The ABCD of smarter spar drilling

December 31, 2010

A configuration change-up on traditional spars could spell out some relief in wind load, drop the unit’s center of gravity, slim down the hull, and cut the amount of tripping time needed for drilling operations from a spar. OE’s Jennifer Pallanich talks to FloaTEC’s John Murray and Seawell’s Chris Magnuson about the Radial Wellbay Spar design.

The Radial Wellbay Spar design, created through a joint industry partnership by the same name, rearranges the spar’s traditional matrix of well slots into a circular pattern around the perimeter of the wellbay. This design opens up the space in the center of the wellbay.

FloaTEC came up with the bones of the design after it began looking at ways to continue revenues from truss spars once patent protection on the design ceases in 2014, notes FloaTEC’s John Murray, director of technology development. When looking at the spar’s hull, Murray says, it’s easy to see a very large interior hole ‘with a lot of volume’ where potential buoyancy exists. ‘How do you capture that buoyancy?’ he asks. After considering various ideas, Murray homed in on the radial wellbay concept.

With the initial concept, the well slots are lined up along the outer perimeter of the square in which the traditional matrix wellbay currently exists, making use of the hull’s traditionally unusable center volume where the risers are located. ‘I thought it may be of some use to a drilling company because we can take the drilling equipment from the top deck and put it down inside the hull,’ Murray explains.

Among the benefits he cites are: a reduced wind load area by using the hull to protect the drilling package; a reduced center of gravity by lowering the heavy drilling package from the deck to the hull and removing the need for a derrick; and an overall smaller hull to accommodate the same payload.

Excited by the new configuration, FloaTEC asked Seawell if the company could drill through the square wellbay perimeter.

‘My first answer was yes [we can drill through it], because that’s exactly what I was looking for,’ says Chris Magnuson, Seawell’s general manager of drilling facility engineering. The open center would allow for the setback and pipe racking system to be lowered 160ft. He also recalls his next reaction to seeing the new configuration: ‘“Instead of making it square,” I asked, “can you make it round?”’ The move from square to round meant the setback and racking system could be placed on a rotating system, allowing for pipe racking support for all wellbays. A circular skidding system of the drilling module would allow access for multiple wellbays for coil tubing and wireline operations. According to Magnuson, this would be a significant improvement as the traditional X-Y skidding system restricted access to wellbays. The equipment will simply move around the perimeter until it reaches the correct well slot or riser.

FloaTEC – the 50/50 JV formed by subsidiaries of Keppel Fels and McDermott – and Seawell created the Radial Wellbay Spar JIP in May.

Going below deck
The drilling rig structure constitutes about 30% of the spar’s mass, and putting it on top of the spar has significant motion and dynamics ramifications. ‘The more we can reduce [weight] on the top, the more we can reduce overall’ in terms of steel, Magnuson says.

The huge 145ft-high derrick, traditionally about 40ftx40ft and 60ft above the upper deck, becomes unnecessary with the reconfiguration, Magnuson says. ‘We don’t need to store drill pipe in there any more because we store it down below. Now you’re talking much smaller weight packages,’ he adds, noting the reconfiguration reduces the drilling equipment weight by about 20%. ‘We’re looking at truly integrating the drilling system into the spar . . . so that it can more efficiently drill.’ Moving the setback and racking system from the top deck to the hull also reduces the windload while increasing safety.

‘Putting all this down inside the hull shelters it,’ Murray points out. The drilling mast and offline standbuilding frame are the only two elements of the drilling package that remain on the deck. Further, Magnuson says, placing the setback and racking system below the driller improves safety where dropped objects are concerned.

By dispensing with the derrick, the reconfiguration relies on two systems – the racking system and drilling module – that handle racking and pipe handling or drilling operations, and each can work independently. ‘It brings in a lot of efficiency,’ Magnuson says.

‘Current spars have some limitations on design. One of those is wellbay access. So if you’re drilling, that’s all you can do,’ Magnuson says about how the traditional design limits concurrent drilling operations. ‘Most of the existing spars don’t have much integration. They keep the drilling in one section, the topside in another,’ Magnuson says. The reconfiguration offers more storage and ‘allows us to do multiple operations because the wellbay is in a radial layout’, he says. ‘Your casing crew can work on building casing offline and racking them into the setback while still drilling.’

With the reconfiguration, the driller will be able to deal with drilling while the assistant driller deals with racking.

The reconfiguration and Adjustable Buoyancy Center Well Device (ABCD) move the setback 160ft lower and allow the use of a larger setback. Traditionally, a setback could hold about 20,000ft of drillpipe, but the lower setback on the ABCD has a larger setback and will accommodate double that. Lowering the setback simplifies the drilling system zone management and means the system can be designed to handle Range 3 triple lengths, versus the traditional Range 2 triples, Magnuson says.

The design calls for two PRS-4 column rackers – working as a racker and two pipe chutes – to allow for double redundancy and greater uptime, resulting in the spar’s performance to ‘equal fifth and sixth generation MODUs in its ability to drill. Spars in the past couldn’t do that,’ Magnuson says. ‘This will equal and has the potential to exceed it.’ The tripping rate will be limited by the 6000hp drawworks. A PRS-8 can handle a maximum of about 19 stands, or 1800ft/h, while the PRS-4 unit and pipepipe chute can handle a maximum of 34 stands, or about 3100ft/h. The JIP estimates this could translate to a savings of over $5 million per well versus drilling by a traditional drilling spar.

The second PRS-4 unit also provides redundancy. ‘If you lose a pipepipe chute, rotate it 180°, and you can go on. We’ve tried to focus on where issues have been in the past so you can go on if there’s a problem,’ Magnuson explains.

The focus behind the design, Magnuson says, is to increase safety, maximize efficiency and offer redundancy to drive up uptime. ‘We’re looking at being able to offer a 10-15% decrease in tripping time versus a traditional spar,’ he says. Spars are already appealing in that their traditional day rates are about 35% to 40% that of MODUs, but they lack the efficiency of MODUs and have the associated capex costs. ‘We’re saying you’re going to have a reduced drilling time on this spar because you’re going to have more efficiency and increased uptime.’

The design provides ‘unfettered well access’, Magnuson says. ‘It’s a much smaller drill floor now, much less complex.’ Ultimately, he notes, the goal will be to modularize the drilling equipment, including the mast, so cranes on the spar can lift it. ‘We’re pretty close to that. We can now make this equipment on top truly modular.’ The reconfiguration, Magnuson says, also answers the big challenge of what to do with the mass of drillpipe necessary for drilling operations. The design uses a pipe chute, which ‘allows the drawworks to be the limiting piece of equipment for tripping’.

The reconfiguration also improves safety in hurricane zones. According to Murray, hurricane response will be faster for a spar of the Radial Wellbay design. A traditional spar may take up to four days to prepare for a hurricane to blow through by moving the pipe around and tying it to the deck. The radial layout by design shelters the pipe from the wind, so all that’s left ahead of a hurricane is to lay down the drilling mast, an operation expected to take about a day, he adds.

Design details
The ABCD, made of steel, makes the design work. The compartmentalized unit features slits, allowing the risers passage.

Murray says the ABCD offers the opportunity to configure the drilling equipment in this way. ‘It moves everything down into the wellbay rather than leaving it at the top elevation or the top deck.’ He acknowledges it’s possible to use a radial design in the wellbay without the ABCD: ‘but it will require an enormous hull. The hull can have a 150ft diameter hull instead of a 135ft diameter hull. The ABCD makes the design economically feasible.’

Supplies are another requirement for drilling, and the reconfiguration is expected to decrease the frequency of supply vessel visits. The system can store 10,000ft3 of cement, 6500ft3 of barite, 8600 barrels of liquid mud, 3500 barrels of diesel, 3000 barrels of base oil and 5000 barrels of water. The storage capacity means ‘you’re not worrying about your resupply vessels. You’re not tied to them,’ Magnuson says, so drillers can work longer between supply top-ups.

Those same quantities are now needed in a spar, but the room is not available for storage, Magnuson notes. ‘By utilizing the ABCD, we’re able to store our liquids down below and our solids and bulks elsewhere within the structure.’ The reconfiguration has also seen a mud return system be built around the drilling system. ‘We’ve integrated our shakers into the spar,’ Magnuson says.

Eye on pre-FEED
The Radial Wellbay Spar, with its ABCD, is still in the concept phase. Model tests are planned for early 2011. Both companies have patents pending on the design.

So far, the JIP has shown the design to a number of operators, including supermajors, national oil companies, majors and independents. Murray says the reaction to the design has been ‘consistently very, very positive’.

The next step in the process, Magnuson says, is a pre-FEED with an operator. The JIP will continue to develop the Radial Wellbay Spar concept, but is looking to work with an operator to further the design.

Murray estimates it will take 24-36 months from the signing of the EPC contract to get a Radial Wellbay Spar into the water. ‘It’s the standard schedule for any spar,’ he says, noting the hull would take about 18 months to fabricate while the topsides and equipment would take longer. ‘All the risers systems are identical to the ones that are presently in operation in the Gulf of Mexico.’

Because the JIP intends to fabricate new orders at FloaTEC parent McDermott’s Altamira yard in Mexico, the truss for the spar can be longer than those that have historically been built in Pori, Finland, and transported by heavylift vessel into the Gulf of Mexico.

‘The traditional transport parameter of hull design is the transportation on a heavylift vessel, and this has constrained the overall length of spars,’ Murray explains. Using the yard in Altamira removes the transportation constraint on length. And the longer truss means an ultimate reduction in steel, he adds.

Any hulls fabricated at the Altamira yard can be wet-towed to location in the Gulf of Mexico, which is where the JIP expects the design to find its first application before being considered for deployment elsewhere in the world. The hull would go from skid rails onto a loadout vessel, which would move over a deep pit near the end of the rails before descending to allow the hull to float off. The ship would move away, and the hull would be ready for the wet-tow to its installation site.

Circle gets the square
Both the circular wellbay and the square bay design have their own advantages, Murray notes. The square is readily adaptable to the existing spar hull architecture, he says, and fabricators find it easier to make flat rather than curved surfaces. The circle, he says, is more adaptable. It isn’t too difficult to fabricate, and it offers the ability for equipment to reach any of the risers in the wellbay without X-Y skidding.

Murray estimates the reconfiguration could reduce a traditional 150ft-diameter spar hull by 15ft. The spar can work in waters of 3000-10,000ft and support dry trees.

The design doesn’t depart significantly from the traditional spar design. To the contrary, Murray says, it actually lightens the equipment and topsides weight, and the reconfiguration will allow the spar to use a smaller hull to accommodate the same payload.

All the technology included in the Radial Wellbay Spar is ‘pretty standard’, he says. ‘It’s not a revolutionary design. It’s just an enhancement of an existing product that obviously offers a lot of advantages, particularly a significant economic advantage.

‘The novelty comes in configuration and what that offers to efficiencies in operations,’ Murray adds.

Magnuson agrees: ‘The new layout requires no new drilling rig technology.’ The radial design ‘just packages it in a new format’ to promote efficient drilling, he adds. OE



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