Today a significant percentage of the production equipment is not installed on the surface of the sea but on the sea bottom. As with all equipment the equipment on the sea bottom needs regular maintenance. Also during the lifetime of an oilfield the bor holes and the oilfield itself need maintenance to keep the production as high as possible.

Maintenance of the oil field and the production equipment on the sea bottom is a difficult task, which is both time intensive and very expensive.

To perform these maintenance special vessels are being used such as semi-submersibles or drill vessels. These vessels have a number of disadvantages such as a low transit speeds or high daily running cost.

New builds or converted non-dedicated vessels the so called"well intervention vessels"are increasingly being used to install equipment on the sea bottom and to perform maintenance. The advantages of small vessels are low running cost and acceptable transit speeds. However the disadvantage is that small vessels tend to have bad motion characteristics. They move a lot more compared to the bigger units limiting the weather window. This might not be an issue in areas where only mild weather conditions prevail but in areas where often- severe weather conditions are encountered a small weather window severely limits the operational capabilities of the small vessel.

Well intervention involves everything from lowering a ROV to do a visual check to lowering entire production or maintenance units to the sea bottom and retrieving the units. During the intervention

operation these units have to be moved over the deck of the vessel from and to storage areas, moonpool and maintenance areas. Often these units are big and heavy, which makes handling the units difficult. Sometimes those modules are required to be stacked on top of each other prior to lowering them to the seabed. Often crewmembers have to work on elevated levels to be able to reach all parts of the units. Current practice is the use of man-riding winches. This is both dangerous and time consuming. Scores of accidents have occurred with the use of man riding winches.

This requires cranes and moving systems that can handle these heavy objects under the condition imposed by the vessel. Moving and lifting modules on a moving deck can be quite dangerous and numerous accidents have occurred during this kind of activities.

Apart form moving objects on the decks of the vessel these lowering and lifting of the units through moonpools located in the vessel generates some specific problems. When lowering units through the moonpool these objects tend to swing form side to side. There is a considerable risk of damage to the unit or the vessel when the modules are not constrained in some way. Retrieving objects through the moonpool is equally dangerous. The relative motion s of the vessel and the modules can be such that there is also the danger of the module hitting the vessel. Endangering the vessel and the lives for the crew.

On well intervention vessel according to prior art standard drilling derricks are used which have the shape of an inverted U to lower to and lift objects of the seabed. This severely limits the size of the modules that can be handled because every module has to pass through the V-door of the drilling derrick. Additionally the two vertical support structures of the derrick severely limit the area that can be reached by other cranes and equipment of the vessel. Due to the construction of the drilling derricks according to prior art the drilling derricks must be p laced at specific locations in order not to hinder other equipment. This limits the freedom in the design of

the vessel considerably. Also removing the drilling derrick from the vessel when it is not used is a difficult task due to the size and the weight of the drilling derrick.

It is an object of the invention to provide an improved vessel, in particular with regard to the handling of loads which are to be raised and/or lowered through the moonpool.

It is a further object of the present invention to provide measures which provide improved safety for personnel involved with the loads to be raised and/or lowered through the moonpool, in particular when working at elevated levels.

The invention provides a monohull vessel, preferably an offshore vessel, according to claim 1.

Preferably the tower is loacated adjacent a single side of the moonpool. Hereby the access to the moonpool when raising or lowering loads is greatly improved in comparison to the prior art design wherein vertical structural parts of the derrick were present on opposed side of the moonpool.

Preferably the tower is provided with one or more trolleys and associated vertical trolley guides, which allow for up and down movement of the one or more trolleys along at least a part of the height of the tower. More preferably the guides extend dwonswards such that the trolleys can be lowered into the moonpool, preferably till the bottom of the vessel or even beneath said bottom.

Preferably said trolleys are adapted to engage on a load hoist wire and/or directly onto the load, preferably also when the load is (partly) submerged in the moonpool. This allows for a stabilization of the load and avoid collisions and damage of the load as well as increases the safety for operating crew.

Preferably the tower is provided with a moveable work platform in order to allow the crew to work safely at elevated heights, e. g. when

caarying out work on a load held by the hoist system in or above the moonpool.

Preferably the vessel comprises a hatch assembly for covering (a part of) the moonpool.

In a preferred embodiment the hatch assembly includes multiple hatches having fenders for allowing contact of the fenders with a load held in or above the moonpool. Hereby the load can be stabilized using the hatches.

As an alternative or in combination with the fendered hatches a separate fender assembly can be provided at the moonpool, inclusing moveable fender to engage on the load.

The invention further relates to a tower for such a vessel and also to a hatch assembly of a vessel having a moonpool.

The invention also relates to methods for operating a vessel having a moonpool.

Advantageous embodiments of the vessel according to the invention, the tower and the hatch assembly as well as the methods according to the invention are disclosed in the subclaims and in the description referring to the drawings.

In the drawings: Figure 1 shows a side view of a vessel with a module handling system installed.

Figure 2 shows a top view of a vessel with module handing system installed.

Figure 3 shows a top view of a vessel with Multipurpose tower in another direction.

Figure 4 shows a top view of a working platform.

Figure 5 shows a perspective view of a working platform.

Figure 6 shows a side view of an auxiliary trolley.

Figure 7 shows a side view of a main trolley.

Figure 8 shows a side view of a multipurpose tower.

Figure 9 shows a detailed view of a mast head of a multipurpose tower.

Figure 10 shows a top view of a moveable hatch.

Figure 11 shows a detailed top view of a moveable hatch.

Figure 12 shows a side view of a moveable hatch.

Figure 13 shows a top view of two hatches with rails.

Figure 14 shows a side view of the rail system.

FIG 1 shows a side view of a vessel (7) on which a Multi Purpose Tower (1) is mounted on deck (10). Also a Knuckleboom Crane (3) is mounted on the vessel (7). Knuckleboom crane (3) can be used to pick up equipment and tools from the quay and load them onto the vessel.

Multi Purpose Tower (1) is located next to moonpool (9). It can be seen that first trolley guiding rail (13) and second trolley guiding rail (14) which are connected to the front of Multi Purpose Tower (1) run into moonpool (9) to the bottom (8) of the vessel (7). In this embodiment main trolley (29) and auxiliary trolley (27) can move from the top side (113) of the Multi Purpose Tower (1) to the bottom (8) of the vessel (7). It is now possible for main trolley (29) and auxiliary trolley (27) to be connected to module (5) while module (5) is lowered into moonpool (9). This prevents any free movement of module (5) while it is being hoisted of lowered into of from moonpool (9). Only when module (5) reaches bottom (8) of vessel (7) both trolleys disconnect. Since the module is now under water level (123) the motions of module (5) are considerably reduced thereby reducing the risk of module (5) hitting first side wall (125) or second side

wall (126) of the moonpool. Also module (5) has to be lowered only a small distance in order for the module (5) to clear from vessel (7) completely. Secured to skid cart (111) is sub-sea module (5).

Fixably mounted on side (115) of Multi Purpose Tower (1) is storage area (117), which holds in this particular embodiment riser (119).

-However storage is not limited to risers only. Also other equipment such as hoses, drill pipe and casing can be stored. It should be noted that although in the preferred embodiment centerline (129) of Multi Purpose Tower (1) coincidences with centerline (131) of moonpool (9) other placements are also possible.

FIG 2 shows a top view of deck (10) of vessel (7). Multi Purpose Tower (1) is located next to moonpool (9) in such a way that first trolley guiding rail (13) and second trolley guiding rail (14) which are not visible in this figure can run directly from topside (113) of the Multipurpose Tower (1) to bottom (8) of vessel (7). Also visible are first hatch (23) and second hatch (21). First hatch (23) and second hatch (21) can move over deck (10) in directions indicated with A and B respectively. Movement of First Hatch (23) and Second Hatch (21) respectively is always perpendicular to second multi purpose centerline (133) of Multi Purpose Tower (1) otherwise the hatches would collide with the Multi Purpose Tower. Module (5) can be moved over deck (10) of vessel (7) with the aid of transversal push pull unit (17) and longitudinal push pull unit (19). Not visible in the drawing is skid cart (111) on which module (5) is secured. Skid cart (111) skids over longitudinal skid rails (101) and transversal skid rails (103), which are removably mounted on deck (10). Push pull unit (17) moves over skid rail topside (91), which is not visible in this view while push pull unit (19) moves inside longitudinal skid rails (101). By using longitudinal push pull unit (19) and transversal push pull unit (17) module (5) can be moved over the whole area of deck (10) provided that a multitude of longitudinal skid rails (101) and transversal skid rails (103) are installed.

FIG 3 shows a different placement of Multi Purpose Tower (1) on vessel (7) in which first Hatch (23) and Second Hatch (21) move perpendicular to the longitudinal axis (135) of vessel (7). The choice which placement of moonpool (9) and Multi Purpose Tower (1) is governed by operational and technical conditions.

Working on well intervention vessels is dangerous and demanding.

Often the deck of the vessel is moving considerably in all directions.

During bad weather wind and water are making working difficult.

Especially when crewmembers have to work on elevated heights the wind load increases considerably. Also any motions of the well intervention vessel are amplified when working at elevated heights.

On most vessels crewmembers who have to work on elevated levels are being hoisted by so called"man riding winches". The use of man- riding winches no matter how reliable they are has caused a large number of accidents often with deadly consequences.

FIG 4 shows working platform (31), which can move on the outside of Multi Purpose Tower (1) over first rail (157), second rail (158), third rail (159) and fourth rail (160) which makes main riding winches superfluous. The size of working platform (31) is such that it can pass main trolley (29), auxiliary trolley (27) without interference.

Also working platform (31) can pass module (5), which is not shown in this figure when it is being hoisted by Main Hoist (59). Located on working platform (31) are movable plates (43) and (44) which can move in the directions indicated with the letters F and G. Often different modules have different sizes. In order to allow the crew to work on the modules in a safe and efficient matter movable plates (43) and (44) can move to minimize the gap between the modules and working platform (31). Minimizing this gap prevents crewmembers and tools from falling down. To protect the crew working platform (31) can be fitted with railing (151). In another embodiment working platform (31) can be fitted with a wind wall or other protection devices.

FIG 5 shows a perspective view of working platform (31).

It is desirable that crew and tools can be moved from deck (10) to working platform (31) when the working platform is hoisted to elevated levels. In this embodiment auxiliary trolley (27) is being used for this purpose.

FIG 6 shows a side view of auxiliary trolley (27) in which first moving arm (47) and second moving arm (48) are visible. Due to the parallelogram construction any load that is picked up by moving arm (47) and moving arm (48) does not rotate when said moving arms move outward. Said moving arms can connect to basket (153) to transport crew and equipment to working platform (31). Moving arms (47) and moving arm (48) can move inward in order to let auxiliary trolley pass main trolley (29) and module (5) without interference. Auxiliary trolley (27) is also used to stabilize module (5) when module (5) is being hoisted or lowered. In this case said moving arms move outward until a connection can be made with module (5). Auxiliary trolley (27) moves on the inside of first trolley guiding rail (13) and second trolley guiding rail (14) mounted on front side Multi Purpose Tower (1) guided by first wheel set (50) and second wheel set (51) which are fixably connected to auxiliary trolley main structure (49). Moving said moving arms (47) and (48) is accomplished by hydraulic cylinder (45). In this embodiment cylinder (45) can be controlled in such a way that it can act as a damper. This is advantageous when modules, which are being hoisted in the moonpool have to be stabilized.

Stabilization is for a part done by using auxiliary hoist (27) by controlling hydraulic cylinder (45) to act first as a damper until movement of module (5) is decreased such that a fixed connection between moving arms (47) and (48) and module (5) can be made. After connection moving arms (47) and (48) extend or retract to align module (5) to Multi Purpose Tower (1). It is clear that large forces can occur during the damping phase and auxiliary trolley (27) has a relative heavy construction to cope with these forces.

FIG 7 shows a side view of main trolley (29). The main purpose of main trolley (29) is to center the main hoist wire (59) to prevent it from swinging and consequently to prevent module (5) from swinging

when it is hoisted. This is accomplished by letting main hoist wire (59) to run through a fitting hole in main trolley (29). Main trolley (29) can move freely on rails (13) and (14) without interference with the auxiliary trolley (27) and working platform (31). Hoisting the main trolley is done by the ball weight (161), which is located at the end of the main hoist wire (59) and connects to a catching cone (163) fixably mounted on main trolley (29). During lowering of loads in the moonpool main trolley (29) moves to bottom (8) of the vessel where it disconnects from the ball weight (161) while the load is being lowered further. During hoisting of loads when the ball weight (161) at the end of the main hoist wire (59) passes the main trolley (29) it connects to it again and lifts the main trolley together with any load that is connected to the main hoist wire. The shape of the ball weight (161) is such that it centers when it enters the catching cone (163) of the main hoist (29). No locking devices are necessary in this embodiment although operational demands could make additional locking of the ball weight (161) and catching cone (163) necessary.

FIG 8 shows a side view of Multi Purpose Tower (1). Multi Purpose Tower (1) is mounted on deck (10) of vessel (7). It can be seen that first trolley guiding rail (13) and second trolley guiding rail (14) are mounted on front side (165) of Multi Purpose Tower (1) and are also mounted on moonpool side wall (131) and are finally running to vessel bottom (8). This makes it possible for auxiliary trolley (27) and main trolley (29) to move from Multi Purpose Tower Top Side (113) to vessel bottom (8) through moonpool (9). Inside Multipurpose Tower (1) are multiple winches installed first winch (167), second winch (168), third winch (169), fourth winch (170) and fifth winch (171).

Said winches are installed at a low elevation level. This is advantageous because it contributes to a lower Centre Of Gravity of the vessel. Said winches (167), (168), (169), (170) and (171) are used to hoist auxiliary trolley (27), working platform (31) and a plurality of wires of which three are shown first wire (175), second wire (177) and fourth wire (179) that can be connected to module (5) and are lowered with module (5) to the sea bottom. Said wires (175),

(177) and (179) run from third winch (169), fourth winch (170) and fifth winch (171) to first compensation system (181) and second compensation system (183) and third compensation system (187) which is not visible and over a multitude of sheaves located in masthead (191).

Between winches (167), (168), (169), (170) and (171) and module (5) heave compensation systems can be installed. Two of those heave compensation systems (181) and (183) are visible. Heave compensation systems (181) and (183) are fixably mounted to Multi Purpose Tower (1) near multi purpose tower top side (113) Near said heave compensation systems (181) and (183) are first pressure vessel (197) and second pressure vessel (199) fixably mounted to multi purpose tower (1) which are connected to said heave compensation systems. The number of pressure vessels does not need to be the same as the number of heave compensation systems. Main hoist wire (59) runs from the inside of vessel (7) to the inside (193) of Multi Purpose Tower (1) and then first sheave (195) and over second sheave (196). First sheave (195) is fixably connected to masthead (191) while second sheave (196) is rotatebly fixed to masthead (191). Ladder (199), which is connected to masthead (191) and makes it possible for the crew to move from the working platform (31) onto the masthead (191) in a safe and orderly manner. This is advantageous because large pieces of equipment can now be transported to the masthead without the restrictions of the limited space, which exists inside multi purpose tower (1). Also the crew does not need to climb a large number of stairs or ladders to reach masthead (191). Also visible is riser storage (117) fixably connected to Multi Purpose Tower (1) with a riser stored (119) in it.

Figure 9 shows masthead (191) with an optional positions of sheave (196). Sheave (196) is rotably connected to masthead (191). In the first position denoted with the roman capital I the main hoist wire (59) runs directly to module (5). In the second position denoted with the roman capital II the main hoist wire runs from sheave (196) to third sheave (201) which is connected to ball weight (161) and at its is finally connected to masthead (191). In sheave position II a heavier load can be hoisted compared to sheave position I although at

a lower speed. Changing from position I to position II is relatively easy and takes little time. The advantage is that with the same Multi Purpose Tower a wide range of loads can be hoisted.

FIG 10 shows a top view of first hatch (23) located next to moonpool (9). First hatch (23) and second hatch (21) are in a preferred embodiment identical in construction. First hatch comprises first structural beam (207) and second structural beam (208) fixably connected to each other which can be moved in direction A by first hydraulic cylinder (203) and second hydraulic cylinder (204). Said cylinders are connected on one side to deck (10) of vessel (7) and on the other side to first hatch (23). Also visible is fender (222), which can move in direction A. Fender (222) is connected with hydraulic cylinders (232) and (234) to structural frame (207). Fender (222) is located on the side, which is oriented to the center of moonpool (9). Fender (222) can be moved in three ways, first by moving structural frame (207), second by extending or retracting hydraulic cylinders (232) and (234) while structural frame (207) is not moving and thirdly by moving both the structural frame (207) and hydraulic cylinders (232) and (234). All hydraulic cylinders can be controlled to act as shock dampers. When module (5) is hoisted into moonpool (9) its movements have to be minimized in order to prevent damage to the moonpool. This is accomplished by moving second hatch (21) and first hatch (23) simultaneously to the center of moonpool (9) with the fenders fully extended and damping out any excess movement of module (5). On fender (222) a shock absorbing material is mounted to minimize impact damage. In a preferred embodiment this material is wood or rubber but other materials can be used as well. Once module (5) is stationary auxiliary trolley (27) can dampen out the remaining movements of module (5).

Often modules are lowered to the seabed, which require a power supply or other services from the vessel to function properly. It is also possible that another module is lowered on top of the first module already installed on the sea bottom. To guide any extra modules wire guides are used which are to guide the modules to the correct place

without the need for alignment from a ROV. These wire guides run from the module on the sea bottom through the moonpool to the top of the Multi Purpose Tower in this specific embodiment. It is conceivable that up to 7 wires and two umbilical wires are running down at the same time. When a new module is lowered to the seabed first it has to be connected to the guide wires. Not always is the size of a module such that it can move without interference between the wires. When this is not the case these wires have to be moved apart to create the space needed for passage of the new module. Another problem that occurs is that the movement of the vessel causes the wires to move inside the moonpool of the vessel thus making it difficult to catch and secure the wires. All above mentioned actions and functions have to be incorporated in the moving hatches.

FIG 11 shows a detail of first hatch (23) with first secondary hatch (303) and second secondary hatch (305) movably mounted to it. Located on first secondary hatch (303) and second secondary hatch are first wire catching system (307) and second wire catching system (309). The purpose of said catching systems is twofold, gripping the wires and moving the wires. This is accomplished by gripping system (311) mounted on first secondary hatch (303), which can move in direction indicated by the capital K by hydraulic cylinder (313). Likewise gripping system (315) is mounted on second secondary hatch (305).

FIG 12 shows a side view of first hatch (23) on which first secondary hatch (303) is visible in the open position and second secondary hatch (305) is visible in the closed position. In the open position of secondary hatches (303) and (305) the secondary hatches do not interfere with the stabilization procedure of module (5) using fenders. In the closed position secondary hatches form a safe working platform over the moonpool. Also visible are first hatch moving system (203) and second hatch moving system (204). In a preferred embodiment these systems are hydraulic cylinders but other moving systems can be used as well. First hatch (23) slides on rails indicated by numbers (331) and (333).

When Multi Purpose Tower (1) is oriented on vessel (7) as indicated in FIG 3 the rails in which longitudinal push pull unit (19) moves can be an integral part of the movable hatches. Once the hatches are fully opened moonpool (9) is completely cleared. Orienting Multi Purpose Tower (1) as indicated in FIG 2 making rails an integral part of the movable hatches is not possible anymore.

FIG 13 shows first rail (345) and second rail (347), which can slide over first hatch (23) moved by first rail cylinder (349) and second rail cylinder (351). First rail (345) and second rail (347) move in the same direction as first hatch (23) indicated by the capital A.

First rail (345) is shown in fully retracted position while second rail is shown in fully extended position. Third rail (351) and fourth rail (353), which can slide over second hatch (21) moved by third rail cylinder (355) and fourth rail cylinder (357). Third rail (351) and fourth rail (353) move in the same direction as second hatch (21) indicated by the capital B. Third rail (351) is shown in fully retracted position while fourth rail is shown in fully extended position. It can be seen that in when the all the rails are in fully retracted position moonpool (9) is completely cleared. When first rail (345), second rail (347), third rail (351) and fourth rail (353) are fully closed it is possible for longitudinal push pull unit (19) to move over moonpool (9) to transport module (5) to the centerline () of moonpool (9) or to the other side of moonpool (9).

In FIG 12 first rail (345), first rail cylinder (349), second rail cylinder (351) and second rail (347) are also visible.

FIG 13 shows a side view of transversal rail (103) and a cross view of longitudinal rail (101). In this specific embodiment transversal push pull unit (17) is moving on the top side of rail (103) while longitudinal push pull unit (19) is moving inside longitudinal rail (101). It can be seen that transversal push pull unit can slide over the topside of longitudinal push pull unit indicated by (401) to be able to pass over longitudinal rail (101). The moving system to move longitudinal push pull unit (101) and push pull unit (103) is well

known from previous art. Both longitudinal push pull unit (101) and transversal push pull unit (103) are fitted with locking devices indicated by (405) and (407) to prevent module (5) from moving in unwanted directions. It is contemplated that the power to drive the push pull units in a preferred embodiment is delivered by a central power unit. Although each push pull unit can be fitted with an independent power unit as well.

The invention is by no means limited to the exemplary embodiment described herein above, but comprises various modifications hereto, in so far as they fall within the scope of the following claims.

This invention contemplated methods for the following: Catching and stabilizing sub sea equipment in a moonpool comprising the following steps : Hoisting equipment from the sea bottom into the moonpool Catching of the lifting hook by a main trolley which is located at the bottom of the moonpool Further hoisting the equipment into the moonpool together with the main trolley * Stabilizing the equipment in one direction by using two movable hatches Stabilizing the equipment in a direction perpendicular to the first direction by using an auxiliary trolley with movable arms Lifting stabilized equipment into the multi purpose tower together with the main trolley and the auxiliary trolley Lowering equipment through the moonpool to the sea bottom comprising the following steps:

Equipment is skidded on movable moonpool hatches by using the transversal and longitudinal push pull units Connection of lifting wire and guiding wires to the equipment Connection of auxiliary trolley to the equipment Hoisting of equipment by using the main hoist Moving the longitudinal push pull unit and skid carts out of the way Clearing the moonpool by moving the movable hatches to the sides of the moonpool Lowering the equipment together with the auxiliary trolley and main trolley into the moonpool Disconnection of the main trolley and auxiliary trolley when the equipment has reached the bottom of the moonpool Lowering the equipment to the seabottom Hoisting the auxiliary trolley out of the moonpool Closing of the movable hatches Handling suction piles comprising the following steps: Skidding of the suction pile in horizontal positino to the centerline of the moonpool Connection of the suction pile to the main hoist and connection of any umbilical cables

Hoisting of the suction pile to a vertical position by the main hoist Connection of auxiliary trolley to the suction pile Moving the longitudinal push pull unit and skid carts out of the way Clearing the moonpool by moving the movable hatches to the sides of the moonpool Lowering of the suction pile into the moonpool Disconnection of auxiliary trolley and main trolley when suction pile reaches the bottom of the moonpool Lowing of the suction pile to the sea bottom Hoisting the auxiliary trolley out of the moonpool Closing of movable hatches Handling ROV's comprising the following steps: Skidding ROV with catching basket to the centerline of the moonpool 'Connection of ROV to main hoist and auxiliary hoist Hoisting of ROV Moving the longitudinal push pull unit and skid carts out of the way Clearing the moonpool by moving the movable hatches to the sides of the moonpool

Lowering of the ROV into the moonpool Disconnection of ROV from basket when the ROV reaches the bottom of the moonpool Hoisting of basket Closing of movable hatches Catching and spreading wires running through the moonpool Closing of movable hatches Stabilizing wires with movable fenders of the movable hatches in a first direction Stabilizing wires in a second direction perpendicular to first direction by moving secondary hatches Locking wires in wire spreaders Spreading the wires Handling equipment modules on the vessel comprising the following steps : Locking of module to transversal push pull unit Moving of module over the transversal rail to the longitudinal rail by the push pull unit Skidding over the longitudinal push pull unit Locking of longitudinal push pull unit to module Unlocking of transversal push pull unit of module Moving the module by the longitudinal push pull unit in longitudinal direction of the vessel These and other embodiments and variations are contemplated to be within the scope of the invention.