Gisle Rørvik

Influence of sigma-phase precipitation on the impact behaviour of duplex stainless steel pipe fittings

In this study, the effect of sigma-phase precipitation in duplex stainless steel (grade 2205) pipe fittings collected from top-side process systems offshore was investigated under realistic loading conditions. This was done by conducting quasi-static and dynamic impact tests on 3” fittings with a nominal outside diameter of 88.9 mm, nominal wall thickness of 3.05 mm and D/t-ratio of nearly 30. The fittings had a sigma-phase level between 0 and 15 vol.%, and were loaded by a massive steel indenter with a hemispherical nose of 25 mm radius. In the quasi-static tests, the loading rate was 2 mm/min, while in the dynamic tests the impact velocity was varied between 5 and 10 m/s. A few dynamic impact tests on 2” fittings with an increased wall thickness and low sigma-phase levels were carried out for comparison, while some 3” fittings were pre-charged with hydrogen before dynamic testing to examine the effect of sigma phase on cathodic protected subsea components. When the sigma-phase level was low (i.e. σ ⩽ 5 vol.%), no evidence of fracture was found in any of the pipe fittings, whereas at higher sigma-phase levels (σ > 5 vol.%), fracture occurred in all tests. At the highest sigma-phase level (σ ≈ 15 vol.%), extensive fracture and crack growth took place independent of the loading rate.

Fasteners in Subsea Applications: End User Experiences and Requirements

Fasteners used for critical applications subsea represent a challenge in terms of material selection, quality control, traceability and documentation for manufacture and delivery to ensure adequate performance, integrity and avoidance of costly failures. The definition of critical subsea fasteners are both pressure containing and primary load bearing members, but may in a wider sense also include fasteners which may affect functionality of valves, equipment and instruments for safe, reliable and cost effective operation.The major failure mechanism experienced during the last decades is so-called hydrogen induced stress cracking (HISC) or hydrogen embrittlement (HE) primarily resulting from the hydrogen charging conditions of the cathodic protection (CP) system. The main bulk of fasteners used subsea is of low alloyed steel grade type (e.g. ASTM 320 Grade L7, L7M, L43 bolts with ASTM 194 Grade 4, 7M, 7 nuts) which relies on electrical continuity to the cathodic protection system to avoid sea water corrosion, but may be prone to HISC/HE if the fastener hardness and strength level exceed the specified and established limits. If higher strength materials are needed from design, either cold formed or precipitation hardened CRA’s are usually selected (e.g. Grade 59, 660, 625, 686, 718, 725), but these may also be susceptible to HISC/HE depending on strength level and manufacturing practice.The present paper will give some field experience and typical examples of recent HISC failure case histories for low alloyed steel grades, but also for some CRA type of subsea fasteners. Furthermore, some in-house low alloyed steel grade (L7, L43) fastener HISC test results will be presented to indicate the robustness of today’s hardness and strength level restrictions, and at which loading and pre-tension level a given strength grade may be prone to HISC failure. Finally, some experience from manufacturing stage as well as focus areas in materials specification, quality assurance / quality control and design will be discussed.Copyright