Investigation of vortex-induced-vibration of self-elevating offshore platforms with tubular legs under combined waves and currents

Abstract

The effects of the oscillatory lift forces acting on the legs and the consequent Vortex-Induced-Vibration (VIV) are customarily neglected in the structural design of self-elevating offshore platforms, hereinafter called jack-ups. Severe vibrations of shallow water jack-ups due to vortex shedding about its circular cylindrical legs have led to many unsuccessful operations at aggressive tidal locations. This research project aims to understand the intricacies involved with the VIV of the cylindrical legged jack-ups and investigate the effect of vortex induced forces on its strength and life. This PhD project consists of analysis of industry design data, development of a simplified single-degree-of-freedom (SDOF) analysis methodology, physical experiments, numerical simulations and propositions towards VIV suppression. The analysis of the design data revealed that many of the jack-ups currently in commission are vulnerable to inline, crossflow and yaw VIV in currents and regular waves. A simple mathematical model is developed based on the SDOF analogy and principle of conservation of energy evaluating various modes of VIV of the jack-up with cylindrical legs in uniform current and regular waves. Mass ratio, damping ratio and mode factor are found to be the important parameters controlling the crossflow VIV and radius of gyration also for the yaw VIV. Criteria for the initiation of the multiple VIV modes are also developed for the jack-up experiencing current and regular waves. SDOF analysis of the jack-ups indicated extreme yield and fatigue strength utilisations during lock-in vibrations. The newly developed mathematical VIV model is validated by a set of experiments conducted in a wind, wave and current flume. Model tests revealed that the sway (crossflow) and yaw (torsional) modes of the jack-up VIV could be dynamically excited by the vortex shedding. The experimental investigation further revealed that jack-up experiences significant crossflow VIV in uniform currents and regular waves, and yaw VIV in uniform currents. The jack-up is found not to experience any significant VIV during tests in irregular waves. In regular waves with imposed current also, the jack-up is found to experience crossflow and yaw VIV. The force tests revealed considerable drag and lift amplifications, and additional inertial loading on the legs during VIV. 2D Fluid-Structure Interaction (FSI) simulations carried out in model scale under uniform currents have also resulted in similar response behaviour. It is demonstrated by means of mathematical investigation and physical experiments that streamlined leg fairing can be used as an excellent vortex suppression device for the jack-ups. Designs of two optimum fairing profiles are developed based on NACA0018 profile and successfully tested on the cylindrical legs of the scaled jack-up model, achieving substantial reduction of VIV amplitudes. The finding of the jack-ups experiencing VIVs in currents and regular waves well within its certified operating envelopes is a matter of significant concern. The mathematical method will equip engineers to consider the effect of VIV in jack-up designs. The two fairing section offsets presented can be readily used by Industry for practical applications. Moreover, the comprehensive results from the numerical simulation and experimental measurements will provide reliable benchmark for future study on the topic.