There is increasing demand for pipeline installation, including SCRs, in deeper water, coupled with a requirement for higher operating pressures and temperatures and the need to transport corrosive constituents. For such applications, the use of high strength steel, Grade X80, offers significant benefits including a reduction in pipeline weight and savings in material and fabrication costs. Furthermore the reduction in linepipe weight reduces buoyancy module requirements and facilitates installation by existing pipelay vessels which would otherwise require increased top tension capability if lower strength pipe was used.
Reel-lay offers a cost effective offshore installation method for high strength steel pipe. Hitherto reel-lay installation has been limited to Grade X65/70 strength pipe. Subsea 7, in collaboration with Vallourec and Mannesman Tubes, (refer to hereafter as V&M Tubes) has performed a qualification programme for reelable X80 linepipe. V&M Tubes manufactured seamless X80 pipe of 323.9mm OD × 18mm WT pipe in accordance with DNV OS-F101, supplementary P requirements. Subsea 7 developed and qualified a mechanised girth weld procedure based on the GMAW-CMT/PGMAW welding process. Procedure qualification was successfully performed in compliance with DNV OS-F101, including mechanical, fracture toughness and sour service testing.
In order to address the need to transport more corrosive constituents, Butting manufactured Alloy 625 and 316L mechanically lined or BuBi® pipe(323.9 x17.5+3.0mm) using the X80 pipe supplied by V&M Tubes. Subsea 7 developed a novel girth welding procedure utilising internal welding of the CRA lining and external welding using conventional C-Mn filler wire. The latter facilitated the achievement of overmatching weld metal strength which is necessary for reeled pipe. Girth weld procedure qualification was successfully performed in accordance with DNV OS-F101 including a full scale bending trial.
The development of linepipe material and welding solutions for reelable high strength carbon steel and CRA lined pipe are considered to be key enabling technologies for the exploitation of deep water oil and gas reserves in the future.