Low-Alloy SA-533 Steels as Alternative to ASME SA-516 Carbon Steel for Fabrication of Lightweight FPSO Vessels

One of the biggest challenges in the design of an Floating Production Storage and Offloading (FPSO) plant is managing the weight of the facilities within the limited space and weight capacity of the ship. Weight reduction allows simplification of the load distribution on the ship, which is critical because the motions of the structure due to sea current and waves may affect the vessel performance. In this context, the use of high strength steels for pressure vessels reduces the weight by permitting thinner vessels and lighter supporting steel structures. Compared to ASME SA-516 Grade 70 steel, commonly used for pressure vessels, ASME SA-533 Class 2 steel provides approximately 30–50% greater allowable stress when vessels are designed in accordance with ASME Section VIII Division 1 or 2. Therefore, special considerations are needed to ensure sufficient impact toughness of the welds and to balance toughness, tensile and hardness properties required by the severe service condition. This paper aims at summarizing some technical data on high strength carbon steel SA-537 class 2 and low-alloy steel SA-533 type C class 2. A case study will illustrate the weight savings that can be achieved with these grades. In addition, characterization results obtained on base material from laboratory tests as well as welded joints will be provided, including hardness survey as regard to ANSI/NACE MR0175 / ISO 15156. Finally, a world-class manufacturer of pressure equipment will provide some feedback on the recent fabrication of pressure vessels in ASME SA-533 Type C grade for an offshore project in Western Africa. This contribution demonstrates the interest of high strength steels for the fabrication of lighter pressure vessels, and their potential use for severe conditions such as low-temperature or sour service.