In the offshore sector of the oil and gas industry, material corrosion is a phenomenon with significant risks and high costs. The presence of chlorides in the air in the offshore environment can cause severe corrosion of the exterior of the facility. Moreover, oil and natural gas contain harmful substances, such as carbon dioxide and hydrogen sulfide, which can corrode pipeline components internally.
In addition to the risk of corrosion, other parameters (such as erosion, mechanical strength, material availability and weight) also affect the choice of materials for offshore installations. “Facilities” mainly refer to equipment and piping components. The offshore sector of the oil and gas industry is dedicated to solving the development of “coastal” oil and gas fields under the ocean. Among the material selection difficulties to be discussed, corrosion is the main problem
—Ferguson Lu, Chief Engineer, Shanghai HY Industry Co., Ltd
Stellite alloy overlaid on 22Cr duplex steel for Offshore valve Alloy
Stellite 6 is one of the most commonly used hard-faced alloys in the valve industry due to its excellent toughness, hardness and wear resistance. Stellite 6 is a cobalt alloy that contains other compounds such as chromium, tungsten and carbon. Cemented carbide covers the inside of the valve to prevent corrosion, wear and abrasion. In fact, the internal parts of the valve are located in the water stream and are therefore more susceptible to corrosion, friction and wear than the external parts.
Due to its high mechanical strength and corrosion resistance, about 60% to 70% of the pipelines and valves used in offshore operations are 22Cr duplex steel. It is feasible to use stellite alloy on 22Cr dual-phase steel, but the sigma phase formed during the welding of dual-phase steel makes it difficult to change. Sigma phase is a hard and brittle crystal structure formed at high temperature, which can cause cracking and destruction of duplex stainless steel. The alloying elements in dual-phase alloys (such as chromium and molybdenum) promote the formation of the sigma phase, which increases the hardness and reduces the toughness of the material.
Welding large carbon steel valves to 22Cr duplex steel pipelines
Generally, the pipeline valve located on the oil pipeline is the largest, heaviest and most important valve. They also usually have the longest delivery time for offshore platforms. Material of these valves is carbon steel, not corrosion-resistant alloy (CRA), because these valves deal with non-corrosive and well-treated oil. It should be noted that it is not economical to choose 22Cr duplex stainless steel for such a large valve. However, the pipeline connected to the valve will be made of 22Cr duplex steel material instead of carbon steel to save wall thickness and weight. Because dual-phase steel has relatively high mechanical strength, and does not need to have a corrosion margin.
There are two difficulties in welding the pipe to the valve. The first is the difference in thickness between the valve and the pipe. The second is the difficulty of welding two different materials together. If the wall thickness of the valve is more than twice the thickness of the pipe, the transition piece should be welded to the valve from one side, and welded to the transition piece from the other side. The wall thickness of the transition piece should be equal to the thickness of the pipe. Welded to one end of the pipe, the wall thickness is equal to the thickness of the valve from the end welded to the valve.
When the thickness is different, the main difficulty in welding carbon steel to 22Cr dual-phase steel is that the thickness of the carbon steel is greater than 19mm. This means that according to ASME B31.3 process piping code requirements, post-weld heat treatment (PWHT) is required after welding to eliminate residual stress. However, the application of PWHT will increase the risk of sigma embrittlement in dual-phase steels. The solution is to coat the carbon steel with Inconel 625 or Alloy 59 alloy and apply PWHT only to the carbon steel part. Afterwards, the coated carbon steel is coated with Inconel 625 or Alloy 59 filler into the duplex steel.
What is a small pipe fitting?
A small pipe is a section of pipe that is welded to the valve body before the valve manufacturer assembles the valve. Small pipe fittings will be welded to the connecting pipes on the construction site. If the pipes in the yard are welded directly to the valve without any welds, the welding heat will melt and break the seal inside the valve.
Hard surface corrosion of Stellite alloy and tungsten carbide in seawater
Stellite alloy and tungsten carbide are common hard surface materials used inside valves to reduce corrosion and wear. One of the main limitations of these two materials is that they can be corroded in seawater. In seawater, Ultimet of cobalt alloy has good wear resistance. Therefore, in special cases, Ultimet is applied to hard surfaces because not all valve manufacturers are familiar with this material. Ultimet’s lack of adhesion to the base (core) material may cause Ultimet to wear out after years of use.
Hydrogen-induced stress cracking (HISC) corrosion in Offshore valve Alloy
In the subsea industry of the oil and gas industry, materials such as low alloy steel, duplex steel and super duplex steel, as well as nickel cemented carbide (such as Inconel 718 and Alloy 72 ) are prone to HISC.
The main reason for the failure of HISC is due to the combined effect of stress from the load (for example, tension in the pipeline) and the entry of hydrogen formed on the steel surface due to cathodic protection. HISC analysis should analyze susceptible materials through the following methods: Valve manufacturers to reduce the risk of HISC.
Titanium butterfly valves use titanium bearings
Titanium alloy valves, such as butterfly valves, are common in the offshore industry. Bearings are valve components installed around the valve stem to prevent the valve stem from moving laterally due to the applied load. The bearing of the valve may come into contact with sea water. Generally, the bearing may contain two different materials, one is a softer material inside, such as Teflon (PTFE), and the inside is metal. Although titanium was initially selected for the metal part of butterfly valve bearings in seawater, there is currently no titanium bearing with PTFE inner coating on the market.
Therefore, Hastelloy, a nickel-chromium-molybdenum alloy with high seawater resistance, is often selected as an alternative metal material for bearings. The difficulty is to ensure that the internal PTFE has sufficient adhesion to Hastelloy during the rotation of the valve stem to open and close the valve. Rotation test can be performed to prove that the internal PTFE coating has sufficient adhesion to Hastelloy.
Conclusions and recommendations for offshore valve Alloy
Material selection and valve failure due to corrosion are major issues in the industry. Generally, the most important thing to learn is to use more corrosion-resistant alloys instead of carbon and low-alloy steel materials in valves in the offshore industry. The use of low-alloy steel bolts and coated cast iron gearboxes in offshore environments poses a high risk of failure. In addition, HISC may be one of the most serious corrosion challenges faced by materials such as duplex steel, super duplex steel and hard nickel alloys in subsea services.
Shanghai HY Industry Co., Ltd is qualified Titaniumalloy，Nickelalloy&cobalt alloy materials supplier.
We have more than twenty years experience in kind of High temperature alloy production and have ten years experience in alloy powder. Specialize in surface Protection Technology.
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