WO2016083328A1

WO2016083328A1 - Semisubmersible unit

Info

Publication number: WO2016083328A1
Application number: PCT/EP2015/077408
Authority: WO
Inventors: Martin Mann
Original assignee: Gva Consultants Ab
Priority date: 2014-11-27
Filing date: 2015-11-24
Publication date: 2016-06-02

Abstract

The present disclosure relates to a semisubmersible unit (2, 2a-d, 62, 62a comprising - a float (8) located below a still water level, - a deck structure (4) located above the still water level, and - at least two support columns (14, 16, 18, 20; 64, 65, 66) connecting the float (8) and the deck structure (4), such that the still water level intersects the support columns (14, 16, 18, 20; 64, 65, 66). The semisubmersible unit comprises a guide arrangement (30, 68) for guiding an object during displacement from the deck structure (4) to the body of water, and a support structure (34, 34a-d, 70, 70a) for supporting the guide arrangement. The support structure comprises at least one guide support and a first and a second lower support. The guide support has an elongated shape and is adapted to support the guide arrangement. The lower supports have elongated shapes and form an angle (α₁, α_1a,Y) with each other, wherein 0< the angle < 180°. Each one of the lower supports connect the at least one guide support to the float and/or one of the support columns. The disclosure further relates to a method for displacement of an object, e.g. a well intervention equipment, in relation to a semisubmersible unit.

Description

SEMISUBMERSIBLE UNIT

TECHNICAL FIELD

The present disclosure relates to a semisubmersible unit for use in offshore applications. The disclosure further relates to a method for displacement of an object, e.g. a well intervention equipment, in relation to a semisubmersible unit.

BACKGROUND

Semisubmersible units are widely used in marine applications. Commonly known are semisubmersible production units and semisubmersible drilling units, which are widely used e.g. during drilling for or production of natural resources, such as hydrocarbons, e.g. gas, oil, etc. Such semisubmersible units come in a number of different types and constructions. Generally they provide stable structures which are constructed to be partially submerged in the water, whereby a float, such as for example one or more pontoons, provide buoyancy. Their draught level can be adjusted and controlled in dependence on the situation.

As a part of various operations performed on the semisubmersible unit, an object, such as a piece of equipment, may be launched from the deck structure of the semisubmersible unit into the ambient water or retrieved from the ambient water onto the deck structure. It is known to perform such operations on the outboard side of the deck structure and the float. However, there is a risk that the object is damaged, e.g. when passing the splash zone, especially in hard weather conditions. The object may e.g. be damaged by waves and/or by being thrown against a portion of the semisubmersible unit on its way up or down.

Document WO 2005/056380 A1 discloses a semisubmersible unit for use in offshore applications. The semisubmersible unit comprises a hull configuration including vertical support columns, a central pontoon structure disposed inboard of the columns at a lower end thereof, and a deck structure supported at an upper end of the columns. The vertical columns are adjoined to the outer periphery of the central pontoon and have a transverse cross sectional shape with a major axis oriented radially outward from a centre point of the hull, and a central vertical axis disposed a distance outward from the pontoon outer periphery. In an embodiment, the central pontoon structure is located inboard of the vertical columns and may have a central moonpool opening.

However, when an object is launched from the deck structure of the semisubmersible unit described in WO2005/056380A1 downwards towards the pontoon structure and possibly through the central moonpool opening of the pontoon structure, or displaced in the reverse direction, the object is unprotected when passing the water level. There is then a risk that the object is damaged, e.g. when passing the splash zone, especially in hard weather conditions.

SUMMARY

The objective of the present disclosure is to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative. It is desirable to provide a semisubmersible unit being able to reduce, or preferably avoid, the risk of damaging objects, which are launched from the semisubmersible unit or retrieved to the semisubmersible unit, in particular objects such as well intervention equipment. The objective above may be achieved by the subject-matter of claim 1 .

Thus, in a first aspect of the present disclosure there is provided a semisubmersible unit, adapted to float in a body of water, for use in offshore applications comprising a float located below a still water level, a deck structure located above the still water level, and at least two support columns connecting the float and the deck structure, such that the still water level intersects the support columns. The semisubmersible unit further comprises a guide arrangement and a support structure. The guide arrangement is adapted for guiding an object during displacement from the deck structure to the body of water. The guide arrangement is at least partly located between the deck structure and the float. The support structure is adapted for supporting the guide arrangement. The support structure comprises at least one guide support and a first and a second lower support. The at least one guide support has an elongated shape and is adapted to support the guide arrangement. The first and second lower support have elongated shapes and form an angle with each other, wherein 0< the angle < 180°. Each one of the lower supports connects the at least one guide support to the float and/or one of the support columns. The semisubmersible unit comprises a deck structure, which may comprise a plurality of different decks located at different levels. On the different decks, equipment used in relation to hydrocarbon drilling and production operations may be provided, as common in the field. Other structures, such as accommodation modules and control rooms, may be provided on the deck structure.

The semisubmersible unit also comprises a float, e.g. one or more pontoons, usually fully immersed in the water at a distance below the still water level. The float provides buoyancy, which keeps the semisubmersible unit partially submersed in the water.

The semisubmersible unit further comprises two or more support columns, connecting the deck structure with the float. There may be a support column located at each corner of the semisubmersible unit, e.g. four support columns for a square or rectangular semisubmers- ible unit or three support columns for a triangular semisubmersible unit. The still water level will intersect each support column during normal operation of the semisubmersible unit. One or more of the support columns may also provide buoyancy.

The term "still water level" is commonly used in the field and may be defined as average water surface elevation at any instant, excluding local variation due to waves and wave set-up, but including the effects of tides, storm surges and long period seiches. The term "splash zone" is used herein to denote a zone, which is regularly splashed by waves, e.g. +/- 5 metres from the still water level. The guide arrangement is intended for guiding the object during displacement from the deck structure to the body of water, preferably at least when passing the splash zone. Further, the guide arrangement may help to protect the object during displacement, preferably at least when passing the splash zone. The guide arrangement is at least partly located between the deck structure and the float. The guide arrangement may be arranged to extend downwards from the deck structure. A portion of the guide arrangement may extend above the deck structure. Preferably, the guide arrangement at least extends through the splash zone, such that it can guide and/or protect the object being launched or retrieved at least when passing the splash zone. The guide arrangement does preferably not go all the way to the bottom of the sea. In parti- cular, the guide arrangement is not a riser going all the way to the bottom of the sea. Further, the guide arrangement is preferably located between the support columns, e.g. within an envelope defined by the support columns. The guide arrangement may have a vertical position, which is adjustable in relation to the rest of the semisubmersible unit. The guide support may be utilized when adjusting the vertical position of the guide arrangement. Thereby the vertical position of the guide arrangement may be adjusted, when changing a draught level of the semisubmersible unit. The guide arrangement may be arranged to be movable between a non-operating, or parking, position and an operating position. The parking position may be located at, within, or on, the deck structure of the semisubmersible unit. The guide arrangement may be moved vertically at least partially within and/or through an opening of the deck structure, with which the guide arrangement is preferably aligned, during movement between the parking position and the operating position. In the operating position, the vertical position of the guide member may be adjustable in relation to a draught level of the semisubmersible unit. The guide arrangement may be arranged to be movable such as to extend at least through a range of +/-3 metres in relation to the still water level. The guide support may support the guide arrangement during adjustment of its vertical position. The vertical position of the guide arrangement may therefore be adjustable such that in the operating position the lower part of the guide arrangement is supported by the first and second lower supports.

The guide arrangement may be permanently attached to the guide support. The guide support may thereby be movable together with the guide arrangement.

The guide arrangement may comprise a guide rail, guide chain and/or guide wire for guiding and/or protecting the object during displacement. One or more of the guide rails, guide chains and/or guide wires may be used. It /they may be located on an inside of a side wall or a framework forming portions of the guide arrangement.

The deck structure of the semisubmersible unit may comprise an opening. In that case, the guide arrangement is preferably aligned with the opening, such that an object being launched through the opening can move downwards at least partly by means of the guide arrangement towards the ambient water. The guide arrangement may be located substantially centrally in relation to the deck structure. Alternatively, the guide arrangement may be located off-centre in relation to the deck structure. However, the guide arrangement and hence the guide arrangement are preferably located inboard, e.g. inboard of the support columns, e.g. within an envelope defined by the support columns. This is since the relative motions in the area located inboard of the support columns are lower than the relative motions outboard of the support columns.

The object to be displaced may e.g. be a well intervention equipment. A well intervention equipment is an equipment used for a well intervention operation. The well intervention equipment may thus be used for operations such as: inspection e.g. by a camera, perforating/re-perforating, zone isolation, inspection/repair/ installation of an insert downhole safety valve, milling of short scale bridges, well killing, pumping, scale treatment, selective tracer injection or sampling, exchange of gas lift valves, sleeve operations for DIACS valves (Downhole Instrumentation And Control System valves), plug and abandonment operations of subsea wells, exchange/installation of subsea trees, data gathering e.g. production logging tool string. The term well intervention equipment as used herein does not include drill pipes or drill risers. The support structure comprises at least one guide support and a first and a second lower support, all having elongated shapes. Having an elongated shape means that its length dimension is greater than its cross dimension, e.g. 5 times greater, preferably 10 times greater or more preferably 20 times greater. They may comprise a bar or a tubular structure.

A cross-sectional area of the lower support is defined as the cross-sectional area enclosed by an outer wall of the lower support. If the lower support comprises a tubular structure, a material cross-sectional area of the lower support, i.e. a cross-sectional area of the outer wall itself, is defined as the cross-sectional area enclosed by the outer wall minus the cross-sectional are of the inner cavity of the tubular structure. The cross- sectional area is determined for a cross-section perpendicular to the axial direction coinciding with the elongated shape. If the lower supports are shaped as circular tubes or bars, the cross-sectional area would be defined by the outer diameter d₂ as A= (d₂/2)²*TT. Similarly, a cross-sectional area of the support column is defined as the cross-sectional area enclosed by an outer wall of the support column. The material cross-sectional area of the support column, i.e. the cross-sectional area of the outer wall itself, is defined as the cross-sectional area enclosed by the outer wall minus the cross-sectional area of the inner space enclosed by the outer wall.

Further, the cross-sectional area of the lower support is preferably less than 10% of the largest cross-sectional area of the support columns, more preferably less than 5% and most preferably less than 1 %. Similarly, the material cross-sectional area of the lower support is preferably less than 10% of the largest material cross-sectional area of the support columns, more preferably less than 5% and most preferably less than 1 %.The lower supports are no wing pontoons. They may be shaped such that they will not take up much load from the water, but yet provide an appropriate support for the guide arrangement.

The angle formed between the first and second lower supports is defined as the angle between their respective axial directions coinciding with the elongated shape. If the shape of the lower support deviates from a straight shape, the angle is defined in relation to a straight line passing between the point where the lower support is connected to the float and/or the support column and the point where the lower support is connected to the guide support.

The lower support may be attached to the support column, e.g. to an edge of the support column, to a side wall of the support column or inside the support column. If the lower support is attached to the edge or the side wall of the support column, it may be fastened to the support column via a bracket, which is arranged such that the bracket takes substantially no vertical loads. As an alternative, or a complement, the lower support may be attached to a pontoon between a pair of support columns or to a bracing between a pair of support columns, with or without utilizing brackets.

The first and second lower support may form the angle with each other in a plane that is substantially parallel to the still water level. The plane may hence be substantially horizontal. The first and/or second lower support may connect a lower portion of the at least one guide support to the float and/or one of the support columns. Preferably the first and/or second lower support may connect an end of the at least one guide support to the float and/or one of the support columns. This will help to keep the guide support, and thereby the guide arrangement, in a desired position.

At least one of the lower supports may be adapted to take both tensile and compressive loads in the axial direction of its elongated shape. It may e.g. comprise a bar or a tubular structure. This differs from a wire, which is adapted to take tensile load, e.g. in

substantially the axial direction, but will bend out of the axial direction, if subjected to compressive loads in substantially the axial direction.

The angle may fulfil 30°< the angle < 150°, preferably 45°< the angle < (360 (number of support columns) +45°), more preferably 60°< the angle < (3607(number of support columns) +30°), most preferably the angle is within 3607(number of support columns) +/- 20°.

The at least one guide support may extend substantially parallel to the guide arrangement, preferably in a vertical direction. The guide support may be as long as the guide arrangement, or alternatively, shorter or longer. The guide support may e.g. extend up to at least the deck structure and in particular to the opening of the deck structure. The guide support may also extend vertically above the deck structure. The guide support may extend downwards from the deck structure. The float may comprise a pontoon connecting the at least two support columns, wherein the first lower support connects the at least one guide support to a first of the support columns and the second lower support connects the at least one guide support to a second of the support columns, wherein the first and second support columns are connected by the pontoon. This implies that the lower supports do not transfer any substantive load that may occur between pontoons of a semisubmersible unit.

Moreover, if the semisubmersible unit comprises a pontoon connecting the at least two support columns, a portion of the support structure together with a portion of the pontoon and, optionally, a portion of one or both of the at least two support columns may form a closed circumference. This is beneficial for providing the guide arrangement with a stable support.

The axial direction of the elongated shape of the first and/or second lower support may be substantially parallel to the still water level, e.g. horizontal.

The lower supports may be located above a transit draught level of the semisubmersible unit and/or below an operational draught level of the semisubmersible unit. Thereby they are out of water when transporting the semisubmersible unit, i.e. not influenced by the forces of the water. Then, when the semisubmersible unit is in an operational position, the first and second lower supports are below the still water level. They can thus help to support the guide support, which is preferably located below the still water level, such that the guide arrangement can help guiding and protecting the object to be launched or retrieved through the splash zone.

The guide support may comprise at least two guide support elements. In that case, the at least two guide support elements may be connected to each other by at least one connecting element, preferably located at substantially the same vertical level as where the lower supports are connected to the guide support. The at least one connecting element will help to keep the two guide supports in desired position relative to each other. Thereby the guide support elements together form a stable guide support. Preferably, the at least two guide support elements are parallel with each other. There may also be additional connecting elements between the two guide support elements at other vertical levels.

The guide support, the first support and/or the second lower support may be tubular. They are thus able to take both tensile and compressive loads. The tubular structure is known to have good tensile properties in relation to its weight, e.g. as regards bending stiffness. The guide support, the first support and/or the second lower support may be made of a material having a minimal yield strength of at least 355 MPa, such as structural steel having a minimal yield strength of at least 355 MPa.

In case the deck structure comprises an opening and the guide arrangement is aligned with the opening, the opening may have a cornered shape, e.g. being square or rect- angular and at least one of the at least two guide support elements may be located at a corner of the opening. Preferably the at least two guide support elements are located at a respective corner of the opening.

A first of the at least two guide support elements may be located at a first corner of the opening and a second of the at least two elements may be located at a second corner of the opening. In that case, the connecting element, which is described above, may have a length substantially corresponding to the distance between the first and second corners. Thereby, the two guide support elements may be connected at a lower portion by the connecting element and at an upper portion by the opening in the deck structure, which will help to keep the guide support elements in a fixed position relative to each other. If the connecting element has a length corresponding to the distance between the corners of the opening, the two guide support elements will extend in parallel, preferably in parallel with the guide arrangement. There may also be additional connecting elements between the two guide support elements at other vertical levels.

The guide arrangement may be arranged to engage with the object during displacement below the deck structure. If the object is to be launched into the ambient water, the object is preferably releasably attachable to the guide arrangement, such that the object can be released when it reaches a lower end of the guide arrangement. Alternatively, or in addition, the object may be released through an opening in a side wall of the guide arrangement.

The guide arrangement may extend at least through a range of +/-3 metres in relation to the still water level, i.e. from 3 metres above the still water level to 3 metres below the still water level. Preferably, the guide arrangement extends +1-5 metres in relation to the still water level. Thereby the object is protected and guided when passing the actual water level and through the splash zone. However, the guide arrangement does preferably not go all the way to the bottom of the sea. The guide arrangement and/or the support structure may be adapted to be used for holding or storing one or more instruments, e.g. in a vertical position. The instruments may be held in ready-to-use position, e.g. in a position such that they are parallel to the axis of the guide arrangement and/or the axis of the guide support. The instruments may have a diameter between 150 and 250 millimetres. Suitable holding or storing positions are along one of the lower supports or at the guide arrangement, either on the inside or outside thereof. The instruments may be located within a tube, which may comprise one or more instruments and which may function as a revolving magazine exposing one instrument at a time. The tube may be used to protect the instruments when being in the splash zone or passing through the splash zone. The instrument and/or the tube may be adapted to be lowered by means of a winch.

The semisubmersible unit may in addition to the first and second support columns mentioned above comprise a third support column and an additional support structure for supporting the guide arrangement. The additional support structure comprises at least one additional guide support and at least one additional lower support. The at least one additional guide support has an elongated shape and is substantially aligned with the guide arrangement. The at least one additional lower support has an elongated shape and form an angle β with the closest of the first and second lower supports, wherein 0< β< 180°, wherein the at least one additional lower support connects the at least one additional guide support to the float and/or the third support column. The semisubmersible unit may comprise three, four, five, six or more support columns. In particular, it is useful for a semisubmersible unit comprising three support columns.

In an embodiment, the semisubmersible comprises a first and a second pair of support columns. The support columns of each pair are connected with a respective first and a second pontoon. The semisubmersible unit includes at least two of the support structures, as described above. Each support structure comprises at least one guide support and a first and a second lower support forming an angle with each other, the first lower support connecting the at least one guide support to a first of the pair of support columns and the second lower support connecting the at least one guide support to a second of the pair of support columns. In particular, this is useful for a semisubmersible unit comprising four support columns.

In a second aspect of the present disclosure, there is provided a method for displacement of an object, e.g. a well intervention equipment, in relation to a semisubmersible unit, e.g. launching or raising the object into or from the ambient water. The semisubmersible unit comprises a float located below a still water level, a deck structure located above the still water level, and at least two support columns connecting the float and the deck structure, such that the still water level intersects the support columns. The method comprises

- displacing the object by means of a guide arrangement being at least partly located between the deck structure and the float. The guide arrangement is supported by a support structure comprising at least one guide support and a first and a second lower support. The at least one guide support has an elongated shape and is adapted to support the guide arrangement. The first and second lower supports have elongated shapes and form an angle with each other, wherein 0< the angle < 180°. Each of the lower supports connects the at least one guide support to the float and/or one of the support columns.

The method may further comprise

- engaging the guide arrangement with the object during displacement. The method may also comprise

- releasing the object from the guide arrangement, when reaching a lower end of the guide arrangement.

Alternatively, or in addition, the object may be released through an opening in the side wall of the guide arrangement.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be further explained by means of non-limiting examples with reference to the appended drawings wherein:

Fig. 1 is a schematic view of a semisubmersible unit according to a first embodiment of the invention, viewed in cross section from the side,

Fig. 2 is a horizontal cross-section of the semisubmersible unit of Fig. 1 ,

Fig. 3 is a schematic view of a semisubmersible unit according to a second embodiment, viewed in cross section from the side,

Fig. 4 is a horizontal cross-section of the semisubmersible unit of Fig. 2, Fig. 5 is a horizontal cross-section of a semisubmersible unit according to a third embodiment,

Fig. 6 is a detail of a horizontal cross-section of a semisubmersible unit according to a fourth embodiment, and

Fig. 7 is a horizontal cross-section of a semisubmersible unit according to a fifth

embodiment. Fig. 8 is a horizontal cross-section of a semisubmersible unit according to a sixth

embodiment, and

Fig. 9 is a horizontal cross-section of a semisubmersible unit according to a seventh embodiment.

It should be noted that the appended drawings are not necessarily drawn to scale and that the dimensions of some features of the present disclosure may have been exaggerated for the sake of clarity. DETAILED DESCRIPTION

The disclosure will, in the following, be exemplified by embodiments. It should however be realized that the embodiments are included in order to explain principles of the invention and not to limit the scope of the invention, defined by the appended claims. Details from two or more of the embodiments may be combined with each other.

Figures 1 and 2 schematically illustrate a semisubmersible unit 2 according to a first embodiment of the invention: Figure 1 illustrates a cross section of the semisubmersible unit 2 viewed from the side and Figure 2 illustrates a horizontal cross-section. The semisubmersible unit 2 comprises a deck structure 4, which may comprise a plurality of different decks located at different levels. On the different decks, equipment used in relation to hydrocarbon drilling and production operations may be provided, as common in the field. Also spaces such as accommodation spaces and control rooms may be provided on the deck structure 4. A main deck 6, often also referred to as operation deck, is located relatively high above a still water level of the sea, preferably so high that waves will normally not reach the main deck 6. The semisubmersible unit 2 further comprises a float 8, here illustrated in the form of a plurality of pontoons 10, 12, usually fully immersed in the water at a distance below the still water level. The float 8 provides buoyancy, which keeps the semisubmersible unit 2 partially submersed in the water. The semisubmersible unit 2 further comprises two or more support columns 14, 16, 18, 20, connecting the deck structure 4 with the float 8. One or more of the columns 14, 16, 18, 20 may also provide buoyancy.

The deck structure 4 of the illustrated semisubmersible unit 2 comprises an opening 22 for launching or retrieving equipment. The illustrated semisubmersible unit 2 of Figures 1 and 2 comprises four support columns 14, 16, 18, 20, one at each corner of the semisubmersible unit 2, but it would, according to the invention, also be feasible to use three support columns, or five, six or more. The still water level intersects each column 14, 16, 18, 20 during normal operation of the semisubmersible unit 2.

The semisubmersible unit 2 is further provided with a ballast system, which is known in the art. By using the ballast system, the draught level of the semisubmersible unit can be adjusted. That is, the degree of submersion of the semisubmersible unit can be adjusted, i.e. its intersection with the still water level can be adjusted. During transport of the semi- submersible unit 2, the semisubmersible unit 2 is usually located relatively high in the water, often with the draught level close to the upper level of the float 8. This is commonly referred to as transit position, having the draught located at a transit draught level 24. When the semisubmersible unit 2 is not being transported, for example when it is used during operations, such as drilling or oil production, it is lowered to an operation position. The still water level now intersects the support columns 14, 16, 18, 20 at the operation draught level 26. The position of the operation draught level 26 may vary depending on the type of operation performed, weather conditions, structure and dimensions of the semisubmersible unit, etc. In the illustrated example, the operation draught level 26 may be in the range of 10-20 metres measured from a base line 28 of the semisubmersible unit 2, i.e. from the underside of the float 8.

The semisubmersible unit 2 comprises a guide arrangement 30, here in the form of a framework structure. As an alternative, or a complement, the guide arrangement may comprise a substantially closed side wall. The guide arrangement 30 extends downwards from the deck structure 4. The guide arrangement 30 is aligned with the opening 22 in the deck structure 4, such that an object (not illustrated) being launched through the opening 22 can move downwards while being guided by the guide arrangement 30. The object may e.g. be a well intervention equipment (not illustrated).

5 In the illustrated example, the guide arrangement 30 is located centrally in relation to the deck structure 4, but it would also be feasible with an off-centre location. The guide arrangement 30 is anyway preferably located inboard the support columns 14, 16, 18, 20, i.e. between the support columns 14, 1 6, 18, 20, e.g. within an envelope defined by the support columns 14, 1 6, 1 8, 20. The guide arrangement 30 preferably extends at least 10 through a range of +/-3 metres in relation to the still water level, i.e. from three metres above the still water level to three metres below the still water level and more preferably extends +1-5 metres in relation to the still water level. Thereby the object is protected and/or guided when passing the actual water level and through the splash zone.

15 The guide arrangement 30 is adapted to guide equipment being launched into the water or raised from the water. The guide arrangement 30 extends substantially vertically downwards. A portion of the framework, or the side wall, may be used to guide the object to be displaced, or there may, optionally, be a guide rail, a guide chain, a guide wire or similar located at the framework, or the side wall, forming part of the guide arrangement

20 30. The guide arrangement 30 is arranged to engage with the object during displacement below the deck structure 4 in order to safely guide the object through the air gap and the splash zone. If the object is to be launched into the ambient water, it is preferably releasably attached to the guide arrangement 30, such that the object can be released from the guide arrangement 30, when it reaches a lower end 32 of the guide arrangement

25 30. Alternatively, or in addition, the object may be released through an opening at the side of the guide arrangement 30. The guide arrangement 30 makes it possible to guide the object through the splash zone with a reduced influence from weather, wind and waves, as compared to a semisubmersible unit having no guide arrangement.

30 A support structure 34 supports the guide arrangement 30. The support structure 34 helps to reduce, or avoid, any lateral swaying of the guide arrangement 30. The support structure 34 comprises a guide support 36, which has an elongated shape with an axial direction A1 being substantially parallel to an axial direction A2 of the guide arrangement 30. The guide support 36 may e.g. have a tubular cross-section. Preferably, the guide support 36 is located at one of the sides of the framework of the guide arrangement 30, e.g. at its outside as illustrated for the first embodiment in Figures 1 and 2.

The support structure 34 further comprises a first lower support 38 and a second lower support 40. The first and second lower supports 38, 40 have elongated shapes with axial directions A3 and A4. The first lower support 38 connects the guide support 36 to the first support column 14 by means of a first bracket 39 connected to a side wall of the first support column 14. Similarly, the second lower support 40 connects the guide support 36 to the second support column 16 by means of a second bracket 41 connected to a side wall of the second support column 16. The lower supports 38, 40 are hence fixedly connected to the support columns. The first and second lower supports 38, 40 thus connect two support columns 14, 16 being joined by the pontoon 10. The brackets 39, 41 are preferably adapted such that they substantially do not transfer vertical loads from the support column 14, 16 to the lower support 38, 40.

It is preferred that the support structure 34 is arranged such that the guide arrangement 30 is supported at a lower portion, more preferably at its lower end 32, or close to its lower end 32, by the first and second lower supports 38, 40. The first and second lower supports 38, 40 extend in a substantially horizontal direction. Further, the first and second lower supports 38, 40 are located above the transit draught level 24 and below the operation draught level 26. Thereby they are out of water when transporting the semisubmersible unit 2, i.e. not influenced by the forces of the water. Then, when the semisubmersible unit is in an operational position, the first and second lower supports 38, 40 are below the still water level. They can thus help to support the lower end 32 of the guide support 36, which is preferably located below the still water level, such that the guide arrangement 30 can help guiding and protecting the object to be launched or retrieved through the splash zone. In the illustrated embodiment, the lower support is shaped as a circular tube having an inner diameter and an outer diameter d₂. The cross-sectional area of the lower support 38, 40 is defined as the cross-sectional area enclosed by the outer wall of the lower support 38, 40, hence defined by the outer diameter d₂ as A= (d₂/2)²*TT. The material cross-sectional area of the lower support 38, 40, i.e. the cross-sectional area of the outer wall itself, is defined as the cross-sectional area enclosed by the outer wall minus the cross-sectional area of the inner cavity, in this case defined by (d₂/2)^2*n -(αΊ/2)^2*ττ.

Similarly, the cross-sectional area of the support column 14, 16, 18, 20 is defined as the cross-sectional area enclosed by the outer wall. The support column has a maximal transverse inner dimension d₃ and a maximal transverse outer dimension d₄. In the illustrated embodiment, the maximal transverse dimension coincides with the diagonal. The material cross-sectional area of the support column, i.e. the cross-sectional area of the outer wall itself, is defined as the cross-sectional area enclosed by the outer wall minus the cross-sectional are of the inner space enclosed by the outer wall.

The lower supports 38, 40 are adapted to be able to take both tensile and compressive loads. They may comprise a bar or a tubular structure. The elongated shape means that its length dimension is greater than its cross dimension, e.g. 5 times greater, preferably 10 times greater or more preferably 20 times greater. Further, the cross-sectional area taken as a cross-section perpendicular to the axial direction A3, A4 is less preferably less than 10%, more preferably less than 5% and most preferably less than 1 % of the largest cross- sectional area of the support columns 14, 16, 18, 20. Similarly, the material cross- sectional area of the lower support 38, 40 is preferably less than 10% of the largest material cross-sectional area of the support columns 14, 16, 18, 20, more preferably less than 5% and most preferably less than 1 %.The lower supports 38, 40 are hence shaped such that they will not take up much load from the water, but yet provide an appropriate support for the guide arrangement 30.

The axial directions A3 and A4 form an angle ai with each other when seen in a plane that is substantially parallel to said still water level, as in Figure 2. The angle ai is greater than 0° and less than 180°. In the illustrated example the angle ch is about 100°.

The first and second lower supports 38, 40 form together with portions of the first column 14, the first pontoon 10 and the second column 16 a closed circumference, which is beneficial for providing the guide arrangement 30 with a stable support.

Even if the illustrated example of Figures 1 and 2 shows that the first and second lower supports 38, 40 connect the guide support 36 to an edge of the first and second support columns 14, 16, it would also be feasible to connect the guide support 36 to a side wall of the first and second support columns 14, 16 or to the first pontoon 10 between the first and second support columns 14, 16.

Further, even if the illustrated example of Figures 1 and 2 shows that the first and second 5 support columns 14, 16 have an almost square cross-section, the support columns may also have a circular, elliptic, oval, rectangular, hexagonal, octagonal or any other polygonal cross-section. The lower supports may in that case connect to a corner, if the support column has a polygonal cross-section, or they may connect to a side wall of the support column.

10

The support structure 34 of the first embodiment is an example of a 3-dimensional connection, which is able to restrict movements in all three dimensions, holding the guide arrangement 30 in a defined 3-dimensional position relative to the first and second support columns 14, 16.

15

The guide arrangement 30 and/or the support structure 34 may be adapted to hold one or more instruments. The instruments may have a diameter between 150 and 250 mm. The instruments may be held in ready-to-use position, e.g. in a position such that they are parallel to the axis A2 of the guide arrangement 30 and/or the axis A1 of the guide support

20 36. Suitable positions are P1 along one of the lower supports 38, 40, or at the guide

arrangement 30, either on the inside or outside P2. The instruments may be located within a tube, which may comprise one or more instruments, or which may function as a revolving magazine exposing one instrument at a time. The tube may be used to protect the instruments when being in the splash zone or passing through the splash zone. The

25 instrument and/or the tube may be adapted to be lowered by means of a winch.

The semisubmersible unit 2 further comprises a third support column 18 and a fourth support column 20 connected by the second pontoon 12. The semisubmersible unit 2 may as a complement, or an alternative, comprise an additional support structure 42, which is

30 similar to the support structure 34 described above. The additional support structure 42 comprises an additional guide support 44, which has an elongated shape with an axial direction A5 being substantially parallel to the axial direction A2 of the guide arrangement 30. The additional support structure 42 further comprises a first lower support 46 and a second lower support 48 connected to the third support and fourth support columns 18, 20

35 by means of brackets 47 and 49. The third support column 18 and the fourth support column 20 have axial directions A6 and A7 forming an angle a₂ with each other, wherein 0< a₂ < 180°. In the illustrated example the angle a₂ is about 100°, i.e. substantially as large as the angle ai . Further, the axes A3 and A6 of the first lower supports 38, 46 of the respective support structure 34 and additional support structure 42 form an angle βι with 5 each other and the axes A4 and A7 of the second lower supports 40, 48 of the respective support structure 34 and additional support structure 42 form an angle β₂ with each other. In the illustrated example these angles βι , β₂ are about 80°.

The first and second lower supports 46, 48 form together with portions of the third column 10 18, the second pontoon 12 and the fourth column 20 a closed circumference, which is beneficial for providing the guide arrangement 30 with a stable support.

By connecting the first and second lower supports 38, 40 of the support structure 34 to the first and second support column 14, 16 connected by the first pontoon 10, and connecting 15 the first and second lower supports 46, 48 of the additional support structure 42 to the third and fourth support columns 18, 20 connected by the second pontoon 12, the risk of introduction of global loads to the semisubmersible unit 2, such as racking, is reduced or avoided.

20 Figures 3 and 4 schematically illustrate a semisubmersible unit 2a according to a second embodiment of the invention comprising a support structure 34a. Figure 3 illustrates a cross section of the semisubmersible unit 2a viewed from the side and Figure 4 illustrates a horizontal cross-section. Components and structures being common for the first and second embodiments are denoted with the same reference numbers.

25

Similar to the semisubmersible unit 2 of the first embodiment, the semisubmersible unit 2a of the second embodiment also comprises four support columns 14, 16, 18, 20, one at each corner of the semisubmersible unit 2a. The support columns 14, 16, 18, 20 are pairwise connected by the first 10 and the second pontoon 12.

30

The support structure 34a comprises a first and a second lower support 38a, 40a and a guide support 36a. The guide support 36a of the second embodiment differs from that of the first embodiment in that it comprises two guide support elements 50, 52 instead of one guide support 36 as in the first embodiment. The two guide support elements 50, 52 are 35 preferably arranged such that they are located in a respective corner of the opening 22 in the deck structure 4, which in the second embodiment is assumed to be square or rectangular. The first and second lower supports 38a, 40a are connected to the respective support columns 14, 16 by means of brackets 39a, 41 a. The support structure 34a further comprises a connecting element 54 located between the two guide support elements 50, 52, preferably at the same level, or substantially the same level as where the first and second lower supports 38a, 40a are connected to the respective guide support elements 50, 52. Thereby the two guide support elements 50, 52 are connected at a lower portion by the connecting element 54 and at an upper portion by the opening 22 in the deck structure 4, which will help to keep the guide support elements 50, 52 in a fixed position relative to each other. Preferably the connecting element 54 has a length corresponding to the distance between the corners of the opening 22, such that the two guide support elements 50, 52 extend in parallel, preferably in parallel with the guide arrangement 30, as is illustrated. There may also be, however not illustrated, additional connecting elements between the two guide support elements 50, 52 at other vertical levels.

The first and second lower supports 38a, 40a have elongated shapes with axial directions A3a and A4a. The axial directions A3a and A4a form an angle a-i_a with each other when seen in a plane that is substantially parallel to said still water level, as in Figure 4. The angle a-i_a is greater than 0° and less than 180°. In the illustrated example the angle a-i_a is about 90°.

Similar as for the first embodiment, the guide arrangement 30 and/or the support structure 34a may be adapted to hold one or more instruments. The instruments may be held in ready-to-use position, e.g. in a position such that they are parallel to the axis A2 of the guide arrangement 30 and/or the axes of the guide support elements 50, 52. Suitable positions are P1 along one of the lower supports 38a, 40a, at the guide arrangement 30, either on the inside or outside, see position P2, or along the connecting element 54. The instruments may be located within a tube, which may comprise one instrument or which may function as a revolving magazine exposing one instrument at a time. The tube may be used to protect the instruments when being in the splash zone or passing through the splash zone. The instrument and/or the tube may be adapted to be lowered by means of a winch. The semisubmersible unit 2a may as a complement, or an alternative, comprise an additional support structure 42a being similar to the support structure 34a already described. The additional support structure 42a comprises two guide support elements 56, 58, which are located at a respective corner of the opening 22 in the deck structure 4. Hence, assuming that the opening 22 is square or rectangular, there will be a guide support element 50, 52, 56, 58 in each corner. The guide support elements 56, 58 of the additional support structure 42a are connected by means of lower supports 46a, 48a and brackets 47a, 49a to the third and fourth support columns 18, 20. A connecting element 59 connects the guide support elements 56, 58. As an option, however not illustrated, there may be a connecting element also between a guide support element of the support structure 34 and an adjacent guide support element of the additional support structure 42, e.g. 50 to 56, or 52 to 58.

Figure 5 illustrates a horizontal cross-section of a semisubmersible unit 2b according to a third embodiment of the invention comprising a support structure 34b. Components and structures being similar with the first and second embodiments are denoted with the same reference numbers. The third embodiment differs from the second embodiment in that the first and second lower supports 38b, 40b of the support structure 34b are attached to the side wall of the first and second 14, 16 support columns instead of to the corner as for the second embodiment. Also the third embodiment may comprise an additional support structure 42b.

Figure 6 illustrates a detail of a horizontal cross-section of a fourth embodiment of the invention. The fourth embodiment differs from the second embodiment in that the lower supports 38c, 40c, 46c, 48c of the support structure 34c, 42c extend through the side wall of the support columns 14, 16, 18, 20 such that the lower supports 38c, 40c, 46c, 48c are fastened at the inside of the support columns 14, 16, 18, 20.

Figure 7 illustrates a horizontal cross-section of a fifth embodiment of the invention. The fifth embodiment differs from the second embodiment in that the first lower support 38d of the support structure is connected to a first bracing 60, which connects the first support column 14 and the third support column 18. In a similar way, the second lower support 40d of the support structure is connected to a second bracing 61 , which connects the second support column 16 and the fourth support column 20. Figure 8 illustrates a semisubmersible unit 62 according to a sixth embodiment of the invention having three support columns 64, 65, 66. The semisubmersible unit 62 has an opening in the deck structure and a guide arrangement 68 aligned with the opening. The guide arrangement 68 may have a round shape or oval shape or it may be polygonal e.g. 5 hexagonal. In the illustrated example, the guide arrangement 68 is round and is supported by a support structure 70 comprising a guide support 71 with three guide support elements 72, 74, 76. Each guide support element 72, 74, 76 is connected to the closest of the three support columns 64, 65, 66 by means of a first lower support 78, a second lower support 80 and a third lower support 82. Further, the three guide support elements 72, 74, 10 76 are connected to each other, e.g. by means of a connection element, in the form of a connection ring 84, which may have the same shape as the opening in the deck structure. The three support columns 64, 65, 66 may be pairwise connected, e.g. by pontoons 86, 88, 90. The axial directions of the three guide support elements 72, 74, 76 form angles β_{1 ;} β₂ of about 120° with each other.

15

Figure 9 illustrates a semisubmersible unit 62a according to a seventh embodiment of the invention also having three support columns 64, 65, 66 like the fourth embodiment and a support structure 70a. The fifth embodiment differs from that of the fourth embodiment in that each of the three guide support elements 72, 74, 76 is connected by a pair of lower 20 support elements 92, 94 to the support columns 64, 65, 66 and in that the connection ring 84 may be omitted. The axial directions of the lower support elements 92, 94 form an angle γ to each other, 0< γ < 180°.

In another aspect of the invention, there is disclosed a method for displacement of an 25 object, e.g. a well intervention equipment, in relation to the semisubmersible unit e.g. launching or raising the object. The method comprises

- displacing the object by means of the guide arrangement 30, 68, which is at least partly located between the deck structure 4 and the float 8. The guide arrangement 30, 68 is supported by a support structure 34, 34a-d, 70, 70a comprising at least one guide support

30 36, 36a, 71 and a first 38, 38a-d, 78, 92 and a second 40, 40a-d, 80, 94 lower support, as described above.

The method may further, as an option, comprise

- engaging the guide arrangement 30, 68 with the object during displacement.

35 In that case, the method may comprise

- releasing the object from the guide arrangement 30, 68 when reaching the lower end 32 of the guide arrangement 30, 68. Further modifications of the invention within the scope of the appended claims are feasible. As such, the present invention should not be considered as limited by the embodiments and figures described herein. Rather, the full scope of the invention should be determined by the appended claims, with reference to the description and drawings. In particular, any of the shown attachments of the lower support to the edge of the support column, to the side wall of the support column, inside the support column, to the bracing or the pontoon between support columns, with our without utilizing brackets, may be combined with any of the guide supports and/or any guide arrangement.

Claims

A semisubmersible unit (2, 2a-d, 62, 62a), adapted to float in a body of water, for use in offshore applications comprising

- a float (8) located below a still water level,

- a deck structure (4) located above said still water level, and

- at least two support columns (14, 16, 18, 20; 64, 65, 66) connecting said float (8) and said deck structure (4), such that said still water level intersects said support columns (14, 16, 18, 20; 64, 65, 66),

characterized in that

said semisubmersible unit (2, 2a-d, 62, 62a) further comprises

- a guide arrangement (30, 68) for guiding an object during displacement from said deck structure (4) to said body of water, said guide arrangement (30, 68) being at least partly located between said deck structure (4) and said float (8), and

- a support structure (34, 34a-d, 70, 70a) for supporting said guide arrangement (30, 68), said support structure (34, 34a-d, 70, 70a) comprising:

- at least one guide support (36, 36a, 71 ) having an elongated shape and being adapted to support said guide arrangement (30, 68), and

- a first (38, 38a-d, 78, 92) and a second (40, 40a-d, 80, 94) lower support having elongated shapes and forming an angle (ai , αι_&,γ) with each other, wherein 0 < said angle (α-ι , αι_&,γ) < 180°, each one of said lower supports connecting said at least one guide support to said float (8) and/or one of said support columns (14, 16, 18, 20; 64, 65, 66).

The semisubmersible unit (2, 2a-d, 62, 62a) according to claim 1 , wherein said first (38, 38a-d, 78, 92) and second (40, 40a-d, 80, 94) lower support form said angle (α-ι , a_1a,Y) with each other in a plane that is substantially parallel to said still water level.

The semisubmersible unit (2, 2a-d, 62, 62a) according to claim 1 or claim 2, wherein said first (38, 38a-d, 78, 92) and/or second (40, 40a-d, 80, 94) lower support connects a lower portion of said at least one guide support (36, 36a, 71 ) to said float (8) and/or one of said support columns (14, 16, 18, 20; 64, 65, 66), preferably connecting an end of said at least one guide support (36, 36a, 71 ) to said float (8) and/or one of said support columns (14, 16, 18, 20; 64, 65, 66). The semisubmersible unit (2, 2a-d, 62, 62a) according to any one of the preceding claims, wherein at least one of said lower supports (38, 38a-d, 78, 92; 40, 40a-d, 80, 94) is adapted to take both tensile and compressive loads in an axial direction (A3, A4, A3a, A4a) of its elongated shape.

The semisubmersible unit (2, 2a-d, 62, 62a) according to any one of the preceding claims, wherein 30°< the angle (ai , αι_&,γ) < 150°, preferably 45°< the angle < (360 (number of support columns) +45°), more preferably 60°< the angle < (360 (number of support columns) +30°), most preferably the angle is within 3607(number of support columns) +/- 20°.

The semisubmersible unit (2, 2a-d, 62, 62a) according to any one of the preceding claims, wherein said at least one guide support (36, 36a, 71 ) is extending substantially parallel to said guide arrangement (30, 68), preferably in a vertical direction.

The semisubmersible unit (2, 2a-d, 62, 62a) according to any one of the preceding claims, said float (8) comprising a pontoon (10, 12; 86, 88, 90) connecting said at least two support columns (14, 16, 18, 20; 64, 65, 66), wherein said first lower support (38, 38a-d, 78, 92) connects said at least one guide support (36, 36a, 71 ) to a first of said support columns (14, 16, 18, 20; 64, 65, 66) and said second lower support (40, 40a-d, 80, 94) connects said at least one guide support (36, 36a, 71 ) to a second of said support columns (14, 16, 18, 20; 64, 65, 66), wherein said first and second support columns (14, 16, 18, 20; 64, 65, 66) are connected by said pontoon (10, 12; 86, 88, 90).

The semisubmersible unit (2, 2a-d, 62, 62a) according to any one of the preceding claims, said float (8) comprising a pontoon (10, 12; 86, 88, 90) connecting said at least two support columns (14, 16, 18, 20; 64, 65, 66) wherein a portion of said support structure (34, 34a-d, 70, 70a) together with a portion of said pontoon (10, 12) and, optionally, a portion of one or both of said at least two support columns (14, 16, 18, 20; 64, 65, 66) form a closed circumference. The semisubmersible unit (2, 2a-d, 62, 62a) according to any one of the preceding claims, wherein said axial direction (A3, A4, A3a, A4a) of said elongated shape of said first (38, 38a-d, 78, 92) and/or second (40, 40a-d, 80, 94) lower support is substantially parallel to said still water level.

The semisubmersible unit (2, 2a-d, 62, 62a) according to any one of the preceding claims, wherein said lower supports (38, 38a-d, 78, 92; 40, 40a-d, 80, 94) are located above a transit draught level (24) of said semisubmersible unit (2, 2a-d, 62, 62a) and/or below an operational draught level (26) of said semisubmersible unit (2, 2a-d, 62, 62a).

1 1 . The semisubmersible unit (2a-d, 62, 62a) according to any one of the preceding claims, wherein said guide support (36a, 71 ) comprises at least two guide support elements (50, 52; 72, 74, 76), preferably said at least two guide support elements (50, 52; 72, 74, 76) being substantially parallel.

12. The semisubmersible unit (2a-d, 62, 62a) according to claim 1 1 , wherein said at least two guide support elements (50, 52; 72, 74, 76) are connected to each other by a connecting element (54, 84) preferably located at substantially the same vertical level as where said lower supports (38a-d, 78, 92; 40a-d, 80, 94) are connected to said guide support.

13. The semisubmersible unit (2, 2a-d, 62, 62a) according to any one of the preceding claims, wherein said guide support (36, 36a, 71 ), said first (38, 38a-d, 78, 92) lower support and/or said second (40, 40a-d, 80, 94) lower support are tubular.

14. The semisubmersible unit (2, 2a-d, 62, 62a) according to any one of the preceding claims, wherein said cross-sectional area of each one of said first lower support (38, 38a-d, 78, 92) and said second lower support (40, 40a-d, 80, 94) is preferably less than 10%, more preferably less than 5% and most preferably less than 1 % of the largest cross-sectional area of the support columns (14, 16, 18, 20; 64, 65, 66).

15. The semisubmersible unit (2, 2a-d, 62, 62a) according to any one of the preceding claims, wherein said guide support (36, 36a, 71 ) and/or said lower supports (38, 38a-d, 78, 92; 40, 40a-d, 80, 94) are made of material having a minimal yield strength of at least 355 MPa, such as steel having a minimal yield strength of at least 355 MPa. 16. The semisubmersible unit (2, 2a-d, 62, 62a) according to any one of the preceding claims, wherein said deck structure (4) comprises an opening (22), said guide arrangement (30, 68) being aligned with said opening (22).

17. The semisubmersible unit (2a-d, 62, 62a) according to claim 1 6 when dependent on claim 1 1 , said opening (22) having a cornered shape, e.g. being square or rectangular, wherein at least one of said at least two guide support elements (50, 52), is located at a corner of said opening (22), preferably said at least two guide support elements (50, 52) are located at a respective corner of said opening (22). 18. The semisubmersible unit (2a-d, 62, 62a) according to claim 1 7 when dependent on claim 1 2, wherein a first (50) of said at least two guide support elements is located at a first corner of said opening (22) and a second (52) of said at least two guide support elements is located at a second corner of said opening (22), said connecting element (54) having a length substantially corresponding to the distance between said first and second corners.

19. The semisubmersible unit (2, 2a-d, 62, 62a) according to any one of the preceding claims, wherein said object is a well intervention equipment.

20. The semisubmersible unit (2, 2a-d, 62, 62a) according to any one of the preceding claims, wherein said guide arrangement (30, 68) is arranged to engage with said object during displacement below said deck structure (4), said object being releasably attachable to said guide arrangement (30, 68).

21 . The semisubmersible unit (2, 2a-d, 62, 62a) according to any one of the preceding claims, wherein said guide arrangement (30, 68) and/or said at least one guide support (36, 36a, 71 ) extend at least through a range of +/-3 metres in relation to said still water level, preferably +1-5 metres in relation to said still water level.

22. The semisubmersible unit (2, 2a-d, 62, 62a) according to any one of the preceding claims, wherein said guide arrangement (30, 68) and/or said at least one guide support (36, 36a, 71 ) extend downwards from said deck structure (4).

23. The semisubmersible unit (2, 2a-d, 62, 62a) according to any one of the preceding claims, wherein said guide arrangement (30, 68) is located substantially centrally in relation to said deck structure (4).

24. The semisubmersible unit (2, 2a-d, 62, 62a) according to any one of claims 1 to 22, wherein said guide arrangement (30, 68) is located off-centre in relation to said deck structure (4).

25. The semisubmersible unit (2, 2a-d, 62, 62a) according to any one of the preceding claims, wherein said guide arrangement (30, 68) and/or said support structure (34, 34a-d, 70, 70a) are adapted to be used for holding or storing one or more instruments, e.g. in a vertical position.

26. The semisubmersible unit (2, 2a-d, 62, 62a) according to any one of the preceding claims further comprising a third support column (18; 66) and an additional support structure (42, 42a, 42b, 42c, 42d) for supporting said guide arrangement (30, 68), said additional support structure (42, 42a, 42b, 42c, 42d) comprising

- at least one additional guide support (44) having an elongated shape and being substantially aligned with said guide arrangement (30, 68), and

- at least one additional lower support (46, 46a, 48, 48a, 82)

having an elongated shape and forming an angle β with the closest of said first and second lower support (38, 38a, 78; 40, 40a, 80), wherein 0< β< 180°, said at least one additional lower support (46, 46a, a, 82) connecting said at least one additional guide support (44) to said float (8) and/or said third support column (18, 66).

27. The semisubmersible unit (2, 2a, 2b) according to any one of the preceding

claims, said semisubmersible unit (2, 2a, 2b) semisubmersible comprising a first and a second pair of support columns (14, 16 and 18, 20), wherein said support columns (14, 16 and 18, 20) of each pair are connected with a respective first and a second pontoon (10, 12), said semisubmersible unit (2, 2a, 2b) including at least two of said support structures (34, 34a, 34b, 34a, 34b), each support structure (34, 34a, 34b) comprising at least one guide support (36, 36a) and a first and a second lower support (38, 38a, 38b; 40, 40a, 40b) forming an angle (ai , ai_a) with each other, said first lower support (38, 38a, 38b) connecting said at least one guide support (36, 36a) to a first of said pair of support columns (14, 18) and said second lower support (40, 40a, 40b) connecting said at least one guide support (36, 36a) to a second (16, 20) of said pair of support columns.

28. A method for displacement of an object in relation to a semisubmersible unit (2, 2a-d, 62, 62a), said semisubmersible unit (2, 2a-d, 62, 62a) comprising

- a float (8) located below a still water level,

- a deck structure (4) located above said still water level, and

characterized in that said method comprises

- displacing said object by means of a guide arrangement (30, 68) being at least partly located between said deck structure (4) and said float (8), said guide arrangement (30, 68) being supported by a support structure (34, 34a-d, 70, 70a) comprising

- a first (38, 38a-d, 78, 92) and a second (40, 40a-d, 80, 94) lower support having elongated shapes and forming an angle (α-ι , αι_&,γ) with each other, wherein 0< the angle (ai , αι_&,γ) < 180°, each one of said lower supports connecting said at least one guide support (36, 36a, 71 ) to said float (8) and/or one of said support columns (14, 16, 18, 20; 64, 65, 66).

29. The method according to claim 28 further comprising:

- engaging said guide arrangement (30, 68) with said object during displacement.

30. The method according to claim 29 further comprising:

- releasing said object from said guide arrangement (30, 68), when reaching a lower end (32) of said guide arrangement (30, 68).