Reducing the cost of subsea wellhead removal

Norse Cutting and Abandonment (NCA) has recently completed a rigless removal operation of 3 suspended subsea wells in the North Sea. The cutting and retrieval operation was done using Abrasive Waterjet Cutting (AWJC) from a subsea construction support vessel and eliminated the use of a drilling rig or use of explosive cutting, which have been the traditional solutions to this challenge. NCA used the new Subsea Wellhead Picker on the project, which combines an Internal Multistring Cutting Tool (IMCT) and a hydraulic wellhead connector and enable cutting and pulling of the subsea wellhead in one single deployment and recovery operation, only assisted by ROVs. This technology breakthrough opens up for more efficient and environmentally friendly abandonment of wells, but also more efficient drilling of exploration wells.

The three wells were located in a subsea template in the Central North Sea at 240 feet water depth. The wells had previously been plugged using a drilling rig and left behind on the seabed for potential future use. The cuts were done 15 feet below the seabed and the wellheads were recovered independently from the template using the subsea construction support vessel’s heave compensated crane. The template was later recovered by the same vessel.

The Subsea Wellhead Picker operation

After arriving on location, the wells were surveyed, corrosion caps removed and the wellbore cleaned and drifted to cutting depth in order to secure access for the Internal Multistring Cutting Tool. The Subsea Wellhead Picker was then deployed and stabbed into the well assisted by a work class ROV. When the IMCT was at cutting depth, the connector was activated and load tested and the cutting started.

The highly efficient Abrasive Waterjet Cutting uses high pressure water mixed with abrasives to cut through the combined steel and cement compound that you will find just below the wellhead. A typical casing configuration is a 30” conductor, 20” or 18 5/8” surface casing, 13 3/8” casing and 10 3/8” or 9 5/8” casing, which results in a 4-string cemented multistring conductor.

NCA’s patented IMCT enables cutting through all the casing strings from the inside in one go. This is now proven technology with a track record of more than 90 multistring conductors cut, ranging from wells on 100 feet water depth to 5 string conductors on 1000 feet water depth. The cutting process is typically completed in 2-4 hours, which gives a roundtrip time for the Subsea Wellhead Picker of only 8-12 hours from it leaves the deck of the vessel until the wellhead is safely landed on deck.

The advantages compared to using a costly drilling rig should be obvious; the rig rates for semis are typically 4-5 times higher than a construction support vessel of opportunity, and the Subsea Wellhead Picker is in general more efficient than even mechanical cutting run on drill string. The IMCT can also go inside casing as small as 7 inch casing and still cut all the way through to a 36 inches conductor, an operation which would require several drill pipe runs and days to perform with a drilling rig.

The alternative cutting method if you use a vessel is explosives cutting which has proven to have a very negative impact on marine life in the area, and not least for the integrity of downhole cement plugs. In addition, the explosive cutting method creates a flared cut which can make it hard to recover the conductor string, and also complicates the deck handling when it is recovered to surface.

The operation was done during the winter season in the North Sea, which highlights another advantage with the system. The first wellhead was cut and retrieved in a significant wave height (Hs) of 2.5-3.0m, while the two last ones were retrieved in Hs of approximately 1.0m. All 3 wellheads were successfully cut and safely retrieved on the first attempt. The system’s capability to work in heavy weather makes it attractive to perform these non-value-adding operations during the low season over the winter.

Releasing rig days

So far the Subsea Wellhead Picker has been used for removing old suspended wells that have been abandoned in the past. Even though there still are hundreds of these suspended wells in the North Sea and in other regions, another big potential is to replace the subsea wellhead removal operation on new exploration being drilled by semisubmersible drilling rigs. Many exploration wells are drilled for the sole purpose of exploration and are not intended for future production. Consequently, the last phase of the drilling program is to plug the well and remove the subsea wellhead before the rig leaves location.

Depending on well complexity, the subsea wellhead removal operation can take several days. The Subsea Wellhead Picker can replace current wellhead removal tools run from the rig and save time in the rig operation, however with the current rig rates, the significant savings can be had by letting the drill rig leave the wellhead behind for later recovery by the Subsea Wellhead Picker from a vessel. These wellheads can then be grouped together and picked up by a vessel in campaigns, releasing many rig days for drilling of more wells and reducing the well cost for our clients.

In the future NCA will continue to develop innovative abandonment techniques that reduce cost and improve HSE in decommissioning. As shown in this article, these methods can also in many cases be used to increase efficiency and save cost in the exploration and development phases of a field.

Achievements

• The operation described in this article was the first ever done without use of traditional methods like explosives or mechanical cutting on these water depths.
• This was a technological breakthrough promoting an environmental, cost efficient and safe method for cutting and retrieving abandoned subsea wells.
• NCA’s IMCT system won the OTC Spotlight award in 2005.
• NCA’s Subsea Wellhead Picker was nominated for the ONS Innovation Award in 2004 and won the NOF Energy Innovation and Technology award in 2007.
• The IMCT technology is patented.