This paper presents the technical and economic feasibility analysis of the subsea shuttle tanker (SST). The SST is proposed as an alternative to subsea pipelines and surface tankers with the primary purpose of transporting CO₂ autonomously underwater from onshore facilities to subsea wells for direct injection at marginal subsea fields. In contrast to highly weather-dependent surface tanker operations, the SST can operate in any condition underwater. The technical–economic analysis is performed in two steps. First, the SST’s technical feasibility is evaluated by investigating designs with lower and higher capacities. The purpose is to observe the appearance of technical limits (if present) when the SST is scaled down or up in size. Second, an economic analysis is performed using the well-reviewed cost models from the publicly available Zero Emissions Platform (ZEP) and Maritime Un-manned Navigation through Intelligence in Networks (MUNIN) D9.3 reports. The scenarios considered are CO₂ transport volumes of 1 to 20 million tons per annum (mtpa) with transport distances of 180 km to 1500 km in which the cost per ton of CO₂ is compared between offshore pipelines, crewed/autonomous tanker ships, and SST. The results show that SSTs with cargo capacities 10,569 m³, 23,239 m³, and 40,730 m³ are technically feasible. Furthermore, the SSTs are competitive for short and intermediate distances of 180–750 km and smaller CO₂ volumes of 1–2.5 mtpa. Lastly, it is mentioned that the SST design used the DNVGL Rules for Classification for Naval Vessels, Part 4 Sub-surface ships, Chapter 1 Submarine, DNVGL-RU-NAVAL-Pt4Ch1, which is primarily catered towards military submarine design. It is expected that a dedicated structural design code that is optimized for the SST would reduce the structural weight and corresponding capital expenditure (CAPEX).

A subsea shuttle tanker (SST) concept for liquid carbon dioxide transportation was recently proposed to support studies evaluating the ultra-efficient underwater cargo submarine concept. One important topic is the position keeping ability of SST during the offloading process. In this process, the SST hovers above the well and connects with the wellhead using a flowline. This process takes around 4 h. Ocean currents can cause tremendous drag forces on the subsea shuttle tanker during this period. The flow velocities over hydroplanes are low throughout this process, and the generated lift forces are generally insufficient to maintain the SST’s depth. The ballast tanks cannot provide such fast actuation to cope with the fluctuation of the current. It is envisioned that tunnel thrusters that can provide higher frequency actuation are required. This paper develops a maneuvering model and designs a linear quadratic regulator that facilitates the SST station-keeping problem in stochastic current. As case studies, the SST footprints at 0.5 m/s, 1.0 m/s, and 1.5 m/s mean current speeds are presented. Numerical results show that the designed hovering control system can ensure the SST’s stationary during offloading. The required thrust from thrusters and the propeller are presented. The presented model can serve as a basis for obtaining a more efficient design of the SST and provide recommendations for the SST operation.

This work investigates the hydrodynamic performance of a subsea shuttle, an extra-large freight submarine, during near-seabed operation. The three-dimensional Reynolds-averaged Navier–Stokes method combined with the k − ω shear stress transport model is used to predict the pressure, skin friction, drag, and lift forces acting on the subsea shuttle. The present numerical model is verified and validated against the experimental and numerical data from the SUBOFF-1 project, a standard submarine model developed by the Defense Advanced Research Projects Agency. Two operational scenarios are considered in this study: (1) the subsea shuttle traveling near the seabed with a forward speed; (2) the subsea shuttle hovering close to the seabed and subject to an incoming current flow. A representative seabed boundary layer profile is considered in the analyses. A fully developed boundary layer profile is generated using 1D simulations and implemented as the inlet boundary condition in the 3D simulations. The effects of the gap ratio between the subsea shuttle and the seabed, and the inflow speed of the boundary layer flow on the hydrodynamic properties of the subsea shuttle are evaluated and discussed in detail.

A novel subsea shuttle tanker (SST) concept was proposed as a cost-effective alternative to subsea pipelines and tanker ships for liquid CO₂ transportation between a source facility and a subsea well. The SST will be deployed to transport CO₂ to marginal subsea fields with an annual CO₂ storage capacity around 1 million tonnes. A baseline design was recently developed by the authors to support research work aimed at assessing large and ultra-efficient subsea cargo drone technology. One crucial aspect is the development of SST's operation envelope, i.e., the safe depth versus speed regions. The development of this envelope entails comprehensive and detailed studies of SST's dynamic load-effects under all expected operating scenarios which in the early concept development phase can be performed using suitable computational models. In this technical brief, the initial development of such a model is unveiled. This fully coupled 2D planar model considers the most relevant load-effects which are from hydrodynamics, hydrostatics, and control surface induced loads. The most important features of the model such as the derivation of hydrodynamic derivatives and model verification are also discussed. As an example, this model is then used to study the depth control problem which is a key aspect in the determination of the safety operational envelope. The results show that unsuitable control schemes that do not look ahead in the trajectory lead to undesirable results. In contrast, a feed-forward heading control method achieves a good and fast control response.