ABSTRACT

Ferritic-austenitic (duplex and superduplex) stainless steels (DSSs and SDSSs) are an attractive option for subsea equipment and connecting spools, in which the internal corrosion threats can be difficult to manage, using corrosion inhibition and in-line inspection. However, whilst these materials are advantageous for managing internal corrosion, they still require external cathodic protection (CP) to prevent localized corrosion in seawater at typical operating temperatures. This CP results in hydrogen charging of the duplex stainless steel, either at uncoated locations or, in locations where the coating has lost integrity. In the presence of a critical hydrogen concentration, all duplex and super duplex stainless steels are susceptible to hydrogen embrittlement, via a cracking mechanism commonly referred to as HISC: if critical stresses/strains are exceeded, subsea failure can occur. Whilst mitigations such as increased (less negative) CP potential, tailored microstructure and high-reliability coatings can be considered, these are challenging and are rarely a complete barrier to the threat.

Instead, the industry assumes that hydrogen charging will happen and relies instead on design approaches that avoid equipment being exposed to stresses or strains exceeding the nominal thresholds for HISC. This approach is captured by DNV Recommended Practice F112: 2008 (DNV RP F112: 2008). Full scale test validation of this approach is limited. A comprehensive test program is described quantifying the HISC performance of retrieved superduplex stainless steel subsea components and, comparing the actual performance against the limits derived following DNV RP F112: 2008. To facilitate this comparison, comprehensive material characterization, small, intermediate and full scale environmental-mechanical testing, and finite element analyses were conducted. This work has demonstrated that both stress and strain-based approaches of DNV RP F112: 2008 can be overly conservative.

INTRODUCTION

Ferritic-austenitic (duplex) stainless steels (DSS) are often used to manufacture subsea components for their high strength associated with a good corrosion resistance. Typical uses are short flowlines, welded hubs and flanges, manifolds, towheads, flowbases, sensors and valves. As subsea facilities are protected from seawater corrosion by cathodic protection (CP), (S)DSS components are exposed to hydrogen uptake, especially when a coating system is not in place or has failed at a certain location.

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