Modified reliability reassessment of fixed offshore structures for Malaysian water

Abstract

Fixed offshore platform structures are subjected to external loadings such as gravity loads, environmental loads, hydrodynamic loads and accidental loads. The structures continuously undergo modifications and upgrading in order to meet safety and production demands or due to structural damage on critical components. The aim of this research is to propose an improved Structural Reliability Assessment (SRA) methodology for fixed offshore platforms in Malaysian waters by including the effect of wave-in-deck and to quantify the impact of key design parameters on overall platform structural response. Case studies on the sensitivity of marine growth profile, current blockage factor, drag and inertia coefficients on overall structural loading are performed to quantify the overall impact of these key parameters. The platform reliability based on proposed improved method is compared against an existing method, the Simplified Structural Reliability Assessment (SSRA). For the case and sensitivity studies, the examples of existing fixed offshore jacket structures in Malaysian water region are used. In structural integrity assessment of existing platforms, structural utilization factor is used to measure the level of stress on structural members and foundation. The study on structural integrity assessment shows that the drag coefficient and current blockage factor have significant impact on the overall loading of jacket structures in Malaysian waters as compared to marine growth profile and inertia coefficient. The sensitivity study on drag coefficient quantifies the percentage increases on base shear for operating and storm conditions, ranging from 17% to 72% and 18% to 70% respectively. The current blockage factor sensitivity study for operating and storm conditions shows the base shear increase from 10% to 16% and 6% to 9% respectively. The existing platforms may not be able to meet today’s integrity requirement without performing costly strengthening. Therefore, a modified SRA method is proposed to demonstrate the fit-for-purpose of the platforms based on As Low As Reasonably Practical (ALARP) concept which has been proven to be a cost effective solution to non-compliant existing and ageing platforms. The advantage of the method is its ability to estimate the magnitude of wave height that causes platform failure and takes into consideration the wave-in-deck in the calculation of platform Reserve Strength Ratio (RSR). The study demonstrates that including additional loading contribution from wave-in-deck in the RSR calculation is more practical, realistic and accurate. The case study demonstrates the platform RSR decreases from 4.13 to 3.40 which is equivalent to 17% reduction. In the final phase, this study proposes a unique parameter Alpha, (??) which is used to calculate the wave height that causes platforms to collapse. The ?? depends on platform geometrical configuration. The sensitivity study concludes the values of a for fixed offshore structures at Malaysian water ranging from.1.5 to 2.5.