008 FPSO Forum Mooring Reliability v1

Jan Mathisen

2011-10-26

Reliability of Deep Water Mooring Systems

FPSO Forum

© Det Norske Veritas AS. All rights reserved.


2011-10-26

Contents

Brief introduction to structural reliability analysis

JIP’s from 1990 to 2013 & some developments along the way

- Limit states for mooring lines

- Calibration

Future developments

Conclusion

Related papers

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2011-10-26

What do I mean by structural reliability analysis (SRA) It computes the probability of some (failure) event

- As defined by a limit state function

- E.g. ultimate limit state for mooring line failure

- E.g. the probability that the tension will exceed a series of tension levels

Makes explicit use of probability distributions of random variables

- Referred to as level 3 SRA

Overall approach

- Both load and capacity

- Mooring line tension and strength

First and second order reliability methods are used to compute probability integrals (FORM & SORM)

- Monte Carlo analysis is ineffective for these problems

- Direct numerical integration is not feasible in practice

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2011-10-26

SRA

PROSProbabilistic models

provide a more accurate version of reality

Overall model of event

Probability of failure is a transferable measure to other events with similar consequences

CONSUnfamiliar

- Easy to make mistakes

Additional input required

More computations

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2011-10-26

5

Ship in waves – random or deterministic description?



2011-10-26

6

Chain strength – random or deterministic description?

From 2010 Mooring

and DP seminar

– presentation by

Øystein Gabrielsen,

Statoil

– Norne top chain,

pitting in splash zone

– tests show

99-100% of

break test load

(BTL or MBL)



2011-10-26

SRA – by-products Design point

- Values of random variables most likely to lead to failure event

- Very useful for checking SRA model

- Insight into failure event

Uncertainty importance factors, indicate:

- which random variables contribute most to probability of failure event

- which random variables could be simplified as deterministic variables with median values

Sensitivity factors

- How the probability of the failure event changes with small changes in input parameters

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2011-10-26

SRA applications Calibration of design standards for a specified

target reliability target failure probability

- Format of design equation/inequality

- Recipes for characteristic load and capacity

- Safety factors

Analysis and design of novel structures

Reassessment of aging or damaged structures

Risk-based inspection planning

Analysis of actual structural failures

Not normally used in design, when good, applicable design standards are available

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2011-10-26

FPS 2000 R&D Programme – organised by Marintek Part-project 1.8 on Reliability of Station-Keeping Systems (1990-1992)

Initial development of DNV’s reliability analysis of mooring lines

Ultimate limit state (ULS) – for line failure under extreme loads

- Limit state function = breaking strength – line tension

- Starts with a single component (e.g. a chain link)

- Proceeds to many components in a line segment

- and to several segments in one mooring line

Frequency domain analysis of line tension (MIMOSA)

- Separate low-frequency and wave-frequency tension components combined by Turkstra’s

hypothesis

- Separate model uncertainties on tension components

- Integrated analysis of tension distribution and reliability

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2011-10-26

FPS 2000 – cont. (1) - capacity “A chain is no stronger than its weakest link.”

- Assumes all links are subject to same tension

- Not precise for mooring lines

SRA model allowed for linearised variation of tension within a line segment

- Explicitly includes load and strength of each component (link)

- Using published & in-house data for breaking strength of chain links & steel wire rope

- Overall variability in component strength is split into:

- a global component due to variability between link types & manufacturers

- & a local part due to variability between similar links

Subsequent simplification assuming same tension in all components (links) within a segment

- Only includes strength of weakest link and largest tension (at top)

- Strength of weakest link is dependent on the number of links

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2011-10-26

FPS 2000 – cont. (2) – response surface FORM & SORM search through the domain of stochastic variables for the design

point

- Short-term distribution of tension required at each iteration point

Started with direct link between reliability analysis (PROBAN) and response analysis (MIMOSA)

Developed adaptive response surface, performing additional MIMOSA calculations as needed

Subsequently separated reliability analysis and response analysis with interface through a data file for the response surface

11

PROBAN MIMOSA

PROBANMIMOSAInterface

file

Response

surface

module



2011-10-26

12

Response surface for

tension parameter as

a function of wave

direction(NorMoor pilot unit A, pltfil23b_5)



2011-10-26

13

Response surface for

tension parameter as

a function of sign.

wave height(NorMoor pilot unit A, pltfil23b_5)



2011-10-26

PROMOOR JIP (1993-1995) Reliability-Based Design of Mooring Systems

- Organised by DNV in cooperation with Marintek

Initial development of DNV’s fatigue limit state (FLS) for mooring lines – for line failure under cyclic loads

Based on Miner-Palmgren hypothesis with T-N capacity function

- Using published & in-house fatigue capacity data for chain & steel wire rope

- Assumes same fatigue load applied to all components in a mooring line segment

- Splits overall variability in fatigue capacity into:

- global part due to variation between component types & manufacturers, which does not vary between

similar components

- & local part that that varies between similar components

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2011-10-26

DeepMoor JIP (1996-2000) Reliability-Based, Cost-Effective, Design Methods for Deep Water Mooring Systems

- Organised by DNV in cooperation with Marintek

Initial development of DNV’s accidental limit state (ALS or progressive collapse limit state PLS) for mooring lines

- Transient analysis found unnecessary in presence of wave loads

- 1st failure in a line weakened by unknown causes

- 1st and 2nd failure considered in same storm, so that no adjustment is feasible

- Difficult to condition properly on 1st failure

Calibration of ULS, ALS & FLS based on test cases:

- Semisub in 70, 350, 1000 m depths & FPSO in 70, 350, 2000 m depths

- at Haltenbanken & in Gulf of Mexico

Extension for fibre rope

15

DET NORSKE VERITAS AS

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Case Identification

Lin

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)QS1 PSF2 PSFTarget



2011-10-26

NorMoor JIP (2011-2013) ULS

Investigate recipes for directional characteristic tensions

Time-domain response analysis (SIMO & RIFLEX)

- Short-term tension dstn. estimated from time series

- Simplified model uncertainty for tension response analysis

- Separate analysis of tension dstn. & reliability

Simple corrosion model for chain included time dependent reliability

Test cases

- Semisubs and turret-moored fpso

- Water depths from 150 to 1500 m

- Chain, steel wire rope and polyester fibre rope (Syrope model)

- Haltenbanken & Gulf of Mexico + maybe Brazil

17



2011-10-26

Potential future developments ??? Time-dependent reliability of polyester rope and for other fibre types

Calibration of ALS and FLS for time-domain analysis (NorMoor extension?)

Improved model for corrosion (based on SCORCH JIP?)

Reliability analysis of FLS for shackles using fracture mechanics (based on mooring component assessment JIP?)

Reliability analysis of chain links including additional loads near upper termination (based on out-of-plane bending JIP?)

System reliability analysis of mooring system (all mooring lines)

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2011-10-26

Conclusions Reliability analysis of mooring lines is nearing maturity

- (agreement on details of a recommended practice?)

Reliability analysis has already provided valuable insights into mooring line design

- Variation in reliability with water depth

- Effect of line grouping on fatigue reliability

- Effect of distribution of metocean directions on reliability

Reliability analysis is essential in calibration of design methods for mooring lines

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2011-10-26

Papers FPS 2000

- Mathisen, Mørk, “Reliability-Based Design Methods for Mooring Lines,” 8th Int. Conf. on FPS, London, Dec. 1993

ProMoor

- Larsen, Mathisen, “Reliability-Based Mooring System Design for a Drilling Semisubmersible,” 6th

ISOPE, Los Angeles, May 1996.

- Larsen, Mathisen, “Reliability-Based Fatigue Analysis of Mooring Lines,” 15th OMAE, Florence, June

1996

DeepMoor

- Hørte, Lie, Mathisen, (1998), “Calibration of an ultimate limit state for mooring lines,” OMAE-1457,

Lisbon.

- Mathisen, Hørte, Larsen, Lie, (1998), “Calibration of a progressive collapse limit state for mooring lines”,

OMAE-1458, Lisbon.

- Sogstad, Mathisen, Hørte, Lie, (1998) “Modifications to DNV mooring code (POSMOOR) and their

consequences “,OMAE-1460, Lisbon.

- Mathisen, Hørte, (2005), “Calibration of a fatigue limit state for mooring lines,” Int. Conf. Computational

Methods in Marine Engineering, CIMNE, Oslo.

NorMoor pilot

- Mathisen, Okkenhaug, Larsen, (2011), “On the probability distribution of mooring line tensions in a

directional environment”, OMAE2011-50104, Rotterdam.

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2011-10-26

21

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