An Application of New Wave-Free Columns to the Tension Leg Platform

Abstract

Floating marine structures have emploied a slack mooring system. In deep waters, this system needs a wide mooring area and motions of the floating body are larger than those in shallow waters. A taut mooring system has appeared to solve these disadvantages in deep waters. The tension leg platform (TLP) employing the taut mooring system has some advantages that heaving motions are very small and the mooring area needs not to be so wide. But in using a TLP, some peculiar phenomena are observed. One of the serious problems is the unstable motions arised from sub-harmonic resonance of swaying motions in low frequency zone. Other is the springing phenomenon in the mooring system due to super-harmonic resonance of heaving motions in high frequency zone. Vertical wave exciting forces are closely related with these phenomena. When the vertical wave exciting forces are small at these resonance frequencies, the unstable motions and the springing phenomenon will be reduced. In this report, a new wave-free column whose vertical wave exciting force vanishes in low and high frequency zones, is applied for the TLP. It is shown that the unstatable motions and the springing phenomenon are reduced in both theoretical and experimental investigations.