Design of paired column semisubmersible hull

• Main Author: Odijie, Charles
• Other Authors: Ye, Jianqiao
• Published: Lancaster University 2016
• Subjects: 620
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.718645

There is a constant effort to reconfigure column stabilized semisubmersible unit to meet the challenging demands associated with deep water exploration. Paired column semisubmersible platform is one of the recent column stabilized semisubmersible hull configured to allow top-deck well head compatibility for oil reserves in deep waters. Its unique ability to maintain reduced vertical motion in extreme weather conditions despite its hull size and payload create a high payload to motion ratio, as compare to conventional semisubmersible hulls. This unique feature makes it recommendable for other hull applications in ocean engineering. A study has been carried out to harness this high payload to motion ratio offered by this new hull concept in the development of drilling and production platforms in deep waters, support and foundation systems. Numerical models were developed to understand the semisubmersible hull (dynamics of the reduced vertical motion and its ability to withstand bending and twisting behaviour from extreme wave conditions). Prior to this, a preliminary CFD model was developed in to understand the vortex shedding effect on the arrayed columns. An experimental setup was also put together to understand this motion behaviour, alongside a detailed review of the first model. The motion response of a scaled hull model was studied in a wave tank with a Digital Image Correlation (DIC) system known as Imetrum. To further investigate its application for other ocean depths and support systems, series of hydrodynamic models were developed in ANSYS AQWA with weather conditions as recommended by API, DNV, and ABS. The AQWA model was validated with results recorded by Imetrum system from the wave tank experimental test. The wave forces and moments were studied for different draft sizes and ocean conditions, and their response where checked in ORCAFLEX. A finite element model was finally developed in APDL to understand the nature and effect of stresses from wave, current and wind loads, alongside topside integration. The results obtained from the FE model was use to postulate reinforcement during scantling, for different hull applications. The results for motion response showed favourable heeling moment for smaller draft sizes as recommended by regulatory bodies, but a reconfiguration for heave displacement might be required for smaller draft size. In such case, an increase in pontoon area or an additional heave plate attachment has been recommended. Furthermore, the effect of wavecurrent interactions was observed to create unique motion behaviour for all draft sizes at resonance frequency range. A fluid-structure interaction model of multi-phase flow will be required to understand this behaviour. The stress concentration on the columns generated from hydrodynamic loads was observed to be higher on the inner columns, relative to the outer ones.