Design of Flexible Pipe for High-Temperature Fluid (1991)

ABSTRACT This paper describes the design of a new plastic/metal flexible pipe for high-temperature gas or oil. If a high-temperature fluid is carried through a conventional pipe, its plastic layer would be degraded or softened, shortening remarkably the life of the flexible pipe. For this new flexible pipe, its plastic inner pipe is constructed of two layers, fluoroplastic with high oil resistance and polyamide resin (nylon resin) with adequate rigidity even at high temperature, so that it may allow transportation of higher-temperature fluids. INTRODUCTION Flexible pipes feature ease of installation, storage, and transportation. Furthermore, when used as a riser, flexible pipes can easily follow the movement of the floating vessel. Incorporating these features, flexible pipes have come to be widely used in undersea oil fields. With the spread of flexible pipes, there has been a growing need for pipes usable under various severe conditions. To meet this demand various types of pipes have been developed.1,2) The upper working temperature limit of conventional flexible pipes is considered to be between 90 and 1OO°C. However, the demand for flexible pipes for carrying fluids whose temperature is higher than the above limit has grown as the petroleum developmentat the marginal field has been promoted. To fill this demand, a flexible pipe for high-temperature fluids has been newly developed. BASIC CONSTRUCTION OF FLEXIBLE PIPES The flexible pipe consists basically of the following layers (from inside to outside).Interlocked conduit: A flexible metal spiral pipe of interlock structure. The main function of this interlocked conduit is to enhance the resistance to external pressure and prevent the pipe from being transformed into elliptic shape when it is bent. Since high corrosion resistance is required, the interlocked conduit is normally made of stainless steel.Plastic inner pipe: A plastic layer covering the interlocked conduit. This plastic layer, which is in contact with the internal fluid, is designed so that the internal fluid may not leak through it. This plastic layer therefore has to withstand various chemicals used in oil fields. Furthermore, adequate flexibility and fatigue resistance are required.Pressure reinforcing armor: A spiral of a steel strip or steel tape. This armor serves to enhance the resistance to the axial force and hoop stress. Generally, short- and long-pitch spirals are provided to withstand the hoop stress and axial force, respectively.External sheath: A plastic laver covering the pressure reinforcing-armor. This sheath serves to protect the pressure reinforcing armor against damage or abrasion and isolate it from sea water for corrosion protection. HEAT RESISTANCE OF FLEXIBLE PIPES As described previously, flexible pipes have a metal-plastic composite structure. Since the working temperature is at most one hundred and several tens degrees, no problem will arise with the heat resistance of the metal parts. In addition, the external sheath has no thermal problem because it is exposed to sea water or air. Consequently, the problem of the heat resistance of a flexible pipe may be regarded as that of the plastic inner pipe.