Developments in structural system reliability assessments of fixed steel offshore platforms

Incremental wave analysis and its application to performance-based assessment of jacket platforms

Reliability assessment of foundations for offshore mooring systems under extreme environments

In this study, the technical papers on structural condition assessment of aged fixed-type offshore platforms reported over the past few decades are presented. Other ancillary related works are also discussed. Large numbers of researches are available in the area of requalification for life extension of offshore jacket platforms. Many of these studies involve reassessment of existing platforms by means of conducting pushover analysis, a static nonlinear collapse analysis method to evaluate the structure nonlinear behaviour and capacity beyond the elastic limit. From here, the failure mechanism and inherent reserve strength/capacity of the overall truss structure are determined. This method of doing reassessment is described clearly in the industry-adopted codes and standards such the API, ISO, PTS and NORSOK codes. This may help understand the structural behaviour of aged fixed offshore jacket structures for maintenance or decommissioning.

https://link.springer.com/content/pdf/10.1007/s11804-020-00181-z.pdf

Condition Assessment Techniques for Aged Fixed-Type Offshore Platforms Considering Decommissioning: a Historical Review

In recent time, the offshore oil and gas industry is facing many challenges like ferocious competition from shale field operators and alternative energy sources, production drop, rising cost etc. All these factors play major role in low crude oil prices. To combat these challenges, the operators are focusing on several untapped field with low production capacity, known as “Marginal Fields” that require innovative design approach to make it economically viable. Minimum facility platform is promising solutions for marginal fields; however, reliability of such structure is a major concern among the operators. In most of the past studies, the reliability technique is effective applied on four (4) legged jacket structure for optimization. This research has attempt to apply reliability analysis method to Mono-tower structure, as minimum facility platform for marginal fields around Malaysia region. The structure was designed as per API RP 2A (WSD). The maximum wave height and current data from Malaysian offshore is using to generate random variables as per Weibull distribution and Monte Carlo Simulation (MCS) have been developed. The surface modelling and curve fitting is done to develop quadrilateral equation in MATLAB for environmental load modelling. Design cases developed as per API RP 2A or ISO19902 must provide adequate levels of reliability throughout the service life. A combination of engineering technique i.e. component based reliability analysis and safety factors, used to ensure integration of all types of uncertainties such as loads, structural properties as resistance, failure modes. FEM method is use for accurate modelling. The probability of failure and reliability index of critical members and other structural members of interest was determined with First Order reliability Method (FORM). The fluctuation in loads and variation material properties were accounted in the assessment. The target reliability level can be achieved either by comparing with existing traditional jacket structure or by following analytical probabilistically models. The partial factors evaluated using reliability assessment is in accordance with concept of Load Resistance Factor Design (LRFD) presented in ISO-19902. Further, the ISO recommendations are followed to calibre the factors as per regional climatic conditions. The estimated reliability Index is 3.95 and probability of failure (Pof) is 5.3x10-5 as per reliability analysis results. Therefore, the conclusion is that Mono-tower as minimum facility platform is suitable for marginal field development that is fulfilling the requirement of reliability, safety and certification. The minimum structure demonstrates equal or higher reliability index for selected members as per ISO. The environmental load calibration has result in factor of 1.26, as against 1.35 suggested by ISO, indicates the potential reduction for Malaysian region without compromising the safety level of structure.

Reliability assessment and calibration of ISO-LRFD factors for minimum facility offshore platfrom in Malaysia

During the past three decades, an immense amount of effort has been devoted to development of sophisticated computer programs to enable the assessment of storm wind, wave, and current loadings and the ultimate limit state capacity characteristics of conventional, pile-supported, template-type offshore platforms. These programs require high degrees of expertise to operate properly, are expensive to purchase and maintain, and require large amounts of manpower and time to complete the analyses. Due to the sophistication of these programs, experience has shown that it is easy to make mistakes that are difficult to detect and that can have significant influences on the results. This paper summarizes the second phase of development and verification of simplified procedures to evaluate environmental loadings and ultimate limit state lateral loading capacities of template-type platforms. Reasonable simplifications and high degrees of user friendliness have been employed in development of the software to reduce the engineering effort, expertise, and costs associated with the analyses. The computer program that has been developed to perform the simplified analyses has been identified as ULSLEA (ultimate limit state limit equilibrium analyses) (Mortazavi and Bea, 1994).

ULSLEA: A Limit Equilibrium Procedure to Determine the Ultimate Limit State Loading Capacities of Template-Type Platforms

This paper describes development of a reliability-based screening procedure for platform assessments and requalifications The objective of this procedure is to identify the critical structures and structure components that need to undergo a detailed assessment so that limited resources can be efficiently used The reliability-based screening procedure helps identify potential failure modes and weak links in the structure, and provides information on the effects of load reduction and strengthening measures on the reliability of the platform The reliability analysis screening procedure is based on a first-order second-moment (FOSM) formulation A study is summarized of the implications of the simplified FOSM method In the case of an eight-leg drilling and production platform located in the Gulf of Mexico, the results from FOSM reliability analysis are compared with those from first and second-order reliability methods (FORM and SORM) There is excellent agreement between the FOSM and FORM/SORM results

M. M. Mortazavi

Papers

ULSLEA: A Limit Equilibrium Procedure to Determine the Ultimate Limit State Loading Capacities of Template-Type Platforms

Reliability-Based Screening of Offshore Platforms