WO2000021825A1

WO2000021825A1 - A loading buoy

Info

Publication number: WO2000021825A1
Application number: PCT/NO1999/000310
Authority: WO
Inventors: Sigmund Askestad; Vidar Aanesland
Original assignee: Den Norske Stats Oljeselskap A.S.
Priority date: 1998-10-14
Filing date: 1999-10-13
Publication date: 2000-04-20
Also published as: NO984778L; AU6374099A; NO984778D0; NO319945B1

Abstract

A loading buoy for the transfer of hydrocarbon fluids to a floating vessel (10) via a loading hose (9) floating in the sea, comprises a bottom-anchored main unit (13) having one or more through-going passages for connection of risers (8) to a swivel means (21) for fluid transfer via a pipe connection (26) to the loading hose. The loading buoy comprises a tail member (14) projecting laterally outwards from the main unit (13) in a connection therewith which is rigid in the vertical plane. The tail member (14) constitutes a hull member floating in the water next to the main unit (13) in buoyancy equilibrium therewith. The tail member (14) is provided with a connecting point for a hawser (11) for mooring of the vessel (10) to the buoy (1), and said pipe connection (26) preferably extends along the tail member (14) and is arranged for connection to the loading hose (9) at the outer end of the tail member at a distance from the main unit (13).

Description

A loading buoy

The invention relates to a loading buoy for the transfer of hydrocarbon fluids to a floating vessel via a loading hose floating in the sea, comprising a bottom- anchored main unit having one or more through-going passages for connection of risers to a swivel means for fluid transfer via a pipe connection to the loading hose.

Customary loading buoys of this type, i.e. buoys for the loading of standard tankers, comprise a main unit having a round hull, wherein the associated mooring lines are connected along the circumference of the buoy. The hull typically has a diameter of 12 - 20 m and a depth of 5 - 8 m from the bottom to the deck of the boy. At the centre of the buoy there are arranged passages for introduction of risers which are coupled to a swivel at the centre of the buoy. The tanker is moored by means of a hawser to a rotating table at the top of the buoy, the rotating table being centrally mounted in connection with the swivel and supporting an outer rotating part thereof. The inner, stationary part of the swivel is supported by the buoy deck and connects the risers to export pipes on the rotating table via the outer rotating part.

The rotating table on these buoys thus covers the entire deck of the buoy. Usually it consists of three arms of which a first arm extends in the direction of a hawser which is connected to the tanker, a second arm supports the export pipes and is arranged for connection of the floating loading hose, and a third arm extends in the opposite direction to that of the hose connection and forms a basis for mooring of small boats.

These conventional buoy structures are encumbered with several drawbacks which can be summarised as follows:

• The rotating table sweeps over the whole deck, something which entails problems in carrying venting of possible tanks up from the buoy hull.

• It is likely that there have to be installed electrical collector shoes to protect the rotating table bearing against lightning strokes, because of the fact that the buoy forms the best earth connection to the sea, and the highest point is mounted on the rotating table.

• The rotating table with the arms with the export pipes is subject to damages as a result of collisions. With a deformation as a result of a smaller collision the rotating table may be locked, or the swivel for transfer of oil to the tanker in more serious cases may be damaged.

• There is a limited space for equipment on the buoy deck.

• A major part of the deck area will be region classified, where demands are made on the equipment with respect to fire and explosion safety. • The buoy is subject to a great overturning moment with appurtenant heeling in case of large hawser forces, since the hawser attacks at a point high above the fixing of the mooring lines.

The object of the invention is to provide a buoy structure making it possible to eliminate the above-mentioned drawbacks, whereby the structure inter alia entails a substantially increased stability of the buoy under the influence of external forces, there is provided an extra area which is suitable for placing of equipment and which implies reduced demands on equipment with respect to fire and explosion safety, and there is provided a safe and reliable structure which also gives operational advantages.

The above-mentioned object is achieved with a loading buoy of the intro ductorily stated type which, according to the invention, is characterised in that it comprises a tail member projecting laterally outwards from the main unit in a connection therewith which is rigid in the vertical plane, the tail member constituting a hull member floating in the water next to the main unit in buoyancy equilibrium therewith, and the tail member being provided with a connecting point for a hawser for mooring of the vessel to the buoy.

The invention will be further described below in connection with exemplary embodiments with reference to the drawings, wherein Figs. 1 and 2 show a side view and a plan view, respectively, of a bottom- anchored buoy according to the invention, wherein a tanker is moored to the buoy by means of a hawser, and a floating loading hose extends between the buoy and the tanker;

Fig. 3 shows a schematic plan view of the buoy in Figs. 1-2 on an enlarged scale, and with connected hawser and loading hose during a loading operation; Fig. 4 shows a schematic vertical longitudinal section of the buoy; Fig. 5 shows a schematic sectional view along the line V-V in Fig. 3; Fig. 6 shows a schematic sectional view along the line VI-VI in Fig. 3; Fig. 7 shows a plan view of the buoy, with a tender vessel moored along a side of the deck of the buoy;

Figs. 8 and 9 are a side view and a plan view, respectively, illustrating how forces from the hawser act on the buoy in relation to anchoring and buoyancy forces; Fig. 10 shows a corresponding plan view to that of Fig. 3, but shows the buoy as it floats with hawser and loading hose in the sea, when the tanker is disconnected;

Figs. 11 and 12 show a side view and a plan view, respectively, of an alternative buoy structure according to the invention; and

Fig. 13 shows a more detailed perspective view of a buoy structure of the type shown in Figs. 1-10. Corresponding parts are designated by similar reference numerals in the different Figures in the drawings.

Figs. 1 and 2 show a buoy 1 floating on the surface 2 of a body of water 3 (sea or ocean) and which is anchored to the seabed 4 by means of anchoring or mooring lines 5 and associated anchors 6. Along the seabed there extends a pipeline 7 passing into a riser 8 carrying oil from the pipeline up to the buoy 1, through the buoy and further via a loading hose 9 to a vessel 10 which, e.g., may be a usual shuttle tanker. Fig. 1 shows only one riser 8, but several risers could possibly be coupled to the buoy. As shown in Fig. 2, which applies to the conditions during a loading operation, the loading hose 9 floats on the water and follows current and weather in the direction towards the vessel 10 which, in its bow portion, is moored to the buoy 1 by means of a hawser 11. The end of the hose at the vessel is coupled to the loading manifold 12 thereof, for distribution of the oil to the loading tanks of the vessel. As shown in Figs. 3 and 4, the buoy 1 comprises a main unit 13 and a tail member 14 which projects laterally from the main unit in a connection therewith which is rigid in the vertical plane, and which forms a hull member floating in the water in buoyancy and weight balance with the main unit. In the embodiment according to Figs. 1-10, which is a preferred embodiment, the main unit 13 is of the type comprising a bottom-anchored, central member 15 and an outer buoyancy member 16 rotatably mounted thereon, the central member 15 having one or more through-going passages or guide tubes (not shown) for the receipt of risers 8.

In the illustrated embodiment, the buoyancy member 16 is a circularly cylindrical member having a central opening 17 (Fig. 6) for receipt of the central member 15 which is usually designated "rotating tower" or "turret". The buoyancy member 16 may be divided into a number of watertight compartments. The rotating tower 15, which may be designed in many different ways, is provided with a suitable means (not shown) for suspension of the risers 8, of which only one is shown in Fig. 4 whereas two are shown in Fig. 6. As suggested in Fig. 6, the rotating tower is mounted by means of a bearing 18 which is situated just above the water line 2.

As further shown in Figs. 4 and 6, the risers at their upper end are connected to one or possibly more shut-off valves 19 which are mounted on the rotating tower deck 20. After the shut-off valves, the risers are brought together and connected to a swivel means 21 which is mounted on an upper part of the rotating tower, more specifically on a so-called pipe deck 22.

On the buoyancy member 16 there is mounted a supporting means 23 (not shown in Fig. 6) for the support of a first part of the pipe connection between the swivel means 21 and the loading hose 9. The pipe connection is shown to comprise a first pipe member 24 which, via a shut-off valve 25 (omitted in Fig. 6), is connected to a rigid loading pipe 26 extending along the tail member 14 and at an outer end thereof being connected to the loading hose 9 via a reinforced and soft hose member 27.

In Fig. 4 there is also shown a winch 28 arranged on the tail member 14, for pulling in mooring lines via a guide disc 29 in connection with anchoring of the buoy. As shown, the mooring lines 5 are suspended at the lower outer peripheral edge of the rotating tower 15.

The tail member 14 is rigidly connected to the main unit 13 of the buoy and in the shown embodiment is an elongated member extending radially out from the main unit and having a width which is substantially less than the width of the main unit, i.e. the outer diameter thereof in the illustrated, circularly cylindrical embodiment. As shown in Figs. 3 and 5, a generally rectangular, relatively lightweight deck 30 having a width generally corresponding to the width of the main unit, is mounted over the tail member 14, and possibly over a part of the main unit 13. Along each longitudinal side edge of the deck there is arranged a depending fender means 31. The deck is partly supported by the tail member 14 and partly by a number of inclined struts 32 on each side of the tail member.

The advantage of the fact that the deck is equally wide as the main unit 13 of the buoy 1, is that it is then easier for a larger boat 33 to moor along one side 34 of the buoy, as shown in Fig. 7. Simultaneously, the deck will constitute a rejecting means for the tanker 10, and also act as a fender for the rear part of the buoy hull, and as a fender and protection for the loading pipe 26 which is placed within a side edge of the deck and partly under the deck, as appears from Figs. 3 and 5.

As shown in Fig. 3, the loading pipe 26 is mounted such that it diverges laterally at a chosen angle of 25° - 30° with the longitudinal axis of the tail member 14 in a direction rearwards and away from the main unit 13. This entails that the connection between the pipe 26 and the hose 9 is situated so far as possible outwards towards one side of the buoy, but still within and protected by the fender means 31.

The direction of the buoy 1 during loading operations is controlled essentially by the tension in the hawser 11 , which is directed through the centre of the rotating tower of the buoy. Fig. 3 shows how the buoy will orient itself in relation to the turning point of the rotating tower, more specifically so that the longitudinal axis of the tail member 14 will coincide with the hawser direction, and so that the connection point for the hose 9 will be turned laterally outwards. Because of the mentioned inclined position of the loading pipe 26, the hose is directed slantingly outwards and away from the tanker 1. This is an essential and advantageous feature of the buoy structure, as the loading hose gets a favourable direction laterally away from the hawser 11, so that the distance between the hose and the ship becomes more favourable. Figs. 8 and 9 show the forces acting on the buoy 1 during a loading operation.

As mentioned above, the tanker 10 in its bow portion is moored to the buoy 1 by means of the hawser 11 , so that the ship "hangs" in the buoy at the leeward side thereof. The "weather direction", i.e. the wind and current direction, is indicated by the arrow W in Fig. 8. The hawser secures that the buoy adopts a stable direction in relation to the ship in that it is fastened and guided through a guide at the rear edge of the buoy, so that there is achieved a moment-giving arm for turning of the buoy about the centre axis of the rotating tower. The hawser thereby gets a controlling effect because of the moment-giving arm which is designated Lj in Fig. 9. Since the force from the hawser 11 is substantial in relation to the force from the loading hose 9 during a loading operation, the buoy will take a position which is generally controlled by the fixing point of the hawser and the centre of the rotating tower. In the illustrated embodiment both are situated in the longitudinal axis of the buoy. As shown in Fig. 8, the hawser force F_H acts in a direction slantingly upwards which is determined by the bow height H_B of the ship, the fixing height of the hawser on the tail member 14 of the buoy, and the distance L₂ between the ship and the tail member. The hawser force may be up to 200 - 300 tons, which is substantial in relation to the displacement of the buoy, which typically can be between 500 and 1500 tons for such loading buoys. It is therefore of importance that the forces from the ship attack the buoy in such a manner that too large heeling moments on the buoy do not arise.

In Fig. 8 the extension of the hawser force F_H and the tension from the mooring lines 5' and 5" on the leeward side and weather side, respectively, of the buoy are shown with stippled lines. The most advantageous heeling angle for the buoy is obtained when the force directions meet in a common point, as in Fig. 8.

It is seen from Fig. 8 that the fixing point for the hawser 11 on the tail member 14 instead of on the edge of the cylindrical member 13, gives a smaller moment arm for the hawser force. It is further seen that it is advantageous with a fixing of the mooring lines 5 on a rotating tower or turret having a small diameter, typically 2 - 6 m, in relation to a fixing at the outer circumference of a conventional buoy which typically has a diameter of 12 - 20 m, such as mentioned in the introduction. With a conventional buoy the heeling moments will be substantial.

Because of the hawser force there will be induced an increasing force in the mooring line 5" at the weather side, which force is larger than the force relief in the mooring line 5" on the leeward side. This entails that the buoy sinks somewhat down under the influence of the hawser force F_H. In Fig. 9, the water line area of the main unit 13 is designated A_M, whereas the water line area of the tail member 14 is designated A_τ. Advantageously, the area A_M is larger than the area A_τ, something which entails that the buoy 1 essentially sinks parallel to itself when applying a vertical load to the buoy.

The water line area A_τ in practice will be controlled by the requirement for a buoyancy volume of the tail member, and this buoyancy volume will be adapted to the weight of the tail member and the equipment placed thereon. This water line area will entail that the buoy will be influenced by a little upwards directed force with an increase of the draft of the buoy. This force causes a small trim change of the front portion of the buoy (the draft increases ahead and decreases astern) with an increased vertical load from the anchor system. This tendency is strengthened by the upwards directed hawser force. Said water line area will act stabilising on the buoy in case of a heeling moment from the hawser.

Thus, the optimum placing of the fixing point for the hawser will be dependent on the water line area of the tail member. In order to compensate for the small trim angle forwards from the tail member, it will be advantageous with a small "eccentricity" between the hawser force direction and the point of intersection between the mooring line forces, so that the hawser force will contribute with a small aft trim moment.

As will be clear, the hawser force direction will change with increasing draft of the tanker as it is filled with oil. The same hawser force thereby will give a trim change of the buoy as the oil load of the ship increases. The most favourable placing of the fixing point for the hawser will be a point giving a small aft trim when the tanker is fully loaded, and a small trim of the bow of the buoy when the ship is in ballast.

Fig. 10 shows how the buoy 1 orients itself in the sea in inactive condition, i.e. when the tanker 10 is disconnected and the hose 9 as well as the hawser 11 float passively with the current. The tension in the floating loading hose and the effect of the current and the waves along the sides of the buoy are then essential for turning of the buoy with the weather. The tail member will then position itself in the direction with the weather, and it is avoided that the hawser and the hose twist themselves around the buoy or get entangled in bad weather. An essential reason why the buoy takes the advantageous direction with the fixing points of the hose and the hawser in the weather or co-current direction, is that the pull in the hose acts far astern with a large moment arm about the centre axis of the rotating tower or turret.

Another effect which is of importance for the weather-turning property of the buoy, is the length of the buoy, and that the tail member behind the main unit of the buoy will act as a fin for the orientation of the buoy. The effect is that when the buoy turns outwards, the current will turn the buoy back in the direction of the current. In addition to the advantages with respect to the directional stability of the buoy, the hull shape of the buoy is also advantageous for the motion and behaviour of the buoy when it is subjected to loads from the hawser 11.

The principle of the shape of the hull in the water line and under water is shown in Fig. 9, wherein the water line areas A_M and A_τ are shown as hatched fields. The main purpose of the outer part 16 of the main member 13 of the buoy is to produce buoyancy for supporting the tension forces from mooring lines and risers, together with the weight of the equipment on this part of the buoy. As appears from Fig. 4, a great part of the equipment and the corresponding weight will be concentrated in this region of the buoy. It is therefore in this region that there is a need for buoyancy.

In the illustrated example the tail member has a relatively narrow, rectangular water line area A_τ. The water line area may of course also have other shapes, and for instance be trapezoidal or a combination of rectangle and trapezium shape, or possibly have a partly curved shape. In the embodiment according to Figs. 3-6, the tail member has transverse ribs having a rectangular shape, but they could alternatively be e.g. V- or U-shaped. A V-shaped cross-section of the underwater hull can be used to reduce the buoyancy in the region without loosing too much water line area. This could be an advantage in cases where there is no need for large weights in this aft part of the buoy. In other cases there may be a need for extra buoyancy, so that it will then be of interest to increase the width of the tail member in relation to the embodiment in Fig. 5.

The buoy structure described above has a number of advantages which may be summarised as follows: • The buoy has an internal rotating tower or turret that protects the buoy with risers and anchoring terminations against impact.

• All equipment is placed within the buoy. The loading pipe (export line) is placed under the overhanging deck, so that the rigid pipe part is also protected against interaction with the tanker. • The buoy has an extent allowing the equipment, such as a cabinet for a control system and winches, to be located outside of zone-classified regions (as opposed to a standard buoy wherein the export system will sweep over the whole circumference of the deck).

• The length of the tail member of the buoy gives an advantageous vector diagram for the mooring forces, so that the trim angle changes with a hawser force change will be minimal.

• The tail member has a certain extent and width, so that a balance is obtained with respect to weight and buoyancy, also in the longitudinal direction of the buoy. • The rotating tower is placed centrally in the buoyancy member, so that the large vertical forces from anchoring and risers attack at the centre, without causing any trim angle or moments in the hull.

• The tail member with its extent in the longitudinal direction sees to it that the buoy achieves a weather-vane effect also when the buoy is inactive, i.e. when the tanker is disconnected and the loading hose floats in the sea downstream of the buoy.

• The extent of the buoy provides good mooring possibilities for relatively large boats, with a landing platform having a length of 15 - 20 m, as opposed to landing platforms having a length of 3 - 4 m for a standard buoy.

• With the rotating tower built in, the highest point on the buoy will be naturally earthed to the hull and the water, so that lightning will be guided past rotating tower bearing and swivel without expensive brush systems.

Figs. 11 and 12 show schematic views of an alternative embodiment of the buoy according to the invention. This buoy 35 comprises a main unit in the form of a buoyancy body 36 which, in this embodiment, is shown to be circularly cylindrical, and wherein a number of mooring lines 5 are fastened to the outer circumference of the body. The rotating tower in the embodiment described above is omitted, and is replaced by a bearing 37 for a rotating table 38. The rotating table is rigidly connected with a tail member 39, and on the rotating table and the tail member there is mounted a deck 40 corresponding to the deck 30 in the preceding embodiment. A loading hose and a hawser (not shown) will be connected to the tail member 39, in a corresponding manner as in the embodiment according to Figs. 1-10.

The relation between the buoyancy of the main member 36 and the tail member 39 is as for the embodiment described above, and the buoyancy of the tail member is adapted to the load which is to be placed on the deck. Further, the length of the tail member is adapted to the requirements for minimising heeling forces from the hawser on the buoy, in accordance with the principles described above, and also in accordance with the above-mentioned weather-vane principles. This embodiment of the buoy has a rotating table 38 which, because of a smaller weight, will rotate more easily with changes in weather directions than the buoy described above. The rotating tower buoy will, however, have a bearing which is more protected against impact loads from the tanker against the buoy, and against fatigue forces from the buoy, as a result of high-frequency buoy-induced loads acting in the hawser between the buoy and the tanker.

As regards the tail end 39, this may have different alternative geometries, as viewed from above. For example, it may be trapezoidal, with an increasing width in the rearward direction. The advantage of such a shape is that it gives a little additional hydrodynamic resistance at the rearward end, something which is advantageous for weather-vane action. In addition, the water line area contributes to a larger resistance against trim changes of the buoy as a result of a trimming force (heeling in the longitudinal direction) from e.g. the hawser. Another possible tail shape is a rearwards tapered shape which, together with the main unit of the buoy, gives a streamline drop shape with favourable flow conditions. The tail member may also have cylindrical or elliptic shape, or any shape provided it contributes to buoyancy in order to carry the load of the buoy, or to achieve the desired weather- vane properties for the buoy.

Fig. 13 shows a more detailed perspective view of a buoy 1 of the type shown in Figs. 1-10. In the Figure there are used corresponding reference numerals to those used in Figs. 1-10.

In the same manner as shown in Fig. 7, a tender vessel 33 may be moored in the longitudinal direction of the buoy along the side 34 of the deck 30, to bring elements and equipment between the loading buoy and the vessel, via a lifting means (not shown) which is displaceable along a crane path 41 mounted on the main unit 13 of the buoy. This crane path extends over the whole main unit and ahead to a lay- down area 42 on the deck 30.

The shown crane 28 may be used to pull in the mooring lines 5 over the guide pulley 29 without the buoy having to be turned in order to handle the individual lines.

In the embodiment of Fig. 13, the tail member 14 as shown has a front portion having a trapezoidal water line area, and a rearward rectangular portion. This configuration is advantageous with respect to building costs.

Claims

Patent Claims

LA loading buoy for the transfer of hydrocarbon fluids to a floating vessel

(10) via a loading hose (9) floating in the sea, comprising a bottom-anchored main unit (13) having one or more through-going passages for connection of risers (8) to a swivel means (21) for fluid transfer via a pipe connection (26) to the loading hose, characterised in that it comprises a tail member (14; 39) projecting laterally outwards from the main unit (13; 36) in a connection therewith which is rigid in the vertical plane, the tail member (14; 39) constituting a hull member floating in the water next to the main unit (13; 36) in buoyancy equilibrium therewith, and the tail member (14; 39) being provided with a connecting point for a hawser (11) for mooring of the vessel (10) to the buoy (1 ; 35).

2. A loading buoy according to claim 1, characterised in that said pipe connection (26) extends along the whole tail member (14; 39) and is arranged for connection to the loading hose (9) at the outer end of the tail member at a distance from the main unit.

3. A loading buoy according to claim 1 or 2, characterised in thatthemainunit (13) is of the type comprising a bottom- anchored central member (15) and an outer buoyancy member (16) rotatably mounted thereon, the central member (15) comprising the through-going passages for risers (8), and that the tail member (14) is rigidly connected to the buoyancy member (16).

4. A loading buoy according to claim 1 or 2, characterised in that the main unit (36) on its upper side is provided with a bearing means (37) for bearing of a rotating table (38) which is rigidly connected to the tail member (39).

5. A loading buoy according to one of the claims 1-4, characterised in that the tail member (14; 39) constitutes an elongated member extending radially outwards from the main unit (13; 36), and having a width which is substantially less than the width of the main unit.

6. A loading buoy according to claim 3, characterised in that the tail member (14) has a rectangular or trapezoidal water line area, or a combination thereof.

7. A loading buoy according to claim 3 or 6, characterised in that the tail member (14) is rectangular, V-shaped or U- shaped in cross-section.

8. A loading buoy according to one of the preceding claims, characterised in that, over the tail member (14; 39), there is mounted a deck (30; 40) having a width essentially corresponding to the width of the main unit (13; 36).

9. A loading buoy according to claim 2, characterised in that said pipe connection (26) comprises a rigid loading pipe which is mounted on the tail member (14) so that it diverges laterally at a chosen angle with the longitudinal axis of the tail member (14) in the direction away from the main unit (13).

10. A loading buoy according to one of the preceding claims, characterised in that the connection point of the tail member (14) for the mooring hawser (11) is arranged at the outer end of the tail member, and is placed at such a height on the tail member (14) that forces from the hawser (11) during operation exert an insignificant heeling force on the buoy (1).

11. A loading buoy according to claim 8, characterised in that, on at least one longitudinal side (34) of the deck (30), there are mounted fender means (31) for protection of the buoy and equipment thereon against external influences.