Blowout preventers are designed to shut in a well under pressure so that formation fluids
that have moved into the wellbore can be contained and circulated out while continuous
control of the well is maintained. Control Systems for the BOPs are of necessity highly
efficient hydraulic systems. The objective is to operate functions, such as closing rams,
on the BOP stack in as short a time as possible. Supplying enough volume of pressured
hydraulic fluid to operate those emergency functions is essential. To have the necessary
quantity of control fluid under pressure requires storing this fluid in accumulators. These
accumulators operate by the expansion and compression of nitrogen gas that is separated
from hydraulic fluid by either rubber bladders or pistons.
Accumulators are used both on the surface and at the seafloor. As long as you use
accumulators on the surface or in relatively shallow waters, you may not have a problem
with the volume of hydraulic fluid capacity of gas charged accumulators. The problem
may arise when the wellhead is at water depth of more than 3500 ft. In deep water
drilling, the accumulators should be placed on the subsea blowout preventer stack to
reduce hydraulic response times and provide a hydraulic power supply in case of
interruption of surface communication. Accumulators are also used in subsea production
control systems to provide local storage that allows smaller line sizes in control
umbilicals. Hydraulic fluid capacity of an accumulator drops to 15% of its capacity on
the surface and even less, depending on the water depth. A large number of
accumulators are needed to perform BOP functions that could have been done by just a
few of them on the surface or at relatively shallow water depth.
Gas inside gas charged accumulators does not behave like an ideal gas as we go to very
deep water, due to high hydrostatic pressure at that water depth. The higher the ambient
pressure, the more the gas behaves like a real gas rather than an ideal gas and the lower
the fluid capacity of the accumulators. Compressed gas has energy in it, and can release
this energy at the time desired, thatÂs why it is used in accumulators. Now, we have to
look for something that is able to store energy, but unlike the nitrogen, its functionality
must not be affected by the increasing hydrostatic pressure of water as a function of
water depth. Springs and heavy weights will be discussed as two options to replace
nitrogen in accumulators. Efficient deep water accumulators would reduce the number of
accumulators required in deepwater and cut the cost of the project. With the advent of
such efficient accumulators, we can hope that one of the numerous problems of
deepwater drilling has been solved and we can think of drilling in even deeper waters.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/3346 |
| Date | 12 April 2006 |
| Creators | Mir Rajabi, Mehdi |
| Contributors | Juvkam-Wold, Hans C. |
| Publisher | Texas A&M University |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | Book, Thesis, Electronic Thesis, text |
| Format | 943010 bytes, electronic, application/pdf, born digital |
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