A new method for constructing jacket type offshore platforms is presented. The method takes advantage of the integrated deck concept in conjunction with an offshore mating of deck to jacket. The mating operation is discussed on the theoretical and practical points of view.

A description of the EKOUNDOU accommodation platform construction - the world's first offshore mating - is given.

Introduction

Offshore operating oil companies are looking for methods permitting to reduce the overall cost of platforms construction. Among the cost parameters involved in such a construction, one of the most significant is parameters involved in such a construction, one of the most significant is the amount of work to be performed offshore for hook-up and pre-commissioning of the facilities. This single factor explains the pre-commissioning of the facilities. This single factor explains the present trend toward very large derrick barges, able to lift heavy modules present trend toward very large derrick barges, able to lift heavy modules weighing 2000 tons and above, in order to minimize the number of modules to be hooked up.

Another trend is to take full advantage of that heavy modules concept and build the platform topside as a "single module", so-called integrated deck, and mate it with a purposely designed substructure. This method has already been used several times for construction of concrete gravity platforms, mating of the integrated deck on the substructure being made in platforms, mating of the integrated deck on the substructure being made in sheltered water and the whole platform towed offshore afterwards.

However the bulk of offshore platforms is of the steel jacket type. Such platforms can be built using the integrated deck concept which means, in platforms can be built using the integrated deck concept which means, in that case, performance of the mating operation offshore in unsheltered waters.

Construction of this type of platform brings new conceptual problems relatively to their architecture, and to the mating operation.

Such problems have been solved and solutions experienced for the first time with the construction of an accommodation platform on the EKOUNDOU field operated by ELF SEREPCA offshore Cameroons.

ARCHITECTURAL PARTICULARS
a. General platform architecture

The platform architecture derives directly from the deck installation principle. After construction, the deck is loaded out and transported on a principle. After construction, the deck is loaded out and transported on a flat deck ocean-going barge from which it will be directly transferred on the jacket substructure, when on site. For doing so, the barge will be positioned in a slot arranged inside the jacket and deck legs connected to positioned in a slot arranged inside the jacket and deck legs connected to jacket piles. Then weight of the deck will be transferred from the barge to the jacket.

b. The integrated deck

The integrated deck is a self-supporting structure spanning from one side of the jacket to the other with overhangings. It incorporates several legs protruding outboard on each side of the barge which will later fit with protruding outboard on each side of the barge which will later fit with the jacket piles.

The deck comprises several floors, generally two, on which all the equipment and systems are fitted.

Main particulars of the integrated deck are:

Structurals: Structure of the deck can be built using two different systems:

  • deep truss beams making maximum use of the structural steel with little interference with equipment arrangement

  • shipbuilding-type hull construction using plates and stiffeners elements. In both cases, the total amount of steel included in the construction is far less than the equivalent platform modularly built.

Equipment layout: Equipment can be installed in an orderly manner as the size and interface constraints of modular construction do not apply. Main functions of the platform can be more easily separated, generally, process equipment on the platform can be more easily separated, generally, process equipment on the lower floor, and utility equipment on the upper floor, resulting in great improvements in safety and operation ease.

Piping and cabling: Piping and cabling: Piping and cabling can be run straighter and need fewer fittings, valves, Piping and cabling can be run straighter and need fewer fittings, valves, flanges and connection boards.

Deck construction: Owing to the above features, the deck is easier and cheaper to construct than the equivalent number of modules. As it is constructed in a single yard, it is also easier to control. The result will be that the achieved facility will be of a better overall quality.

c. The substructure

The substructure consists of a purposely designed steel jacket and piling. Geometry and scantlings of jacket and piling will depend on deck weight, water depth, bottom nature and environmental conditions.

However, in each case, the jacket upper part will be open in one direction for constituting the barge slot.

Due to this particular, small jackets with a single line of piles on each side (two to four) may suffer a lack of transversal rigidity, principally in the case of high air gap platforms and should be specially considered. When design weather conditions are severe, it can be necessary weld additional transverse bracings after deck installation.

However, in most cases, platform rigidity is fully satisfactory and meets accepted standards.

THE MATING PROBLEM

The mating problem lies in the difficulty to transfer the heavy weight of the deck, from a floating body (the barge) subject to erratic movements, onto a fixed body (the substructure) without damaging them.

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