Qualification of Mechanically Lined Pipe MLP with High Frequency Welded HFW Host Pipe for Subsea Applications with Reeling Installation

Abstract

Known as HFW-MLP, Mechanically Lined Pipe (MLP) with High Frequency Welded (HFW) host pipes are potentially the most cost-effective bi-metallic pipes for subsea pipelines when corrosion resistant alloys (CRA) are required. However, HFW-MLP has a very limited track record for subsea applications. This paper details a recent programme to qualify MLP with HFW host pipes. The qualification programme has been performed in accordance with DNVGL-ST-F101 (2017) and internal supplementary requirements for reelability and weldability. It considers material testing of HFW-MLP at each manufacturing stage and product qualification including full-scale reeling simulation, anti-corrosion coating simulation and girth welding. Qualification is supplemented with a detailed evaluation of the manufacturing process HFW-MLP is compared to traditional MLP which is supplied with seamless carbon steel as the host pipe or backing steel. This novel product lowers the supply cost, reducing the Capital Expenditure (CAPEX) for subsea pipeline projects. Detailed evaluation of mechanical test results and dimensional inspections using laser profiling assess the impact of the HFW host pipe longitudinal seam weld and conclude that there is no detrimental effect on the performance of the completed MLP. A comprehensive review of full-scale reeling simulations, coating simulations and welding trials is completed, with the conclusion that an HFW host pipe does not adversely affect the ability of the MLP product to be girth welded and to withstand the plastic deformation exerted upon the product during reel-lay installation. It is concluded that HFW-MLP is qualified for subsea pipeline static applications via the reel-lay, S-Lay or J-lay installation methods. The qualification of HFW-MLP provides a more cost-effective solution for the development of corrosive subsea fields by reducing overall product supply and installation costs.