WO2015200084A1 - Improved liftboat - Google Patents

Abstract

The present invention is directed to a an improved liftboat with: a plurality of extensible non-tubular and tubular legs capable of pinning in adverse weather conditions limited to only tubular leg liftboats but in water depths normally reserved to non-tubular liftboats; more flexibility in accessing the platform/structure by making side as well as end approaches to a platform/structure; more working deck space for hot work than existing liftboats; a leg encircling crane; and a propulsion system capable of dynamically positioning the liftboat in reference to a user defined location.

Description

IMPROVED LIFTBOAT

This application claims the benefit of U.S. Provisional Patent Application 62/018,572 filed 06/28/2014.

TECHNICAL FIELD The present invention relates to liftboats.

BACKGROUND ART

Liftboats are a subset of the jack-up vessel category of mobile offshore units ("MOU"). Liftboats are typically chartered to perform work on or over oil and gas well platforms and other structures in, above and/or below the water. The typical liftboat is a vessel with at least three extensible legs to make contact with the bed of a surrounding body of water. The bed can be a seabed, river bed, lake bed or a combination thereof. Pads or cans are fixed to the lower ends of the legs to make secured contact with the bed and thereafter support the weight of the elevated liftboat, the legs and any loads placed on the liftboat or legs. The typical liftboat is self-propelled and capable of maneuvering under its own power to position one end of the liftboat (usually the stern) into a location adjacent to an unobstructed side of a platform. Once in position, the liftboat lowers one of its legs to make contact with the bed and "pins" the liftboat into a temporary position. Thereafter, the liftboat's position is refined with additional maneuvering and the remaining legs lowered to make contact with the bed. Once the liftboat is in the desired position (and the legs are in contact with the bed), the hull is lifted (elevated or jacked up) out of the water to a user selected height just above the water surface. Once in this preload position, her ballast tanks are loaded for the purpose of pushing the pads or cans firmly into the bed. After a user defined period of time with the pads/cans supporting the preload weight, her ballast tanks are emptied and the vessel is elevated to the user defined work elevation. Work typically requires the use of the liftboat's crane and portions of the main deck for work and especially hot work (such as but not limited to welding). Upon completion of the work, the hull is lowered into the water and the pads/cans are worked free through lifting of the legs and/or water jetting away the surrounding bed material. Once the pad/cans are free, then the legs are elevated to a user defined height and the liftboat proceeds to its next location.

The number of locations next to the platform/structure for work are limited given the typical liftboat has only one working end free of obstructions (typically the stern). Furthermore, the lack of an alternative to one working end results in a particular pad/can penetration pattern being formed in the bed next to the platform - thereby forcing the use of a liftboat with the same pad/can pattern; orienting the liftboat in favor of preventing bed instability over access to the platform/structure and/or using a liftboat with a pad/can pattern that does not cause instability of the bed. Clearly the use of only one end of existing liftboats limits the type of work to be performed by a liftboat as well as the type of liftboat to perform the desired work. The legs of an existing liftboat are either tubular or non-tubular. Existing tubular legs have a generally circular cross section and are typically constructed to be buoyant. In contrast, existing non-tubular legs are a construction of steel members in a lattice with either a triangular or quadrangular cross section and have very little, if any, buoyancy. Tubular legs, while buoyant, exhibit approximately seven times the deflection and oscillation of non-tubular (or lattice) legs of similar length and diameter. Such deflection and oscillation may prove useful during the pinning of a liftboat to the bed during adverse weather/water conditions, but becomes a detractor once the liftboat is elevated. In particular, such deflection and oscillation adversely affects habitability and crane operations on an elevated liftboat. Tubular legs can be stiffened to reduce or eliminate such characteristics, but alters its usefulness during pinning in adverse weather and further increases the liftboat's vertical center of gravity due to the added weight of the stiffeners. Which in turn would require the use of transverse offsetting weights (typically in the form of ballasted wing tanks) to counteract the increased weight of stiffened tubular legs. In contrast, lattice legs, while stiffer are not required for shallow water work. Furthermore, leg damage is likely while attempting to pin a liftboat with lattice legs in water and weather conditions acceptable for a comparable liftboat with tubular legs. This is mainly due to the stiffness of lattice legs. As such, experience has shown that liftboats with tubular legs most effectively operate in water depths of approximately 220 feet; whereas liftboats with non-tubular legs typically operate in water depths up to 425 feet.

Liftboats with lattice (or non-tubular) legs come with certain limitations not found in liftboats with tubular legs. With all liftboats, the need for a crane is essential to perform work alongside a platform. However, the legs (tubular or otherwise) projecting upward from the main deck present as obstacles to a liftboat's crane. Leg encircling cranes (LEC's) provide a partial solution to this limitation through the use of a slewing bearing fixed to a crane pedestal encircling a leg. By doing so, the center of the crane's slew (rotation) coincides with the center of the leg, thereby eliminating one leg as an crane obstacle. Clearly, the use of a LEC on a liftboat has its advantages. However, fitting a LEC on a lattice leg is problematic. A lattice leg is approximately 40% greater in its effective diameter than an equivalent tubular leg. This greater diameter dictates the use of a larger pedestal and a multipart slewing bearing to surround the leg; all of which significantly increases the cost of acquisition and ownership. To avoid the complexity of a larger, multipart slewing bearing, existing liftboats with lattice (non-tubular legs) are constructed with a separate crane pedestal with a one part slewing bearing. However, such a pedestal significantly reduces the liftboat's useable working deck space and the crane's operating envelope when compared to a liftboat with a LEC.

Leg and pad/can strikes are a common problem with current liftboats. Offshore Supply/Support Vessels ("OSV's") allide with and cause damage to the legs (or pads/cans) of a liftboat. Leg and pad/can damage can also be caused by adverse weather, operator error, inaccurate information about the bed, normal wear and tear or a combination thereof. Regardless of how the leg/can damage is caused, the requisite repairs/replacements necessitate the use of shipyards or other facilities with sufficient crane capacity. Repairs/replacements at such facilities may require significant transits, waiting times or staging all of which reduce the earning capacity of a liftboat. In short, none of the existing liftboats are configured to replace a damaged leg and/or pad/can with its own crane.

Another common issue with existing liftboats focuses upon the cooling of closed loop fluid systems (e.g. hydraulic power systems or engine cooling systems) when the liftboat is elevated. Unlike a displacement vessel where the surrounding water is readily accessible to cool closed loop fluid systems, a liftboat elevated above the water must elevate the cooling water (typically by pumps and pipes). Such machinery requires maintenance and repairs over the worklife of a current liftboat and significantly contributes to the cost of acquisition and maintenance.

Finally, current liftboats are configured with a propulsion system of sufficient horsepower to propel the liftboat through water with retracted legs. And while current liftboats may be able to move at a reduced speed with its legs partially extended, none are configured to sustain extensive periods of dynamic positioning in relation to a platform (or other user defined location).

In summary, existing liftboats have extensible legs that generally limit the weather conditions in which pinning can occur; the type of crane that can be used; and the amount of available work deck. Furthermore, none of the existing liftboats use its extensible legs for operations other than attachment to the bed and support the liftboat.

DISCLOSURE OF INVENTION The present invention as more fully described below is generally directed to a an improved liftboat with: (a) a plurality of extensible legs capable of pinning in adverse weather conditions limited to only tubular leg liftboats but in water depths normally reserved to non-tubular liftboats; (b) more flexibility in accessing the platform/structure by making side as well as end approaches to a platform/structure; (c) more working deck space for hot work; (d) a LEC; and/or (e) a propulsion system capable of dynamically positioning the liftboat in reference to a user defined location. More specifically, the improved liftboat comprises at least one is a tubular leg with the remaining legs being non-tubular legs. Each extensible leg is fitted with a pad or can designed to support, with other legs, the weight of the improved liftboat when in contact with the bed of a body of water (including but not limited to a seabed, river bed or lake bed). An example would be a four leg improved liftboat wherein one leg is tubular and the remaining three legs are lattice (1 -3 Leg Combination). See Figure 2. A four leg improved liftboat variation contemplates two tubular legs and two lattice legs (2-2 Leg Combination). Another embodiment of the improved liftboat comprises three legs, wherein one or two legs are tubular and the remaining one or two legs are lattice (1 -2 Combination or 2-1 Combination). In a preferred embodiment, extension and retraction of the legs and elevation and lowering of the vessel is handled by at least one jacking motor. The novel extensible leg combination of the present invention overcomes the limitations of both all tubular and all non-tubular leg arrangements. The novel extensible leg combination allows an owner to use the greater deflection of a tubular leg to its advantage in "pinning" the liftboat during adverse weather/water conditions as well as mount and operate a LEC resulting in a liftboat capable of operating in harsher weather/water conditions with improved deck work space over existing liftboats.

In one embodiment, the pads/cans of the extensible legs of the improved liftboat are watertight and fitted with at least one watertight chamber. In another embodiment, pads/cans are easily removed from a stump weld joint at a lower end of a leg thereby exposing an internal flat plate capable of functioning as a support structure like a pad or can in contact with a bed. With a LEC, an improved liftboat with four extensible legs and appropriate ballasting would be able, by itself, to remove and/or install a damaged leg, pad and/or can without, or at least reduce, the need for shore side support or assistance. This novel arrangement significantly increases the number of locations where pad/can repairs can be performed.

The improved liftboat further contemplates positioning tanks, including but not limited to service tanks and closed loop tanks, within the hull of the liftboat on the longitudinal centerline of the hull between two longitudinal bulkheads running at least the length of the machinery space. Such a tank placement eliminates or at least reduces tank breaches into the adjacent water due to collisions or allisions between the improved liftboat 10, platform, structure and/or OSV. A vent trunk is formed in the machinery space above the service tanks to accommodate vent heads protruding from the service tanks. Furthermore, the bilge of the machinery space is configured to become a spill containment system for the service tanks. This arrangement eliminates or at least reduces the need for penetrations of the main deck. Which in turn allows more of the main deck to be configured by an owner/charterer for use as a welding or hot work area. Furthermore, this service tank arrangement provides for air cooling of the tanks on two opposing sides as well as the top thereby eliminating, or at least reducing, the need for water cooling systems as used in existing liftboats. An added benefit of this tank arrangement is the reduction of piping and cross overs required between the service tanks and the machinery.

In another embodiment, the function of the leg combination described above is extended beyond load support to motion dampening by installing a propulsion system capable of self-propulsion, maneuvering, dynamic positioning or a combination thereof with its legs in any position between and including full retraction and full extension when the liftboat is in a body of water. Such a propulsion system overcomes the static (e.g. fix in place) limitations of traditional liftboat legs. The use of a traditional radial blade propeller system, ducted propeller system, azimuth thruster system, cycloidal thruster system, in-hull thruster system or a combination thereof for dynamic positioning allows the improved liftboat to operate without regard to waterdepth. Furthermore, such an improved liftboat with its legs fully extended becomes a very stable dynamically positionable MOU capable of providing crane and/or gangway services in deep water far better than those of traditional dynamically positionable vessels.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 a and 1 b liftboat types according to the prior art; Figure 2 isometric view of an improved liftboat (10);

Figure 3 cross sectional views of an improved liftboat's aft cross section and forward cross section along line A-A in Figure 2;

Figure 4 side view of an improved liftboat;

Figure 5 top cross section view of an improved liftboat along line B-B in Figure 4; and

Figure 6 detailed view of extensible leg (50).

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention represents a vast improvement over the prior art as described above. In the following description, numerous specific details are set forth in order to provide a thorough understanding of examples of the present invention. It will be understood by one skilled in the art however, that examples of the present invention may be practiced without some or all of these specific details. In other instances, well known elements have not been described in detail in order not to unnecessarily obscure the description of the present invention.

In a preferred embodiment, the improved liftboat is designed, constructed, maintained and operated in accordance with applicable rules, requirements and/or guidelines of governmental agencies (like the U.S. Coast Guard) and/or nongovernmental organizations (like the International Maritime Organization or classification societies). The improved liftboat comprises a main deck 202 fixed to a hull 201 to form a vessel 20. A plurality of extensible legs 50 are operatively coupled to the vessel 20 by at least one jacking system 503. See Figure 6. At least one of the extensible legs 50 is a tubular leg 502 and the remaining extensible legs 50 are non- tubular legs 501 . See Figure 2. The remaining extensible legs 50 are preferably legs with a lattice construction. See Figures 2-4. Tubular legs 502 can be buoyant or non- buoyant, but the preferred embodiment is non-buoyant tubular legs 502. This hybrid extensible leg configuration allows the improved liftboat 10 to be pinned during adverse weather/water conditions exceeding the limits of a liftboat with all non- tubular legs of similar dimensions. This arrangement also provides the improved liftboat 10, when elevated, a stiffer leg foundation during adverse weather/water condition exceeding the limits of a liftboat with all tubular legs of similar dimensions. In a preferred embodiment, the improved liftboat 10 has a total of four extensible legs 50 of which one is a tubular extensible leg (502) and the remaining three are non-tubular extensible legs (501 ). See Figure 2.

A leg tower 205 for each extensible leg 50 is fixed to and extends up from the main deck 202 of the improved liftboat 10. See Figures 2-4. In a preferred embodiment, an accommodation structure 206 is fixed at one end of the vessel 20. See Figures 2-4. At least one leg passage 203 is formed through each of the towers 204 and extends through the vessel 20. In a preferred embodiment, leg towers 205 are positioned on deck to maximize work deck space 2021 and allow the improved liftboat 10 to approach, position and access a platform or structure from each side and at least one end (bow or stern) of the improved liftboat 10. See Figures 2 and 3.

At least one extensible leg 50 is operatively coupled to and passes through a leg passage 203. See Figures 2 and 5. As described above, the jacking system 503 provides the operative connection between the plurality of extensible legs 50 to the vessel 20. The jacking system 503 comprises at least one of each of the following for each extensible leg: a pinion 5032 fixed to a planetary drive 5033 which in turn is fixed to a jacking motor 5033. See Figure 6. The jacking motor 5033 is fixed to a leg tower 205 adjacent to a leg passage 203. See Figure 6. Pinion 5032 is operatively coupled to a rack 5031 fixed to an extensible leg 50. See Figure 6. For each extensible leg 50, at least three sets of leg rack guides 5035 are fixed to a corresponding leg tower 204 and in sliding contact with an extensible leg 50. Each leg rack guide set 5035 comprises an upper leg rack guide 50351 and a lower leg rack guide 50352. See Figure 6. In a preferred embodiment, the distance between the upper and lower leg rack guides 50351 , 50352 is sufficient to prevent dynamic motion or whipping of an extensible leg 50 fully retracted, fully extended or in any position therein between.

The improved liftboat 10 includes at least one crane 60. See Figures 2 and 4. In a preferred embodiment, at least one of the cranes 60 is a leg encircling crane (LEC) 601 . See Figures 2 and 4. As mentioned above, the LEC of the improved liftboat 10 increases the amount of work deck 2021 space over existing liftboats. In yet another preferred embodiment, the LEC 601 encircles a tubular extensible leg 50, thereby reducing the cost of acquisition and maintenance over existing liftboats with a LEC 601 encircling a non-tubular leg. See Figures 2 and 4. It is further contemplated that the LEC 601 of the improved liftboat 10 would be of sufficient capacity and dimensions to remove or insert any one of the plurality of extensible legs 50 into a leg passage 203.

Each extensible leg 50 of the improved liftboat 10 has a foot end 504 capable of making securing contact with a bed or floor beneath the water to support the improved liftboat 10. See Figures 2 and 3. In a preferred embodiment, each foot end

504 comprises a flat plate 505, pad 5041 or spud can 5042 and a stump weld 5043. The pad 5041 or spud can 5042 is connected to and removable from an extensible leg at a stump weld 5043. In a preferred embodiment, at least one pad 5041 or spud can 5042 is fitted with at least one watertight compartment 5041 1 for floating said pad 5041 or can 5042 when removed from an extensible leg 50 of an improved liftboat 10. Above the stump weld 5043 is a flat plate 505 capable of supporting a portion of the improved liftboat when a pad 5041 or spud can 5042 is removed from an extensible leg 50 at stump weld 5043. Use of a stump weld 5043 and flat plate

505 allows easier removal and replacement of a pad 5041 or spud can 5042, especially when the pad 5041 or spud can 5042 comprises at least one watertight compartment 5041 1 .

In a preferred embodiment of the improved liftboat 10, a tank system 2015 is fitted within vessel 20. The tank system 2015 comprises a plurality of tanks 20151 , piping 20152 and a plurality of ballast tanks 20156. See Figure 3. The plurality of ballast tanks 20156 are positioned in and at the outer regions the vessel 20 and fluidly connected to the surrounding water by piping 20152 and at least one pump 20157 to alter the improved liftboat's 10 draft and/or provide weight for preloading, counter-balancing, listing, trimming or a combination thereof. The plurality of tanks 20151 are positioned in a machinery space 30 formed in the vessel 20. Piping 20152 and pumps 20157 are used to convey fluids to and from the plurality of tanks 20151 and ballast tanks 20156.

The plurality of tanks 20151 described above can be, but are not limited to, service, storage and/or closed loop cooling tanks for fuel oil, liquid natural gas, lubricating oil, hydraulic oil, water or a combination thereof. In one embodiment, a plurality of tanks 20151 are positioned on or near the longitudinal centerline of the hull 201 between two longitudinal bulkheads 2014 running the length of the machinery space 30. See Figures 3 and 5. This arrangement protects the plurality of tanks 20151 from collisions and/or allisions with platforms, structures, OSVs or a combination thereof. Furthermore, the centerline placement reduces the amount and complexity of the piping 20152 between the plurality of tanks 20151 and related machinery. The machinery space 30 may comprise at least one vent trunk 20153 formed between main deck 202 and the tops of tanks 20151 . Each tank 20151 comprises at least one vent head 20154 positioned within a vent trunk 20153 and fluidly connected to a tank 20151 . See Figure 3. Such an arrangement eliminates the need for a penetration through the main deck 202 for venting and enlarges the work deck 2021 area for operations, including but not limited to hot work. Furthermore, this novel arrangement allows for air cooling of the exposed sides and top of each tank 20151 thereby eliminating, or at least reducing, the need for water cooling systems as used in existing liftboats. In another embodiment, a bilge 302 is formed between bottoms of tanks 20151 and hull 201 to create a spill containment system 20155 to capture fluid from vent head(s) 20154. See Figure 3.

In a preferred embodiment of the improved liftboat 10, the function of the plurality of extensible legs 50 is expanded beyond load support to motion dampening by installing a propulsion system 40 capable of self-propulsion, maneuvering, dynamic positioning or a combination thereof with its extensible legs 50 in any position between and including full retraction and full extension when the improved liftboat 10 is in a body of water. See Figures 4 and 5. Such a propulsion system 40 overcomes the static (e.g. fixed in place) limitations of traditional liftboat legs.

It is contemplated that propulsion system 40 comprises at least one power train 402 between at least one engine 401 and at least one propulsion unit 403. Propulsion unit 403 can be a traditional propeller 4033, cycloidal drive 4031 or an azimuth thruster 4031 such as a Kort nozzle 4032 or an electric podded azimuth thruster (also known as an AZIPOD) 40322. If a propulsion unit 403 is mechanically driven, then power train 402 comprises a power shaft 4021 mechanically connected between engine 401 and propulsion unit 402. Likewise, if a propulsion unit 403 is electrically driven, then power train 402 comprises an electric generator 4022 mechanically coupled to engine 401 and electrically connected to propulsion unit 402 by an electric cable 4023. Furthermore, if a propulsion unit 403 is hydraulically driven, then power train 402 comprises a hydraulic pump 4024 mechanically coupled to engine 401 and fluidly connected to propulsion unit 402 by hydraulic piping, hoses or a combination thereof 4025. Engine(s) 401 can be an internal combustion engine configured to operate on gas, diesel, LNG, oil or a combination thereof to provide the requisite mechanical power.

In a preferred embodiment, propulsion system 40 comprises four engines 401 and four cycloidal drives 4031 , wherein each engine 401 is mechanically connected to a corresponding cycloidal drive 4031 by a power shaft 4021 . See Figure 5. In another embodiment, propulsion system 40 comprises four engines 401 and four Kort Nozzles 40321 , wherein each engine 401 is mechanically connected to a corresponding cycloidal drive 4031 by a power shaft 4021 . In another embodiment, propulsion system 40 comprises four engines 401 , four electric generators 401 1 and four electric podded azimuth thrusters (also known as AZIPOD) 40322; wherein each engine 401 is coupled to a corresponding electric generator 401 1 and each podded azimuth thruster 40322 is electrically connected to a corresponding electric generator 401 1 by a power cable 4022.

Traditionally, propulsion units 403 on vessels capable of dynamic positioning (hereinafter DP) are positioned as far outboard as possible from the transverse and longitudinal centerlines of the vessel 20 to maximize maneuvering torque. Furthermore, the propulsion units 403 of a traditional DP vessel are positioned to extend below the lowest part of the hull 201 to maximize water flow across the propulsion unit 403. However such an arrangement would expose the propulsion units 403 to bed strikes and mechanical damage for a liftboat operating in shallow water. To eliminate or at least reduce such strikes and damage, the propulsion units 403 of the improved liftboat 10 are positioned in hull recesses 2013 formed in the bottom of the hull 201 . See Figure 3. In another embodiment, propulsion units 403 are configured to be retractable for ease of maintenance and upkeep. In yet another embodiment, propulsion units 403 are positioned aft of the extensible legs 50 and beneath the work deck 2021 to eliminate or at least reduce main deck obstructions (like leg towers 205 and/or accommodation structure 206) should a propulsion unit 403 have to be removed from the vessel 20 for service. See Figure 4. In those embodiments where power shafts 4021 are used to drive propulsion units 403, the engines 401 and power shafts 4021 are positioned in an "X" arrangement to facilitate propulsion unit 403 placements as far outboard as possible while maintaining the engines 401 and power shafts 4021 clear of the extensible legs 50. See Figure 5. The "X" arrangement of the propulsion system 40 with the use of longitudinal centerline tank arrangement described above further reduces the complexity and length of piping 20152 between vented tanks 20151 and engines 401 .

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Claims

1 . A liftboat for operation in a body of water having a surface and a floor, said liftboat comprising: a main deck fixed to a hull to form a vessel; a plurality of tanks fixed in said vessel; and a plurality of extensible legs operatively coupled to said vessel by at least one jacking system, each said extensible leg comprising a foot end, at least one of said extensible legs is a tubular leg and remaining said extensible legs are non-tubular legs, said jacking system further comprising at least one jacking motor for each of said extensible legs configured to extend said extensible legs to contact said foot ends with the floor of the body of water and thereafter raise the liftboat above the surface of the body of water.

2. A liftboat as claimed in claim 1 further comprising a machinery space formed in said vessel and said plurality of tanks are positioned within said hull on the longitudinal centerline of said vessel to at least reduce listing.

3. A liftboat as claimed in claim 2, further comprising a machinery space formed in said hull 201 and comprising at least one vent trunk formed between said main deck and tops of said plurality of tanks; and each of said tanks further comprising a vent head fluidly connected to a tank and positioned within said at least one vent trunk for venting said plurality of tanks below the main deck.

4. A liftboat as claimed in claim 3, said machinery space further comprising at least one bilge formed between bottoms of said plurality of tanks and said hull to form a spill containment system.

5. A liftboat as claimed in claim 1 , wherein arrangement of said tanks and said extensible legs allow the liftboat to stand on three of said extensible legs.

6. A liftboat as claimed in claim 1 , further comprising a propulsion system for self-propulsion, maneuvering, dynamic positioning in relation to a user defined fixed point or a combination thereof.

7. A liftboat as claimed in claim 6, said propulsion system comprising a cycloidal drive.

8. A liftboat as claimed in claim 1 , further comprising a crane fixed to said vessel and encircling at least one of said extensible legs.

9. A liftboat as claimed in claim 8, said crane capable of safely replacing each of said extensible legs to or from the liftboat.

10. A liftboat comprising: at least four towers and an accommodation structure fixed to a watertight vessel and arranged to form a working deck with at least one open side between said towers and said accommodation structure; at least one leg passage formed through each said towers and extending through said watertight vessel; at least one extensible leg operatively coupled to and passing through each said leg passage and terminating in a foot at a foot end of an extensible leg; and at least one said extensible leg is tubular and at least three said extensible legs are non-tubular.

1 1. A liftboat as claimed in claim, each said foot end comprises a stump weld connection located between a flat plate and a said foot, wherein said flat plate supports in part the liftboat when said foot is removed and said flat plate is in contact with the bed of a water body.

12. A liftboat as claimed in claim 10, said feet arranged and sized to provide the greatest acceptable combinations of liftboat orientations to an existing structure, user defined location or a combination thereof with previous footprints formed in the bed of a waterbody.

13. A liftboat as claimed in claim, further comprising a propulsion system for self- propulsion, maneuvering, dynamic positioning or a combination thereof.

14. A liftboat as claimed in claim 13, said propulsion system comprising at least one cycloidal drive fixed within a recess formed in an underside region of said watertight vessel.

15. A liftboat as claimed in claim 10 further comprising at least one crane fixed to a said tower and encircling one of said extensible legs.