Failed austenitic SS bolts in valves

In recent years, we have experienced a number of cases with valves in offshore environments, in which the internal bolting in austenitic stainless steel failed after some time in the environment. Typically, it has proven to be valves operating in temperatures of 50–60˚C or more, and the failure mechanism was chloride-induced stress corrosion cracking in the austenitic bolts.Not least for valves in hydrocarbon service, such bolt failures may result in a major loss of containment with extensive gas leaks possessing a high risk potential of ignition resulting in fi re or explosion.

Keywords: Gas leak • Internal bolting • Offshore

By Kristian Lund Jepsen and Per Grumsen, Ramboll Oil & Gas, Esbjerg, Denmark

Figure 1: Norwegian case from 2012. Bolts were in 316 SS.

Figure 2: Norwegian case from 2012.

The phenomenon has been seen in both the Middle East (Persian Gulf) and in the Danish and Norwegian North Sea.In one case, offshore Qatar (reported in 2008), a two-piece 2” ball valve on a fuel gas skid, in which all four internal bolts keeping the two parts together failed. The failure resulted in the release of almost 500 kg (= 500 m3 at atmospheric pressure) of natural gas during a 3 minute release. Fortunately no ignition of the gas took place. The bolts were in 304 SS.In that case, the owner identifi ed a high number of equivalent valves in equivalent service conditions. The valves were prioritised in 3 groups (1, 2 and 3) depending on the risk potential. All valves in group 1 (the ones with the highest risk rating) were replaced within months after the incident. The new valves were with bolts in super duplex stainless steel.A rather similar event was reported from a platform in the Norwegian North Sea

During disassembling the valve onshore, it was observed that the upper fi ve stud bolts had fractured already before the pressure test, and that one bolt had partly cracked too. This bolt fractured totally during the pressure test, which caused opening of the valve housing – and with that the leak – see Figure 4. A metallurgical examination of a cross-section through one of the fractured AISI 316 stud bolts suggested that the root cause of the fractures was chloride-induced stress corrosion cracking – see Figure 5. Based on this result of the examination, a warning was sent out, and an inspection of the pipe system for the presence of possible other valves of the same type was initiated.Several other valves of the same type, which had suffered the same type of bolt cracking/fracture, were detected during this inspection, and immediate replacement of these valves was carried

in September 2012. A two-piece ball valve with austenitic stainless steel bolts failed as the bolts broke. The failed valve was operating at service temperatures exceeding 60˚C and had bolts in 316 SS.In the Norwegian case, further 17 similar valves in equivalent services were identifi ed in the same plant before any incident took place.The case from the Danish North Sea turned up as an 8” super duplex stainless steel process valve failure (valve for produced water, temperature approx. 70°C), which – fortunately - occurred during a standard leak test. The valve was equipped with AISI 316 austenitic stainless steel bolts (A4-bolts).The valve started to leak due to opening of the top part between body and fl ange before full test pressure was reached – see Figure 4. The valve was then sent onshore for repair.

Figure 3: The valve in opened condition – immediately after pressure testing.

Loose AISI 316stud bolts

opened assembly

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out in order to reduce the risk of catastrophic leaks occurring during normal service.New valves and repaired valves are equipped with super duplex stainless steel bolts.

Chloride-induced stress corrosion cracking (SCC)Chloride-induced stress corrosion cracking is a type of environmental cracking caused by the simultaneous action of a corrosive agent (aqueous chlorides) and sustained tensile stress in the material.This actual type of cracking primarily occurs in austenitic stainless steels, if the following criteria are fulfi lled:

• High tensile stress level• High temperature• High chloride concentration

A high tensile stress level in a material is e.g. present as the result of the manufacturing process (rolling, cold-forming, welding, tightening of bolts etc.), which results in a high static stress level in the stainless steel material.In the actual cases, primarily the tightening of the bolts has resulted in a high tensile stress level in the bolt material. A high temperature (in practice approx. 50-60°C may be suffi cient for standard austenitic stainless steel) most often originates from a medium present inside pipes and vessels etc. – however, especially at locations, where a warm climate is present (sub-tropical or tropical), the temperature of external surfaces may reach heights, in which austenitic stainless steel suffers from chloride-induced stress corrosion cracking,

if the other parameters necessary are present too.In the actual cases from Norway and Denmark, the temperature of the medium inside the valves during normal service is reported to be suffi ciently above the “lower level”, at which this cracking mechanism may occur. The conditions, under which SCC (stress corrosion cracking) may take place due to chlorides, are shown in Figure 6 below.Chlorides may be present in various types of mediums transported in process plants – however, common to the valve failures described above is that chlorides necessary for the actual failure type have been added from the surroundings – thus resulting in the chlorides coming into contact with external surfaces of valves and piping.

Figure 4: Failed valve with six fractured stud bolts in the upper part – after disassembly.

Figure5: Fractured stud bolt and micro-photo of cross-section through fractured surface. Note the comprehensive cracking of the bolt material up to the fracture surface. The crack image to the right (trans-granular cracks) suggests the cracking mechanism to be chloride-induced stress corrosion cracking. Red-colouring of the bolt threads due to corrosion is seen.

Five bolts already fractured

One partly cracked bolt fractured during the test

Fractured stud bolt Fracture surface

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Especially in marine environments, this may be a problem, which must be taken into consideration, when selecting materials.Also at indoor installations, chlorides may be added to stainless steel external surfaces – e.g. during deluge tests carried out using seawater or from dripping water from leaking pipe systems etc.Adding water to heated surfaces will result in evaporation of the water - to a certain extent – resulting in increased risk of high chloride concentration on the surfaces in question. During the years, several failures in stainless steel items caused by chloride-induced stress corrosion cracking have

Figure 6: Conditions under which SCC may be experienced. It is seen for the austenitic stainless steels (304/304L and 316/316L), that the conditions may be present from about 50˚C service temperatures, if high chloride concentrations are present (courtesy of Sandvik). Figure 7: Transgranular stress corrosion cracking.

been reported. The cracking has been caused by evaporation of water containing only small – and often insignifi cant - amounts of chlorides on heated surfaces.Thus evaporation of such water added continuously to warm surfaces may over time lead to the accumulation of chlorides - fi nally resulting in a chloride level, which makes stress corrosion cracking possible – even despite a “harmless environment”.

RecommendationsIt is strongly recommended that operators of offshore oil/gas installations investigate, if they have valves with austenitic stainless steel bolts in service conditions with

About the author

Kristian Lund Jepsen is Senior Chief Consultant at Ramboll Oil & Gas in Esbjerg, Denmark. Mr. Jepsen holds an M.Sc. in mechanical engineering from the Technical University of Denmark, Copenhagen, from 1981. He has been with Ramboll Oil & Gas for about 18 years. Mr. Jepsen is a technical expert within the oil and gas production and processing areas, with special focus on mechanical equipment and systems including valves. Earlier in his career, he has held positions in large energy companies such as Maersk Oil and Halliburton.

Per Grumsen is Senior Chief Consultant at Ramboll Oil & Gas in Esbjerg, Denmark. Mr. Grumsen was educated as marine engineer in Esbjerg, Denmark in 1980. He has been with Ramboll Oil & Gas as material expert since 2012 and had formerly a long career (approx. 30 years) at the material department of FORCE Technology in Esbjerg. Fields of activities have among other things been: Failure investigation and analysis (corrosion problems, cracks and fractures, breakdown of engines, turbines, valves etc.), metallurgical examinations, material selection, NDE-methods, corrosion prevention e.g. including specifi cation and inspection of surface treatments (coatings, paint systems, hot-dip galvanizing etc.).

temperatures above 50˚C on their installations and replace such valves or at least the bolts. For stainless steel valves, super duplex stainless steel bolts are often a good alternative.It is also recommended to change piping and valve specifi cations to avoid purchasing valves with austenitic stainless steel bolts in the future.It may even be required to introduce checks of received new valves into stock to ensure that the specifi ed bolting material has been implemented. Some of the cases above are cases, in which austenitic bolting had not been specifi ed in the purchase specifi cations.

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