System and method for improved heave compensation

Abstract

A damping system for a heave compensator for an off-shore oil rig includes a hydraulic cylinder having a piston and a housing. The hydraulic cylinder is configured for accepting a hydraulic fluid. There is a flow passage for restricting the flow of the hydraulic fluid during movement of the piston in the housing. The hydraulic fluid is a magnetic fluid and the damping system includes a magnetic fluid management system for controlling a magnetic field at the flow passage. A heave compensator including such a damping system, and a method for controlling the damping of a heave compensator are also disclosed, the method including subjecting a magnetic fluid to a magnetic field at a flow passage for restricting the flow of the magnetic fluid.

Claims

1. A method for controlling the damping of a heave compensator, wherein the method comprises: moving a piston of a hydraulic cylinder in a housing of the hydraulic cylinder to damp relative motion between a first structure coupled to the piston and a second structure coupled to the housing, wherein the housing comprises a permanent magnet disposed in an interior of the housing, and the piston comprises an inductive coil disposed in the interior of the housing and which is inductively coupled with and moves relative to the magnet in response to the movement of the piston in the housing; placing a magnetic fluid management system (MFMS) of the heave compensator in an energy harvesting mode; generating electrical energy in the inductive coil by electromagnetic induction in response to the relative movement of the inductive coil and the magnet in the housing with the MFMS in the energy harvesting mode; reconfiguring the MFMS from the energy harvesting mode to a separate damping mode such that a flow of electrical energy to an electromagnetic coil in the piston in is provided; and subjecting the hydraulic magnetic fluid in fluid communication with the hydraulic cylinder to a magnetic field at a flow passage for restricting the flow of the magnetic fluid and to adjust the damping between the first structure and the second structure in response to altering the viscosity of the hydraulic magnetic fluid by supplying the electromagnetic coil with electrical energy with the MFMS in the damping mode.

2. The method according to claim 1, further comprising monitoring movement of a load on the heave compensator.

3. The method according to claim 1, further comprising generating electrical energy by electromagnetic induction during damping of the heave compensator.

4. The method according to claim 1, further comprising: tuning a programmable logic controller by selecting a heave compensation system mode selected from the group consisting of a recoil mode, a lift-off mode, and a drilling mode; measuring a payload weight of a submerged drill string; measuring at least one of velocity and displacement of an off-shore rig, the hydraulic cylinder being coupled between the off-shore rig and the drill string; and changing the viscosity of the magnetic fluid with the programmable logic controller based on the heave compensation system mode, the payload weight, and the at least one of velocity and displacement of the off-shore rig.

5. The method according to claim 1, wherein the magnet is coupled to the housing such that relative movement between the magnet and the housing is restricted.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) In the following is described exemplary embodiments illustrated in the accompanying drawings, wherein:

(2) FIG. 1 shows a heave compensator comprising an integrated damping system according to this disclosure;

(3) FIG. 2 shows a heave compensator comprising another integrated damping system according to this disclosure;

(4) FIG. 3 shows a heave compensator comprising a separate damping system according to this disclosure;

(5) FIG. 4 shows in a larger scale the separate damping system of FIG. 3;

(6) FIG. 5 shows a hydraulic cylinder which can be used for harvesting electrical energy;

(7) FIG. 6 shows a heave compensator comprising yet another damping system according to this disclosure;

(8) FIG. 7 shows a flow chart demonstrating the overall control scheme of one exemplary embodiment of a magnetic fluid management system (MFMS); and

(9) FIG. 8 shows a control system block diagram for a magnetic fluid cylinder, valve, and/or port.

DETAILED DESCRIPTION OF THE DISCLOSED EXEMPLARY EMBODIMENTS

(10) In the drawings, the reference numeral 1 indicates a heave compensator comprising a damping system according to this disclosure. Identical reference numerals indicate identical or similar features. The drawings are presented in a simplified and schematic manner, and the features therein are not necessarily drawn to scale.

(11) FIG. 1 shows a schematic drawing of a heave compensator 1 with a damping system according to this disclosure. The heave compensator 1 comprises two plunger type hydraulic cylinders 2, each comprising a piston rod 5 in a cylinder housing 7. The hydraulic cylinders 2 may also be of another type. The rods 5 are attached to a load 9 which movement is to be damped, while the housings 7 are attached to a main structure (not shown), for example an offshore oil rig. The cylinders 2 are filled with a magnetic fluid 3 functioning as a hydraulic fluid. The fluid is in fluid communication with an accumulation tank 11 via a flow path 15 and an isolation valve 13. The damping system comprises a valve 17 functioning as a flow passage for restricting the flow of the magnetic fluid 3 during the movement of the rod 5 in the cylinder housing 7. The valve 17 comprises an electromagnet (not visible), which can provide a magnetic field at the flow passage. The amplitude of the magnetic field, and thereby the viscosity of the magnetic fluid 3 at the valve 17, can be controlled via a magnetic fluid management system (not shown). If the viscosity of the magnetic fluid 3 at the valve 17 is increased, the flow of said magnetic fluid 3 through the valve 17 is restricted further, thus increasing the damping effect of the hydraulic cylinders 2 and the heave compensator 1. The valve 17 can be placed elsewhere, but the position along the main flow path 15 ensures equal distribution of the damping to the individual hydraulic cylinders 2.

(12) FIG. 2 shows a similar heave compensator 1 as in FIG. 1, but in the embodiment shown in FIG. 2 the damping system comprises a port 19 on each hydraulic cylinder 2, where the ports 19 function as flow passages. The ports 19 comprises electromagnets (not visible) for providing a magnetic field. The magnitude of the electric field may be controlled by a magnetic fluid management system (not shown), thereby controlling the damping of the damping system.

(13) FIG. 3 shows a heave compensator 1 similar to the ones in FIGS. 1 and 2, however, in the embodiment shown in FIG. 3 the magnetic fluid damping system comprises a separate hydraulic cylinder 23 added to a prior art heave compensator 1. The magnetic fluid hydraulic cylinder 23 comprises magnetic fluid 3 while the main hydraulic cylinders 2 comprises a standard hydraulic fluid 21. Details of the magnetic fluid hydraulic cylinder 23 are shown in FIG. 4. This embodiment of the disclosure has the advantage that it may be added to existing heave compensators 1 for improved damping and additional features as described above.

(14) FIG. 4 shows in a larger scale the magnetic fluid hydraulic cylinder 23 from FIG. 3. The hydraulic cylinder 23 comprises a cylinder housing 25 and a piston rod 27 with a piston 29. The piston 20 has openings 31 which functions as flow passages for restricting the flow of the magnetic fluid 3 through the openings 31 in the piston 29 when the rod 27 and piston 29 are axially displaced in the cylinder housing 25. The piston also comprises an electromagnetic coil 33 for subjecting the magnetic fluid 3 at the openings 31 to a magnetic field. The magnitude of the magnetic field may be controlled using a magnetic fluid management system (not shown), and the electromagnetic coil 33 can be connected to said magnetic fluid management system via a cable in the piston (also not shown). The magnetic fluid hydraulic cylinder 23 comprises a gas chamber 35 having a variable volume and comprising a compressible gas. The volume of the gas chamber 35 changes when the piston 29 and rod 27 moves within the cylinder housing 25, since less volume is available for the magnetic fluid 3 in the rod side of the cylinder 23 than on the piston side due to the volume of the rod 27.

(15) FIG. 5 shows a magnetic fluid hydraulic cylinder 23 which can be used for harvesting electrical energy. The shown embodiment is the same type as in FIG. 4, but the principle may be used for other types of cylinders as well, and for hydraulic cylinders with hydraulic fluids which are not magnetic fluids. In addition to the features shown in FIG. 4, the cylinder 23 comprises a central permanent magnet 37 connected to the cylinder housing 25 and a coaxial inductive coil 39 located in the piston rod 27. Movement of the piston rod 27 in the cylinder housing 25 will thus have the effect of a relative movement of the permanent magnet 37 through the inducing coil 39, whereby electrical energy is induced in said coil 39.

(16) FIG. 6 shows another exemplary embodiment of a heave compensator 1 comprising a magnetic fluid damping system according to this disclosure. In this embodiment, the heave compensator 1 comprises two hydraulic cylinders 2 which are differential cylinders that have a low-pressure side on the rod side 41. The rod side 41 comprises magnetic fluid 3, while the piston side 43 may comprise regular hydraulic fluid 21 for cost-saving. The magnetic fluid 3 on the rod side 41 of each cylinder 2 is connected to a magnetic fluid accumulator 45, and a valve 17 comprising an electromagnet (not shown) functions as a flow passage between the accumulator 45 and the rod side 41 of the cylinder 2. The degree of damping can be controlled by subjecting the magnetic fluid 3 to a magnetic field at the valve 17.

(17) FIG. 7 shows a flow chart demonstrating the overall control scheme of one exemplary embodiment of a magnetic fluid management system (MFMS). The MFMS may be interfaced with the heave compensator control system for optimal performance of the total system. The MFMS process may include a possibility of energy harvesting as described above.

(18) If the process does not have energy harvesting, or if a decision is taken not to use this feature, the first step of the MFMS process is to start vibration attunement for low force variation. Initiation of the MFMS process may require equipment power up before participation in the heave compensation operation, possibly involving controllers, electrical interfaces, and HMI (Human-Machine Interface) being online and in stand-by mode ready for operation. The process which follows is the automatic mode where preset vibration attunement algorithms for low force variation are run. This may involve a certain input voltage setting being applied to the electromagnetic coils with a specific characteristic vs the velocity and position of the piston in order to minimize variation of force which is beneficial for a desired process, e.g. the drill bit during drilling. Damping will be lower with higher piston velocity, with damping also increasing near the cylinder end stops. If energy harvesting is activated, the device or circuit will go into a configuration in which the induced voltage is supplied back to the control circuit for powering it or to an energy storage system.

(19) The next step is to reconfigure the control scheme, where the software will check for any changes to the control scheme based on input from user. This can be the activation of energy harvesting or changes to parameters of the automatic mode. The damping force will be monitored and controlled to keep it within acceptable limits in terms of force variation to protect the payload from excessive damping forces. The load and speed is monitored, and if it is within a prescribed limit, the process returns to the vibration attunement step for low force variation and repeats the following steps. On the contrary, if the limit is exceeded, e.g. due to a loss-of-load, the damping is altered to an anti-recoil damping profile to avoid high kinetic energy of the load. The damping force will be increased according to the recoil acceleration in an attempt to decelerate the recoiling load as much as possible. A decision is thereafter taken on whether to stop the process or restart it from the beginning. For system shutdown, e.g. when the drilling operation is over, the active circuits are deactivated.

(20) FIG. 8 shows a block diagram of a magnetic fluid management system 100 for a magnetic fluid cylinder, valve, and/or port. This is a simplified, but representative magnetic fluid management system 100 for controlling the MF devices in relation to vibration attunement. The magnetic fluid management system 100 can be considered a lower-level control scheme that covers the steps of vibration attunement for low tension variation and anti-recoil damping profile from FIG. 7. The desired damping is derived from the heave compensator's force characteristics, which may change depending on compensator stroke and speed. There will be additional control schemes for end stop damping.

(21) The load cell measures the tension below the hoisting wire ropes, thus giving an indication of the payload wet weight, e.g. of a submerged drill string. This signal goes to a programmable logic controller 50 which processes it according to a force control algorithm, e.g. a P, PI, PD or PID (where P is an abbreviation for proportional, I is an abbreviation for integral, and D is an abbreviation for derivative) feedback control system which outputs an adjustment signal based on deviation from a reference tension setting. The engagement of this mode, as well as its standard settings, will be dependent on the overall heave compensation system mode. For example, if the overall system is in drilling mode, the force controller will be the dominant contributor to the control signal with specific tuning parameters favourable for drilling.

(22) The motion reference unit is an inertial measurement unit for sensing linear and rotational accelerations, which are then processed to give velocity and displacement values. The MRU feeds the displacement and velocity values to a programmable logic controller, e.g. as the one which processes load cell signal, and the values are then processed as part of a feed forward control circuit. This circuit will be tuneable through adjustment of the feedforward gain among other parameters. The engagement of this mode, as well as its standard settings, will be dependent on the overall heave compensation system mode. For example, if the overall system is in recoil or lift-off modes, the feed forward controller will be the dominant contributor to the control signal with specific tuning parameters favourable for these scenarios.

(23) The reference signal for desired damping will have a standard value but can be adjusted by user input. This reference signal will be used as part of a feedback control loop together with the computed damping force output from the MF device.

(24) The MF unit/EM coil represents the unit itself, i.e. the magnetic fluid cylinder, valve, or port, which takes a voltage input signal and outputs a change in magnetic field which will then affect fluid damping.

(25) It should be noted that the above-mentioned exemplary embodiments illustrate rather than limit the invention, the invention being defined by the claims set out below, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb comprise and its conjugations does not exclude the presence of elements or steps which are not stated in a claim. The article a or an preceding an element does not exclude the presence of a plurality of such elements.

(26) The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.