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Content archived on 2022-12-27

CONAT-DEEPWATER PRODUCTION TOWER

Objective

Development of an articulated tower used in waterdepths between 300 m and 450 m in the North Sea as an alternative for fixed platforms or as satellite platforms to them. A parameters study performed shows the deckweight that can be carried by towers in the waterdepth range and the deflections these towers undergo. A structure that is optimum with regard to operational conditions and the design of which is guided by practice, safety and cost was subject to the R&D work. A component of special importance of the R&D work was the design of a joint for the articulation which is able to transfer high loads from the column into the base and which is, at the same time, maintenance free and resistant to wear and tear. Construction methods and configuration of the tower have been developed and worked on to assure a vertical construction of the tower and a partial installation of the topsides close to shore in sheltered areas.
Comprehensive investigations have shown that the system has a high degree of reliability and integrity.
The articulated production tower has an Eigenperiod of To = 89 sec. The conservative calculation of the cumulative frequency occurence of inclinations during the lifetime shows that inclinations more than 1 deg will occur less than 10 %, inclinations more than 2 deg less than 0,05 % of the time. Thus the expected downtime of the operation of the system due to inclination is relatively low. An analysis of earthquake response based on specified frequency content and peak ground acceleration for the probable event with a return period of 100 years as well as for the extreme event with a return period of 10.000 years have been performed with the result that the PLS code and the ULS code provisions can be observed. The effects of a subsea gas blow out affecting the motion behaviour of the column have been investigated. The loss of buoyancy leads to a reduced restoring coefficient which does not endanger the integrity of the structure. Due to its articulated connection the structure is insensitive to non uniform settlements. Extensive fatigue calculations show good fatigue resistance. The hydrostatic pressure acting on the conical section of the bottle-shaped column together with the circumferential hydrostatic pressure are producing a biaxial compression state in the column's shaft which is favourable with respect to fatigue resistance.
In the ball joint no high local stress variations are to be expected under all SLS/ULS loading conditions. Ther permanent vertical force acting on the ball joints results in small variation according to wave loading. Therefore the ball joint is in a state of stress that is also favourable with respect to fatigue resistance. By the evaluation of long term statistical data on wave loading a lifetime for the production risers of 10.000 years is predicted being 400 times as much as the planned operational lifetime. The design of conductors and guiding pipes inside the column provides a double barrier to the inside of the column.
The althernative design of a production tower in 335 m waterdepth with offshore installed topsides shows a saving of about 30 % of the concrete masses of the column versus a production tower where the system has to be towed out with prior installed topsides. The design of a production tower in 400 m waterdepth has a very improved motion behaviour versus the 335 m version.
The main components of the substructure are the bottle shaped column uprighted by its buoyancy and attached by a ball joint to the base structure.
The concrete column has a circular cross section and is due to its shape in biaxial compression without the requirement of prestressing. In case that higher deckloads have to be installed inshore, the column has to be surrounded by a multicell buoyancy body, closed at its upper and lower ends by domes. This buoyancy body is monolithically concreted together with the column and serves for floating stability during tow out. Several as risers are installed inside the shaft.
The joint of articulation is of the ball - and socket-type made of stainless steel and under permanent compression without any tension member. The horizontal and vertical forces affecting the column are transferred from the upper spherical shell to the lower sperical shell into the foundations. The moments about the vertical axix are transferred by the torsion lock intothe base. All slide faces are plated with a metallic matrix comparable with a tin-lead-bronce alloy in which solid lubricants are embedded.
The material has low friction coefficient and has excellent self lubricant features which are further improved by an oil which will be injected into the sliding faces. All parts subject to wear are designed with a sufficient safety margin so that during 50 years lifetime no replacement is necessary. The inside of the joint is separated from the inside of the column by a gastight and pressure resistant bulkhead. The ball joint is permanently filled with compressed air balancing the outer water pressure. No sealings for the operation of the joint are required. Access into the joint is provided. The ball joint is surrounded by a spherical air bell which keeps the ball joint under "dry" conditions and prevents the permanent contact of sand-contaminated seawater with the outer surface of the joint. The concrete base is designed as an arrangement of vertical cylinders surrounding a central thick-walled cylinder to which the ball joint is fastened. Each of these cylinders is closed at its lower and upper ends by spherical domes. The base structure is resting on a skirt of 16,00 m long sheel piles penetrating the subsoil. The base is equipped with installations for waterballast, input of compressed air and vent pipes. Whether or not the base has storage capacity depends on the field conditions.
All production - and export risers are installed inside the column. The productiontrees are installed on the deck.

Call for proposal

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Coordinator

BILFINGER + BERGER BAUAKTIENGESELLSCHAFT
EU contribution
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Address
OFFSHORE DIVISION POSTFACH 760 240
2000 Hamburg
Germany

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Total cost
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