Essential valve material selection challenges in the offshore sector
Material corrosion is a significant risk and costly phenomenon in the offshore sector of the oil and gas industry. The offshore environment is a chloride containing marine atmosphere that can cause severe external corrosion of facilities. Moreover, oil and gas contain undesirable products such as carbon dioxide and hydrogen sulfide that corrode piping components internally.
In addition to corrosion risk, other parameters such as erosion, mechanical strength, availability of materials, and weight, affect the material selection for offshore facilities. ‘Facilities’ refers mainly to equipment, as well as piping components. The offshore sector of the oil and gas industry addresses the development of oil and gas fields ‘off the coast’ under the sea/ocean. Among the material selection challenges to be discussed, corrosion is the main concern.
By Karan Sotoodeh, Lead Engineer, Baker Hughes
Stellite overlay on 22Cr duplex
Stellite 6 is one of the most common hard facing alloys used in the valve industry, due to its outstanding toughness, hardness and wearing resistance. Stellite 6 is a cobalt alloy that contains other compounds such as chromium, tungsten, and carbon. Hard facing alloys are overlaid on valve internals to prevent erosion, wearing, and galling.2 In fact, the internal parts of valves are positioned in the flow stream and are thus more prone to corrosion, abrasion, and wearing than the external parts.
Approximately, 60% to 70% of the piping and valves used in the offshore industry are in 22Cr duplex due to its high mechanical strength and corrosion resistance.3 Applying stellite on 22Cr duplex is possible, but it is challenging due to the formation of sigma phase in the duplex during welding. Sigma phase is a hard and brittle crystal structure which is formed at high temperatures and can lead to duplex stainless steel cracks and failure. Alloy elements such as chromium and molybdenum in the duplex promote the formation of sigma phase,3 which increases the hardness and reduces the toughness of the material. Figure 1 illustrates a stellite 6 overlay on the 22Cr duplex body of a dual plate check valve used in the offshore industry.
Welding a large carbon steel valve to 22Cr duplex piping
Pipeline valves located on oil export pipelines are normally the largest, heaviest and most important valves; they also typically have the longest delivery time for the offshore platforms.4,5 The material of these valves is carbon steel, which is not a corrosion resistant alloy (CRA) since the valves handle non-corrosive and well-treated oil. It should be noted that selecting 22Cr duplex for such a big valve is not economical. However, the pipeline that connects to the valve will be made of 22Cr duplex material, instead of carbon steel, to save wall thickness and weight. As duplex has relatively high mechanical strength and is not required to have corrosion allowance.4
There are two challenges associated with welding pipeline to valves. The first is the thickness difference between the valves and the pipeline; the second is the difficulty of welding the two different materials together. If the valve wall thickness is more than double that of the pipe thickness, a transition piece should be welded to the valve from one side and to the pipeline from the other side.4 The transition piece should have a wall thickness equal to the pipeline thickness on the end welded to the pipeline, and a wall thickness equal to the valve thickness from the end welded to the valve.
The main challenge related to welding carbon steel to 22Cr duplex when thicknesses differ is that carbon steel is thicker than 19 mm. This means it requires post weld heat treatment (PWHT) after welding to remove the residual stresses, according to the ASME B31.3 process piping code requirement.4,6 However, applying PWHT increases the risk of sigma brittle phase in the duplex. The solution is to butter the carbon steel with alloy 625 or 59 (nickel alloy) and apply PWHT to only the carbon steel section. Once those have been buttered, the buttered carbon steel is buttered to the duplex with alloy 625 or 59 filler. Figure 2 illustrates a pipeline valve body in 38" and CL1500 in carbon steel welded to a transition piece in carbon steel and a 22Cr duplex pup piece. The pup piece can be welded to the pipeline in 22Cr duplex in the fabrication yard.
WHAT IS A PUP PIECE?
A pup piece is a piece of pipe which is welded to the body of the valve before assembly of the valve by the valve manufacturers. The pup piece will be welded to the connected pipe in the construction yard. If the pipe in the yard would be welded directly to the valve without any pup piece, the welding heat can melt and destroy the seals inside the valve.
Stellite and tungsten carbide hard facing corrosion in seawater
Stellite and tungsten carbide are common hard facing materials that are applied to valve internals to mitigate erosion and wearing.2 One main limitation of these two materials is their corrosion in seawater service. In cases of seawater service, Ultimet, which is a cobalt alloy, provides very good wear resistance.2 Applying Ultimet to hard facing is therefore done in special cases, as not all valve manufacturers are familiar with this type of material. Lack of Ultimet adhesion to the base (core) material could lead to Ultimet wearing away after years of operation.
Hydrogen induced stress cracking (HISC) corrosion
Materials such as low alloy steel, duplex, and super duplex, as well as hard nickel alloys such as alloy 718 and 72, are prone to HISC in the subsea sector of the oil and gas industry. Figure 3 illustrates low alloy steel valve bolting cracks due to HISC corrosion in a subsea environment.
The main reason for HISC failure has been attributed to a combination of stresses coming from loads (e.g., tension in a pipeline), and ingress of hydrogen formed at the steel surface due to cathodic protection.7 HISC analysis should be conducted for susceptible materials by the valve manufacturer to mitigate the risk of HISC.
Using titanium bearings for titanium body butterfly valves
Titanium valves, such as butterfly valves, are common in the offshore industry. A bearing is a valve component installed around the stem to prevent lateral movement of the stem from applied loads. The bearing of the valve could have contact with seawater. Normally, a bearing may contain two different materials, one soft such as Teflon (PTFE) as the inner part, and a metallic outer part. Although a titanium material was initially selected for the metallic part of the butterfly valves bearings in sea water service, titanium bearings with PTFE inner coatings are not currently available on the market.
Therefore, Hastelloy, which is a nickel-chromium and molybdenum alloy with high seawater resistance, is often selected as an alternative metallic material for the bearing. The challenge is to make sure that the inner PTFE has sufficient adhesion to the Hastelloy during stem rotation for opening and closing the valve. A spinning test can be performed to prove the adequate adhesion of the inner PTFE coating to the Hastelloy.
Conclusions and recommendations
Material challenges and the failures of valves due to corrosion are leading problems in the industry. In general, the most important lesson to learn is to use more corrosion resistant alloys instead of carbon and low alloy steel materials for valves in the offshore industry. There is a high risk of failure involved with using low alloy steel bolts and coated cast iron gear boxes in offshore environments. In addition, HISC is probably one of the most important corrosion challenges in subsea service for materials such as duplex, super duplex and hard nickel alloys.
1. Sotoodeh K. (2020). Requirement and calculation of corrosion allowance for piping and valves in oil and gas industry. Journal of bio and tribo corrosion 6,21. Springer. https://doi.org/10.1007/s40735-019-0319-4
2. Sotoodeh K. (2019). Application of hard facing alloys for valves. Stainless steel world magazine. Volume 31. pp 54-56
3. Sotoodeh K. (2016). Piping and valve materials for offshore use – part 1. Stainless steel world magazine. Volume 28. pp 40-44
4. Sotoodeh K. (2019). Welding a carbon steel body valve to 22Cr duplex pipe: a case study. Valve world magazine. 24(5) pp. 38-41
5. Sotoodeh K. (2018). Pipeline valves technology, material selection, welding and stress analysis (a case study of a 30” class pipeline ball valve). Journal of pressure vessel technology. ASME. Paper No. PVT-18-1043; Doi:10.1115/1.4040139.
6. American Society of Mechanical Engineers (ASME) B31.3 (2012). Process piping. NY. USA
7. Det Norske Veritas (DNV) RP-F-112. (2008). Design of duplex stainless-steel subsea equipment exposed to cathodic protection. Hovik, Norway: Det Norske Veritas.
About the author
Karan Sotoodeh works as a Valves and Actuators Senior/Lead Engineer for Baker Hughes in Høvik, Norway. He has approximately 15 years of experience in piping and valve engineering in Iran and Norway.