Anti-Spin Control for Marine Propulsion Systems Øyvind N. Smogeli 1 , Jostein Hansen 2 , Asgeir J. Sørensen 1 and Tor Arne Johansen 2 Abstract— An anti-spin controller for marine propulsion systems in rough seas is developed. From measurements of motor torque and propeller shaft speed, an observer providing an accurate estimate of the propeller load torque is used to calculate an estimate of the torque loss. A monitoring algo- rithm utilizing the estimated torque loss detects ventilation incidents, and activates the anti-spin control action. When a ventilation situation is detected, the anti-spin control action will reduce the propeller shaft speed to some optimal value, using a combined power/torque controller. The ultimate goal is to minimize the effect of ventilation, and hence increase the thrust production, limit the transients in the power system and reduce the mechanical wear and tear of the propulsion system components. Simulations are provided to validate the performance of the control scheme. I. I NTRODUCTION Presently, dynamic positioning (DP) systems have limi- tations in rough seas. The reasons for this are limitations in the thrust capability and the available power, and reduced performance of the control system due to thrust losses and unmodelled nonlinearities. The most severe thrust loss effects that may be experienced are due to ventilation, which in this work is used to describe air suction to a submerged propeller and in-and-out-of water effects. Ventilation leads to an abrupt and large loss of propeller thrust and load torque [1]. This is challenging for the local thruster controller, which is trying to fulfill the high-level control commands from the DP system. The effects of ventilation are loss of thrust, excessive wear and tear of mechanical components and undesired power transients. It is typically induced by large vessel motions and large waves encountered during harsh weather conditions, and is frequently experienced by e.g. tunnel thrusters and main propellers on offshore supply vessels and shuttle tankers. In order to counteract the problem of ventilation, the concept of anti-spin thruster control was introduced in [2]. Model tests and simulations of a marine thruster showed the feasibility of increasing thrust production, reducing wear and tear on the mechanical propulsion system and limiting transients in the power system during severe thrust loss incidents by means of controlling the thruster spin. The anti-spin concept was divided in three: Detection, Switching 1 Ø. N. Smogeli and A. J. Sørensen are with Department of Marine Technology, Norwegian University of Science and Tech- nology, NO-7491 Trondheim, Norway. E-mail: [oyvind.smogeli, as- geir.sorensen]@marin.ntnu.no. 2 J. Hansen and T. A. Johansen are with Department of Engineering Cybernetics, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway. E-mail: [jostein.hansen, tor.arne.johansen]@itk.ntnu.no. and Control. However, no explicit control scheme for a general operational setting was presented. This paper provides an anti-spin control scheme that im- proves all three parts of the concept, utilizing recent results in thruster control. There are many similarities to car wheel anti-spin control [3], which has served as motivation for the research presented here. The ventilation detection scheme, which triggers the anti-spin control action, is implemented by monitoring an estimate of the propeller torque loss. The estimated torque loss is based on a propeller load torque observer [4]. The torque loss estimate is a convenient variable for detecting the ventilation situations, as it gives explicit and instantaneous information on the thruster load condition. In this work the switching algorithm is by a large part bypassed, as the thruster controller presented here is valid for all load conditions. This simplifies the overall control scheme, and removes any concerns about ensuring bumpless transfer between the various controllers. The torque loss estimate is also used as a basis for the anti- spin control action. This work is a continuation of the research on propulsion control, where contributions have been made by for exam- ple [5], [6] and the references therein. The local thruster controller used in this work is a combined power/torque control scheme. Power and torque thruster control in marine propulsion systems were first introduced by [7], and recently refined to the combined power/torque control scheme by [4]. II. THRUSTER MODELLING The thruster is modelled as an electric motor, a shaft with friction and a hydrodynamically loaded propeller. The rotational dynamics are described by the following equations: ˙ Q m = 1 T m (Q c − Q m ), J ˙ ω = Q m − Q p − K ω ω, Q p = f Q (θ,ξ), (1) where T m is the motor time constant, Q c is the commanded torque, Q m is the motor torque, J is the rotational inertia of the propeller including added mass, shaft, gears and motor, K ω is a linear friction coefficient, ω is the rotational speed of the propeller in rad/s, and Q p is the propeller load torque. The load torque is modelled as a general function f Q of fixed thruster parameters θ (i.e. propeller diameter, position, number of propeller blades, pitch ratio, propeller blade expanded-area ratio) and variables ξ (i.e. shaft speed,