High-Pressure Equipment

It is not possible to control a well without equipment that can handle the potentially high pressures experienced in a well control situation. The BOPE is designed for different pressure ratings, and appropriate well planning include planning for worst case scenarios that insure the proper equipment for the operation.

Casing, Well Heads and Spools

Interestingly, casing is not commonly a topic included in well control equipment discussions. Watson et al. bring up the last casing string set in the well as the most important piece of well control equipment.³ This is very logical, as a casing rupture would render all the other BOPE obsolete. The casing design of a well needs to be based on worst case scenarios for burst and collapse. A proper casing design should ensure the correct pressure rated casing for the well, leading to the kick being guided through the correct channels for a kill procedure. Like other BOPE, the casing should be tested for wear and pressure.

In order for BOPE to be attached to a well, there needs to be a wellhead on the casing.

The wellhead serves as the supportive structure on which to attach the “BOP stack, tubing head and Christmas tree.”²⁹ It is the foundation for the surface equipment and thus needs to be sturdy enough to support this and still hold pressures to their working pressure rating. When strings are hung after having attached the wellhead to the surface casing, additional casing spools are added to support these. The spools serve as a way of sealing the different strings from each other, and are often used to add flexibility with regard to circulation through choke or kill lines, or for added spacing for stripping operations.²⁹ The spools need to be able to resist similar pressure to those of the BOPs.

Annular Preventers

At the top of the BOP stack we normally find an annular type blowout preventer.

Annular preventers are often shut-in first, due to their “versatility and position in the stack.”³ As with the other components in a BOP stack, annular preventers are designed to seal the off the well to the surroundings. Specifically, it is designed to seal the annular space of the well, and in certain cases, the open hole.

Annulars come in different varieties, but are generally designed to seal off the well through the use of a “circular rubber packer element, a piston, a body and a head (cap).”²⁹ The packer is closed by the hydraulic force applied by fluid to the closing chamber, provided by the pressure from the accumulator system. The use of this flexible rubber packer allows the annular preventer to seal against many different shapes and sizes of tools in the well, thus leading it to be very versatile compared to the other BOP stack components. The annular is unique in another sense, in that it is possible to move or strip the pipe in and out of the hole while closed. Examples of annular preventers are shown in Fig. 3.5.

Fig. 3.5 – Two types of annular preventers.²⁹

Ram Preventers

In a normal stack configuration, you will find ram preventers below the annular. This type of BOP does not have the versatility of the annular, but is specifically designed to close around, over, or through the pipe in the hole. There are different kinds of ram preventers.

Pipe rams are designed to close around the pipe in the hole, thus sealing off the annulus, to serve as a back-up for the annular.³ They work on a similar principle, where a closing chamber is pumped full of hydraulic fluid, forcing a set of opposing rams to be forced toward the pipe to seal around it. Similarly, they are opened with hydraulic fluid being pumped to the opening chamber, which brings the pistons and rams out to the retracted position. Fig. 3.6 illustrates one model of pipe rams. Another version of the pipe ram is the variable bore ram, which has the ability to close on different size pipes. These are particularly useful when a tapered string is in the hole.

Another type of ram preventer is the blind ram. Working in the same way, they differ in that the sealing element aims to seal on an open hole. The opposing rams are flat, and seal against each other with the pressure of the hydraulic fluid. Another ram that can close on an open hole is the blind/shear ram. This ram works like the blind ram when there is no pipe in the hole, but has cutting ability to shear off the pipe and seal the cut if necessary. This type of ram is mandatory for subsea BOP stacks.³ Shear rams only have the shear capacity, and are not as versatile as the blind/shear ram.

Fig. 3.6 – Example of pipe ram preventer.²⁹

The components of a BOP stack can be organized in many different ways. These often depend on company policy and preference, and the different configurations have advantages and disadvantages. Depending on the pressure rating needed for the BOPE, components are added or omitted. One example of a BOP stack is show in Fig. 3.7, which represents the company policy of Saudi Aramco for operation in the Kingdom of Saudi Arabia.²⁶

Fig. 3.7 – Example of BOP stack configuration from Saudi Aramco.²⁶

Kill and Chokeline Equipment

When dealing with a well control event, the chokeline acts as the conduit for circulating out the kick while remaining in control of the backpressure kept on the well. This line will bring the fluids safely to the surface, and direct them away from the rig and

personnel through the choke manifold. Similarly, the kill line is used to pump fluids into the well in a kick situation, if normal kick handling methods are not possible. Both lines serve as a entry and exit points for the fluid in well control events, and both lines need to be rated to the same pressures as the BOP stack.³ Other high-pressure components like valves and chokes also have to be of sufficient pressure rating, and should be able to withstand the erosional wear of the high velocity fluids. These are used to direct the fluid flow, and control the velocity at which it flows.