Skid-on topsides solution may alter Spar - TLP preference lineup

Pins similar to design for offshore airfield

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Alternatives to heavy lifts such as McDermott's skid-on method may lower the cost of Spar installation.
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Following the breakup of Spars Inter national, J. Ray McDermott, through its new SparTEC division, is exploring novel solutions for heavy lift operations for Spar-type units. If these efforts are successful, it could alter the ranking of preferences among Spar and TLP-based production solutions in deepwater.

Spars International has been dissolved with Aker and McDermott going their separate ways. There are a number of transition contracts still in the works, but for the future it looks as if each of these companies will go it alone. For its part, J. Ray McDermott plans to add hull construction to its portfolio of services.

It is a natural extension, and Daniel Houser Vice President and General Manager of Subsea and Deepwater Technology for J. Ray McDermott, contends it allows customers a one-stop-shopping option. He said SparTEC has equal access to all the patented technology developed under the Deep Oil Technology and Spars International names, including key software and analytical tools for the modeling and design of Spar hulls. Other capabilities include everything between the construction of topsides and hull facilities to precommissioning. Houser said the addition of hull construction adds to J. Ray's overall understanding of Spar development projects, allowing for accurate risk assessment. In anticipation of this need, McDermott purchased the TNG yard in Veracruz, Mexico.

Offshore assembly

SparTEC has developed several variations on the classic Spar design. Houser says Spar hulls are relatively simple and cost effective to build. They can be built in either McDermott's or a third party's fabrication facility or shipyard, making them a versatile floating solution for deepwater production. The only detriment is that a Spar requires an offshore location for installation of the topsides.

Because of its deep draft, a Spar hull must be shipped out from shore on its side. Once on site, the hull is righted and moored in place, so the topsides can be set. A typical installation requires up to three heavy lifts in the range of 3,000 tons each. There are only a handful of vessels available to do such work, including McDermott's own DB-50. The day rate for such vessels can be high.

Equally expensive is the actual offshore commissioning of the topsides. Because the modules are so heavy, they must be lifted into place one at a time and then connected, once secure on the hull. This complex process is subject to weather delays and the availability of a heavy lift vessel. That means, on top of the actual cost of such operations, there is the added risk of delays.

Offshore mating is one of the critical differences between Spars and tension leg platforms (TLP). The TLP costs more to construct, but can be precommissioned onshore for much less money. To overcome this obstacle, McDermott is looking at a number of alternatives to the traditional heavy lift. One option in particular has moved to the model-testing phase.

Using what Houser refers to as the skid-on method, it may be possible to precommission an entire topsides system and skid it onto the Spar in one move. This would be a topside in the range of 15,000 to 20,000 tons, which would cover a variety of field development scenarios. If successful, this technique would dramatically lower the cost of installing a Spar, making it more competitive with the TLP across a broader range of development sizes and water depths.

Skid-on design

According to Tom Carr, Project Manager with SparTEC, the new design involves a hinge connection between the specially equipped transport barge and the Spar hull. This marine connector allows the two to remain connected in sea states where they need to move independently. The design would require modifications to the launch barge, as well as the Spar hull. For example, McDermott would use the Intermac 650 as the transportation barge for this work. A rectangular male connector would extend from the bow of the barge with two pins, one on each end, which are retracted hydraulically.

These pins are designed to fit into corresponding sockets on each end of the female connector, mounted on the Spar hull. The barge is maneuvered near the connection point, then a series of mooring lines is used to draw the two together and hold the barge in place while hydraulic rams are actuated to engage the pins into the receiver. Once the connection is made, the relative motions between the two floating structures are the same, with the exception of relative pitch, which is carried across the pins like a hinge.

Carr said the pins are tapered on the end, and there are no shoulders on the sockets. This design allows the connection to be disengaged quickly in high seas. He said the core technology of the hinge connection is an offshoot of work McDermott did for the US Navy in the design of the Mobile Offshore Base (MOB) project. The MOB is a floating logistics base concept that would link five huge semisubmersible platforms together to form an offshore airfield. The connection between the vessels would be made using this same hinge design.

Once the connection is made between the hull and vessel, the topsides would be skidded slowly from the barge to the Spar. As the weight of the topsides is transferred from one floating structure to the other, ballasting must be adjusted to keep the two level. There is an elastomer incorporated into the skids to absorb and distribute the differential energy as the weight is transferred.

Once the skid is complete, the topsides would be hooked up to the Spar. The majority of precommisioning will already have been done before load out, Carr said. "You would literally tow out with the lights running," he said.

Because of the long, 10-14 sec, period waves offshore West Africa, the payload capacity of the skid-on will be less than in the Gulf of Mexico, Carr said. In the Gulf of Mexico, Carr predicts this method could be used to transfer loads as great as 25,000 tons.

Remote installations

In addition to the obvious savings and convenience of shipping topsides in one integrated piece, this concept also simplifies installations in remote areas. Houser said he believes the skid-on method will first be used in the Gulf of Mexico, where spar-based production systems are proliferating rapidly, but in areas such as West Africa, there are even more advantages.

Typically, topsides installed in West Africa have to be shipped across the Atlantic Ocean on a transportation barge. They then meet with a heavy-lift vessel at the installation site. This means two independent and very expensive activities. With the skid-on method, the topsides could be transported and installed using the same barge, eliminating the need, and expense, of a second vessel.

Houser said the detailed engineering on this process could be finalized by summer, but it is not the only process the company is looking at. There are a number of alternatives to conventional heavy-lift vessels that could make Spars more competitive and reduce the cost of installing topsides offshore. The topsides are specially designed to accommodate the skid procedure, which Houser said is very similar to the conventional methods used when skidding topsides onto transport barges.

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