Comparative Study of Structural Reliability Assessment Methods for Fixed Offshore Structures

Abstract

The oil and gas sector has recognised structural integrity assessment of ageing platform for prospective life extension as a rising concern, particularly in encountering the randomness of the harsh ocean environments. This condition leads to uncertainty in wave-in-deck load estimates and a high load level being imposed on offshore structures. This emphasises the necessity of enhanced reliability, as failure might result in inaccessibility because of the uncertainties related to long-distance services, such as accuracy of predictions of loads and responses. Even though the established guidelines present a fundamental assessment, additionally, comprehensive rules are required. This paper performed a reliability analysis incorporating practical approaches that can more accurately represent time-dependent structural deterioration. The following two procedures have been adopted by a majority of significant oil and gas operators to monitor the safety and integrity of these structures: a) Ultimate Strength Assessment (USA) method and b) Reliability Design Assessment (ReDA) method. A comparison of these two reliability approaches was performed on selected ageing jacket structures in the region of the Malaysian sea. The comparative findings, namely, reserve strength ratio (RSR) at various years of the return period (RP) and ratio value for risk of failure regarding the probability of failure (POF), provided a check and balance in strengthening confidence in the results. The findings showed that the structural components might safely survive either using the USA and ReDA method in such conditions, as the reliability indexes were determined to be satisfactory compared to allowable values from ISO 19902 design specifications. Therefore, these evaluations were determined to control the risk level of the structure during the remaining of its lifetime and undertake cost-effective inspections or mitigation strategies when necessary.

Probability of Failure for Fixed Offshore Structures Determination: An Analysis of a Braced Monopod and 4-Legged Platforms with Respect to Industry Standards

“…The method is similar to the demandsupply or load-strength principles. Therefore, it is beneficial for general demand and supply (Bea, 1992;Mat Soom et al, 2019;Onoufriou & Forbes, 2001). Demand is the load model, while supply is referred to as the resistance model or strength model.…”

“…In general, probability is a tool to quantify the uncertainty of events and reason in a principled manner (Barltrop & Adams, 2013;Brebbia & Walker, 2013;Leimeister & Kolios, 2018). It is essential to know that this integral equation is applicable only if the load and strength models are statically independent for a continuous variable (Mat Soom et al, 2023;Sundararajan, 2012;Zentuti et al, 2018). The process of finding an integral is called integration.…”

“…The process of finding an integral is called integration. In basic reliability engineering, the meaning of reliability is the performance at or above a given standard (rated from 0 to 1, where 0 is the least reliable) (Abu Husain et al, 2013;Alati et al, 2013;Mat Soom, 2018;Mohd Zaki et al, 2016;Mourão et al, 2020;Rathod et al, 2011). On the other hand, the probability of failure is the performance below a given standard (rated from 0 to 1, where 1 is an absolute failure).…”

“…According to Mat Soom et al (2015, the most commonly used procedures to evaluate the integrity level of offshore platforms are Global Ultimate Strength Assessment (GUSA) and Risk-Based Design and Assessment (RBDA). PETRONAS developed GUSA in 2014 to evaluate and assess more than 100 platforms under PETRONAS Carigali Sdn Bhd (PCSB) in the region of Malaysian offshores, while Shell developed an RBDA in 1997 (Ayob et al, 2014;Azman et al, 2017;Efthymiou & Van de Graaf, 2011;Mat Soom, 2018). Several oil and gas companies apply non-linear analysis using modern software to obtain the reserve strength ratio (Mat Soom, 2018).…”

“…PETRONAS developed GUSA in 2014 to evaluate and assess more than 100 platforms under PETRONAS Carigali Sdn Bhd (PCSB) in the region of Malaysian offshores, while Shell developed an RBDA in 1997 (Ayob et al, 2014;Azman et al, 2017;Efthymiou & Van de Graaf, 2011;Mat Soom, 2018). Several oil and gas companies apply non-linear analysis using modern software to obtain the reserve strength ratio (Mat Soom, 2018). The reserve strength ratio is based on the ultimate base shear over the design of the return period.…”