Inline IR-Cured XLPE Technology for Flexible Pipes
Authors: Procida I.-M. , Nielsen N.J. Rishøj 
Published: October 2008

To 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.

Introduction

The function of the polymeric pressure sheath in flexible pipes is to make the pipe leak tight. Therefore, the integrity of this layer is critical for the performance of the pipe during its design life. It is manufactured in an extrusion process that produces a tube with a diameter range of 2 to 19inches and corresponding typical tube wall thickness of 5 to 12mm. Also, the length of the flexible pipes typically varies from a few hundred metres for riser applications to several kilometres for flowline applications. Thus, owing to this challenging largescale and long-term extrusion requirement there are only a limited number of commercially available materials that can provide a sufficiently high processing stability thus preventing degradation of the material in the extrusion equipment. Consequently, the material grades normally used as pressure sheath in flexible pipes are high density polyethylene (HDPE), polyamide (PA) and polyvinylidene fluoride (PVDF).

The choice of material grade for a specific application is promarily governed by temperature and chemical compatibility requirements. Typical maximum design temperatures are 65°C for HDPE, 95°C for PA and 130°C for PVDF. However, it is well known that PA is susceptible to hydrolysis when subjected to water and/or methanol at elevated temperatures. This often limits the operating temperature of PA to 60-65°C to achieve a 20-year design life. As polyethylene material does not suffer from hydrolysis degradation, there is an incentive to enhance its mechanical properties thus increasing its temperature range while keeping the cost at a competitive level.