NO338418B1 - Floating unit - Google Patents

Description

LIQUID UNIT

TECHNICAL AREA

The present statement relates to a floating unit for offshore use according to the preamble in claim 1.

BACKGROUND

Floating devices, such as partially submersible devices, e.g. platforms or rigs, and other types of vessels, e.g. ships, are largely used in offshore applications. They can be used during drilling for and/or production of natural resources, such as hydrocarbons, e.g. gas, oil etc. Such units are available in a number of different types and different designs. What they all have in common is that they are designed to be partially submerged in the water.

As part of various operations performed on the floating unit, an object, such as a piece of underwater equipment, may be launched from the floating unit's deck structure into the surrounding water or retrieved from the surrounding water onto the deck structure. It is known to perform such operations on the outboard side of the floating unit. However, there is a risk that the object will be damaged, e.g. when it passes the spray zone, especially in harsh weather conditions, e.g. the object is damaged by waves and/or is thrown against the floating entity.

The underwater equipment can be an ROV, a remotely operated vehicle. Such vehicles are largely used in offshore underwater applications, e.g. in well intervention, particularly when inspecting before, during or after drilling for, or production of, the natural resources mentioned above. The ROV can also be used for various maintenance and repair operations. With this in mind, the ROV can be equipped with various tools and other underwater equipment. ROVs are usually lowered into the sea from the floating unit by hanging freely suspended in the air on the side of the unit. In such a position, the ROV is unprotected from the surroundings, which are often harsh. The ROV may be exposed to wind and water waves splashing against the ROV which may cause damage to the ROV, as well as making it difficult to control the ROV's movement. In particular, the ROV may be forced towards the floating unit by the wind and possibly have a collision with the floating unit with high pressure. Floating units are also themselves prone to move in response to wave movements in the sea, which further increases the risk of damage to the ROV.

According to its summary, patent document NO 861784 A describes a partially submersible platform of the type comprising a deck or a composition of decks carrying various pieces of equipment, connected via an intermediate structure to a base comprising one or more rafts, in addition to means for mooring and coupling means capable of connecting the deck to the seabed. The intermediate structure comprises at least one double-walled column with an open lower end, which consequently internally offers a protected access passage to the sea.

However, there is still a risk that water movements at the lower opening or in the access passage could damage equipment, such as an ROV, which is launched or picked up via the access passage explained in NO 861784 A.

SUMMARY

The purpose of the present disclosure is to overcome or improve at least one of the disadvantages of the known technique, or to provide a useful alternative.

It is desirable to provide a floating unit that is able to reduce, or preferably avoid, the risk of damaging objects launched from the floating unit or brought up to the floating unit, in particular underwater equipment such as well intervention equipment or an ROV.

The purpose above can be achieved by the object in claim 1.

Accordingly, in a first aspect of the present disclosure there is provided a floating device for offshore application. The floating unit comprises a cavity wall which at least partially delimits a cavity. The cavity wall comprises a lower part which at least partially defines a lower cavity opening which is adapted to open into surrounding water. The lower portion is at least partially surrounded by a proximal enclosing portion of the bottom of the floating unit. The proximal enclosing portion is vertically located above the lower portion.

By configuring the bottom part of the cavity wall as explained herein, the entrance loss for water flowing towards the cavity will increase. Consequently, less water will enter the cavity. Therefore, the maximum water level in the cavity and/or the velocity of the water in the cavity may decrease. Thus, the risk that water movements at the lower opening of the cavity could damage underwater equipment that is launched or retrieved is reduced or avoided.

The floating unit may be a partially submersible unit or any other type of vessel commonly used in offshore applications. The term bottom of the floating unit as used herein is not limited to a flat bottom, but is instead defined as a surface that is more horizontal than vertical.

The cavity can be adapted for launching and/or retrieving underwater equipment. Such a cavity arranged in a floating unit to provide protected access to the surrounding water, through which cavity equipment can be launched from the unit into the surrounding water, and retrieved from the water to the unit, is often referred to as a moon pool. The underwater equipment to be moved through the cavity can e.g. be well intervention equipment. Well intervention equipment is equipment used in a well intervention operation. The well intervention equipment can therefore be used for operations such as: inspection e.g. with a camera, perforating/reperforating, zone isolation, inspection/repair/installation of an inline downhole safety valve, painting of short scale bridges ("seale bridges"), well killing, pumping, weight treatment, selective tracer injection or sampling, exchange of gas lift valves ( gas lift valves), sleeve operations for DIACS valves, i.e. Downhole Instrumentation And Control System valves, plug and abandon operations in subsea beans, exchange/installation of subsea trees, data collection eg tool string for production borehole measurement. The term well intervention equipment as used herein does not include drill pipes or mud risers. The cavity wall comprises an internal portion extending inside the floating unit, and a projecting portion extending from the near surrounding portion of the bottom. Accordingly, the protruding portion extends into the surrounding water below the floating unit. The internal and the protruding part can form an integral unit, so that the cavity wall is one part. Alternatively, the cavity wall may comprise a telescoping assembly so that the protruding part can be lifted into the internal part, e.g. when transporting the floating unit to a new location. When the cavity is to be used for launching and retrieving underwater equipment, the protruding part can be moved to the desired position which has the bottom part of the cavity wall in a position in relation to the nearby surrounding part as explained herein. The protruding part of the cavity wall preferably extends in the vertical direction, but it can also deviate from the vertical direction, e.g. be somewhat inward-sloping. However, the lower cavity opening is suitably large for launching and/or retrieving the underwater equipment.

The lower part of the cavity wall is at least partially surrounded by the near surrounding part of the bottom of the floating unit, i.e. at least half of the circumference of the lower part is surrounded by the near surrounding part. Preferably, most of the circumference of the bottom portion is surrounded by the proximate surrounding portion, more preferably substantially the entire circumference of the bottom portion is surrounded by the proximate surrounding portion, and most preferably the entire perimeter of the bottom portion is surrounded by the proximate surrounding portion . In other words, the proximate surrounding surrounds the lower part preferably completely or substantially completely.

If the lower part does not completely surround the lower opening, the vertical level of the lower part is defined by its lowest point.

The near surrounding part is located vertically above the lower part of the cavity wall. Accordingly, the near surrounding part has a higher vertical level than the bottom part. However, the proximal surrounding portion is located on the outside of the cavity wall such that the proximal surrounding portion is not directly above the bottom portion. Accordingly, part of the cavity wall may protrude from the nearby surrounding part.

The bottom part can be symmetrically located in relation to the nearby surrounding part, e.g. both parts have a common center point. As an alternative or complement, the bottom part may be asymmetrically located in relation to the nearby surrounding part. By way of example only, the bottom portion may be symmetrically located relative to the near surrounding portion viewed in a first direction of the floating unit, but asymmetrically viewed in another direction. However, it will also be possible for the bottom part to be symmetrically located in the nearby surrounding part in both directions, or asymmetrically in both directions. The two directions can e.g. be a transverse and a longitudinal direction in the floating unit.

The near surrounding part is defined as the part of the bottom of the floating unit that has the greatest vertical difference from the bottom part. The proximate surrounding portion may form a bent line that at least partially extends around the lower portion. The bent line may be open if the lower portion does not completely surround the lower opening and/or if the adjacent surrounding portion may comprise a planar surface, in the event that the portion of the bottom located adjacent to the lower portion is flat. The near surrounding portion may be a combination of one or more curved line segments and/or one or more planar surfaces.

The proximal surrounding portion may extend directly from the cavity wall or there may be a first transit zone, further described below, between the cavity wall and the proximal surrounding portion. The extension of the proximal surrounding portion is determined as a horizontal distance between the proximal surrounding portion and the closest point in the cavity wall. If the near surrounding part comprises a ring or a ring segment, the extension is determined as the horizontal distance between the ring and the closest point in the cavity wall. If the near surrounding part comprises a planar surface, the extension is fixed to the limit of the planar surface farthest from the cavity wall. The horizontal distance to the nearest point on the cavity wall and hence the extension may vary along the circumference if the circumference of the nearest surrounding part is followed. However, a maximum extension may be determined as the greatest horizontal distance for the entire circumference of the adjacent surrounding portion. The maximum extension can be at least 1 meter, preferably at least 2 meters, more preferably at least 3 meters.

If the proximate surrounding portion includes a planar surface, the width of the proximate surrounding portion may be determined as the width of the planar surface along a line passing through the closest point in the cavity wall and the boundary of the planar surface furthest away from the cavity wall. The width can be in the range of 1-5 metres, preferably in the range of 2-3 metres.

However, the portion of the bottom located proximate to the bottom portion may instead have a bent surface. In that case, the nearby surrounding portion may form a ring around the bottom portion. The ring may be open if the lower part does not completely surround the lower opening.

The lower cavity opening may have a cross-sectional area of at least 0.5 m<2>, preferably of at least 1 m<2>, more preferably of at least 1.5 m<2>, and most preferably of at least 2 m2. The abbreviation m used herein means Sl unit meter. The lower cavity opening is consequently larger than a pipe or pipeline that terminates in the surrounding water. The area can be chosen to be large enough for launching and/or retrieving underwater equipment, such as an ROV. An ROV can be sized so that one side of it extends between 0.5 and 5 meters, preferably between 1 and 4 meters and more preferably between 2 and 4 meters. The cavity may have a different cross-sectional area at a higher vertical level than the lower cavity opening.

The lower part of the cavity wall can have a wall width in the range 1-50 cm, preferably in the range 2-40 cm, more preferably in the range 5-30 cm, where cm is an abbreviation for centimeter.

The maximum vertical distance between a portion of the near surrounding portion and the bottom portion may be at least 0.3 meters, preferably at least 0.5 meters, more preferably at least 1.0 meters and most preferably at least 1.5 meters.

The proximal surrounding portion may be at least partially surrounded by a distal surrounding portion of the bottom of the raft. The distal surrounding portion is vertically located below the proximal surrounding portion at a maximum vertical distance which may be at least 0.3 meters, preferably at least 0.5 meters, more preferably at least 1.0 meters and most preferably at least 1.5 meters.

The distal surrounding portion may comprise or be constituted by a horizontal plane, or its surface may slope upward. The distal surrounding portion may form part of the bottom of the remaining portion of the floating unit, excluding the proximal surrounding portion and the bottom portion. The proximal surrounding portion may be symmetrically located in the distal surrounding portion, e.g. have a common center point. Alternatively, the proximal surrounding portion may be asymmetrically located in the distal surrounding portion. By way of example only, the proximal surrounding portion may be symmetrically located relative to the distal surrounding portion viewed in a first direction of the floating unit, but asymmetrically viewed in another direction. However, it will also be possible for the proximal surrounding part to be symmetrically located in the distal surrounding part in both directions or asymmetrically in both directions. The two directions can e.g. be a transverse and a longitudinal direction in the fluid unit.

The distal surrounding part can essentially have the same vertical level as the lower part of the cavity wall. The bottom portion may also be at a lower vertical level than the distal surrounding portion or at a higher vertical level, as long as the bottom portion is at a lower vertical level than the proximal surrounding portion. If the lower part is at a higher level than the distal surrounding part or at the same level, the lower part is protected by the distal surrounding part. If the vertical portion of the lower portion coincides with that of the distal surrounding portion, the lower cavity opening is as low as possible while being protected by the distal surrounding portion. This can be advantageous during e.g. manufacture, maintenance or repair of the device, since the part of the device with the lower part can be placed on the ground without damaging the lower part.

The distal surrounding portion may have substantially the same vertical level as a baseline in the floating unit. The distal surrounding portion may form part of the bottom of the floating unit, e.g. a bottom of a pontoon if the floating unit is a partially submersible platform or part of the hull if the floating unit is of another vessel type, such as a ship.

The lower cavity opening may have a square, rectangular or polygonal cross-section viewed in a horizontal plane. As an alternative, the lower cavity opening may have a circular, oval or elliptical cross-section viewed in a horizontal plane. In any case, it is preferred that the lower cavity opening is adapted for launching and/or retrieving underwater equipment, such as an ROV.

The floating unit may further comprise a first transit zone located between the bottom portion and the adjacent surrounding portion. Accordingly, the bottom part can gradually transform into the nearby surrounding part. The configuration of the first transit zone can be selected to help achieve a desired water deflection. The width of the first transit zone taken in the horizontal direction can be in the range 0-2 metres, preferably in the range 0-1 metres. The areas are given with the endpoints included in the area. The width 0 meters corresponds to no transit zone, i.e. a gradual transition between the bottom part and the near surrounding part, which has been found advantageous in certain applications.

The first transit zone may comprise or be constituted by a continuous surface, such as a bent plate. The surface may support at least 25%, preferably at least 50%, more preferably at least 75% and most preferably substantially the entire length of a portion of the cavity wall, which projects downwardly relative to the adjacent surrounding portion. The continuous surface can help improve the mechanical stability of the protruding part of the cavity wall.

The floating unit may comprise at least one support member extending between the portion of the cavity wall, which portion extends downwardly relative to the proximal surrounding portion, and the proximal surrounding portion. The support element can support at least 25%, preferably at least 50%, more preferably at least 75%, and most preferably essentially the entire length of the protruding part of the cavity wall. The at least one support element may help to improve the mechanical stability of the protruding portion of the cavity wall. A plurality of support elements are preferably used along the circumference of the projecting portion. They can e.g. be located at a center-to-center distance in the range of 0.3-1 meter, preferably in the range of 0.5-0.7 meters along the circumference of the projecting part. A support element can be located in at least one of the corners if the cavity opening is square, rectangular or polygonal.

The support element can be an essentially two-dimensional support element with a shape such as a plate. The plate may be located so that it's surface is perpendicular to both the vertical direction and a circumferential extension direction of the nearby surrounding portion. The plate can be shaped so that a cross-section through the protruding part of the cavity wall in the plane of the support element has a general bell shape or funnel shape. Alternatively, the support member may form a general conical shape.

An essentially one-dimensional support element can be used as an alternative to or as a complement to an essentially two-dimensional support element, e.g. a pole or pole. Its lower end may then be located at a place at least 25% of the entire length of the protruding part of the cavity wall counted from the vertical level of the nearby surrounding part, preferably at least 50%, more preferably at least 75%, and most preferably at i substantially the entire length of the protruding portion of the cavity wall.

The floating unit may further comprise a second transit zone located between the proximal surrounding portion and the distal surrounding portion. Accordingly, the proximal surrounding portion may gradually transition into the distal surrounding portion. The second transit zone may comprise or be constituted by a continuous surface, such as a bent plate. The width of the second transit zone can be in the range 0-3 metres, preferably in the range 1-2 metres, in which 0 corresponds to a gradual transition. The configuration and shape of the second transit zone can be chosen to help achieve a desired water deflection, similar to that of the first transit zone. The configurations and shapes of the first transit zone and the second transit zone can be selected so that together they help achieve the desired water deflection.

A part of the first transit zone and/or the second transit zone can have a bell-shaped, funnel-shaped, cone-shaped or cylindrical cross-section, seen in a vertical plane. In that case, the first transit zone and/or the second transit zone may comprise or be constituted by a continuous surface.

The projecting part of the cavity wall may comprise or be made up of or be supported by a wobble keel-shaped element. The rocker keel-shaped element can form a skirt

around the lower cavity opening. The oscillating keel-shaped element can have a triangular cross-sectional shape, e.g. comprising two walls, one of which is the cavity wall, which meet at or are adjacent to the lower part. The lower part of the bilge keel-shaped member may form the lower part of the cavity wall. The bilge-keel-shaped

the element may surround at least half the circumference of the projecting part, more preferably substantially the entire circumference of the projecting part and most preferably the entire circumference of the projecting part. The bilge keel-shaped member may form the continuous surface of the first transit zone.

The floating unit may be a partially submersible platform comprising a raft located below a still water level, a deck structure located above the still water level and at least two columns connecting the raft and the deck structure so that the still water level crosses the columns. The raft includes the bottom.

Partially submersible units are largely used in offshore applications. Partially submersible production units and partially submersible drilling units are generally known, which have been largely used e.g. during drilling for and/or production of natural resources, such as hydrocarbons, e.g. gas, oil etc. Such partially submersible rigs and units are available in a number of different types and different designs. Common to all is that they provide stable structures that are constructed so that they are partially submerged in the water, whereby a raft, such as one or more pontoons, provides buoyancy. For some partially submersible units, the draft level can be adjusted and controlled depending on the situation.

The partially submersible unit typically comprises a deck structure which may comprise a plurality of different decks located at different levels. Equipment used in connection with hydrocarbon drilling and production operations can be provided on the various decks, as is customary in the field. Rooms, such as living quarters and control rooms can also be provided on the deck structure. The partially submersible unit also includes a raft that is usually fully submerged in the water at a distance below the still water level. The partially submersible unit further comprises two or more columns, which connect the deck structure to the raft. By way of example only, a partially submersible unit may include four columns, one in each corner of the unit. The still water level will cross each column during normal operation of the unit. One or more columns can also provide floating power.

The term "quiet water level" is commonly used in the field and can be defined as the average water surface elevation at any instant, excluding local variations due to waves and wave formations, but including tidal effects, storm surges and long period shoals.

The water plane crosses the columns during normal operation. The unit's water plane area is thus small compared to the area of the deck structure's main deck. For example, the unit's water plane area is less than or equal to 50%, alternatively less than or equal to 30% of the main deck area. A partially submersible unit as defined herein therefore differs from a unit such as a ship, where the water plane area is within the same area as the area of the main deck.

For a partially submersible unit, the draft level is defined as the distance from the baseline, e.g. keel line, of the partially submersible unit to the still water level. The level of a desired draft will depend on the specific type and design of the partially submersible unit. As a non-limiting example, the length of the cavity wall may be at least twice the draft level of the partially submersible unit. By way of example only, the draft level may be in the range of 10-20 meters measured from the unit's baseline, i.e. from the bottom of the raft, for a partially submersible unit floating in an operational condition. The length of a cavity can be in the range of 20-40 metres. The term length is the extension of the cavity in its longitudinal direction, which generally coincides substantially with a vertical direction of the partially submersible unit.

The column is adapted to send cargo from the deck structure to the raft. Such a column usually includes various features, such as a structural frame or other structural elements that provide the required strength to the column. The column may also include a ballast system that may be used to adjust the draft of the semi-submersible unit, in addition to other systems known to provide seawater in various types of applications, such as firefighting, to the semi-submersible unit. Hoist arrangements for seawater pumps can also be arranged in the column.

The deck structure may comprise a plurality of decks at different vertical levels, among which is a main deck, sometimes referred to as the service deck. The main deck may include lifts, winches and other devices for hoisting equipment on board the platform and for moving such equipment on or between different decks.

It is advantageous to provide the lower cavity opening vertically low, as it results in the underwater equipment, e.g. The ROV, will be launched in the water below the water level, which provides a safer launch and retrieval of the underwater equipment, e.g. The ROV, especially during rough weather, since the level of dynamic wave pressure decreases rapidly with distance from the still water level.

The nearby surrounding portion may form a depression in the bottom of the raft. Then the lower part of the cavity wall will be located in the recess. Thus, the lower cavity opening can be protected by the rest of the bottom of the raft. The size of the depression and its volume are chosen so that it is suitable to be large enough to ensure suitable water flow away from the lower cavity opening.

The near surrounding portion may extend to at least one side of the raft, e.g. two opposite sides of the raft. This configuration can help direct water flow away from the lower cavity opening.

The raft may include at least one pontoon. The pontoon can extend between the lower ends of two of the columns. By way of example only, the partially submersible platform may comprise four columns connected in pairs by pontoons. The partially submersible platform in that example then comprises two pontoons. The nearby surrounding part may form a depression in the pontoon. The near surrounding portion may extend to at least one side of the pontoon, or on two opposite sides of the pontoon.

The cavity wall can at least partially extend into one of the at least two columns. In some partially submersible units, more than one or each column may be provided with a cavity wall. There may also be more than one cavity wall in the same column. The cavity may be arranged to extend over at least part of the column, although it may be advantageous for it to extend over the entire length of the column, such that an upper cavity opening is located on a deck, such as the main deck, and the cavity extends all the way through the column. The positioning of the cavity in one of the columns reduces the amount of reconstruction of the partially submersible unit since no additional columns have to be provided. It further reduces additional weight in the device, as well as the amount of additional features required on the partially submersible device. The cavity can be formed by the internal structure of the column. In other embodiments, the cavity can be formed by arranging, for example, a pipe, trunk or caisson in the column, whereby the cavity is formed in the interior of the pipe, trunk or caisson.

As an alternative to using one of the columns to house the cavity, the partially submersible unit may be provided with a column whose main purpose is to house a cavity to thereby provide an arrangement for launching and/or retrieving underwater equipment into and out of, respectively surrounding water. This is often referred to as a dedicated column.

Each of the above-discussed columns is vertically oriented, but in some types of partially submersible units and/or in certain designs, may tilt with respect to the vertical direction.

The lower part may be located in an area in the raft corresponding to an outlet from the column, in a longitudinal direction from the column, at the bottom of the raft. The outlet is in a vertical direction if the column is vertically oriented.

As mentioned above, the floating unit can be a ship. The lower cavity opening is then an opening in the ship's hull at the bottom of the hull. The near surrounding part may form a depression in the bottom of the hull.

BRIEF DESCRIPTION OF THE FIGURES

In the following, the present invention will be further described by means of non-limiting examples with reference to the attached figures in which: Fig. 1 is a schematic section of a cross section of a floating unit according to a first embodiment of the invention, exemplified by a partially submersible unit viewed from the side. Fig. 2 is a perspective section from the underside of part of a partially submersible

unit according to a second embodiment of the invention.

Fig. 3-6 illustrate a number of different cross-sections of the partially submersible of it

first embodiment as marked in figure 2.

Fig. 7 is a detailed section of a part of Fig. 5 which illustrates a support element.

Fig. 8 illustrates an oscillating keel-shaped element.

Fig. 9 is a perspective section from the underside of part of a partially submersible

unit according to a second embodiment of the invention.

Fig. 10-11 illustrate two different cross-sections of the partially submersible of the other

the embodiment as marked in Figure 9.

Fig. 12 is a perspective section seen from the underside of part of a partially submersible

unit according to a third embodiment of the invention.

Fig. 13-14 illustrate two different cross-sections of the partially submersible of the third

the embodiment as marked in Figure 12.

Fig. 15 is a side section of a ship according to a fourth embodiment of

the invention, and

Fig. 16 is a schematic cross-section of the ship.

It should be noted that the attached figures are not necessarily drawn to scale and that the dimensions of some of the features of the present account may have been exaggerated for the sake of clarity.

DETAILED DESCRIPTION

In the following, the invention will be exemplified with embodiments. However, it should be clarified that the embodiments are included to explain the principles of the invention and not to limit the scope of the invention, which is defined by the appended claims. Details from two or more of the embodiments can be combined with each other.

Figures 1 and 2 schematically illustrate a floating unit for use in offshore applications in the form of a partially submersible unit 1, for example for purposes as described above, according to a first embodiment of the invention. Figure 1 illustrates a cross-section of the partially submersible unit 1 seen from the side. Figure 2 schematically illustrates the partially submersible unit 1 seen in a perspective section from the underside. The unit 1 comprises a deck structure 2 which may comprise a plurality of different decks located at different vertical levels. Equipment used in connection with e.g. hydrocarbon drilling and production operations may be provided on the various decks common in the art. Areas such as living quarters and control rooms can also be provided on the deck structure.

A main deck 4, often also referred to as the operating deck, is located relatively high above the still water level of the sea, preferably so high that waves will not usually splash over the surface of the main deck. The partially submersible unit further comprises a raft 5, here illustrated in the form of a plurality of pontoons 6, one of which can be seen in Figure 1, which is fully submerged in the water at a distance below the still water level 12 during operation of the unit. The raft 5 provides buoyancy which keeps the partially submersible unit 1 afloat and only partially submerged in the water. The unit 1 further comprises one or more columns 8 which connect the deck structure 2 to the raft 5. Just as an example, at least one of the columns 8 can also provide buoyancy. In Figures 1 and 2, the partially submersible unit 1 is illustrated to comprise four columns 8, one at each corner of the unit 1. The still water level 12 intersects each column 8 so that the columns 8 will define the water plane area of the partially submersible unit 1.

The partially submersible unit 1 is further provided with one or more ballast systems which are known in the field. By using this ballast system, the draft level 13 and/or the slope of the unit 1 can be adjusted. That is, the degree of immersion of the partially submersible unit 1 can be adjusted, i.e. its intersection with the still water level 12 can be adjusted. The draft level 13 can for example be in the range 10-20 meters measured from the unit's baseline 14, i.e. from the bottom 15 of the raft 5 and the length of a cavity 20, which is further described below, can be in the range 20-40 metres. The term length is the extent of the cavity 20 along its longitudinal direction which in Figure 1 coincides with the partially submersible unit 1's vertical direction V.

The columns 8 are illustrated in Figure 1 to be essentially vertically oriented. In other embodiments, however, the columns 8 may be inclined with respect to the vertical direction V. The cavity 20 may extend in a direction parallel to or even coaxial with the direction of extension of the column 8. The cross-sectional area of the cavity can mainly be circular, square or rectangular, but other geometries are also possible. Although Figure 2 illustrates a floating unit 1 comprising four columns 8, the present account is not limited to partially submersible units comprising four columns, but the number of columns may be more, or in some embodiments also fewer. However, the number of columns is often three or more.

One or more control systems 16 can be provided to control the operation of the various components related to the cavity 20 and to launch and/or retrieve equipment therethrough.

According to the invention, at least one of the columns of the partially submersible unit 1, e.g. one of the columns 8 or a dedicated column, provided in its interior with a cavity 20. The cavity 20 can be used for launching underwater equipment, such as a remote-controlled vessel, i.e. an ROV, 18, from the unit 1 into the surrounding seawater the unit 1. Although an ROV is used as an example of underwater equipment that can be launched in the cavity 20, it is also thought that other types of equipment can be launched. The partially submersible unit 1 illustrated in figures 1 and 2 is provided with the cavity 20 in one of its columns 8. In some partially submersible units, more than one of each column may be provided with a cavity 20. There may also be more than one cavity in the same column. Such a cavity 20 which is arranged in a floating unit 1 to provide a protected access to the surrounding water, through which equipment can be launched from the unit 1 into the surrounding water and retrieved from the water to the unit 1, is often referred to as a moon pool.

The cavity 20 is delimited by a cavity wall 21. The cavity 20 has an upper end 22 and a lower end 24, and a lower cavity opening 26 at its lower end 24. It can correspondingly have an upper cavity opening 23 at the upper end 22. The lower cavity opening 26 represents the outlet from the cavity 20 into the surrounding water. The upper cavity opening 23 is located towards the upper end 22 of the cavity, which end opens into the surrounding air or into a space in or on a structure in or on which cavity 20 is arranged.

In the first embodiment, the cavity 20 extends along the full height of the column 8 and through the pontoon 6 located below the column 8 in which cavity 20 is arranged, so that the lower cavity opening 26 is located at or at least close to the bottom 15 of the raft 5. By providing the lower the cavity opening 26 so low, the ROV will be launched into the water below the water level, which provides a safer launching and retrieval of the underwater equipment, e.g. The ROV, especially during severe weather, since the level of dynamic wave pressure decreases rapidly, e.g. exponentially, with the vertical distance from the still water level.

Figure 1 illustrates that the ROV 18 is launched into the seawater via the cavity 20. The ROV 18 can be retrieved from the water and to the unit 1 in a similar way via the cavity 20. Since the ROV 18 is launched and retrieved via the cavity 20, it is protected from wind and water waves in the surrounding sea, which reduces the risk of damage to the ROV during launch and retrieval. Positioning the cavity 20 in one of the columns 8 reduces the amount of reconstruction of the unit 1, since no additional columns need to be provided. Furthermore, it reduces additional weight of the device, as well as the amount of additional features required on the partially submersible device. The cavity 20 can be formed by the internal structure of the column 8, and in the embodiment illustrated in Figures 1 and 2, by the raft 5. The cavity 20 can be formed, for example, by taking up

openings in the top and bottom of the column 8, while the rest of the column 8 is isolated from water penetration.

The partially submersible unit 1 illustrated in figure 1 further comprises a house which forms an ROV workshop 28, in which the ROV 18 and other underwater equipment can be stored and where repair, service, modifications or preparations on the ROV can be carried out.

Figures 3-6 illustrate a number of different cross-sections through a lower end of the column 8

which houses the cavity 20 and part of the pontoon 6 of the partially submersible unit in Figures 1-2. The cross-sections in figures 3 and 5 cross the cavity 20, while the cross-sections in figures 4 and 6 are located on the outside of the cavity 20. The cross-sections in figures 3 and 4 are in the transverse direction T of the pontoon 6, while the cross-sections in figures 5 and 6 are in that of the pontoon 6 longitudinal direction L. The cross-section is marked with corresponding Roman numerals in Figure 2.

Figure 3 illustrates a cross-section through the cavity 20 in a plane above the pontoon 6. Accordingly, the cross-section passes through the lower cavity opening 26. A lower part 30 of the cavity wall 21 defines the lower cavity opening 26. The lower cavity opening 26 can have a cross-sectional area of at least 0.5 m<2>, preferably at least 1 m2, more preferably at least 1.5 m<2>, and most preferably at least 2 m<2>. The lower part 30 forms an edge around the lower cavity opening 26. The lower part 30 can comprise or be constituted by the lower end wall part 32 of the cavity wall 21. In general, the wall width of the lower part 30 is narrow in relation to the cross-sectional area of the lower cavity opening 26. Only as an example the wall width can be in the range 1-50 cm, preferably in the range 20-40 cm, more preferably in the range 5-30 cm.

A near surrounding part 34 surrounds the bottom part 30. The near surrounding part 34 is defined as that part of the bottom 15 of the floating unit 1 which has the greatest vertical difference with respect to the bottom part 30. If a part of the floating unit's 1 bottom 15 is horizontal on the outside of the bottom part 30, as it is in the first embodiment, the near surrounding part 34 forms a horizontal plane, as illustrated in Figures 2-6. The lower part 30 is located symmetrically in relation to the nearby surrounding part 34 in the longitudinal direction L of the pontoon 6 but asymmetrically in the transverse direction T. However, it is also possible that the lower part 30 is located symmetrically in relation to the nearby surrounding part 34 in both directions or asymmetrically in both directions.

The near surrounding portion 34 is vertically located above the bottom portion 30 by a first vertical difference designated Ayi in Figures 3-6, which may be at least 0.3 meters, preferably at least 0.5 meters, more preferably at least 1.0 meters and most preferably at least 1.5 meters.

In the first embodiment, the proximate surrounding portion 34 comprises a planar surface and accordingly an extension l_iav the proximate surrounding portion 34 is fixed to the planar surface boundary 35 which is farthest away from the cavity wall 21. The horizontal distance to the closest point in the cavity wall 21 , and consequently the extension L1, can vary if the circumference of the near surrounding part 34 is followed, as seen in figure 3 as a difference between the left side and the right side. However, a maximum extension may be determined as the greatest horizontal distance for the entire circumference of the near surrounding portion 34. The maximum extension may be at least 1 meter, preferably at least 2 meters, more preferably at least 3 meters.

If the near surrounding portion comprises a planar surface, as in the first embodiment, a width w of the near surrounding portion 34 can be determined as the width of the planar surface along a line passing the closest point of the cavity wall 21 and to the planar surface's border 35 which is furthest away from the cavity wall 21. The width w can be in the range 1-5 metres, preferably in the range 2-3 metres.

In the first embodiment, the planar surface begins at the cavity wall 21 and consequently the width w is the same as the extension L1. The lowermost part 30 is converted directly into the nearby surrounding part 34, i.e. with a step in the illustrated cross-section. Alternatively, a first transit zone, which is further described below, may be located between the bottom portion 30 and the adjacent surrounding portion 34.

By configuring the bottom part 30 of the cavity wall 21 according to the invention, the entrance loss for water flowing towards the cavity 20 will be increased. Water streams coming from below, illustrated by arrows in Figure 3, can flow away from the lower cavity opening. This may result in less water entering the cavity 20. Consequently, the cavity 20's maximum water level and/or the speed of the water in the cavity 20 may decrease. This reduces or avoids the risk that relative water movements at the lower opening 26 of the cavity 20 may damage underwater equipment that is launched or retrieved.

The proximal surrounding portion 34 is in the illustrated embodiment surrounded by a distal surrounding portion 38, which forms the bottom 15 of the remaining portion of the pontoon 6. The distal surrounding portion 38 may comprise or be constituted by a horizontal plane, as illustrated by the illustrated embodiments , or its surface may slope upwards. The distal surrounding portion 38 is located at a lower vertical level than the proximal surrounding portion 34, by a second vertical difference designated Ay2 in Figures 4 and 6, which may be at least 0.3 meters, preferably at least 0.5 meters, more preferably at least 1.0 meters and most preferably at least 1.5 meters. In the illustrated example, the distal surrounding portion 38 is located at substantially the same vertical level as the baseline 14.

A second transit zone 40 is located between the proximal surrounding portion 34 and the distal surrounding portion 38. In the illustrated first embodiment, the second transit zone 40 is formed by a continuous surface. The configuration and shape of the second transit zone 40 can be selected to help achieve a desired water deflection of water streams 31, see Figure 3.

The distal surrounding part 38 has an inner boundary at a distance L2 from the cavity wall 21. The distance L2 is determined in a similar way to the extension L1 of the nearby surrounding part 34. The distance L2 can be in the range 2-10 meters, preferably in the range 4-7 meters.

The difference between the distance L2 and the extension Li of the nearby surrounding part 34 defines a horizontal width d2 of the second transit zone 40. If the second transit zone 40 is omitted, the extension Li of the nearby surrounding part 34 and the distance L2 will essentially coincide. The ratio l_i/L2 can be between 0.5 and 1, preferably between 0.7 and 1.

In the illustrated first embodiment, the vertical level of the bottom portion 30 is higher than that of the distal surrounding portion 38. Accordingly, the first vertical difference Ayi is less than the second vertical difference Ay2. However, this is optional. The bottom portion 30 may also be at a vertically lower level than the distal surrounding portion 38 or at the same vertical level, as long as the bottom portion 30 is at a lower vertical level than the proximal surrounding portion 34.

It may be advantageous to have the vertical level of the lower part 30 coincide with that of the distal surrounding part 38, since then the lower cavity opening 26 is as low as possible, while at the same time it is protected by the bottom of the pontoon 5. This can be advantageous during e.g. manufacture, repair or maintenance of the device, since the part of the device with the lower part can be placed on the ground without damaging the lower part 30.

The ratio Ai/Ay2 can be in the range 0.5 to 2, although it can, as described above, be advantageous if the ratio 1 is less than or equal to 1, e.g. 0.5< Ai/Ay2£ 1, or 0.7< Ai/Ay2£1 or 0.8< A^Ay^l.

The nearby surrounding part 34 can be seen as an indentation in the pontoon 5 at a location corresponding to that of the column 8. The indentation can be as large as the cross-sectional area of the column 8. It can also be smaller, as seen in Figure 2, or larger, as seen in Figure 9, which is described further below. Figure 4 illustrates a cross-section through the pontoon 6 below the column 8, but on the outside of the cavity 20. The cross-section illustrates the vertical level of the nearby surrounding part 34. This cross section illustrates the second vertical difference Ay2 between the proximal surrounding portion 34 and the distal surrounding portion 38. Figure 5 illustrates a cross section through the cavity 20 along the pontoon 6. The lower portion 30 defines the lower cavity opening 26, as already described for Figure 3. Figure 6 illustrates a cross-section along the pontoon 6 below the column 8 but outside the cavity 20. The cross-section illustrates the vertical levels of the proximal surrounding portion 34 and the distal surrounding portion 38.

If the first vertical difference denoted Ayi in Figures 3-6 is greater in relation to the stiffness properties of the protruding part of the cavity wall 21, it may be appropriate to provide the protruding part of the cavity wall 21 with one or more support elements, as illustrated in Figure 2 and further described in connection with figures 7 and 8.

Figure 7 illustrates that the protruding part of the cavity wall 21 of the embodiment illustrated in Figures 1 - 6 is supported by essentially two-dimensional support elements in the form of plates 42. They can e.g. be located with a centre-to-centre distance in the range of 0.3-1 metres, preferably in the range of 0.5-0.7 metres, around the circumference of the cavity wall 21. In the illustrated embodiment, the plates 42 are shaped so that a cross-section of the lower end 24 of the cavity wall 21 through the plates 42 has a general funnel shape. Alternatively, it may have a general conical shape. In the illustrated embodiment, the support elements support the entire length of the protruding part of the cavity wall 21 which results in a suitable support effect. In other embodiments, however, the support elements can support at least 25%, preferably at least 50%, more preferably at least 75% of the entire length of the protruding part of the cavity wall 21.

A substantially one-dimensional support element may be used instead of using a substantially two-dimensional support element, e.g. a pole or pole. Its lower end can then be located in a place at least 25% of the entire length of the protruding part of the cavity wall 21 counted from the vertical level of the nearby surrounding part 34, preferably at least 50%, more preferably at least 75%, and most preferably essentially the entire length to the protruding part of the cavity wall 21.

As an alternative or as a complement, as illustrated in Figure 8, the projecting part of the cavity wall 21 and the at least one support element can be made up of, comprise or be supported by a swing keel-shaped element 44 which can surround at least two sides of the cavity opening, and preferably all four sides . The wobble keel-shaped element 44 forms a skirt around the lower cavity opening 26. The wobble keel-shaped element has a triangular cross-sectional shape, e.g. comprising two walls, one of which is the cavity wall 21, which meets at or is close to the lower part 30. The triangular cross-section of the rocker keel-shaped element 44 has an angle a which is in the range of 15-45 degrees, preferably 20-35 degrees.

Figures 9-11 illustrate part of a partially submersible unit according to a second embodiment of the invention. Figure 9 shows a perspective section from the bottom. Figure 10 shows a transverse cross-section through the cavity opening 26 and Figure 11 shows a longitudinal cross-section through the cavity opening 26. The nearby surrounding part 34' surrounds the entire circumference of the lower part 30. The near surrounding portion 34' extends all the way to the lateral sides 46, 48 of the pontoon 6 so that the near surrounding portion 34' is not limited by the pontoon 6 in one direction, in the transverse direction T in Figure 9. This configuration can help water to flow away from the lower cavity opening, as illustrated by arrows in Figure 10.

The distal surrounding part 38 is formed by the bottom 15 of the pontoon 6, similarly to the first embodiment. In this case, however, the distal surrounding portion 38 surrounds only two sides of the proximal surrounding portion 34', i.e., the distal surrounding portion 38 surrounds approximately 60% of the circumference of the proximal surrounding portion 34'. The protruding part of the cavity wall 21 illustrated in Figure 9 does not include any support elements. However, similar support elements as illustrated for the first embodiment or an oscillating keel-shaped element can be used to support the protruding part of the cavity wall 21. Furthermore, there are no transit zones in Figures 9-11. However, it will be possible to use a corresponding first and/or second transit zone as for the first embodiment. The extension Li of the proximal surrounding portion 34 and the distance L2 to the distal surrounding portion 38 essentially coincide since the second transit zone 40 is omitted, see Figure 11.

Figures 12-14 illustrate a part of a partially submersible unit according to a third embodiment of the invention. Figure 12 shows a perspective section from the bottom. Figure 13 shows a transverse cross-section through the cavity opening 26 and Figure 14 shows a longitudinal cross-section through the cavity opening 26. Similar to the second embodiment, the proximal surrounding portion 34' extends all the way to the lateral sides 46, 48 of the pontoon 6. The distal surrounding portion 38 is formed by the bottom 15 of the pontoon 6, similar to the first and second embodiments. The protruding part of the cavity wall 21 is supported by a continuous surface 50, which surrounds the entire circumference of the lower part 30. Accordingly, the continuous surface 50 forms a first transit zone 36 having a horizontal width d1 between the bottom portion 30 and the near surrounding portion 34'. Furthermore, there is no other transit zone in the third embodiment. However, it will be possible to use a corresponding second transit zone as in the first embodiment.

The extension L1 of the proximal surrounding portion 34 and the distance L2 to the distal surrounding portion 38 essentially coincide. Furthermore, the width w of the near surrounding portion 34 is smaller than the extension L1 of the near surrounding portion 34 due to the first transit zone 36. The horizontal width d1 of the first transit zone 36 and the width w of the near surrounding portion 34 constitute the extension L1 of the near surrounding portion section 34.

In Figures 1-14 above, the cavity 20 is illustrated as located in a partially submersible unit 1. However, it is also possible to have a cavity 120 that corresponds to the cavity 20 described above in a ship 101, see Figures 15 and 16. The cavity 120 is surrounded of a cavity wall 121 and has a lower cavity opening 126 in a bottom 115 of the ship's 101 hull 103. Many details correspond to the first embodiment and will not be described above.

The cavity wall 121 comprises the lowermost part 130 which defines the lower cavity opening 126 which is adapted to open into surrounding water. The lower portion 130 is surrounded by a proximal surrounding portion 13 of the floating unit 101 bottom 115, the proximal surrounding portion 134 is vertically located above the lower portion 130. The proximal surrounding portion 134 is surrounded by a distal surrounding portion 138 which coincides with the bottom 115 of the ship 101. There is a second transit zone 140 between the proximal surrounding portion 134 and the distal surrounding portion 138. The proximal surrounding portion 135 extends all the way to the sides 146, 148 of the ship 101. Alternatively, the proximal surrounding portion 134 may form a recess in the hull 103. Although the embodiment in Figures 15 and 16 illustrates the protruding part of the cavity wall 121 without a support element, the protruding part of the cavity wall 121 can be supported in any of the ways described above. There may also be a first transit zone, which corresponds to the one described above.

Further modifications of the invention within the scope of the appended claims are possible. As such, the present invention should not be considered limited by the embodiments and figures described herein. Rather, the full scope of the invention shall be determined by the appended claims, with reference to the description and figures.

Claims (21)

1. A floating unit (1, 101) for offshore use, said floating unit (1, 101) comprises a cavity wall (21, 121) which at least partially delimits a cavity (20, 120), said cavity wall (21, 121) comprises a lower part (30, 130) which at least partially defines a lower cavity opening (26, 126) which is adapted to be opened into surrounding water, characterized by said bottom part (30, 130) is at least partially surrounded by a nearby surrounding part (34, 34', 134) of said floating unit's (1,101) bottom (15), said nearby surrounding part (34, 34', 134) is vertically located above said lower part (30, 130). 2. The floating unit (1, 101) according to claim 1, wherein said nearby surrounding part (34, 34', 134) completely or substantially completely surrounds said lower part (30, 130). 3. The floating unit (1, 101) according to any one of the preceding claims, wherein said lower cavity opening (26, 126) has an area of at least 0.5 m<2>, preferably at least 1 m<2>, more preferably at least 1.5 m<2> and most preferably at least 2 m<2>. 4. The floating unit (1, 101) according to any of the preceding claims, wherein said lower part (30, 130) of said cavity wall (21, 121) has a wall width in the range 1-50 cm, preferably in the range 2 -40 cm, more preferably in the range 5-30 cm. 5. The floating unit (1, 101) according to any one of the preceding claims, wherein said near surrounding part (34, 34', 134) extends at least 1 meter from said cavity wall (21, 121), preferably at least 2 meters , more preferably at least 3 metres. 6. The floating unit (1, 101) according to any one of the preceding claims, wherein the maximum vertical distance (Ayi) between a part of said near surrounding part (34, 34', 134) and said bottom part (30, 130 ) is at least 0.3 meters, preferably at least 0.5 meters, more preferably at least 1.0 meters and most preferably at least 1.5 meters. 7. The floating unit (1, 101) according to any of the preceding claims, wherein said proximal surrounding portion (34, 34', 134) is at least partially surrounded by a distal surrounding portion (38, 138) of said floating unit (1, 101) bottom (15), said distal surrounding part 838,138) is vertically located below said proximal surrounding part (34, 34', 134). 8. The floating unit (1, 101) according to claim 7, wherein said distal surrounding part (38, 138) has substantially the same vertical level as said cavity wall (21, 121) bottom part (30, 130). 9. The floating unit (1, 101) according to claim 7 or 8, wherein said distal surrounding portion (38, 139) has substantially the same vertical level as said floating unit (1, 101) baseline (14). 10. The floating unit (1, 101) according to any one of the preceding claims, wherein said floating unit (1, 101) further comprises a first transit zone (36) located between said bottom part (30, 130) and said near surrounding part ( 34, 34', 134). 11. The floating unit (1, 101) according to any one of the preceding claims, wherein said floating unit (1, 101) comprises at least one support element (42) extending between a part of said cavity wall (12, 121), the portion of which extends downward in relation to said nearby surrounding portion (34, 34', 134), and said nearby surrounding portion (34, 34', 134). 12. The floating unit (1, 101) according to claim 11, wherein said support element is a plate (42) or a rod. 13. The floating unit (1, 101) according to claim 7, or any of claims 8-12, when dependent on claim 7, wherein said floating unit (1, 101) further comprises a second transit zone (40, 140) located between said nearby surrounding part (34, 34', 134) and said distal surrounding part (38, 138). 14. The floating unit (1, 101) according to any one of claims 10-13, wherein a part of said first transit zone (36) and/or said second transit zone (40, 140) has a bell-shaped, funnel-shaped, conical or cylindrical cross-section seen in a vertical plane. 15. The floating unit (1, 101) according to any one of the preceding claims, wherein said part of said cavity wall (21, 121), which part extends downwardly with respect to said near surrounding part (34, 34', 134 ), comprises, or is made up of, or is supported by, an oscillating keel-shaped element (44). 16. The floating unit (1, 101) according to any of the preceding claims, wherein said cavity (20, 120) is suitable for launching and/or retrieving underwater equipment, such as an ROV (18). 17. The floating unit (1) according to any of the preceding claims which is a partially submersible platform, said partially submersible platform comprising - a raft (5) located below a still water level (12), said raft (5) comprising bottom (15), - a deck structure (2) located above said still water level (12), and - at least two columns (8) which connect said raft (5) and said deck structure (2) so that said still water level crosses said columns ( 8). 18. The floating unit (1) according to claim 17, wherein said near surrounding part (34) extends to at least one side (46, 48) of the raft (5). 19. The floating unit (1) according to any one of claims 17-18, wherein said cavity (20, 120) at least partially extends into one of said at least two columns (8). 20. The floating unit (1) according to claim 19, wherein said lower part (30, 130) is located within said raft's (5) area which corresponds to an outlet from said column (8) in a longitudinal direction of said column ( 8) on the bottom (15) of said raft (5). 21. The floating unit (1) according to claim 19 or 20, in which said nearby surrounding part (34) is at least partially, preferably completely, located in said raft (5)