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Decommissioning involves the safe plugging of the hole in the earth’s surface and disposal of the equipment used in offshore oil production. Decommissioning is a rapidly developing market sector in the petroleum business, with major potential and major risks. It is a source of major liability for counties, operators, contractors and the public and it must be understood if it is to be managed cost effectively.
Process of decommissioning
Offshore decommissioning involves 10 steps: project management, engineering, and planning; permitting and regulatory compliance; platform preparation; well plugging and abandonment; conductor removal; mobilization and demobilization of derrick barges; platform removal; pipeline and power cable decommissioning; materials disposal; and site clearance. Each step is discussed below.
Project management, engineering, and planning for decommissioning an offshore rig generally starts three years before a well runs dry. The process involves:
- Review of contractual obligations
- Engineering analysis
- Operational planning
Because derrick barges are limited in number, many operators contract these vessels two to three years in advance. Additionally, much of the decommissioning process requires special contractors who focus on a specific part of the process. Most operators will contract out project management, cutting, civil engineering, and diving services.
Permitting and regulatory compliance
Obtaining permits to decommission an offshore rig can take up to three years to complete. Operators often contract local consulting firms to ensure that all permits are in order prior to decommissioning because those firms are familiar with the regulatory framework of their regions.
To prepare a platform for decommissioning, tanks, processing equipment and piping must be flushed and cleaned and residual hydrocarbons have to be disposed of; platform equipment has to be removed, which includes cutting pipe and cables between deck modules, separating the modules, installing pad eyes to lift the modules; and reinforcing the structure. Underwater, workers prepare the jacket facilities for removal, which includes removing marine growth.
Well plugging and abandonment
Plugging and abandonment is one of the major costs of a decommissioning project and can be broken into two phases.
The planning phase of well plugging includes:
- Data collection
- Preliminary inspection
- Selection of abandonment methods
- Submittal of an application for BOEMRE approval
In the GOM, the rig-less method, which was developed in the 1980s, is primarily used for plugging and abandonment jobs. The rig-less method uses a load spreader on top of a conductor, which provides a base to launch tools, equipment and plugs downhole. Well abandonment involves:
- Well entry preparations
- Use of a slick line unit
- Filling the well with fluid
- Removal of downhole equipment
- Cleaning out the wellbore
- Plugging open-hole and perforated intervals(s) at the bottom of the well
- Plugging casing stubs
- Plugging of annular space
- Placement of a surface plug
- Placement of fluid between plugs
Plugs must be tagged to ensure proper placement or pressure-tested to verify integrity.
According to BOEMRE, all platform components including conductor casings must be removed to at least 15 ft below the ocean floor or to a depth approved by the Regional Supervisor based upon the type of structure or ocean-bottom conditions. To remove conductor casing, operators can chose one of three procedures:
- Severing, which requires the use of explosive, mechanical or abrasive cutting
- Pulling/sectioning, which uses the casing jacks to raise the conductors that are unscrewed or cut into 40 ft-long segments.
- Offloading, which utilizes a rental crane to lay down each conductor casing segment in a platform staging area, offloading sections to a boat, and offloading at a port. The conductors are then transported to an onshore disposal site.
Mobilization/demobilization and platform removal
Mobilization and demobilization of derrick barges is a key component in platform removal. According to BOEMRE, platforms, templates and pilings must be removed to at least 15 ft below the mudlline.
First, the topsides are taken apart and lifted onto the derrick barge. Topsides can be removed all in one piece, in groups of modules, reverse order of installation, or in small pieces.
If removing topsides in one piece, the derrick barge must have sufficient lifting capacity. This option is best used for small platforms. Also keep in mind the size and the crane capacity at the offloading site. If the offloading site can't accommodate the platform in one piece, then a different removal option is required.
Removing combined modules requires fewer lifts, thus is a time-saving option. However, the modules must be in the right position and have a combined weight under the crane and derrick barge capacity. Dismantling the topsides in reverse order in which they were installed, whether installed as modules or as individual structural components, is another removal option and the most common.
Topside can also be cut into small pieces and removed with platform cranes, temporary deck mounted cranes or other small (less expensive) cranes. However, this method takes the most time to complete the job, so any cost savings incurred using a smaller derrick barge will likely be offset by the day rate.
Removing the jacket is the second step in the demolition process and the most costly. First, divers using explosives, mechanical means, torches or abrasive technology make the bottom cuts on the piles 15 ft below the mudline. Then the jacket is removed either in small pieces or as a single lift. A single lift is possible only for small structures in less than 200 ft of water. Heavy lifting equipment is required for the jacket removal as well, but a derrick barge is not necessary. Less expensive support equipment can do the job. Pipeline and power cable decommissioning Pipelines or power cables may be decommissioned in place if they do not interfere with navigation or commercial fishing operations or pose an environmental hazard. However, if the BOEMRE rules that it is a hazard during the technical and environmental review during the permitting process, it must be removed.
The first step to pipeline decommissioning in place requires flushing it with water followed by disconnecting it from the platform and filling it with seawater. The open end is plugged and buried 3 ft below the seafloor and covered with concrete.
Costs of Decommissioning
The cost of the removal of a platform can be affected by an ever-evolving list of factors. Logic points to determinants such as location, water depth and function such as oil or gas. However, one study that looked at decommissioning platforms in the North Sea, a market seen as the industry’s best practices example, points towards a simpler explanationSweet, Rod. “At Last... Some Visibility Into North Sea Decommissioning Costs” DecomWorld, DecomWorld, http://analysis.decomworld.com/structures-and-maintenance/last-some-visibility-north-sea-decommissioning-costs..
The main factor in decommissioning a platform is directly related to the weight of the materials that need to be removed. Operations that critically focus on disconnection and removal will save extra costs during the decommissioning process. Almost two thirds of the decommissioning costs come from the removal cost, making it imperative to find a contractor with great equipment, experience and service.
The costs of decommissioning can be reduced if the initial design has studied the methodology of decommissioning and amended the design to suit
Alternatives to Decommissioning
Due to the cost of decommissioning a platform, many operators choose to reuse portions of their offshore facilities. Depending on factors such as intent of use, parameter and wear and tear, operators must choose between entire platform reuse and reuse of select components. Examples of these components include such things as production manifolds and wellheads. One major consideration when making this decision is whether or not corrosion of the structure has occurred. This factor for reuse and other ideas below were studied and reported on by Gorman and Neilson in “Decommissioning Offshore Structures.”
Wind and water power generation are more recently developed alternatives to decommissioning a platform. Wind power generation is the concept of placing a wind turbine on the top of a inactive platform, producing between 10 and 20 MW of energy. Cost considerations and maintenance make this option implausible in many cases.
For water power generation use, platforms are modified to use waves sequences to create electrical power. Major alterations must be done to the structure of a platform to make it viable for this type of use, and even then failure would be likely.
In the Gulf of Mexico, success has been seen with the “Rigs to Reefs” policy. Allowance is made for decommissioned platforms to be modified into a permanent, artificial reef environment on the ocean floor.
Another alternative involving an oceanography solution is the introduction of fish cages into the jacket of the decommissioned platform. In the North Sea, this has the potential of creating an environment for farming of coastal fish. However, this alternative brings its own obstacles including placing and retrieving the cages from within the structure, overall safety considerations and maintaining a farm staff.
Alternatives to decommissioning a platform must be evaluated on a case-by-case basis and weighed against many different factors, eligibility for decommissioning and local environmental and safety policies.
Materials disposal and site clearance
Platform materials can be refurbished and reused, scrapped and recycled or disposed of in specified landfills.
To ensure proper site clearance, operators need to follow a four-step site clearance procedure.
Pre-decommissioning survey maps the location and quantity of debris, pipelines, power cables, and natural marine environments. Post decommissioning survey identifies debris left behind during the removal process and notes any environmental damage ROVs and divers target are deployed to further identify and remove any debris that could interfere with other uses of the area. Test trawling verifies that the area is free of any potential obstructions.
- Gorman, Prof. D.G, Murphy & Neilson, Dr. J. (Eds). (1998). Decommissioning Offshore Structures. Berlin, Heidelberg, New York: Springer-Verlag.
- “Rigs to Reefs.” Bureau of Safety and Environmental Enforcement. Bureau of Safety and Environmental Enforcement. http://www.bsee.gov/Exploration-and-Production/Decomissioning/Rigs-to-Reefs/.
Noteworthy papers in OnePetro
N.R. Anthony (Andersen Consulting), B.F. Ronalds (U. of Western Australia), E. Fakas (U. of Western Australia), SPE Asia Pacific Oil and Gas Conference and Exhibition, Brisbane, Australia, 16-18 October, SPE-64446-MS. http://dx.doi.org/10.2118/64446-MS P.B. Mount II and M. Voskanian, California State Lands Commission, Offshore California Decommissioning: Past, Present, Future, SPE-94390