Contingency measures for subsea drilling operations

Article published in the Regulator | Issue 4: 2018

Preventing the escape of petroleum is paramount and continues to be of the upmost importance for the oil and gas industry and NOPSEMA. Following well blowout incidents such as Macondo, community and stakeholder interest in offshore petroleum activities, incident prevention and contingency measures for drilling operations has increased including discussion and debate regarding the use of blowout preventers and capping stacks. The following information seeks to clarify the differences between the role of blowout preventers and capping stacks.

What is a blowout?

A blowout is a very rare event that occurs when all of the means to control the well have failed resulting in an uncontrolled and unintended release of hydrocarbons from the well.

How is this pressure controlled?

When drilling for oil and gas the wellbore opens up the naturally confined spaces where the hydrocarbons are located and creates a potential pathway for them to migrate to the surface. Maintaining control of the well is an integral part of all well drilling operations. The principle means of maintaining control of well pressure is to take advantage of the hydrostatic pressure provided by the drilling mud. Drilling mud is a mixture of fluids and solids which is used downhole in the drilling process. The weight and friction of the drilling mud in the well provides equalising pressure in the well as drilling proceeds. Where there is a balance between the hydrostatic pressure of the drilling mud and the formation pressure in the surrounding rocks sand and shale, no oil and gas will be able to enter the well. If, however, the well’s hydrostatic pressure falls below the formation pressure a ‘kick’ can occur; this is when formation fluids (including gas, oil, and water) enter the well. This initial influx, if allowed to escalate, would then result in a blowout if it were not for the blowout preventer.

What is a blowout preventer?

A blowout preventer (BOP) is an assembly of specialised safety valves put in place in the early stages of drilling (before drilling into any reservoir zones) to be used in the case of an emergency to control well pressure and prevent a blowout. It is installed between the wellhead system and the drill floor. On floating offshore drill rigs the BOP is placed on the well at the sea bed. Once installed, all further drilling is conducted through the BOP. In an emergency the BOP can be operated by a variety of means including through the use of a remotely operated vehicle (ROV). When activated, the powerful hydraulic ram and annular preventers inside the BOP can cut through drill pipe, seal, control and monitor oil and gas wells to prevent blowouts. There can be between six to eight individual blow out preventers installed in a single assembly to make up the BOP. .Rapidcap has been reproduced with permission from Halliburton Boots and Coots. BOPs were first developed for oil and gas drilling in the 1920s and have been subject to continuous improvement ever since; this was accelerated following the Macondo incident in the Gulf of Mexico in 2010. Since BOPs are critically important to the safety of the crew, the rig and the well itself there are now more stringent industry standards as well as regulatory oversight for the operation and maintenance of BOPs.

Can a blowout preventer fail?

While failure of BOPs is very rare the blowout of the Deepwater Horizon drilling rig in an example of where a BOP was installed and failed to contain the escape of petroleum. Since this incident the design and use of BOPs has received increased oversight from industry and regulators around the world. There are now more stringent industry standards for design operations and maintenance of BOPs, as well as greater regulatory oversight of equipment and processes. Advancements in testing and maintenance of BOPs have made drilling for oil and gas safer. International standards for BOPs require that in the event one component of the BOP fails, the failure will not affect the overall ability of the BOP to shut in a well. Subsea BOPs have a system that, when armed, automatically ‘shuts-in’ the well in the event of a total loss of hydraulic supply. These are some examples of the updated measures that increase the reliability of BOPs for offshore drilling activities.

The potential use of capping stacks if the BOP fails

In the unlikely event that the BOP fails to prevent a blowout, a further response option available for drilling operations is the use of a capping stack. Where the BOP is located on the seafloor and is in sufficiently deep water, a subsea capping stack can be lowered onto the BOP or wellhead. Once the capping stack is firmly installed over the existing BOP or wellhead, the flow of hydrocarbons will be diverted through the capping stack and valves will be slowly shut in order to contain the flow of oil. In the best case, the capping stack can close off or ‘shut in’ the well entirely, to stop the release of oil while a relief well is drilled at a safe distance to intercept and ‘kill’ the well from below as was the case during the Macondo incident. Capping stack systems may also be used to reduce and potentially redirect flow.

What is NOPSEMA’s role?

NOPSEMA has functions and powers conferred on it under the Offshore Petroleum and Greenhouse Gas Storage Act 2006 (OPGGS Act) and relevant regulations and provides regulatory oversight on safety, well integrity and environmental management matters. Prior to any drilling activity occurring in Australian Commonwealth waters, oil and gas companies must submit a safety case for the facility, a well operations management plan and an environment plan to NOPSEMA for approval.

What is the difference between a subsea blowout preventer and a capping stack?


• Both blowout preventers (BOPs) and capping stacks contain valve sets that can ‘shut in’ a well.

• Both BOPs and capping stacks can be hydraulically powered and operated remotely for example by a remotely operated underwater vehicle

• Both BOPs and capping stacks can be used offshore and onshore.


• BOP’s are very large pieces of equipment and can be up to five storeys high and weigh up to 400 tonnes. Capping stacks are smaller with less valves allowing for easier transport and can range from 80-180 tonnes.

• BOPs are used to prevent blowouts; they are to be closed when formation fluids begin to flow into the well. BOPs can cut through pipe. Capping stacks are used to respond to flowing blowouts; they are not used to prevent them. Capping stacks cannot cut through pipe but can be operated on flowing wells.

• A BOP is a standard piece of drilling safety equipment that is used on all wells. Every offshore drilling rig carries one or more on board when it is deployed to start a new drilling operation. The term capping stack is used to describe additional blow out control capabilities contained within a highly specialised piece of emergency response equipment that is only brought to site once a blowout has occurred. There are 15-20 capping stacks available worldwide strategically positioned to service the oil and gas industry.

• Subsea capping stacks cannot be used in shallow waters or on fixed platforms where the wellhead is at or above the sea surface because there is no physical way of safely manoeuvring the capping stack onto the wellhead.

• Capping stacks can be partially closed, for example, if the blowout pressure is too high for surviving well infrastructure or surrounding formation geology.

• Capping stacks can be used to divert the flow of oil rather than simply shutting the flow of oil off. This requires associated processing and safety equipment to receive, process, and store recovered oil.