Effects of spacing ratio on the FIV fatigue damage characteristics of a pair of tandem flexible cylinders

Abstract

Estimating the fatigue damage caused by flow-induced vibration (FIV) is an important issue when considering groups of cylindrical structures in the offshore industry. However, cylinders may interact with each other over relatively short distances, complicating their fatigue damage characteristics. Furthermore, different spacing ratios (the ratio of the centre-to-centre distance to the cylinder diameter) among cylinders significantly affect their FIV fatigue damage distributions. To quantify the effects of spacing ratio on FIV fatigue damage, experimental data of two tandem flexible cylinders with a high aspect ratio and a low mass ratio under five different spacing ratios (T/D = 4.0, 6.0, 8.0, 10.0 and 16.0, where T is the centre-to-centre distance in the direction of incoming flow and D is the cylinder diameter) are used to analyse their fatigue damage employing the rain flow counting method and S-N curves. The five spacing ratios are classified into three cases (Case A for T/D = 4.0, Case B for T/D = 6.0 and 8.0, and Case C for T/D = 10.0 and 16.0) in terms of three types of wake flow patterns. It can be found that the fatigue damage of the upstream cylinder is less susceptible to various spacing ratios, especially in the cross-flow (CF) direction. Nevertheless, in the case of T/D = 4.0, the two cylinders tend to experience similar fatigue damage when Vr≥20.04. In contrast, the fatigue damage of the downstream cylinder is significantly reduced owing to the wake shielding effect when T/D = 6.0 and 8.0. Interestingly, the dominant modal lag of the downstream cylinder causes it to suffer more serious in-line (IL) fatigue damage than the upstream cylinder when 13.78≤Vr≤18.79 under T/D = 10.0 and 16.0.