Assessment of offshore platforms under subsidence - Part II: Analysis and results

An assessment of three offshore platforms in the Valhall field of the North Sea is described in this paper. Ultimate strength and reliability analyses were performed for the jacket-type platforms for four levels of sea floor subsidence. The nonlinear structural models of the platform elements, joints, and associated soil-structure interaction are summarized in a companion paper by Gebara et al., 2000, "Assessment of Offshore Platforms Under Subsidence-Part I: Approach.'' ASME J. Offshore Mech. Arct. Eng., 122, pp. 260-266. Similarly, environmental loads and reliability acceptance criteria are presented by Pawsey et al., 1998. "Characterization of Environmental Loads on Subsiding Offshore Platforms, " OMAE98, Lisbon, Portugal, and Stahl et al., 1998, Acceptance Criteria for Offshore Platforms, " OMAE98, Lisbon, Portugal, respectively. This paper describes the procedures employed and results obtained for ultimate strength and reliability analysis. Ultimate platform strength was calculated using a pushover analysis in which environmental loads for increasingly rare events were applied incrementally until the platform failed. Failure was defined as formation of a limiting mechanism in the platform structure or foundation. Structural reliability analysis was performed to estimate the annual probability of platform failure as a function of subsidence level. The platform structural system was modeled in terms of two major subsystems that were assessed as likely to fail: (i) the platform deck, and (ii) the platform jacket and piles. Uncertainties were included in the loads structural components and soil strengths based on industry experience and data. First and second-order reliability methods (FORM/SORM) were used to estimate the annual failure probabilities. Consideration was given to the correlated nature of the failure modes due to the common environmental loads. The reliability results were compared with predefined acceptance criteria to judge the suitability of continued operations.