Offshore Concrete Structures Are Back

After a period with low demand for concrete offshore installations, the market is booming with concrete projects for LNG receiving terminals and offshore platforms. Norway is a world leader on offshore concrete platforms with 15 of the world ’s 30 larger offshore concrete structures located on the Norwegian continental shelf. The first of these platforms where the Ekofisk Tank installed in 1974 and the last was Troll gravity based structure installed in 1996. Now the Norwegian offshore concrete cluster, including DNV, has mobilised in a remarkable way. AkerKv ærner is responsible for design and construction of two offshore platforms for Shell ’s Sakhalin II project in Russia. In addition they are responsible for the design of Port Pelican (California) LNG receiving terminal for ChevronTexaco. They are also responsible for the design of the North Adriatic LNG receiving terminal for ExxonMobil in Italy. The Sakhalin project is located in a harsh and artic environment where concrete structures have proven excellent characteristics with a minimum of maintenance. The engineering is carried out in Norway. However, previous projects have shown that a local content of 70 percent may be expected.

The cost-optimal water depth for fixed concrete LNG terminals is approximately 20 m, while floating terminals often require a minimum water depth of approximately 60 m.

Less harm to the environment The driving force behind the development of the offshore LNG receiving terminals is the concern for long -term US gas supply combined with a general safety concern for increased handling of LNG. By bringing terminals offshore, it is possible to receive LNG in shallow waters without dredging environmentally sensitive areas, as well as removing LNG inventory away from populated areas. As an example, Port Pelican is located 64 km off the Louisiana coast line. Both ChevronTexaco and ExxonMobil have chosen a fixed gravity based concrete structure with an integrated LNG storage system and a regasification facility on top. Potential spill of LNG on a local concrete surface is not considered critical as concrete has excellent properties at low temperatures. So far, only fixed concrete LNG terminals are being designed, but the future calls for floating terminals. Typical challenges for floating terminals are the need of flexible LNG loading systems and to predict sloshing loads in containment tanks due to wave movements. DNV is involved in these areas by qualifying flexible loading systems and performing R&D programs for sloshing loads. DNV is prepared for the concrete future with updated offshore codes and participation in important technology development projects. Date: 2004-08-26 Author: Eva Halvorsen

Facts: To assist owners and designers in developing sound and reliable concrete structures, DNV has recently renewed offshore codes: • DNV-OS-C502 Offshore Concrete Structures • DNV-OS-C503 Concrete LNG Terminal Structures and Containment Systems • DNV-OSS-309 Verification, Certification and Classification of Gas Export and Receiving Terminals >>