Instability analysis of offshore towers in waves

Abstract

This paper investigates the instability of an upright buoyant tower subjected to surface waves. In this regard, Duffing’s equation and Mathieu type instability are discussed in the analysis of moored floating structures. In particular, the stability of a special purpose single point mooring (SPM) concept is discussed in detail. A single leg inclined mooring (SLIM) tower was developed as a concept for an articulated structure to moor tankers with a single hawser in shallow water. The tower is not given enough buoyancy to stay upright and assumes an inclined position under equilibrium and thus possesses a circle of equilibrium. Inside this circle, the tower is found unstable. The interaction of this tower with incident waves is analyzed here. It is shown that the frequency domain analysis applicable to upright articulated tower is generally not applicable for SLIM. The amplitude of inline oscillation predicted by this solution is found to be conservative and time domain solution is desired that can retain the nonlinearities. The time domain solution explained the observed SLIM oscillation in waves. In the case of SLIM attached to a tanker, the frequency-domain analysis for a coupled system is shown to correlate the experimental data well and generally no instability is observed.