TPi January 2010

enable pipes to be produced with end dimensions sufficiently accurate to ensure the quality of fit needed for welded SCR connections, ” said John Hardie, staff engineer, pipeline systems at Shell International Exploration and Production Inc. This is attributable to the unavoidable deviations in diameter caused during the rolling of the pipes. An even more serious problem is that the pipes exhibit a degree of ovality. The effect of the two shortcomings is cumulative. AllenMcNickle, supply chain representative at Shell Exploration & Production, commented, “ As a result, the welded pipes do not fit together sufficiently accurately, so that the stabilisation of the welding process is problematic and there are wall thickness variations in the weld area. This has a highly adverse affect on inspectability and dynamic strength. ” The preferred solution was matching the pipes after machining and precision measurement of each pipe end. This was an enormously demanding logistical task, as all the material had to be measured, sorted with the help of a computer, and fed into the welding process in an exactly defined sequence and at an exactly aligned angle of rotation. Besides the demanding technical and logistical aspects, there are other problems. Machining reduces the wall thickness in the weld area, where fatigue strength is especially critical. This narrows the tolerance specifications for the welding process and increases the stress levels in the endangered area. Moreover, a given production batch may not contain enough ‘matching’ pipes. As a consequence, pipes from different production batches may have to be machined to make their inside and outside diameters match, or pipes with a thicker wall may have to be used. The logistical problems of welding onshore or, even more difficult, on the pipe-laying vessel, can be huge.

The pipe end is heated to 1,280°C before being forced into the slit between a two-part die and a mandrel

reason, they have to cover the largest possible area of the sea bed, ” explained Volker Rohden, product manager of riser and flowlines at V&M Tubes. URSA is a tension-leg platform around 130km south of New Orleans. The depth of the water there is 3,995ft (1,218m). URSA floats with the help of the buoyancy provided by four giant vertical steel cylinders and is anchored to the seabed by steel tendons (tension legs). In addition to the conventional risers, which run vertically from the platform to the template on the seabed, it has steel catenary risers that link it to pipelines running at right angles to it. Steel catenary risers consist of pipe strings that are welded together onshore, provided with the necessary surface coating, and then reeled onto the giant drum of a pipe-laying vessel. Steel catenary risers descend in a curve to the touchdown zone on the seabed, where they run horizontally. The pronounced curvature and the swings caused by the relative motion between the platform and the kilometres-long string impose

considerable dynamic stresses. This is especially critical in the area of welds. As a result, the targeted design life of traditional steel catenary risers is 20 years. One target of the development of the PURE risers was to increase this targeted design life from 20 to 30 years. Not only the requirements for the risers were extreme: the whole URSA platform was designed to exceed the highest industrial requirements for hurricane force wind and waves. The crucial problem was the dimensional tolerances of the pipe ends achievable with the formerly used production methods. “ Previously, neither our own specifications nor those of API5L were adequate to

PURE pipe ends are carefully machined both inside and outside after the upsetting process

61

www.read-tpi.com

January 2010 Tube Products International