The main control unit is called an accumulator. It controls all systems that interconnect to prevent emergency situations. The system activates based on hydraulic pressure, and the typical accumulator houses pumps, a hydraulic reservoir, a control manifold, control valves, compressed gas bottles, and several other facets.
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Often, an accumulator holds enough power to allow for BOP units and their backup functions to run when other systems fail, so it is mounted on top of the BOP stack itself.
An annular BOP is what closes the space around the diameter of the drill pipe, but since the actual system cannot be closed with a value due to the drill string, this controls the air flow for the mud without cutting off any other systematic elements.
The annular BOP sits at the top of the BOP stack, to provide the most efficient airtight option. A RAM BOP also works to prevent any unwanted materials from exploding in the system, but several are needed in the BOP stack to ensure optimum safety.
All the parts of a RAM BOP (and sometimes an annular BOP component) make up a BOP stack. Types can include:
RAM valves can be stacked a number of different ways, for multiple types of efficiencies. Some BOP systems can reach massive heights, especially in deep-water drilling where a BOP system has a completely different purpose for successful extraction.
BOPs and pressure control equipment are typically checked to verify meeting OEM and API specifications, including specification 16A for drill-through equipment.
Many preventers are also certified under DNV, ABS (American Bureau of Shipping), and Lloyd’s of London.
I do not want to pretend to be a subject matter expert and I am open to criticism and corrections but thought it is an interesting topic to write about.
A lot of students from my alma matter have been asking me about the different configurations of the BOP. One student asked me about the different configurations of the BOP’s and why some floaters have five, six, and even seven cavities in the rams.
To start off let us refer to some standards
API STD 53 is the go-to standard that looks at well control systems requirements for Land Rigs, Jack-ups, Floaters in terms of BOP and controls configuration. This is also referred to by many countries' specific requirements such as BSEE, NPD, etc.
BSEE Well Control Rule is a Regulatory requirement for BOP and control systems required to meet in order for Rigs to drill in the US GoM.
Referring API STD 53 (2018), Point 6.1.3.8
Subsea BOP stacks shall be Class 5 or greater and consist of the following:
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a) A minimum of one annular preventer;
b) A minimum of two pipe rams (excluding the test rams);
c) A minimum of two sets of shear rams for shearing the drill pipe and tubing in use, of which at least one shall be capable of sealing.
To simplify further, API states that you require a minimum of Four cavities. (There are exceptions to the above rule but in principle to maintain a class 5 stack you need to have Four sets of ram cavities and one Annular)
So why two Annular BOPs? Well this is something which requires further elaboration
The biggest advantage of the Annular BOP is the ability to allow to strip which is very useful in well control situations. Though the Variable Rams were designed to accommodate the Tool joints these are not designed for stripping which brings the discussion back to Annular.
This is in conjunction with the experience of the field engineers who as a way of single-point failure led to the reason of having 2 Annular in the BOP these days.
Going back to the reason why we are seeing Five, Six, and even Seven ram cavity BOP’s, There are specific instances where the operation demands the need for shearing the Casing then we need to include this as a function and the discussion began on Point 6.1.3.8 where the requirement was for two shear rams out of which only one needs to seal. Different engineers have different interpretations and there is to date no unified conclusion.
To elaborate, there were a set of engineers which did not see the use of adding another cavity as they interpreted that the casing shear could serve as a shear only ram which is able to shear casing as well as Drill pipe and another set of engineers who looked at the intent of the said API standard and concluded the requirement for adding another cavity as casing shear function independent of the two Drillpipe shear rams.
Taking an example, Refer to BSEE Well Control Regulations 250.734(a)(1)(ii)
Both shear rams must be capable of shearing at any point along the tubular body of any drill pipe (excluding tool joints, bottom-hole tools, and bottom hole assemblies such as heavy-weight pipe or collars), workstring, tubing provided that the capability to shear tubing with exterior control lines is not required prior to April 30, 2018, appropriate area for the liner or casing landing string, shear sub on subsea test tree, and any electric-, wire-, slick-line in the hole no later than April 30, 2018; under MASP. At least one shear ram must be capable of sealing the wellbore after shearing under MASP conditions as defined for the operation. Any non-sealing shear ram(s) must be installed below a sealing shear ram(s).
OEMs have designed Rams to shear Cables (e.g. CDVS-II rams from Cameron) which are designed to shear pipe and cable. However, these rams cannot shear casing hence the need for a separate casing shear functions hence necessitate requirement for two cavities dedicated for Blind shear rams which handle Pipe and Cable and one cavity for casing shears.
Another reason for an additional cavity is for the purpose of testing Deepwater BOP. From 2011-2014 when the oil prices were high and so were the Rig rates, there was an increased push for saving rig time which necessitated the need for making rigs efficient, One such case was the use of test rams. Test Rams are nothing but pipe rams that run upside down which is meant to seal downwards instead of upwards. This allowed the rig to test the BOP without running a test tool to the wellhead. The reason why this was important was the running time involved running a test tool at 12000 ft water depth and retrieving it back as well. This time lost was costing the operator a lot of money and necessitated the requirement for test rams which at the time were practical as there was a lot of rig time saved by using a test Ram.
Operators do have a requirement for seven cavity ram BOP which are more common in the Gulf of Mexico than the rest of the world however the rest of the world runs on either a Four/Five ram approach for midwater moored floaters or a Five/Six ram approach for DP Floaters depending on the water depth.
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