Material Selection for HP/HT Developments

Abstract

Abstract This publication examines the material selection challenges for offshore High Pressure (HP) and High Temperature (HT) developments, both present and future. The materials are examined from corrosion/cracking, mechanical performance and economic perspectives. The scope of this publication includes subsea equipment and pipelines and is oriented towards subsea developments. It does not include wells, wellheads and trees, though the material selection experience and literature for these have been reviewed. The parameters examined include material resistance to corrosion and cracking from a variety of mechanisms, costs of materials, mechanical strength, retention of mechanical properties at elevated temperatures, and requirements for dealing with forces associated with longitudinal stress such as upheaval buckling. Methodologies for control of temperatures and pressures, such as cooling spools, are also discussed. Different operators have different philosophies towards material selection for HP/HT developments, and the chosen materials range from carbon steel to super duplex stainless steels and nickel alloys. The duplex type stainless steels can offer excellent corrosion resistance and mechanical strength, especially 25% Cr super duplex. However, as temperatures increase, these materials suffer from a steep decline in mechanical strength. Also, the susceptibility to hydrogen induced stress cracking (HISC) can place severe limits on the allowable stress levels. They are also expensive materials with long delivery times. Carbon steel suffers in terms of corrosion resistance, even with corrosion inhibition, as inhibition becomes problematic at elevated temperatures. However, the loss of mechanical strength is less marked, and corrosion can be mitigated by the use of corrosion resistant alloy (CRA) cladding if required. The super modified Martensitic 13% Cr materials have excellent mechanical strength, very good retention of strength even at high temperatures, and low cost. However, they are less resilient in terms of corrosion and cracking resistance and are highly susceptible to HISC. Materials selection is affected by a large number of variables, and by the relative conservatism of the operator. Pipeline and subsea equipment design needs to be flexible in response to these requirements. This becomes especially challenging for future developments, where increased temperatures may require solutions out with those traditionally utilized.