The Tension Buoyant Tower: A Design for Deep Water

Abstract

The Tension Buoyant Tower (TBT) consists of a cylindrical spar type buoyancy module moored to the sea bottom by a group of tendons arranged in a pattern close to the centerline of the buoyancy module. The buoyancy module consists of buoyant hard tanks extending to a depth of 250 feet or more depending on the specific requirements of the design. An upper stem extends below this point to a depth of 500 feet or more. Seven tendons pass through a centerwell in the buoyancy module and extend to the seafloor, passing through a lower stem before template. Local tendon bending stresses are limited by tapered tendon sleeves at the entrance points to the upper and lower stems. The tendons and sleeves are allowed to pivot at these points, eliminating the need for elastomeric flex joints. Coupled analysis of the design was performed using the finite element program COPIPE. The key parameters in the design include: base tension, location of platform CG and CB, sleeve design and location of tendon support points within the stems. The results of analysis showing the sensitivity of the design to these and other parameters will be discussed. Application of the TBT to Gulf of Mexico applications ranging from 1,000 feet to 10,000 feet will be reviewed. INTRODUCTION Deep water Gulf of Mexico development will undoubtedly involve the use of innovative structures for drilling and production. We have already seen the first deep water tension leg well platform at Jolliet, the World's deepest floating drilling and production platform at Green Canyon Block 29 and in 1992 will see another deep water drilling and production platform at Auger. All of these were preceded by the guyed drilling and production tower at Lena. As water depth increases it becomes ever more difficult to provide structures which allow surface drilling and completion. The TBT allows drilling from the structure coupled with traditional surface trees. The TBT (Figure 1), is a compliant offshore structure used for drilling and producing oil and gas in water depths ranging from 1,000 feet to 10,000 feet. Low cost and simplicity are its principal features. The structure relies on buoyancy to keep it upright and on bundled vertical tendons to maintain its position. The lower ends of the tendons are fixed to a base which is pile fastened to the seafloor. Although the TBT can be designed to support any deck load, it is well suited to supporting drilling and production facilities in the range of 5,000 to 25,OOO tons. The drilling and production equipment is similar to that used on conventional fixed or compliant platforms. Conductors are supported by sleeves and/or spacers from the well deck to the mudline. They are pretensioned to level required to prevent buckling under the extreme pitch motions of the TBT, approximately 6 degrees. Wells can be drilled from, and completed on the deck of the TBT as is done on fixed platforms.