The present invention relates to an offshore system for performing subsea wellbore related activities, e.g. workover activities, well maintenance, installing an object on a subsea wellbore, drilling a subsea wellbore, etc. while the offshore system is subjected to heave motion due to waves.

The present invention also relates to a vessel including an offshore system and methods that are performed using the vessel according to the invention.

In the art, e.g. as marketed by the present applicant, offshore vessels are known that comprise:

a tower positioned on the vessel and defining a substantially vertical firing line along the drilling tower;

a mobile working deck which is moveable with respect to the tower along the firing line within a motion range including a heave compensation motion range; and

a heave compensation system being adapted to provide heave compensation of the mobile working deck within the heave compensation motion range.

In a known embodiment, e.g. as disclosed in WO2016/062812, the heave compensation system is also configured to provide heave compensation to a traveling block being part of a hoisting device allowing synchronous motion of the mobile working deck and the traveling block during operation.

As a result thereof it is possible to keep both a drill string suspended by the traveling block or the mobile working deck as well as a riser suspended from the mobile working deck in a more or less similar position with respect to the subsea wellbore independent of heave motion due to waves. This improves for instance wellbore pressure control during drilling of the subsea wellbore.

However, the known systems also have some disadvantages. One of these disadvantages is that heave compensation of the mobile working deck as disclosed in WO2016/062812 also introduces a safety hazard, namely, that the mobile working deck is subjected to a relatively big acceleration caused by the heave compensation system itself when the loads applied to the mobile working deck suddenly and unintentionally change. An example of such a sudden and unintentional change is when the relatively heavy drill string that is initially supported by the mobile working deck is unintentionally but suddenly released, for instance when a clamp suddenly fails or a component, possibly in the drill string itself, loses structural integrity. The tension inherently present in the heave compensation system may then launch the mobile working deck like a catapult as the tension in the heave compensation system cannot be lowered sufficiently quick to compensate for the sudden change in load to the mobile working deck.

It is therefore an object of the invention to provide an improved offshore system that allows to heave compensate the mobile working deck with a lower risk of unintentionally big accelerations of the mobile working deck caused by the heave compensation system.

To achieve the object of the invention, the present invention provides an offshore system for performing subsea wellbore related activities, comprising:

a tower to be positioned on a floating body and defining a substantially vertical firing line along the tower;

a mobile working deck which is moveable with respect to the tower along the firing line within a motion range which is the sum of an elevation motion range and a heave compensation motion range;

a heave compensation system being adapted to provide heave compensation of the mobile working deck within the heave compensation motion range; and

an elevation system being adapted to position the mobile working deck within the elevation motion range,

wherein the offshore system further comprises an overload protection system configured to detect an undesirably big acceleration of the mobile working deck, and wherein the overload protection system, in the event of a detected undesirably big acceleration of the mobile working deck, is configured to control the elevation system such that the mobile working deck motion in the elevation motion range is opposite to the mobile working deck motion in the heave compensation motion range in order to lower or limit the acceleration of the mobile working deck.

The invention is based on the insight of the inventors that the previously combined functionality of the working deck compensators to elevate the mobile working deck and to apply heave compensation once elevated should be separated from each other thereby allowing to use the elevation system in case of a sudden change in load applied to the mobile working deck to lower or limit the acceleration of the mobile working deck and prevent or at least limit the damage to the mobile working deck or any other component of the offshore system or floating device the offshore system is positioned on.

In the event that the heave compensation system is used to compensate the heave motion due to waves within the heave motion range and a downwardly directed load applied to the mobile working deck suddenly decreases, the heave compensation system will cause the mobile working deck to rapidly accelerate upwards and is not able to quickly counteract this acceleration. However, upon detection of this undesirably big acceleration of the mobile working deck, the elevation system can be used to quickly lower the mobile working deck within the elevation motion range to counteract the acceleration thereby giving time to the heave compensation system to adjust to the new load situation and keep the acceleration within limits to avoid or minimize damage to the offshore system. The same applies in case an upwardly directed load is suddenly increased.

Similarly, in the event that the heave compensation system is used to compensate the heave motion due to waves within the heave motion range and an upwardly directed load applied to the mobile working deck suddenly decreases, the heave compensation system will cause the mobile working deck to rapidly accelerate downwards and is not able to quickly counteract this acceleration. However, upon detection of this undesirably big acceleration of the mobile working deck, the elevation system can be used to quickly raise the mobile working deck within the elevation motion range to counteract the acceleration thereby giving time to the heave compensation system to adjust to the new load situation and keep the acceleration within limits to avoid or minimize damage to the offshore system. The same applies when a downwardly directed load is suddenly increased.

Although the invention is described as working both ways, the overload protection method may be configured to only cope with one situation, for instance the situation that the mobile working deck is rapidly accelerated upwards due to the undesirably big acceleration, preferably because a downwardly directed load is suddenly decreased. Hence, in that case, the overload protection system is configured to detect an undesirably big upwards acceleration of the mobile working deck, and the overload protection system is configured to control the elevation system such that the mobile working deck motion in the elevation motion range is downwards.

In an embodiment, the heave compensation system includes one or more hydraulic heave compensators, wherein the elevation system includes one or more hydraulic devices, and wherein the one or more heave compensators of the heave compensation system are connected to the floating body and to the one or more hydraulic devices of the elevation system with the one or more hydraulic devices being connected to the mobile working deck, or the one or more hydraulic devices of the elevation system are connected to the floating body and to the one or more heave compensators of the heave compensation system with the one or more heave compensators being connected to the mobile working deck.

It is noted here that connected to in the above embodiment also means an indirect connection instead of a direct connection, but that in any way the one or more hydraulic heave compensators are arranged in series (operatively seen) with the one or more hydraulic devices between the floating body and the mobile working deck.

It is further noted that the above described embodiment does not require that the number of hydraulic heave compensators is equal to the number of hydraulic devices. It may well be possible, but it is also possible that there is a difference in number of compensators and devices.

In an embodiment, one or more actuators are provided between the floating body and the mobile working deck, each actuator being formed by a heave compensator of the heave compensation system in series with a hydraulic device of the elevation system. The actuator may be embodied as a stack formed by a heave compensator arranged on top of a hydraulic device or a hydraulic device arranged on top of a heave compensator, but in an alternative embodiment, each heave compensator of the heave compensation system comprises a hydraulic cylinder and a piston rod moveable within and extending from said hydraulic cylinder, wherein each hydraulic device of the elevation system comprises a hydraulic cylinder and a piston rod moveable within and extending from said hydraulic cylinder, and wherein the piston rod of the heave compensator forms the hydraulic cylinder of the hydraulic device. Alternatively, an embodiment in which the piston rod of the hydraulic device forms the hydraulic cylinder of a corresponding heave compensator is also envisaged.

In an embodiment, each heave compensator of the heave compensation system comprises a hydraulic cylinder, a piston and a piston rod moveable within and extending from said hydraulic cylinder, said piston dividing a space inside the hydraulic cylinder in a cap side chamber and a piston rod side chamber, and wherein the overload protection system is configured to open at least one release valve to release hydraulic pressure from the cap side chamber of each heave compensator in the event of a detected undesirably big acceleration of the mobile working deck. This reduces the load applied to the mobile working deck by the heave compensation system and thus lowers the acceleration of the mobile working deck in addition to the motion caused by the elevation system to lower the acceleration of the mobile working deck.

In an embodiment, each hydraulic device of the elevation system comprises a hydraulic cylinder and a rod moveable within and extending from said hydraulic cylinder, wherein the hydraulic cylinder includes a pressure chamber for receiving pressurized hydraulic fluid to position the rod relative to the hydraulic cylinder, and wherein the overload protection system is configured to open at least one release valve to release hydraulic pressure from the pressure chamber of each hydraulic device in the event of a detected undesirably big acceleration of the mobile working deck. Releasing the hydraulic pressure results in lowering of the mobile working deck within the elevation motion range and is thus one of the examples of control of the elevation system in case of an undesirably big acceleration of the mobile working deck.

In an embodiment, the offshore system further comprises a riser tensioning system adapted to connect a riser extending along the firing line between the subsea wellbore and the floating body, and a riser connecting system adapted to connect the riser to the mobile working deck, wherein the overload protection system is configured to detect the undesirably big

acceleration of the mobile working deck in the riser connecting system.

In an embodiment, the overload protection system comprises a cylinder in the riser connecting system, which is in fluid communication with a release valve of the elevation system, such that in case of an undesirably big acceleration of the mobile working deck, the cylinder will retract or extend and cause the release valve to open.

The invention also relates to a vessel comprising a floating body and an offshore system according to the invention, wherein the tower of the offshore system is positioned on the floating body. The vessel may for instance be a semi-submersible.

The invention further relates to a method for performing subsea wellbore related activities using the above described vessel, comprising the following steps:

a. positioning the mobile working deck within the elevation motion range using the

elevation system to allow heave compensation of the mobile working deck;

b. providing heave compensation of the mobile working deck using the heave

compensation system; and

c. in the event an undesirably big acceleration of the mobile working deck is detected, controlling the elevation system such that the mobile working deck motion in the elevation motion range is opposite to the mobile working deck motion in the heave compensation motion range in order to lower or limit the acceleration of the mobile working deck.

In an embodiment, during the heave compensation the mobile working deck supports a drill string, wherein the undesirably big acceleration is caused by the support of the drill string suddenly and undesirably failing.

In an embodiment, a riser is connected to the mobile working deck during heave

compensation, and wherein the undesirably big acceleration is detected at the location where the riser is connected to the mobile working deck.

The invention may also be summarized as an offshore system for performing subsea wellbore related activities, comprising:

a tower to be positioned on a floating body and defining a substantially vertical firing line along the tower;

a mobile working deck which is moveable with respect to the tower along the firing line within a motion range including an elevation motion range and a heave compensation motion range;

heave compensation system being adapted to provide heave compensation of the mobile working deck within the heave compensation motion range; and

an elevation system being adapted to position the mobile working deck within the elevation motion range,

wherein the heave compensation system and the elevation system are arranged in series such that either the heave compensation system is adapted to provide heave compensation of the combination of mobile working deck and elevation system or the elevation system is adapted to position the combination of heave compensation system and mobile working deck, wherein the offshore system further comprises an overload protection system configured to directly or indirectly detect an undesirable lowering of predetermined minimal size of the weight supported by the mobile working deck, and wherein the overload protection system is configured such that, in the event of a detected undesirable lowering of the weight that cannot be compensated for by the heave compensation, the elevation system is controlled to lower the mobile working deck within the elevation motion range.

Features and/or embodiments described above may be applied to the offshore system according to the above summary where applicable and will not be unduly repeated here. The invention will now be described in a non-limiting way by reference to the accompanying drawings in which like parts are indicated using like reference symbols, and in which:

Fig. 1 depicts a semi-submersible vessel representing an exemplary embodiment of a vessel according to the invention;

Fig. 2 schematically depicts the mobile working deck of the vessel of Fig. 1 supported by a heave compensation system and an elevation system according to an embodiment of the invention; and

Fig. 3 schematically depicts the mobile working deck of the vessel of Fig. 1 supported by a heave compensation system and an elevation system according to another embodiment of the invention.

Fig. 1 depicts a vessel 1 , here of the semi-submersible type. The vessel 1 comprises a floating body or hull 3 with a deckbox structure 3a having an upper or main deck 6 and a box bottom, one or more pontoons 3b, here two parallel pontoons, and multiple, here four, support columns 3c extending upward from the one or more pontoons 3b and supporting thereon the deckbox structure 3a.

In the deckbox structure 3a there is a moonpool 4 that extends through the deckbox structure 3a up to the main deck 6 of the deckbox structure.

For example, the height of the deckbox structure 3a between the main deck 6 and the box bottom is between 11 and 15 meters, e.g. about 12.5 meters.

The vessel 1 comprises an offshore system with a tower 30 erected above the main deck 6, here of the deckbox structure 3a, and adapted to perform subsea wellbore related activities along at least one firing line 9 of the offshore system that vertically extends through the moonpool 4 into the sea. The mentioned wellbore related activities may include workover activities, well maintenance, installing an object on a subsea wellbore, drilling a subsea wellbore, etc.

The tower 30 is provided with a crown block 31 at the top thereof and a travelling block 32 is suspended from the crown block 31 in a multiple fall arrangement of a cable 33. The cable is connected to one or more winches allowing the travelling block 32 to be moved up and down along an operative side of the tower 30 and in the firing line 9, so generally in a range above the main deck 6. In a drilling operation the travelling block 32, for example, carries a topdrive 34 adapted for rotary drive of a drill string that is suspended from the topdrive and extends into a subsea wellbore.

As preferred, the travelling block 32 is coupled to a trolley 35 that travels up and down along one or more vertical rails 36 provided on the tower 30.

The vessel 1 is further provided with a winch system comprising a hoisting winch and a hoisting cable 12 connected to the hoisting winch. A travelling hoist block 13, distinct from the travelling block 32, is able to travel up and down along a side of the tower 30, and is supported by the hoisting cable 12. The block 13 may be used for connecting with an object to be lowered to, or lifted from, the seafloor. Therefore, the length of the cable 12 is sufficient to reach the seafloor.

Preferably, the cable 12 is arranged in a single fall arrangement, so with hoist block 13 at the end thereof as shown in Fig. 1. In an alternative one could envisage a two-fall arrangement, wherein the block 13 has at least one sheave around which the cable 12 passes. In a two-fall arrangement the cable 12 may have a terminal end that is embodied as a dead end. In another embodiment of a two-fall arrangement the cable 12 is connected at either end thereof to a winch, e.g. to allow for increase speed and/or redundancy. In yet another embodiment one end of the cable is connected to a winch and the other end to a heave motion

compensation system.

In an embodiment the cable 12 is a synthetic fibre rope.

The tower 30 here is provided with a top sheave 37 from which the cable 12 is suspended in the firing line 9. The top sheave 37 may be arranged in proximity of the crown block 31 or even combined therewith

The vessel 1 is provided with a mobile working deck 15 that is moveable with respect to the tower along the firing line within a motion range which will be elucidated in more detail below. The mobile working deck 15 has a working deck surface, which working deck in a lowered position covers the moonpool 4, and which working deck surface in the lowered position of the mobile working deck 15 is preferably level with the main deck 6.

As will be explained in more detail below the mobile working deck 15 is supported by a heave compensation system being adapted to provide heave compensation of the mobile working deck within a heave compensation motion range, and an elevation system being adapted to position the mobile working deck within the elevation motion range, wherein the heave compensation system and the elevation system are arranged such that a position of the mobile working within the motion range is the sum of the position of the mobile working deck within the elevation motion range and the position of the mobile working deck within the heave compensation motion range. In other words, the motion range is the sum of the elevation motion range and the heave compensation motion range.

The elevation system can be used to raise the mobile working deck 15 from the lower stationary position and to bring the mobile working deck 15 at a level where the heave compensation range is positioned above the lower stationary position allowing heave compensation without the risk of a collision with the main deck 6.

The heave compensation system may further be adapted to provide heave compensation for the cable 33 to provide a heave compensated motion of the travelling block 32 and thus of any drill string or the like suspended from the topdrive 35 during a drilling operation. The heave compensation may be active and/or passive, and may include one or more sheave compensators each supporting one or more cable sheaves that engage the cable from which the travelling block is suspended. The heave compensation may also, alternatively or in combination with sheave compensator(s), act directly on the winch(es), e.g. an active heave compensation control via a variable frequency electric drive of the winch(es).

Hence, as is preferred, the heave compensation system is able to effect a heave

compensating motion of the mobile working deck within the heave compensation motion range synchronized with the heave compensation of the travelling block 32 that travels along a side of the tower 30.

In a lower stationary, so non-compensated, position the mobile working deck 15 (see figure 1) is envisaged for use as a stationary drill floor covering the moonpool 4 and generally level with the adjoining deck 6 of the hull.

In an embodiment, the mobile working deck 15 may be provided with a personnel access platform supported underneath the mobile working deck 15 that facilitates access to equipment underneath the mobile working deck 15 during operations.

In an embodiment the vessel 1 is provided with a drilling tubulars storage rack 120 that is mounted on the hull 3, e.g. the deckbox structure, e.g. multi-joint drill pipe stands storage rack, e.g. a rotary storage rack. The drilling tubulars storage rack is adapted for storage of drilling tubulars in vertical orientation therein. The vessel, e.g. the mast structure 30, is provided with a racker system that is adapted to move a drilling tubular between the storage rack and a position aligned with the firing line. The racker system is heave compensated and is configured to bring a drilling tubular removed from the storage rack in a heave

compensation motion that is synchronized with the heave compensation motion of the mobile working deck 15. For example the racker comprises a vertical motion arm assemblies rail, wherein at least one, e.g. multiple, motion arm assembly 125 is mounted on said vertical motion arm assemblies rail, each motion arm assembly having a base that is vertically mobile along said vertical motion arm assemblies rail by a drive configured to provide said heave compensation motion that is synchronized with the heave compensation motion of the mobile working deck. For example each motion arm assembly 125 further having an extensible, e.g. telescopic, arm that is mounted via a vertical axis slew bearing on said base so as to allow for extension and retraction of said arm as well as slewing motion of said telescopic arm about said vertical slew axis, wherein said arm is adapted to support a tubulars gripper tool at an end of said arm, so as to allow for gripping of a drilling tubulars by means of the tubular gripper tool.

Fig. 2 schematically depicts a cross-sectional view of the mobile working deck 15 of Fig. 1 with the working deck surface indicated by reference numeral 16. Fig. 2 depicts the situation that a diverter 110 is supported on the underside of the mobile working deck 15 and a riser 115 is connected to the diverter 110. The diverter 110 is configured to divert a hydrocarbon and/or drilling mud stream from a subsea wellbore to the vessel. Commonly a hose or pipe connects the diverter to a mud handling facility onboard the vessel 1 , e.g. located within the deckbox structure 3a.

Also shown in Fig. 2 is a slip device 20 having for instance mobile clamping jaws, which slip device 20 is configured to support a suspended drill string 130 or the like.

In an embodiment, the mobile working deck 15 supports a riser spider device configured to support a suspended riser, e.g. during assembly and disassembly of a riser. For example, the riser spider device has radially moveable dogs that engage underneath a flange of a riser joint to support the weight of the riser string.

The vessel 1 of Fig. 1 , as an option, allows for performing of drilling activities wherein the heave compensated mobile working deck 15 rests on the top of the rigid riser that extends down to a subsea well. In an embodiment, the hull 3, here in the deckbox structure 3a adjoining the moonpool 4, comprises one or more wireline riser tensioner equipment rooms accommodating therein wireline riser tensioner equipment, e.g. comprising vertically oriented wireline riser tensioner cylinders. This equipment is configured to provide top tension to the riser 115 that extends along the firing line between the seafloor and the vessel 1.

In this embodiment, the diverter 110 is connected to the mobile working deck 15 via a connecting structure 140. This connecting structure 140 may for instance include cylinders 145 operatively connected at one end to the diverter 110 and with the opposite end to the mobile working deck 15. The connecting structure 140 may provide an additional tension to the riser 115 albeit that in a practical embodiment, this additional tension may be relatively small compared to the tension applied to the riser using the aforementioned wireline riser tensioner equipment.

As already indicated above, the mobile working deck 15 is supported by the heave compensation system and the elevation system. Fig. 2 depicts an embodiment thereof.

Shown are two hydraulic actuators 150, but a practical embodiment may include any number of such hydraulic actuators, e.g. 1 , 3, 4 or more hydraulic actuators 150 depending on the demands, available space, etc.

The hydraulic actuators 150 are thus provided between the floating body, i.e. the hull 3, and the mobile working deck 15. Each hydraulic actuator 150 comprises a hydraulic compensator 151 connected to the hull 3 and a hydraulic device 152 arranged between the hydraulic compensator 151 and the mobile working deck 15.

In this embodiment, the hydraulic compensator 151 is part of the heave compensation system allowing heave compensation in a heave compensation motion range indicated by the stroke ST 1 of the hydraulic compensator 151.

In this embodiment, the hydraulic device 152 is part of the elevation system allowing to position the mobile working deck 15 in an elevation motion range indicated by the stroke ST2 of the hydraulic device 152.

In other words, the heave compensation system and the elevation system are arranged in series, so that the position of the mobile working deck 15 in its motion range is determined by the sum of the relative position Z1 within the heave compensation motion range and the relative position Z2 within the elevation motion range. In a practical embodiment, the heave compensation motion range may be 5-10 meters, e.g. about 7.5 meters, and the elevation motion range may be 4-8 meters, e.g. about 6 meters, so that the motion range of the mobile working deck 15 may be 9-18 meters, e.g. about 13.5 meters.

In this embodiment, the hydraulic compensator 151 is configured as a double-acting compensator allowing to actively move in both directions, i.e. up and down, to achieve the desired heave motion compensation speeds in both directions.

Hence, the hydraulic compensator 151 includes a hydraulic cylinder 151a, a piston 151b, and a piston rod 151c connected to the piston 151 b and being moveable within and extending from the hydraulic cylinder 151a. The piston 151b divides a space inside the hydraulic cylinder 151a in a cap side chamber 151 d and a piston rod side chamber 151e. Forcing hydraulic liquid in the cap side chamber 151 d and letting hydraulic liquid simultaneously be allowed to leave the piston rod side chamber 151e will actively extend the hydraulic compensator 151. Forcing hydraulic liquid in the piston rod side chamber 151e and letting hydraulic liquid simultaneously be allowed to leave the cap side chamber 151 d will actively retract the hydraulic compensator 151.

In this embodiment, the hydraulic device 152 is configured as a single-acting device allowing to actively extend the device, but passively retract the device using gravitational forces or other downwardly directed loads. As the hydraulic device 152 is used to elevate the mobile working deck 15, a double-acting device can be used, but is not necessary per se.

Hence, the hydraulic device 152 includes a hydraulic cylinder 152a and a piston rod 152b being moveable within and extending from the hydraulic cylinder 152a. The piston rod 152b and the hydraulic cylinder 152a delimit a space 152c. Forcing hydraulic liquid in the space 152c will actively extend the hydraulic device 152. When an appropriate valve is opened, gravitational forces acting on the hydraulic device, e.g. the weight of the mobile working deck and any loads applied thereon, will push hydraulic liquid out of the space 152c thereby allowing to passively retract the hydraulic device 152.

In the situation of Fig. 2, the mobile working deck 15 is in an elevated position Z2 within the elevation motion range allowing to heave compensate the mobile working deck within the heave compensation motion range. The main load applied to the mobile working deck 15 is the weight of the drill string 130 supported by slip device 20. During heave compensation, the hydraulic compensator 151 has more or less the behaviour of a spring, which spring has to provide a sufficiently large upwardly directed spring force to the mobile working deck to counteract the weight and loads applied to the mobile working deck 15.

In case of a sudden failure of the slip device 20 or sudden breaking of the drill string, the weight, i.e. the loads, applied to the mobile working deck 15 suddenly decreases significantly. The result is that the hydraulic compensator 151 , which is acting like a spring, will tend to launch the mobile working deck 15 by applying a relatively large acceleration to the mobile working deck 15 caused by the spring force applied to the mobile working deck 15 by the heave compensation system. In most instances, the applied spring force of the hydraulic compensator cannot be lowered sufficiently quick to prevent the launch of the mobile working deck 15. Whether the mobile working deck will actually be launched is not of relevance, because the damage caused by the relatively large acceleration of the mobile working deck and subsequent impact with other components of the offshore system, e.g. the tower, is similarly undesired.

To cope with the abovementioned situation, the offshore system in Figs. 1 and 2 includes an overload protection system. The function of the overload protection system is to prevent or minimize damage to the offshore system. The overload protection system is therefore configured to detect an undesirably big acceleration of the mobile working deck. In this embodiment, an undesirably big acceleration of the mobile working deck is detected in the cylinders 145 of the connecting structure 140 between the diverter 110 and the riser 115.

When an undesirably big acceleration of the mobile working deck occurs, the mobile working deck 15 will start to move upwards relative to the riser string 115 causing the cylinders 145 to retract. The retraction of the cylinders 145 or an associated pressure increase (or decrease) in a chamber in the cylinders may be used to detect the undesirably big acceleration.

It is of course noted that any other detection principle may alternatively or additionally be used, e.g. using a dedicated detection system with one or more detectors or sensors.

Examples thereof are:

measurement of tension in the connecting structure using e.g. strain gauges;

measurement of acceleration of the mobile working deck using e.g. acceleration sensors;

measurement of a relative position of the riser/diverter relative to the mobile working deck using e.g. a laser. The cylinders 145 are in this embodiment hydraulically connected (see dashed lines) to release valves 170 arranged on the respective hydraulic devices 152 of the elevation system. In the event of an undesirably big acceleration of the mobile working deck, the cylinders 145 will retract, so that the increase in hydraulic pressure in the cylinders 145 will cause the release valves 170 to open allowing hydraulic liquid to escape from the spaces 152c caused by the upwardly moving hydraulic cylinder 152a and the mass inertia of the mobile working deck 15. The result is that the mobile working deck moves downwards in the elevation motion range, i.e. the Z2 value decreases, while the motion in the heave compensation motion range is upwards, i.e. the Z1 value increases, such that the overall acceleration of the mobile working deck 15 is lowered or at least limited preventing or at least minimizing the damage caused by undesired motion of the mobile working deck 15.

The above embodiment describes a passive overload protection system. The cylinders 145 and release valves 170 may operate without requiring any active input or power, so that in substantially all situations the overload protection system may function. However, it is also envisaged that the overload protection system is an active system or a combination of active and passive components. Hence, the overload protection system may comprise a detection system configured to detect an undesirably big acceleration of the mobile working deck, e.g. using detectors and/or sensors, and/or may comprise a control system to control the elevation system and/or the heave compensation system in dependence of an output of the detection system.

Although not depicted in Fig. 2, such a detection system and control system may be present such that the detection system detects movement of the working deck relative to the diverter 110 or by measuring the pressure inside the cylinders 145 and the control system may be connected to the heave compensation system, e.g. release valves in fluid communication with the cap side chambers 151 d of the heave compensators 151 to release hydraulic pressure in these chambers quickly reducing the spring force applied by the heave compensators 151 to the mobile working deck 15 to further reduce and/or limit the acceleration applied to the mobile working deck 15 in addition to the passive overload protection system part. However, the cylinders 145 may also be connected to the release valves of the heave compensators 151 in a passive manner similar to the release valves 170.

In Fig. 2, the heave compensation system and the elevation system are arranged in series by arranging the respective heave compensators and hydraulic devices in series and combining a heave compensator 151 and a hydraulic device 152 into one actuator 150. Fig. 3 schematically depicts the mobile working deck 15 of the vessel of Fig. 1 supported by a heave compensation system HCS and an elevation system ES according to another embodiment of the invention. Again, the heave compensation system HCS and the elevation system ES are arranged in series, seen in vertical direction. Shown are two heave compensators 151 as part of the heave compensation system HCS and arranged between the floating body, hull 3 in Fig. 1 , and an intermediate member IM. The elevation system ES comprises a hydraulic device 152 arranged between the intermediate member IM and the mobile working deck 15. Hence, one hydraulic device 152 is used per two heave compensators 151. The offshore system may for instance comprise two configurations as shown in Fig. 3 at opposite sides of the moonpool.