Study on behavior of carbon fiber-reinforced composite for deep sea applications
Authors: Venkatesan R. , Dwarakadasa E.S. , Ravindran M. 
Published: October 2008

In-situ test results on tensile, compression, flexure and ILSS properties of carbon fiber-reinforced composite in deep-sea are presented for the first time. Results revealed that apparent weight gain increases with depth. Mechanical properties are not affected by deep-sea environment and deterioration or degradation or bio-film formation was not observed.

Introduction

Composite materials are used in a large variety of applications in many industries due to their lightweight, good corrosion resistance, high mechanical strength, high strength-to-weight ratio and excellent fatigue strength. On the other hand, applications of advanced composites that exploit high strength and fatigue resistance are still limited to the aerospace industry. This is because of structural complexity of such products and the costs involved. In addition to cost savings in terms of weight, composites have several other properties that make them attractive alternatives to steel in deep water drilling applications. Composites have excellent fatigue properties, will not corrode in seawater and have strengths similar to that of steel. Also, the fact that composites can be designed with certain properties in a specified direction can be exploited in the production riser where a low axial stiffness is desirable. Spool able composites pipes, giving cost savings of the order of 20-30 % with superior corrosion resistance have been developed by a number of major oil companies. A drawback to using composites in marine applications is the lack of information concerning the long-term effects of seawater on composites. Many materials when fabricated into a system failed in seawater service in unexpected manners different from the predicted behavior established using data obtained from tests in the laboratory saline conditions. In designing deep water systems, the performance of these composites at such an environment is required. Such test data are scarce and are not available in public domain. This work involves very expensive deep sea mooring system and ship for long-term observations. Hence it was decided to study behavior of most widely used carbon fiber reinforced composite in deep water up to 5100m depths by exposing coupons for corrosion and mechanical properties

Application

Once the potential of the deep seabed is fully realized, it promises to make an enormous contribution to the world's resource base. Manganese nodules., methane gas deposits and cobalt-rich crust are resources of current interest for exploration and are deposited over and beneath the ocean floor at depths ranging from 800 to 6000 meters. The oil industry is presently moving to deep and ultra deep locations for drilling oil in the ocean at depths of 2000m and more. To recover oil economically from such depths, several new technologies must be developed. The use of composites in production risers could potentially reduce the platform size by reducing the riser weight, since the buoyant weights of polymeric composite materials are approximately 0.8 g/cm3 versus 6.9 g/cm3 for steel. Alternatively, existing platforms could be used at much greater ocean depths if steel risers are replaced by composite risers1–4.