Patent Publication Number: US-6655312-B1

Title: Active semi-weathervaning anchoring system

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to an anchoring system comprising a vessel, at least two anchors and an anchor line connecting the anchors to the vessel, wherein the anchor line extends from a first anchor, to a pulling device on the vessel, and from the pulling device to the second anchor. 
     From WO98/45874 it is known to moor a vessel, such as an FPSO, to the sea bed with mooring lines which are secured to the ship&#39;s hull in the central area. By pulling in or paying out the anchor line via securing devices on the side of the vessel, the vessel may be rotated through 180° around a vertical axis to be aligned with prevailing wind and current conditions. 
     From WO-97/47516 an anchoring system for positioning an offshore vessel is known wherein bow and stem anchor lines are connected to respective winches at the bow and the stem of the vessel. The vessel may be rotated by varying the lengths of the anchor lines by means of the winches. 
     From European patent application EP-A-878389 an anchoring system is known in which bow and stern anchor lines are interconnected and extend in the length direction across the deck of the vessel via a winch for allowing passive semi-weathervaning of the vessel. 
     The known mooring systems have as a disadvantage that a relative large number of winches need to be applied, which winches have to be of a relatively large size to generate a lifting force directed against the tension in the mooring lines, which may amount to 100 tons or more. Furthermore, the possibilities for station keeping and directional adjustments are relatively limited in the known passive semi-weathervaning system. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a semi-weathervaning anchoring system which utilises a relatively small number of pulling devices of relatively small pulling power. It is a further object of the present invention to provide an anchoring system by which heave and roll motions of the vessel can be significantly reduced, and by which active weathervaning is possible. 
     Thereto the anchoring system according to the present invention is characterised in that the anchor line is located at a distance from the longitudinal or transverse centreline of the vessel. 
     By running the anchor line across the vessel at a distance from the longitudinal or transverse centrelines of the vessel through a pulling device, one part of the anchor line can be taken in while the other is paid out to cause a rapid and simple adjustment of the vessel position and/or direction. As one pulling device operates on two anchor lines, the number of pulling devices remains relatively low. Furthermore, as the anchor line parts are tensioned against each other, the pulling device only has to operate against the difference in anchor line tension so that the pulling power of the pulling device can be relatively small. 
     The vessel of the present invention may be a floating production storage and offloading (FPSO) vessel which may or may not have drilling capabilities (FPDSO) or may be a drilling vessel with a tension leg deck (TLD) such as described in European patent application No. 99200823.5. The anchor lines may be comprised of anchor chains, steel cables, polyester cables, or combinations thereof. The anchor lines may be formed of one piece or may be interconnected at the position of the vessel. The anchor line may comprise two separate sections that each are connected to the pulling device. 
     The pulling device may comprise a winch, or other equivalent pulling means such as an hydraulic arm or a pinion and rack combination. 
     The anchors may comprise pile, drag or suction anchors connected to the seabed, and the mooring lines may comprise clump weights or other anchoring equipment for maintaining a stable position of the vessel. 
     In one embodiment, the anchors are placed on each side of the longitudinal centreline, the pulling device being placed on the forward or rear part of the vessel. By placing the pulling device on the rear part of the vessel, the bow of the vessel can be rotated through about plus or minus 45°. A 30° weathervaining adjustment can already reduce heave and roll motions by as much as 50%. 
     In a further embodiment both the front and rear parts of the vessel comprise a pulling device such that also lateral displacement and yaw motions of the vessel can be adjusted and/or corrected. Near midship of the vessel, a number of anchor lines that extend in the transverse direction may be fixedly connected or may be connected via additional pulling devices for lateral displacement of the vessel. 
     Another embodiment comprises an anchor line configuration in which two anchor lines extend in the longitudinal direction of the vessel, on both sides of the longitudinal centreline thereof. Each anchor line comprises a pulling device at the side of the vessel, the anchor lines being guided through fairleads at the bow and at the stem and from thereon continuing towards the anchors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A number of embodiments of an active semi-weathervaning anchoring system according to the present invention will be explained in detail with reference to the accompanying, non-limiting drawings. In the drawings; 
     FIG. 1 shows an embodiment wherein two adjustable anchor lines extend transversely to length direction at the rear end of the vessel, 
     FIG. 2 shows an embodiment of an anchoring system with adjustable anchor lines at the bow and stem of the vessel, 
     FIG. 3 shows an embodiment wherein additional adjustable anchor lines are provided near midship, 
     FIG. 4 shows an embodiment wherein the vessel near midship is provided with fixed anchor lines, 
     FIG. 5 shows an embodiment wherein the adjustable anchor lines extend on both sides of the longitudinal centreline, generally in the length direction of the vessel, 
     FIG. 6 shows an embodiment wherein the anchor lines cross the deck of the vessel, and 
     FIG. 7 shows an embodiment comprising two pairs of adjustable anchor lines extending in the length direction and in the transverse direction. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows an anchoring system  1  according to the present invention comprising a vessel  2  and bow anchor lines  3 ,  4  that are on one end connected to anchors  5 ,  6  and that with their other ends are fixedly connected to the vessel  2  via fairleads  7 ,  8 . At the stern  9  of the vessel two anchor lines  11 ,  12  extend between anchors  13 ,  14 ,  15 ,  16  which are located on respective sides of a longitudinal centreline  17 . Each anchor line  11 ,  12  is movably guided via a respective fairlead  18 ,  19 ,  20 ,  21  and is connected to a respective pulling device  22 ,  23 , at the stem of the vessel. By actuating the pulling devices  22 ,  23 , the stem  9  can be laterally displaced such that the bow of the vessel can be headed against the prevailing wind and current directions by rotation of the vessel around the point R, such as schematically indicated by the arow A. 
     FIG. 2 shows the embodiment of a vessel  24  wherein at the bow  25  and the stern  26  each time two anchor lines  27 ,  28 ;  29 ,  30  are connected to respective pulling devices  31 ,  32 ,  33 ,  34 . In this embodiment, the lateral position of the vessel may be adjusted and yaw motion around the vertical centreline of the vessel may be corrected. 
     FIG. 3 shows an embodiment of a vessel  35  comprising a bow anchor line  36  and a stem anchor line  37  each connected to respective pulling device  38 ,  39 . At a midship position  40 , three anchor lines  41 ,  42 ,  43  run across the deck of the vessel in a transverse direction via pulling devices  44 ,  45  and  46 . This configuration is especially useful for FPDSO-vessels with drilling/work over rigs, or in combination with a tensionleg deck such as described in European patent application number 99200823.5 in the name of the applicant. 
     FIG. 4 shows an embodiment of a vessel  84  suitable for rotational positioning and small sideways translations, as schematically indicated by the arrows. In addition to adjustable bow anchor line  85  and stern anchor line  86 , two pairs of fixed anchor lines  87  or  88  are provided, connected to the midship portion  89  of the vessel. The vessel can be rotated by pulling in and paying out the anchor lines  85 ,  86  while the holding power of the fixed lines  88 ,  87  can be retained without winching. This configuration is also favourable for use in conjunction with an FPDSO with drilling/work over rigs or with a tension leg deck (TLD constructions). 
     FIG. 5 shows an embodiment of a vessel  50  suitable for rotational positioning and large lengthwise translations as schematically indicated by the arrows. Two anchor lines  51 ,  52  are connected to anchors  53 ,  54 ,  55 ,  56 . Each anchor line  51 ,  52  is connected to pairs of anchors  53 ,  56 ;  54 ,  55  located on either side of the transverse centreline  57 . At the vessel  50 , the anchor line sections extend in the length direction of the vessel between pairs of fairleads  58 ,  59 ;  60 ,  61 . The pulling devices  62 ,  63  are located near the sides of the vessel  50  between the position of the fairleads  58 ,  59 ,  60 ,  61 . 
     FIG. 6 shows an embodiment of a vessel  65  suitable for small rotations and large lengthwise and sideways translations, as schematically indicated by the arrows. The vessel comprises two anchor lines  66 ,  67  which extend crosswise across the deck of the vessel  65 , each anchor line  66 ,  67  being connected to a pulling device  68 ,  69 . This arrangement is particularly useful for moving the vessel laterally. If for instance lines  66  and  67  are both winched toward the bow of the vessel, the vessel will move to the stern. If lines  66  and  67  are both winched toward one side, the vessel will move to the opposite direction. With differential winching the vessel can also be induced to turn while being translated. 
     FIG. 7 shows an FPSO vessel  70  which is moored according to the present invention and which is connected to a shuttle tanker  75  in a tandem mooring configuration. The shuttle tanker  75  is with its bow connected to the stem of the vessel  70 , via a hawser  76 . The weather direction is indicated schematically by arrow W. The active weathervaning capabilities of the vessel  70  via anchorline  71  and pulling device  72 , by plus or minus 45°, provide sufficient clearance for weathervaning of the shuttle tanker  75  and for keeping both vessels  70 ,  75  aligned with the weather. Weathervaning through plus or minus 45° of the vessel  70  and mooring of the shuttle tanker  75  to the bow when necessary, can keep the shuttle tanker  75  within a 180° sector. 
     In FIGS. 1-7 a detector, such as a wave signalling buoy can be used, in the vicinity of the moored vessels to send a radio signal to the vessel with prevailing wind and current directions, on the basis of which, by means of a control system known in the art, the pulling devices are actuated to adjust the position of the vessel.