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

Abstract

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.