| dc.description.abstract | A subsea shuttle tanker has been proposed as a multipurpose, versatile transport and storage
system. This paper presents the station keeping challenge of the subsea shuttle tanker design
during underwater loading and offloading at a subsea well under an extreme current
environment. Understanding the behaviour of the proposed subsea shuttle tanker during
offloading in extreme currents is vital for both the design of the subsea shuttle tanker itself
but also the required actuator effort needed to uphold the demanded station keeping abilities.
During the offloading process, the hoovering subsea shuttle tanker would current-vane in a
water depth of approximately 70 metres. Recent studies have shown that the drag force
exerted on the subsea shuttle tanker body is up to 80 times larger for side-ways current
compared to the head-on current. With current-waning capabilities, the generated lift forces
are low, and thus the subsea shuttle tanker will use less effort to maintain its desired position
and water depth. The paper further investigates the movement of the subsea shuttle tanker
during offloading with extreme current speeds, i.e., above 1.6 m/s, in the surge, heave, and
pitch motions, respectively. The planar model is built up using a Luenberger observer, where
the vessel motions are measured and fed into a linear quadratic regulator (LQR) for
calculations of the control input. The LQR control’s primary focus is to hold and achieve the
target for the subsea shuttle tanker during the offloading process, i.e., minimize the horizontal
and vertical motion. Finally, a state-of-the-art probabilistic method is used to predict the
maximum potential displacement during offloading, i.e., the Average Exceedance Rate
Method. | |
