Nonlinear Wave Load Effects On The Stochastic Behavior Of Fixed Offshore Platforms

ABSTRACT A two-dimensional multiple mode stochastic formulation, which accounts for the relative motion between the fluid and the structure, and addresses the non-linear wave loading effects has recently been developed. Four similar fixed offshore platforms designed for water depths ranging between 400 ft (122 m) and 1000 ft (328 m) are used to illustrate the methodology. The importance of considering both the wave-structure interaction and a third order approximation for the non-linear wave loading effects is discussed. The role of hydrodynamic damping as compared with the usual viscous damping approximations is also discussed. The numerical results consistently demonstrated that each of these factors plays a significant role when predicting the response of fixed offshore platforms in water depths in excess of 400 ft (122 m). INTRODUCTION As the offshore petroleum recovery activities have moved from the protected shallow water coastal areas to the more hostile open water areas around the world, the structural analysis procedures used in the design of these offshore structures have become increasingly more sophisticated. Initially, offshore platforms were assumed to be rigid bodies subjected to design waves in a static or quasistatic manner. For shallow water platforms this still can be a reasonable approach, but for fixed platforms designed for deeper water depths this becomes a less realistic assumption. Steel jacket platforms are constructed using a lattice work of long steel cylindrical members. During the wave induced oscillatory flows, the structure and/or its members may vibrate out of phase with the fluid motion. Wave-structure interaction is the technical term used to indicate this type of relative motion structural response due to wave-induced hydrodynamic loading. Over the years, relative motion effects have become recognized as an important factor in the prediction of deep water offshore platform response. Discussion of this concept can be found most recently in articles describing the response of marine risers and compliant platforms, although early articles on the earthquake excitation of offshore platforms provided the basis for these more recent developments. Wave-structure interaction can be introduced into the analysis of offshore structures through the use of a kinematical transformation and a generalized form of the well-known Morison equation. Analytical manipulation of the forcing function leads to the identification of two very important terms into the governing equations. The first is the added-mass term and the second the hydrodynamic, or radiation damping, is a direct result of the linearization of the viscous drag term in the forcing function. It is this viscous drag term which once again draws our attention. Generally, the added-mass term is evaluated using standard approximations while the hydrodynamic damping is a non-linear factor in the response computations which is obtained during the required iterative solution procedure. The response predictions can be obtained either in the time or frequency domains. Time domain simulations have the advantage of easily treating any problem non-linearities but, the numerical techniques are rather inefficient when treating the long time histories associated with the offshore wave environment. Frequency domain simulations are quite attractive computationally but, generally are restricted to linear structural dynamics.