mm R4 chain

• Chain Fatigue Life (current API RP 2SK edition):– 27 yrs for “South”– 110,000+ yrs for

“North”

Midships Offsets in Squalls

-60

-40

-20

0

20

40

60

-60 -40 -20 0 20 40 60

Offset in meters EastO

ffse

t in

met

ers

No

rth

Extreme Midships Offsets5% of Water Depth Envelope

DOT 2004 New Orleans 10/22

Case 2 Required Chain Size:

Chain Size: 114 mm R4 162 mm ORQ MinimumBased on: Max Load Fatigue RequiredCurrent API RP 2SK 27 212 200API RP 2SK for 2005 9 57 60POSMOOR 7 63 160

Fatigue Life in years

162 mm is min. to meet API,Other codes need larger chain.

DOT 2004 New Orleans 11/22

Case 1 EPS=WD, No Buoy Case 2 EPS=WD

103 mm R42218 yrs API521 yrs new API376 yrs POSMOOR

162 mm ORQ212 yrs API57 yrs new API63 yrs POSMOOR

DOT 2004 New Orleans 12/22

Case 3 EPS=WD+10%

Case 4 EPS=WD+20%

Case 5 EPS=WD+30%

Case 6 EPS=WD+40%

143 mm ORQ210 yrs API57 yrs new API107 yrs POSMOOR

131 mm ORQ216 yrs API60 yrs new API103 yrs POSMOOR

117 mm ORQ211 yrs API59 yrs new API92 yrs POSMOOR

107 mm R3S203 yrs API58 yrs new API84 yrs POSMOOR

DOT 2004 New Orleans 13/22

Case 7 (EPS=WD) Fatigue Results for N legs:

• 136 mm Chain, based on API Fatigue:– 220 yrs current API RP

2SK– 63 yr API RP 2SK for

2005– 57 yrs POSMOOR

• Note legs away from swell can have fatigue problems

DOT 2004 New Orleans 14/22

Total Weights Required for Cases 1-6:

Total Mooring Chain and Wire Weights

0

200

400

600

800

1000

1 2 3 4 5 6

Case Number

To

tal W

eig

ht,

ton

nes

Chain Weight

Wire Weight

DOT 2004 New Orleans 15/22

Relative Chain + Wire Costs for Cases 1-6:

0%

25%

50%

75%

100%

1 2 3 4 5 6

Rel

ativ

e C

om

po

nen

t Co

st

DOT 2004 New Orleans 16/22

“South” Leg Pretensions as% of Chain BS & Wire MBL:

Case Pretension Chain BS Percent of Wire MBL Percent ofNumber tonnes (@10yrs) tonnes Chain BS tonnes Wire MBL

1 107 989 11% 892 12%2 222 1685 13% 1106 20%3 214 1367 16% 1065 20%4 209 1174 18% 1004 21%5 202 960 21% 944 21%6 199 816 24% 892 22%

DOT 2004 New Orleans 17/22

Why does the fatigue life improveas EPS increases?

• WF damage is >> LF damage• D = N * (21/2 * Rrms)M * Γ(1+ M / 2) / K• D Q (∆T/BS)3.36

• As EPS increases, ∆T/BS decreases

DOT 2004 New Orleans 18/22

Tension Change as FPSO Heaveswith a 2 meter wave:

Line Tension with Vertical Offset

190

200

210

220

230

240

-1.0 -0.5 0.0 0.5 1.0

Vertical Offset, meters

Ten

sio

n, t

on

nes

Case 2

Case 3Case 4

Case 5Case 6

Case 2

Case 3

Case 4

Case 5

Case 6

DOT 2004 New Orleans 19/22

Horizontal Tension Components Balance:

• Offloading Lines exert 130 tonnes Th each

• “North” lines are at lower tensions – add to pull of flowlines

• “South” lines at higher Thtension to balance pull to North

134 t each

134 t each

48 t each

48 t each

130 t each

DOT 2004 New Orleans 20/22

Line Tension has Vertical & Horizontal Components:

Case 2 Case 6

134 t

176 t 221 t

134 t

147 t199 t

DOT 2004 New Orleans 21/22

Conclusions

• Analyzing a mooring system w/o export flowlines is not equivalent to one with flowlines.

• Increasing the allowable anchor radius (EPS) is an effective means of increasing the fatigue life of the mooring chain.

• Fatigue damage can occur in mooring legs away from the swell, too.

DOT 2004 New Orleans 22/22

Conclusions

• New API RP 2SK Guideline (2005):– fatigue life factor of safety is SF=3. – Automatically assuming SF=10 will require

greatly increased chain sizes.