- Published on Tuesday, 16 April 2013 09:41
- Written by Alyssa Canales and James L. Browne
Oil wells drilled in deep waters typically face many difficulties with pressure, especially as the wells are placed on the bottom of the ocean floor. Oil and gas recovered from this depth can see pressures ranging 5,000 PSIG to 30,000 PSIG and possibly higher when flowing out of the well into the riser pipes. This is where many of the problems begin, and is the inspiration for development of valve technology that can withstand these extremely high pressures.
Offshore Process 101
Ocean bottom wells, also known as Christmas trees, are located several miles away from the production platform. In some instances, it can be as far as seven miles. Not only are these located several miles under the ocean, but one production platform may contain as many as six or more wells. Additionally, each well has a riser pipe ranging four to six inches in diameter, depending on well flows and pressures, which must flow the oil or gas back to the production platform.
As the fluids flow through the well from the deep bottom up to the surface, pressures can drop suddenly, causing the hydrates to freeze and plug the riser pipes. To prevent this, each well is equipped with several large valves that can be opened and closed by a remote operating underwater vehicle that runs back to the production platform. Methanol and/or chemical fluids are injected into the well head through high pressure pumps that exceed the well head pressure.
It is vital to the process to keep corrosive gases, asphalt, and excess hydrates from freezing the well, which would plug and prevent the oil/gas from flowing. The methanol/chemical fluids run from the production platform to the bottom of the ocean flow through an umbilical of stainless steel tubes, each possessing as many as six to nineteen tubes which operate the well valves.
Also, while drilling, the pumps must be protected from extremely high pressures and the umbilical stainless steel tubes must not exceed the burst or yield pressures of the tubing. If for any reason a tube becomes blocked, the tube pressure could experience an excursion, meaning the tubing will experience swelling before bursting. The tube line can only take a few excursions before it will inevitably burst. If the pressure does exceed the amount the valve can withstand, disaster can occur. Such was the case in the historic BP incident when the blowout preventor failed to close off flow.
It is for this reason that the speed and accuracy of opening the relief valve is essential. Also, it is paramount that the burst must be replaced as quickly as possible to prevent the flow of methanol/chemical fluids to the well head from being stopped or plugged. This is also why controlling the pressures is critical, and why the valve that controls these processes can handle extremely high pressures. It must be functioning properly to allow the methanol and chemical injection system to perform its task and keep the riser pipes from freezing.
Davy Jones Discovery
A recent discovery has found the pressures at the well head do not always depend on the depth of the waters being drilled. While you would expect deep waters to have the highest pressures, this has proved otherwise by a recent drill in the 20 ft. of water off Louisiana’s Gulf Coast. This field, nicknamed “Davy Jones”, has reached a depth of 29,000 ft. into the earth where temperatures and pressures have been said can melt and disable conventional drilling equipment.
This shallow water has created pressures exceeding the standard high of 30,000 PSIG. Pressure of this height can be ruinous to the valve operating at that juncture to inject chemicals to keep the pipes from freezing and rupturing. This is an instance where the industry norm of using a spring relief valve may not suffice.
When using a spring relief valve, as soon as the device opens, the high fluid velocity wire draws the seat and seal, causing it to leak methanol to the suction side of the pump. If the leak is bad, this reduces the flow of the methanol to the well. As stated previously, if this happens, the well could freeze the fluids, causing the well to plug. To unplug a frozen well can cost into the millions of dollars and to have one tube rupture in the umbilical multitude could cost multimillions.
One recent innovation to help address all of these issues is an angle valve that has been designed without the typical spring relief. The rupture pin valve is designed to hold a bubble-tight, closed position and is unaffected by pulsating pressure or changing ambient temperatures, which can change quite suddenly and unexpectedly with offshore drilling. The valve is capable of withstanding 30,000 PSIG and possibly higher depending on the needed pressure set point. Once the pressure reaches an exact point, the valve will open within milliseconds and the pin will buckle within an accuracy +/- 5% to set point. This accuracy is accomplished by using Euler’s Law of Compressed Columns, which states the pin diameter, length and modulus of elasticity can be used to determine the exact set point.
The valve can also be equipped with a proximity device for remote open indication to alert when the pin has reached its set pressure point and has buckled. This can be extremely beneficial when dealing with more than one well, as many of these platforms do.
The pin is located external to the flow, accomplishing limited-to-no product loss once the pin buckles, as the line does not need be opened in order to change the pin and reseat the piston. This process can be accomplished by one person in under a minute reducing downtime and improving safety.
With the dangers inherent in offshore drilling, it is fundamental to have the correct instruments in place to withstand the high pressures, especially when the high pressure may come unexpectedly. The danger of the riser pipes freezing is a problem many drillers face and one that causes loss of product, time and money. The rupture pin valve is one of the innovations which could make an enormous impact on the industry.