Extending the TLP brand
  from: Offshore Engineer
  by: Jennifer Pallanich
  Friday, April 04, 2008

Another extended tension leg platform is inching its way toward the Gulf of Mexico. It is shorter on steel requirements than its sister design, engineering on the ETLP is moving forward, and basin tests are nearly complete. Jennifer Pallanich reports.

Because of new requirements to model test all future projects according to the revised environmental criteria, FloaTEC carried out model tests on an ETLP designed for post-Katrina conditions. FloaTEC carried out wind tunnel testing on a 1:216 model before the wave basin test. The company is considering a second optional test.

‘This is the first time this has been tested in post-Katrina conditions,’ says John Murray, FloaTEC director of technology development.

The hull of the ETLP model is fiberglass with a foam core, and the deck is aluminum. The 1:92 scale model weighs in at 150lb.

A model of the ETLP was undergoing a basin test December 2007 to ensure it meets metocean criteria for the central Gulf of Mexico, which is the most severe weather zone in the Gulf, revised after the 2005 hurricane season.

The American Petroleum Institute (API) revision of its 2INT-MET guidance for designs of future offshore structures in the Gulf of Mexico changed the design basis requirements used when designing new structures and affects the configuration of TLPs, spars and semisubmersibles for use in the Gulf of Mexico.

The wave basin tests, performed at the Offshore Technology Research Center in Texas, include static pulling, free decay, white noise, regular wave, and irregular wave. Houston Offshore Engineering is working for FloaTEC as a strategic partner on the model test as part of a Gulf of Mexico ETLP project.

The irregular wave tested for one-year winter storm and the extreme 1000-year hurricane. In the 1000-year hurricane tests, both wave dominant and wind dominant events were considered with the variations of the wave peak periods from short, middle and long per the latest API guidelines. The 1000-year hurricane, wave dominant with the short wave period created the steepest waves, which Jun Zou, manager of naval architecture at Houston Offshore Engineering, says challenge ETLP design in terms of air gap and tendon tensions with the existing design tool.

Computer modeling led the team to predict there would be a minimum of 3.36ft of air gap, Zou says. The test results showed an 8.08ft clearance between the bottom of the deck steel and the wave crest, he adds. Factoring in high tide and subsidence brings the minimum air gap down to 3ft to 5ft, he says, which should be adequate under the revised API guidelines.

‘So far we haven’t seen any green water on the deck,’ Zou says. ‘Some splash, but that’s not green water.

As of mid-December, FloaTEC had completed a significant portion of the model testing. ‘So far we haven’t seen any green water on the deck,’ Zou says. ‘Some splash, but that’s not green water.’

This particular ETLP was designed around a full production, drilling and quarters concept to give operators flexibility in field development planning.

‘You’re getting the facility that carries the drilling kit practically for free,’ says Eric Namtvedt, president of FloaTEC, adding there will be a marginal cost increase related to added payload. That’s not the only reason FloaTEC sees the design as a cost saver.

‘We have centered the columns while we have retained the footprint,’ he says. The tethers, he adds, have the same footprint at the seabed as a regular TLP.

The ETLP has a structural deck that supports the integrated topsides on top of four vertical columns, connected to conventional pontoons underwater. The tendons attach to ‘extended’ porches fitted to the pontoons. Moving the columns in saves on steel requirements. ABB developed and later sold the ETLP design to joint venture company FloaTEC, formed by Keppel Fels and J Ray McDermott in 2005.

The ETLP concept has been deployed for Kizomba A and B offshore Angola and Magnolia in the Gulf of Mexico.

The design tested features 18 slots for drilling and dry tree production, two export SCRs, two WI and four future tieback SCRs. It has a 2 million pound hook load for drilling and can drill 35,000ft wells. It can handle production rates of 120,000b/d, 110mmcf/d, 80,000b/d of water and 100,000b/d of water injection.

Column spacing is at 220ft. It has a 106ft still water air gap, a 113ft in-place draft, 95,400 short tons in-place displacement, and 35-40ft wet tow draft.

FloaTEC is marketing the design as a solution for fields requiring dry trees.

Namtvedt says the company is progressing on the engineering to the point of being ready to order long lead items. He estimates, on receiving client commitment to proceed, the unit will require a two-year fabrication cycle from steel delivery.

‘It’s a flexible and proven design,’ he says. ‘The key word is robust.’ In other news, FloaTEC, one of several firms working on a dry tree semi, aims to roll out its solution in 1Q 2008. OE

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