On the Challenges of Analysis and Design of Turret-Moored FPSOs in Squalls (2)

Presented by: Amir IzadparastCo-Authors: Arun Duggal

Overview

• Background –– Squalls Yet Another Environmental Phenomena– Current Design Practice– Our Proposal (previous paper)

• Further Discussion on Design Value

Background – Squall Characteristics • Squall: A sudden onset of Strong Winds with speeds increasing to at least

8 m/s and sustained at 11 m/s for at least 1-minute. The Intensity and Duration is longer than that of a Gust.

Background – Squall Characteristics

0 2000 4000 6000 8000 10000 120005

10

15

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35

40

Time (s)

1-M

in W

ind

Spe

ed (m

/s)

Squall 1-Min Wind Speed

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-140

-120

-100

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-40

-20

0

20

Squall Wind Direction

Background – Analysis

• Analysis For Squalls– Scale the measured squalls (from 9 to 100+ timeseries) to 100yr

RP wind speed (commonly up to 36m/sec) – Scale the time as well?

• Run squalls from all plausible headings.– Squall Heading is commonly defined as the heading at peak

velocity• Various vessel loading conditions• Various ambient wave and current combinations (zero wave and

current!)• Various riser arrangements

Background – Design Value (Class Society)

• DNV-OS-E301 (Oct. 2010):– The extreme value of the mooring line tension and offset shall be taken as

the maximum value for the time series of the actual responses.

• DNV-OS-E301 (Oct. 2013): – “As a minimum the maximum tension from at least 20 representative

squalls is required.”…– “If a much better basis in data is available a 95% fractile as the characteristic

load may be accepted. The set of scaled squalls considered shall be much larger than 20 representative cases (e.g. 100 representative squalls).”

• NR 493 DT R02 E (April 2012):– “Design response (of tension, offset,...) is the maximum responses obtained

over all the squall cases except otherwise agreed.”

Background – Design Value (Argument)

• From a Designer Perspective:– Most critical value is the most sensitive estimate

• Input squalls (scaling, quality of measurements, etc.)• Numerical modeling

– Most critical value is the most unreliable (uncertain) statistics• Could significantly change in time• Tends to increase with sample size increase

Background – Design Value (Argument)

Most critical value is the most sensitive estimate 

Input squalls (scaling, quality of measurements, etc.)Numerical modeling

Most critical value is the most unreliable (uncertain) statistics

Could significantly change in timeTends to increase with sample size increase

• Maximum overall response is the most sensitive estimate

• Input squalls (scaling, quality of measurements, etc.)

• Numerical modeling

• Maximum overall response is the most unreliable (uncertain) statistics

• Sample size effect• Could significantly change in

time• Tends to increase with sample

size increase

From a Designer Perspective

Most critical value is the most sensitive estimate 

Input squalls (scaling, quality of measurements, etc.)Numerical modeling

Most critical value is the most unreliable (uncertain) statistics

Could significantly change in timeTends to increase with sample size increase

• Most critical expected maximum response of large enough sample is not significantly sensitive to input uncertainties.

• Most critical expected maximum response of large enough sample is significantly less uncertain and less sensitive to sample size.

Background – Design Value (our proposal)

• 19th SNAME TX Offshore Symposium Feb. 2014: – Choose ~ 35 squalls that require minimal “scaling” for desired

return period– Run simulations changing other variables (loading condition,

ambient environment, squall heading, riser arrangements, etc.)– Treat the squalls as independent realizations of the same

phenomena and estimate the Expected Maximum of the 35 observed maxima

• The only random variables are squall intensity and directionality– The design value is the most critical Expected Maximum

Question?

• “Are we fair to spread moored FPSOs?”– The design value for spread moored FPSOs is estimated as the

expected value of response to squalls approaching the vessel from a certain angle (e.g. beam on).

– The design value for turret moored FPSOs is estimated as the expected value of response to squalls approaching the vessel with a specific heading but individual maximum response could happen at different relative headings.

Spread Moored vs. Turret Moored

Spread-Moored Turret-Moored

Response – First Reaction

Spread-Moored Turret-Moored

Earth Fixed vs Vessel Fixed Angles

Further Discussion on Design Value

• Case Study– A typical deep-water turret moored FPSO designed for West

Africa environment– Taut chain-polyester-chain mooring legs– 3G*3L mooring leg arrangement– 73 squalls, 24 headings (360deg, 15deg resolution)

• Most critical draft• Most critical riser arrangement• Most critical ambient environment• 1752 cases

N

A More Careful Look at Offset

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50

100

t (sec)

Off

set (

m)

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10

20

30

40

t (sec)

Squa

ll Sp

eed

(m/s

ec)

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200

300

400

t (sec)

Squa

ll H

eadi

ng (d

eg)

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90

180

270

360

t (sec)

Rel

. Hea

ding

(deg

)

A More Careful Look at Offset

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20

40

60

80

t (sec)

Off

set (

m)

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 300010

20

30

40

t (sec)

Squa

ll Sp

eed

(m/s

ec)

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000150

200

250

300

350

t (sec)

Squa

ll H

eadi

ng (d

eg)

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 30000

90

180

270

360

t (sec)

Rel

. Hea

ding

(deg

)

A More Careful Look at Offset

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 30000

50

100

t (sec)

Off

set (

m)

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 30000

10

20

30

40

t (sec)

Squa

ll Sp

eed

(m/s

ec)

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000100

200

300

400

t (sec)

Squa

ll H

eadi

ng (d

eg)

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 30000

90

180

270

360

t (sec)

Rel

. Hea

ding

(deg

)

A More Careful Look at Offset

At the time of Max Offset

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Squall - Vessel Rel Heading (deg)

Max

. Obs

erve

d O

ffset

Rad

ius (

m)

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26

28

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Squall - Vessel Rel Heading (deg)

Win

d Sp

eed

(m/se

c)

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30

60

90

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150

180

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270

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Squall - Vessel Rel Heading (deg)

Squa

ll H

eadi

ng (d

eg)

A More Careful Look at Offset

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Y (m)

X (m

)

-100-80-60-40-20020406080100-100

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-60

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-20

0

20

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100

Y (m)X

(m)

300deg0deg60deg120deg180deg240deg

N

Offset Design Value – Original Proposal

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Squall Heading (deg)

Offs

et R

adiu

s (m

)

Expected (Max)Min (Max)Max (Max)

10-3

10-2

10-1

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40

60

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120

Probability of Exceedance

Offs

et (m

)

Data PointsGEVSquall Angle = 300deg

N

Offset Design Value – Offset Direction

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Offset Direction (deg)

Offs

et R

adiu

s (m

)

Expected (Max)Min (Max)Max (Max)

10-3

10-2

10-1

10020

40

60

80

100

120

Probability of Exceedance

Offs

et (m

)

Data PointsGEVOffset Direction = 300deg

Method E [Max Offset] (KN)

Expected Value – Squall Heading 63.9Expected Value – Offset Direction 64.4

N

A More Careful Look at Tension

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3000

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t (sec)

Tens

ion

(KN

)

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 300010

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30

40

t (sec)

Squa

ll Sp

eed

(m/s

ec)

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100

150

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t (sec)

Squa

ll H

eadi

ng (d

eg)

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 30000

90

180

270

360

t (sec)

Rel

. Hea

ding

(deg

)

A More Careful Look at Tension

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 30001000

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t (sec)

Tens

ion

(KN

)

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 300010

20

30

40

t (sec)

Squa

ll Sp

eed

(m/s

ec)

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 30000

50

100

150

t (sec)

Squa

ll H

eadi

ng (d

eg)

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 30000

90

180

270

360

t (sec)

Rel

. Hea

ding

(deg

)

A More Careful Look at Tension

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 30001000

2000

3000

4000

t (sec)

Tens

ion

(KN

)

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 30000

10

20

30

40

t (sec)

Squa

ll Sp

eed

(m/s

ec)

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000-100

0

100

200

t (sec)

Squa

ll H

eadi

ng (d

eg)

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 30000

90

180

270

360

t (sec)

Rel

. Hea

ding

(deg

)

A More Careful Look at Tension

0 30 60 90 120 150 180 210 240 270 300 330 3602000

2500

3000

3500

4000

Squall - Vessel Rel Heading (deg)

Max

. Obs

erve

d Te

nsio

n (K

N)

0 30 60 90 120 150 180 210 240 270 300 330 36024

26

28

30

32

34

36

38

Squall - Vessel Rel Heading (deg)

Win

d Sp

eed

(m/se

c)

0 30 60 90 120 150 180 210 240 270 300 330 3600

50

100

150

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250

300

350

Squall - Vessel Rel Heading (deg)

Squa

ll H

eadi

ng (d

eg)

At the time of Max Tension in Most

Critical Line

Tension Design Value – Original Proposal

0 30 60 90 120 150 180 210 240 270 300 330 3601000

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Squall Heading (deg)

Tens

ion

(KN

)

Expected (Max)Min (Max)Max (Max)

10-3

10-2

10-1

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2000

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3500

4000

4500

5000

Probability of Exceedance

Tens

ion

(KN

)

Data PointsGEVSquall Heading = 120deg, Leg 2

0 30 60 90 120 150 180 210 240 270 300 330 3601500

2000

2500

3000

3500

4000

Squall Heading (deg)

Tens

ion

(KN

)

Expected (Max)Min (Max)Max (Max)

N

Tension Design Value – Most Critical Line

0 30 60 90 120 150 180 210 240 270 300 330 3602000

2500

3000

3500

4000

Squall Heading (deg)

Tens

ion

of M

ost C

ritic

al L

eg (K

N)

Expected (Max)Min (Max)Max (Max)

10-3

10-2

10-1

1001500

2000

2500

3000

3500

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4500

5000

Probability of Exceedance

Tens

ion

(KN

)

Data PointsGEVSquall Heading = 120deg, Most Critical Leg

10 20 30 40 50 60 701

2

3

4

5

6

7

8

9

Squall No

Mos

t Crit

ical

Lin

e N

o.

Method E [Max Tension] (KN)

Expected Value – Individual Leg 2835Expected Value – Most Critical Leg 2863

Next Steps

• Response – Based Analysis of Turret Moored FPSOs in Squalls– Challenges

• Scaling Issues• Synthetic Squalls (intensity and directionality) • Extrapolating Inputs or Responses

Thank You!