Subsea/Surface Systems

William Furlow • Houston

New TLP design overcomes mating limits

In the choice between a TLP solution and a Spar, one of the major considerations has always been the cost of offshore mating. A conventional TLP is towed to an offshore well site upright, and then ballasted down to be locked off to the tendons. This means the topsides can be mated and commissioned dockside before the hull is taken offshore.

Because of its length, a Spar hull must be towed offshore on its side, ballasted to a vertical attitude, and anchored in place. Only then can the topsides modules be mated and the commissioning performed. While the cost to engineer and fabricate a Spar is less than a conventional, four-column TLP, the cost of mating and commissioning topsides offshore is higher.

Atlantia, makers of the SeaStar TLP, have found themselves in the middle of this dilemma. Their design is a minimal TLP with a single column, rather than the four seen on other TLP designs. The SeaStar is transported offshore in the vertical position and anchored in place as a traditional four-column TLP hull. But, because of its low center of buoyancy and small waterplane area, an advantage in a minimal design, the topsides must be mated and commissioned offshore.

The earlier versions of this design were for smaller fields where a single heavy lift could place the topsides. These topsides designs were similar to those seen on fixed-bottom jackets in conventional water depths. The costs and complexity of these types of installations were small enough to make the platform less expensive, despite the offshore mating and commissioning.

Now, with the 10,000-ton payload of the new Matterhorn platform (the first dry-tree SeaStar design), Atlantia is clearly competing head-to-head with conventional TLPs. The single-column hull is still simpler and less expensive to design and fabricate than a four-column TLP, but the offshore mating is approaching the limits of heavy-lift equipment in the Gulf of Mexico due to the larger, more complex topsides.

Increasing the size and weight of the topsides further would require multiple heavy lifts, meaning more risk and more complexity, which further increases costs. In addition to these considerations, in more remote areas it is sometimes difficult to secure a heavy lift vessel. It would be much simpler and less expensive to install a fully mated platform.

Using much smaller vessels, it would be possible to preinstall the anchor piles, freestanding tendons, and other equipment so that a self-installing platform could be towed to site and hooked up without the need for a heavy-lift vessel. Thus, the goal of the company's new design is a hull that can be mated quayside like a conventional TLP, but still maintain the simplicity and low cost of a mono-column design.

The limitations of the SeaStar are not its deckload or stability in transit. On previous installations, the SeaStar TLP has used a heavy-lift vessel to provide additional stability for some phases of submergence.

This requirement is now addressed by adding temporary buoyancy outboard of the pontoons to provide additional stability. These Temporary Buoyancy Modules are connected to the tips of the three pontoons using a standard latching mechanism. They can easily be installed at the dock and, once the platform is securely in place, the TBMs can be removed. Because the tanks are located outboard of the pontoons, they are off the critical path and do not interfere with any other operations.

According to the new design, these tanks would be installed quayside, used during the platform installation, and then removed once the platform is in place. By using TBMs, Atlantia's SeaStar can be installed with the topsides already completed.

This design would also allow the TLP to be relocated without removing the topsides, another advantage over Spar and other SeaStar designs. The design has been completed for Atlantia's third-generation SeaStar, which incorporates this new installation method as well as a much larger-diameter hull accommodating a payload of 30,000 tons.

The design package has been sent to a number of fabrication yards worldwide for construction cost estimates. Atlantia envisions this new design will be applicable not only for hub facilities in the Gulf of Mexico, but also for remote projects offshore West Africa and Southeast Asia.

Multiphase flow meter completes field trials

Halliburton Energy Services has completed field trials in Algeria of its non-radioactive multiphase flow meter system – FastQ. The skid-mounted system measures the flow rates of oil, water, and gas continuously without mixing or separating of fluids.

The flow meter system is used for exploration and production well testing, production monitoring, and allocation metering. It is free of radioactive sources and has been successfully tested at independent flow test laboratories in Norway and the UK.