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, joint
s, and associated soil-structure interaction are summarized in a companion
paper by Gebara et al., 2000, "Assessment of Offshore Platforms Under Subsi
dence-Part I: Approach.'' ASME J. Offshore Mech. Arct. Eng., 122, pp. 260-2
66. 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 a
l., 1998, Acceptance Criteria for Offshore Platforms, " OMAE98, Lisbon, Por
tugal, respectively. This paper describes the procedures employed and resul
ts obtained for ultimate strength and reliability analysis. Ultimate platfo
rm strength was calculated using a pushover analysis in which environmental
loads for increasingly rare events were applied incrementally until the pl
atform failed. Failure was defined as formation of a limiting mechanism in
the platform structure or foundation. Structural reliability analysis was p
erformed to estimate the annual probability of platform failure as a functi
on of subsidence level. The platform structural system was modeled in terms
of two major subsystems that were assessed as likely to fail: (i) the plat
form deck, and (ii) the platform jacket and piles. Uncertainties were inclu
ded 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 envi
ronmental loads. The reliability results were compared with predefined acce
ptance criteria to judge the suitability of continued operations.