With increasing water depth, the weight of offshore structures for drilling or production systems becomes a major problem, and analysis shows that the weight of the risers is one of the critical points. This paper presents the principal results of a research programme carried out in the field of lightweight, high-performance tubes made of composite materials, to be used as risers in the oil offshore industry.
Advanced composite materials are made of fibres embedded in a matrix of resin. The principal fibres used by IFP/Aerospatiale are glass, carbon, or aramid fibres. Their main mechanical properties are summarized in the following table:
Table (available in full paper)
It can be seen from this table that they have very high strength properties, along with low density, especially when compared with metallic materials. Their other properties are very high fatigue, creep and corrosion resistance. They have been initially developed for the aerospace industry, but they now find a growing use in all the industries where such qualities of lightweight, mechanical performance, and corrosion resistance are needed1,2.
The basic composite material (laminate) is made of parallel fibres embedded in a matrix of resin, with a volume ratio of fibres to resin of about 60 to 66%. The material is highly anisotropic, all the properties of mechanical strength and axial stiffness being concentrated in the direction of the fibres. Consequently, composite structures are made by carefully reinforcing the directions of principal stress with adequate layers of orientated laminates.
Tubes are structures well suited to composite material technology. The major problem is the end fittings, where heavy loads have to be transmitted from the steel connectors to the filament wound body of the tube
High Performance Composite Tubes Design
The tube body is made as follows (Fig. 1):
Circurnferential layers wound at an angle of nearly 90°. These layers are calculated to withstand internal and external pressure loads.
Longitudinal layers helically wound at a small angle with the tube axis to withstand axial and bending loads.
Internal and external liners to ensure water tightness of the structure. (When in service, microcracks are induced in the matrix of high-performance composites. This does not weaken their mechanical properties but prevents the structure from being watertight.)
The composite structure appears similar to that of a flexible pipe In fact composites are far more rigid, since there is no sliding between the layers.
The end fitting is a key point of the technology of composite tubes. In the case of ordinary low-performance glass tubes, a composite fitting with threaded connector is bonded to the current length of the tube. But, due to the weak shear strain resistance of the basic material, it is always very limited in performance, especially in tension. In the case of the high-performance IFP-Aerospatiale tubes, the fitting is made of a metal insert fully integrated into the composite and designed to have the same mechanical properties as the current length.