Dynamic behaviour of jackets exposed to wave-in-deck forces

Abstract

During the last decade, wave-in-deck loading on fixed offshore structures has increasingly been acknowledged as an issue of concern to the offshore oil and gas industry. Being mainly an issue for existing structures, the reason is partly that some offshore fields experience seabed subsidence due to reservoir compaction and partly that the data we possess on environmental conditions indicate that certain extreme events are not as rare as previously estimated. This work deals with the dynamic effects of wave-in-deck loading on jacket platforms. Focus has been on the underlying mechanisms of the global structural response and on dynamic versus static response in the elastic as well as the plastic response domain. The evaluation of different methods for the calculation of wave-in-deck loading, comprising both magnitude and time variation, came naturally as a part of the work. Dynamic and static response to external loading has been studied by carrying out analyses of jacket models using a simplified model as well as a full finite element model. The simplified model is a single degree of freedom (SDOF) type of model that utilises results, i.e. load-displacement or resistance curves, from nonlinear static pushover analysis to calculate dynamic response. The SDOF model used herein is not to be confused with e.g. commonly used generalised SDOF models. The applicability of the simplified model to predict dynamic response of complex structural systems is particularly investigated. The application of the SDOF model and development of a modified model has contributed to important understanding of the nature of jacket response to wave-in-deck loading. The type of SDOF model used in this work is found unsuited for use as an analysis tool in case of loading involving a distribution which varies with time, however, it is believed to have a potential for (nonlinear) problems of non-varying load distribution. The examination of the inherent differences in dynamic and static behaviour by use of the different analysis methods has made it clear that improved performance detected by dynamic analysis compared to static can mainly be attributed to 1) ductility reserves of the structure beyond ultimate capacity — as opposed to response reduction caused by inertia of the mass — and 2) the change in load distribution immediately prior to deck impact. With respect to the former, the author will recommend explicit attention to be paid to ductile design for new structures. Although existing jackets are not explicitly designed to resist the loads generated by wave impact on deck, this work has shown that ductile North Sea jackets may be able to resist considerable wave-in-deck loading. Further, the levels of acceleration detected during the analyses identifies acceleration response as an important indicator of dynamic performance for jackets exposed to wave-in-deck loading.