A New Generation of Flexible Pipe

ABSTRACT: Flexible pipe, an unbonded, composite construction consisting of steel and thermoplastic layers manufactured in long continuous lengths, was first introduced into the market in 1972. To date, over 2300 km (1400 miles) of flexible pipe have been installed worldwide, with continuous service in excess of 15 years for some pipe. Due to continuing Research and Development efforts, new materials (cross-linked polyethylene, ferritic stainless steels, aluminum, composite fibreglass and thermal insulation foam) and improved design (wire profile) have recently been developed and an entirely new generation of flexible pipe with equal or better performance and reduced cost will be available from the early 90's. INTRODUCTION: This paper will only consider unbonded flexible pipe which has demonstrated long term reliability when subjected to severe service conditions. Thanks to its performance and durability, flexible pipe has become a key element in the successful development of floating production systems, subsea completions and deep water applications (ref. l and 2). Basically, the pipe structure is composed of steel and thermoplastic layers: the internal thermoplastic layer ensures the leakproofness of the transported fluid (crude oil, gas and water), the external thermoplastic jacket offers corrosion protection and the steel layers provide mechanical resistance to pressure, tension, torque and mechanical loads (ref. 3). A typical flexible pipe structure is composed of 4 or 5 layers as illustrated in Fig. 1:an interlocked stainless steel carcass,an internal thermoplastic sheath,possibly, one interlocked pressure layer made of steel wires for high pressure applications,two crosswound flat steel armours,an external thermoplastic sheath. Foam layers may be added to provide additional thermal insulation. The purpose of this paper is to present the newly developed materials and design which are drastically enhancing the performance and reducing the cost of flexible pipe. NEW MATERIALS: Cross-linked Polyethylene: Since 1986, three different thermoplastics are used in the manufacturing of flexible pipe, namely (ref. 3):A proprietary grade of polyamide 11 (RILSAN* BESNO P40TL+) is extensively used in crude oil applications when the temperature does not exceed 100 deg.C and has demonstrated extremely good ageing characteristics. As shown in Table 1, the two limitations of polyamide 11 are its resistance to strong acids and a maximum operating temperature range varying between 70 deg.C and 100 deg.C for applications with hot water due to the hydrolysis of the plastic.A proprietary grade of polyvinylidene fluoride PVDF (COFLON*) may be used up to a maximum temperature of 150 deg. C in crude oil applications. Its mechanical performance remains very stable even when subjected to highly corrosive fluids and it has excellent resistance to acids and to hot water. However, the cost of COFLON* which is twice as expensive and denser than RILSAN*, limits its use to severe temperature conditions.High density polyethylene (HDPE) can only be used in dead oil applications and with a maximum temperature of 60 deg.C. HDPE has an adverse swelling behavior, a poor blistering resistance and has shown limited ageing performance with gas. It is therefore not recommended for applications with gaseous hydrocarbons.