BOP Shear Rams for Hydrogen Sulfide Service
- J.R. Canal (Shaffer)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- December 1989
- Document Type
- Journal Paper
- 347 - 350
- 1989. Society of Petroleum Engineers
- 5.2.1 Phase Behavior and PVT Measurements, 4.2.3 Materials and Corrosion, 1.10 Drilling Equipment, 1.7 Pressure Management, 4.1.2 Separation and Treating, 1.6 Drilling Operations, 1.11 Drilling Fluids and Materials
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The shear rams in a blowout preventer (BOP) are critical components of the drilling system, which is expected to shear drillpipe and to seal the wellbore in all types of drilling fluids. The brittleness of high-strength tool steels when exposed to wellbore fluids containing H2S has been recognized for many years by the drilling industry. The development in recent years of high-strength materials that are resistant to hydrogen embrittlement has made possible the manufacture of H2S-resistant shear rams. This paper describes the fabrication of such shear rams. The cutting edge is made from a cobalt base (UNS R30035) and an iron-based superalloy (UNS S66286) joined by an electron-beam (EB) welding process. The discussion continues with the shearing tests of API 5A Grades E, G, and S-135 drillpipe of two wall thicknesses with a 13 5/ 8-in. [346-mm], 10,000-psi [69-MPa] working-pressure (WP) ram BOP. Introduction
The Natl. Assn. of Corrosion Engineers' (NACE's) MR-01-75 Standard covers metallic material that is resistant to sulfide stress cracking (SSC) for petroleum production and drilling equipment to be used in H2S-bearing hydrocarbon service. According to NACE, SSC may be controlled by using the metallic materials and processes described in the MR-01-75 Standard, controlling the environment, and/or isolating the components from the sour environment.
To promote efficient shearing of a tool joint, the cutting edge is made from high-strength tool steel. These materials are hardened to Rockwell hardness (HRC) 50 to 55 and will crack within minutes when exposed to H2S. The shear rams are outside the scope of the NACE MR-01-75 Standard. Therefore, users of ram BOP's must isolate the shear blades from the H2S environment to prevent SSC failures.
SSC appears to be a combination of general corrosion and hydrogen embrittlement of materials in fluids containing water and H2S. The general corrosion is caused by the reaction of the H2S with the iron-based material, which produces iron sulfide and free atomic hydrogen. This atomic hydrogen enters the steel and collects or is trapped in regions of the lattice structure that are highly stressed, drastically reducing the ductility of the metal. This is especially true for ferrous materials, which depend on the martensitic transformation to achieve their strength. The internal lattice strain must be reduced by tempering the material to hardnesses below HRC 22 to be used safely in an H2S environment. Use of nonferrous materials that derive their strength from cold work or an aging heat-treatment reaction--e.g., precipitation hardening to increase material strength--is permitted to hardness levels of HRC 33 to 35. Multiphase alloys (e.g., Co/Ni/Cr/W), which are composed of intermetallic constituents that are to be cold-worked and aged to achieve their strength, can be used in hardness levels up to HRC 48 to 50.
This paper describes the fabrication of H2S-resistant shear blades that can be exposed to wellbore fluids containing H2S and used to shear drillpipe without any hydrogen embrittlement of the material. The shear ram is made from an iron-based superalloy, UNS S66286, and the cutting edge is made from a cobalt multiphase alloy, UNS R30035. The cutting edge is inserted into the UNS S66286, and both materials are joined by an EB welding process. The EB weld joint was tested after the NACE TM-01-77 test procedure to determine the integrity of the weldment to the NACE test solution, which was loaded to 90 to 100% of the weldment yield strength.
A prototype H2S shear ram was built for a 13 5/8-in. [346-mm], 10,000-psi [69-MPa] WP ram BOP and was tested shearing 19.5-lbm/ft [29-kg/m] API 5A Grades E, G, and S-135 drillpipe and 25.6-lbm/ft [38.1-kg/m] API 5A Grade G drillpipe. A total of 27 shearing cuts was performed, and the shear edge was dressed between cuts. Fig. 1 shows a schematic of the shearing action on drillpipe with Type 72 shear rams. The top sketch shows the shear/blind rams in the open position. The center sketch shows the beginning of the shearing cycle. The bottom sketch shows the completed shearing action and wellbore pressure sealing. Note that the shearing and the wellbore-sealing actions occur in one stroke of the rams.
Shear Ram Design
The shear ram design basically consists of upper and lower ram blocks supported in a holder that transmits the shearing force to the blades from the hydraulic cylinders. The upper shear ram block forms part of the shearing blade, and the lower shear ram block supports a cutting blade. The blind ram is sealed by an elastomer located between the block and the ram block holder that seal against the ram cavity when the rains are closed. The leak path between the shearing blades is sealed by an elastomer strip that is energized after the shearing action has occurred. The upper ram block is machined to accommodate a square insert as the cutting edge. The lower shear blade is also machined to accommodate a square insert.
A 1/2 x 1/2 -in. [12.7 x 12.7-mm] square corner corresponds to the location of the cutting edge. An insert of UNS R30035 is located and joined to the upper shear block; the same is done to the lower shear blade by EB welding. Fig. 2 shows a section of the insert, which serves as the cutting edge joined to the backing material, at 4X magnification. Fig. 2 illustrates the effect of the heat-affected zone on the cold-worked and aged UNS R30035.
The EB welding process was used because it minimizes weld distortion and shrinkage and permits the joining of metals that depend on hardening by cold-working for their strength without significant deterioration of their mechanical properties at the welded joint. In EB welding, the joint to be welded is heated to the melting point of both materials by bombardment of a dense stream of high velocity electrons. The kinetic energy of the electrons is changed into heat on impact with the work piece causing the melting of both materials.
The EB weld is characterized by its long and thin weld-joint appearance (Fig. 3). The equipment power determines the depth of electron penetration into the material, which, in turn, determines the material thickness that may successfully be welded. For example, practical welding requirements for beam penetrations of 0.375 and 0.625 in. [9.53 and 15.88 mm] are 2 and 3 kW of electrical power, respectively.
The choice of material for the cutting edge is dictated by the high strength requirement of the cutting edge. The multiphase UNS R30035 alloy was chosen as the cutting edge because of its unique combination of high mechanical strength, good stress-corrosion resistance, and low-temperature properties. This multiphase alloy is a simple quaternary alloy system, consisting of 35% Ni, 35% Co, 20% Cr, and 10% Mo, that can be strengthened to 260,000-psi [17.9-GPa] tensile strength with good ductility. The high strength can be achieved by combining a phase transformation induced by cold-working and an aging reaction.
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