ABSTRACT

This study numerically compares TLP(Tension Leg Platform) motions and fatigue lives of tendons with or without including TLP VIM(Vortex Induced Motion) or tendon VIV(Vortex Induced Vibration) in coupled analysis of TLP with tendons in waves and currents. Equation of motion was solved by HOBEM(Higher Order Boundary Element Method) and convolution for the floater part and by FEM(Finite Element Method) for tendon part. The fatigue life of tendon was calculated by Rainflow method with the tension results from coupled analysis. A cylindrical TLP with tendons was analyzed as a numerical example and the floater motions and tendon tensions were examined for various current speeds to see the trend versus current speed. Four analysis cases such as coupled analysis without TLP VIM nor tendon VIV, coupled analysis without TLP VIM and with tendon VIV, coupled analysis with TLP VIM and without tendon VIV, coupled analysis including both TLP VIM and tendon VIV were compared to discuss which case has larger effects on TLP motion or tendon fatigue.

INTRODUCTION

VIM or VIV by current is a main design load in in deep draft platform such as spar, TLP or slender bodies such as risers, tendons. And, many studies were done by many researchers(Gopalkrishnan 1993, Bourguet 2011, Jain 2012, Chen et al. 2015) about vortex induced responses. VIM or VIV induces lateral motion perpendicular to current direction. So, it induces sway and roll of floating body and lateral vibrations of tendons. In some situation, lock-in happens if VIM frequency is identical to sway frequency. In that case, lock-in of heave, pitch, yaw as well as sway, roll may occur due to coupled effect if the floating body is strongly connected with tendons. And, it may affect fatigue damage of tendons.

The objective of this study is to compare TLP motions and fatigue lives of tendons with or without including TLP VIM or tendon VIV in coupled analysis of TLP with tendons in waves and currents to see which case more affects TLP motion or tendon fatigue. A floating wind turbine studied by Robertson & Jonkman(2011) was analyzed as a numerical example. The sample sturucture is a TLP structure connected with 8 pretensioned tendons. The TLP motions and tendon tensions were examined for various current speeds to see the trend versus current speed. Four analysis cases such as coupled analysis without TLP VIM nor tendon VIV, coupled analysis with tendon VIV, coupled analysis with TLP VIM, coupled analysis with both TLP VIM & tendon VIV were compared.

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