Model proves semisubmersible design

June 1, 2004
A new option for production facility design may be entering the competition for the deepwater Gulf of Mexico.

A new option for production facility design may be entering the competition for the deepwater Gulf of Mexico. The Octabuoy Classic has been around the industry since 2001, but has not yet been used in the GoM, though it has been tested.

"We have done extensive model testing of the Octabuoy Classic, which is the design we started with four years ago," Ida Husem, Moss Maritime manager of Marine Technology, said. "The model testing has confirmed the motions characteristics concept."

The Octabuoy SDM is the proposed solution when motion requirements are slightly less stringent but exceed what can be me by conventional semi (SCR fatigue).
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The Octabuoy's special octagonal ring pontoon takes a different approach to the semisubmersible design, which solves the parametric motions problem – heave, pitch, and roll – of traditional deep-draft floaters.

"It is the hydrodynamic behavior of the hull and the special shape of the hull and column that make it better than all the other concepts," Husem said.

Husem and her team have since developed other alternatives based on the first Octabuoy design. Moss Maritime now has a full range of Octabuoy floaters, "from the Octabuoy Classic, which is the ultimate solution when it comes to motions that can accommodate a dry tree, all the way to the conventional semi with improved motions compared to a conventional semi that can accommodate steel catenary risers, which is the main feature of the Octabuoy Shallow Draft."

The Octabuoy SDM (shallow draft mooring) serves wet tree applications, but the motions are good enough to accommodate dry trees in certain applications, Husem says. "Octabuoy SDM can also be used in the GoM with wet trees only. It can accommodate steel catenary risers, which is cost-efficient. Conventional semis have problems dealing with steel catenary risers in the GoM environments, which is why we need the motion control concept."

Moss Maritime says the Octabuoy line is a cost-effective alternative to spar and TLP designs. Compared to a spar, the Octabuoy line is competitively priced, but the Octabuoy out performs the spar's motions, the company says, resulting in better performance of steel catenary risers, drilling, and operations. Compared with TLPs, the Octabuoy's motions are similar, but the cost is much lower, Husem says, adding that a TLP's tethers create issue that become more complicated as water depth increases.