Seeking additional gains from subsea wells on a tight budget has paved the way for the implementation of a new technology to the market. Elaine Maslin reports.
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Helix’s Compensated Coiled Tubing Lift Frame in its operational position. |
When Shell found the potential to recover more oil from one of its subsea wells in the UK central North Sea, it was not inevitable that a suitable method could be found in the current low oil price environment to make the sums add up to move forward with extraction.
While there was the potential to get more out of the well, existing methods to unlock that potential: perforations via wireline using a vessel or coiled tubing using a semisubmersible, made the job uneconomical due to the time it would take by one method, and overall cost by the other.
Aware of the industry’s need for alternative approaches to performing low cost interventions, Helix Well Ops (UK) approached Shell and proposed using its coiled tubing deployment technology, developed in conjunction with Altus Intervention, as a possible option – using coiled tubing from a vessel – to overcome the economics of traditional methods.
The system is comprised of an Altus Intervention provided coiled tubing unit, which incorporates a bespoke fatigue management system, called Talos. This system is combined and interfaced with Well Ops’ own compensated coiled tubing lift frame (CCTLF) technology, which virtually eliminates the effect of vessel motion and reduces coil fatigue towards that of a fixed installation or platform.
“The riser system is designed to operate in 80-200m water depth and to deploy up to 23,000ft of 2.375in diameter coil. The system was first launched in 2010 and while it was proven successful, the job was curtailed due to severe weather conditions. The market at the time was extremely buoyant and the demand for riserless intervention was so great that the CT equipment was placed into hibernation not long after the initial deployment,” says Steve Nairn, vice president, Helix Well Ops (UK).
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The Well Enhancer in Aberdeen at the end of the successful coiled tubing operation. |
Going back further than the initial deployment of the Well Enhancer system in 2010, the concept of coiled tubing from a vessel was originally performed on the vessel that pioneered vessel interventions, the Seawell, another Helix asset, in the late 1990s. That vessel, which recently went through an extensive re-fit, had some limitations with its moon pool size and deck space so accommodating the equipment required for coiled tubing was a long-term challenge.
Realizing the potential for this application resulted in Well Ops designing and commissioning its new build Well Enhancer. This vessel, which like Seawell also supports dive operations, was designed with coiled tubing in mind where often the opposite is true: the vessel already exists and technologies or applications are reverse engineered onto them. Here, the thinking was different and started on paper following the experiences of the Seawell. “One of the biggest challenges is managing coil fatigue,” Nairn says. Fatigue, a result of repeated bending and straightening of the tube itself, is the enemy of coiled tubing. The greater heave from a vessel compared to a rig only serves to magnify the problem.
On the Well Enhancer, there are three compensation systems. The Talos system provides a length of slack coil tube by using a secondary injector between the coil drum at the rear of the vessel and the main injector. The main injector, pre-built and tested in the Well Ops CCTLF prior to mobilization, sits directly above the moon pool during operations. The CCTLF incorporates a passive compensation system which operates in conjunction with the Talos system to maintain the slack coil thus reducing fatigue.
The CCTLF, in turn, is held by the 150-tonne capacity Huisman tower, which is used in active heave mode during the deployment and recovery of the equipment subsea, but also acts as a failsafe should any of the other elements of the system fail, which provides a level of redundancy over rigs. It is also an integral part of the emergency shut down (ESD) system, which enables the vessel to close the valves and shut the well in and allows the vessel to move away within 35 seconds of an ESD being initiated.
While it takes longer to set up than a riserless system, it’s more efficient once you’re there, especially for projects like scale milling or accessing long or horizontal wells, Nairn says. The benefit, he adds, is not having to reactivate a rig for work on a short duration project or even a single well with intervention taking less than a month.
It should also be noted that rigs are not necessarily designed to conduct coiled tubing operations. They may require additional equipment including the coiled tubing itself to be transported to the rig by a supply vessel, and then assembled and tested before being used. Well Enhancer can do all this in port as part of its normal mobilization activities and as such is largely self-sustaining throughout the campaign.
To run the system, Well Ops’ 7.375in subsea intervention lubricator (SIL) is deployed in wireline mode through the moon pool. Once landed out in active heave on the subsea Xmas tree on top of the well, the system is split at the “high-angle” disconnect package and the vessel steps away a short distance from the well, while the upper half of the system is recovered back to surface and suspended below the vessel hull.
The lower section remains connected to the tree and provides control of the well preventing any environmental release while the upper section is progressively lowered back to depth by the addition of sections of 6.625in riser, riser monitoring components and stress joints.
Once deployed back to depth the vessel then moves to take up position directly over the well and the upper half is landed and latched back onto the lower section, an operation which can be done in sea states up to 5-6m, according to Nairn. Then, the coiled tubing frame is brought into the tower, with the injector head but not before the tooling is loaded into the riser.
Known as the bottom hole assembly, these are the tools that are going to perform the work within the well and are connected to the end of the coiled tubing. On a rig, these are inserted into the riser at some height above the rig floor and require men on harnesses working at height in order to do so. With the Well Ops system this is all conducted at deck level on a stable flat work site without any need for harnesses or working at height which greatly reduces the risks associated with coiled tubing operations.
Shell decided to use the system on its Pierce well this past summer, working closely with Well Ops and Altus Intervention. Because this was initially just one job, other operators were persuaded to join Shell and Well Ops to make the investment needed to re-commission the system.
The project included focus on familiarization time and training, as the system hadn’t been used for some time. Offshore deployment was in June.
Pierce is produced via Bluewater’s Haewene Brim FPSO, over blocks 23/22a and 23/27a in the central North Sea, 265km east of Aberdeen close to the Norwegian maritime border. The field was discovered in 1975, with first oil in 1999.
Working in 83m water depth, the project involved drifting and milling to 4400m (14,500ft) and performing a reservoir saturation tool log before perforating 267m (877ft) of reservoir section in four runs, followed by a venturi clean-up.
The operation was on budget with no HSE incidents and production gains 25% above expectations.
The project on Pierce has been billed as the first live well coiled tubing operations from a mobile offshore drilling unit class light well intervention vessel. It has been shortlisted for an ICOTA European Charter Innovation award and been described as a potential game changer for Shell and the wider industry.
Winners
Helix Well Ops, Shell and Altus Intervention received this year’s Intervention & Coiled Tubing Association (ICoTA) Innovation Award during the SPE ICoTA Well Intervention Conference in Aberdeen early November. The three firms had been up against CannSeal and Centrica/Island Offshore as finalists.
Neil Greig of Helix Well Ops said: “Helix Well Ops, are delighted to be joint recipients of this prestigious award along with Shell UK and Altus Intervention. It is an acknowledgement of the investment in a concept developed over many years by Helix Well Ops in conjunction with Altus Intervention and we are grateful for the support of Shell UK who committed to using the system and enabling it to be successfully deployed for the first time in a live well.”
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