WO2004001193A2

WO2004001193A2 - Compensation device

Info

Publication number: WO2004001193A2
Application number: PCT/NO2003/000207
Authority: WO
Inventors: Christian Magnus Pedersen
Original assignee: Hydralift ASA
Current assignee: Hydralift ASA
Priority date: 2002-06-21
Filing date: 2003-06-19
Publication date: 2003-12-31
Anticipated expiration: 2004-12-21
Also published as: NO315435B1; AU2003274836A8; AU2003274836A1; NO20023047A; WO2004001193A3; NO20023047D0

Abstract

A compensation system, for example for compensation of a drill string (2) which is subjected to wave motion, the drill string (2) being mounted, possibly by means of a wire arrangement (3), on a crown block (10) on board a floating vessel (1), such as, for example, a drilling vessel, drilling platform, etc. The compensation system comprises : at least one main cylinder (6), comprising a cylinder cavity (8) and a piston rod (7), where at the end of the piston rod that is located inside the cylinder space (8), a piston is attached. The main cylinder (6) is arranged in such a manner that it is secured relative to a deck structure (9) on board the vessel, the second end of the piston rod (7) that is located outside the main cylinder’s cylinder space (8) being attached to the crown block (10). At least one auxiliary cylinder (11) comprising a cylinder cavity (13)and a piston rod (12), where at the end of the piston rod (12) that is located inside the cylinder cavity (8), a piston is attached. At least one arm link comprising at least a first arm (14) and a second arm (15), characterised in that the first arm (14) is rotatably connected at one end to the deck structure (9) and at the other end is rotatably connected to the first end of the second arm (15), the second end of the second arm (15) being rotatably mounted on the block element (10). The auxiliary cylinder (11) is rotatably mounted on the deck structure (9) and moreover the second end of the piston rod (12) that is located outside the auxiliary cylinder’s cylinder space (13) is rotatably attached to the first arm (14). The main cylinder (6) and the auxiliary cylinder (11) are connected to the same power supply unit, where the power supply unit may be composed of an accumulator (18), which may, for example, be connected to a fluid reservoir (22).

Description

Compensation device

The invention relates to a compensation system which can be used in connection with compensation of, for example, drill strings on board floating vessels, such as drilling ships, drilling platforms and other types of vessels.

The use is known of different types of compensation systems for compensating for movements that influence a system or a device. In connection with oil and gas- related activity, special systems have been developed that compensate for the wave motion to which the vessels and their related equipment are subjected. An example of this that may be mentioned here is compensation systems that can be used for vessels which are anchored to the seabed, or which are connected to the seabed by risers or drill strings.

When drilling for oil down in an oil well, at least two different requirements will have to be satisfied in order to alter the position of the drill string relative to the vessel. From the vessel, lengths of the oil string will require to be passed down to the seabed as the drill string's lower end penetrates further down into the crust of the earth. Thus it is necessary for the vessel to be provided with equipment that enables this to be done. In addition, equipment will be required to ensure that the drill string can be kept stationary relative to the seabed. A commonly known method of achieving this is to move the drill string relative to the wave motion influencing the drilling vessel.

When conducting oil and gas-related operations from floating vessels, the vessel and the equipment employed from the vessel down in the well will be subjected to wave influence that causes both the vessel and the equipment to be moved relative to the seabed. When drilling from a floating vessel, the vessel is often permitted to be moved with the waves, while the drill string is kept stationary relative to the seabed. Particularly if the vessel maintains its position by dynamic positioning and hawsers are not employed from the vessel to the seabed, there will be no need for the actual vessel to be compensated for the wave motion to which it is subjected. In this instance, therefore, it is sufficient to employ equipment only in order to compensate the drill string. The drill string is connected to the floating vessel and extends from the vessel down to the borehole concerned. An objective is to keep the tension in the drill string as constant as possible during the wave compensation, in order to keep the pressure the bit has in the borehole against the earth's crust as constant as possible.

Different types of compensation systems are known for compensating for the wave- induced movements, where the object of the compensation systems is to keep the drill string stationary relative to the seabed. A compensation system of this type must satisfy certain conditions in order to maintain as far as possible an optimised level of drilling. The compensation system may be arranged in such a manner that it enables the drill string to be moved in a vertical direction in the opposite direction to the direction in which the wave motion attempts to move the drill string and the vessel. The compensation system will therefore move the drill string upwards when the wave motion attempts to move the drill string downwards and vice versa. The object is thereby achieved that the drill string is kept stationary and influenced as little as possible by the wave motion. It is important for the compensation system to have the ability to maintain the tension in the drill string during these wave-induced compensation movements, and it is therefore vital to keep the force variations at a minimum over the distance over which the drill string is moved during the compensation movement.

It is a problem that the known compensation systems are not capable of keeping the tension in the drill string constant in a simple manner during the vertical compensation movement. In the known compensation systems, hydraulic cylinders are often employed to maintain a constant tension in the drill string during the compensation movements. The greatest force is exerted in the cylinder when the cylinder piston is completely inserted in the cylinder space. When the cylinder piston is located in an outer position in the cylinder space and the piston rod is moved out of the cylinder space, the force is at a minimum and the cylinder will consequently not necessarily deliver the force to the drill string that is necessary to maintain a constant tension in the drill string. It is desirable for a compensation system to provide an additional force that is uniform over the whole force range, i.e. that the variation in force is minimised.

The hydraulic cylinders are often connected to a fluid reservoir where the fluid is pressurised, for example by means of an air battery. The fundamental principle that applies is that the pressure in a fluid reservoir connected to a large gas volume changes relatively little when the piston moves from completely compressed to full stroke, and the force in the cylinder thereby also changes relatively little over the piston's travel. This is utilised in order to keep the tension in the drill string as constant as possible. The greater the gas volume, the less the force variations over the cylinder's stroke length. In a mathematically ideal situation where the gas volume is infinitely large, it is possible to achieve a situation where the force does not vary at all. A particular drawback of the known systems, therefore, is that in many cases very large air batteries have to be provided in order to obtain a constant force over the cylinder's displacement. Since it is not possible to provide infinitely large air batteries, the problem with such a system will be that the cylinder force varies over the stroke length.

Several different mechanical devices are employed today for compensation of drill strings. We refer in particular to US 5520369, which describes a known system for compensation of a drill string, where the drill string is moved in a vertical direction relative to the vessel as compensation for the wave motion to which the vessel is subjected. In the publication, the drill string is shown suspended in a block which in turn is connected to hydraulic cylinders and connected to an air battery with a large volume, where the cylinder displacement of the hydraulic cylinders follows and compensates for the vessel's wave motion. A mechanism of arms and wires ensures equalisation of the power variation that occurs in the hydraulic cylinders. The size of the air battery must be adapted in relation to the ability of the arm/wire mechanism to equalise the force variation in the cylinders in order for the system to function optimally. A disadvantage of this system is that the wire length is not kept completely constant over the stroke length and that the size of the air battery is determined by the ability of the arm/wire mechanism to equalise the force variations in the cylinders.

We also refer to US 3791628 in which an arrangement of arms, wires and hydraulic cylinders constituting a compensation system is also employed. In this compensation system the wire length is kept constant over the entire stroke length of the cylinder, as opposed to the system disclosed in US 5520369 where the wire length varies. The drawback of this system is that the arms and wires together do not constitute an arrangement that compensates for the force variation that occurs over the piston travel of the hydraulic cylinders.

We also briefly mention WO 00/18350 which discloses a compensation device where wires and discs are employed in an arrangement for achieving compensation.

For the sake of simplicity, the compensation system according to the invention is described herein usedϊn connection with drill strings employed from floating vessels. However, this compensation system will also be suitable for use in connection with other types of equipment, where the object is preferably to keep a motion-influenced device stationary relative to a reference plan or another object. As an alternative area of application for a compensation system according to the invention, the compensation system may, for example, be used to move structures from a wave-influenced floating unit down to the seabed. The areas of application for the invention that are mentioned herein should not be considered limiting for the invention, as it is the fundamental principles achieved by the compensation system that will be important for the person skilled in the art when he attempts to find areas of application for the compensation system.

The compensation system according to the invention comprises an arrangement with at least one main cylinder connected to a power supply unit which may comprise an accumulator and a fluid reservoir, at least one auxiliary cylinder and one arm link comprising at least a first and a second arm. The arrangement is mounted on a crown block on board a floating vessel, where the drill string is attached to a crown block possibly by a wire arrangement. It is an object of the invention to provide a compensation system where the force variations over the main cylinder are equalised, thus permitting the drill string to be kept at as stable and uniform tension as possible. The special assembly of the compensation system's components according to the invention permit the main cylinder combined with the auxiliary cylinder to exert a tension in the drill string that is uniform over the entire stroke length without requiring the fluid reservoir, with which the main cylinder is connected, to have as large a volume as that which is customary in known compensation systems. Nor is it according to the invention an essential condition for equalisation of the force variations over the main cylinder's stroke length that the wires together with the arms should form a system corresponding to the compensation system in US 5520369.

The size of the fluid reservoir, for example the air battery, is of great importance with regard to both space and weight for the floating vessel. It is therefore an advantage of the compensation system according to the invention that where an air battery is employed as fluid reservoir, a reduction can be obtained in the gas volume compared with the gas volume required when using the system in US 5520369.

One or more main cylinders are included in the compensation system. Each main cylinder comprises a cylinder cavity and a piston rod, where a piston is attached at the end of the piston rod located inside the cylinder cavity. The main cylinder is attached in or to a deck structure on board the vessel and in addition the crown block is attached to the end of the piston rod located outside the main cylinder's cylinder space. The drill string is attached, possibly by means of a wire arrangement, to the crown block. The vessel and the deck structure where the main cylinder is mounted will be moved with the wave motion, while the piston rod with the crown block attached to the end of it will be moved out of and into the cylinder cavity in the opposite vertical direction to the wave motion, with the result that the crown block and the drill string are kept stationary relative to the seabed. The deck structure may, for example, be a derrick.

The compensation system further comprises at least one arm link and according to a preferred embodiment of the invention at least two arm links, where each arm link comprises at least a first arm and a second arm which are movable both relative to each other and relative to the deck structure and the crown block. The first arm is rotatably connected at one end to the deck structure on board the vessel or to a mounting structure attached to the deck structure. The other end of the first arm is rotatably attached to the first end of the second arm. Furthermore, the second end of the second arm is rotatably mounted on the crown block.

The main cylinder and arm link components can be found in the hitherto known compensation systems. The same also applies per se to the auxiliary cylinder which forms part of the compensation system according to the invention, but the auxiliary cylinder's function and configuration will be new in the invention, as will now be explained in greater detail.

In a preferred embodiment of the invention the compensation system comprises two auxiliary cylinders, each auxiliary cylinder comprising a cylinder cavity and a piston rod. At the end of the piston rod located inside the cylinder cavity there is attached a piston. The auxiliary cylinder, moreover, is attached to the deck structure, the cylinder housing being rotatably mounted at its lower end on to the deck structure. The auxiliary cylinder is attached to the arm link, the end of the piston rod located outside the auxiliary cylinder's cylinder space being rotatably attached to the first arm. This method of mounting an auxiliary cylinder is new in relation to the prior art and by means of this configuration an effect is achieved that is not achieved by means of the known compensation systems. In an alternative embodiment of the invention the auxiliary cylinder may, for example, be horizontally oriented almost parallel to the vessel's deck structure.

In order to obtain a compensation system that is easier to optimise than, for example, the system disclosed in US 5520369, the main cylinder and the auxiliary cylinder are connected to the same power supply unit with the result that the main cylinder and the auxiliary cylinder have the same pressure. Different types of cylinders may be included in the compensation system, and the choice of type of cylinder or other suitable structures with power-supplying properties, which are suitable in the individual compensation system, is naturally left to the person skilled in the art. The power supply unit may be composed of an oil accumulator which, for example, may be connected to a fluid reservoir such as an air battery. It will be obvious to the skilled person that different types of accumulators and fluid reservoirs may be appropriate for use in the compensation system.

Discs or other suitable elements may be provided that are designed for receiving and transferring one or more wires at the pivot points between the deck structure and the first arm, the first and the second arm, the second arm and the crown block respectively. The drill string can be mounted on the crown block by means of a wire arrangement in such a manner that the drill string remains suspended slightly below the crown block. The wire employed in suspension of the drill string can be passed over the discs to attachment on the deck on one side of the vessel and to a winch located on the other side of the vessel. The wire can be tightened and slackened by the winch as required, for example when the drill string has to be passed further down into the crust of the earth as drilling progresses.

This wire and disc system as described above is necessary in order to make the compensation system work in the same way as that described in US 5520369. The additional force from the wire arrangement as it is utilised in US 5520369 is not necessary in the compensation system according to the invention, since additional force from the auxiliary cylinder is used instead in order to achieve as constant force as possible in the compensation system's crown block. The wire arrangement is mentioned in this context since it is often used as a preferred method of connecting the drill string to the vessel. It will therefore be possible to use the compensation system without this wire arrangement, or the wire arrangement may be replaced by another suspension arrangement for the drill string according to the choices of the skilled person.

When the vessel and the drill string are influenced by wave motion, as mentioned earlier the compensating movements cause the piston rod in the main cylinder to be moved in and out of the cylinder space, with the result that the drill string remains stationary relative to the seabed, even though the vessel is moving vertically with the waves.

In order to utilise the compensation device according to the invention so as to achieve an optimised situation where the force variations on the crown block are minimised over the stroke length of the main cylinder, the compensation system is arranged in such a manner that a correcting additional force is provided from the auxiliary cylinders when the main cylinder's piston moves from an upper to a lower position, or vice versa, in the cylinder space.

When the piston rod in the main cylinder's cylinder space is moved to an upper position, preferably from a position where the second arm is horizontally positioned, this movement of the piston rod causes the arm link to be straightened out. This straightening causes the angle between the first and the second arm to increase and the auxiliary cylinder and the arm link's first and second arm to be positioned in a favourable geometrical configuration, where the auxiliary cylinder's tractive power, or downwardly-acting additional force, acts on the first arm and the second arm in the arm link acts with an increasing force upwards, when the crown block moves upwards. The auxiliary cylinder's additional force is therefore instrumental in further straightening out the arm link. As described above, with the compensation system according to the invention the object is achieved that when the force in the main cylinder decreases, the force from the auxiliary cylinder is instrumental in keeping the total force acting on the crown block as constant as possible when the main cylinder's piston rod moves towards an upper position. The drill string will therefore be kept at as constant tension as possible.

When the piston rod in the cylinder cavity of the main cylinder is moved to a lower position, preferably from a position where the second arm is horizontally positioned, this positioning of the piston rod causes the first and the second arm in the arm link to be moved towards each other, thus reducing the angle between the first and the second arm. The arm link's first and second arm and the auxiliary cylinder attain a favourable geometrical configuration, where the auxiliary cylinder's tractive force or downwardly-acting additional force acting on the first arm is instrumental in causing the second arm to act with an increasing force downwards on the crown block when the crown block moves downwards. This additional force thereby counteracts the increasing force in the main cylinder, thereby permitting the total force acting on the crown block to be kept constant in the main cylinder, even when the main cylinder's piston rod is located in a lower position, with the result that the drill string is also kept at a constant tension even in this lower position.

When the auxiliary cylinder is horizontally oriented, as described earlier, a highly favourable geometrical configuration is obtained between the auxiliary cylinder and the arm link, since the additional force transferred from the auxiliary cylinder to the arm link will exert a greater perpendicular force on the crown block than with other orientations of the auxiliary cylinder. For practical reasons, however, it is not easy to achieve this positioning of the auxiliary cylinder.

By means of this compensation system the object is achieved that the force variations over the main cylinder's piston travel are reduced from 10% in a known system as illustrated in US 5520369 to 2% according to the invention.

The compensation system acts as a passive system where the drill string is automatically self-regulating in relation to the wave motion acting on the vessel and the drill string. In an alternative application of the compensation system it will be possible to connect the compensation system to a sensor-controlled regulation, where the compensation is undertaken on the basis of information from a control system.

Compared with the compensation system known from US 5520369, the compensation system according to the invention is easy to adapt to any altered external conditions. This is in contrast to the compensation system in US 5520369, which applies a principle where the wire is used for equalising the force variations in the hydraulic main cylinder, and where the rigidity of the wire and the rigidity of the main cylinder must be adapted to each other. There is no connection between the wire system and the cylinder, and it is a relatively cumbersome process to adapt them to each other in order to obtain an optimum benefit from the compensation system.

In comparison, optimisation of the compensation system according to the invention takes place in a much simpler and more efficient manner. When the pressure is determined for the main cylinder and the auxiliary cylinders, the dimensions of the auxiliary cylinders are adapted according to the diameter of the main cylinder, whereupon the system acts in a self-regulating fashion as described above. When adapting the dimensions of these two pistons in relation to each other, an approximately constant force can be obtained over the entire piston travel. The compensation system will then function optimally without any further kind of control and will therefore be highly stable. With the device a compensation system is obtained which is passively self-regulating, where the system itself ensures that the force variation in the system is minimised.

In a preferred embodiment both the main cylinders and the auxiliary cylinders are hydraulically operated. Many types of cylinders will be suitable for use in the compensation system and as an alternative embodiment mention may be made here of cylinders that are pneumatically operated. In an embodiment of the invention the main cylinder will be in the form of a plunger cylinder. In this case the main cylinder's piston is designed with openings, thus enabling fluid to be transferred from one side of the piston to the other. In a preferred embodiment of the invention, fluid is supplied in the cylinder space of the main cylinder and the auxiliary cylinder respectively on the side of the piston where the area of the piston is least.

The invention will now be described in connection with the attached figures, in which:

Figure 1 is a principle view of a floating vessel provided with a drill string.

Figure 2 illustrates a section of the drill string's suspension corresponding to the illustration in figure 1.

Figures 3a and 3b illustrate the device according to the invention and a graph illustrating the basis for the invention respectively.

Figure 4 is a principle view of the compensation system according to the invention.

Figure 5 illustrates the compensation system as in figure 4, where a wire system is shown provided in connection with the compensation system.

Figure 6 illustrates the compensation system's geometrical configuration, where the main cylinder's piston is located in a lower position in the piston cavity.

Figure 7 illustrates an alternative geometrical configuration of the compensation system, where the main cylinder's piston is located in an upper position in the piston cavity.

Figure 8 illustrates the force transfer that takes place when the compensation system is located in the same situation as that illustrated in figure 6.

Figure 1 is a general view of an area of application for the invention, where the compensation system can be used in connection with mounting a drill string 2 on a floating drilling vessel 1. The drill string is lowered into an oil well in the crust of the earth.

Figure 2 illustrates a wire system 3 employed for suspension of the drill string 2. The wire system runs over discs 4 whose purpose is to ensure that tension is exerted on the drill string 21. In figure 2 it can be seen that the wire system is attached to the vessel at one end thereof, while the wire is connected to a winch 5 at the other end of the wire system. When the drill string 2 is lowered into the crust of the earth as drilling progresses, the winch 5 is employed to pay out wire and pull it in again as required.

Figure 3b is a diagram giving a graphic illustration of how the force in the main cylinder varies over the main cylinder's piston travel. The force F is indicated along the y-axis, while the piston travel is shown along the x-axis. Curve a illustrates an ideal situation in a cylinder where the force is constant over the entire piston travel, while curve b illustrates the real situation in the main cylinder where the force varies with the piston travel. These two situations have formed the basis for producing a compensation system according to the invention. The additional force that must be supplied in the compensation system in order to achieve a compensation system where the force variations are minimal is illustrated by curve c. (The real curve c will deviate from the illustrated linear curve c by +/- 2% and with a deviation form approximating a sine curve). It has been an object of the invention to provide a device which can contribute in a simple and stable manner with the force/power supply illustrated by curve c. With a compensation system according to the invention, therefore, the auxiliary cylinder provides additional force that reduces the force variations to a minimum over the main cylinder's piston travel. This is illustrated in figure 3a where arrow a illustrates the reaction force from tension in the drill string as demonstrated by curve a in figure 3b. Arrow b illustrates force in the cylinder as demonstrated by curve b. The arrows c illustrate vertical component of force in the second arm as demonstrated by curve c.

Figure 4 illustrates a view of a compensation system according to the invention where a main cylinder 6 is mounted in a deck structure 9. The main cylinder 6 is mounted with piston and piston rod 7 in the main cylinder's cylinder space 8. A crown block 10 is attached to the end of the piston rod 7 that projects outside the cylinder space 8. Furthermore, an auxiliary cylinder 11 is rotatably mounted on the deck structure 9. From figure 4 it can be seen that the auxiliary cylinder 11 is also mounted with piston and piston rod 12 in the auxiliary cylinder's cylinder space 13. The compensation system further comprises an arm link composed of a first arm 14 and a second arm 15, which are rotatably interconnected at an attachment point 16. The second arm 15 is also rotatably connected with the crown block 10 at an attachment point 17, and the first arm 14 is rotatably connected to a structure 23, which is located in connection with the deck structure 9 or which is a part of the deck structure 9 at an attachment point 24.

The main cylinder 6 and the auxiliary cylinder 11 illustrated in figure 4 are hydraulic cylinders connected with hose connections 19, 20 extending from an oil accumulator 18 to the main cylinder 6 and the auxiliary cylinder 11 respectively. The oil accumulator 18 has hose connection 21 to a gas battery 22. From figure 4 it can be seen that the hose connections 19 and 20 convey fluid into the main cylinder 6 and the auxiliary cylinder 11 respectively in the part of the cylinder space where the area of the piston is least. Hose connection 19 is equipped with a closing valve 27 for closing off the fluid flow in the hose.

In figures 4-7 HB stands for hovedbjelke (main beam) which follows the movements of the platform. KB stands for kronblokk (crown block) which follows the movements of the seabed.

In figure 5 a compensation system is illustrated similar to that in figure 4, but in addition the compensation system is illustrated equipped with wire system 3 and discs 4. The discs 4 are shown located at the connecting points 16, 17, 24. The wire is arranged over the discs 4, where the end 25 of the wire runs over the crown block 10 to an arrangement for suspension of the drill string 2. This arrangement is not illustrated in figure 4, but will be similar to that illustrated in the section in figure 2. The end 26 of the wire 3 extends down to attachment in a winch 5, which is mounted on the vessel, and this can also be seen in figure 2.

When the drill string is arranged in the wire arrangement 2, which is connected to the crown block 10, and the compensation system is in operation, the crown block 10 and the drill string will compensate for the wave motion to which the vessel 1 and the drill string 2 are subjected, thus permitting the crown block 10 to be kept stationary relative to the seabed.

The deck structure 9 is a part of the vessel 1 and will therefore follow the wave- induced movements of the vessel 1. The main cylinder's housing 6' is permanently mounted on the deck structure. When the vessel is subsequently subjected to wave motion, the main cylinder's housing 6' will follow the vessel's movements, while the main cylinder's piston is moved out of and into the cylinder space 8, thus permitting the crown block 10 and the drill string to be kept stationary relative to the seabed.

In figure 6 the compensation system is illustrated in a situation where the main cylinder's piston rod 7 is moved into a lower position in the cylinder cavity 8. In this lower position the first and the second arm are moved towards each other, the angle between these two arms being reduced. In figure 6 the auxiliary cylinder 11 is illustrated without hose connection for fluid supply, but the auxiliary cylinder 11 will be arranged in such a manner that the piston rod 12 acts with a force in the form of a tractive force or downwardly-directed force that has a force component that acts perpendicularly on the first arm 14. This force results in a force in the second arm's 15 longitudinal direction consisting of a horizontal and a vertical force component, where the force's vertical component acts downwards on the crown block 10. The effect of the force transfer from the auxiliary cylinder 11 to the crown block 10 is that as constant tension as possible is maintained in the drill string 2 over the stroke length of the main cylinder 6 when the piston moves towards a lower position. In figure 7 the main cylinder's piston rod 7 is illustrated in an upper position in the cylinder cavity 8. In the upper position, the first and the second arm are moved away from each other, thus increasing the angle between the arms. In this position the influence of the auxiliary cylinder's force in the form of a tractive force or downwardly- acting force will have a force component that acts perpendicularly on the first arm, which in turn results in a force in the second arm's 15 longitudinal direction consisting of a vertical and a horizontal component. This force in the second arm has a vertical component that acts upwards on the crown block 10. The effect of this force transfer from the auxiliary cylinder 11 to the crown block 10 is that a constant tension is maintained in the drill string over the stroke length of the main cylinder 6 when the piston moves towards an upper position.

In figure 8 the influence of the additional force from the auxiliary cylinder 11 on the crown block 10 is illustrated by means of arrows at the second arm 15. It can be seen from the figure that the main cylinder is located in a situation where the piston 7 is located in a lower position in the cylinder space 8. The force acting in the main cylinder 6 is illustrated by an upwardly-directed arrow in the main cylinder 6.

Claims

PATENT CLAIMS

1. A compensation system, for example for compensation of a drill string (2) which is subjected to wave motion, the drill string (2) being arranged, possibly by means of a wire arrangement (3), on a crown block (10) on board a floating vessel (1), such as, for example, a drilling vessel, drilling platform, etc., where the compensation system comprises:

- at least one main cylinder (6) comprising a cylinder cavity (8) and a piston rod (7), where at the end of the piston rod that is located inside the cylinder space (8) a piston is attached, and that the main cylinder (6) is arranged in such a manner that it is secured relative to a deck structure (9) on board the vessel, the second end of the piston rod (7) that is located outside the main cylinder's cylinder space (8) being attached to the crown block (10),

- at least one auxiliary cylinder (11) comprising a cylinder cavity (13) and a piston rod (12), where at the end of the piston rod (12) that is located inside the cylinder cavity (8), a piston is attached,

- at least one arm link comprising at least a first arm (14) and a second arm (15), characterised in that

- the first arm (14) is rotatably connected at one end to the deck structure (9) and at the other end is rotatably connected to the first end of the second arm (15), the second end of the second arm (15) being rotatably mounted on the crown block

(10),

- the auxiliary cylinder (11) is rotatably mounted on the deck structure (9) and moreover the second end of the piston rod (12) that is located outside the auxiliary cylinder's cylinder space (13) is rotatably mounted on the first arm (14),

- the main cylinder (6) and the auxiliary cylinder (11) are connected to the same power supply unit, where the power supply unit may be composed of an accumulator (18), which may, for example, be connected to a fluid reservoir (22).

2. A device according to claim 1, characterised in that the fluid reservoir (22) may be composed of a pressurised fluid, for example a gas battery.

3. A device according to one of the claims 1-2, characterised in that the accumulator is composed of an oil accumulator or a pneumatic accumulator.

4. A device according to one of the claims 1-3, characterised in that the main cylinder's piston is designed with openings, thus enabling fluid to be transferred from one side of the piston to the other.

5. A device according to one of the claims 1-4, characterised in that the deck structure (9) is composed of a derrick structure.

6. A device according to one of the claims 1-5, characterised in that the auxiliary cylinder (11) is horizontally oriented.

7. A device according to one of the claims 1-6, characterised in that the system comprises a main cylinder (6), two arm links and two auxiliary cylinders (11).

8. A device according to one of the claims 1-7, characterised in that discs (4) or other suitable elements designed for receiving and transferring one or more wires are provided at the pivot points (24, 17, 16) between the deck structure (9) and the first arm (14) respectively, the first and second arm (14, 15) respectively, and the second arm (15) and block element (10) respectively.

9. A device according to one of the claims 1-8, characterised in that the fluid is supplied to the cylinder cavity of the main cylinder (6) and the auxiliary cylinder (11) respectively on the side of the piston where the area of the piston is least.

10. A method for utilising the compensation device according to one of the claims 1 -9 in order to achieve an optimised situation where the force variations over the piston travel in the main cylinder are minimised, characterised in that the piston rod (7) of the main cylinder (6) is moved to an upper position, thus increasing the angle between the first and the second arm (14, 15), and the auxiliary cylinder and the two arms (14, 15) are positioned in a geometrical configuration where the auxiliary cylinder's piston rod (12) acts with a force on the first arm (14) that results in a force in the second arm's (15) longitudinal direction consisting of a horizontal and a vertical component, the vertical component of the force in the second arm (15) acting upwards on the crown block (10), with the result that the total force acting on the crown block (10) is as constant as possible, or that the piston rod (7) of the main cylinder (6) is moved to a lower position, thus reducing the angle between the first and the second arm (14, 15), and the auxiliary cylinder (11) and the two arms (14, 15) are positioned in a geometrical configuration where the auxiliary cylinder's piston rod (12) acts with a force on the first arm (14) that results in a force in the second arm's (15) longitudinal direction consisting of a horizontal and a vertical component, the vertical component of the force in the second arm (15) acting downwards on the crown block (10), with the result that the total force acting on the crown block (10) is as constant as possible.