Qualification and Use of IR-Cured XLPE for Flexible Pipes

Abstract

Abstract Unbonded flexible pipes constitute key components in floating production systems for the offshore oil and gas industry and it is important to continuously improve the performance of these pipes to meet the demands of production regularity under increasingly severe operational conditions. The polymeric pressure sheath is a critical component in flexible pipes as the function of the layer is to make the pipe leak proof. Therefore, it is important to carry out material research to enhance the performance of this layer thus further securing the long term integrity of the pipe. A new method to cross-link HDPE pressure sheath in flexible pipes using Peroxide in combination with infrared radiation (IR) has been developed. This method is unique in the sense that it is an inline process where extrusion and cross-linking is done in one step. The IR-cured XLPE technology will be described in this paper with emphasis on presentation of detailed results from the comprehensive qualification programme which has been carried out according to API Spec 17J / ISO 13628-2 [1]. Also, actual field applications of these pipes will be presented including ID15?? flowlines. The newly developed IR cross-linking method significantly expands the application envelope of the conventional XLPE flexible pipes, for example with respect to increase of allowable bore pressure and temperature, and the manufacture of large size pipes up to ID16?? or more with uniform cross-linking of the entire thick-walled pressure sheath. Further novel applications which this technology enables include the manufacture of cross-linked intermediate and outer sheath layers. The paper will present comprehensive qualification test results from both material and mid-scale pipe testing which document that the new IR cross-linking technology is superior to the conventional cross-linking technology previously used for flexible pipes. In particular, it is shown that the new XLPE material can sustain considerably higher pressure and temperature loads compared to existing industry standards, for example as specified in API RP 17B / ISO 13628-11 [2]. Introduction Flexible pipes are complex multi-layered structures built from a number of helically wound metallic wires or strips combined with concentric layers of polymers, textiles, fabric tapes and lubricants. The number, type and sequence of the component layer depend on the specific design requirements. Figure 1 shows a typical unbonded flexible pipe build-up, which according to API RP 17B / ISO 13628-11 [2] is categorized as a Family 3 pipe structure, with carcass (inner-most layer), internal pressure sheath, pressure and tensile armour layers, and outer sheath.