TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractTo day Silane cross-linked HDPE is used as pressure sheath in unbonded flexible pipes for the offshore industry. This method is based on a two step process where the first step is the extrusion of the sheath and the second step the cross-linking using hot water or steam. However, this process is time consuming and it is difficult to cross-link the crystalline part of the polymer. Furthermore, it is required that the flexible pipe be finished with end fittings before hot water can be circulated in the pipe. Finally, it will not be possible to verify the actually obtained cross-linking of the pressure sheath unless re-terminating an end fitting.NKT has developed a new method to cross-link HDPE pressure sheath in flexible pipes using Peroxide in combination with infrared radiation (IR). This method is unique in the sense that it is an inline process where extrusion and cross-linking is done in one step. Thus, it is possible to measure the degree of cure as soon as the material has cooled down and no additional process is needed. Therefore, upon finalizing the extrusion of the pressure sheath test samples can be taken from the pipe to document the degree of cross-linking which can be as high as 90% compared to typically only 75% for the Silane-based process.The IR-cured XLPE technology will be described in this paper and results from the qualification programme which has been carried out according to API Spec 17J [1] and ISO 13628-2 [2] will be addressed.
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].
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