Vessel-based decommissioning system offers economic alternative

Well decommissioning is traditionally performed from drilling rigs due to the complexity of operations and the available technologies and methods, which often include risers, divers, and explosives. But increasingly, operators are looking to a vessel-based approach to not only keep decommissioning project costs down, but to reduce non-productive time and safety risks to workers.

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Innovative approach employed on North Sea campaigns

Francis McGeehan, David Coull, and Javier Falcon, Oceaneering

Well decommissioningis traditionally performed from drilling rigs due to the complexity of operations and the available technologies and methods, which often include risers, divers, and explosives. However, rigs are expensive to operate compared to vessels, and often do not have the flexibility to change locations on short notice.

Increasingly, operators are looking to a vessel-based approach—as well as the innovative application of tooling solutions—to not only keep decommissioning project costs down, but also to reduce non-productive time and safety risks to workers. A vessel-based approach could reduce costs by as much as 50%, in part due to lower spread rates and the ability of vessels to quickly move. Additional cost reductions are achieved through the ability to live stream operations, which enables faster decision making and allows some operations to be controlled remotely. Further cost reductions also can be realized with multi-client campaigns where mobilization and transit costs are shared.

Oceaneering employed a vessel-based approach in 3Q 2018 for the most recent phase of a six-year decommissioning campaign. Two major North Sea operators needed a plan for an eight-well plug and abandonment (P&A) and well severance campaign in the Danish and Norwegian sectors of the North Sea. Because a wide range of equipment was needed for different workscopes, the campaign was divided into two distinct phases to reduce the operators’ costs without compromising operational efficiency or safety.

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A vessel-based decommissioning approach could reduce costs by as much as 50%, in part due to lower spread rates. (All images courtesy Oceaneering)

Phase one

The project’s first phase focused on two wells offshore Denmark. Work involved casing cutting and pulling and annulus perforation on a mudline suspension well. However, the most significant challenge of the campaign’s first phase was presented by the second well. This well had a temporary abandonment (TA) cap that needed to be removed, and the potential existed for retained pressure to be found below the TA cap.

Previous attempts to safely gauge pressure below the TA cap using standard procedures had failed due to an unusual field modification made 30 years ago. In case operations had to be suspended, the physical tooling solution’s final arrangement also needed to be capable of being left in situ without concern for an uncontrolled release of hydrocarbons into the environment.

The decommissioning team and supply chain partners determined they would need to source existing oilfield equipment that could be quickly and cost-effectively repurposed to offer a competent technical and commercial solution. Instead of employing normal drill pipe intervention techniques, the team used an annular BOP as a stripper, mating it to a proprietary slip-lock conductor connector, in a new tooling configuration called the conductor stripper assembly (CSA).

The CSA was tested and then deployed in combination with a specially constructed workstring arrangement. The workstring arrangement allowed for the subsea containment, monitoring, venting, fluid diversion, and automatic pressure relief of any potential well fluids—all while allowing the workstring to operate with a mixture of manual-operated equipment (chain tong) and mechanical-operated (drill pipe tongs) equipment and without returns to the vessel.

The CSA was deployed with theMVIsland Valiantvessel. The project team also enabled 24/7 live streaming of subsea operations direct to the operator’s project team. The live streaming improved efficiency as it allowed the onshore teams to review what was occurring in real time. It also eliminated the need to send video clips and images and descriptions back and forth for discussion, or for an operator to send a representative offshore to monitor operations.

The first well that the team worked on was prepared for the subsequent setting of a surface cement plug. The second well was inspected, the condition verified and original equipment manufacturer documentation provided to the client.

The first phase not only marked a new application of existing tooling, but also was conducted from a vessel instead of a jackup drilling rig. This particular operator first approached Oceaneering in 2012 for information on the flexibility and cost-effectiveness of a vessel-based solution for the P&A of North Sea wells. Multiple rig moves would have been cost prohibitive as these wells were not in the same area.

Using a vessel not only saved the operator the costs of renting and moving a jackup, but also provided the ability to change locations if an unexpected challenge arose. This capability allowed work to continue while a solution for a challenge was simultaneously developed. During the campaign, the vessel had to change locations when the TA cap encountered proved to be a different type than that specified in records. Due to this discovery, the TA cap assessment had to be performed onshore. The top of an existing bridge plug was also found deeper than expected, requiring a bridge plug be mobilized from shore.

Since starting work for this operator in 2012, Oceaneering has conducted P&A operations for 28 suspended exploration and appraisal wells, mostly mudline suspension wells drilled between 1966 and 1998 in water depths ranging from 110 ft to 237 ft.

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A vessel-based approach was employed in 3Q 2018 for two North Sea operators that required a wide range of equipment.

Phase two

The project’s second phase—conducted from the offshore construction vesselOlympic Zeus—centered on well cementation, well severance, wellhead recovery, debris clearance and seabed clearance survey activity for six wells offshore Denmark and Norway. The integrated project team was provided with well data, well-specific work programs, wellhead severance tooling, the Olympic Zeus and a Magnum work-class ROV.

Beyond the normal scope of supply, the team also deployed an existing cement support tool (CST) in a new manner. To prevent cement from slumping inside a well, a retainer is typically set—usually a mechanical plugging device such as a bridge plug—inside the wellbore at the lowest elevation that the cement plug will be set. However, this method can be both complex to run and expensive due to the one-time use of the component, additional equipment requirements and need for additional personnel offshore.

First, the CST was placed inside a deployment stinger tool. Next, a water hose was then attached to the top of the stinger. Finally, water was pumped down the hose into the stinger, which in turn pumped water out of the CST tool, allowing it to open and set at the required depth. The CST was deployed with the vessel’s 20-ton offshore construction crane and guided into place by the ROV.

Finally, all the wellheads were cut 10 ft below the mudline (seabed) using the patented abrasive water jet cutting technology developed by Oceaneering, and all debris was recovered so the seabed was returned to its original state. All project HSE, operational and KPI objectives of no harm to workers, no high potential incidents, no environmental harm, and no falling objects also were achieved without the use of divers, guide wires, or explosives.

The campaign’s first phase was completed in 16.5 days, with an average of five days per well (excluding transits and mobilizations). The second phase was completed in 15 days, with an average of 16 hours per well (excluding transits and mobilizations). Using a vessel-based approach to decommissioning substantially reduced the transit time to and from the site.

As the oil and gas industry continues to focus on cutting costs and reducing workers’ exposure to safety risks, the use of a vessel-based approach to decommissioning will likely continue to grow in the years to come. •

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