The working environment of offshore oil and gas mining equipment is very harsh. The equipment must first contact with seawater, crude oil and natural gas, and withstand the corrosion of high temperature chlorides, sulfides, ammonia and other media. The highest corrosion temperature can reach 300℃. Secondly, the equipment must withstand the working load inside the system and the impact load generated by the sea wind and waves. Thirdly, the size of deep-sea oil and gas exploration and mining equipment is very large, such as the lifting pipe length of several hundred meters, the equipment weight is very large, and the strength, toughness and fatigue resistance of materials are very high requirements. The titanium devices involved mainly include drilling riser, conical stress joint, underground operation flow pipe, pressure pipe, drilling tool lifting device, seawater piping system, cooling system, fire extinguishing system, etc. The materials used are industrial pure titanium (Grade l, Grade2), Ti-6Al-4V alloy (Grade 5, Grade23, Grade28), Ti-0.15Pd(Grade7), Ti-3A-2.5V(Grade9, Grade 18). And Ti-38644 alloy, many titanium parts are prepared with large size or large size titanium pipe.

Comparison of plane fracture toughness and stress corrosion fracture toughness of Ti-6A-4V base alloy with different alloying elements and under different forging conditions. It can be seen that the Ti-6A1-4V ELI alloy with Ru added has the best stress corrosion fracture toughness after β forging. The planar strain fracture toughness (KIC) and stress corrosion fracture toughness (KISCC) of Ti-6Al-4ⅤELI alloy forged by β forging technology are much higher than that of ordinary Ti-6A1-4V alloy. Therefore, Ti-6A1-4V ELl alloy and T-6A1-4V with Ru added have been widely used in offshore oil exploitation.

There are four main titanium components for offshore oil and natural gas operation: riser, drill pipe, conical stress joint and downhole flow pipe (coil).

1. Riser

The titanium alloy riser used in the UK North Sea oil field is Grade23, a super large high pressure pipe made of Ti-6Al-4VELI(Grade23) titanium alloy. It is made of 30 giant titanium alloy pipes with a total length of 438m and a diameter of 599mm(inside diameter)×25mm(wall thickness)× 14.6m(length) connected. See Figure 2 for the photo of the product. The riser is manufactured by RMI Inc. Replacing steel with titanium tubing reduces lift by 63% and manufacturing costs by 40%. It is suitable for production in 350m water depths.

Figure 2 Grade23 Titanium Alloy water riser

2. Drill the pipe

Titanium alloy has been successfully used in mining equipment with small curvature radius. It is Grade5(Ti-6Al-4V) titanium alloy made of ultra-deep drilling pipe, with vertical depths of up to 9200m. It is manufactured with Φ93~168mm tube. Using it instead of steel drill pipe, the lifting force of drill tool can be reduced by 30%, and the torque can be reduced by 30%~40%. In addition, since titanium alloy is non-magnetic, it does not interfere with the detection signals of oil well detectors. FIG. 3 shows the application site of titanium alloy in a production well with a small curvature radius. Table 1 shows a comparison of the effects of titanium and steel on oil drilling equipment with a radius of curvature less than 20m. It can be seen that after using titanium metal, the cycle failure life of the equipment is greatly improved, about 50~100 times that of steel. The number of titanium tubes being drilled far exceeds the number of steel tubes being drilled.

3. Conical stress joint

The titanium conical stress joints are Grade23(Ti-6A-4V ELI) and Grade29(Ti-6a-4V-Ru) titanium alloys Grade23(Ti-6A-4V Eli) and Grade29(Ti-6a-4V-Ru) titanium alloys that are currently used in Gulf of Mexico oil fields and are only 1/3 the length of the steel joints. The specifications are Φ(80~559) mm x (6.4~15.2)m, and the costs are similar.

4. Downhole operation flow pipe

Underground operation flow pipes are usually Grade2, Grade7, Grade9, Grade12, Grade18, and Grade28. The current maximum outside diameter is 63.5mm, the minimum wall thickness is 3.2mm, and the coil length can reach 1630~3050m. The target outer diameter of future coils is reportedly 152mm. When titanium alloy coil is used instead of steel coil, its coil fatigue life is 5~10 times that of steel coil. At the same time, the corrosion fatigue life of Wells containing acid or sulfur is also improved. Titanium alloy coil product photo and application diagram.