Recent Experiences With Cracking of Load Bearing Dissimilar Metal Welds on Subsea Production Systems

Abstract

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).

Influence of post-weld heat treatment on the microstructure and hardness of dissimilar buttered and welded joints with nickel alloys

“…Despite several years of successful use, a sequence of failures has occurred in recent years. One of the most reported cases of failure occurred at the interface of AISI 8630/Inconel 625 dissimilar joint, in manifolds [5], failures of this nature have also been reported at dissimilar AISI 8630/Inconel 725 joints [6], and at the interface of the butter layer between ASTM F22M/Inconel 625 joints [7].…”

“…Zone M, composed of martensite, is susceptible to hydrogen embrittlement. Therefore, for an application in an environment favorable to hydrogen embrittlement, the AISI 4130/Inconel 725 buttered interface has been shown to have a higher possibility of failure [7].…”

Butter-weld Interface Microstructural Analysis Employing NCS and LAMS Sections

“…Among these materials Nibased superalloys stand out for their good performance in terms of mechanical and corrosion resistance 12 . Among the components of the offshore system that are constituted by dissimilar welding, there are components such as Manifolds, Flowlines and Risers 10,13 . These have used high-strength low-alloy (HSLA) steels, for example 8630 and F22, which receive buttering weld multipasses with Ni-based weld metal (e.g.…”

“…The most notable case was that of the Thunder Horse platform operated by BP in the Gulf of Mexico in 2006 10 . Among the materials cited in the literature, the joints between 8630M / 625 are mentioned in the history of failures 13 . The F22 / 625 joint has shown promise due to the lower carbon content in its composition.…”