Dynamic Behaviour of Compliant Towers in Deep Sea

Abstract

For oil and gas production in deep water (> 500m) compliant structures are designed. By dedicated reduction of stiffness and optimization of mass (including added mass contributions) the fundamental period is tuned to about 30s, which is well above the period range of significant wave energy. This paper presents the results of a comprehensive numerical analysis of the hydroelastic behaviour of compliant towers in deep water, investigating the dynamic response of a flexible structure in a given sea state. The numerical program system is based on ADINA with an integrated hydrodynamic module for wave/structure interaction. Nonlinear effects of viscous forces are considered using Morison’s vector equation. In addition nonlinear characteristics of soil/structure interaction are also included according to API regulations. At first, for verification of the program system, the numerical results of a monotower are compared to analytical solutions obtained by modal analysis of the structure in regular and irregular seas. Based on the validated program a compliant tower installed in 503 m water in the Gulf of Mexico (Baldpate tower) is modeled in 3D, and the characteristic dynamic behavior is evaluated. Finally, the tower is exposed to a real “freak” wave (the 25.6m high New Year Wave which has been registered at the North Sea Draupner platform on January 1, 1995), and the associated loads and motions are evaluated.