The key issue addressed in this paper is the accuracy of structural reliability models for the case of fixed steel offshore structures under extreme storm loading. The emphasis is on engineering accuracy for the purpose of use in decision-making, and more specifically to achieve sufficient accuracy to enable the use of reliability models in deriving design criteria for fixed offshore platforms. These reliability models are used to derive partial load factors for use in conjunction with API LRFD to achieve a target reliability level appropriate for permanently manned installations. These load factors are location-dependent. Further load factors are proposed for the design of new, not normally manned installations.

Reliability-Based Criteria for Fixed Steel Offshore Platforms

Marine hydrodynamics is characterised by both weak nonlinearities, as seen for example in drift forces, and strong nonlinearities, as seen for example in wave breaking. In many cases their relative importance is still a controversial matter. The phenomenon of particle escape, seen in linear theory, appears to offer a guide to when strongly nonlinear effects will start to become important, and what will happen when they do. In the case of the “ringing” of vertical cylinders in steep waves, particle escape is shown to correspond approximately to local wave breaking, which leads to the cavitation responsible for “ringing”. Another example is rogue waves, where recent results from weakly nonlinear theory are disappointing, and also fail to explain the rogue waves seen in relatively shallow water, as in the data from the Draupner and Gorm platforms. Recent laboratory experiments, too, show wave crests continuing to grow in height after all frequency components have come into phase, which is inconsistent with weakly nonlinear theory. Particle escape, which is more frequent in shallow water, offers a simple alternative explanation for these observations, as well as for the violent motion at the wave crests, which often confuses rogue-wave data. Extreme wave crests have long been known to be strongly nonlinear, so it appears possible that rogue waves are primarily a strongly nonlinear phenomenon. Fully nonlinear computations of two interacting regular waves are presented, to explore further the connection between particle escape and wave breaking. They are combined with Monte-Carlo simulations of particle escape in hurricane conditions, and the very few measurements of large breaking waves during hurricanes. It is concluded that large breaking waves will have occurred about once per hour, and once per 100 h, respectively, in the recent hurricanes LILI and IVAN. These findings call into question the use of non-breaking wave models in the design codes for fixed steel offshore structures.

Weak or strong nonlinearity: the vital issue

Fixed and Jack-up platforms : Basis for reliability Assessment

Chapter 5 – Probabilistic Design of Offshore Structures

Forces on a single pile caisson in breaking waves and current

Since results from the Ocean Test Structure were presented at Civil Engineering in the Oceans IV in 1979, there have been numerous laboratory experiments and a few full-scale measurements of local and global wave forces on offshore platforms. The laboratory experiments have shed considerable light on such phenomena as current blockage within dense structures, wave and current shielding in conductor arrays, wave and current interaction, three-dimensional wave kinematics, and the effects of wake encounter, surface roughness, and cylinder orientation on C d and C m . From these data, an engineering procedure for calculating the static wave/current global load on a typical space frame structure has been synthesized and proposed for inclusion in the 20th edition of API-RP2A. This procedure is still grounded on the Morison equation for calculation of local forces and a higher-order regular wave theory for calculation of two-dimensional wave kinematics, but involves a sequence of simple modifications to the traditional methodology.

John C. Heideman

Papers

Static Wave Force Procedure for Platform Design