A SINGLE POINT MOORING BUOY WITH RIGID ARM

Abstract: The buoy (3) and rigid arm (15) are free to rotate about the vertical axis (A-A) of the buoy. The rigid arm (15) is connected to the single point mooring buoy by a hinge arrangement (9) and an axial rotational connection (10) to allow for independent pitch and roll motions of the buoy in relation to the rigid arm. The vessel (1) is connected to the rigid arm by two tension members (6,7), which could be chains, cables or bars. The rigid arm is ballasted (13,14) at the vessel end. In addition, the vessel and the single point mooring buoy may be connected using a conventional hawser (18). The weight of the rigid arm and its ballast, and the loading on the tension members, combine to produce a restoring force and moment which prevent the vessel from riding up to the buoy or jack-knifing of the assembly when the rigid arm moves out of line with the centre line of the vessel.

Description: The invention relates to a mooring system for mooring a vessel on the surface of a body of water comprising a mooring point having buoyancy and being anehored to the floor of the body of water and a rigid arm structure, connected at one end to said mooring point and at the other end to the respective, ballasted, ends of two tension members, one on each side of the vessel hill. Such a mooring system is known from FR-A-2.420.475, fig.7 and 8. In this known mooring system the mooring point is a tower structure which is kept upright by its own buoyancy and anehored to the floor of the body of water by means of a universal joint. The upper end of this tower supports by means of a horizontal pivot axis two rigid arms which are at their respective other ends via respective horizontal pivot axes connected to the vertical tension members of which the other ends are the bow of the vessel interconnected by a further horizontal axis. Such a mooring system allows a limited movement in the longitudinal direction of the vessel and provides a restoring force when the vessel moves towards the mooring point and away from it. However, for rolling movements of the vessel this mooring system is in fact a rigid structure so that rolling movements of the moored vessel impart very severe bending moments on the entire structure resulting in a heavy loading of the various parts and pivots and a rapid wear therof, the more so because they are constantly subjected to the corrosive influence of the seawater. The invention aims to provide an improved mooring system of the abovementioned kind. To this end according to the invention the mooring point is a floating buoy having a limited lateral stability and anchored by means of anchor chains or anchor cables, the tension members are pivotable tension members such as chains of cables and the rigid arm structure is Y-shaped each vessel end therof being connected to a tension member, the arrangement being such as to allow a rotational movement around the longitudinal axis of the Y-shaped structure. This results into a mooring system which has, as far as the counteracting of movements toward and away from the mooring point is concerned, the same advantages as the prior art mooring system but of which the structure is subjected to much smaller forces and loadings than in the prior art mooring system; moreover the whole has less pivot points and is considerable simpler. Rolling movements of the vessel do not result into heavy loading and the resultant wear and tear of the structure, the buoy simply rolling together with the rigid arm structure around the longitudinal axis thereof. Such rolling movements of the vessels are smoothed out by the up and down movements of the arms of the Y-shaped structure. Another, however far more impractical, solution would be to provide an axial rotational connection in the rigid arm. It is observed that mooring systems with a floating buoy are known in itself from NL-A-66.00321 and 72.07903. These known mooring systems lack the combination of the rigid arm and in the mooring system according to NL-A-6600321 the floating buoy has a considerable lateral stability so that the rigid arm is subjected by very heavy forces resulting from rolling movements of the vessel. Consequently this structure must have considerable inherent strength and stiffness and a heavy hinge structure at the end which is connected to the vessel to provide adequate strength and stability, resulting in an expensive construction. In the mooring system according to NL-A-7207903 the buoy has less lateral stability but this mooring system has the drawback of a considerable resistance against sideways (swaying) movements of the vessel so that here, too, the structure must satisfy the demands as outlined above. The invention is elucidated on the hand of the drawing. The figure shows the vessel 1 floating on a body of water 2, and moored by means of two pivotable tension members 6, 7, a rigid arm 15 and a floating mooring buoy 3 which is anchored by one of more anchor chains 5. The single point mooring buoy 3 is of a type known as such and consists of a body 4 on which the turntable 8 is free to rotate about the vertical axis A-A of the buoy body 4. The buoy body 4 is attached to the floor of the body of water 2 by means of one or more anchor chains 5. The rigid arm 15 is in this embodiment attached to the mooring buoy 3 by means of the hinges 9. These hinges 9 allow relative movement between the buoy 3 and the rigid arm 15 about the horizontal axis B-B. Furthermore, the arrangement allows rotational movement of the rigid arm 15 around its longitudinal axis by virtue of the limited lateral stability of the mooring buoy 3. A less practical solution would be to incorporate an axial rotational connection in the rigid arm 15 such as the one indicated with reference numeral 10. The rigid arm 15 is 6-shaped and the two side arms 11-12 incorporate ballasting arrangements 13, 14. This implies that the two side arms 11, 12 can be ballasted or alternatively deballasted, in which case rigid arm 15 can become independently booyant. Two pivotable tension members 6, 7 such as chain or cables connect the two side arms 11, 12 to the vessel 1. These tension members 6, 7 run from the ballasting members 13, 14 upwards through hawses 19 to fixing points 22 on deck 21 of the vessel 1. Any rolling movement of the vessel 1 is thus smoothed out by the up-and-down movement of the side arms 11, 12 causing the middle leg of the Y-shaped rigid arm to oscillate about its axis. In addition, a mooring hawser 18 connects the single point mooring buoy 3 to a fixing point 24 on deck of the vessel 1. This is not essential but provides both an additional mooring facility and a factor of safety in the mooring arrangement. The product fluid is carried to the single point mooring buoy 3 by means of the submarine riser 16. It is then transported to the vessel 1 by means of the floating hose string 17.

Claims: 1. Mooring system for mooring a vessel (1) on the surface of a body of water (2), comprising a mooring point (3) having buoyancy and being anchored to the floor of the body of water (2) and a rigid arm structure (15, 11, 12) connected at one end to said mooring point (3) and at the other end to the respective, ballasted, ends of two tension members (6, 7) one on each side of the vessel hull, characterized in that said mooring point (3) is a floating buoy (4) having a limited lateral stability and anehored by means of anehor chains or anchor cables, the tension members (6, 7) are pivotable tension members such as chains or cables and the rigid arm structure (15, 11, 12) is Y-shaped, each vessel end (13, 14) thereof being connected to a tension member (6, 7), the arrangement being such as to allow a rotational movement around the longitudinal axis of the Y-shaped structure. 2. Mooring system according to claim 1, characterised by an axial rotational connection (10) in the rigid arm (15).

Inventors: DE BAAN, JACOB
Applicants: BLUEWATER TERMINAL SYSTEMS N.V.
Application year: 1981
Main classification: B63B27
IPC codes: B63B22