Station Keeping of a Subsea Shuttle Tanker System Under Extreme Current During Offloading
Master thesis
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https://hdl.handle.net/11250/3014781Utgivelsesdato
2022Metadata
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Sammendrag
A subsea shuttle tanker has been proposed as a multipurpose, versatile transport and storagesystem. This paper presents the station keeping challenge of the subsea shuttle tanker designduring underwater loading and offloading at a subsea well under an extreme currentenvironment. Understanding the behaviour of the proposed subsea shuttle tanker duringoffloading in extreme currents is vital for both the design of the subsea shuttle tanker itselfbut 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 awater depth of approximately 70 metres. Recent studies have shown that the drag forceexerted on the subsea shuttle tanker body is up to 80 times larger for side-ways currentcompared to the head-on current. With current-waning capabilities, the generated lift forcesare low, and thus the subsea shuttle tanker will use less effort to maintain its desired positionand water depth. The paper further investigates the movement of the subsea shuttle tankerduring offloading with extreme current speeds, i.e., above 1.6 m/s, in the surge, heave, andpitch motions, respectively. The planar model is built up using a Luenberger observer, wherethe vessel motions are measured and fed into a linear quadratic regulator (LQR) forcalculations of the control input. The LQR control’s primary focus is to hold and achieve thetarget for the subsea shuttle tanker during the offloading process, i.e., minimize the horizontaland vertical motion. Finally, a state-of-the-art probabilistic method is used to predict themaximum potential displacement during offloading, i.e., the Average Exceedance RateMethod.