Magnolia extends TLP water depth

New designs such as the Magnolia extended tension-leg platform (ETLP) have made installation of TLPs in deeper water practicable.

ConocoPhillips operates the Magnolia field in Gulf of Mexico Garden Banks Blocks 783 and 784. In August 2004, it installed the platform in 4,674 ft of water and expected production to begin this month.

ConocoPhillips holds a 75% working interest in the field, with the remaining 25% interest held by Devon Energy Corp.

Extended reach TLPs

The Magnolia TLP (Fig. 1) is a smaller version of the ELTP installed on the ExxonMobil Corp.-operated Kizomba A field off Angola in November 2003. A second similar ELTP will be installed on the Kizomba B field in 2005. Water depth of both ELTPs off Angola will be 3,863 ft.

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The Magnolia ETLP ([top] photo) set a water depth record of 4,674 ft for a TLP when it was installed in the Gulf of Mexico last August (Fig. 1). Photo from ABB Lummus Global. The extended porches on the Magnolia ETLP ([bottom] photo) allow the tendons to be connected farther from the columns (Fig. 2). Photo from ABB Lummus Global.Click here to enlarge image

ABB Lummus Global designed the ELTP. Unlike previous TLPs, the ELTP has extended porches or pontoon extensions outboard of the columns that provide the tendon connection points (Fig. 2). Previous TLPs had tendons that were connected to the lower portion of the hull at the columns.

On a ETLP, the columns have been moved inboard, allowing a more favorable support for the deck and its associated riser and drilling related loads without requiring increased hull size. On the Kizomba TLP, the columns are about 50 ft inboard, while on the Magnolia ETLP the columns are about 26 ft inboard.

The temporary boxes or flotation tanks on the columns of the Kizomba A ETLP add stability during the wet tow (Fig. 3). Photo from ABB Lummus Global.Click here to enlarge image

Also unlike the typical TLP, the ETLP design requires only 8 tendons compared with the 12 tendons on previous full-sized TLPs.

ABB indicates that the ETLP has a 40% weight savings for the hull and deck steel and a high structural-weight-efficiency ratio of 0.8 in the Gulf of Mexico and a 1.15 for West Africa. This ratio quantifies the payload weight that can be supported by a given weight of a hull and deck platform. Magnolia topsides weigh about 15,230 tons and will accommodate a workover rig.

Another ETLP advantage is that it can be installed as a completely integrated and commissioned unit. The shorter and less complex installation operation is less likely to be hindered by inclement weather.

For the Kizomba A ETLP, the installation required the addition of temporary boxes or flotation tanks to the columns. The boxes were removed after installation. The Magnolia ETLP, on the other hand, required no boxes.

Production

Magnolia is designed to handle 50,000 bo/d with 150 MMscfd of gas. Two pipelines will connect the platform to Shell Exploration & Production Co.'s operated hub at Garden Banks 128, about 50 miles away.