Blow out Preventer (BOP) is a very important part of well control equipment and the first thing which we would like to discuss in this article is the BOP stack organization. The BOP stack can be configured in various configurations which must be suitable for the operation.
API has the recommended component codes for BOP as listed below:
A = Annular Preventer G = Rotating Head
R = single ram type preventer with one set of rams, blind or pipe.
Rd = double ram type preventer with two sets of rams, blind or pipe.
Rt = triple ram type preventer with three sets of rams, blind or pipe.
CH = high remotely operated connector attaching well head or preventers
CL = low pressure remotely operated connector attaching; the marine riser to the BOP S = spool with side outlet for choke and kill lines
M = 1000 psi
How can I know the BOP configuration and rating from the codes?
When you see the code, you need to read upwards from the bottom of BOP stack. Let’s take a look at the following example:
15M 13-5/8” – RSRRA
This BOP stacks has pressure rating of 15,000 psi with a bore size of 13-5/8” inch. There are following BOP component from bottom to top
Rams – Spool – Rams – Rams – Annular (see the figure below)
You need to keep in mind that the BOP stack is able to shut the well in and allow you to perform well control operations with the greatest flexibility. Considerations of how to arrange the BOP stack are as follows:
• The BOP stack must be suitable for the drilling operation.
• The stack should be able to serve the stripping operation not just only shut the well in.
• Pressure rating must be higher than expected surface pressure on surface when the well control situation is happened.
• Excessive BOP rams cause difficulty to handle and maintenance. Additionally, the cost of BOP stack is more expensive.
• Sour gas and temperature on surface directly affects the element in the BOP’s.
• The best BOP stack arrangement is the one that is suite for the operation within safety limit.
Blow-out Preventers Stack Arrangements:
Depending on expected pressures, a combination of one annular preventer, one or more ram-type preventers and a drilling spool can be used as wellhead control equipment. API1 recommends the use of a single designation to distinguish various BOP’s stack arrangements. The designation uses the working pressure of the stack, the through-bore of the preventers and the type of
arrangements. Thus, a stack designated as 3M-135/8
-SRA means that the related pressure is 3000 psi (1M=1000psia) , the through-bore is 135/8in and an 97arrangement of one drilling spool (S), one ram type preventer (R) and one annular preventer (A) is used. A drilling spool is normally used as a crossover spool between the BOP and the casing housing.
Typical blow-out preventer arrangements for 2 M (2000psi) to 13 M (13000psi) working pressures are given in fig.
The BOP’s Control System:
The BOP’s are designed to be closed remotely using hydraulic pressure supplied by an operating or control unit. The control unit is designed to close and open each individual BOP through a system of piping and remotely controlled valves. The control unit is normally built on a skid-mounted assembly and placed at a safe distance from the rig floor.
The main components of a control system include:
a-) an accumulator bank; b-) charging pumps;
c-) fluid reservoir;
d-) a manifold and piping for directing the fluid to the appropriate preventer.
Either the rig generators or separate power source can power the pumps. A separate power source is normally used, to allow the unit to be used even when the rig engines are shut down.
The prime function of the control system is to store energy which can be released within 30 s or less. This energy is used to close the BOP’s. The rigs air-operated pumps or any manually operated pump can be used to effect the closing 98of the preventers, but these devices are very slow-acting and used as a back-up to the main accumulator pumps.
Accumulators: The heart of the operating system is the bank of accumulators. An accumulator is a high-pressure cylinder containing pre-charged nitrogen gas and hydraulic fluid. The gas is a separated from the fluid by a rubber diaphragm or a float. The hydraulic fluid can be just water or hydraulic oil with anti-corrosion additives.
Air-operated or electrical pumps are used to force hydraulic fluid from a reservoir into the bank of accumulators until the working pressure of the system is achieved. For example, the working pressure required to close an annular preventer is 1500 psi. Entry of the hydraulic fluid into the accumulator causes the nitrogen gas to occupy a much smaller volume and in turn, have a much higher pressure. The increased gas pressure will help in releasing the fluid at a much faster rate than can be achieved by most pumps, thereby allowing the preventers to be closed quickly. The accumulator is provided with a valve on the outlet connection, which closes when the useable fluid charge is exhausted. This is required to preserve the precharge nitrogen gas.
The useable fluid charge is normally 2/3 of the total fluid charge, and is defined as the amount of fluid which can be recovered as the total pressure in the accumulator drops from the 99working pressure to 1200psi. The 1200psi represents the pressure required to hold annular preventer closed.
Charging Pumps: Charging pumps can be air- or electrically driven and are normally powered by two independent sources. An air compressor or an air storage tank can supply the air power.
A separate generator is required to provide the electric power.
Fluid Reservoir: The fluid reservoir contains the hydraulic fluid used to charge the
accumulators and acts to receive the total hydraulic fluid upon opening the preventers. The total capacity of the reservoir should at least be equal to twice the useable fluid capacity of the
accumulator system. Hydraulic oil or fresh water-soluble oils are normally used with glycol as an additive when wor
Manifold and Piping: Each preventer is provided with 2 lines, an opening and a closing line and a four-way valve. The lines are made of seamless steel with a working pressure equal to or greater than the working pressure rating of the BOP’s stack up to 5000psi.