Tethraflex - New Generation Zero Permeation Hose for Umbilicals
Authors: Donnison C. , Dutton A. 
Published: October 2008

This paper discusses the design and development of a new hose construction suitable for applications in umbilicals installed in deep-water. The hose is suitable for operations involving high temperature, gas transportation and chemically aggressive fluids. The hoses incorporate a continuous metallic flexible liner, which prevents low molecular weight fluids such as methanol and gas permeating from the hose into the interstices of the umbilical. The new hose construction is suitable for deep-water applications down to 2,000 metres where the hose can be subject to external hydrostatic pressures up to 200 bar with zero internal pressure.

Introduction

This paper discusses the development of a new type of hydraulic hose, which is suitable for use with aggressive fluids and gases. The hose is suitable for use in dynamic and static umbilicals used for subsea installations in deep-water applications for the offshore oil and gas industry.

Conventional hoses used in umbilicals are constructed with a liner produced from an extrudable thermoplastic material. The liner material chosen has to be resistant to the fluids and chemicals used by the offshore oil and gas industry. The liner can also allow low molecular weight fluids to permeate into the surrounding environment albeit in relatively low volumes. When installed underwater, the hose may or may not be pressurised and may or may not be filled with a liquid.

In such cases the hose is subjected to an external pressure differential due to the hydrostatic pressure caused by the depth of the water.

In a conventional hose construction this external pressure is resisted by the collapse resistance of the thermoplastic liner and to a lesser extent by the thermoplastic outer sheath. The collapse resistance is a function of the material thickness and the modulus of the thermoplastic inner liner, the thermoplastic outer sheath and the diameter of the hose. As the diameter increases, the resistance to collapse decreases.

Collapse resistance is only a problem when hoses are unpressurised and filled with a fluid of specific gravity that is less than seawater. A standard 1/2" thermoplastic hose filled with a liquid of specific gravity of 0.8 will have a maximum operating depth of 520 metres. And if the hose is filled with gas at atmospheric pressure the maximum operating water depth will be 126 metres.

With the move to subsea production in deeper water, conventional hoses are no longer viable. For this reason carcass lined hose has been developed in which the carcass provides an internal support for the thermoplastic liner. The carcass does not prevent the liner coming into contact with the fluids in the hose. The liner therefore requires the chemical and fluid resistance as for conventional thermoplastic hoses. The ideal solution to these two problems would be to utilise a steel tube either as a hose liner or on its own, but a steel tube tends to be rigid which results in a lack of flexibility. The new hose construction embodies the collapse resistance and zero permeation of a steel tube and the flexibility of a thermoplastic hose.