Improved Thermoplastic Materials For Offshore Flexible Pipes
Authors: Dawans F.A. , Jarrin J.A. , Lefevre T.O. , Pelisson M.A. 
Published: October 2008

Long-term ageing tests representative of field operating conditions have been conducted on various thermoplastic materials proposed for the inner tube of flexible pipes for offshore drilling and production applications.

In particular, experimental data are provided about the changes of the mechanical properties of selected thermoplastic materials owing optimized formulation, when exposed over time to crude oil in presence of gas and water.

INTRODUCTION

Flexible pipes are usually made of heterogeneous materials to provide flexibility and strength at the same time. This is accomplished by a suitable design of the cross-sectional structure. Flexible pipes with high pressure rating and large diameters may, be of two types: bonded and unbonded (Fig. 1). Flexible pipes of the bonded structure are made of textile and steel reinforcements which are embedded in elastomers allowing shear but-no slip between the two.. On the other hand, flexible pipes of unbonded structure are-made of steel components ensuring the mechanical performance and-of plastic components making it leakproof. Such an unbonded structure allows slipping among the steel and plastic layers.

When IFP (French Petroleum Institute) research in flexible pipes had started more than twenty years ago, both structures were considered. Several bonded structures have been tested in order to obtain a combination of the properties of steel-belted radial tires-and helical hoses. But, finally, we decided to renounce to such a structure taking mainly into account of manufacturing and testing reasons. For instance the long-term reliability of rubber-based pipes is rather limited, mainly due to the swelling and blistering trend of rubbery polymers in the presence of gas and crude mixtures, even if the elastomeric materials are contained and constrained between an interlocked tube and steel layers. Then, because of their high oil and gas absorption under associated environmental pressure and temperature, bonded structures appeared unsuitable to avoid any failure on rapid depressurization which may have serious consequences.

The bulk of our gathered .data to assist failure prediction has shown that unbonded structure is the most appropriate for severe service requirements. Such unbonded pipes have been developed and patented worldwide by IFP and since 1972 they are currently manufactured in continuous lengths of several thousands of feet and selled by COFLEXIP. Numerous field experiences have fully confirmed their longterm reliability, even when subjected to maximum service conditions.

DESIGN CONSIDERATIONS

COFLEXIP pipe structures are basically composed of steel end thermoplastic material. The steel components which ensure the mechanical performances consist of helical steel wire or interlocked steel strip. As these steel armors are not leakproof, internal impermeability is provided by continuously extruded sheaths of thermoplastic materials. Each section of COFLEXIP pipe is fitted, at both ends, with an alloy steel coupling within the steel armors being anchored and bonded. This concept of the structure offers long unit sections, wide ranges of physical and mechanical characteristics, high resistance to corrosion and practically no restriction as to fluids transported.