A stabilizing influence

October 3, 2014

When the Varg A monotower offshore Norway required stabilization, IK came up with a novel, remote, subsea solution. Elaine Maslin found out more after hearing about the project at the Underwater Technology Conference in Bergen earlier this year.

The clamp being lowered into place.
Photos from IK. 

Varg A is something of an anomaly on the Norwegian Continental Shelf (NCS). It is a dwarf compared to some of the other offshore giants there.

Yet, its stability was still crucial to the operator of Varg Talisman Energy Norge AS. Varg A is a monotower, normally unmanned wellhead platform, piled to the seabed in 84m water depth, through four grouted sleeves, with oil produced via the Petrojarl Varg production vessel.

The field, discovered in 1984, is about 200km west of Stavanger in the Norwegian sector of the North Sea and has been operating since 1998. It is destined for a new lease of life after Talisman took it over in 2005 and started drilling new wells in the area.

In March 2013, Talisman through scheduled monitoring discovered that a crack had developed around the circumference of one of its four foundation piles, indicating there was vertical movement between the pile and the sleeve, raising uncertainty over the structural integrity of the platform. Talisman moved fast to find an alternative to permanently stabilize the facility and went to Stavanger-based IK Norway for a solution.

A task force of specialist engineers from IK and Talisman performed a pre-study, in which several repair options were defined and reviewed. The thorough concept evaluation performed in the study was crucial for the project success, says IK’s project manager, Kenneth O. Rosén.

Based on the study, IK Norway was tasked with finding a diverless arrangement, which had to be completed by Autumn of that year, without needing to stop production on the platform. It would mean temporarily fixing and elevating the effected sleeve, then permanently securing it, taking into consideration environmental loads, installation loads and safety.

IK’s solution was a remotely and hydraulically operated clamp, capable of taking a 1600-tonne load, to elevate the sleeve, and then, to permanently secure the facility, producing 35, 300mm-diameter structural plugs, and the means to install them—all by August of the same year. It was a unique, fast, and challenging, but ultimately successful project, Rosén says.

“We had never made a clamp like this before and I do not think it has been done anywhere else in the world either,” he says. “It was a very comprehensive and intense project and new challenges came up all of the time. A safety and risk review highlighted the risk of the sleeve buckling, so we developed shear plugs to stop this from happening during the elevation, but then we needed a new tool to install these shear plugs subsea. The concept for permanently securing the platform required 35, 300mm diameter plugs, weighing 250kilos each. We had to make a new tool to drill through the steel and grout, in order to install these. To make sure the plugs were installed correctly, we built a 7m-tall fixation template, with a common docking interface on each of the 35 plug slots, for which the drilling and installation tool, guided by the ROV, could latch.

“The critical element was being able to specify and design the equipment, while carrying out the necessary procurement and quality control, as well as manufacturing and testing in the available time.”

The clamp lowered in place and closed. 

Clamp

A key part of the project was the clamp. IK looked at a number of options, taking into consideration operational constraints, as well as manufacturing limitations, ease of installation and operation.

The final design comprised, in effect, of two parts – the upper clamp, to grip on to the pile, and a lower split clamp that closed around pile sleeve, all to be hydraulically controlled, via ROV panels, using an ROV. To meet the project deadline, manufacturing started before the design was complete, using early design information to order materials. Manufacturing was also carried out off IK’s own promises, at a quayside manufacturing facility, in order that the 50-tonne clamp could be shipped directly offshore, instead of needing road haulage, which would require permissions, adding to the project timescale. Another manufacturing challenge was sourcing components, at a significantly busy time of the year, which meant IK had to source components from across Europe, because some of its usual suppliers were already fully booked.

Plugs

The clamp on deck, with the Varg A monotower in the background.

The plugs, to permanently secure the pile, were made using carbon steel and include wedges, to enable them to be torqued tight by an ROV. This was no mean feat. “We needed to be able to handle the plugs on deck in a controlled manner, drill and insert the plugs accurately subsea while managing the buoyancy once each 250kilo plug was released by the ROV,” Rosén says.

A fixation template was designed and fabricated with uniform docking interfaces, to allow the ROV to position itself and then drill and place each plug correctly. A drilling tool was then developed, to drill through the steel and concrete sections of the sleeve, grout and then pile, including dealing with weld beads, and removing swarf from the bit. A 300mm-diameter hole saw was developed, with special profile cutting teeth to handle the sleeve, pile and grout. Before deployment, it was tested off the quayside, using a work class ROV, which helped refine the tool and make any improvements where necessary. Despite initially not seeming a critical part of the project, the drill tool, and ancillary equipment, was key, Rosén says. “When we started, we thought the clamp would be the most critical element, but the drill alone could have been a nine month project,” he says.

Offshore campaign

Project execution was fast. Just the month after the project team was set up, detailed repair concepts were established, and by mid-May a preferred option was chosen following evaluation, which included looking at operational constraints and procurement and manufacturing. The fixation template and clamp started fabrication in June with ancillary tool development and tested in July, before early August factory acceptance testing and offshore mobilization mid-August.

A cross section of one of the 35 plugs. 

The offshore campaign started in August and lasted around five weeks in total. First, the fixation template was installed and initial drilling carried out, through the sleeve. Then clamp was lowered into place and closed to pull up the sleeve. The holes were pre-drilled through the sleeve to reduce the operational time it would take to drill through the piles. The sleeve was then jacked up, and the holes drilled through the pile, ready for the plugs to be inserted.

“A lot of engineering, testing and planning went in to preparing as much as we could before we went offshore to reduce the amount of time it would take to insert the plugs—as the more plugs inserted the more secure the structure would be,” Rosén says. “As soon as we start drilling through the pile, there would be no going back.” By mid-September, the campaign was complete, with the platform’s structural integrity successfully secured.

Team working was a critical part of the project, Rosén says. “Managing such a big project in such a short period of time requires team work,” he says. “Everyone, all the companies involved, in different locations and from different cultures, working together towards a common goal, with an ethos where you share problems, to solve them. Talisman had a very proactive approach in this respect. You also need people who are prepared to take on a challenge and put other things aside, because it’s not all straight forward.”

Engineering consultancy Atkins provided verification services and DeepOcean was the subsea contractor. The clamp and auxiliary tooling is now in storage and Varg A is safe and stable.

The partners on Varg are Talisman Energy Norge AS, with 65%, Petoro (30%), and Det norske oljeselskap (5%).

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