The use of mechanically lined pipe (MLP) using a thin liner, i.e. 2.5mm, can provide a more cost effective linepipe material solution relative to a standard 3.0mm liner. This is especially the case for the more expensive liner materials with higher corrosion resistance, including Alloy 625. Thin liners, i.e. 2.5mm, can be used without compromising pipeline integrity and performance, whilst still fulfilling design requirements defined in most pipeline design standards, including DNVGL-ST-F101. The suitability of 2.5mm liner MLP has previously been demonstrated in service over a range of pipeline bundle projects installed with the controlled depth tow method, but not to date for risers installed by reel-lay. This paper presents the details and test results of the qualification programme to support its use for both flowlines and risers installed by reel-lay.
The qualification MLP test pipes, which comprised an outer diameter (OD) 219.1mm × wall thickness (WT) 15.9mm X65 + 2.5mm Alloy 625, were manufactured using established manufacturing procedures and facilities. Reeling and fatigue test strings were fabricated using qualified welding solutions. The fabricated test strings were subject to internal visual inspection and dimensional measurement using laser metrology in order to provide a benchmark for comparison post reeling. The test strings were given a simulated reeling procedure using bending and straightening formers representative of a reel-lay vessel with the smallest reel hub diameter, this being a conservative material straining condition. An internal pressurisation technique, as per standard installation practice for the present pipe lay contractor for MLP, was applied during the simulated reeling procedure. Post reeling the internal laser metrology inspection procedure was repeated in order to confirm the integrity of the liner and to check for the presence of any evidence of liner wrinkling or damage. Subsequently, full scale fatigue testing was performed using the high frequency resonance bending procedure. Testing was performed to ultimate failure to determine the fatigue endurance limit of the thin liner MLP.
Additionally Finite Element Analysis (FEA) was performed to further validate the satisfactory reeling performance of the thin liner MLP. The FE numerical analysis embraced manufacture of the MLP pipe and test samples coupled with the reeling procedure. Sensitivity analysis on pipe strength and geometrical mismatch was performed to demonstrate the robustness of the linepipe material solution and reeling procedure.
All of the critical qualification activities were performed and verified by DNVGL and in accordance with the guidance of DNVGL-RP-A203 Technology Qualification process.
The paper highlights the qualification programme performed to enable the cost effective use of thin liner MLP, specifically Alloy 625, for risers installed by reel-lay.