Assessment of offshore platforms under subsidence - Part I: Approach

This is the first of four companion papers that present a comprehensive assessment of the effect of sea floor subsidence on the Valhall complex of platforms. The study has included an estimate of the increase in platform failure probability as a function of increased subsidence. Subsidence raises the effective mean still water level and increases the potential for inundation of the deck for extreme storm conditions. Deck wave slam forces generate significantly greater platform loading and lend to: (a) higher levels of structural inelastic response and increased risk of structural failure as well as (b) water reaching the cellar deck, and hence affecting operators as well as equipment, The paper focuses on addressing the first of these two issues. A structural assessment study was performed to address the significance of present and future. levels of subsidence on the safety of three North Sea platforms. The study included a systematic assessment procedure that addressed each of the factors that impacted structural integrity issues and reliability concerns, Such factors included: ultimate strength analysis methodologies. tubular joint formulations, group pile effects, and soil-structure interaction. which are described in this paper as well as deck impact force formulations (Pawsey et al,, 1998, ''Characterization of Environmental Loads on Subsiding Offshore Platforms, "17th International Conference Offshore Mechanics anti Arctic Engineering, Lisbon, Portugal, July) component and system reliability modeling (Jha et al., 2000, "Assessment of Offshore Platforms Under Subsidence-Part II: Analysis and Results,'' ASME J. Offshore Mech, Arct, Eng,, 122, pp. 267-273), and acceptance criteria (Stahl et al,, 1998, "Acceptance Criteria for Offshore Platforms," 17th International Conference on Offshore Mechanisms and Arctic Engineering. Lisbon, Portugal, July), This paper presents the assessment procedure, as well as the modeling approach. The paper also discusses the consequence classification of the three platforms and stare-of-the-art soil mechanics techniques that lead to a significant increase in the tensile capacity of the foundation.