This paper presents a new concept for offshore unloading of cryogenic pressurized CO_2. The offshore unloading system will transport the liquid CO_2 from the dedicated CO_2 ship to the wellhead on the platform at the required temperature and pressure. During the unloading phase the ship is connected to a submerged turret loading, STL, system. The CO_2 is pumped to a pressure high enough to avoid phase transition in the transfer lines. A flexible riser, a sub-sea pipeline and an insulated pipeline in the platform shaft bring the CO_2 from the unloading location to the topside of the platform. The CO_2 is pumped to the injection pressure and heated to avoid operational problems before it is injected into the reservoir for IOR (Increased Oil Recovery) using the existing water injection wells. In the North Sea a typical wellhead pressure for injection of CO_2 is 200-300 bar. The temperature should be high enough to avoid hydrate formation downstream the wellhead. CO_2 hydrates cannot form at temperatures higher than 15 °C; hence by heating the CO_2 to a higher temperature the hydrate problem should be omitted. The CO_2 can be heated at the ship by using seawater-based heat exchangers or, in case of low seawater temperature by other heating systems. On the platform the CO_2 can be heated by excess heat if available. The CO_2 will also be heated from the wellhead to the reservoir and in the transfer lines from the ship to the platform. Several challenges have been discussed, among these icing problems when unloading CO_2 at -50°C and how to avoid dry ice formation in depressurisation of the CO_2 pipeline. Due to the large investment costs of the ship a high unloading rate of the CO_2 is crucial to lower the total transportation costs to a competitive level. However, an intermediate offshore storage is costly. Therefore, in contrast to pipeline transport, the CO_2 will be unloaded in batches. Despite the technical and economical challenges with offshore unloading of CO_2, the ships flexibility to deliver CO_2 directly from the source to the oil fields might be a key element in providing CO_2 for IOR to the North Sea.
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