ExxonMobil has chosen a non-traditional solution for the Kizomba field off the Angolan coast. The solution was to use ABB's tension leg platform specially designed for great ocean depths, Extended Tension Leg Platform (ETLP). The advantage of an ETLP is that even at a depth of 1,200 metres, the wellheads are still dry. This better facilitates continuous well work compared with subsea solutions. The concept was developed through the Deepstar programme, in which several of the major oil companies have teamed up to develop technology for extracting oil and gas in the Gulf of Mexico.
Conoco, which also was first to engineer a Tension Leg Platform (TLP), has also chosen an ETLP solution for its Magnolia field in the Gulf of Mexico. "The advantage of an ETLP is that it reduces the amount of steel in the hull by 30 percent. Therefore it can tolerate being installed in deeper water compared with ordinary TLP solutions," says Rune Strømquist, director of research at ABB.
Reduced steel weight in the hull also reduces the total cost of development. This is because this structure provides the same area on deck. The legs that the deck rests on, however, are centred differently from those on a traditional TLP. This reduces the load resulting from the movements of the large columns that the deck rest on. The total saving in weight for the ETLP is in the overall weight of the platform.
ABB's contract with ExxonMobil for the Kizomba field is approx. USD 600 million. It comprises topsides plus hull. The topsides are being manufactured by Grootint in Swijndrecht, the Netherlands, whereas the hull is being produced by Daewoo in Korea.
During the actual towing-out phase of the platform, stability tanks will be mounted on the hull. Although these tanks are watertight, they contain systems for ballasting the platform. Once the tensions legs are mounted and the platform itself deballasted, the stability tanks will be removed.
Anders J. Steensen
