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Reliability of mooring lines and piles for a permanently manned vessel in a tropical cyclone environment

Reliability of mooring lines and piles for a permanently manned vessel in a tropical cyclone environment
Reliability of mooring lines and piles for a permanently manned vessel in a tropical cyclone environment

The factors of safety for stationkeeping systems in current standards (ISO, API) are not derived or validated using reliability analyses. As the oil and gas exploration and production is breaking new boundaries, deploying new floating systems and moving into regions with harsher environments, it is of paramount importance to understand what level of reliability these new marine structures are achieving. This paper presents a reliability analysis of the mooring system of a Floating LNG (FLNG) vessel permanently moored and permanently manned offshore North West Australia in a tropical cyclone environment. The reliability analysis addresses both the mooring chain and the pile foundation. The analysis accounts for the long term characteristics of the environment, including the short term variability, in response to a given sea state and the variability and uncertainty in strength of the mooring chain and the pile. The stationkeeping system was analysed using detailed time domain simulations, capturing system non-linearities and low frequency oscillations as well as wave frequency responses and, thereby, reducing modelling uncertainties to a minimum. It is found that for the conditions modelled, neither the chain nor the pile meet a target reliability of 10−4/annum using the factors of safety commonly used in design following current ISO and API standards. New factors of safety are proposed to achieve this target reliability. For the pile design, one complicating factor is that current design standards do not explicitly define the exceedance probability that should be associated with the characteristic value of the undrained shear strength to be used in the design. It is demonstrated that the required factor of safety is crucially dependent on the definition of this characteristic value and on the level and the type of uncertainty in the soil strength profile. A recommendation is made regarding the definition of this characteristic value and the associated factor of safety. Furthermore, it is found that designing the mooring system to an environmental condition with a return period of 10,000 yr (as an Abnormal Limit State event), and setting the factor of safety to unity, meets the target reliability of 10−4/annum for the pile, if the characteristic undrained shear strength is a lower bound, defined in this paper by the 10th percentile value. For the chain however, this target reliability is not achieved.

Factor of Safety, FLNG, Mooring chain, Pile reliability, Probability of failure, Reliability, Soil strength variability
0141-1187
430-446
Stanisic, Dunja
cd348460-7097-49e3-9f5d-765354e4eb53
Efthymiou, Mike
acaa9a55-c6d9-41b5-a01d-0b7cb4ed40c6
White, David J.
a986033d-d26d-4419-a3f3-20dc54efce93
Kimiaei, Mehrdad
8324495a-122a-43d8-a8d0-d38182a39104
Stanisic, Dunja
cd348460-7097-49e3-9f5d-765354e4eb53
Efthymiou, Mike
acaa9a55-c6d9-41b5-a01d-0b7cb4ed40c6
White, David J.
a986033d-d26d-4419-a3f3-20dc54efce93
Kimiaei, Mehrdad
8324495a-122a-43d8-a8d0-d38182a39104

Stanisic, Dunja, Efthymiou, Mike, White, David J. and Kimiaei, Mehrdad (2019) Reliability of mooring lines and piles for a permanently manned vessel in a tropical cyclone environment. Applied Ocean Research, 82, 430-446. (doi:10.1016/j.apor.2018.10.019).

Record type: Article

Abstract

The factors of safety for stationkeeping systems in current standards (ISO, API) are not derived or validated using reliability analyses. As the oil and gas exploration and production is breaking new boundaries, deploying new floating systems and moving into regions with harsher environments, it is of paramount importance to understand what level of reliability these new marine structures are achieving. This paper presents a reliability analysis of the mooring system of a Floating LNG (FLNG) vessel permanently moored and permanently manned offshore North West Australia in a tropical cyclone environment. The reliability analysis addresses both the mooring chain and the pile foundation. The analysis accounts for the long term characteristics of the environment, including the short term variability, in response to a given sea state and the variability and uncertainty in strength of the mooring chain and the pile. The stationkeeping system was analysed using detailed time domain simulations, capturing system non-linearities and low frequency oscillations as well as wave frequency responses and, thereby, reducing modelling uncertainties to a minimum. It is found that for the conditions modelled, neither the chain nor the pile meet a target reliability of 10−4/annum using the factors of safety commonly used in design following current ISO and API standards. New factors of safety are proposed to achieve this target reliability. For the pile design, one complicating factor is that current design standards do not explicitly define the exceedance probability that should be associated with the characteristic value of the undrained shear strength to be used in the design. It is demonstrated that the required factor of safety is crucially dependent on the definition of this characteristic value and on the level and the type of uncertainty in the soil strength profile. A recommendation is made regarding the definition of this characteristic value and the associated factor of safety. Furthermore, it is found that designing the mooring system to an environmental condition with a return period of 10,000 yr (as an Abnormal Limit State event), and setting the factor of safety to unity, meets the target reliability of 10−4/annum for the pile, if the characteristic undrained shear strength is a lower bound, defined in this paper by the 10th percentile value. For the chain however, this target reliability is not achieved.

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More information

Accepted/In Press date: 26 October 2018
e-pub ahead of print date: 5 December 2018
Published date: 1 January 2019
Keywords: Factor of Safety, FLNG, Mooring chain, Pile reliability, Probability of failure, Reliability, Soil strength variability

Identifiers

Local EPrints ID: 428978
URI: http://eprints.soton.ac.uk/id/eprint/428978
ISSN: 0141-1187
PURE UUID: 533f75c9-1712-4c26-a9d1-54965b2f9ed2
ORCID for David J. White: ORCID iD orcid.org/0000-0002-2968-582X

Catalogue record

Date deposited: 15 Mar 2019 17:30
Last modified: 07 Oct 2020 02:17

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