New Composite Construction Technology for Semisubmersibles and TLP's

Abstract

ABSTRACT The paper describes the development of composite construction technology for application to the large pontoon-type components of TLP's and semi submersibles which hitherto have been designed using expensive stiffened plate methodologies. Composite technology (ie. double skin) has been used for fixed platforms but cannot be app1ied yet to stiffened plate and shell components of TLP's and semi-subs because of differences in design criteria, required resistance levels, load cases, size and safety requirements. The development and app1ication of the new technology (partly funded by the European Commission) is described with reference to available back-up data and information. Technical and economic evaluations are also presented. With the envisaged increased use of TLP's and floating production systems, the new technology described in this paper can be deployed as a viable and cost-effective alternative to stiffened plate and shell construction. The development of the new technology is presented, including appraisal of existing stiffened plate technology, current app1ications of composite technology, creation of knowledge bases, development and application of the new design method. Case study scenarios are presented, covering technical and economic comparisons between conventional stiffened plate solutions and composite section designs. At a time when the use of cost-effective systems remains of paramount importance, the use of sandwich sections with a cement matrix infill presents significant economic advantages. The consequences of using the new composite technology described in this paper are improved engineering designs, materia1 weight savings and reduction of fabrication and installation costs. 1. INTRODUCTION This paper concerns the use of composite steel/concrete sandwich construction for the construction of the major elements of the hulls of floating production facilities such as semi submersibles or tension leg platforms. Steel and concrete have both been used separately, or have been proposed, for the construction of floating vessels including ships and offshore production facilities, but have not been used in combination in the form proposed for floating production systems. Composite steel/concrete technology is, of course, widely used in building and civil engineering but not normally in double skin form. In steel jacket structures grout or concrete filling has seen increasing use to strengthen members and joints, to provide impact resistance and to improve stress distribution in steel components and hence improve fatigue life. The use of annular grouting for these app1ications and also to achieve axia1 connection between concentric steel tubular is widely developed and includes grouted pile/sleeve connect ions, 1eg pins to join together jacket sections and joints to add additional braces underwater for strengthening purposes. The present authors have been extensively involved in such applications as described in References 1 to 5. Composite double skin panel construction has been proposed for a wide range of structural uses particularly in the marine environment where structures tend to be 1arge and the construction sequence is easily adapted to benefit from this form of construction.