At the center of stability
  from: Offshore Engineer
  by: Jennifer Pallanich
  Friday, April 04, 2008

The continued march into offshore production, combined with smaller field sizes and remote locations, means producers are scrutinizing their options for floating production. US editor Jennifer Pallanich talks to two engineering firms about their latest offerings.

With devastating hurricanes still fresh on operators’ minds, OPE is banking on its Satellite Services Platform, designed for stability amid fierce waves, to find a deepwater home. Drawing inspiration from the spar, Houston-based OPE uses a center column counterweight to keep the SSP vertical in the face of a big wave, says OPE president Gary Quenan.

‘That’s the secret to its stability,’ he says.

Quenan notes that most waves and winds can’t generate enough force to affect the SSP because of the center column but that a big wave could have some effect.

‘This vessel can stay on station during a Katrina (hurricane category 5) storm,’ Quenan says, adding that unlike a semi, TLP or spar, the SSP does not have an air gap. Instead, it rides the waves, eliminating concerns about green water damage to topsides.

OPE says the results from the basin test carried out at Marin on the SSP 320 in July 2007 indicated less than 4° of significant pitch/roll in category 5 storms and no vortex-induced motions associated with a loop current.

‘That’s pretty phenomenal,’ Quenan says of the roll and pitch in a Katrinatype storm. ‘You don’t have to move. You can stay online.’

He likened the heave motion acceleration of the SSP to an elevator, about 0.07G’s in winter storms and 0.3G’s in hurricanes. ‘The acceleration of your heave motions is very bearable,’ he says.

The test looked at wind, current and waves, Richard Haun, senior vice president of technology and project development, says. ‘The wind didn’t matter. The current didn’t matter. It came down to waves.’

The design doesn’t require a turret. ‘It’s omnidirectional, if you will,’ Quenan says.

Under the design, Haun says, the SSP FPSO will stay on station with its 12-point mooring system, even during a major hurricane.

‘We can safely ride out the storm,’ says Nico Vandenworm, vice president of business development.

Quenan says the vessel’s deep keel provides counterbalance to prevent capsizing.

Tangible progress

When OPE unveiled the concept in August in Houston, Quenan said the company believed ‘we will have an order’ but that the company was ready to build on spec. The SSP 320 model testing had just been carried out in July 2007 and detailed design was scheduled to follow. After design, he said in August, it would go to a fabricator, where it would likely take 18 to 24 months to build.

OPE has contracted Worley Parsons Sea to prepare the final design details to obtain fabrication quotations for the SSP 320 hull, which is 320ft in diameter with 1.25 million barrels storage capacity.

‘We are in the initial stage of working through the engineering process and will get it to maturity, where we’ll have drawings and specifications for this particular size, which will allow us to get quotes and negotiate with a fabricator to build it,’ Quenan says, noting it will take between six and eight months to design the hull.

He says OPE has funding for the engineering.

‘We’ll have something tangible to put in front of our clients to show them that it works,’ Quenan says. OPE hopes to gain a commitment from an operator for the unit. ‘We’re prepared to build on speculation, but we’re hoping that’s not necessary.We’re moving forward with the detailed design,’ he says.

The adaptability of the SSP opens it up for use as an FPSO or FSO, a floating drilling and/or production platform, a control buoy, an LNG unloading terminal, an early production platform, and an FPSO with workover or drilling capabilities as well as being suitable for marginal fields. OPE sees the FPSO design as the closest competition to the SSP.

‘Because it has so much weightcarrying capacity, you could also use it for a GTL facility . . . or a floating LNG vessel,’ Quenan says.

It can also be reused in different regions, which could prove useful for a vessel designed for a 35-year lifespan.

‘You can tune this vessel to adapt to different environments,’ Quenan says. Tuning the SSP involves lowering or raising the center of gravity to set a GM that increases the natural pitch period and shifting the natural period outside the area of the wave energy. A longer center column increases the added inertia of roll/pitch and reduces the pitch response.

Quenan sees the design finding use in deepwater locales like the Gulf of Mexico,West Africa, Brazil, and Southeast Asia.

Designing elements

The SSP uses some characteristics of the spar, such as the center column for inherent stability, Quenan says. It also uses the efficient liquid storage shape of a sphere, which the design truncates at the top.

OPE has designed the SSP in several sizes, but only the SSP 120 and SSP 320 have gone through basin testing.

‘The vessel is scalable,’ Quenan says. ‘We’ve looked at numerous sizes. For an FPSO application, it’s anywhere from a 256ft in diameter to 420ft.’

A 256ft diameter design would offer storage of 500,000 barrels, while a 420ft diameter design would hold 2 million barrels. At a quarter-mile in circumference, the SSP 320 will be able to accommodate many risers, Quenan says.

‘We have plenty of room for access around the platform,’ Haun says.

The SSP is expected to weigh in at up to 40% to 50% of a comparable ship shape vessel, Quenan says. He sees a 50% savings advantage over an FPSO because of lower materials needs.

‘The 320 design can support more than 10 times its weight, and it weighs about 17,000t,’ Quenan says. For comparison, Vandenworm says, the structural weight of a Suez max tanker with similar storage levels tips the scales at 27,000t.

Loading and offloading hydrocarbons in addition to changes in draft will not affect stability, Haun says. ‘A lot of this has to do with the fact that we have the center column . . . which gives us good stability’ without requiring active ballasting, he says.

That center column pulls double duty: it can hold chemicals, diesel or potable water.

Since it doesn’t have to be built in a drydock, a unit could be built anywhere in the world, Haun says. Additionally, it will be stable for wet towing, Quenan notes.

The engineering firm consulted with ABS and the US Minerals Management Service while developing the design, Haun says.

Vandenworm notes OPE also consulted with operators to learn what they want in platforms and invited representatives from operating companies to watch the basin test. OE

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