Stochastic Evaluation of Offshore Carbon Fibre Reinforced Concrete Platforms on Aluminum Girders

A stochastic evaluation of the performance of Carbon Fibre Offshore Plastics (CFRP) offshore platform consideringsubmerged and partially submerged environmental conditions wasanalyzed usingSwedish code,Boverket (2004). A Probability-based analysis using First Order Reliability Method (FORM) was used to determine the safety index of the deck considering varied load ratios, effective depths of the deck, and ultimate strength of Fibre Reinforced Plastics (FRP) tendons. The results generated from FORM indicates that the theoretical framework for risk assessment based on the Joint Committee for Structural Safety JCSS (2003) showed that the maximum safety index of the CFRP deck was shown to be 3.49 which is higher than the limit set by the JCSS (2003) code. Hence the deck can adequately transmit the given loading conditions when designed in accordance with Boverket (2004). Also the resultsof the Finite Elementanalysis carried out on the deck showed that the von-Mises stress was within acceptable limits, implying that the resisting moment of the CFRP deck was adequate. Hence, it is shown that the CFRP deck can be used in marine environment with increasing tidal loading as the CFRP wasalso able to resist failure due to compression. The flexural as well as shearing resistance are also within safety limits; and is about 500% greater than that of a steel reinforced concrete platform.However considering the serviceability limit state of deflection, the CFRP platforms did not show noticeable deformation in the geometry of the deck from the finite element analysis. The imposed load that can be sustained on a 150mm thick CFRP deck is 20kN/m2; while that of 200mm thick CFRP deck is 30kN/m2.

Corrosion is a major problem on reinforcedconcrete structures especially in the case of macrocell formation, because it can cause local loss of the reinforcement cross-section inconjunction with subsequent cracking and spalling of concrete cover in the structure. With increasing deterioration,the serviceability will be impaired and the load bearing capacity decreases. The inspectionand maintenance strategies to detect such damages can be costly and economicplanning is mandatory. In off-shore structures, structural components are built to be embeddedinwater as in the case of harbours, oil rig platforms just to mention a few. It is however essential that proper design to safely carry the imposed loadingsbe considered. Mostimportantly, design for the durability and resistance to: chloride effect, salty water, de-icing salt, freeze thaw etc. must be thoroughly considered (Humphrey, 2003).

Once corrosion gets started due to chloride ingress, anodic areas can be detected throughpotential mapping. Potential mapping provides two-dimensional information about a structure.This kind of information can be used for a spatial evaluation of corroding areas. It has to be considered that potential fields are influenced by several parameters such as concretecover and resistivity, which always will have an effect on the spatial variability.