Fidelity of computational modelling of offshore jacket platforms

The development of oil and gas exploitation offshore has a history of about half a century. Many platforms have been built since to facilitate the production of hydrocarbons oil and gas, of which fixed offshore jacket type structures are the most commonly adopted rigs for shallow water depths. The present paper focuses on the modelling of a 4-legged X-braced jacket type platform, representative of a typical fixed platform in the North Sea using nonlinear finite element analysis. Normally, offshore platforms are conservatively designed using linear-elastic models to determine the effects of applied actions. The nonlinear effects of joint flexibility, piled foundations and geometrical imperfections on the platform behaviour are investigated in this paper. Joint flexibility is studied by modelling the jacket using beam elements and introducing rigid or flexible joints. A hybrid model, with the critically loaded leg and connected joints built using shell elements, is applied for the investigation of localised effects on increasing joint flexibility. The soil-pile interaction is modelled implicitly using sets of decoupled springs distributed along the piles. The geometrical imperfections are introduced in the compression legs of the jacket. The imperfect leg shapes are generated based on the failure modes of the platform. The platform is loaded by operational and environmental loads. The environmental loads are gradually increased until platform failure occurs. Eight load cases are considered, where the environmental loads are applied in 4 end-on and 4 broadside directions. The findings of the paper indicate that incorporation of joint flexibility and piled foundation result in the reduction of platform yielding and ultimate strength. The piled foundation affects platform stiffness severely. The imperfections increase platform deformability in the elastic rage and lead to dramatic reduction of jacket base shear capacity.