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WITH HIGH SEAS and winds offshore, it is easier to construct oil and gas platforms on land.  

But then the challenge is moving the massive structures out to sea. French company Technip has solved that problem with the floatover method, a technique for moving a platform deck to its permanent offshore location.

Trelleborg's Orkot material is used for lowfriction pads that facilitate the “skidding” of oil and gas decks onto transportation vessels and from them to the support structures at sea. The decision to use Orkot bearings was based on the material's properties such as a high load capacity and compression strength, a low friction coefficient and strong chemical resistance. Demands are high in a floatover operation. The material must remain stable during the skidding operation, while continuously withstanding high loads during the time the decks are under construction, about two years. “Installing a large platform deck, or topside, onto its jacket, or legs, using the floatover method is gaining popularity,” says David Emery, a Project Manager in the Offshore platforms Product Business Unit of Technip.

“This is because the operational cost is much lower than other methods of installation, such as modular lifts or a single piece installation by a heavy lift barge. Also, a high proportion of the pre-commissioning work can be completed onshore prior to loadout, significantly reducing the duration and cost of the offshore commissioning phase.” Orkot® TLMM from Trelleborg Sealing Solutions is used for low-friction pads that facilitate the skidding of oil and gas platform decks onto transportation vessels. “It was chosen as a more economical alternative to Teflon® pads,” continues Emery. “Bound inside and below the supports of the topsides (substructures), it's a critical part of the loadout procedure for topsides onto the transportation vessel and then onto their offshore installation.”

Orkot’s excellent characteristics The decision to use Orkot bearings was based on the material's excellent characteristics. These included high load capacity and compression strength, a low friction coefficient and good chemical resistance, along with ease of machining and bounding. Demands are high in a floatover operation The material has not only to remain stable during the skidding operation but also to continuously withstand high loads for the time the topsides take to construct, around two years. Initially Orkot TLLM was fitted on the AMENAM 1 Platform (11,000 metric ton topsides), offshore Nigeria, in 2003.

Then it was successfully used for the East Area project for the GN Platform, also offshore Nigeria, (18,000 metric ton topsides) and recently for the AMENAM 2 Platform (9,600 metric ton topsides) installed in 2006. The next use will be for the OFON 2 project in OML 102 for the OFP2 topsides (14,000 metric ton topside),” says Emery. “We continue working with Trelleborg Sealing Solutions because the Orkot material is excellent and effective in the application. It is also due to the quality of the service we receive.” “An example of this is how they worked with us on the design of the pads. These were adapted to include a machined groove below the bearing to allow a better flow of grease on the skid way. Their specialists have also trained our personnel in pad bounding at our facility.” Transportation: Loadout (fig. 1) - The platform deck's journey begins at the construction site. The entire deck rests on Orkot pads laid on a runway to “skid” the deck onto its transport barge. Due to the pads' low friction properties, hydraulic cylinders (stand jacks) can push the platform deck from the quay and onto the barge.

The deck is then towed to the offshore installation site .Docking, load transfer and separation:Docking: Vessel Entry - At the installation site, the vessel is docked to the support structure, called the jacket. Vessel positioning - At the final destination, the vessel is moored inside the jacket by eight mooring lines to ensure accurate positioning of the deck's legs over the jacket legs. Load transfer: The vessel is ballasted to make contact with the support structure and then prepared to transfer the deck. Separation: After completion of the deck load transfer, a separation occurs between the deck and the vessel and the vessel is retrieved from the jacket with the assistance of tugboats. World's first The Platform AMP1 of the Amenam/Kpono Phase 1 project was 60 m long, 40 m wide and 50 m high and had three working levels each the size of a football field. At 11,500 metric tons, the floatover installation on-site offshore constituted a major achievement for a facility of that size.

The extremely critical floatover operation involved pinpoint accuracy. This was followed by further installations, including the current record holder in the Gulf of Guinea using the Unideck® system, the 18,000 metric ton gas compression GN platform for the East Area project. Techip is involved in moving complete oil & gas platform decks that are 15 storeys high and as big as soccer fields from shore to their jackets out at sea using a floatover method. A low-friction product from Trelleborg Sealing Solutions facilitates skidding from dock to support structure. The platform deck structures that are moved from land to their jackets offshore can be as high as a 15-story apartment building and each of the working levels is the size of a soccer field. Giant moves

Unideck floatover method Unideck enables a very short installation time allowing it to be used even in severe swell conditions. Conventional floatover by ballasting is too slow and can cause excessive impact between the topside and the jacket. The technology developed by Technip combines ballasting and jacking to improve the stability of the heavy transport vessel during the transportation phase and uses jacking to provide a quick transfer of the integrated deck weight onto the pre-installed jacket. This avoids high dynamic impact loads. An offshore home away from home The platform deck structures that are moved from land to their jackets offshore are massive. They can be as high as a 15- storey apartment building and each of the working levels is the size of a soccer field. The platform is a complete factory made of steel, fitted with all the equipment to extract and store oil and gas before transferring it to transport ships. For the personnel living on the offshore rigs for months on end, they are also a home away from home. There is an average of about 100 individual bedrooms and bathrooms on a platform. Much like a giant hotel in the middle of the sea, they also feature restaurants, fitness centres and occasionally even running tracks. The decks are fully fitted with everything from beds to plumbing before they are transported out to sea.

Ilustration: Lönegåtf & Co/Trelleborg