Modelling of a Subsea Shuttle Tanker Hovering in Ocean Currents

Abstract

Abstract A baseline design of subsea shuttle tanker (SST) for liquid carbon dioxide transportation was recently proposed by authors 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. When the subsea shuttle tanker is offloading at the storage site, it hovers above the well in the vicinity and connects with the wellhead using a flowline. The entire offloading process takes around four hours. Ocean currents can cause tremendous drag forces on the subsea shuttle tanker during this period. Preliminary studies show that the drag force introduced by the sideways current is over 80 times that of the heading current. Therefore, the SST would current-vane to reduce load effects from currents and maintain its position. During this process, the flow velocities over hydroplanes are low, and the generated lift forces are generally insufficient to maintain the SST’s depth. In the baseline design, compensation tanks are used to maintain the depth when hovering. However, the ballasting and de-ballasting of tanks are generally slow. It is, therefore, envisioned that although it can compensate for the weight change, additional thrusters that can provide actuation with higher frequencies are still required. This paper presents a planar model to study the SST’s vertical position keeping ability in current using its propeller and tunnel thrusters. The ocean current follows a first-order Gauss-Markov process. SST motions are first measured by a Luenberger observer and then delivered to a Linear Quadratic Regulator to calculate the control input. This work facilitates the SST station keeping problem and helps with the design of SST control units. Effects from controller gains and observer gains on the SST motions are discussed. Numerical results show that the designed hovering control system can ensure the SST’s stationary during offloading. Finally, the SST footprints at 0.5 m/s, 1.0 m/s, and 1.5 m/s mean current speed are presented.