BACKGROUND

The installation of sub-sea constructions, such as Christmas trees, modules and other connections to oil wells, are typically done using a vessel, for example a module handling vessel or a well-intervention or well-testing vessel. Preferably, installation is done mid-ship through a moonpool, due to the reduced motion mid-ship. A tower or frame is installed with a winch, a heave compensation system and other systems for the lowering and installation of structures through the moonpool.

Additionally, most vessels are also equipped with a crane with a winch and a heave compensation system, for example a knuckle boom crane. The crane is used for installations that are too big to be launched through the moonpool.

One apparatus for mobile installation is shown in W099/11518, which includes a crane arranged on the deck of a vessel and a derrick on the vessel. The derrick is positioned over a moonpool, and the crane wire is able to operate a lifting means inside the derrick. This is done through positioning the crane such that an outer end of the crane jib or a racking arm can project over the derrick, and then only the wire can actually enter and/or connect to parts of the derrick. This set-up also includes one or more stabilizing frames moved along guide lines and rails inside the derrick to provide sideways stabilization of the load hanging from the wire, and prevent it from being flung against the derrick structure. Only wire holding the load connects to the derrick and the one or more stabilizing frames, keeping the crane and the derrick physically separated.

When launching pipes for a pipeline, a variety of different structures can be used. One such structure is disclosed in WO2014/148907, and is a vessel with pipe storage reels, a pipeline launch tower to launch pipeline in a firing line along the tower, and a guide to provide a circular or semi-circular guide surface to guide the pipe from the storage reels to the firing line. The tower can optionally have a crane with a base of the crane at the top of the tower to help move the pipeline guide between positions. The crane is always connected to the tower in this set-up. Additionally,

WO2014/148907 discloses that the vessel can also have another crane for hoisting large items, but it does not connect to the tower. SUMMARY

According to a first aspect of the invention, an offshore crane tower system for use on a vessel with a moonpool comprises a tower structure which extends in a vertical direction and is for placement on the vessel adjacent to and/or above the moonpool; a crane with a hoist system able to lift and lower a load; and a crane support at an upper portion of the tower structure. The crane can selectively connect to the crane support so that the hoist system of the crane can be used to lift and lower a load through the moonpool.

Such a system allows for the hoist system of the crane to be used as the hoist system for the tower structure, to lift or lower objects through the moonpool. This eliminates the need for a separate hoist system for the tower structure by allowing the crane to connect to the crane support and be used in combination with the tower structure.

According to an embodiment, the crane can connect to the crane support at a tip of the crane arm.

According to an embodiment, the system further comprises a control unit to control the offshore crane tower system.

According to an embodiment, the connection between the crane and the crane support is a solid connection.

According the an embodiment, the hoist system comprises a heave

compensation system.

According to an embodiment, the hoist system comprises at least one winch.

According to an embodiment, the offshore crane tower system can be part of a vessel.

According to an embodiment, the tower structure extends vertically above a moonpool on the vessel.

According to an embodiment, the crane is secured to the vessel. Optionally, the crane is secured adjacent a side of the vessel.

According to a second aspect of the invention, a method of manufacturing a vessel with an offshore crane tower system includes securing a tower structure extending in a vertical direction adjacent to and/or above a moonpool on the vessel; and securing a crane with a hoist system to the vessel in a position so that crane can selectively connect to the crane support to be able to hoist a load through the moonpoool using the tower structure as support. An upper portion of the tower structure comprises a crane support.

According to an embodiment, the crane can selectively connect to the crane support at a tip of the crane, for example, a tip of the crane arm.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. la is a side view of a vessel with an offshore crane tower system.

Fig. lb is a top view of the vessel and offshore crane tower system of Fig. la. Fig. 2 is a perspective view of a tower structure of the offshore crane tower system of Figs, la-lb.

DETAILED DESCRIPTION

Fig. la is a side view of a vessel 10 with an offshore crane tower system 12, and Fig. lb is a top view of vessel 10. Vessel 10 includes tower structure 13 with crane support 14 and crane 16 (with hoist system 17, winch 18, heave compensation system 19 and tip 20), control unit 21 and moonpool 22. Fig. la shows crane 16 connected to tower structure 13, and Fig. lb shows crane 16 not connected to tower structure 13.

Moonpool 22 is an opening in vessel 10 through which objects, for example, modules, can be lifted and lowered. Moonpool 22 is generally in a center portion of vessel 10.

Tower structure 13 extends in a vertical direction from vessel 10 deck, and is located above and/or adjacent to moonpool 22. Tower structure 13 provides structural support for lifting or lowering objects through moonpool 22. Crane support 14 is located on an upper portion 24 of tower structure 13. Crane support 14 is able to support a tip 20 of crane 16 arm, for example, with a plurality of supporting braces or mounts positioned and shaped to receive the crane arm at tip 20.

Crane 16 is secured to vessel 10, and includes hoist system 18, heave compensation system 19 and an arm with tip 20 for lifting or lowering objects. Crane 16 can sit adjacent to a side of the vessel 10 and near tower structure 13. Hoist system 17 includes winch 18 for letting in or out the cable used to lift or lower objects. Other systems could include more than one winch 18 for additional lifting power. Heave compensation system 19 works to compensate for movement of the waves on vessel 10 when lifting or lowering objects. While hoist system 17 and heave compensation system 19 are shown as supported by crane 16 above the deck on vessel 10, in other embodiments, one or both systems could be located elsewhere, for example on a lower deck of vessel 10. Control unit 21 can control the movements of crane 16, as well as hoist system 18 and heave compensation system 19.

In operation, offshore crane tower system 12 is able to lift and lower loads through the moonpool 22 using crane 16 supported by tower structure 13. Crane arm at tip 20 can connect to crane support 14. The hoisting cable or wire can then be guided to the right position within tower structure 13, and the control unit 21 can control the hoisting system 17 of the crane 16 to work with the tower structure 13. This enables crane 16 to be used as the lowering and lifting system for moonpool 22, with tower structure 13 providing support for the lower and lifting operations.

In past systems, vessel 10 used a tower with a hoist system for lifting or lowering objects through the moonpool 22. Additionally, a crane with a hoist system was used for lifting or lowering obj ect that were too large to fit through the moonpool 22. Crane tower system 12 allows for the use of crane 16 hoist system 17 for lifting and lowering objects both through the moonpool 22 when connected to crane support 14 (Fig. la) and at a side of the vessel 10 when not connected to crane support 14 (Fig. lb). This eliminates the need for separate hoisting and/or heave compensation systems for tower structure 13. By forming tower structure 13 with crane support 14, crane 16 is able to connect to tower structure 13, which provides structural support so that crane 16 is able to operate to lift and lower large and/or heavy objects through moonpool 22 even at a fully extended position. Tower structure 13 is able to distribute forces on the horizontal plane of the vessel, ensuring that crane 16 is not loaded in a way that may lead to failure of crane 16. Additionally, when lifting and lowering modules, the use of tower structure 13 enables stacking of modules in a vertical position and aligning with moonpool 22. This facilitates connecting modules to subsea orientation equipment systems ("SOES") and testing modules, if desired. The ability to use crane 16 hoisting system 17 and heave compensation system 19 in combination with tower structure 13 to lift and lower objects through the moonpool 22 provides for a sturdy and more economical, compact and lighter overall system by eliminating the space and weight taken up on vessel 10 for separate tower hoisting and/or heave compensation systems for lifting and lowering objects through moonpool 22. Fig. 2 is a perspective view of tower structure 13 of offshore crane tower system 12. Tower structure 13 includes crane support 14, which in this case is formed by two support structures 26 which receive crane tip 20. This can be a solid connection, and can be secured by one or more securing devices. In some embodiments, the weight of crane 16 can be the securing force which holds crane 16 in place for lifting or lowering through moonpool 22 once connected to crane support 14. Other embodiments could have different structures and or designs for crane support 14, depending on crane 16, tower structure 13 and the expected loads for system 12.

In summary, offshore crane tower system 12 allows for the use of crane 16 hoisting system 17 and/or heave compensation system 19 in combination with tower structure 13 to lift or lower objects through the moonpool 22 of vessel 10. Tower structure 13 provides the support for distributing load on vessel 10 and ensuring that crane 16 is not loaded in an undesirable way. Crane tower system 12 eliminates the need for separate hoist and/or heave compensation systems for tower structure, thereby making for an overall more smaller, lighter weight and less costly system without losing functionality. Additionally, the offshore crane tower system 12 can result in an increased payload capability of vessel 10.

While the invention has been described with reference to exemplary

embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.