The world’s first rigless platform well abandonment
Conventionally, such a project necessitates a jackup rig. But, on this project, it was out of the question – there was no time or budget – so the integrated team hatched an ambitious plan. To unlock cost efficiencies and expedite the process, a totally rigless well abandonment was planned for the normally unmanned platform.
It was a daunting challenge to perform certain of the activities in the North Sea for the first time without a rig. The lack of platform cranes, deck space and accommodation can pose considerable challenges to rigless well decommissioning, and the team had to consider their operational plans carefully for the series of tasks required. These included
- downhole perforating and cementing operations
- well tree removal
- tubing severance and recovery
- severance of the casing strings below the mudline and their recovery
- recovering the 20-in. conductor below the fracture point
- back-loading the cut casing sections owing to deck restrictions.
InterAct developed the downhole programme for wireline perforation and cementing of the well, which owing to a contractual agreement, the existing contractor carried out. Once this work was complete, Claxton began recovery of the conductor.
The retrieval and handling of well trees and tubulars is a key area in rigless decommissioning that demands special attention. For this, Claxton designed and manufactured a bespoke conductor reaction recovery system that would interface with the Leman platform (Figure 2). This comprised lifting support beams, boom cranes and a hydraulic jacking system, complete with reaction slips and suspended false rotary.
The full Claxton casing recovery package was also required. This includes a positive-grip slip-type tension ring, drilling and pinning apparatus, and bandsaw machines to achieve a multiple casing string recovery and bring the 95⁄8-, 133⁄8- and 20-in. casings to surface. Lifting equipment and kit to recover the production tubing with the platform’s crane were also required.
One of the first tasks was to recover the 20-in. conductor section below the fracture point. The key equipment for this was a proven, Claxton-designed packer anchor system, which is run on drillpipe or casing and tubing.
The 200-t-rated load spreader beams were designed and positioned to span the original platform drilling skid beams. The spreader beams provided the structural support to suspend the hydraulic reaction recovery package and to support the false rotary table and the work platform over the well’s centre.
The multiple casings were severed internally using Claxton’s SABRE™ abrasive cutting system, which can simultaneously sever all the casings in a well, regardless of the casing loading, eccentricity or annulus contents. SABRE™ has seen successful use on some of the most significant abandonment campaigns. Its abrasive jet exits the tool at transonic speeds to make light work of cutting even composite materials such as cemented casings.
Using a jet of naturally occurring cutting components (garnet, water and air), SABRE™ has a low environmental impact. Significantly, the system can be deployed from a vessel or platform to enable simultaneous abrasive cutting of multiple well casings without impacting on adjacent infrastructure.
SABRE™ cut the multiple casings 4.5 m below the mudline, in accordance with the regulations applying to the UK Continental Shelf. Then the combined 9.5⁄8-, 13.3⁄8- and 20-in. casing strings were recovered down to the sub-mudline abrasive cut. This left the 20-in. conductor below its fracture point retained with the platform guides.
The combined string recovered, the next step was to recover the 20-in. conductor section below the fracture point. The plan was to do this using the Claxton internal inflatable packer anchor system, which had been designed for a similar recovery carried out for ExxonMobil. The packer system was to be run in hole using 5-in. drillpipe.
In fact, it was not possible to fully recover the 20-in. pipe stump because an undetected full fracture and separation of the 20-in. conductor caused the 20-in. section to fall to one side within the guide. This prevented further access to the bore of the 20-in. casing. The 20-in. stump was later recovered from below the lowermost guide using a dive vessel that was already in the field.
The recovery of the combined conductor and casing strings required hydraulic cylinders complete with a positive-grip tension ring to provide a continuous reaction recovery process. Two hydraulic cylinders retracted while two cylinders extended. This was used in combination with the suspended false rotary to support the string until it had been recovered to the work-deck false rotary. The hydraulic reaction system used the space between the main deck and the cellar deck, which removed the need for a mast or a recovery system placed above the platform’s main deck.
The drilling and pinning operation
Before each cut, the inner casing strings were attached to the conductor by boring a hole though all the conductor and casing strings (95⁄8-, 133⁄8- and 20-in.) and pinning the strings together with a certified load pin.
The Claxton drilling and pinning system provides an efficient method for securing multiple casing strings for safe recovery operations and enables their simultaneous retrieval. It is a cold-work technique and is therefore especially suited to abandonment operations where adjacent wells may still be in production. Crucially, compared with conventional methods, less deck space is required for the recovered casings, which simplifies disposal logistics and reduces costs.
The combined strings were recovered and cold cut into lengths to fit in a conventional 20 ´ 8 ft half-height container. The lengths were determined by the back-load requirement, the crane’s capacity and the team’s material handling practices.
Since this project, it has become widely accepted that operators can derive major cost efficiencies from rigless decommissioning and slot recovery operations. In the intervening years, Claxton has been involved in decommissioning more than 60 North Sea platform wells, in most cases without using a drilling rig or a platform crane.
There is no doubt that the absence of a rig and the limited crane facilities on many platforms pose a series of unique technical challenges, but, with an inventive and committed team, substantial cost savings can be made compared with rig-based operations.
Many of the solutions pioneered at Leman have since become commonly used. Moreover, the industry’s rigless well decommissioning capability has been further developed to meet the growing need for safe, low-cost well decommissioning services. Key to this has been the development of a range of equipment and techniques, especially around the retrieval and handling of well trees and tubulars.
The rigless project in Leman field was a success. The bespoke equipment package performed as expected in the field, and a total of 316 work-hours was recorded offshore without incident. The only lost time was due to inclement weather. The job was also completed on time and on budget: Perenco calculated that the costs for overall operation were less than 50% of those for an equivalent rig-based solution.
And finally, a note about data accuracy. Operators and service companies alike know that the quality of their information can make or break a project’s budget. At Leman, there were no concerns about this once Bob Claxton’s 38-year-old tally books from the original well installation job were dusted off. These were inordinately detailed and proved extremely useful.
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