Abstract

ETPM successfully installed the Ekpe Gas Compression deck offshore Nigeria for the NIGERIA NATIONAL PETROLEUM CORPORATION AND MOBIL PRODUCING NIGERIA JOINT VENTURE in June 1997, using for the first time the shockless Smart-Leg system. The Ekpe Gas compression platform is located in a water depth of 50 meters and is subjected to long period swell conditions common to offshore West Africa, which causes the movements of cargo barges to have large amplitudes. This activity involved positioning the cargo barge within the jacket structure, centering the deck over the piled jacket, and mating the legs of the 4 100 tonne deck to the piles. For the first time, passive hydraulic devices were used to neutralize the vertical movements of the barge and to transfer the deck weight from the cargo barge to the piled jacket structure. The Smart-Leg jacks allowed the mating of each deck leg onto the corresponding jacket pile to take place at the precise time when the deck leg vertical speed is zero, therefore eliminating the kinetic energy and the risk of impact. The deck mating was completed in a few seconds, less than the swell period. After the deck was installed, all the hydraulic devices were retrieved, ready to be used for another project. This paper presents the installation procedures, an operational description of the major hardware employed, and details of the on-site monitoring systems used to accomplish float-over deck installation. Factors which influenced the selection of the float-over installation method are also discussed.

Introduction

In 1993, Smart-Leg existed only as a concept patented by ETPM. It took 4 years to develop the system, build the equipment and perform the first operation. The advantages of the integrated deck concept for large offshore platforms are well known. These decks, however, can be quite heavy and some areas in the world may be far away from the usual operating scenes of the large crane vessels necessary if a conventional lift technique is selected for the installation. Floatation methods offer an alternative solution for installation, but few methods permit the installation of heavy integrated deck in long swells where the movements of the cargo barge have large amplitudes. Floatation methods are limited by the difficulty of maintaining the cargo barge steady inside the jacket or by the shocks to be absorbed during the mating operation. Rather than absorbing the shocks, the Smart-Leg floatation method aims at eliminating them. Passive and retrievable jacks are used to progressively freeze the motion of the cargo barge, thus allowing a very large deck to be installed in significant swell conditions. During the abandonmentphase, the deck can be removed by reversing the operation. Offshore West Africa, the crane capacity of local construction vessels is a limitation to heavy deck installation. For a deck installation above 1,200 tonne, one would have to mobilize a heavy duty Semisubmersible Crane Vessel (SSCV) from the North Sea, where these units normally are located. ETPM was determined to find an alternative to using SSCVs, and preferably a solution that can accomodate a wide range of deck sizes and seastate condition.

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