Critical offshore pipelines require extremely low D/t (Outer diameter to wall thickness ratio) pipes, that pose a manufacturing challenge using conventional practices and operational toolings. Such pipes are most often manufactured through the seamless route, due to the D/t ratio. But seamless pipe are more expensive as compared to welded pipes, along with having higher lead times due to fewer manufacturers. There is also a concern with the uniformity of wall thickness throughout the pipe body due to the manufacturing process, but with an advantage of better properties citing homogenous composition due to absence of a weld seam. Though in the last couple of decades, owing to advances in the welding technology, the weld seam of a welded pipe has proved to be superior to the base metal plates/coils in terms of its strength integrity.
To manufacture such pipes through the LSAW (Longitudinal Submerged Arc Welded) process requires major overhauling and process modifications at practically all the stages of the process flow. The low diameter and high wall thickness become very demanding when it comes to bending the plate and then welding (SAW) from the inside due to space constraints.
This paper describes in detail the challenges faced and overcoming of the difficulties in manufacturing an 18″ diameter, 35 mm wall thickness LSAW pipe (D/t ratio of 13) for a major oil & gas player, by development of new designed toolings for various equipment like plate crimping, pipe forming and mechanical expander; new lubrication oil; and optimizing the process parameters, as the major factors contributing to success. The tooling design modifications were done in-house, along with the development of a new lubrication oil and its treatment in plant. This enabled Welspun Corp to surpass the equipment capacities prescribed by the OEMs, and set a new benchmark in the industry for the production of extremely low D/t welded pipes, with excellent mechanical properties along with impeccable pipe dimensions.