Investigation on the Use of Different Approaches to Mooring Analysis and Appropriate Safety

Factors

Sojan VasudevanPaul Westlake

2

Objectives

• Present the results of a Mooring Analysis– Frequency Domain - Quasi-Static– Time Domain - Uncoupled– Time Domain - Coupled

• Differences in Results• Investigate Reasons• Recommendations

3

Mooring System

4

Mooring System

X

Y

Z20 m

X

Y

Z

OrcaFlex 9.4b: Base - with s truts .dat (modified 14:43 on 30/03/2011 by OrcaFlex 9.4b) (azimuth=335; elevation=30)Reset

5

Methodology

• Mooring analysis in ROMEO (frequency domain)• Mooring analyses in Orcaflex

– Uncoupled, without struts– Uncoupled, with struts– Coupled, without struts– Coupled, with struts

6

Results – Line TensionN

NE

E

SE

S

SW

W

NW

0

200

400

Romeo Quasi-Static

7

Results – Line Tension

N

NE

E

SE

S

SW

W

NW

0

200

400

Romeo Quasi-Static

Orcaflex Uncoupled w/o struts

8

Results – Line Tension

N

NE

E

SE

S

SW

W

NW

0

200

400

Romeo Quasi-Static

Orcaflex Uncoupled w/o struts

Orcaflex Uncoupled with struts

Orcaflex Coupled w/o struts

Orcaflex Coupled with struts

9

Results – Line Tension

N

NE

E

SE

S

SW

W

NW

0

200

400

Romeo Quasi-Static

Orcaflex Uncoupled w/o struts

Orcaflex Uncoupled with struts

Orcaflex Coupled w/o struts

Orcaflex Coupled with struts

Maximum Permissible

10

Summary

• Huge differences in worst line tensions depending on the method used.

1. Why?

2. Is the design acceptable?

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Environment

Env Direction (from)

One hour mean wind speed

[m/s]

Significant wave height

Hs [m]

Peak period Tp [s]

Zero-crossing period Tz [s]

N 40.5 16.4 17.0 12.6 NE 32.8 10.2 13.4 9.9 E 30.6 9.5 13.0 9.6

SE 39.9 14.2 15.9 11.7 S 40.5 16.4 17.0 12.6

SW 38.9 16.4 17.0 12.6 W 40.5 16.4 17.0 12.6

NW 40.5 16.4 17.0 12.6

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Line Tension vs Environment

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%0

50

100

150

200

250

300

350

400

450

ROMEO - quasistatic Orcaflex Uncoupled w/o cross bracings

Orcaflex Uncoupled with cross bracings Orcaflex Coupled w/o cross bracings

Orcaflex Coupled with cross bracings

% 100-yr Return Period Weather (from SW)

Wo

rst

line

te

ns

ion

[t]

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Comments

• For up to the 50% weather, none of the mooring lines go slack.

• For the 75% weather, some lines start going slack.

• For the 100% weather, more lines go slack over large lengths.

• Slack lines affect line tension in coupled analyses, but not in uncoupled / Q.S analysis.

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Acceptance Criteria

• Line Tension Safety Factors– API / DNV / ISO etc– Separate Safety Factors for Q.S and Dynamic

• E.g. API RP 2SK– Q.S 2.00– Dynamic 1.67

– No mention about Coupled / Uncoupled– Coupled more accurate lower safety factor may do!

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Conclusions

• The coupled analyses give significantly higher line tensions compared to the frequency domain analyses in ROMEO and the uncoupled analyses in Orcaflex.

• The higher tensions from the coupled analyses are probably caused by the mooring lines going slack.

• Inclusion of struts in the analyses increases the worst line tension by about 15% in the case of displacement RAOs and 20% in the case of load RAOs.

• Consider lower safety factors for coupled dynamic analysis.

Questions?