Patent Application
20110100279
The invention relates to an active heave compensation system and to an active heave compensation method.
Heave compensation has been known for many years. Many solutions have been provided, some of which will be discussed below. In general, heave compensation provides for a compensation of wave motion on a load. The load may be submerged or partially submerged, thereby being subjected to the wave motion. Also, or instead thereof, it may be the case that the load is held by a floating platform (such as a ship), which is subjected to the wave motion. Further, many other cases may be imaginable where heave motion may be desired, such as a situation where a load is to be taken from or placed on a floating platform, the floating platform being subjected to wave motion. Heave compensation may be provided for any kind of load, e.g. a load to be carried by a crane or other lifting installation, constructions submerged under water such as pipeline laying equipment, etc. It is to be understood that the above examples are for illustration only, and are not intended to limit the scope of this document in any way.
Heave compensation systems can be subdivided inactive and passive heave compensation systems. Combinations of active and passive systems may be provided too. In a passive heave compensation system, a compressible medium is provided in a form of a gas spring, hydraulic system, etc. to provide for a compensation. In an active heave compensation system an actuator is provided to actively compensate for effects of the wave motion. Many constructions have been described in the literature. In general, in an active heave compensation system, use is made of a hydraulic system. As an example, a hydraulic cylinder may be provided which extends and compresses synchronously with the wave motion, thereby interacting with for example a cable holding the load. In each wave, energy is to be supplied to the hydraulic system to exert a force onto the load. Some of the energy may be regained in the other part of the heave motion cycle and e.g. stored by compression of a gas. In the next cycle, the compressed gas can then be applied to drive the load or at least to contribute thereto.
Although hydraulic/gas pressure active heave compensation has been extensively used in many configurations, a disadvantage is that this setup leads to a complex system and involves a risk of leakage of hydraulic fluid, resulting on the one hand in a relatively complex and costly system, while on the other hand requiring regular and secure maintenance to avoid leakage and risks of environmental pollution caused thereby.
In order to at least partly compensate the above-mentioned drawbacks of active heave compensation systems, the inventors have devised an active heave compensation system comprising a motor-generator to interact with a load and a control unit which is arranged to control operation of the motor-generator, the control unit being arranged to:
The active heave compensation system according to the invention thus comprises a combination of a motor-generator and an electrical storage element. In a first part of the wave motion cycle, the motor-generator acts as a motor and drives the load. In a second part of the wave motion cycle, energy is regained and the motor-generator acts as a generator thereby regenerating at least part of the energy with which the load has been driven in the first part of the wave motion cycle. The regenerated energy is stored in the electrical storage element. The stored energy can now be used in a first part of a following wave motion cycle to power the motor-generator. Within the scope of the invention, for the motor-generator, use can be made of a separate motor and a separate generator which both interact with the load, however in an advantageous embodiment, use is made of a motor type which acts as a generator, thus a motor which, when not provided with electrical energy, but when mechanically driven by a corresponding motion of the load, generates electrical energy thereby acting as a generator. Any type of motor-generator may be provided, as an example, use may be made of an three-phase asynchronous motor. The term motor-generator may in general terms be defined as an arrangement which is adapted to convert electrical energy into motion and to convert motion into electrical energy. Any type of electrical storage element can be used, however it is preferred that a capacitor is applied as a capacitor can provide for a low loss storage, thereby enhancing energy efficiency of the heave compensation system. Preferably, the capacitor comprises a super capacitor, as thereby a high capacitance value, and consequently a high energy storage capacity can be provided in a comparably small volume. Furthermore, a super capacitor may provide for a low series resistance, hence allowing a low loss energy storage, may allow a quick charging and discharging, may provide a high efficiency, and may provide a long operating life. Also a combination of a battery and capacitor, such as a super capacitor can be used as electrical storage element. While a capacitor can provide a high output energy and a battery can provide energy during a relatively long period, a combination could benefit from both features.
According to the invention, a control unit may be provided to control the motor-generator so as to drive the load in a first part of the wave motion cycle and to regenerate energy in the second part of the wave motion cycle. The control unit (comprising e.g. a microcontroller, microprocessor, or any programmable logic device, e.g. being provided with suitable program instructions to perform the actions as described) may thereto e.g. control a power supply associated with the motor-generator. The control unit may thereby control the power supply such as to power the motor-generator to drive the load in the first part of the cycle and to regenerate at least part of the energy in the second part of the cycle.
The storage element, such as the super capacitor, can be electrically connected in many ways. An advantageous configuration is achieved when the storage element is electrically connected parallel to a electrical power source to power the motor-generator. The power source may e.g. be formed by a mains voltage, a supply voltage of an installation in which the active heave compensation system is comprised, etc. Thereby, peaks on the power supply a voltage of the electrical power source true to the drawing of electrical power in the first part of the wave motion cycle and the regeneration in the second part of the wave motion cycle may be reduced due to a buffering of the electrical power source by the storage element, in particular the super capacitor.
In another advantageous embodiment, a converter may be provided which is electrically connected between the motor-generator and the storage element. The converter may convert a motor-generator voltage into a charging respectively discharging voltage of the storage element and vice versa. The converter may thereby provide for a voltage level conversion to take account of a difference in voltage level of the motor-generator or other element of the power supply, and the storage element. In particular, when making use of a capacitor, such as a super capacitor in the storage element, the converter may provide for a conversion towards a suitable charging voltage of the capacitor/super capacitor and for a discharging thereof, possibly allowing the (super) capacitor to be used over a wide voltage range, and thus over a wide charging/discharging range. The converter may comprise any suitable converter, in a preferred embodiment a bidirectional direct current-direct current converter, such as a switching converter may be applied, as thereby a low loss conversion may be provided.
Where in this document reference has been made to a capacitor or a super capacitor, this may be understood such as to include a plurality of capacitors/super capacitors, connected in series, connected in parallel, or any combination thereof.
When the storage element comprises a plurality of (super) capacitors, the converter may comprise a switching network to switch the capacitors in series and/or parallel combinations. Thereby, a low loss conversion may be provided: as an example, the lower the voltage provided to the converter for charging the capacitors, the more capacitors are put in parallel, while the higher the voltage provided to the converter, the more capacitors are connected in series. Thereby, by switching the capacitors to be in series/parallel configurations, an operating voltage range of the individual capacitors may be adapted to the voltage provided for charging. For discharging, the same principle may be applied.
In another embodiment when applying a (super) capacitor as the storage element, the converter may comprise an inductor to form an inductor-capacitor resonance circuit with the super capacitor. To obtain optimum results, a resonance frequency of the resonance circuit may be adapted to a cycle frequency of the wave motion. Thereby, a low loss conversion may be provided, in particular when the resonance frequency has been adapted to the cycle frequency of the wave motion, as thereby the cycle of providing energy and storing regenerated energy may be synchronised with the resonance mode of the resonance circuit.
The control unit may comprise a voltage measurement device for measuring a voltage of the power source. The control unit may thereby be arranged to compare the measured voltage with a low and a high threshold voltage value for driving the converter in order to charge the electrical storage element when the voltage exceeds the high threshold voltage value, and in order to discharge the electrical storage element when the measured voltage succeeds the low threshold value. Thereby, a simple control algorithm may be provided, as in case of a low supply voltage, i.e. in case of a high current drawn by the motor, the electrical storage element is discharged, thereby providing energy for driving the motor-generator, and in case that the power supply voltage is high, indicating that energy is regenerated by the motor-generator, the converter is operated to charge the (super) capacitor, thereby storing regenerated energy.
Alternatively, the control unit may comprise a current measurement device for measuring a current, provided by the power source. The control unit may thereby be arranged for comparing the measured current with a current set point, for driving the converter in order to discharge the electrical storage element when the measured current exceeds the current set point and in order to charge the electrical storage element when the current set point exceeds the measured current. A simple control algorithm may be provided, and in case of a high supply current, i.e. in case of a high current drawn by the motor, the electrical storage element is discharged, thereby providing energy for driving the motor-generator. In the case that the power supply current is low, indicating that energy is regenerated by the motor-generator, the converter is operated to charge the (super) capacitor, thereby storing regenerated energy.
In a further, advantageous embodiment, the control unit may be arranged to measure an operating voltage of the electrical storage element and to connect an electrical power dissipater when the operating voltage of the storage element exceeds a maximum operating voltage, thereby preventing the storage element from being overloaded by dissipating part of the energy stored therein in case that a maximum voltage is exceeded. In another embodiment, the excess energy may be fed back to the power supply. It may be advantageous to use the energy on other places on board of the ship
In a further embodiment, the control unit is arranged for comparing a time average of the voltage of the storage element with a predefined storage voltage set point. The predefined storage voltage set point represents the desired operating voltage of the storage element. It may be expected that the average operating voltage of the storage element will decrease, as in each cycle of the wave motion energy will be dissipated in cables and inside the motor-generator. When, in that case, the control unit may modify the current set point, the control unit may drive the power source to provide energy to compensate for the losses.
In another embodiment of the invention, the control unit comprises a measurement device for measuring a variable, representative of a heave motion, which is to be compensated. This variable can be, among others, a wave related variable, a heaving related variable or a motor-generator related variable. This can relate to any suitable parameter, such as a depth of the water as measured by a suitable sensor such as an ultrasound sensor, an acceleration of the cable, load etc as measured by an acceleration sensor, an angle of the cable etc as measured by an angle meter, or an air speed velocity as measured by an air speed meter. The control unit is arranged for driving the converter in order to charge or discharge the electrical storage element to provide or to buffer at least a part of the electrical energy involved with the heave compensation on the basis of the measured variable. The control unit can also be arranged for driving the power source to provide or to receive at least a part of the electrical energy involved with the heave compensation. Thereby, the storage element and/or the power source may quickly start to supply or buffer the energy involved with the heave motion compensation, resulting a better heave compensation or lower energy losses.
The same or similar advantages and preferred embodiments as achieved with the heave compensation system according to the invention, may also be provided by a heave compensation method according to the invention. The method according to the invention provides an active heave compensation method for at least partly compensating for an effect of a wave motion on a load, the method comprising:
Further features effects and advantages of the invention will become clear from the appended drawings and corresponding description, in which non-limiting embodiments of, invention are disclosed, wherein:
a depicts a functional layout of the control unit according to an aspect of the invention;
b depicts another functional layout of the control unit according to an aspect of the invention; and
As schematically illustrated in
The control unit may be formed by a separate control unit, however, it is also possible that the control unit forms part of an existing control unit of the lifting installation or of any other installation. It is for example possible that the control unit is provided with sensors to sense the wave motion, the sensors thereby providing a suitable signal to the control unit to enable it to control the motor-generator accordingly.
The power supply may comprise any suitable configuration for powering the motor-generator: as an example, the power supply PS may comprise an inverter. Many alternatives are possible: it is for example imaginable that the power supply comprises a plurality of switches to electrically connect the motor-generator either the power line PL and/or with the capacitor C for storing energy. Many implementations are possible, some of which will be described below.
Thus, in the first part of the wave motion cycle, the energy storage element is at least partly discharged, while in the second part of the wave motion cycle, energy that is regenerated by the motor-generator is buffered by the energy storage element. As a consequence, in the setup according to
A further example is shown in
In a further embodiment, the control unit may comprise a voltage measurement device for measuring a voltage of the power line PL or power source SRC. The measured voltage is then compared by the control unit, by a suitable comparator thereof, with a low threshold voltage and a high threshold voltage value. The converter (such as the converter in
In a further embodiment, the control unit may comprise a current measurement device for measuring a current of power source SRC. After comparison with a current set point (which is discussed below) the control unit will then drive the converter CONV in order to discharge the electrical storage element when the measured current exceeds the current set point and in order to charge the electrical storage element the current set point exceeds the measured current. A high supply current (i.e. higher than average) would indicate that a high current is drawn by the motor. In that case, it would be advantageous if the electrical storage element is discharged and provides energy for driving the motor-generator. In case the power supply current is low (i.e. lower than average), the converter may be operated to charge the (super) capacitor.
In
Since energy losses will occur in the cables, converters and motor-generators, the energy level of the energy storage will decrease even in the case of a perfect wave compensation. Therefore, the powers source should compensate for these losses. This can be accomplished by the “slow voltage control loop”. A function of such slow voltage control loop may be to control the voltage of the capacitors around the constant desired operating voltage of the storage element. This predefined storage voltage set point (input 3 in
It may be advantageous to take in account the expected heave compensation. This would yield a faster response and thereby a more effective compensation, i.e. less energy losses. The expected heave compensation may be calculated on the basis of measurements of waves, tractive forces on the load, and/or acceleration of the driving axes of the motor-generator. Also the movements of the ship itself may be used to calculate an expected heave compensation. A value representative of the expected heave compensation (input 3 in
In all above embodiments, as well as in any other possible embodiment, an electrical power dissipater, such as a resistor or any power consuming device, may be connected to the electrical storage element for dissipation of energy, when the operating voltage of the electrical storage element would exceed a maximum operating voltage. Thereby, safe operation of the electrical storage element may be provided for. In other embodiment, safe operation is established by feeding back the excess of energy to the power source.
Since all electrical devices described in the embodiments above have their own operating characteristics for safe operation, it can be understood that several control systems can be applied to measure the operation voltages and currents and to take action when safe operation is in danger. Devices can, for example, be disconnected from the system when voltages are too high. These safety control systems have not been shown in the figure or described for clarity reasons.
In the context of this document, the term “heave compensation” is to be understood as to comprise any form of wave motion compensation, including among others vertical movement compensation, horizontal movement compensation and roll compensation.
It may also be understood that use of a super capacitor, possibly in combination with a “slow voltage control loop” and a “fast voltage control loop”, as described above may be advantageous for other systems where, in one part of a cycle, energy is required, while in another part of the cycle energy is produced and stored.
An example of such a system is described in the international patent application PCT/NL2008/000221. It discloses a mono hull vessel with a heavy lift crane. In
The drive and control system may be provided with a motor/generator M/G and a energy storage C (such as a super capacitor with a converter) as described above to drive the solid roll damping mechanism. The movements of the solid roll damping ballast can be described as cycle, as the ballast may be moved from larboard to starboard and vice versa. In the cycle, energy may be produced and stored in a first part the cycle and may be required in another part.
Any of the above mentioned embodiments according to the invention may be applied on the active roll damping mechanism, the motor/generator M/G and the energy storage C. In particular, a “slow voltage control loop” and a “fast voltage control loop” may be applied to control buffering of energy in the energy storage and providing energy from the energy storage to drive the motor/generator, which is driving the solid roll damping ballast.
It may be understood from the above, that similar embodiments may be applied to other kind of ships, such as drill ships. Since drill ships are often positioned at one position in the ocean, they may experience rolling of the ship as a disturbing factor. A anti-rolling system to counteract the rolling of the drill ship may be based on the roll-stabilization system described above and in the international patent application PCT/NL2008/000221. Also in this case, any of the embodiments of the invention may be applied to drive a motor/generator M/G and a energy storage C, which may comprise a super capacitor, in order to buffer energy in the energy storage and to provide energy from the energy storage to drive the motor/generator, which is driving the solid roll damping ballast.
It may be understood that in the embodiments and applications of the invention as described above, it also possible that a motor interacting with the load is not generating or regenerating energy and that a power supply is providing energy to the energy storage. When the motor requires energy, the energy storage may provide at least a part of the energy required by the motor to interact with the load. Thus, the energy required by the motor is provided completely by the power supply, for example in a continuous way during the whole cycle. In the cycle part, when the motor does not require energy, the energy provided by the power supply is stored in the energy storage. In the cycle part, when the motor requires energy, the energy is provided at least for a part by the energy storage.
Therefore, an active heave compensation system may be provided comprising
a motor to interact with a load;
a control unit which is arranged to control operation of the motor, the control unit being arranged to operate the motor to drive the load in a first part of a wave motion cycle; and
an electrical storage element arranged and electrically connected to the motor for buffering energy for powering the motor in a following cycle of the wave motion.
This compensation system may also be applied compensate for roll movements, i.e. an anti-rolling system as is described above and in the international patent application PCT/NL2008/000221. Embodiments of the invention, as are described in claims 2-16, may also be applied to this active heave compensation or anti-rolling system.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008/000091 | Mar 2008 | NL | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/NL2009/000070 | 3/26/2009 | WO | 00 | 12/10/2010 |
