Deck Installation by Floatover Method in the Arabian Gulf

Abstract

Abstract Decks installation using the floatover method in lieu of the traditional derrick lifting is a well known method all over the world and specially in the Arabian Gulf. This method provides an attractively cost effective method for the decks installation specially when the deck weight exceeds 2,000 tonnes. The floatover method eliminates the use of heavy lift derrick crane barges since it uses the cargo barge itself as an installation barge for entering inside the jacket, mooring and lowering the barge to transfer the deck load to the piles then retrieve the barge from under the installed deck. This method was successfully used in numerous deck installations in the Arabian Gulf out of which three cases are addressed in this paper. The three cases cover decks having weights of 2,500, 6,700 and 6,400 tonnes for a gas production deck, a gas compression/ production deck and an oil wellhead/process deck respectively. This paper presents the philosophy, method of performance, control systems used, technical aspects for the floatover installation method performed for the installation of offshore decks and modules in the Arabian Gulf and a comparative study with the derrick lifting method. The paper also addresses the types of technical analyses performed as well as the structural and marine aspects followed for the operation. Introduction In recent years the development plans of the oil and gas fields in the Arabian Gulf have required the installation of heavy offshore decks to meet the production requirements of those fields. These decks vary in size and geometry according to the type of services they are intended for. The first deck under investigation by this paper is the KPP deck installed in the ABK field offshore, Abu Dhabi, U.A.E. This is a six legged gas production deck weighing 2,500 tonnes and installed in a water depth of 29m. The second is the PU deck installed in the North Field offshore, Qatar. This is an eight legged gas production and utilities deck weighing 6,700 tonnes and installed in a water depth of 53m. The third is the AB deck installed in Al Shaheen Field offshore, Qatar also. This is an eight legged wellhead and process deck weighing 6,400 tonnes and installed in a water depth of 61m. The above mentioned three cases were studied at the early phases of the projects with respect to the installation technique and found that the floatover method is the more advantageous and cost effective in all cases when compared with the derrick lifting method. Moreover, the last two installations, PU and AB decks, comprised the largest single piece installation for a gas and an oil deck in the Arabian Gulf respectively, which clearly demonstrates the viability and cost effectiveness of the method. The floatover method can be done in two ways. The first way is by entering the installation barge inside the jacket, mooring it inside and then lowering the installation barge to transfer the deck load to the piles using ballast water only. This method is called floatover installation by barge ballasting. The second way is typically the same as the first except that hydraulic jacks are used to lift the deck prior to entrance in the jacket and then used again to rapidly transfer the deck load to the piles. This method is called floatover installation by jacking system. This second case is mainly used when the deck is required to be transported with a low center of gravity above installation barge deck and is then required to be raised prior to entrance in the jacket to maintain sufficient clearance for the entrance. The selection of the way to perform the floatover installation is a very important factor in the deck design and also in the selection of the installation barge. Both