I’ll be the first to say it.I’ve been watching closely( since I’m laid up in bed with 2 crushed heels from a fall). This cap is not working. As a fabricator it looks pretty lame to me. I don’t know who is doing the engineering here but IMHO there needs to be a change.
The top of the cut above the flange is very rough, and there is no way to clamp the fixture to the flange, so how can a proper seal be attained at 13,000psi.
I don’t understand why they didn’t just cut the bolts and remove the top half of the flange.
From the live thread posted by Repeal The 17th Who brought it to our attention from elsewhere.
A perfect seal in not desired. The leakage around the bottom perimeter keeps sea water out
The live tread........
http://www.freerepublic.com/focus/news/2524815/posts?page=4491#4459
An essay on the cap design
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And then, just one more:
“It is obvious that most people do not understand the basics of how the top hat is supposed to operate. And BP, per usual, has not thought it necessary to explain anything.
The Top Hat Seal
For a number of reasons the top hat seal is NOT a pressure seal. It is designed to try to keep seawater out, not to keep oil in.
Let me repeat - The top hat seal is NOT a pressure seal. It is designed to try to keep seawater out, not to keep oil in.
Any water that can get in at the bottom of the top hat will form methane hydrates and probably block up the pipe. If that happens as they are beginning to start a slow flow it just means another setback.
But it is much more likely to happen when there is substantial flow going up the line and they are starting to “pull suction”. At that point there is a high flow rate and the “water hammer” effect of suddenly stopping a mile long slug of oil and gas could easily start tearing the equipment apart, probably at the top hat or onboard the ship so it becomes a safety issue not just another failure.
The top hat is not designed to take any significant pressure, certainly not the pressure that could result from sealing to the BOP so that pressure must be able to escape - through the seal area. Even 1,000 psi would blow the top hat apart and there is potentially about 9,000 to 13,000 psi at the BOP
And at this point I think they are scared enough of the integrity of the well head and BOP connection that they don’t want to have any pressure build up which would happen if you sealed the top hat to the flange.
There are other safety issues that are solved by not having a solid seal.
1. The rig must be able to shut off the flow on deck at any time and the resultant flow has to go somewhere - which is out the top hat seal
2. The rig must be able to pull away from the well at any time in an emergency and just raising the top hat off the BOP solves this problem.
Flow to the Surface
The flow to the surface is through a drill pipe from the top of the top hat. The drill pipe should be able to flow between 20,000 to 30,000 bpd or more if it was 100% oil – NO PUMPS NEEDED. With gas in the flow the amount of potential flow is even greater. In any case the processing system on the ship cannot handle as much as the pipe can transport.
The oil is about 0.85 specific gravity. If the drill pipe was filled just with oil the buoyancy of the oil will raise the pressure at the surface to close to 400 psi. If filled with gas the pressure would be about 2,000 psi.
The problem won’t be to get the oil to flow but to keep it from flowing too fast. They will throttle (choke) the flow back to get the volume they are comfortable with and then pipe the oil and gas, still under some pressure, into a separator vessel where the pressure will be reduced and the gas will go to the flare to be burned and the oil will go into a storage tank.
The product flowing from the bottom will be a mixture of oil (with dissolved gas), NGLs and super-critical methane gas. Hopefully there will be no water as that can really mess things up. During its journey to the surface, and through the processing system there will be a number of changes as gas dissolves out of the oil, the methane goes from super-critical to gas, some of the NGL will turn to gas and all the gas will eventually expands about 150 times before it hits the flare.
The optimum flow at any time will have to be determined by trial and error on the rig. If they were to open it up quickly they might get lucky and obtain a stable flow quickly. The downside of trying to do it quickly is that you could suck in water setting back the whole process for hours or days or worst case end up with an uncontrolled flow on the ship resulting explosion and fire with fatalities and another disaster.
So the fact that it could take a period of days to reach maximum flow is no surprise.
The oil gas ratio in the flow from the well will probably keep varying all the time and coupled with the phase changes and gas expansion will be a continuing problem for the processing crew on the rig. I expect that is the reason we saw daily changes in the amount of oil recovered by the RITT.”
They aren’t going for a positive seal.