OA10932A

OA10932A - Method and system for anchoring and positioning ofa floating vessel and a vessel including such a s ystem

Info

Publication number: OA10932A
Application number: OA9800232A
Authority: OA
Inventors: Kaare G Syvertsen; Sigurd Falch
Original assignee: Den Norske Stats Ojeselskap A
Priority date: 1996-06-11
Filing date: 1998-12-04
Publication date: 2002-02-18
Also published as: BR9710847A; NO962454L; EP0904231B1; EP0904231A1; NO962454D0; WO1997047516A1; ID19491A; AU3195297A; NO315265B1

Abstract

A method and a system for positioning of a floating vessel (1) against the wind and/or wave/current direction prevailing at any time, especially a vessel for the production of hydrocarbons from a subsea source. The system comprises a number of anchor lines (4-9) which, at one of their ends, are fastened to respective winches (10-15) placed in the bow (16) and stern (17) of the vessel, and at their other ends are fastened to respective anchors (18-21) anchored in the seabed at places located at chosen horizontal distances from the vessel (1) and having a chosen mutual angular distance measured along a horizontal circle with the centre in the vessel. The method consists in that the vessel by hauling-in and slackening, respectively, of chosen anchor lines by means of the associated winches is turned and oriented with its longitudinal axis (L) within an angular range of about 180 DEG , so that the bow (16) or the stern (17) is oriented against the prevailing wind and/or wave/current direction.

Description

1 010932

Method and System for anchorinq and positioninqof a floatinq vessel, and a vessel includinq such a System

The invention relates to a method for positioning of afloating vessel against the wind and/or wave/current direction 5 prevailing at any time, especially a vessel for the productionof hydrocarbons from a subsea source.

Further, the invention relates to a System for anchor-ing and positioning of such a vessel, and a vessel including sucha System. 10 In connection with the operation of production vessels for offshore production of hydrocarbons there are known differenttypes of Systems for anchoring and/or positioning of the topicalship or vessel. Such ships usually are provided with a so-called"moonpool", i.e. a vertical opening through the hull of the ship, Ί5 in which there is arranged equipment for connection and securingof the topical risers extending between the ship and e.g. one ormore wellheads on the sea bed, for transport of the topical wellflow from the subsea source. From known anchoring Systems in thisconnection there may be mentioned the so-called SAL System 20 (Section Anchor Loading), where the topical ship is provided witha mooring means in its bow portion, and where one or more anchorUnes extend between the mooring means and an anchor device atthe sea bed. The anchor device, such as a suction anchor or apile anchor, is provided with a swivel means for the anchoring 25 line, to allow turning of the ship without any adverse influenceon the anchor line. By use in exposed areas it may be necessaryto use a dynamic positioning System as a supplément to theanchoring System.

According to other prior art, production vessels and/or 30 storage vessels are kept anchored and positioned by means of acombined mooring and loading buoy which is anchored to the seabed and in operation is introduced and secured in the moonpoolof the ship, the construction being carried out in such a mannerthat the ship may turn or rotate about the buoy under. the 1 010932 influence of wind, waves and water currents. Said turning of theship is admitted in that the buoy is mounted in a rotating bodyor so-called "turret" which is permanently installed in themoonpool of the ship, or in that the buoy is of the bipartite 5 type comprising a submerged, bottom-anchored centre member andan outer buoy member which is rotatably mounted thereon and inoperation is introduced and secured in a submerged receivingspace at the bottom ’ of the ship. The last-mentioned buoystructure is known under the désignation STP buoy, where theabbreviation STP stands for "Submerged Turret Production".

The Systems according to the above-mentioned prior artare intended for operation under various climatic conditions,including also very changeable and partly adverse weatherconditions, with storm from changing directions at shorter or 15 longer intervals. In offshore production of hydrocarbons inrégions having a relatively calm weather with stable, prevailingwind directions over large parts of the year, it would, however,be désirable to be able to use an anchoring and positioningsystem which is simpler and cheaper than the above-mentioned 2Q known Systems.

Thus, the object of the invention is to provide amethod and a system for anchoring and positioning of a floatingvessel, especially a vessel for the production of hydrocarbons,wherein the method enables the use of a simple anchoring and 25 positioning system where a relatively expensive buoy anchoringsystem, and possibly an expensive dynamic positioning system, isavoided.

For achieving the stated object there is provided amethod of the introductorily stated type which, according to the 30 invention, is characterized in that the vessel has a number ofanchor Unes which at one of their ends are fastened to respec-tive winches placed at the bow and stern of the vessel, and withtheir other ends are fastened to respective anchors anchored inthe sea bed at places lqcated at chosen horizontal distances from 35 the vessel and with a chosen mutual annular distance measuredalong a horizontal circle with its centre in the vessel, and thatthe vessel by hauling-in and slackening, respectively, of chosenanchor lines by means of the associated winches is rotated andoriented with its longitudinal axis within an angular range of 3 010932 about 180°, so that the bow or the stern is oriented against theprevailing wind and/or wave/current direction.

The above-mentioned object is also achieved with aSystem of the stated type which, according to the invention, ischaracterized in that it comprises a number of anchor lines whichat one of their ends are fastened to respective winches placedat the bow and stern of the vessel, and with their other ends arefastened to respective anchors anchored in the sea bed at placeslocated at chosen horizontal distances from the vessel and witha chosen mutual angular distance ' measured along a horizontalcircle with the centre in the vessel, so that the vessel byhauling-in and slackening, respectively, of chosen anchor linesby means of their associated winches can be turned or orientedwith its longitudinal axis within an angular range to a desiredorientation.

According to the ' invention there is also provided avessel for the production of hydrocarbons from a subsea sourceconnected to the vessel via a number of risers, the vesselincluding a System for anchoring and positioning of the vessel,which vessel is characterized in that the System comprises anumber of anchor lines which at one of their ends are fastenedto respective winches placed at the bow and stern of the vessel,and with their other ends are fastened to respective anchorsanchored in the sea bed at places located at chosen horizontaldistances from the vessel and with a chosen mutual angulardistance measured along a horizontal circle with the centre inthe vessel, so that the vessel by hauling-in and slackening,respectively, of chosen anchor lines by means of the associatedwinches can be turned ca. 90°, so that the bow or the stern canalways be oriented against the prevailing wind and/or wave/cur-rent direction.

The invention will be further described below inconnection with exemplary embodiments with reference to thedrawings, wherein

Fig. 1 shows a schematic view of a first embodiment ofan anchoring and positioning System according to the invention;and

Fig. 2 shows a schematic view of a second embodiment ofsuch a system. 4 010932

The invention in the following will be described inconnection with a production ship. It will be clear, however,that the invention can also be used in connection With othertypes of floating vessels, such as storage ships or the like. -'5 In Fig. 1 there is schematically shown a production vessel 1 for the production of hydrocarbons from a subsea sourcefrom which hydrocarbons are supplied to the ship via a number ofrisers which, at their upper ends, are secured in a suitableraanner in relation to the ship. As suggested in Fig. 1, the 10 risers 2 in this embodiment are shown to be fastened in amounting unit "3 arranged at one side of the ship. The mountingunit 3 may be fastened to the ship side in a suitable manner, orit may be provided on or constituted by a separate floating unit,e.g. a suitable barge, which can be secured to or released from 15 the ship as required. The risers are coupled, in a manner notfurther shown, to process equipment on the ship, for example viathe midship manifold thereof (not shown).

The vessel is anchored by means of a number of anchorUnes. 4-9 which, at one of their ends, are fastened to 2Q respective winches 10 - 12 and 13-15 placed at the bow portion16 of the vessel and at the vessel stern 17, respectively, andat their other ends are fastened to respective anchors 18-21anchored in the sea bed at places located at chosen horizontaldistances from the vessel 1. More specifically, the anchor Unes 25 comprise a forward pair of anchor lines 4, 5, a rearward pair ofanchor lines 6, 7, an additional rearward anchor line 8, and anadditional forward anchor line 9. The first pair of anchor lines4, 5 has inner ends (i.e. the ends nearest to the vessel) whichare connected to respective winches 10, 11 at the bow 16 of the 30 vessel, and outer ends (i.e. farthest from the vessel) which areconnected to a respective one of two anchors 18, 19 on oppositesides of the vessel. The rearward pair of anchor lines 6, 7 hasinner ends which.are connected to respective winches 13, 14 atthe stern of the vessel, and outer ends which are connected to 35 a respective one of said two anchors 18, 19. As shown, the winches 13 and 14 are arranged on either side of the vessel. Therearward anchor line 8 is connected to a winch 15 at the stern17 of the vessel, and to a third anchor 20 at a distance asternof the vessel, and the forward anchor line 9 is connected to a 5 010932 winch 12 at the bow 16 of the vessel, and to a fourth anchor 21at a distance ahead of the vessel.

The anchors 18 - 21 may be suction anchors or anchorsof another suitable type. The winches 10 - 14 may be conventional 5 motorized winches or windlasses of any suitable type.

Generally stated, the anchor s 18-21 are arranged with a chosen mutual angular distance measured along a horizontalcircle having its centre in the vessel. The centre of the circlesuitably may be situatèd in the yaw axis of the vessel, i.e. itsvertical axis of rotation. In the illustrated embodiment theanchors are arranged at a mutual annular distance of ca. 90°. Thetwo anchors 18, 19 at opposite sides of the vessel 1 are locatedsymmetrically about the longitudinal axis L of the vessel whenthe anchor Unes 4, 5 and 6, 7 in each of said pairs are equally /|5 long. Further, both the rearward anchor line 8 and the forwardanchor line 9 are essentially parallel to the longitudinal axisL of the vessel when the anchor Unes 4, 5 and 6, 7 in each ofsaid pairs are equally long.

As can be seen when considering Fig. 1, the vessel 1 by 2Q means of the described System may be turned and oriented with itslongitudinal axis L within an angular range of near up to 180°,so that the vessel can be positioned in a desired orientation,more specifically so that the bow or the stern is orientedagainst the topical wind and/or wave/current direction. This

25 turning is achieved by carrying out hauling-in or tightening ofchosen anchor lines at the same time as there is carried out asuitable veering-out or slackening of other anchor lines, in bothcases by means of the associated winches: If the vessel isprovided with a supplementary, dynamic positioning system (DP 3Q System), this may possibly automatically pull in or slacken awayanchoring lines.

In Fig. 1, the vessel 1 is shown in a first position(with solid lines) in which the bow of the vessel may be orientedagainst the prevailing wind or current direction, and a second 55 position (shown stippled) in which the vessel is turned about 75°from the first position. This turning is achieved by a tighteningof the anchor lines 5 and 6, and a simultaneous suitable slacke-ning of the remaining anchor lines 4, 7, 8, 9. .The obtainableturning here will be somewhat less than 90° in each direction, 6 010932 but in practice this will be sufficient on the topical places ofuse.

An additional turning of the vessel would be able to beachieved if the winches were movably mounted on the vessel, sothat they were able to be moved to or placed in a more favourable 5 position in relation to the hull or deck of the vessel in orderto achieve the desired turning.

As shown, the stern of the vessel is formed with arounded or bow-like shape, for the achievement of good flowconditions there, when the wind/current cornes in from the sternof the vessel;

In Fig. 2 there is shown a modified embodiment of theSystem according to the invention. Corresponding éléments aredesignated by similar reference nuraerals in Figs. 1 and 2.

In contradistinction to the vessel 1 in Fig. 1, the15 vessel 22 in Fig. 2 is provided with a through-going hull openingor moonpool 23 in which the topical number or the bundle ofrisers 24 is introduced and connected in a conventional manriernot further shown. As will be clear, the shown riser arrangements also might hâve been interchanged in relation to the vessels.gg As regards the anchoring and positioning System in Fig. 2, this largely corresponds to the System of Fig. 1, apart fromthe fact that the forward anchor line 9 in Fig. 1, and therewithalso the associated winch 12 and the associated anchor 21, areomitted. As appears, this implies that the anchoring position of 25 the vessel 22 relative to the anchors 18 and 19 becomes somewhatdifferent from the configuration in Fig. 1. This implies in turnthat the angular range within which the vessel 22 may be turnedby hauling-in and slackening of the topical anchor lines, becomessomewhat reduced in relation to the embodiment according to Fig. •jg 1. However, the embodiment according to Fig. 2 may be of currentinterest in offshore régions where the wind and current direc-tions are particularly stable, and where it is very unlikely thatthere will occur a strong wind coming from a direction lyingoutside of the possible angular range of vessel turning. 25 The anchoring and positioning System according to the invention may be used also on relatively large océan depths, forexample up to about 1000 m and more. The horizontal distancebetween the vessel and the anchor will vary, according to the 010932 topical operational conditions and the depths at theuse, and dépendent on the type of anchor lines usedhorizontal distances are expected to lie in the range2 km. place ofTopicalof 0,5 - f

Claims

8 010932 Patent claims

1. A method for anchoring of a floating vessel and forpositioning of the vessel against the wind and/or wave/currentdirection prevailing at any time, wherein there are used a numberof anchor Unes which at inner ends are connected to winches atio the bow ànd stern of the vessel, and at outer ends are connectedto respective anchors- anchored in the sea bed at a distance fromthe vessel, the vessel being turned to a desired position by hauling-in and slackening, respectively, of chosen anchor Unesby means of the associated winches, CHARACTERIZED IN that a i5 forward pair of anchor Unes at their inner ends are connectedto respective winches at the bow of the vessel, and at theirouter ends are connected to a respective one of two anchors. atopposite sides of the vessel, that a rearward pair of anchorlinés at their inner ends are connected to respective winches at 20 the stern of the vessel, and at their outer ends are connectedto a respective one of said two anchors, and that an additionalanchor line is connected to an additional winch either at the bowor at the stern of the vessel, and to a third anchor either aheador astern of the vessel, so that the vessel with suitable 25 hauling-in and slackening of the anchor Unes can be turnedwithin an angular range of 160 - 180°.

2. A System for anchoring and positioning of a floatingvessel (1; 22), especially a vessel for the production of hydrocarbons from a subsea source, comprising a number of anchor 3o Unes (4-9) which at inner ends are connected to winches (10 -15) at the bow and stern of the vessel, and at outer ends areconnected to respective anchors (18 - 21) anchored in the sea bedat a distances from the vessel (1; 22), so that the vessel canbe turned by hauling-in and slackening, respectively, of chosen 35 anchor Unes by means of the associated winches, CHARACTERIZEDIN that it comprises a forward pair of anchor lines (-4, 5) whichat their inner ends are connected to respective winches (10, 11)at the bow (16) of the vessel (1; 22), and at their outer endsare connected to a respective one of two anchors (18, 19) at 9 010932 opposite sides of the vessel (1; -22), a rearward pair of anchorUnes (6, 7) which at their inner ends are connected to respective winches (13, 14) at the stern (17) of the-vessel (1;22 ), -and at their outer ends are connected to a respective one 5 of said two- anchors (18, 19), and an additional rearward anchorline (8) which is connected to an additional winch (15) at thestern ( 17 ) of the vessel and to a third anchor ( 20 ) astern of thevessel.

3. A System according tô claim 2, CHARACTERIZED IN thatio the respective winches (13, 14) at the stern (17) of the vessel are placéd on either.side of the vessel (1; 22).

4. A System according to claim 2 or 3, CHARACTERIZED INthat it comprises an additional forward anchor line (9) which isconnected to a winch (12) at the bow (16) of the vessel and to is a fourth anchor (21) ahead of the vessel.

5. A System according to any of the claims 2-4, CHARACTERIZED IN that the two anchors (18, 19) on opposite sidesof the vessel (1; 22) are situated symmetrically about "the longitudinal axis (L) of the vessel when the anchor Unes (4, 5 2o resp. 6, 7) in each of said pairs are essentially equally long.

6. A System according to claim 5, CHARACTERIZED IN thatthe additional rearward anchor line (8) is essentially parallelwith the longitudinal axis (L) of the vessel (1; 22) when theanchor Unes (4, 5 resp. 6, 7) in each of said pairs are 25 essentially equally long.

7. A System according to claims 4 and 5, CHARACTERIZEDIN that the additional forward anchor line (9) is essentiallyparallel with the longitudinal axis (L) of the vessel (1) whenthe anchor Unes (4, 5 resp. 6, 7) in each of said pairs are 3o essentially equally long.

8. A System according to any of the claims 2-7,CHARACTERIZED IN that said anchors (18-21) are placed at a chosenmutual angular distance measured along a horizontal circle havingits centre in the vessel (1; 22). 35

9. A vessel for the production of hydrocarbons from a subsea source connected to the vessel (1; 22) via .a number ofrisers (2; 23), the vessel (1; 22) comprising a System for anchoring and positioning of the vessel, wherëin the Systemcomprises a number of anchor lines (4-9) which at inner ends are 010932 10- connected to winches (10-15) at the bow and stern of the vessel,and at outer ends are connected to respective anchors (18-21)anchored in the sea bed at a distance from the vessel (1; 22),so that the vessel can be turned by hauling-in and slackening, 5 respectively, of chosen anchor Unes by means of the associatedwinches, CHARACTERIZED IN that the System comprises a forwardpair of anchor Unes (4, 5) which at their ,inner ends are connected to respective winches (10, 11) at the bow (16) of thevessel ( 1 ; 22 ), and at their outer ends are connected to aio respective one of two anchors (18, 19) at opposite sides of thevessel (1; 22), a rearward pair of anchor lines (6, 7) which attheir inner ends are connected to respective winches (13, 14) atthe stern (17) of the vessel (1; 22), and at their'outer ends areconnected to a respective one of said two anchors (18, 19), andi5 an additional rearward anchor line (8) which is connected to anadditional winch (15) at the stern (17) of the vessel and to a third anchor (20) astern of the vessel. 20 -· ··" 25 30 35