WO2018134255A1

WO2018134255A1 - Mooring system with intermediate buoy floating on water surface

Info

Publication number: WO2018134255A1
Application number: PCT/EP2018/051111
Authority: WO
Inventors: Christian Raymond Bauduin; Francois CAILLE; Gilles TOMAS; Regis BOULARD
Original assignee: Single Buoy Moorings Inc.
Priority date: 2017-01-19
Filing date: 2018-01-17
Publication date: 2018-07-26

Abstract

The invention relates to a mooring system (1) for a FPSO (10) comprising a mooring buoy (20) anchored to the sea bed (40) and an intermediate buoy (30) arranged floating between the mooring buoy (20) and a point of connection (52) of a riser (50) to the sea bed (40), the intermediate buoy being provided with a portion (31) extending above the water and a submerged portion (32) under the water, said submerged portion comprising a support (33) for supporting a completely submerged section of said riser (50) thereon, the intermediate buoy (30) being connected to said mooring buoy (20) by a catenary connecting line (60) and is connected to the sea bed (40) by a catenary anchor line (61) which curves towards the sea bed (40), said catenary connecting line and said catenary anchor line together allowing horizontal movement of the intermediate buoy relative to the mooring buoy. The invention further relates to a method of installing a mooring system which has a mooring buoy and an intermediate buoy.

Description

Mooring system with intermediate buoy floating on water surface

Field of the invention

The present invention relates to a mooring system for a vessel for storing hydrocarbons, such as a Floating Production Storage and Offloading (FPSO) vessel, Floating Storage and Offloading (FSO) vessel or Floating Liquefied Natural Gas (FLNG) vessel, said system comprising a mooring buoy which is provided with a pivotable arm and a fluid connection supported by said arm and connected on one end to a riser, in particular a steel catenary riser, and connectable on another end to said vessel, for transporting said fluid hydrocarbons from the riser to the vessel.

Background art

EP 0 059 499 in the name of applicant describes a mooring system with a tanker carrying an arm at the bow, wherein a body is suspended from said arm around a vertical shaft and attached through a universal joint. The body is connected via anchor chains to the sea bottom. From the body hoses are supported through a curved piece that is suspended from an auxiliary float. The hoses are connected to pipe lines which run to a bottom anchor. Due to currents and waves, the elevation of the part of the tanker carrying the arm varies, causing the hoses to flex. In relatively deep waters, e.g. waters having a depth of 200 m or more, this flexing can be distributed over a corresponding long length of each hose. In shallow water, e.g. having depth of 200 m or less, or even 100 m or less, flexible hoses have to be used if such flexing is to be accommodated. However, flexible hoses are expensive and prone to wear. Alternatively, in shallow waters, the problem of flexing of the hoses can be mitigated by tightly confining the position of the vessel, in particular by strictly controlling the variation in elevation of the part of the vessel that is connected to the arm. This however leads to high loads on the mooring system and corresponding high costs.

GB 2 043 008 describes a permanent single-point mooring system comprising a buoy which is rotatable around a central shaft moored by lines to the sea bed and a rigid mooring arm pivoted to the buoy, wherein vertical motions of the buoy and arm are dampened by dampening members. Though this system dampens quick variations in elevation of the vessel an helps to prevent these from being transferred from the vessel to the buoy, hoses or risers between the buoy and the sea bed are still subjected to considerable flexing when the system is used in shallow waters.

It is an object of the invention to provide a mooring system in which flexing of the risers is reduced.

It is a further object of the invention to provide such a mooring system in which the load on the mooring system is reduced.

The invention further aims to provide a mooring system which increases the allowable range of excursion for a vessel. It is a further object of the invention to provide a mooring system which is easy to install / maintain, and a method for installing such a mooring system.

Summary of the invention

To this end, according to a first aspect, the present invention provides a mooring system for a floating vessel for storing hydrocarbons, said system comprising: a mooring buoy anchored to the sea bed via anchor lines, said mooring buoy comprising a submerged buoyancy member, a support frame connected to the top of the buoyancy member and projecting above the water surface, the support frame carrying an arm above said water surface which arm is pivotable relative to said frame around a vertical axis of rotation, wherein the arm projects outboard from said support frame and is adapted for connecting to said vessel to allow said vessel to weathervane around the mooring buoy, wherein at least one hydrocarbon riser is attached to the mooring buoy, and wherein said mooring buoy comprises a fluid connection supported by said arm and connected at one end to said riser and connectable at another end to said vessel, for transporting said fluid hydrocarbons from the riser to the vessel; an intermediate buoy arranged floating between the mooring buoy and a point of connection of the riser to the sea bed, wherein said intermediate buoy is provided with a portion extending above the water and a submerged portion under the water, said submerged portion comprising a support for supporting a completely submerged section of said riser thereon, wherein said intermediate buoy is connected to said mooring buoy by a catenary connecting line and is connected to the sea bed by a catenary anchor line which curves towards the sea bed, said catenary connecting line and said catenary anchor line together allowing horizontal movement of the intermediate buoy relative to the mooring buoy. Preferably the anchor lines which anchor the mooring buoy to the sea bed curve towards the sea bed to allow some movement of the mooring buoy relative to the sea bed.

The intermediate buoy in effect splits the riser up into two sections both of which can flex along a length thereof and are submerged in the water. The length of one or both of these sections is typically greater than the depth of the water, at least in shallow waters, so that flexing of the riser is distributed over a much longer length of riser than if the riser were free hanging from the mooring buoy down to the sea bed. The catenary connecting line and the catenary anchor line enable horizontal movement between the buoys substantially without causing a vertical movement of either of these at the same time, effectively increasing the range of excursion of the vessel. No anchor line or lines extend along a straight line, such as a vertical straight line, from the intermediate buoy to the sea bed, as this would hinder said horizontal movement. The range of horizontal movement over which the buoys may move relative to each other, without substantially moving vertically relative to each other, depends on the configuration of the catenary anchor line, the catenary connecting line and the both buoys, but is preferably a range of 3 m or more, preferably between 3 m and 8 m. The curvature of the catenary connecting line and/or of the catenary anchor line can be adjusted, for instance by adjusting the length of said line(s) and/or by attaching weights to said line(s), in order to set the bounds of the horizontal range of movement. Such adjustments are easily carried out as the intermediate buoy is visible from above the water surface, and at least those portions of the lines that are arranged near the intermediate buoy can be reached from a position close to the water surface. The catenary connecting line and catenary anchor line which are both connected to the intermediate buoy together control the excursion of the intermediate buoy and consequently control the configuration of the riser or risers supported thereby.

A further advantage of the intermediate buoy being visible from above the water surface, is that the vessel can easily be steered to avoid collision with the buoy and a riser supported by the buoy.

The weight of the catenary connecting line and the weight of the catenary anchor line together with the weight of the riser urge the intermediate buoy to an neutral horizontal position between the mooring buoy and the point of connection where the catenary anchor line is connected to the sea bed. Thus, though the intermediate buoy is horizontally moveable relative to the mooring buoy, it will tend to return to the neutral horizontal position if weather conditions are quiet.

Preferably, the catenary connecting line and the catenary anchor line each have only a single catenary curve, and/or the portion of the riser slopes from the support to the sea bed in a single catenary curve.

In an embodiment the mooring system further comprises the vessel.

In an embodiment the catenary connecting line is arranged for maintaining said intermediate buoy within a shorter distance to said vertical axis of rotation than the length of the arm, preferably wherein the arm has a length of at least 1.3 times said distance. During weathervaning of the vessel around the mooring buoy the arm thus spaces the vessel sufficiently far apart from the intermediate buoy to prevent collision therewith. When viewed in projection onto the horizontal plane, the length of the arm is preferably at least equal to a total length of the catenary connecting line, preferably over the entire range of horizontal movement that is possible between both buoys. The catenary connecting line allows some movement between the two buoys while at the same time imposing a limit on the maximum distance between the intermediate buoy and the mooring buoy.

In an embodiment the intermediate buoy is arranged for letting a portion of the riser slope from the support to the sea bed in such a manner that, in as far as said portion lies outside of a riser clearance radius from said support, it is arranged completely below a level of the bottom of the vessel, wherein, when viewed in projection onto the horizontal plane, said riser clearance radius is smaller than the length of the arm. The level of the bottom of the hull below which the riser portion outside of the riser clearance radius is arranged can be determined relative to the water surface. The arm, when connected to the vessel, keeps vessel at a distance from the intermediate buoy that is greater than the riser clearance radius, thus preventing the vessel from colliding with the riser portion. The level of the bottom of the vessel can be determined when the arm is attached to the vessel or prior thereto. In order to also take into account variations in the level of the bottom of the vessel, e.g. due to loading of the vessel with hydrocarbons, the intermediate buoy is preferably arranged such that the riser portion, outside of said clearance radius, is completely below a maximum draft level of the floating vessel. Once the (maximum) level of the bottom of the vessel is known, then based on an allowable curvature of said portion of the riser and the point of connection thereof to the sea bed, the location of the intermediate buoy between the point and the mooring buoy can be calculated.

In an embodiment, said catenary anchor line is the only anchor line that is attached to the intermediate buoy for anchoring said buoy to the sea bed. This configuration provides the greatest freedom of horizontal movement of the buoys relative to each other. Preferably, the catenary connection line is the only connection line between the two buoys adapted for limiting the horizontal movement therebetween and thus limiting strain on portion of the riser between the two buoys.

In an embodiment the support of said intermediate buoy is arranged at a level in said water above the level of the bottom of the vessel. The riser is thus supported relatively close to the water surface, allowing the system to be used in shallow waters. The level of the support and of the bottom of the vessel are relative to the water surface. The bottom of the vessel is typically formed by the lowest part of the hull of the vessel, and may be determined based on draft level specifications of the vessel, wherein the draft level of a vessel is defined as the vertical distance between the water line and the bottom of the hull of the vessel, including the thickness of the hull. For instance, the level of the bottom of the vessel for this embodiment may be determined as a maximum draft level of the vessel that is achieved when the vessel is loaded, though for the present embodiment the level is preferably determined as a minimum draft level of the vessel when the vessel is substantially completely unloaded. In general such draft levels are predetermined for each vessel.

In an embodiment, when seen in projection onto the horizontal plane, said intermediate buoy is arranged substantially between the mooring buoy and a point of connection of the riser with the sea bed. Though the intermediate buoy may move horizontally relative to mooring buoy, at least in its neutral position it is thus arranged on a line between the mooring buoy and said point of connection. Alternatively or additionally, the intermediate buoy may be arranged substantially between the mooring buoy and a point of connection of the catenary anchor liner with the sea bed.

In an embodiment, when seen in projection onto the horizontal plane, said point of connection of the catenary anchor line with the sea bed is arranged substantially between the intermediate buoy and the point of connection of the riser to the sea bed, preferably on a straight line between the intermediate buoy and the point where the riser is connected to the sea bed. The distance along the catenary anchor line to the sea bed is thus shorter than a distance along the portion of the riser which slopes from the support to the sea bed, resulting in the horizontal movement of the intermediate buoy being substantially more limited by the catenary anchor line than by said portion of the riser. Stress on said riser portion during horizontal movement of the intermediate buoy is thus reduced. Preferably, the catenary anchor line has a length that is at least 1.2 times the length of said portion of the riser.

In an embodiment said catenary connection line is attached to said intermediate buoy at a position below said support, preferably at least 2 m below the support. The catenary connection line is thus connected to the intermediate buoy at a position below where the riser is supported on the support, so that the catenary connection line can extend substantially below a corresponding portion of the riser to the mooring buoy with a greater curvature than said corresponding portion.

In an embodiment the total length of said riser between the sea bed and the mooring buoy is greater than three times the vertical distance between said support and the sea bed. This makes the mooring system of the present invention especially suitable to be used in shallow waters. In an alternative or additional embodiment said total length of the riser is more than three times the distance between the water surface and the sea bed, wherein said distance between the water surface is defined as a smallest distance therebetween within an area where the ship can weathervane around the mooring buoy.

In an embodiment said catenary connection line and/or catenary anchor line is provided with weights, for urging the intermediate buoy and the mooring buoy to a substantially predetermined horizontal distance from each other. By adjusting the weights and/or the position where the end points of these lines are connected, a neutral position for the intermediate buoy can be set in which the horizontal force exerted thereon by the connection line and the corresponding portion of the riser between the support and the mooring buoy is substantially equal, but oppositely directed, to the horizontal force exerted thereon by the catenary anchor line and the corresponding portion of the riser between the support and the point where the riser is connected to the sea bed. The weights may also urge the intermediate buoy to a substantially predetermined position relative to both the mooring buoy and the connection point of the catenary anchor line with the sea bed.

In an embodiment the connection of said fluid connection to the riser is arranged below the water. The riser can thus remain completely submerged in the water where at least part of the weight of the riser is supported by the water. The fluid connection may be flexible and/or provided with a swivel, to allow rotation of the arm around the vertical axis.

In an embodiment the riser preferably is provided with a bend limiter, such as a bellmouth or a bend stiffener, at either or both of its end connected to the sea bed and its end connected to the mooring buoy, for limiting the extent to which the riser can bend.

In an embodiment the riser has an inner diameter of at least 10 inch (25.4 cm), preferably between 10 inch and 26 inch (66.04 cm).

In an embodiment, the riser is substantially comprised of steel riser segments or bonded hose segments. The riser may for instance be of the steel catenary riser type, or may be comprised of bonded hose segments each provided with an elastomeric matrix reinforced with a number of armour layers to give the hose the required strength and which have been bolted together to form a riser. The reduction in strain on the riser, e.g. on the steel segments or on the armour layers of the bonded hose segments, as provided by the invention increases the expected lifespan on the riser.

In an embodiment the portion of the riser between the sea bed and the support slopes in a single catenary curve having its highest point at or near the support, and the portion of the riser between the support and the mooring buoy sloped in another single catenary curve. Movement of the intermediate buoy and strain on the riser can thus be determined relatively easily.

According to a second aspect, the present invention provides a method of installing a mooring system comprising a mooring buoy anchored to the sea bed via anchor lines, said mooring buoy comprising a submerged buoyancy member, a support frame connected to the top of the buoyancy member and projecting above the water surface, the support frame carrying an arm above the water surface which arm is pivotable relative to said frame around a vertical axis of rotation, wherein the arm projects outboard from said support frame and is adapted for connecting to a vessel to allow said vessel to weathervane around the mooring buoy, wherein at least one hydrocarbon riser is attached to the mooring buoy, and wherein said mooring buoy comprises a fluid connection supported by said arm and connectable at one end to said riser and connectable at another end to said vessel, for transporting said fluid hydrocarbons from the riser to the vessel, said mooring system further comprising an intermediate buoy arranged floating between the mooring buoy and a point of connection of the riser to the sea bed, wherein said intermediate buoy is provided with a portion extending above the water and a submerged portion under the water, said submerged portion comprising a support for supporting a completely submerged section of said riser thereon, said method comprising the steps of:

determining a first position where the riser is connected to the sea bed;

determining a second position of the vertical axis of rotation of the mooring buoy;

connecting a catenary connection line on one end to said intermediate buoy and on another end to said mooring buoy, and connecting a catenary anchor line on one end to said intermediate buoy and on another end to the sea bed at a location between the first and second position in such a manner that, during floating movement of said intermediate buoy, the catenary connection line remains completely spaced apart from the sea bed and a maximum horizontal distance between the intermediate buoy and the second position is less than the horizontal length of said arm, lifting the riser from the sea bed and arranging a section thereof on the support of the intermediate buoy so that a portion of said riser curves in such a manner from the support to the sea bed that at any point where said portion is spaced further apart from the second position than the length of said arm, the riser is arranged completely below level of the bottom of the vessel.

The first and second positions generally are substantially geostationary. A mooring system, e.g. a mooring system as described herein, installed in this manner provides increased excursion range for a vessel moored thereto, while avoiding strain on the mooring system and the riser. This makes the method of the invention particularly suitable for use in shallow water, i.e. water having a depth of 200 m or less, or even 100m or less.

It will be understood that as the level of the bottom of the vessel can vary somewhat due to roll of waves, the level of the bottom of the vessel is preferably determined as an expected lowest level of the vessel in the water where the vessel is to operate. The method provides a fast way to connect a vessel to a riser in shallow water, while minimizing bending of the riser and while allowing increased excursion of the vessel.

In summary, the present invention relates to a mooring system for an FPSO, comprising a mooring buoy anchored to the sea bed and an intermediate buoy arranged floating between the mooring buoy and a point of connection of a riser to the sea bed, the intermediate buoy being provided with a portion extending above the water and a submerged portion under the water, said submerged portion comprising a support for supporting a completely submerged section of said riser thereon, the intermediate buoy being connected to the mooring buoy by a catenary connecting line and connected to the sea bed by a catenary anchor line which curves towards the sea bed, said catenary connecting line and said catenary anchor line together allowing horizontal movement of the intermediate buoy relative to the mooring buoy. The invention further relates to a method of installing a mooring system which has a mooring buoy and an intermediate buoy.

Short description of drawings

The present invention will be discussed in more detail below, with reference to the attached drawings, in which:

Fig. 1 schematically shows a side view of a mooring system according to the present invention,

Figs. 2A and 2B schematically shows the system of Fig. 1 in projection on the horizontal plane, but with the intermediate buoy and the vessel in different positions,

Fig. 3 schematically shows another embodiment of a mooring system according to the present invention, in which the intermediate buoy is connected to the sea bed by multiple catenary anchor lines. Description of embodiments

Fig. 1 shows a mooring system 1 for mooring a vessel 10 which floats on the water. The vessel 10 shown is a vessel as used in offshore industry, in this case an FPSO vessel, adapted for storing hydrocarbons, such as natural gas or oil, which have been extracted from the earth from a position below the sea bed 40. The vessel has a hull 2 with a bottom 3 which extends at a depth d1 in the water as measured from the water surface W. The mooring system 1 is provided with a mooring buoy 20 which floats on the water and is kept in a substantially geostationary position relative to the sea bed by means of anchor lines 41 ,42,43, though the anchor lines allow some horizontal movement. The mooring buoy 20 comprises a buoyancy member 21 which is partly submerged in the water, on top of which a support frame 22 is connected which projects above the water surface W. Also projecting above the water surface is a rigid arm 23 which extends beyond the support frame 22 to the vessel 10 and is connected with an end 23a to the vessel 1. The arm 23, which is connected at its end 23b to the support frame 22 and which is pivotable relative to the frame 22 around a vertical axis of rotation V, allows the vessel 10 to weathervane around the mooring buoy 20 without risk of collision between the vessel and the submerged buoyancy 21 member or the frame 22 that is connected to the top of said buoyancy member. A riser 50, which is connected at one end 51 to the mooring buoy 20, and at another end 52 to the sea bed 40, is provided for transferring hydrocarbons from the sea bed to the mooring buoy. The submerged end 51 of the riser 50 is connected to an end 24b of a fluid connection 24 which is substantially supported by the arm 23. At its other end 24a the fluid connection is connected to the vessel 10 wherein the hydrocarbons are stored. Though in Fig. 1 the riser is shown to be directly connected to the fluid connection, it will be clear that the riser and fluid connection instead may be connected to each other by other means, such as a swivel connection.

The vessel 10 may weathervane around the mooring buoy 20, e.g. under influence of wind, waves, currents and/or drifting ice to adopt the position of least resistance with regards to the environment. Additionally, the vessel may move vertically relative to the mooring vessel, due to waves and the like. For absorbing part of the relative movements, the end 23a of the arm is connected to the vessel 10 to be pivotable around a horizontal axis H1 (see Fig. 2A) that is substantially perpendicular to arm 23, and the other end 23b of the arm is connected to the mooring buoy 20 to be pivotable around a horizontal axis H2 that is also substantially perpendicular to said arm. Thus, transfer of motion of the vessel to the riser 50 is partially dampened.

The mooring system 1 is arranged in shallow water having a depth d2 of 100 m or less, due to which the vertical space along which the riser 50 can flex is severely limited. In order to distribute flexing of the riser over a greater distance, the riser is arranged to float in the water in such a manner that its total length along the horizontal direction is greater than its total length along the vertical direction. In Fig. 1 the horizontal distance d3 between the point of connection to the sea bed and the mooring buoy is at least about 2,5 times the depth d2 of the water, allowing any flexing of the riser portion that is not in contact with the sea-bed to be distributed over a length that is greater than the depth of the water.

In order to prevent the riser from hanging free from the mooring buoy 20 to the sea bed 40 and being in contact with the sea bed over a substantial part of the total length of the riser, an intermediate buoy 30 is provided floating between the mooring buoy 20 and the point 52 where the riser 50 is connected to the sea bed. The intermediate buoy comprises a portion 31 which extends above the water and is easily visible above the water surface W. The buoy 30 further comprises a portion 32 which is submerged under the water and which is provided with a support 33 for supporting thereon a section of the riser 50. The support 33 is arranged above the level of the bottom 3 of the vessel, where it can be accessed relatively easily. The support is slightly curved so that the portion of the riser 50 is that is in contact with the support is supported substantially over the length of the curved surface of the support.

A catenary connecting line 60 which hangs in a single catenary curve connects the intermediate buoy 30 to the mooring buoy 20. A corresponding portion 50a of the riser extends from the support 33 to the mooring buoy 20 in a substantially same vertical plane as the connecting line 60. When seen in said vertical plane said portion 50a of riser 50 extends substantially along its entire length above said connecting line 60, with a curvature that is less than the curvature of the connecting line 60.

Only a single anchor line in the form of catenary anchor line 61 is attached to the intermediate buoy 30 for anchoring said buoy to the sea bed 40. The catenary anchor line 61 has an end 62 that is connected to the sea bed in such a manner that the intermediate buoy 30, at least when seen in projection on the horizontal plane, lies between said end 62 and the mooring buoy 20 and such that said end 62 lies between the point 52 where the riser is connected to the sea bed and the intermediate buoy 30. In case of horizontal movement of the buoy 30 relative to the sea bed 40, a limit to such movement is thus imposed by the catenary anchor line 61 rather than by the portion 50b of the riser 50 which extends from the support to the sea bed. Though preferably the riser portion 50b and the catenary anchor line 61 lie substantially in a same vertical plane, this is not required.

Together, the catenary connecting line 60 and said catenary anchor line 61 allow a range of horizontal movement of the intermediate buoy 30 relative to the mooring buoy 20 while minimizing flexing of the riser 50. The mooring system of Fig. 1 in this manner provides an increased allowable excursion of vessel 10 relative to the mooring buoy without excessive strain on the riser 50. The lines are provided with respective weights 71 ,72 and 73 which help define the slope of the lines. The lines 60, 61 , together with the weights and the riser, urge the buoys to move to a neutral position relative to each other in which they are at a substantially predetermined horizontal distance d4 from each other.

In order to negate the risk of collision between the intermediate buoy 30 and the vessel 10, the length of the catenary connection line 60 is selected such that no part of the intermediate buoy can be moved further away from the vertical axis of rotation V of the arm 23 than the length of the arm. To also prevent the bottom of the vessel from colliding with that portion of the riser 50 which extends from the support 33 to the sea bed 40, the intermediate buoy 30 is arranged for letting the portion of the riser slope in such a manner that, in as far as said portion lies outside of a riser clearance radius R1 from the vertical axis of the mooring buoy 20, it is arranged completely below a level of the bottom of the vessel 10. Thus, as long as the bottom of the vessel remains outside the riser clearance radius R1 it cannot collide with said riser portion. The riser clearance radius R1 is smaller than the length of the arm 23 when see projected onto the horizontal plane. In Fig. 1 only a section of the riser portion 50b that lies within a distance d5 of the support 33 is arranged above the level d1 of the bottom of the hull 2. Though in fig 1 the greatest part of riser portion 50a also lies below this level, the entire riser portion 50a between the intermediate buoy and the mooring buoy may alternatively lie at or above this level without risk of colliding with the vessel.

Fig. 2A schematically shows a top view of the vessel and mooring system of Fig. 1 , in which the arm connecting the vessel to the intermediate buoy no longer lies in a same vertical plane as the catenary line 60 between the intermediate buoy 30 and the mooring buoy 20, due to weathervaning of the vessel around the mooring buoy 20 over an angle β. The buoys 20,30 of the mooring system 1 are shown in a neutral position in which the magnitude of the force on the catenary connection line 60 for pulling the intermediate buoy towards the mooring buoy is substantially the same as the magnitude of the force on the catenary anchor line for pulling the intermediate buoy in the opposite direction.

The two buoys 20,30 can move horizontally relative to each other, e.g. due to waves and/or movement of the mooring buoy which results from movement of the vessel 10. As an example the intermediate buoy 30 is also shown dotted lines at two positions in which it has moved in a direction parallel to the arm 23, and possible movement of the mooring buoy 20 is indicated by arrows in Fig. 2A. When the buoys 20,30 are not in the neutral position, the weight of the catenary connection line 60, the catenary anchor line 61 , the riser 50 , and optionally any additional weights connected thereto, will urge the buoys back to the neutral position.

Fig. 2B shows a top view of another configuration of the mooring system of Fig. 1 , in which the vessel has weathervaned around the mooring buoy 20. Solid lines indicate the catenary connection line 60, riser portion 50a, riser portion 50b and catenary anchor line 61 aligned substantially in a same vertical plane. The same lines and riser portions, but in a different orientations due to horizontal movement of the intermediate buoy relative to the mooring buoy, are shown in Fig. 2B in dotted lines and are referred to as catenary connection line 60', riser portion 50a', riser portion 50b' and catenary anchor line 61 '. Regardless of the horizontal movement between the two buoys 20,30, the riser portion 50a' extends generally in a same vertical plane as the catenary connection line 60'. However, in the projection on the horizontal plane shown in Fig. 2B, the catenary connection line 60' is at a non-zero angle relative to both the catenary anchor line 61 ' and the riser portion 50b'. Likewise, the catenary anchor line 61 'is at a slight angle a to its corresponding riser portion 50b'. As the points of connection 52 and 62 of the riser and the catenary anchor line to the sea bed remain in place, and as both the catenary connection line 60' and the catenary anchor line 61 " are shorter than their respective riser portions 50a' and 50b', these lines absorb most of the tension caused by the movement between the buoys, thus reducing the tension on riser portions 50a, 50b'.

Fig. 3 shows an alternative embodiment of the mooring system according to the invention. The vessel, mooring buoy and intermediate buoy are the same as in Fig. 1 , with the exception that the intermediate buoy 30, instead of being connected by only a single catenary anchor line to the sea bed 40, is connected to the sea bed by two catenary anchor lines 63, 65 which respectively are attached to the sea bed at connection points 64 and 66. Both lines, when viewed in projection on the horizontal plane as shown in Fig. 3, are connected to the sea bed on a side of the intermediate buoy which faces away from the mooring buoy, and both points of connection to the sea bed lie closer to the intermediate buoy 30 than the point of connection 52 of the riser 50 to the sea bed. Though in this configuration horizontal movement between the intermediate buoy and the mooring buoy is restricted to a greater extent than in the embodiment of Fig. 2A, this configuration still allows significant excursion of the vessel 10 while reducing strain on the riser. The present invention has been described above with reference to a number of exemplary embodiments as shown in the drawings. Modifications and alternative implementations of some parts or elements are possible, and are included in the scope of protection as defined in the appended claims.

Claims

1. Mooring system (1 ) for a floating vessel (10) for storing hydrocarbons, said system comprising:

a mooring buoy (20) anchored to the sea bed (40) via anchor lines (41 ,42,43), said mooring buoy comprising a submerged buoyancy member (21 ), a support frame (22) connected to the top of the buoyancy member and projecting above the water surface (W), the support frame (22) carrying an arm (23) above said water surface which arm is pivotable relative to said frame around a vertical axis of rotation (V), wherein the arm (23) projects outboard from said support frame (21 ) and is adapted for connecting to said vessel (10) to allow said vessel to weathervane around the mooring buoy (20), wherein at least one hydrocarbon riser (50) is attached to the mooring buoy (20), and wherein said mooring buoy (20) comprises a fluid connection (24) supported by said arm (23) and connected at one end (24b) to said riser (50) and connectable at another end (24a) to said vessel (10), for transporting said fluid hydrocarbons from the riser to the vessel;

characterized in that said system further comprises:

an intermediate buoy (30) arranged floating between the mooring buoy (20) and a point of connection (52) of the riser to the sea bed (40), wherein said intermediate buoy is provided with a portion (31 ) extending above the water and a submerged portion (32) under the water, said submerged portion comprising a support (33) for supporting a completely submerged section of said riser (50) thereon, wherein said intermediate buoy (30) is connected to said mooring buoy (20) by a catenary connecting line (60) and is connected to the sea bed (40) by a catenary anchor line (61 ) which curves towards the sea bed (40), said catenary connecting line and said catenary anchor line together allowing horizontal movement of the intermediate buoy relative to the mooring buoy.

2. Mooring system according to claim 1 , wherein said catenary connecting line (60) is arranged for maintaining said intermediate buoy (30) within a shorter distance (d4) to said vertical axis of rotation (V) than the length of the arm (23), preferably wherein said arm has a length of at least 1.3 times said distance (d4).

3. Mooring system according to claim 1 or 2, wherein the intermediate buoy (30) is arranged for letting a portion (50b) of the riser (50) slope from the support (33) to the sea bed (40) in such a manner that, in as far as said portion (50b) lies outside of a riser clearance radius (R1 ) from said vertical axis (V) of said mooring buoy (20), it is arranged completely below a level of the bottom (3) of the vessel (10), wherein, when viewed in projection onto the horizontal plane, said riser clearance radius (R1 ) is smaller than the length of the arm.

4. Mooring system according to any one of the preceding claims wherein said catenary anchor line (61 ) is the only anchor line that is attached to the intermediate buoy (20) for anchoring said buoy to the sea bed (40).

5. Mooring system according to any one of the preceding claims, wherein the support (33) of the intermediate buoy is arranged at a level in said water above the level of the bottom (3) of the vessel.

6. Mooring system according to any one of the preceding claims, wherein, when seen in projection onto the horizontal plane, said intermediate buoy (30) is arranged substantially between the mooring buoy (20) and a point of connection (52) of the riser (50) with the sea bed (40).

7. Mooring system according to claim 6, wherein, when seen in said projection onto the horizontal plane, the point of connection (62) of the catenary anchor line with the sea bed is arranged substantially between the intermediate buoy (30) and the point of connection (52) of the riser (50) to the sea bed.

8. Mooring system according to any one of the preceding claims, wherein the total length of said riser (50) between the sea bed (40) and the mooring buoy (20) is greater than three times the vertical distance between said support (33) and the sea bed (40).

9. Mooring system according to any one of the preceding claims, wherein said catenary connection line (60) and/or said catenary anchor line (61 ; 63,64) is provided with weights, for urging the intermediate buoy (30) and the mooring buoy (20) to a substantially predetermined horizontal distance from each other.

10. Mooring system according to any one of the preceding claims, wherein the connection of said fluid connection to the riser is arranged below the water.

1 1. Mooring system according to any one of the preceding claims, wherein said riser has an inner diameter of at least 10 inch, preferably between 10 inch and 26 inch.

12. Mooring system according to any one of the preceding claims, the riser is substantially comprised of steel riser segments or bonded hoses.

13. Mooring system according to any one of the preceding claims, wherein the portion (50b) of the riser between the sea bed (40) and the support slopes in a single catenary curve having its highest point at or near the support (33), and wherein the portion (50a) of the riser between the support and the mooring buoy sloped in another single catenary curve.

14. Method of installing a mooring system comprising a mooring buoy (20) anchored to the sea bed (40) via anchor lines (41 ,42,43), said mooring buoy comprising a submerged buoyancy member (21 ), a support frame (22) connected to the top of the buoyancy member and projecting above the water surface (W), the support frame (22) carrying an arm (23) above the water surface which arm is pivotable relative to said frame around a vertical axis of rotation (V), wherein the arm (23) projects outboard from said support frame (21 ) and is adapted for connecting to a vessel (10) to allow said vessel to weathervane around the mooring buoy (20), wherein at least one hydrocarbon riser (50) is attached to the mooring buoy (20), and wherein said mooring buoy (20) comprises a fluid connection (24) supported by said arm (23) and connectable at one end (24b) to said riser (50) and connectable at another end (24a) to said vessel (10), for transporting said fluid hydrocarbons from the riser to the vessel, said mooring system further comprising an intermediate buoy (30) arranged floating between the mooring buoy (20) and a point of connection (50a) of the riser to the sea bed (40), wherein said intermediate buoy is provided with a portion (31 ) extending above the water and a submerged portion (32) under the water, said submerged portion comprising a support (33) for supporting a completely submerged section of said riser (50) thereon, said method comprising the steps of:

determining a first position (52) where the riser (50) is connected to the sea bed (40); determining a second position of the vertical axis of rotation (V) of the mooring buoy (20); connecting a catenary connection line (60) on one end to said intermediate buoy (30) and on another end to said mooring buoy (20), and connecting a catenary anchor line on one end to said intermediate buoy and on another end to the sea bed at a location between the first and second position in such a manner that, during floating movement of said intermediate buoy, the catenary connection line remains completely spaced apart from the sea bed and a maximum horizontal distance between the intermediate buoy and the second position is less than the horizontal length of said arm,

lifting the riser (50) from the sea bed and arranging a section thereof on the support of the intermediate buoy so that a portion (50b) of said riser curves in such a manner from the support to the sea bed (40) that at any point where said portion (50b) is spaced further apart from the second position than the length of said arm, the riser is arranged completely below level of the bottom of the vessel.