Dynamic effects on the performance of the steel offshore platforms in extreme waves

The purpose of this study is to investigate dynamic loading effects on the performance of the offshore platforms in extreme wave conditions.The non-linear structural analysis program USFOS has been implemented to study such dynamic effects. The main concept is to model the structure and pile components by two-node beam elements and the plasticity by plastic hinges and the pile-soil interaction as discrete nonlinear spring elements along the piles. The structural model is an 8-leg jacket with 4 pile connections to the ground.This study is conducted in two parts a)By disregarding foundation failure(pile-soil behavior and the associated non-linearities)b)By including pile-soil effects.Characteristic wave and current loading for a specified sea state is generated by using program WAJAC.Non-linear time-domain analysis is performed for the most severe part of the sea loading in a scaled form.Scaling up the characteristic wave load for 100 year return period is progressively continued until the global collapse of the platform.Ductility and overloading ratios are the main performance characteristics of the structure which are studied in the first part.The dynamic response is compared with that of static pushover analysis;and the dynamic overloading(overcapacity) factor as well as ductility ratio are calculated and compared with those derived by others and the results of one-storey plane frame analyses. In the second part,a simple pile-soil model has been implemented by modelling pile elements as beam elements and soil as non-linear spring elements through the pile depth.The necessary pile-soil data for this part are generated by the API 93 specifications and the modified API models and other existing theories.Both monotonous and cyclic pile-soil load transfer-displacement data for axial,lateral loadings and end bearing are generated by pile-soil programs.Cyclic degradation,strain softening and gapping are considered in the development of these programs.The progressive pile-soil failure from top to bottom and its effect on the overall pile-soil-structural collapse behavior and on the dynamic capacity and ductility ratio of the system are studied.