Objectif On the basis of the tender project for a direct founded concrete platform on the marginal Rolf field in the Danish sector of the North Sea it is proposed to develop methods and to carry out project details in such a way that the platform may be reused on another marginal field after the first one has been depleted.The need for a reusable off-shore platform has been furthered by the development in the North Sea for exploitation of earlier found hydrocarbon reservoirs which at the time of exploration were not considered to be commercially exploitable.The study has shown that for certain geotechnical conditions likely to be found to a reasonable extent in the EEC-part of the North Sea , a reinforced concrete monotower platform of the gravity type may be installed at water depths from approximately 40 m up to 70 m.The platform, which initially is designed for environmental loads governing at the high water level has subsequently been checked for positioning at lower levels with 10 m interval applying the corresponding modifications of the environmental loads.The horizontal extension of the hexagonal caisson is determined in order to allow the major jack-up drilling rigs operating in the North Sea to be placed in a working position over the platform.The platform is made removable by applying a short skirt, which enable the platforms to be pulled free of the sea bed, when water pressure is introduced in the different skirt departments.The platform occupies 3,117 m2 and contains approx. 17,000 m3 of reinforced concrete. The size is determined partly by the stability at the different water depths which are intended to be covered within the range of application partly by the hability of being able to float without application of expensive and unhandy floating tanks. In spite of the considerable amount of concrete required, the platform is considred competitive, due to1) the minimizing of the offshore works proper and2) the possibility of reusing, which may limit the cost for future application to the cost of a part of the marine works only.The platform can advantageously be constructed at water depths as low as approximately 16 m and towed to the ballasting site through shallow water.Due to its flexibility it is specially suited for the marginal oil fields in the North Sea.The cost of the first installation is estimated to be DKK 240 millions, however, subsequent installations are much less. In case more than one platform will be required at the same time, savings can be obtained by applying the same dry dock, slip form and fitting out quay for the different platforms.Even though international standards for the removal and retention of obsolete platforms has not yet been agreed upon, there is no doubt, that the worldwide industry faces a multibillion dollar expenditure as it grapples with the disposal of a large amount of installations as offshore fields stop producing. Considering these prospects, the removable and reusable platform has a great advantage, always being scheduled for reinstallation in another offshore field.The proposed concrete platform may be refloated and moved to another location provided that the following activities can be carried out:- Disconnecting pipe systems etc.- Refloating of platform.- Inspection/cleaning of skirt and skirt departments.- Inspection/cleaning of pipe systems etc. for ballasting /injection.- Transport to another location.- Sinking of platform.- Injection of skirt departments.All the activities mentioned are actually based upon proven techniques but may impede on another. However, it is assumed that working procedures may be developed and that suitable materials may be selected, which allow for the above order of operations. As an example it may be mentioned that an injection system which has been used for cement mortar is not reusable right away. Likewise, comprehensive studies are required to ensure that the material to be used for filling the void between the platform bottom and the sea bottom can be removed in an economical way. Cement mortar ispresumably not suitable and therefore it may be required to find another material.In connection with the refloating of the platform, it is necessary to analyse again and possibly change the proposed sinking procedure to make it fully reversible with due regard to an assumed 2-10 years' interval between the two operations. Likewise, the original platform design will have to be analysed for possible new loading cases.The above studies and investigations shall result in the description of procedures, technical calculations and drawings. Programme(s) ENG-HYDROCARB 1C - Programme (EEC) of Community projects in the hydrocarbons sector, 1973-1985 Thème(s) 1.9 - PRODUCTION SYSTEMS Appel à propositions Data not available Régime de financement DEM - Demonstration contracts Coordinateur CHRISTIANI & NIELSEN A/S Contribution de l’UE Aucune donnée Adresse VESTER FARIMAGSGADE 41 1501 Copenhagen Danemark Voir sur la carte Coût total Aucune donnée
