Semisubmersible Sizing and Modification Strategies
Authors: Hung S. , Mangiavacchi A. 
Published: October 2008

ABSTRACT
A strategic approach towards the desigrn of a newly built semi submersible, or the redesign and selection of an existing vessel is presented. The paper also addresses the practical implementation of this design strategy into a computational tool based on large scale parametric and optimization analyses in a logical sequence, in which a number of knowledge-based rules derived from operational constraints, design experience, codes of practice and other design considerations are utilized and enforced. Application of this strategy to a specific case shows how, by choosing suitable controlling parameters matching environmental constraints and field requirements, an appropriate design may be obtained.

INTRODUCTION
Market Overview
At present there are well over 170 semi submersible vessels in the world market [1]. Widely used as drilling, construction and/or support vessels, they have also been successfully used as floating production system (FPS) since 1975, when the Transworld 58 was deployed in the Argyll field. As deeper water and marginal fields are being developed, their employment as production vessels IS becoming more attractive both technically and economically. In the North sea, the Balmoral [2] and the Ivanhoe/Rob Roy [3] fields have both been recently developed successfully by semi submersible FPS, while the Emerald field FPS is at the time of writing being converted from a drilling vessel [4]. Elsewhere, taking advantage of the short development time and market availability, around ten drilling units have already been converted and employed as production vessels since 1977 in the Campos Basin [5]. As far as drilling and support vessels are concerned, due to the increasing level of offshore activities and to the operational superiority of semi submersibles over other vessel types [6], an increase 10 the number of new vessels being built may also be expected in the future.
The Design Spiral
In the initial design of a new semi submersible or the redesign of an existing vessel for new operational circumstances, a vast variety of choices are open to the designer. Given the constraints imposed by codes of practice, operational requirements and economics, a wide range of design parameters such as hydrostatic stability, hydrodynamic response, allowable payload and cost must be considered, and rational decisions must be made in order to reach the final design.
Within the designs or redesign exercise, the existence of inter-relationships between both constraints and characteristics of different natures makes it inevitable that a final design can only be approached iteratively. A typical top level design spiral with particular attention paid to technical issues is shown in figure 1. While different routes exist to reach the :preliminary design, and while it is possible to perform sensitivity analyses on different design parameters by making use of conventional design tools, difficulties frequently arise when large amounts of costly information are available but are found difficult to digest and interpret. The lack of succinct information, suitable tools and experience can all slow down the convergence of the design significantly and give rise to unnecessary development expense and time wastage.