Patent Application
20230035783
This invention relates to the method of providing a 20,000 p.s.i. blowout preventer stack by using a shared pressure differential on components which cannot individually be rated to 20,000 p.s.i.
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Deepwater offshore drilling requires that a vessel at the surface be connected through a drilling riser and a large blowout preventer stack to the seafloor wellhead. The seafloor wellhead is the structural anchor piece into the seabed and the basic support for the casing strings which are placed in the well bore as long tubular pressure vessels. During the process of drilling the well, the blowout preventer stack on the top of the subsea wellhead provides the second level of pressure control for the well. The first level being provided by the weighted drilling mud within the bore.
During the drilling process, weighted drilling mud circulates down a string of drill pipe to the drilling bit at the bottom of the hole and back up the annular area between the outside diameter of the drill pipe and the inside diameter of the drilled hole or the casing, depending on the depth.
Coming back up above the blowout preventer stack, the drilling mud will continue to travel back outside the drill pipe and inside the drilling riser, which is much large than the casing. The drilling riser has to be large enough to pass the casing strings run into the well, as well as the casing hangers which will suspend the casing strings. The bore in a contemporary riser will be at least twenty inches in diameter. It additionally has to be pressure competent to handle the pressure of the weighed mud, but does not have the same pressure requirement as the blowout preventer stack itself.
As wells are drilled into progressively deeper and deeper formations, the subsurface pressure and therefore the pressure which the blowout preventer stack must be able to withstand becomes greater and greater. This is the same for drilling on the surface of the land and subsea drilling on the surface of the seafloor. Early subsea blowout preventer stacks were of a 5,000 p.s.i. working pressure, and over time these evolved to 10,000 and 15,000 p.s.i. working pressure. As the working pressure of components becomes higher, the pressure holding components naturally become both heavier and taller. Additionally, in the higher pressure situations, redundant components have been added, again adding to the height. The 15,000 blowout preventer stacks have become in the range of 800,000 lbs. and 80 feet tall. This provides enormous complications on the ability to handle the equipment as well as the loadings on the seafloor wellhead. In addition to the direct weight load on the subsea wellheads, side angle loadings from the drilling riser when the surface vessel drifts off the well centerline are an enormous addition to the stresses on both the subsea wellhead and the seafloor formations.
When the blowout preventer stack working pressure is increased to 20,000 p.s.i. some estimates of the load is that it increases from 800,000 to 1,200,000 lbs. The height also increases, but how much is unclear at this time but it will likely approach 100 feet in height.
A second complication is that a 20,000 p.s.i. working pressure requires a 30,000 p.s.i. test pressure. As the actual stresses in material is greater than the bore pressure, the differential between the actual stress level and the yield strength of the material becomes much narrower. Imagine for a 15,000 p.s.i. component the maximum stress is 32,000 p.s.i. at working pressure and 48,000 p.s.i. at the 22,500 p.s.i. required test pressure. If the best reasonably available material has a 75,000 p.s.i. yield strength at that point you are working with a 1.56/1 factor. If you simply increase the working pressure to 20,000 p.s.i. with a 30,000 p.s.i. test pressure, the stress at test pressure goes to 72,000 p.s.i. which has barely a 1.04/1 safety factor. With the complications of stress analysis, even doubling the weight of the components will not get the stress levels back down to a reasonable level.
Another complication is that the annular style blowout preventer which have the ability to seal on anything in the bore have been characteristically pressure limited, with 10,000 p.s.i. being the highest presently achieved working pressure rating. The large mass of rubber in the donut around the pipe simply fails at that point.
This has been a problem especially since the working pressure of blowout preventers have exceeded 10,000 p.s.i. as the 15,000 and 20,000 p.s.i. differentials across the sealing elements has not been sustainable. The standard industry solution to this point is to accept the inability of annular blowout preventers to seal at this high pressure and to solely depend on the capabilities and limitations of ram blowout preventers when the pressure differential exceeded 10,000 p.s.i.
The object of this invention is to give the capability of 15,000 and 20,000 p.s.i blowout preventer stacks to be fully related to 15,000 and 20,000 p.s.i. respectively.
A second object of this invention is to vent the pressure on one blowout preventer above its rating to a downstream blowout preventer.
A third object of this invention is to make the pressure differential between two blowout preventer rams in series adjustable.
Another object of this invention is to make the differential seen by two blowout preventers in series equal.
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Blowout preventer stack 60 is landed on a subsea wellhead system 64 landed on the seafloor 66. The blowout preventer stack 60 includes pressurized accumulators 68, kill valves 70, choke valves 72, choke and kill lines 74, choke and kill connectors 76, choke and kill flex means 78, and control pods 80.
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As 10,000 p.s.i. differential across the lower sealing element 208 puts very high stresses in the resilient materials, it may be preferable to distribute the pressure differential differently that a full 10,000 p.s.i. across the lower sealing element before beginning to load the upper sealing element. Relief valve 214 can be remotely controlled as is illustrated by line 220 going to controller 222 in a different pattern such as the differential being evenly divided between the sealing elements such that at a 10,000 p.s.i. total differential each of the sealing elements withstand is the stress of a 5,000 p.s.i. differential.
The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.
