Assessment of Offshore Platforms Under Subsidence—Part I: Approach

Abstract

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 lead 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 on Offshore Mechanics and 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 Mechanics 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 state-of-the-art soil mechanics techniques that lead to a significant increase in the tensile capacity of the foundation. [S0892-7219(00)00204-1]

Structural Integrity of Fixed Offshore Platforms by Incorporating Wave-in-Deck

“…It is suggested that the component method [45,46] or computational fluid dynamic method [28,[47][48][49] are to be utilized to calculate the wave-in-deck load in order to reduce the conservatism of the silhouette method. The risk of platform damage may be reduced via modification such as localized protection or relocation of exposed sensitive equipment or via procedures such as production shut-in and temporary evacuation during storm season [59,60]. Another method that was successfully performed was by jacking-up the platform in order to increase the air gap of platform [61].…”

Experimental investigation of the wave slam and slap coefficients for array of non-circular section of offshore platforms

“…It has been reported that many existing platforms are under danger due to seafloor subsidence. Gebara et al (2000) reported that some of the platforms in the Valhall field subsided by a depth of 3.2 m, which was further forecasted to increase. Similar seafloor subsidence has been reported in North Sea and Gulf of Mexico also.…”

FEMA approaches in seismic assessment of jacket platforms (case study: Ressalat jacket of Persian gulf)

“…Gebara et al assess the performance of the jacket platform under subsidence and perform ultimate strength and reliability analyses for four levels of sea floor subsidence [5]. The assessment process of building prestandards was studied also.…”