Fibre-reinforced caissons for offshore applications

Abstract

Steel caissons currently used on offshore platforms for the uplift of seawater onto platform topsides, for example, are susceptible to corrosion, and some operators have had to replace corroded caissons during service. The use of fibre reinforced plastics (FRP), in particular glass-fibre (GRP), has been explored by a number of offshore operators as an alternative to steel, principally due to the resistance to corrosion of these materials. In addition, their relative light weight facilitates handling and installation that is particularly beneficial for retrofit applications. However, the low flexural modulus of GRP compared to steel, combined with the large support spacings on conventional steel jacket structures in the splash zone region, means that standard GRP tubes designed for biaxial loading (i.e. with ± 55 ° reinforcement fibre winding angle) will exhibit greater deflections than their steel counterparts, when subjected to the various in-service wave loadings. An investigation has therefore been conducted to determine the improvement in flexural modulus of GRP tubes manufactured with shallower reinforcement winding angles. Tubes manufactured with a range winding angles and different reinforcement types have been obtained and subjected to deflection tests. Experimental results are presented and compared with predictions of flexural modulus generated from laminate analyses software packages.