The process piping on subsea production systems (SPS) is normally made of solid corrosion resistant alloys (CRAs). However, some process components are made of low alloyed steels (LASs) which are internally cladded with a CRA. These components require post weld heat treatment (PWHT) to improve the properties in the LAS heat affected zone (HAZ). In order to avoid PWHT during on-site welding to adjoining piping systems, it has been common to weld a buttering layer (e.g. 15 – 20mm long) on to the connecting end of the LAS. The buttering layer consumable has traditionally been an austenitic nickel alloy, Alloy 625/725. The LAS HAZ and the buttering layer are thereafter PWHT’d and machined prior to on-site welding to the adjoining piping system. By this, it is not necessary to perform PWHT on the on-site (e.g. tie-in or closure) dissimilar welds. In the beginning of the century, some operators experienced cracking along the fusion line interface between the nickel alloy buttering and the LAS. These problems were typically experienced during start-up or prior to first production. An extensive research programme was established in order to determine the causes and remedial actions. A group sponsored project led by TWI was performed to understand the failure mechanisms and essential parameters leading to hydrogen assisted cracking, (HAC) of dissimilar metal welds (DMWs). Recommendations were made related to LASs chemistry, welding parameters, bevel geometry and especially PWHT time and temperature. Based on these recommendations there have been only a few incidents with cracking of such welded combinations before 2013 and onwards. Since then Statoil has experienced four off incidents with cracking of dissimilar welds on subsea LAS components. Common for these incidents are that they have been in operation for about 15 years and the cracking happened during cold shut-down periods. This paper presents key observations made and lessons learnt from the incidents summarized above. The main focus has been on environmental fracture mechanics-based testing of samples charged with hydrogen by cathodic protection (CP). Variables have been pre-charging temperature and time, as well as testing temperature. The testing has revealed strong dependency between the operating temperature (i.e. shutdown versus operation) and the sensitivity to HAC. Further, the investigations have shown that the integrity of the coating, as an effective barrier to hydrogen ingress, is the main feature to prevent HAC on this kind of DMWs. The investigation of the four off cracked welds showed clearly that the insulating polyurethane (PU) coating was heavily degraded by hydrolysis at higher temperatures. This exposed the dissimilar weldments to CP which contributed to the hydrogen charging of the weldments. The paper gives also result that show that it is not only PWHT’d LAS (e.g. type 8630M, 4130 and F22M) with dissimilar welds that may suffer from this failure mechanism. Testing has shown that as-welded F65 steel /Alloy 59 combinations may also suffer when charged with hydrogen and tested at low temperatures (e.g. shut down temperature).

Design of a steel catenary riser (SCR) requires the use of connection hardware to decouple the large bending moments induced by the host floater at the hang-off location. Reliability of this connection hardware is essential, particularly in applications involving high pressure and high temperature fluids. One option for this connection hardware is the metallic tapered stress joint. Titanium (Ti) Grade 29 has been identified as an attractive material candidate for demanding stress joint applications due to its “high-strength, low weight, superior fatigue performance and innate corrosion resistance”2. Titanium stress joints for deep-water applications are typically not fabricated as a single piece due to titanium ingot volume limitations, thus making an intermediate girth weld necessary to satisfy length requirements. As with steel, the potential effect of hydrogen embrittlement induced by cathodic and galvanic potentials must be assessed to ensure long term weld integrity. This paper describes testing from a joint industry project (JIP) conducted to qualify titanium stress joint (TSJ) welds for ultra-deepwater applications under harsh service and environmental conditions. Corrosion-fatigue crack growth rate (CFCGR) results for Ti Grade 29 1G/PA gas tungsten arc welding (GTAW) specimens in seawater under cathodic potential and sour brine under galvanic potentials are presented and compared to vendor recommended design curves.

Duplex stainless steel has been used on subsea facilities since the mid 80-ties. The experiences with these materials have been relative good and only a few failures have been reported. However, BP and Shell experience some serious cracking of duplex steel in the mid 90-ties and in beginning of the century. The root cause of these failures was identified to be Hydrogen Induced Stress Cracking, HISC, where the hydrogen source was the cathodic protection system of the subsea facility. These and other similar failures resulted establishment of Joint Industry Projects, JIPs with financial and technical contribution from leading oil companies, contractors, material suppliers and research institutions as TWI, SINTEF and DNVGL. The objective of the JIPs was to establish practical usage limits for duplex stainless steels. The JIPs resulted in a recommended practice “DNV-RP-F112 - Design of duplex stainless subsea equipment exposed to cathodic protection.” This document minimized the failure rate of duplex steel components used subsea. However, since duplex steels components have been used on subsea facilities long before the guidelines and recommendations were issued, there are lot of components presently in use that may be overloaded compared to guidelines and recommendations. As a part of life time extension of one of Statoil’s long time producing fields, a HISC re-calculation of spools connecting SPSs to infield pipelines showed that many of the spools were exposed to stresses above the recommended stresses given in DNV-RP-F112. Since these recommendations were primarily based on testing at ambient seabed temperature (4°C), Statoil, together with SINTEF, started in 2016 a project where the aim was to evaluate the resistance against HISC as an effect of the operation temperature. The results of this project show that the critical net section stress/AYS (HISC resistance) increases with increasing temperature. Based on this, the before mentioned spools can be considered safe even though the spools are exposed to stresses above the recommendations in DNV-RP-F112. Further, the investigations show that the guidelines and recommendations given in DNV-RP-F112 may be conservative for temperatures above 4°C. It is therefore recommended to perform more testing to confirm and incorporate the findings from the present investigation in future revision of DNV-RP-F112.

Design of steel catenary risers (SCRs) requires the use of connection hardware to decouple the large bending moments induced by the host floater at the hang-off location. Reliability of this connection hardware is imperative, especially in those applications involving high pressure and temperature fluids. One option for connection hardware is the metallic tapered stress joint. Because of its inherent density, strength and stiffness, steel is not well suited for these applications as it would result it excessive length and weight for deepwater applications. Titanium grade 29 (Ti 29) has been identified as an attractive material candidate for demanding stress joint applications due to its unique mechanical properties including greater flexibility, excellent fatigue performance, and high resistance to sour fluids. Industry has successfully used this technology in over 60 SCR applications. Titanium stress joints (TSJs) for deep-water applications are typically not fabricated as a single piece due to titanium ingot/billet volume limitations, thus making an intermediate girth weld necessary to satisfy length requirements. Fracture and fatigue performance of these welds in the presence of cathodic potential in seawater and galvanic potentials in sour production fluids that may produce hydrogen embrittlement effects must be assessed to ensure long term weld integrity. This paper describes a joint industry project (JIP) performed to qualify titanium stress joints welds for ultra-deep water applications under harsh service and environmental conditions. Fatigue crack growth rate (FCGR) results for Ti 29 1G/PA gas tungsten arc welding (GTAW) specimens in air, seawater under cathodic potential and sour brine environments under galvanic potentials are presented and compared to vendor recommended design curves.

Dissimilar metal welds (DMWs), as manifested by low alloy steels welded with nickel alloys, are commonly used in subsea production systems. In such applications, there is a risk of hydrogen embrittlement, due to the use of cathodic protection. Historically, one of the most reliable means of establishing the resistance of DMWs to hydrogen embrittlement has been to conduct fracture mechanics-based tests, under representative service conditions. These tests have proved valuable in ranking the performance of various DMWs and, also, emulating the fracture morphologies observed in many subsea failures. Much of the established data from these tests has been generated using single edge notched bend (SENB) specimens, tested at the seabed temperature, to produce ‘tearing resistance’ J R-curves. However, whilst it is recognized that most failures have occurred after equipment has been shut down, there has been little exploration of how parameters, such as temperature and loading mode, influence the resistance to hydrogen-assisted cracking. This paper uses the results of SENB and single edge notched tension (SENT) tests on DMWs, consisting of 2.25Cr-1Mo Grade F22 welded with ERNiCrMo-3 (Alloy 625), to understand how temperature and level of constraint affect fracture resistance, at a range of temperatures (4 to 80°C). The merits and drawbacks of both techniques are summarized, alongside the practical implications for subsea components currently in operation.