WO2015028636A1

WO2015028636A1 - Power control in marine vessel

Info

Publication number: WO2015028636A1
Application number: PCT/EP2014/068423
Authority: WO
Inventors: Ola Erik Fjellstad; Eirik MATHIESEN; Morten BREIVIK; Arne Rinnan
Original assignee: Kongsberg Maritime As
Priority date: 2013-08-30
Filing date: 2014-08-29
Publication date: 2015-03-05
Also published as: NO20131159A1; NO335595B1

Abstract

The invention relates to a system and method for improving the efficiency of the power plant in a vessel. The system comprises a motion reference unit measuring means for monitoring the vessel motions, and prediction means for predicting the vessel motions based on said measurements, the system also comprising a dynamic load prediction unit calculating the dynamic load on the system at least partially based on said predicted motions.

Description

POWER CONTROL IN MARINE VESSEL

The invention is a method for reducing load and frequency variations induced by variations in heavy consumer load in the power distribution system for a dynamically positioned vessel.

Field of the invention

The present invention relates to a method for reducing load and frequency variations in the high voltage power distribution system while the vessel is controlled by a dynamic positioning (DP) control system or its like by adjusting the available power or torque for the DP controlled thrusters in such a manner that the station keeping conditions are not undermined. The present invention presents way of linking load control and

position/velocity control together that has never been done before. Background of the invention

A dynamic positioning system controls the alongship, athwartship and yaw axis motion of a vessel, rig, barge or its like. A vessel controlled by a DP control system or its like, shall always strive to fulfill a commanded velocity and position in the axes that are controlled. The DP system controls the vessel movements by the means of thrusters, propellers and rudders. For simplicity and without loss of generality we will use the term thruster for any propulsion means in the rest of the description of the invention. The most common power system today is diesel electric, meaning that the thrusters are propelled by an electric drive and that power is provided by power generators such as diesel-generators, fuel cells, gas-turbines, dual fuel engines, etc. The power is taken from a power plant with many consumers, where the thrusters normally being the dominant one. The power is normally supplied to the power plant by diesel-generators. A high number of diesel generators are normally required to be connected to the power plant in order to keep a stable frequency in the occurrence of consumer load variations from e.g. heave compensation, draw-work, winch and crane. Frequency variations can be fatal for the power system and might lead to black-out, fallout of subsystems, synchronization problems for generators that shall be connected to the power grid and increased fuel consumption. The offshore industry has for many years desired to reduce the number of online generators without risk of frequency variations and potential black-out, but no substantial solution has been provided for this problem. There are several benefits from reducing the number of online generators, such as reduced Nox emission, reduced sooting, reduced fuel consumption, and reduced maintenance on the engines.

In the thesis by Damir Radan "Integrated Control of Marine Electrical Power Systems", Department of Marine Technology, Norwegian University of Science and Technology, 28 January 2008, several aspects of control of power systems are discussed, and among them the reduction of load and frequency variations in a power distribution system,

A system for handling this is proposed in Norwegian Patent Application No 20120344, which relates to a method for reducing frequency and/or voltage variations in the power distribution by the use of predicted future load changes used as feed forward towards speed / power control system on a Motor Generator Set (MGS), or turbo jet assist system for the engine, or Automatic Voltage Regulator (AVR) on the generator. Several prediction load subsystems are mentioned in the application including DP thruster output, set-point change or start/stop requests to heavy consumers and cyclic load predictors based on pattern recognition. The solution discussed in the NO20120344 application has the disadvantage that it is based primarily on extrapolations based on occurred incidences, which has a limited value in sometimes chaotic environments.

The present invention is also related to NO20120507 describing a system for dynamic load prediction by integrating overall predicted load changes on heavy consumers or other sudden re-configuration of the distribution system together with the

speed/power/voltage control system combined with methods for predicting load variations.

None of these, however, take into account the effects of external forces affecting the power consumption. This is obtained by a system as presented in the accompanying claims. More specifically the invention includes the use of a Motion Reference Unit (MRU; or Vertical Reference Unit - VRU) providing roll, pitch and heave (RPH) positions and velocities, in order to predict wave motion dynamic load on the ship power system. The DP systems are mainly concerned with motions in surge, sway and yaw (SSY) directions, and use sensor inputs accordingly. Hence, RPH-based subsystems for load predictions will not establish undesirable closed loops or chained reactions together with the DP system. The load predictions are calculated from direct RPH

measurements for motion prediction, in combination with control loop states from all RPH-based heavy consumer subsystems. The RPH-based load predictions can be used in the Predicted Load Allocation of the DLP application in the same way as the other mentioned load predictors.

The load prediction may be performed using two different methods. The short term indicating the immediate need because of wave motions or similar where the trajectory of the vessel motions are predicted and thus being capable of calculating the immediate need for compensation. This may for example be based on known models of the vessel and extrapolation based on the measured movements or simulations.

Long term movements may be based on measurements over longer periods of time providing e.g. the frequency and amplitudes of the RPH movements, which may require changes in the load allocation in the power plant. The statistical methods may be of any suitable kind.

The present invention will be described more in detail below, with reference to the accompanying drawings illustrating the invention by way of examples:

Fig. 1 illustrates schematically the vessel with related equipment.

Fig. 2 illustrates the system function.

Referring to figure 1 the RPH-measurements from a vertical reference unit such as the MRU 1 can be used to provide wave motion load predictions to the power system based on RPH-based heavy consumer subsystems including:

- Hydraulic cranes 2 - Drilling heave compensation / Riser tensioner 3

- Helicopter deck stabilization 4

- Electric winch / Dynamic mooring systems 5

- Roll stabilization systems such as rudder roll stabilization 6 and active fins 7. - Propeller ventilation prediction 8

All of these will have a power consumption depending on the vessel movements and may be predicted based on empirical or calculated information. General wave motion cyclic load variations may be calculated using MRU RHP-measurements directly instead of pattern recognition based on heavy consumer subsystem outputs. This also includes statistical information such as mean motion amplitudes and frequencies.

In figure 2 the motion reference unit 1 provides an RPH signal to the different heavy consumer subsystems 11-16 for dynamic control purposes, and to the dynamic load prediction (DLP) system 19 for motion prediction . The motion prediction combined with heavy consumer subsystem 11-16 outputs is used to estimate the required power in the dynamic load prediction (DLP) system 19, which in turn provides the dynamic load control (DLC) system 20 with information for controlling the available generators 21. A load distribution system is also used for controlling how the predicted load and available power is allocated within the system.

The different power consumers compensating for the movements will be controlled according to their specific needs, based on known information about the characteristics during the operation. The exemplified units 11-16 above may thus be control systems as described below.

Hydraulic Cranes 11

Active heave compensation (AHC) is a technique used in lifting equipment such as hydraulic cranes to reduce the influence of waves upon offshore operations. AHC comprises a control system that actively tries to compensate for any movement at a specific point, using power to gain accuracy. The active heave compensation is typically used for keeping the hydraulic crane load at a fixed position relative to the seabed, and to prevent the vessel motion from being transferred to the load. The AHC control is connected to a motion reference unit (MRU) that feeds the system with accurate realtime information of the crane tip motion. The crane controller will pay in and pay out on the winch to compensate for the crane tip motion.

Marine Riser Systems 14

A marine riser tensioner is a device used on an offshore drilling vessel which provides a near constant upward force on the drilling riser independent of the movement of the floating drill vessel. The marine riser is connected to the wellhead on the seabed and therefore the tensioner must manage the differential movements between the riser and the rig. If there were no tensioner and the rig moved downwards, the riser would buckle; if the rig rose then high forces would be transmitted to the riser and it would stretch and be damaged. The riser control is connected to a motion reference unit (MRU) that feeds the system with accurate real-time information of riser motion. The riser controller will pay in and pay out on the riser to compensate for the heave motion of the drilling rig.

Active Helideck Stabilization 13

Helicopter access in due time is crucial for many offshore support vessels, and delays represent a significant cost driver. Therefore, increasing the weather window available for safe landing and take-off, will have a direct impact on operational efficiency for such vessels. Active helideck stabilization systems solve some of the critical safety issues of landing a helicopter on moving helidecks offshore. The stabilization systems is used to control the deck movement compensating for wave induced roll, pitch and heave vessel motion, in order to ensure gentle helicopter landing in wave conditions. The helideck stabilization system is connected to a motion reference unit (MRU) that feeds the system with accurate real-time information of helideck motion. The helideck controller will regulate the position of hydraulic cylinders connected to the helideck to compensate for the wave induced vessel motion. Electric Winch Systems 12

In an electric winch system, the wave movement is compensated by automatically driving the winch in the opposite direction at the same speed. The hook of the winch will thus keep its position relative to the seabed or any fixed point outside the vessel. AHC winches are used in ROV-systems and for lifting equipment that is to operate near or at the seabed. Other winch applications are active mooring systems for anchoring of the vessel to the seafloor, and towed seismic cable spreads for which active

compensation can include tension control, aiming to keep cable tension at a certain level while operating in waves. The winch control is connected to a motion reference unit (MRU) that feeds the system with accurate real-time information of vessel motion at the location of the winch. Winch pull and speed are automatically controlled to compensate for vessel movement during deployment, positioning or towing of load.

Rudder Roll Stabilization 6,15

The rudder's main function is to correct the heading of a ship; however, depending on the type of ship, the rudder may also be used to produce, or correct, roll motion. Rudder roll stabilization consists of using rudder-induced roll motion to reduce the roll motion induced by waves. Reduced roll motion is important for many reasons; transverse accelerations that occur due to rolling interrupt tasks performed by the crew and increases the amount of time required to complete a mission, roll accelerations may produce cargo damage, roll motion increases hull resistance, and large roll angles limit crew capability to handle equipment on board, and/or to launch and recover systems. The rudder roll stabilization system receives measurements of ship motion from a motion reference unit (MRU) and use these measurements in an automatic control system providing suitable rudder commands.

Active Fin Control 7,15

Active fin stabilizers are normally used to reduce the roll that a vessel experiences while under way or, more recently, while at rest. The fins extend beyond the hull of the vessel below the waterline and alter their angle of attack depending upon heel angle and rate- of-roll of the vessel. They operate similar to airplane ailerons. Cruise ships and yachts frequently use this type of stabilizer system. A motion reference unit (MRU) provides motion data to the active fin controller which calculates the appropriate fin angle reference for the fin actuator. Propeller Ventilation Handling 16

Propeller ventilation occurs when the ship propeller drags air from above the water into the blade. Ventilation, particularly when the propeller rises above the water surface, can damage the motor by allowing it to exceed its maximum permitted speed, or cause ship generator tripping. Roll, pitch and heave measurements from a motion reference unit (MRU) can be used together with an adequate vessel model to predict that propeller ventilation is about to occur, and take the appropriate action in the motor/generator dynamic load controller in advance of the ventilation/non-ventilation transition. The present invention may utilize the system according to NO20120507 in relatively high frequency wave pattern loads. The MRU may thus be used to predict the dynamic loads, as highlighted in each of the following examples, should be included. The said set of examples are discussed in the following text, and illustrated in figure 2. - MRU RPH measurements are used by the heavy consumer subsystems as input for automatic control. Typically, both RPH positions and velocities are used in e.g. PID-controllers running on control-computers for each subsystem. Some vessels will have one MRU or heavy consumer located at the measurement point of interest, whereas other vessels have a reduced number of MRUs such that the motion data must be lever arm compensated for some or all subsystem locations before they are distributed.

MRU RPH measurements are used for the calculation of short term predicted motion and long term statistical motion data such as significant heave amplitude and frequency. Short term predicted motion is an online real-time process with range within milliseconds or seconds, appropriate for dynamic control of the heavy consumers in the wave motion range. The long term statistical motion data may have range within minutes, hours, or even days, and represent useful information for manual or automatic planning of the Load Distribution.

Predicted motion together with heavy consumer subsystem controller outputs are used for short term load prediction by the Dynamic Load Predictor. Predicted motion is transformed to predicted power requirements for each heavy consumer subsystem, before combining it with the control output data. The output signal from various controllers may include both controller states and logic information such as actuator power, control deviations, change of regulator set-points and start/stop commands given by other subsystems or a human operator. Also, the predicted motion can be used to forecast exceptional states that are normally difficult to handle by simple controllers. One such example is the propeller ventilation handler, which will otherwise contribute to the Dynamic Load Predictor in a similar way as the different subsystem controllers. The outputs from the Dynamic Load Predictor are predicted power consumptions, which are allocated feed-forward signals to the Dynamic Load Controllers as described in patent no NO20120507.

Statistical data such as significant heave amplitude and frequency generally describe long term vessel dynamics, which means they are altered only seldom due to change in weather or operational conditions. Based on the vessel dynamics and knowledge of DP and HC requirements, the Load Allocation can decide an optimal distribution of thrusters and other heavy consumers among the different motor generator sets. Preferably, each motor generator should run at a constant load close to its rated level for maximum efficiency and minimum exhaust emission. Hence, some motor generators may be turned off completely in periods of expected lower load, so that a repair and maintenance schedules can be carried out on them more easily. The resulting configuration of thrusters, heavy consumers and generators are provided not only to the Dynamic Load Controller, but also to the DP for use in the thruster allocation algorithm inhere. The Load Allocation scheme comprises allocation of generator power and distribution so as to provide sufficient dynamic response to handle the predicted load requirements. This load allocation is typically performed automatically, but may be overridden manually by a human operator or through another control module if required.

The Dynamic Load Controller receives power requirement inputs from all systems onboard a vessel, with the Dynamic Positioning System normally having the highest priority. The requirements are distributed according to the configuration given by the Load Distributor. The outputs from the Dynamic Load Controller are the set-points for the motor generator sets as described in patent no NO20120344.

To summarize the invention thus relates to a system and method for improving the efficiency of the power plant in a vessel. The system comprises a motion reference unit 1 measuring means for monitoring the vessel motions, and prediction means 17, 18 for predicting the vessel motions based on said measurements. The predictions may be based on pre-programmed models for the vessel motions under different situations. The vessel motions may be monitored relative to a chosen reference, motion or

predetermined position and orientation.

The system also comprises a dynamic load prediction unit 19 calculating the dynamic load on the system at least partially based on said predicted motions.

Preferably the sensors in the motion reference unit 1 are adapted to measure roll, pitch and heave (RPH) positions and velocities, and the prediction means includes short term prediction means 17 being adapted to calculate a short term prediction of the motions such as wave movements or other environmental responses.

The short term motion prediction may include means 16 for predicting propeller ventilation, based on the movements calculating the load change resulting from a propeller ventilation, e.g. by reducing the power to the propeller about to run above water.

The dynamic load prediction unit may also be coupled to at least one load consumer 11- 15 on said vessel, each of which being adapted to provide a signal indicating a predicted power consumption based on its planned operation. Each consumer may also be coupled to the motion reference unit 1 so as to take the vessel motions into account when predicting the power consumption. The dynamic load prediction unit 19 being coupled to said dynamic load control unit 20 controlling the motor generator set 21 of the vessel based on the predicted load.. The prediction means may also include long term prediction means 18 being adapted to gather information about the movements of the vessel and calculate a long term prediction of the movements based on a statistical analysis of said data. This may include a load allocation unit 23 for calculating the load allocation based on the calculated long term motions and providing a signal to the load control system 20 indicating the preferred distribution of load over the generators in the power plant.

The dynamic load control unit may also be coupled to a dynamic positioning (DP) system 22 and being capable of communicating with the DP system so as also to adjust the power consumption in each thruster for adjusting the position of the vessel, and the consumer load control being adapted to assign an available power to each thruster in the system according to the required position or to maintain the vessel within a allowed window relative to an optimal position. The DP control system, if the required power for adjusting the position in at least one thruster exceeds the available power, may thus request an increase in the power made available for the thruster from the consumer load control.

Claims

C l a i m s

1. System for improving the efficiency of the power plant in a vessel, the system comprising

a motion reference unit (1) measuring means for monitoring the vessel motions, and prediction means (17, 18) for predicting the vessel motions based on said measurements,

the system also comprising a dynamic load prediction unit (19) calculating the dynamic load on the system at least partially based on said predicted motions.

2. System according to claim 1, wherein said sensors are adapted to measure roll, pitch and heave (RPH) positions and velocities, said prediction means including short term prediction means being adapted to calculate a short term prediction of the motions.

3. System according to claim 2, including a propeller ventilation prediction means, for based on the movements calculating the load change resulting from a propeller ventilation.

4. System according to claim 2, wherein the dynamic load prediction unit is also coupled to at least one load consumer on said vessel, each of which being adapted to provide a signal indicating a predicted power consumption, said dynamic load prediction unit being coupled to said dynamic load control unit.

5. System according to claim 4, wherein said at least one load consumer is coupled to said motion reference unit thus receiving a signal related to said motion and wherein the load consumer is adapted to provide a signal indicating the predicted power consumption as a function of said motions.

6. System according to claim 1, wherein said prediction means including long term prediction means being adapted to gather information about the movements of the vessel and calculate a long term prediction of the movements based on a statistical analysis of said data.

7. System according to claim 6, including a load allocation unit for calculating the load allocation based on the calculated long term motions and providing a signal to the load control system indicating the preferred distribution of load over the generators in the power plant.

8. System according to claim 1, including a dynamic load control being adapted to control a set of motor generators in the vessel based on the predicted load.

9. System according to claim 8, comprising a DP unit coupled to said dynamic load control unit and being adapted to adjust the power consumption in each thruster for adjusting the position of the vessel, and the consumer load control being adapted to assign an available power to each thruster in the system.

10. System according to claim 9, wherein the DP control system, if the required power for adjusting the position in at least one thruster exceeds the available power, requesting an increase in the power made available for the thruster from the consumer load control.

11. Method for improving the efficiency of the power plant in a vessel, comprising the steps of measuring the motions of a vessel relative to a reference predicting the vessel motions based on said measurements, and

calculating the dynamic load on the system at least partially based on said predicted motions.

12. Method according to claim 11, wherein said measured motions are roll, pitch and heave (RPH) positions and velocities, said motion prediction including short term prediction means based thereon.

13. Method according to claim 11, wherein the dynamic load prediction is also based on a signal received from at least one load consumer on said vessel, each of which being adapted to provide a signal indicating a predicted power consumption, said dynamic load prediction unit being coupled to said dynamic load control unit.

14. Method according to claim 11, wherein said prediction also includes long term prediction, comprising the step of calculating a long term prediction of the movements based on a statistical analysis of said motion data.