A novel subsea shuttle tanker (SST) concept was recently proposed as a cost-effective alternative to subsea pipelines and tanker ships for liquid CO2 transportation between a source facility and a subsea well. It is envisioned that the SST will be deployed to transport CO2 to marginal subsea fields with an annual CO2 storage capacity less than 1 million tonnes; volumes that do not justify a full subsea field development. &nbsp;The SST is designed to be a fully autonomous underwater vessel with a cargo capacity of over 17,000 metric tons. It is 155 m long and it has a 17 m diameter hull. The vessel operates at a fixed water depth of between 50 to 200 m in a weather-independent environment. Furthermore, it travels at a slow speed for minimal energy consumption and maximal range. During the offloading process, the SST will approach the subsea well and land on the seabed just outside the safety radius of the well. After that, a remotely operated vehicle (ROV) will mate the offloading flowline to the SST, and the offloading process will start. The landing sequence is technically challenging for various reasons and warrants detailed analysis. First, the SST would have limited manoeuvrability due to the large inertia of the vessel and low effectiveness of the hydroplanes to provide steering at low speeds. Second, during the final phase before the SST lands, seabed boundary effects will intensify and lead to increased non-uniform, time-varying and drag-dominated load-effects. Last, the impact forces during landing should be minimised to allow for the lowest design load. Solving these technical challenges is crucial to meet SST’s design goals of having the least possible control appendices for maximum efficiency/range, and minimal structural weight for the largest cargo capacity. This paper will explore a hierarchical control scheme applied to the SST using a fully coupled 2D planar model that considers the most relevant load-effects. An optimal design that achieves the lowest control efforts and the smallest landing loads together is then proposed based on the results obtained.

Depth Control Modelling and Analysis of a Subsea Shuttle Tanker

Paper Proceedings: https://asmedigitalcollection.asme.org/OMAE/OMAE2021/volume/85116

OMAE 2021

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