Subsea Manifolds: An Alternate Fabrication Strategy Using HIP PM Near Net Shapes

Abstract

The fabrication of near net shape powder metal (PM) components by hot isostatic pressing (HIP) has been an important manufacturing technology for steel and stainless steel alloys since about 1985. The manufacturing process involves inert gas atomization of powder, 3D CAD capsule design, sheet metal capsule fabrication and densification by HIP in very large pressure vessels. Since 1985, several thousand tonnes of parts have been produced. The major applications are found in the oil and gas industry especially in offshore applications, the industrial power generation industry, the pulp and paper industry and in pharmaceuticals and traditional engineering industries. Typically, the components replace castings, forgings and fabricated parts and are produced in grades such as martensitic steels, austenitic and duplex (ferritic/austenitic) stainless steels and nickel- based superalloys. The application of HIP PM near net shapes to manifolds for medium to high pressure use has a number of advantages compared to the traditional forging and welding approach. First, the need for machining of the components is reduced to a minimum and welding during final assembly is reduced substantially. Manifolds by HIP design reduce the necessary welding by 70–90%. Mechanical properties of the HIP PM part are isotropic and equal to the best forged properties in the flow direction as is demonstrated below. This derives from the fine uniform microstructure of the PM parts. The PM parts are significantly lighter in weight because of the need to stiffen the forged component at the location of the weldment for the intersecting passageway — the PM parts can be smoothly blended into the intersection without need for welding. Furthermore, the PM HIP components can be made with significantly reduced manufacturing lead-time, greater design flexibility and improved cost for the final component. The PM HIP near net shape route has received approval from both ASTM [1,2,3] and NACE [4] for specific steel, stainless steel and nickel base alloys. This paper reviews the manufacturing sequence for PM near net shapes and discusses the details of several successful applications. The application of the HIP PM process to subsea manifolds is highlighted.