Numerical study for the effect of VIV on tendon of TLP

Abstract

TLP (tension leg platform) is often used as a supporting structure to hold offshore structures. TLP is moored vertically by axially pretensioned circular beam called tendon. Since tendon is installed underwater as slender structure, itsbehavior is affected by undersea current. Especially, VIV (vortex-induced vibration) induced by current have an influence on tendon in both in-flow and cross-flow directions. In this work, we focus to study the response in cross-flow direction. When the occurrence frequency of VIV coincide with the natural frequency of tendon, the amplitude of VIV-induced response increases rapidly and this phenomenon is called lock-in. The effect of current velocity on VIV’s amplitude is considered in this work to observe the lock-in. VIV-induced lift force model in previous studies is used in cross-flow direction motion, and coupled analysis for both floating structure and tendon is conducted. BEM (boundary element method) and Convolution integral method are used for floating structure model, and FEM (finite element method) is used for tendon model. In this work lock-in is investigated, and motions of floating structure and tension of tendon with VIV are estimated to verify the effect of VIV. Bending stresses in cross-flow direction are assessed for the analysis of fatigue safety.