WO2011072687A1

WO2011072687A1 - A shallow water system

Info

Publication number: WO2011072687A1
Application number: PCT/DK2010/050323
Authority: WO
Inventors: Claus Dencker Christensen
Original assignee: Nkt Flexibles I/S
Priority date: 2009-12-16
Filing date: 2010-11-26
Publication date: 2011-06-23
Also published as: EP2512911A4; AU2010333428B2; IN2012DN05149A; BR112012014650A2; BR112012014650B1; EP2512911B1; CN102762444A; MY158414A; CN102762444B; EP2512911A1; AU2010333428A1

Abstract

The invention relates to a shallow water system comprising a floating weathervaning vessel (3) with a topside hang-off structure, a subsea structure and at least one flexible transporting unit (2) extending from the topside hang-off structure to the subsea structure (1). The weathervaning vessel (3) has a bow with a lowermost bow midpoint. The weathervaning vessel is moored such that at still water it can weathervane around a weathervaning vertical centre line, such that the position of the lowermost bow midpoint follows an annular lowermost bow line which defines a vertical tube shaped border to a lowermost bow midpoint exclusion zone (LE-zone). The subsea structure (1) is arranged in said LE-zone. The weathervaning vertical centre line (9) is preferably the nominal weathervaning vertical centre line. The weathervaning vessel (3) is generally a line moored weathervaning vessel. The shallow water system of the invention provides a simple system for design with a high safety against damaging of flexible transporting unit (2). The shallow water system is in particular beneficial for use at very shallow water.

Description

A shallow water system

TECHNICAL FIELD

The present invention relates to a shallow water system comprising a floating weathervaning vessel, a subsea structure and one or more flexible

transporting units extending there between.

BACKGROUND ART

Shallow water systems of the present type are well know in the art and are generally used for transporting fluids, electricity, electromagnetic waves and other media which are transferred to or from a floating weathervaning vessel. A floating weathervaning vessel from which one or more flexible transporting units are leading to a subsea structure usually comprises an external turret system or an internal turret system. The internal turret system leads the flexible transporting units through the hull bottom whereas the external turret system leads the flexible transporting unit from a topside hang-off structure extending beyond the hull and the rail of the vessel. The present invention in particular relates to a shallow water system comprising an external turret system or another similar mooring system for anchoring the weathervaning vessel by lines.

The weathervaning vessel is usually moored to the seabed but may also in certain situations be moored by line to one or more fixed structures. The weathervaning vessel is moored such that it can weathervane around a vertical centre line and simultaneously the weathervaning vessel have large freedom to move to adapt to the forces applied to the weathervaning vessel e.g. by wind, water current and waves. The weathervaning vessel may for example move forth (near position) and back (far position) in relation to a nominal position, which is defined as the position of the weathervaning vessel when under no influences by the weather, i.e. wind, water current, waves and etc. In shallow water the weather conditions have a vast influence on movements of a vessel and even though the weathervaning vessel is moored the movements of the weathervaning vessel may become quite considerable.

US 2004/0028477 describes a system comprising a weathervaning vessel with a topside hang-off structure, a subsea structure and at least one flexible transporting unit extending from the topside hang-off structure to the subsea structure. In order to ensure that the weathervaning vessel does not collide with the subsea structure, the subsea structure has to have its uppermost point at a position far below the hull bottom even at low tide. This severely restricts the possibly length of the transporting unit in situations where the system is placed at shallow water.

In order to provide the flexible transporting units with sufficient length for allowing the weathervaning vessel to move due to influence by the weather, the prior art shallow water systems usually are structured to arrange the flexible transporting units to have a wave configuration, such as an ^XS' configuration or similar wave configuration e.g. as described in US 4,793,737

In such wave configuration the flexible transporting units are supported by a supporting unit, such as a fixed seabed structure, for example a mid water arch or a mid water jacket, at a distance from the seabed, The term 'seabed' is generally used to demote the subsea floor.

In order to ensure that the supporting unit does not collide with the weathervaning vessel when it moves the supporting units of prior art shallow water systems are arranged with a safety zone beneath the lowermost part of the weathervaning vessel. The safety zone is calculated in relation to expected and predicted movements of the weathervaning vessel when subjected to extreme weather conditions, normally based on known weather conditions within a range of years e.g. the last 20 or 100 years. A safety zone of 2-20 m is not uncommon. As mentioned above the wave configuration of the flexible transporting units provides the flexible transporting units with a surplus of length compared to the distance between the topside hang-off structure and the subsea structure at nominal position of the weathervaning vessel. However, as the

weathervaning vessel moves due to influence by weather conditions, the distance between the topside hang-off structure and the subsea structure may for certain shallow water systems vary very substantially. This may result in that the flexible transporting units may be over-bent or touch down and drag across the seabed and/or the flexible transporting unit may be stretched which may result in damaging of the flexible transporting unit.

The object of the invention is to provide a shallow water system which provides an increased protection of the flexible transporting units while simultaneously being simple to design and arrange with a desired safety margin. DISCLOSURE OF INVENTION

The object of the invention is solved by the shallow water system as defined in the claims. Additional beneficial solutions which may have additional advantages are defined in the sub claims and are described in the following.

The shallow water system of the invention comprises a floating

weathervaning vessel with a topside hang-off structure, a subsea structure and at least one flexible transporting unit extending from the topside hang-off structure to the subsea structure.

The 'floating weathervaning vessel' or merely the 'weathervaning vessel' is a vessel that is moored by line to the seabed and/or to one or more fixed structures such that it can rotate fully or partly around a vertical centre line due to influence by the weather, i.e. current, wind and waves. In most situations the weathervaning vessel will be moored directly to the seabed by a plurality of mooring lines. A weathervaning vessel may for example be able to rotate such that it can have an optimal position with respect to wind, water current and/or waves to be as stable on the water as possible. In one embodiment the weathervaning vessel and the mooring thereof is arranged such that the weathervaning vessel is always facing the wind.

A vertical centre line (also referred to as the weathervaning vertical center line) is defined as a vertical axis around which the moored weathervaning vessel can be weathervaned at still water and with an equidistant distance between the weathervaning vessel and the vertical axis. The nominal vertical centre line is defined by the weathervaning vessel when it is in its nominal position. The weathervaning vessel is in its nominal position when it is free of influence by any influences of forces generated by weather conditions (wind, water current and waves). When the weathervaning vessel in its nominal position it can be weathervaned - without any other movements - around a vertical axis extending to the seabed in a vertical centre line-seabed point which. This vertical axis is herein defined as the nominal vertical centre line (also referred to as the weathervaning nominal vertical centre line), and the centre line-seabed point is a point cut by the nominal vertical centre line and the centre line-seabed point can therefore be used to show the position of the nominal vertical centre line. The vertical nominal centre line can be calculated once the weathervaning vessel has been moored. The weathervaning vessel may be any kind of weathervaning vessel with an external turret system or another similar mooring system. Examples of weathervaning vessels includes FPSO (Floating Production, Storage, and Offloading systems), FSO (Floating Storage and Offloading systems) and FSRU (Floating Storage and Regasification Unit systems) The topside hang-off structure is the structure from to which the flexible transporting unit or units are connected and hang-off from the weathervaning vessel. In this context the topside hang-off is defined as the riser connection point to the surface vessel. The topside hang-off structure may have any size and shape. Such topside hang-off structures are well known in the art. The flexible transporting unit may be any kind of flexible transporting units which are used in such shallow water system. The flexible transporting unit may for example be a flexible riser or an umbilical. Preferably the shallow water system comprises a plurality of flexible transporting units e.g.

comprising a plurality of rises, such as a plurality of risers and optionally at least one umbilical. Flexible transporting units - sometimes also called jumpers - such as risers and umbilicals are well known in the art. Risers are usually applied for transportation of petrochemical products from the seabed to a sea surface installation such as a weathervaning vessel. Umbilicals are often used for transporting fluids, electricity, signals and other to and/or from installations at or beyond the seabed.

The weathervaning vessel has a hull with a bow a lowermost bow midpoint. The bow is the front region of the weathervaning vessel and the bow

midpoint is the line separating the bow in two substantially equal half. The weathervaning vessel is moored such that at still water it can

weathervane around a weathervaning vertical centre line, such that the position of the lowermost bow midpoint follows an annular lowermost bow line. This annular lowermost bow line defines a vertical tube shaped border to a lowermost bow midpoint exclusion zone (LE-zone). Each vertical line lying in the vertical tube shaped border cuts through the annular lowermost bow line. The subsea structure is arranged in the LE-zone.

The subsea structure may be any kind of subsea structure, for example as explained further below.

The inventors of the present invention have realized that there is a zone around the vertical centre line into which the weathervaning vessel cannot enter. The inventors have further realized that this insight can be used in a beneficial fashion to provide in a simple manner a shallow water system which is improved with respect to protection of the flexible transporting unit(s) and which simultaneously is simple to design with a desired safety margin. Hereforto the sub sea structure of the shallow water system has always been arranged well below the keel of the weathervaning vessel to avoid collision. According to the present invention the subsea structure of the shallow water system of the invention need not be arranged below the keel height, but may be arranged further from the seabed. The keel is the spine of the vessel, which runs along the bottom thereof and usually constitutes the lowermost part of the weathervaning vessel. The structure of the shallow water system of the invention allows a surplus of length of the flexible transporting unit between the hang-off structure and the subsea structure which is much larger than a surplus of length of the flexible transporting unit allowed between hang-off structure and the subsea structure in prior art shallow water systems and accordingly the risk of damaging the flexible transporting unit due to over-bending, touch down and drag across the seabed and/or undesired stretching is decreased substantially. It should be understood that attachment(s) may be applied to the bow and or the keel. If such attachment(s) is/are present care should be taken to provide a safety margin when determining the position of the subsea structure.

The shallow water system is in particular useful in highly shallow waters, but it may also be beneficial in less shallow areas. The shallow water system preferably is applicable in shallow waters with a depth of up to about 200 m, such as preferably with a dept of up to about 100 m, such as with a depth of up to about 80 m.

In one embodiment, to provide an extra safety against collision between the weathervaning vessel and the subsea structure, the subsea structure is arranged with a minimum distance to the LE-zone border of at least about 0.5 m, such as at least about 1 m, such as at least about 2, such as at least about 3 m, such as at least about 4 m such as at least about 5 m. Since the bow midline at the upper part of the weathervaning vessel normally will protrude beyond the lowermost bow midpoint, such a safety margin may be an advantage in particular in situations where the sub sea structure is arranged relatively high in water compared to the weathervaning vessel.

In one embodiment the bow of the weathervaning vessel has an uppermost bow midpoint. The uppermost bow midpoint is defined as the point where the bow midpoint crosses a sea surface plane determined at still water and fully loaded weathervaning vessel. The sea surface plane is a horizontal plane touching the surface of the sea at still water. The position of the uppermost bow midpoint determined at still water, when the weathervaning vessel weathervanes around the vertical centre line, follows an annular uppermost bow line, which uppermost bow line defines a vertical tube shaped border to an uppermost bow midpoint exclusion zone (UE-zone). The subsea structure is arranged in the UE-zone. In this embodiment the risk of collision between the weathervaning vessel and the sub structure is extremely low, and will in practise not occur unless the weathervaning vessel is detached. The weathervaning vertical centre line, also referred to merely as the vertical centre line, is preferably the nominal weathervaning vertical centre line as defined above.

In certain environments the weather conditions is such that the

weathervaning vessel generally will be hold relatively stable slightly displaced with respect to its nominal position. For shallow water systems to be use in such areas the vertical centre line used may the vertical centre line

determined in this relative stable slightly displaced position. However, generally it is desired that the vertical centre line is the nominal vertical centre line. In one embodiment the weathervaning vessel comprises a weathervaning bearing, such as a chaintable bearing for mooring the weathervaning vessel. The vertical centre line preferably crosses the weathervaning bearing. The weathervaning bearing is connected to the weathervaning vessel in a rigid structure which extends to a horizontal distance from the lowermost bow midpoint of the weathervaning vessel. A circle with axis in the vertical centre line and a diameter of 2* the horizontal distance defines a vertical tube shaped border to an ideal lowermost bow midpoint exclusion zone (ILE-zone). In this embodiment the subsea structure is arranged in the ILE-zone. In most situations the ILE-zone will be substantially identical with the LE-zone, but it cannot be excluded that mooring systems which does not comprise a weathervaning bearing connected in a rigid structure to the hull could result in embodiments where the ILE-zone and the LE-zone will differ from each other. In such situations it is desired that the subsea structure is arranged in a zone provided by the overlapping of both the ILE-zone and the LE-zone.

It should be understood that the rigid structure which in the above

embodiment connects the weathervaning bearing to the hull of the

weathervaning vessel may extend both horizontal as well as vertical (normally upwards) from the hull. Only the horizontal distance to the lowermost bow midpoint is used for determine the ILE-zone.

In one embodiment the weathervaning vessel comprises a turret and a weathervaning bearing for rotatably mooring the turret to one or more fixed structures, such as the seabed. The turret may be any kind of turret for external use. Examples of useful turrets and turret systems are for example described in US 5,517,937 and US 6,176,193.

In one embodiment the turret comprises a swivel connected to or comprising the topside hang-off structure. Such swivel is well known in the art and may for example be as described in US 5,517,937.

Unless specifically mentioned all distances and determinations are made at still water at average water level.

In one embodiment the subsea structure is arranged to have an uppermost point determined at still water, arranged at a distance from the sea surface which is about 40 m or less, such as is about 35 m or less, such as is about 30 m or less, such as is about 25 m or less, such as is about 20 m or less, such as is about 18 m or less, such as is about 16 m or less, such as is about 14 m or less, such as is about 12 m or less, such as is about 10 m or less. The optimal arrangement of the uppermost point of the subsea structure may preferably be determined in relation to the depth of the water, the size and draft of the weathervaning vessel.

In one embodiment the subsea structure is arranged to have an uppermost point determined at still water, arranged at a distance from the sea surface which is about 85 % of the depth of the water or less, e.g. immediately below or at the sea surface, such as a distance from the sea surface which is about 80 % or less, such as about 75 % or less, such as about 60 % or less, such as about 65 % or less, such as about 60 % or less, such as about 50 % or less, such as about 25 % or less of the depth of the water

In one embodiment the weathervaning vessel has a lowermost part (usually the keel), the shallow water system is arranged such that the uppermost point of the subsea structure has a distance from the sea surface, determined at still water, which is at least about 20 m less than, such as at least 18 m less than, such as at least 16 m less than, such as at least 14 m less than a first horizontal plane, determined at still water, comprising the lowermost part of the weathervaning vessel or less. In one embodiment the weathervaning vessel has a lowermost part determined at still water, a rail and a hull height, and the shallow water system is arranged such that the uppermost point of the subsea structure has a distance determined at still water from the sea surface which about 2 times the height of the hull or less, such as 1.9 times the height of the hull or less, such as 1.8 times the height of the hull or less, such as 1.7 times the height of the hull or less, such as 1.6 times the height of the hull or less, such as 15 times the height of the hull or less, such as 1.4 times the height of the hull or less, such as 1.3 times the height of the hull or less, such as 1.2 times the height of the hull or less, such as 1.1 times the height of the hull or less, such as the height of the hull or less, such as 0.9 times the height of the hull or less, such as 0.8 times the height of the hull or less, such as 0.7 times the height of the hull or less, such as 0.6 times the height of the hull or less, such as 0.5 times the height of the hull or less , such as 0.4 times the height of the hull or less, such as 0.3 times the height of the hull or less, such as 0.2 times the height of the hull or less, such as 0.1 times the height of the hull or less.

The hull height (height of the hull) is defined as the distance between a first horizontal plane comprising the lowermost part of the weathervaning vessel and a second plane parallel to the first plane and comprising the lowermost part of the rail. In one embodiment the weathervaning vessel has a distance to the seabed determined at still water and average water level, and the uppermost point of the subsea structure has a distance to the seabed which is from about the weathervaning vessel distance to the seabed determined at still water minus 20 m to about the weathervaning vessel distance to the seabed determined at still water plus 20 m, such as from about the weathervaning vessel distance to the seabed determined at still water minus 15 m to about the

weathervaning vessel distance to the seabed determined at still water plus 15 m, such as from about the weathervaning vessel distance to the seabed determined at still water minus 10 m to about the weathervaning vessel distance to the seabed determined at still water plus 10 m, such as from about the weathervaning vessel distance to the seabed determined at still water minus 8 m to about the weathervaning vessel distance to the seabed determined at still water plus 8 m.

In one embodiment the subsea structure is arranged to have an uppermost point closer to the sea surface, wherein the uppermost point of the subsea structure has a distance from the sea surface and a distance of the LE-zone border sufficient to avoid collision with the weathervaning vessel under ordinary operation conditions. Based on the principle of the present invention the skilled person will be able to design the shallow water system such that collision can be avoided with high safety. As it will be explained further in the example below the weathervaning vessel can be subjected to a plurality of motions which comprises the following motions referred to within the art as heave, pitch, roll, surge, sway, yaw, 2^nd order and higher order motion in mooring system and/or motion due to thrusters, drift, draft, heel, trim, mtions due to water level and motion due to influence by shuttle talks. In the present text the term 'tilt' includes all of these motions.

In one embodiment the weathervaning vessel under ordinary operation conditions may be tilted into an intrusion zone which includes a part of the LE-zone. In other words, due to weather conditions the weathervaning vessel may enter slightly into the LE-zone. The part of the LE-zone into which the weathervaning vessel can enter under any conditions (excluding damaging of the system such a damaging to the mooring) is referred to as the LE-intrusion zone. In this embodiment the subsea structure has a safety distance from the sea surface and a distance of the LE-zone border sufficient to avoid collision with the weathervaning vessel in the intrusion zone.

In one embodiment the weathervaning vessel has a length; the subsea structure is arranged at a horizontal distance to the vertical centre line which is less than the length of the weathervaning vessel. In one embodiment the subsea structure is arranged at a horizontal distance to the vertical centre line which is less than about 100 m, such as less than about 90 m, such as less than about 80 m, such as less than about 70 m, such as less than about 60 m, such as less than about 50 m, such as less than about 40 m, such as less than about 30 m, such as less than about 20 m, such as less than about 15 m, such as less than about 10 m.

As mentioned above the subsea structure may be any kind of subsea structure. In one embodiment the flexible transporting unit is connected to the subsea structure. In another embodiment the flexible transporting unit may slide over the subsea structure such that the subsea structure's connection point to the flexible transporting unit may vary. In general it is desired that the flexible transporting unit is connected to the subsea structure.

In one embodiment the flexible transporting unit is terminated at the subsea structure - the flexible transporting unit may for example be connected to a stiff pipe structure which e.g. may be integrated with or connected to the subsea structure.

In one embodiment the flexible transporting unit is attached to the subsea structure or passes in contact with the subsea structure. In this connection it is generally desired that the subsea structure has an upper supporting shape, such as a gutter structure which allows the flexible pipe to move slightly at least with respect to the direction of the flexible transporting unit to and from the subsea structure.

In one embodiment the subsea structure is a fixed structure, preferably fixed directly to the seabed, such as a riser tower and a subsea anchoring device for anchoring the one or more flexible transporting units. Examples of such fixed subsea structures are for example described in US 4,182,584 and US 7,040,841.

In one embodiment the subsea structure is moored to a fixed structure, the subsea structure preferably comprises a buoyancy system, more preferably the subsea structure is a moored mid water arch or a mid water jacket. Such mid water arch and mid water jackets are well known in the art.

In one embodiment the subsea structure is held in position by the one or more flexible transporting units. The subsea structure may preferably comprise a buoyancy system, more preferably the subsea structure is a moored mid water arch.

As indicated above buoyancy systems are well know in the art and include in practice any systems that can hold a part of the flexible transporting unit at a position above seabed by means of having a lower density than the sea water. Buoyancy systems usually employ one or more elements comprising low density materials or elements with enveloped air.

In one embodiment the at least one flexible transporting unit comprises or is supported by a buoyancy system arranging the at least one flexible transporting unit in a wave configuration. The wave configuration preferably comprises a wave shape of the flexible transporting unit which provides an extra length of the flexible transporting unit which is larger than what was possible by arranging the subsea structure beneath the keel of the

weathervaning vessel. In one embodiment the at least one flexible transporting unit is supported by a buoyancy system in hog bend.

The shallow water system of the invention may comprise additional elements such as elements of prior art shallow water systems which may be applied in combination with the shallow water system of the invention, this includes for example one or more of the following pieces of ancillary components: bend stiffener, bell mouth, weight coating, weight modules, clump weights, buoyancy modules, clamps, bend restrictors, gutter structures, abrasion protection, impact protection, fire protection, end fittings, steel pipe, tethers and soft slings. BRIEF DESCRIPTION OF DRAWINGS

The invention will be explained more fully below in connection with a preferred embodiment and with reference to the drawings in which:

FIG. 1 shows a schematically side view of a shallow water system of the invention comprising a weathervaning vessel, a subsea structure and a flexible transporting unit.

FIG. 2 shows the shallow water system of FIG.l with an indication of another position of the weathervaning vessel. FIG. 3 shows the shallow water system of FIG.l with an indication of tilting of the weathervaning vessel.

FIG. 4 shows the shallow water system of FIG.l with an indication of the preferred zone of the subsea structure. The figures are schematic and may be simplified for clarity. Throughout, the same reference numerals are used for identical or corresponding parts.

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

FIG. 1 shows a schematic side view of a shallow water system of the invention comprising a weathervaning vessel 3, a subsea structure 1 and a flexible transporting unit 2. The subsea structure 1 is fixed to the seabed 7. The lowermost part of the weathervaning vessel is the keel 12. The vertical line Turret CL' is the nominal weathervaning vertical centre line. As indicated the weathervaning vessel can be positioned in a far position and in a near position. This will be described further in FIG. 2.

FIG. 2 shows the shallow water system of FIG.l with an indication of a weathervaned position with respect to the fully drawn position of the weathervaning vessel 3. In the weathervaned position the weathervaning vessel is indicated with the no. 11. An intrusion zone 5 is indicated around the hull bottom and bow of the weathervaning vessel. The part of the intrusion zone 5, lying in front of the bow is an intrusion into the LE-zone and is designated the LE-intrusion zone, i.e. an intrusion of the weathervaning vessel into the LE-zone. In the near position of the weathervaning vessel, the weathervaning vessel is closer to the nominal weathervaning vertical line, Turret CL. In the far position of the weathervaning vessel, the weathervaning vessel is as further away from the nominal weathervaning vertical centre line, Turret CL.

The sea surface 8 indicates a preferred upper limit for the subsea structure. The plane 4, which is shown at a safety distance to the keel 12 indicates a preferred lower limit for the uppermost point of the subsea structure 1. A vessel free zone 6 beneath the intrusion zone 5 around the keel, and in front of the intrusion zone 5 in front of the bow, indicates the zone into which the weathervaning vessel will not enter. The part of the vessel free zone 6 below the keel 12 is a horizontal limit below which no part of the, afore mentioned, weathervaning vessel 3 will ever protrude during vessel operational design life. The limit is determined based on vessel draft and vessel motion for vessel offset in a given direction.

FIG. 3 shows the shallow water system of FIG.l with an indication of tilting of the weathervaning vessel 3. As mentioned the tilting may comprise heave, pitch, roll, surge, sway, yaw, 2.order motion in mooring system and/or motion due to thrusters as well as other movements. The intrusion zone 5 as shown in FIG. 2 is defined as the volume around the vessel nominal position which may be intruded by a part of the vessel hull during its operational lifetime, and by providing a theoretical calculation of tilting of the weathervaning vessel this intrusion zone can be determined.

FIG. 4 shows the shallow water system of FIG.l with an indication of the preferred zone of the subsea structure. The nominal weathervaning vertical centre line is here 9-10. The preferred position for the uppermost point of the subsea structure is within the vessel free zone 6 in front of the bow of the weathervaning vessel.

The invention is defined by the features of the independent claim(s).

Preferred embodiments are defined in the dependent claims. Any reference numerals in the claims are intended to be non-limiting for their scope. Some preferred embodiments have been shown in the foregoing, but it should be stressed that the invention is not limited to these, but may be embodied in other ways within the subject-matter defined in the following claims.

Claims

PATENT CLAIMS

1. A shallow water system comprising a floating weathervaning vessel with a topside hang-off structure, a subsea structure and at least one flexible transporting unit extending from the topside hang-off structure to the subsea structure, said weathervaning vessel has a bow with a lowermost bow midpoint, the weathervaning vessel is moored such that at still water it can weathervane around a weathervaning vertical centre line,such that the position of the lowermost bow midpoint follows an annular lowermost bow line, which defines a vertical tube shaped border to a lowermost bow midpoint exclusion zone (LE-zone), said subsea structure is arranged in said LE-zone, said weathervaning vertical centre line preferably is a nominal weathervaning vertical centre line.

2. A shallow water system as claimed in claim 1, wherein said subsea structure is arranged with a minimum distance to the LE-zone border of at least about 0.5 m, such as at least about 1 m, such as at least about 2, such as at least about 3 m, such as at least about 4 m such as at least about 5 m.

3. A shallow water system as claimed in any one of claims 1 and 2, wherein said bow of said weathervaning vessel has an uppermost bow midpoint defined as the point where the bow midpoint crosses a sea surface plane determined at still water and fully loaded weathervaning vessel, the position of the uppermost bow midpoint determined, when the weathervaning vessel weathervanes around said vertical centre line, follows an annular uppermost bow line defining a vertical tube shaped border to an uppermost bow midpoint exclusion zone (UE-zone), said subsea structure is arranged in said UE-zone.

4. A shallow water system as claimed in any one of the preceding claims wherein said weathervaning vessel comprises a weathervaning bearing, the vertical centre line crosses said weathervaning bearing, said weathervaning bearing is connected to said weathervaning vessel in a rigid structure extending to a horizontal distance from said lowermost bow midpoint of said weathervaning vessel, a circle with axis in the vertical centre line and a diameter of 2* the horizontal distance defines a vertical tube shaped border to an ideal lowermost bow midpoint exclusion zone (ILE-zone), said subsea structure is arranged in said ILE-zone.

5. A shallow water system as claimed in any one of the preceding claims, wherein said subsea structure is arranged to have an uppermost point determined at still water, arranged at a distance from the sea surface which is about 40 m or less, such as is about 35 m or less, such as is about 30 m or less, such as is about 25 m or less, such as is about 20 m or less, such as is about 18 m or less, such as is about 16 m or less, such as is about 14 m or less, such as is about 12 m or less, such as is about 10 m or less.

6. A shallow water system as claimed in any one of the preceding claims, wherein said subsea structure is arranged to have an uppermost point closer to the sea surface, said weathervaning vessel has a lowermost part, said shallow water system being arranged such that said uppermost point of said subsea structure has a distance from the sea surface, determined at still water, which is at least about 20 m less than, such as at least 18 m less than, such as at least 16 m less than, such as at least 14 m less than a first horizontal plane, determined at still water, comprising the lowermost part of the weathervaning vessel or less.

7. A shallow water system as claimed in any one of the preceding claims, wherein said subsea structure is arranged to have an uppermost point closer to the sea surface, said weathervaning vessel has a lowermost part determined at still water, a rail and a hull height defined as the distance between a first horizontal plane comprising the lowermost part of the weathervaning vessel and a second plane parallel to the first plane and comprising the lowermost part of the rail, said shallow water system being arranged such that said uppermost point of said subsea structure has a distance determined at still water from the sea surface which about 2 times the height of the hull or less, such as 1.9 times the height of the hull or less, such as 1.8 times the height of the hull or less, such as 1.7 times the height of the hull or less, such as 1.6 times the height of the hull or less, such as 15 times the height of the hull or less, such as 1.4 times the height of the hull or less, such as 1.3 times the height of the hull or less, such as 1.2 times the height of the hull or less, such as 1.1 times the height of the hull or less, such as the height of the hull or less, such as 0.9 times the height of the hull or less, such as 0.8 times the height of the hull or less, such as 0.7 times the height of the hull or less, such as 0.6 times the height of the hull or less, such as 0.5 times the height of the hull or less , such as 0.4 times the height of the hull or less, such as 0.3 times the height of the hull or less, such as 0.2 times the height of the hull or less, such as 0.1 times the height of the hull or less.

8. A shallow water system as claimed in any one of the preceding claims, wherein said subsea structure is arranged to have an uppermost point closer to the sea surface, said weathervaning vessel has a distance to the seabed determined at still water and average water level, and said uppermost point has a distance to the seabed which is from about said weathervaning vessel distance to the seabed determined at still water minus 20 m to about said weathervaning vessel distance to the seabed determined at still water plus 20 m, such as from about said weathervaning vessel distance to the seabed determined at still water minus 15 m to about said weathervaning vessel distance to the seabed determined at still water plus 15 m, such as from about said weathervaning vessel distance to the seabed determined at still water minus 10 m to about said weathervaning vessel distance to the seabed determined at still water plus 10 m, such as from about said weathervaning vessel distance to the seabed determined at still water minus 8 m to about said weathervaning vessel distance to the seabed determined at still water plus 8 m.

9. A shallow system as claimed in any one of the preceding claims, wherein said subsea structure is arranged to have an uppermost point closer to the sea surface, wherein said uppermost point of said subsea structure has a distance from the sea surface and a distance of said LE-zone border sufficient to avoid collision with said weathervaning vessel under ordinary operation conditions.

10. A shallow water system as claimed in any one of said preceding claims wherein the weathervaning vessel under ordinary operation conditions can be tilted into an intrusion zone of the LE-zone, said subsea structure has a distance from the sea surface and a distance of said LE-zone border sufficient to avoid collision with said weathervaning vessel in said intrusion zone.

11. A shallow system as claimed in any one of the preceding claims, wherein said weathervaning vessel has a length, said subsea structure is arranged at a horizontal distance to said vertical centre line which is less than the length of the weathervaning vessel.

12. A shallow system preferably as claimed in any one of the preceding claims and comprising a floating weathervaning vessel with a topside hang-off structure, a subsea structure and at least one flexible transporting unit extending from the topside hang-off structure to the subsea structure, the weathervaning vessel is moored such that at still water it can weathervane around a weathervaning vertical centre line, said subsea structure is arranged at a horizontal distance to said vertical centre line which is less than about 100 m, such as less than about 90 m, such as less than about 80 m, such as less than about 70 m, such as less than about 60 m, such as less than about 50 m, such as less than about 40 m, such as less than about 30 m, such as less than about 20 m, such as less than about 15 m, such as less than about 10 m.

13. A shallow water system as claimed in any one of the preceding claims wherein said weathervaning vessel comprises a turret and a weathervaning bearing for rotatably mooring said turret by lines to the seabed and/or to one or more fixed structures.

14. A shallow water system as claimed in claim 13 wherein said turret comprises a swivel connected to or comprising said topside hang-off structure.

15. A shallow water system as claimed in any one of the preceding claims wherein said subsea structure is a fixed structure, preferably fixed directly to the seabed, such as a riser tower, a mid water arch, e.g. a mid water jacket and a subsea anchoring device for anchoring the one or more flexible transporting units.

16. A shallow water system as claimed in any one of the preceding claims 1- 14, wherein said subsea structure is moored to a fixed structure, said subsea structure preferably comprises a buoyancy system, more preferably said subsea structure is a moored mid water arch.

17. A shallow water system as claimed in any one of the preceding claims 1- 14, wherein said subsea structure is held in position by said one or more flexible transporting units, said subsea structure preferably comprises a buoyancy system, more preferably said subsea structure is a moored mid water arch or a mid water jacket.

18. A shallow water system as claimed in any one of the preceding claims, wherein said at least one flexible transporting unit comprises or is supported by a buoyancy system arranging said at least one flexible transporting unit in a wave configuration.

19. A shallow water system as claimed in any one of the preceding claims, wherein said at least one flexible transporting unit is supported by buoyancy system in hog bend.

20. A shallow water system as claimed in any one of the preceding claims, wherein said at least one flexible transporting unit comprises one or more of a flexible riser, an umbilical, preferably said shallow water system comprises a plurality of flexible transporting units, such as a plurality of risers and optionally at least one umbilical.