Also, the pressure vessel’s increasing susceptibility to cyclic loads must be addressed to assure there are design margins against fatigue failure or a fast-fracture failure in areas of localized stress. These thick-wall designs create greater difficulty in manufacturing, fabricating or handling where there are changes in material properties through the wall sections.
When equipment functional requirements go beyond the defined boundaries of existing industry specifications, problems arise such as the effects loading conditions and working environments have on the equipment and the capability to manufacture it to the highest standards.
Oilfield equipment is of complex geometry, far from a simple cylindrical vessel or piping union design. It is subjected to a variety of extreme external loading conditions and environments that are not defined in ASME codes and standards. Not only must equipment designers possess the correct experience and skillset to overcome issues, but they must rely on sound engineering practices and judgment plus unique validation prototype testing pro-grams. Due to the limits on material yield strengths for sour environments, designs rated for working pressure greater than 20,000 psi require tighter design margins.
Designing for higher pressures
Cameron embarked on meeting this most recent request for ultra-HP/HT equipment by building upon ultra-HP/HT equipment designs created and fielded for a development targeting reserves at more than 9,146 m (30,000 ft) below sea level at 25,000 psi. New critical equipment designs such as valves, chokes, tubing and casing hangers, special lockscrews, and installation tooling had to be developed and qualified in a short time frame.
Material properties represent another challenge. Temperature can change all material properties and geometries. A certain degree of variance needs to be built into temperature and pressure combinations, altering the design and requiring new validation tests for every combination.
For the request at hand, immediate identification of material, validation and qualification was crucial. While the norm would be to validate everything, it would have added years to the program, so a criticality matrix was created. Validation was performed only on certain assemblies because of where these fit within the criticality matrix.
Overcoming surface equipment obstacles
At the time of project inception in 2010, it was common belief that such a feat of engineering could not be accomplished. For one thing, there was no industry specification or guideline.
The bolting criteria played a role in leading the industry to believe that Cameron could not get past the bolting problem and past an actual cycle test of 0 C to 232 C (32 F to 450 F) at that pressure. But because the company understood preload requirements and had already designed the STS hanger, Cameron was confident the bolting issue could be resolved.
But there were still technological gaps for Cameron. The valve and choke, both critical components, did not exist. Also, the tooling required for installation did not exist. Valve geometry, wall thicknesses required, the cladding layer and risk of failure due to fatigue were critical issues.
Once valve and choke designs were completed, it was possible to build a manifold. The challenge was the sheer size of the manifold itself. Washout of the spools and the buffer chamber was of concern. In the past, buffer chambers were fabricated with pressure-containing welds, but due to the pressure, it was also realized that welding would significantly limit the structural integrity of the design. The company preferred that the design have no welds inside it. Manufacturing proved difficult because of the internal profile, size and diameters.
Sealing was also a big issue; Cameron developed a unique tubing hanger tool that could land the hanger while maintaining pressure on the control lines. It would have been easy to come up with a tool that just landed the hanger, but to lock and load and preload the hanger and also maintain pressure on the downhole safety valves posed a design challenge.
Conquering drilling requirements
Drilling solutions were built using existing components. Using Cameron’s 15,000-psi BOP ram design as the starting base point, the 25,000-psi design uses new bonnets and bodies but uses the same cavity, elastomers and rubber seals as existing design elements. All components were then constructed to with-stand loads of 25,000 psi.
The existing API proof test required the equipment to withstand 1.5 times the pressure, so the gigantic body had to withstand 37,500 psi. All inlet and outlet connections were new designs. End connections had to resist 8 MMlb of end load, and the flange on this smaller-bore ram is 16 in. thick.