The invention relates to an off-shore installation vessel. Such a vessel is used to install structures at sea. Such structures are usually related to the energy sector, such as jackets and platforms for oil exploration and exploitation, wind turbines and related jackets, pipelines, and cables. An off-shore installation vessel is used to transport such structures from a production facility to the installation site off-shore, and/or is used to actually install the structure on the sea bottom or on a structure which is already present off-shore. Such vessels include pipe-laying vessels, cable-laying vessels, crane vessels, and dedicated transport vessels. Such vessels comprise an installation device, such as a crane, a gantry, and pipe or cable laying equipment. A dedicated transport vessel comprises an installation device used for shifting the large cargo such as a jacket on deck, for launching it into the sea and/or to lift it.

NL-A-7.514.154 discloses a crane vessel with a single hull, a ballast system, a crane, and pipe-laying equipment. The draft of this known crane vessel may be changed by using the ballast system. The crane vessel has a single hull which is symmetrical with respect to a centreline plane, and it comprises a bottom, a bilge, and sides. According to one

embodiment of NL-A-7.514.154, tanks are provided against the sides of the hull. The tanks have doors at their oblique oriented bottom. These doors may be open to the sea. When closed, the crane vessel has a breadth at the waterline which is greater than the breadth of the hull just below the tanks. This increased breadth increases the metacentre height, which is advantageous during lifting of heavy structures. When the doors are open, water can freely run in and out of the side tanks, so that the effective width is comparable to the breadth of the hull without side tanks. This results in a lower metacentre height and correspondingly less response to a typical sea wave spectrum.

A disadvantage of NL-A-7.514.154 is that the underwater doors are relatively expensive, vulnerable, and laborious to maintain.

The current invention aims to solve at least part of the problems of NL-A-7.514.154, or at least to provide an alternative. In particular, it is an objective of the invention to provide an off-shore installation vessel which is less expensive to build, and/or to maintain.

The invention achieves the objective by means of an off-shore installation vessel according to claim 1 , and a method of operating an off-shore installation vessel according to claim 12.

An off-shore installation vessel comprises a single hull, a ballast system, and an installation device. The installation device is designed to install structures at sea, the single hull has a hull shape and is designed with a transit displacement at a transit draft and a maximum displacement at a maximum draft, the maximum draft being larger than the transit draft. The hull shape is defined by frames, comprising a main frame, and waterlines, comprising a transit waterline at the transit draft, and a maximum waterline at the maximum draft. The hull shape is symmetrical with respect to a centreline plane which extends in a horizontal length and a vertical height direction of the hull. The single hull has a design length along the maximum waterline and a maximum breadth, the main frame being halfway the design length, the part of the single hull in front of the main frame being referred to as fore part of the hull and the part of the single hull aft of the main frame being referred to as aft part of the hull. The single hull comprises a bottom, a bilge, and a side, the bilge being the transition between the bottom and the side, at the main frame. The side comprises an oblique side part and a vertical side part, the vertical side part being at a greater distance from the centreline plane than the bilge. The oblique side part extends from the bilge to the vertical side part. The ballast system comprises ballast water tanks with a capacity that is at least equal to the difference between the maximum displacement and the transit

displacement for changing the draft of the off-shore installation vessel from the transit draft to the maximum draft. The transit waterline intersects the oblique side part.

Taking water into the ballast water tanks results in the draft increasing from the transit draft to an increased draft which is between the transit draft and the maximum draft. Thanks to the oblique side parts, taking in ballast water immediately results in increasing the metacentre height, which is favourable for certain installation operations as a larger metacentre height results in smaller heel angles when a mass is moved in a breadth direction of the vessel. Thanks to the oblique sides, the metacentre height keeps on increasing with the intake of more ballast water until the maximum draft is reached. This enables choosing an optimum metacentre height for the current operation of the vessel. Thanks to the combination of a sufficient ballast water tank capacity and the oblique side parts, there are no doors required. A vessel without such doors, and oblique side parts starting directly from the bilge and extending to the vertical side parts, is cheaper to build and to maintain than the vessel of NL-A-7.514.154.

Another advantage of the oblique side part extending from the bilge to the vertical side part is that this results in a lighter structure for a given displacement than the vessel of NL-A-7.514.154, where the side has first an extra vertical part, and then a buckle from where the oblique side part starts.

It is noted that GB-1.223.608 discloses a hull shape with oblique and vertical side parts per se. This is however a regular freight vessel, which may have a ballast system sufficiently large to keep the propeller under water when it does not carry cargo, but certainly not a ballast system which may change the draft of the vessel from a transit draft to a maximum draft. Moreover, this vessel does not have an installation device, designed to install structures at sea.

Preferably, the off-shore installation vessel further comprises a bilge-keel with a height, measured from attachment at the hull to an outer edge, of at least 17%, preferably at least 20%, of the maximum draft. Such relative large bilge-keels will aid in damping rolling motions of the off-shore installation vessel which might be heavier due to the oblique side parts, than with a more conventional hull shape.

In an embodiment, the height of the oblique side part is at least 50%, preferably at least 60%, more preferably at least 65% of the overall height of the side. The larger the oblique side part is, the better it is possible to fine tune the required metacentre height by taking in ballast water.

In an embodiment, the inclination of the oblique side part with respect to centreline plane is between 35° and 55°, preferably between 40° and 50°, more preferably between 43° and 47°. A relative low value of the inclination within these ranges reduces the risk of slamming of waves against the oblique side part. A relative large value of the inclination within these ranges results in a relative wide range of potential metacentre heights.

In an embodiment, the vertical side part has a breadth which equals to the maximum breadth over at least 90% , preferably 95%, more preferably 99% of the length of the aft part of the hull. Extending the maximum breadth backwards results in a large available deck area at the aft part of the vessel, which is favourable for different types of installation activities. Moreover, it improves the stability of the vessel.

In an embodiment, the vertical side part has a breadth which equals to the maximum breadth over at least 15% of the length of the fore part of the hull. Extending the maximum breadth forwards results in a large available deck area at the mid and fore part of the vessel, which is favourable for different types of installation activities. Moreover, it improves the stability of the vessel.

In an embodiment, the oblique side part extends over at least 90%, preferably over at least 95%, of the full length of the hull. This increases the stability.

In an embodiment, the vertical side part merges into the oblique side part in the fore part of the hull. This results in a smooth bow form of the hull which is favourable when the vessel is travelling in head waves.

In an embodiment, all waterlines of the oblique side part are fairing from the front to the aft of the vessel. Fairing waterlines of the oblique side part results in less resistance when the vessel is moving forward.

In an embodiment, all waterlines of the vertical side part are fairing from the front to the aft of the vessel. Fairing waterlines of the vertical side part results in a more gradual change of the hydrostatic properties of the hull when the draft is increased to a draft which intersects the vertical side part, or when the draft is just below the transition of the oblique side part to the vertical side part and the vessel heels.

Preferably, the maximum draft intersects the vertical side part. The maximum draft with corresponding maximum metacentre height will in many cases be required when lifting or shifting relative large masses on board. Having vertical side parts of this draft, results in smaller changes of some of the hydrostatic properties when the vessel heels than in case the sides at the maximum draft are oblique.

In a preferred embodiment, the upper margin of the bilge is tangent to the lower margin of the oblique side part. This optimises the ratio between displacement on one hand and weight of the ship construction and hull surface area on the other hand.

The invention will be explained in more detail referring to the drawings, in which:

Fig. 1 shows a first embodiment of a vessel according to the invention;

Fig. 2 shows a second embodiment of a vessel according to the invention;

Figs. 3-8 show a lines plan of the hull of fig. 1 , wherein

Fig. 3 shows a profile view with buttock lines of the aft vessel;

Fig . 4 shows a plan view with water lines of the aft vessel;

Fig. 5 shows a body plan view with frame lines of the aft vessel;

Fig. 6 shows a profile view with buttock lines of the front vessel;

Fig. 7 shows a plan view with water lines of the front vessel;

Fig. 8 shows a body plan view with frame lines of the front vessel; and

Fig. 9 shows a ballast system.

An off-shore installation vessel according to the invention is denoted in its entirety by reference numeral 1 in figure 1. The off-shore installation vessel 1 comprises a single hull 2, a ballast system 3 (see fig. 9), and an installation device (not shown). The installation vessel of figure 1 is intended to transport very large structures, such as jackets, which are to be installed on sea. Therefore, the vessel 1 has a large unobstructed main deck 4. In this case, the installation device comprises equipment to shift the large structures over the main deck. Such installation devices may comprise cranes, large winches, slides, and/or a skidding system with skidding rails and cylinders.

Another off-shore installation vessel according to the invention is crane vessel 101 , as shown in figure 2. The crane vessel 101 has a similar single hull as in figure 1 , which is accordingly denoted with the same reference numeral 2. The crane vessel 101 comprises a ballast system (not shown), and an installation device in the form of a heavy-lift crane 6. The heavy-lift crane of this embodiment is capable of lifting up to 5.000 ton. The crane vessel has a large main deck 4 , which is suitable for placing large structures during installation activities and to transport such large structures between a production location and an offshore location where the structures are to be installed.

The single hull 2 of both embodiments is shown in more detail in figures 3-8 and has transit displacement of approximately 28,000 ton at a transit draft, in this embodiment 6.6 meter, and a maximum displacement of approximately 60,000 ton at a maximum draft, in this embodiment 1 1.6 meter.

The hull shape is defined by frames as shown in figs 5 and 8, which comprises a main frame 8. The hull shape is further defined by waterlines, comprising a transit waterline 10 at the transit draft, and a maximum waterline 12 at the maximum draft.

The hull shape is symmetrical with respect to a centreline plane CL which extends in a horizontal length and a vertical height direction of the hull 2. The single hull 2 has a design length along the maximum waterline, which in this embodiment is 157 meter, and a maximum breadth, which is in this embodiment is 49 meter. The main frame 8 is halfway the design length. The part of the single hull in front of the main frame 8 is referred to as fore part 14 of the hull 2 and the part of the single hull 2 aft of the main frame 2 is referred to as aft part 16 of the hull 2.

The single hull 2 comprises a bottom 18, a bilge 20, and a side 22. The bilge 20 is the transition between the bottom 18 and the side 22. The side 22 comprises an oblique side part 24 and a vertical side part 26. The vertical side part 26 is at a greater distance from the centreline plane CL than the bilge 20. The vertical side part 26 ends at its upper side at the main, or upper deck 4. The depth of the hull 2 to the main deck of this embodiment is 15.5 meter. The oblique side part 24 extends from the bilge 20 to the vertical side part 26. The upper margin of the bilge 20 is tangent to the lower margin of the oblique side part 24. Put differently, the bilge 20 merges smoothly into the oblique side part 24. In this

embodiment, the height of the oblique side part 24 is approximately 10 meter, the inclination of the oblique side part with respect to centreline plane is approximately 45°.

The transit waterline 10 intersects the oblique side part 24. The maximum draft 12 intersects the vertical side part 26. Referring to figs. 4 and 5, it shown that the vertical side part 26 has a breadth which equals to the maximum breadth over substantially the whole length of the aft part 16 of the hull 2. The vertical side part 26 has a breadth which equals to the maximum breadth over substantially 27 meter, measured from the main frame 8.

The oblique side part 24 extends over the full length of the hull 2. The vertical side part 26 merges into the oblique side part 24 in the fore part of the hull. Put differently, there is no buckle line in the foremost part of the hull 2. All waterlines of the oblique side 24 part are fairing from the front to the aft of the vessel, and all waterlines of the vertical side 26 part are fairing from the front to the aft of the vessel. Put differently, the hull shape is smooth in the length direction from the aft to the bow. Only a buckle line defining the vertical side part 26 interrupts the smooth hull in vertical direction.

The ballast system 3 comprises pumps, of which one pump 28 is schematically shown in fig. 9, a control unit in the form of a computer 30, and ballast water tanks 32. The ballast water tanks 32 have a capacity which is at least equal to the difference between the maximum displacement and the transit displacement. 32000

The off-shore installation vessel 1 comprises a bilge-keel 40 (shown only on one side of the vessel) with a height, measured from attachment at the hull to an outer edge, of 2,5 meter, which is in this embodiment 21 % of the maximum draft.

In use, the crane vessel 101 will sail to an off-shore location. It may have an structure to be installed on its main deck 4, or it may pick up an installation from another vessel or offshore structure. Arriving a the location, the vessel will take in ballast water to increase the draft from the transit draft to an increased draft, which is preferably the maximum draft. The crane 6 will install the structure at sea at the increased draft. At this maximum draft, the crane vessel 101 has a maximum metacentre height, which results in a relative stable ship during heavy-lifting operations. Typically, heel angles will be 1 ° to 2°. Such small angles are not only achieved by the large metacentre height, but also by the ballast system 3. By pumping water from the side tanks at one side to side tanks at the other side of the vessel, the shifting of the weight of the lifted structure is compensated for. Additionally, or alternatively, the ballast system may comprise a large weight which is move with cables and winches from one side of the vessel to the other side. Such a ballast system is disclosed in WO-2009/048322 by the same applicant, which disclosure is incorporated here by reference.

After installation, the ballast water is discharged to reduce the draft to the transit draft. For relative light structures, it may be not necessary to increase the draft, or it may be sufficient to increase it to a lesser extent. The resulting lower metacentre height will generally result in less vessel motions due to waves.

Another off-shore installation vessel according to the invention may be a pipe-laying vessel. At the transit draft, normal pipe-laying operations may be performed. When installing larger structures on the bottom of the sea, such as [Vakterm voor kleppenunit?], the draft may be increased to decrease the heel of the vessel when lifting these structures. A further type may be a cable-laying vessel, also working usually at the transit draft, unless more heavy installations need to be lifted and/or put on the sea bottom.

Several variants of the invention are possible within the scope of the claims. An off- shore installation vessel according to the invention is typically a large vessel, with sufficient breadth in view of the loads that need to be lifted. Smaller and larger dimensions than the disclosed length, breadth, and depth are possible. In general, the breadth will be at least 30 meter. The maximum draft may intersect the oblique side part too.

The hull may have more or less buckle lines than the one in the preferred

embodiment. Their may be a buckle at the transition of the bilge into the oblique side part, which buckle may be positive, to the outside, or negative, to the inside of the hull. The transition from oblique side part to vertical side part may be rounded. Instead of a rounded bilge as disclosed, a straight bilge may be applied, which is oblique with respect to the centre line plane. Preferably, such a straight bilge has an angle to the centreline plane which is larger than the angle of the oblique side part. The off-shore installation vessel is preferably positioned using a dynamic positioning system with thrusters. However, positioning using anchors and/or tug boats is also possible.

In the shown embodiment, the difference between the transit draft and the maximum draft is 5 meter, which is between 30% and 35% of the depth of the hull. More in general, the difference between the transit draft and the maximum draft is at least 25% of the depth of the hull.