Inertial Sensors for Risk-Based Redundancy in Dynamic Positioning

In this paper we present an alternative configuration of sensors, position and heading reference systems for dynamically positioned (DP) vessels. The approach uses a sensor structure based on low-cost inertial measurements units (IMUs), satisfying fault tolerance against single-point failures that is at the essence of the IMO guidelines for both DP class 2 and 3 vessels. Recent results have shown that dual-redundant position and heading reference systems are sufficient to prevent loss of position within some well-defined time horizons by exploiting sensor fusion of the reference systems and triple-redundant MEMS-based IMUs. These IMUs also function as Vertical Reference Units (VRUs), since vessel motions is obtained using the same IMU configuration and sensor fusion framework. In this proposition, the acceleration measurements provided by the IMUs make wind and other force sensors unnecessary, except possibly for an advisory role. The proposed framework has the potential to significantly reduce the cost of dynamic positioning systems without compromising safety. NOMENCLATURE {b} BODY coordinate frame {t} Tangent frame equivalent to North East Down (NED) where {t} is Earth fixed and rotates with the Earth rate {e} Earth Centered Earth Fixed coordinate frame {i} Inertial coordinate frame ptb ∈ R3 Position in {t} frame vtb ∈ R3 Linear velocity in NED frame ωib ∈ R3 Angular rate of the vessel({b} frame) relative the inertial frame in the BODY frame, given in the BODY frame f ib ∈R3 Spesific force of the vessel({b} frame) relative the inertial frame in the BODY frame, given in the BODY frame φ ,θ ,ψ Euler angles: Roll, pitch, yaw R(ψ) ∈ SO(3) Rotation matrix: For rotating the BODY z-axis to the NED frame Rb ∈ SO(3) Full rotation matrix, describing the attitude between the {b} and the {t} frame. rb ∈ R3 Leverarm from vessel CO to given position reference system (PosRef) px North position components given in {t} (similar to NED) 1 Copyright c © 2017 by ASME py East position components given in {t} (similar to NED) ub tb,v b tb Surge and sway velocity r Yaw rate τ? Generalized force vector M Mass matrix. M > 0 ∈ R3×3. D Linear damping matrix. D > 0 ∈ R3×3. dGNSS Differential Global Navigation Satellite Systems DP Dynamic Positioning FDI Fault Detection and Isolation HPR Hydroacoustic Position Reference IMU Inertial Measurement Unit MEMS Micro-electrical-mechanical System PosRef Position Reference VRS Vertical Reference Sensor VRU Vertical Reference Unit ZUPT Zero Velocity Update INTRODUCTION The International Maritime Organization (IMO) have issued guidelines for vessels with dynamic positioning (DP) systems, [1], to reduce the risk of loss of position during DP operations. Classification societies have defined class notations based on these guidelines. The DP classifications are dependent on vessel type, operation and the potential consequences in the event of loss of position. These range from equipment class 1 to 3, while some societies also operate with equipment class 0. The general functional requirement is that single failure in an active component should not result in loss of position. This is handled by redundancy in all active components, where redundancy, according to [2], is defined as: Redundancy. The ability of a component or system to maintain its function when one failure has occurred. Redundancy can be achieved, for instance, by installation of multiple components, systems or alternative means of performing a function. For equipment class 2 and 3, the following main rules are given by MSC/Circ. 645, [1]: Equipment class 2: Redundancy in all active components. Equipment class 3: Redundancy in all active components and physical separation (A.60) of the components. Equipment class 1 (and 0) allows for loss of position in the event of a single failure. Therefore, these classes will not be dealt with further in the paper. A more detailed description is given in Tab. 1, where class notations from DNV GL, American Bureau of Shipping (ABS) and Lloyds Register (LR) are given. Concerning the sensors system on a DP vessel, both DP notations related to equipment class 2 and 3 often implement the redundancy requirement by enforcing a triple-redundancy requirement related sensor to installation. In the authors’ opinion, this practice has so far not taken advantage of the full potential of MEMS inertial sensors, obtained from recent developments, and the knowledge of upsides and downsides with the different standard onboard sensors. The triple-redundancy requirements may also in certain circumstances impair robustness and safety since the potential downsides outweighs the potential upsides. Especially so related to the wind sensor where the conclusions from [3], state that using the wind sensor is not essential for maintaining positing and, in fact, that using wind-feed-forward control can be detrimental in stationkeeping. This paper proposes an alternative sensor configuration to that of today’s state-of-the-art class notations. The configuration has low-cost redundant MEMS IMUs at its center. With this structure, the redundant IMUs, with appropriate software, have the potential to replace existing sensor solutions. In addition, dual-redundant PosRefs and gyrocompasses are deemed sufficient to maintain position and heading. The latter is due to FDI of the PosRefs and gyrocompases obtained by exploiting the redundant IMUs and estimators. Moreover, the sensor structure is compliant with the main principle of both equipment class 2 and 3; no single point of failure in an active (sensor) component shall results in loss of position. With IMUs, all accelerations, induced by forces affecting the vessel, are measured directly by accelerometers. Therefore, the wind sensors can be circumvented without impeding the DP control performance. Furthermore, the industry-standard VRU solutions can also be replaced since the vessel motions are obtained using the same redundant-IMU configuration and sensor fusion. All in all, the proposed sensor configuration has the potential to significantly reduce the cost of dynamic positioning systems without compromising safety. SENSOR AND CONTROL SYSTEMS