Near-failure behavior of jacket-type offshore platforms in the extreme wave environment

During extreme storms, peak wave loads may exceed an offshore platform's static ultimate strength. Although a static analysis predicts collapse in such a loading environment, a structure may be able to dynamically resist these overloads if the load is limited in duration and the structure has sufficient inertial resistance and ductility capacity.;A basic understanding of the beyond-static-capacity behavior of jacket-type offshore platforms in the extreme wave environment is the principal objective of this study. Insight is facilitated first by the use of single-degree-of-freedom (SDOF) models that represent the global (pushover) force-deformation properties of full 3-D jacket models. Based upon the SDOF models, methodologies are developed to assess the relationship between a wave force history and the implied beyond-static-capacity demand. Insight is furthered by the application of the methodologies to four full 3-D non-linear dynamic models of steel jacket platforms on pile foundations. Provided that several subtleties are appreciated and incorporated into the static pushover analyses, the SDOF models are shown to provide satisfactory predictions of the 3-D dynamic response both in a single-and multiple-wave environment. Depending on the failure mechanism, these subtleties include: (a) modeling the asynchronous build-up of the horizontal and vertical wave forces, (b) considering an overturning moment rather than a conventional gross base shear perspective, and (c) modeling viscous damping forces. A modified static pushover procedure is proposed that includes these effects.;We additionally observed in the 3-D dynamic models unexpectedly significant effects related to (a) the relative velocity formulation of Morison's fluid force equation, and (b) dynamic effects prior to reaching static ultimate capacity. These are counteracting effects that increase and decrease, respectively, the effective wave height capacity to a much larger extent than is predicted from a linear elastic basis.;Accounting for a structure's dynamic, beyond-static-capacity response may provide justification for increasing the structure's force capacity rating 10% or even 20% beyond the static ultimate strength. An explicit form of the probability of failure expression is utilized to demonstrate that for certain ductile structures reductions of as much as 70% in the estimated annual failure probability may be obtained.