GB2041306A

GB2041306A - Squat Seabed Mooring System

Info

Publication number: GB2041306A
Application number: GB7901905A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-01-18
Filing date: 1979-01-18
Publication date: 1980-09-10
Also published as: CA1134154A

Abstract

For mooring a vessel such as an oil tanker, the lower end of an articulated swivelling arm is built into a squat base unit so that the upper end is free to rotate and to move around and over the base unit. A tanker hauls in a hawser and lifts the head of the arm into its reception chamber. It is thus moored by the arm to the base unit. The head of the mooring arm is held rigidly in the reception chamber and moves with the tanker. The base unit can be an oil reservoir and the arm can be hollow and be used to transfer fluid between reservoir and tanker. When not in use the mooring arm lies flat on the base on the sea bed, (or within a depression in the sea bed), where it is in no danger of damage from deep keels. <IMAGE>

Description

SPECIFICATION Squat Seabed Mooring System This invention concerns a mooring system for use generally in rather shallow, and exposed situations, in offshore locations. The mooring system can be used in conjunction with a number of other functions and is characterised by the fact that it fulfils the mooring function and the second function in one main joining member which is an articulated swivel and which I have called the arm. For instance, when the second function is that of transferring a fluid between a vessel and a seabed installation the transfer arm is the joining member and it has the dual function of mooring the vessel and also of carrying the fluid either from the seabed installation to the vessel, or vice versa.

The arm is articulated having elbows and swivels, and the head of the arm is located and lifted by the vessel into a special reception chamber located in the underside of the vessel.

The bottom of the arm is swivelly connected and pivoted to a heavy seabed installation called the base unit. When not in use, the arm is left lying on top of the base unit.

The main characteristic of the system is that it has a very low profile when not in use. The base unit is squat and the arm lies flat on top of the unit in a substantially horizontal position when it is lowered to the seabed after use.

The best application of the present invention is for mooring and loading oil or liquid gas tankers in situations where icebergs or other ice formations may damage existing types of tanker loading installations.

Another application of tbe invention is for mooring, loading, and unloading oil, or liquid gas, tankers in shallow water close to land where the installation can lie flat on the seabed when not in use and will not form a hazard to shipping.

Where the sea depth is such that even when lying flat on the seabed the installation would still form some kind of obstruction or create a hazard, then the seabed is dredged down to a lower level, locally, so as to accommodate the installation (when not in use) in a hazard-free seabed depression.

Another characteristic of the present invention is that the manner of articulation of the loading arm is such that it can be very large (typically 4m.

diameter) so that it can transfer fluid in large quantities quickly. This characteristic is very necessary when loading tankers in the presence of moving winter ice floes in Arctic areas. The shorter the loading time the smaller the risk of being pushed off station by heavy ice formations.

A further characteristic of the present invention is that the system is always below water level both when in use and when at rest on the seabed.

In Arctic regions this is most important because in winter time the environment above water (ice) level is very hostile and full of difficulties. Working wholly below ice level in these conditions has very great advantages.

A further function of the system is seabed storage. It is efficient and convenient to store oil at a low level (on the seabed) so that the forces of gravity can be used to convey the oil to the vessel.

The base unit is used as a seabed reservoir and, where necessary, the capacity is increased by surrounding it with a number of similar squat storage units to which the base unit is connected.

Because the arm has a swivel attachment to the seabed base unit, it can, when connected to the vessel, enable the vessel to move round the base unit in a weather vane fashion so that the vessel assumes a position to leeward of the base unit, the vessel itself weathervaning about the top of the arm so that the whole mooring system is in the most advantageous position in relation to the environmental forces.

When the second function is fluid transfer, the members of the articulated arm and the swivel are all hollow so that the fluid is contained by them as it moves from the base unit to the vessel or vice versa.

The arm has to carry very large mooring forces and is constructed of alternate single and twin members, in order to attain symmetry about one axis so as to reduce torsion forces and bending moments.

Basically a vessel, according to the invention, locates and lifts the head of a heavy articulated arm with one or more hawsers or chains which are permanently attached to the end of the arm and are passed through a reception chamber located in the bottom of the vessel until the top end of the arm is within the reception chamber.

The other (lower) end of the arm being connected to the base unit by a double joint which allows the bottom section to swivel around a vertical axis and rotate about a horizontal axis. The top end of the arm also has a joint which allows the top section of the arm which penetrates the reception chamber to do so with a substantially vertical longitudinal axis.

The arm consists of a number of alternate single and twin members connected together with pivot or swivel joints to give the necessary degrees of flexibility required by the system.

The top end of the arm is held in a suspended position within the reception chamber by the hawser. The vessel is then free to roll and pitch in the sea and can weathervane about the seabed installation, but it is restrained to movement within the limited length of the arm about the sea bed base unit.

In its most useful form the arm is composed of large heavy steel pipe sections linked together in such a way that the bottom section can rotate in its bearings, set in the seabed base unit, while the top section, suspended by the ship with the head restrained within the reception chamber, allows the ship to move with the water and wave movements but restricts it from moving outside of a radius from the centre of the bottom attachment, which radius is determined by the weight and geometry of the arm, the draught of the vessel, and the depth of water.

There are many ways of mooring and loading oil tankers in the open sea. The usual way is to moor the tanker to a buoy and connect a flexible hose from a seabed supply to the tanker either directly, or more usually via a mooring buoy.

Presently used systems are characterised by having separate members to moor the ship (a hawser) and to transfer the fluid (a flexible pipe).

U.K. Pat. App. 16513/76 described a system which had one member which fulfilled the two functions of mooring the tanker and transferring the fluid. This system utilised a large rigid pipe telescopically mounted in a substantially vertical attitude in a flexibly restrained seabed structure.

The heavy rigid pipe was located and lifted by a hawser from the tanker until it engaged within a downward facing reception chamber located in the underside of the tanker. Whereupon the (feed) pipe was gripped by a hydraulic seal on the lip of the reception chamber and the fluid was allowed to flow by gravity from the seabed supply point to the tanker, the (feed) pipe performing the two separate functions of mooring and liquid transfer in one member.

This system works well in deep water but the height of the seabed structure has to be at least half the water depth, which creates collision problems wherever tanker or iceberg draughts can approach this depth.

The present invention aims to overcome this difficulty by an arrangement which enables the system to lie on the seabed with a low profile when not in use, but to extend up to the tanker reception chamber level when required.

The system is particularly useful in the Arctic environment where ice floes are often continually moving and where ice ridges and iceberg keels often reach down to the seabed, and lower, scouring deep furrows in the latter. The system is located so that in its lowered position, its top is below ice keel level. Where seabed furrows show that ice keels have previously penetrated the seabed, a depression is formed in the seabed by dredging or other means and the system is set below anticipated ice keel level.

The invention also provides a vessel which has mooring and oil transfer equipment for cooperation with the mooring apparatus when the latter is in position under the sea, and this equipment on the vessel comprises a reception chamber within the hull of the vessel, an opening in the bottom of the vessel for the entry of the mooring arm head into the reception chamber and means for hauling in the mooring hawser through the entry opening in the reception chamber to draw the mooring arm head into the reception chamber, a resilient gripping ring being fitted around the entry opening to engage with the mooring arm and form a seal.

In Arctic use a dedicated icebreaker tanker is fitted with a reception chamber, sonic buoy location means, buoy retrieval means, lanyard and hawser winches, hawser jacking and gripping means, fast hawser lowering means, reception chamber gate, inflatable annular gate seal, hawser guides, mooring post head stops, hawser direction sensor and base unit direction sensors.

The tanker first shatters the ice along a path upstream of the facility for a distance such as to allow it to be in shattered ice during its loading period. The tanker then returns to the mooring terminal, locates the buoy, feeds the buoy and lanyard through its reception chamber, winches in the hawser, senses the direction of the mooring arm head, manoeuvres over the mooring arm head, lifts the mooring arm head into the reception chamber, and inflates the annular gate seal. Ports in the mooring arm head are then opened and the oil is free to flow into the reception chamber and out through the distribution manifolds and pumps into the ships oil storage tanks. The whole operation is accomplished below ice level where temperatures are reasonable. Temperatures above ice level are intolerable in most Arctic winter conditions.

The articulation of the mooring arm is particularly important.

The seabed installation is the base unit into which the bottom (first) member of the arm is fixed, in such a manner that the bottom member has a substantially vertical axis but has the ability to twist around that axis.

The second member is pivotally connected to the top end of the bottom member, and is a double member, being composed of parallel, handed, twins.

The third member is pivotally connected to the second member. The third member is a single member with a longitudinally swivelling ability such that one end can twist around the longitudinal axis in relation to the other end.

The fourth member is pivotally connected to the third member. The fourth member is a double member composed of parallel handed twins.

The fifth member is pivotally connected to the fourth member. The fifth member is a single member.

The fifth member is generally the top member and forms the mooring arm head.

More than five members can be used if required. The system can also work with four members if the (singie) bottom (first) member is of such a size that the second member (which would be a single swivelling member) can pivot into it. The third member would then be a double member and the fourth member would be a single member and would form the mooring arm head.

The invention will be described in more detail with the aid of an example illustrated in the accompanying drawings in which: Fig. 1 shows an elevation and also a plan of the general situation showing the mooring apparatus and the tanker in accordance with the invention.

Fig. 2 is a more detailed plan of the mooring arm lying horizontally flat on the seabed and attached to the base unit.

Fig. 3. shows a section through the base unit, an elevation of the mooring arm in three characteristic positions, and transverse sections through the tanker in two characteristic positions.

Fig. 4. is a section through the mooring arm head suspended within the tankers reception chamber.

Fig. 5 is a diagram showing the articulations and restrictions applied to the mooring arm.

The mooring apparatus shown in the drawings is intended for the mooring and loading of large ice breaking oil tankers in Arctic regions where winter ice floes with deep keels form a great hazard.

The mooring and loading apparatus comprises a base unit 11 and mooring arm 13 enabling an icebreaking tanker 10 to be moored in position against an advancing ice floe 15, with low ice keels 16, while being loaded with crude oil from the reservoir storage units 1 2.

1 8 is the direction in which the ice is flowing, 1 9 is the length of the tanker and 9 is the area of ice which the ice breaking tanker shatters preparatory to loading oil in order not to be pushed off station. 1 7 is the length of the shattering path necessary to attain this.

14 is the seawater level, and 8 is the seabed.

7 is an echo sounder warning system, consisting of a ring of seabed echo sounders, surrounding the loading terminal, connected to a shore station which continuously monitors ice keel levels during the winter period and warns tankers of approaching danger while loading. Figs.

2 and 3 show more details of the system.

53 is the single bottom (first) member of the mooring arm mounted with a substantially vertical axis in the base unit 11 so that it can revolve around that axis.

54 is the twin second member pivotally mounted into the top of the first unit. 23 are pivot joints allowing rotation.

55 is the single third member pivotally mounted into the upper end of the second member. It incorporates a swivel 20 which allows one end to rotate about its longitudinal axis in relation to the other end.

56 is the twin fourth member pivotally mounted into the top of the third member.

57 is the single fifth member pivotally mounted into the upper end of the fourth member.

57 is the mooring arm head, to the top of which is attached the hawser 60. 57 has a slip sleeve 24 around it to enable it to twist in relation to the tanker as it is gripped by the gate seal 70.

58 is the reception chamber in the bottom of the ships hull.

59 is the reception chamber gate.

The approximate scale is shown in the typical case shown in Fig. 3. Water depth is 30m. 40 is the seabed and 41 is a typical ice keel scour.

42 is the dredged depression to enable the facility to be below expected ice keels and hazard free.

46 shows the height of a storm wave with the tidal range adding to the water level variation.

45 is a typical water level and 47 and 48 are typical ice formations.

49 is a deep ice keel.

43 is the sandy seabed material.

44 is a protective bund built around the installation with dredged material.

61 is the lanyard used to haul in the hawser and 62 is the captive sonic buoy which locates the lanyard.

51 is a laden tanker at lowest water and 50 is an unladen tanker at about highest water.

26 and 27 are slide tracks to enable the mooring arm to rotate easily about its centre pivot when pulled laterally by the hawser.

Fig. 4 shows the mooring arm head and reception chamber in greater detail.

56, 59, 60 and 70 have already been mentioned.

71 is the scour manifold.

72 is the port shown closed, and 86 is it shown open during loading.

87 is the oil/water interface, 88 being the oil and 89 being water.

74 are the manifolds through which the oil flows to the tankers distribution booster pumps.

75 are loose lead-in plates.

73 is the hawser terminal block.

76 is the hawser direction sensing device.

77 is the segmented, removable, hawser guide.

78 and 81 are radial diaphragms.

79 is the port's opening bayonet.

82 is a grillage retaining the hawser guide.

83 points to the port return springs.

84 are the externally replaced port opening jacks.

85 is the annular mooring head stopping ring which is equipped with sensors to sense the direction of the mooring arm base in relation to the tanker so that the tankers propellers, thrusters, and steering can be used to reduce the environmental forces.

90 are the mooring head port fouling guides.

Fig. 4. (and the other figures) are shown diagrammatically for simplicity. The mooring head is in fact symmetrical about its longitudinal centre line.

Fig. 5. shows the linkage system.

ABCD is the seabed.

EF is the base unit.

GH is the bottom (first) member of the mooring arm. It is held by bearings UV and WX with a substantially vertical axis about which it can rotate.

HJ is the twin second member which is pivoted at H (to the first member) and at J to the third member.

JL is the single, third member, It has a longitudinal swivel at K enabling joint L to rotate in relation to joint J about the longitudinal axis.

JL is pivoted at L to the fourth member LM.

LM is a twin member. It is pivoted to the fifth member (the mooring head MQ) at M.

MQ, the single fifth member (the mooring head), is supported by the hawser T, restrained by the reception chamber gate NP and located and sensed by the annular mooring head stopping ring RS.

Fig. 6. is a plan on a four member articulated mooring arm (referred to previously on page 8).

Fig. 7. is an elevation of the same arm in the loading position.

531 is the single bottom (first) member of the mooring arm mounted with a substantially vertical axis in the base unit 11 so that it can revolve around that axis. In this case the single bottom (first) member is made of such a size that a double horizontal pivot joint can be formed on its top structure to enable the single second member 55 to be pivotally connected to it. The other numbers shown have all been designated in previous figures.

Claims

1. Apparatus for mooring a vessel comprising: a base unit for installation on the sea bed, an articulated mooring arm swivelly attached to the said base unit, a mooring hawser attached to the head of the mooring arm, whereby the mooring hawser may apply a basically vertical lifting force to the mooring arm, and means attached to the said hawser for location by a vessel to be moored, whereby the vessel may haul in the hawser and lift the head of the mooring arm into a reception chamber built into the hull of the vessel, the head of the mooring arm being held rigidly by the reception chamber to the vessel, and the mooring arm head having the necessary freedom of movement to enable it to move in unison with the vessel both vertically, horizontally, and, rotationally around the centre of attachment to the base, whilst being held by the base unit within the confines of a circle whose radius is defined by the geometry and weight of the mooring arm and the height and attitude of the mooring arm head in relation to the base.

2. Apparatus as claimed in claim 1 in which the base unit is hollow and is constructed as a reservoir for fluids.

3. Apparatus as claimed in claim 1 or 2 in which the base unit is installed in a shallow depression excavated out of the sea bed, such that, when not in use, the mooring arm and the base unit are below the general sea bed level and are in no danger of damage by deep keels.

4. Apparatus as claimed in claim 1, 2, or 3, in which the members and joints of the mooring arm are hollow, with fluid communication means with both the base unit and the vessel, to enable the mooring arm to be used for fluid transfer between the base unit and the vessel.

5. Apparatus as claimed in claim 4 in which the mooring arm is in the form of alternate sections of single and double steel tubes with pivoted joints.

6. Apparatus as claimed in claim 4, or 5, in which one of the single sections of the mooring arm has a longitudinal swivel joint to enable one end to rotate in respect to the other end about it's longitudinal axis.

7. Apparatus as claimed in any of claims 1 to 6 in which the mooring arm head is fitted with loose lead-in plates threaded onto the hawser.

8. Apparatus as claimed in any of claims 1 to 6 in which the mooring arm head has a conical top for engagement with a stopping ring built into the reception chamber.

9. Apparatus as claimed in any of claims 1 to 6 in which the mooring arm head has a slip sleeve for cooperating with a gate seal built into the reception chamber.

10. Apparatus as claimed in any of claims 1 to 6 in which the mooring arm head has controllable outward opening ports to allow fluid communication with the vessel through the reception chamber.

11. An improved method of mooring a vessel with the apparatus of claim 1 comprising the steps of: a) locating said mooring hawser from the vessel, b) passing said hawser through a reception chamber in the hull of the vessel, c) hauling in the hawser to lift the head of the mooring arm into the reception chamber until the conical head engages with the stopping ring and the base of the mooring arm head is secured by the reception chamber gate.

12 An improved method as claimed in claim 11 comprising the further step of inflating the gate seal.

13. An improved method as claimed in claim '1 or 12 comprising the further step of transferring fluid between the base unit, which is constructed as a reservoir, and the vessel, by means of the mooring arm which is hollow and allows fluid communication.

14. A vessel having mooring equipment for cooperation with the mooring apparatus of claim 1, the mooring equipment incorporating improvements comprising a stopping ring and hawser direction sensing means, in addition to having a reception chamber within the hull of the vessel with an opening in the bottom of the vessel for entry of the mooring arm into the reception chamber, hawser haulage means, a reception chamber gate, and a gate seal.

15. A vessel as claimed in claim 14 in which the stopping ring, working in conjunction with the reception chamber gate and the mooring hawser, secures the mooring arm head rigidly to the hull of the vessel through the reception chamber so that the mooring arm head is locked into the vessel and moves in unison with it.

16. A vessel as claimed in claim 15 in which the stopping ring is equipped with sensing means to sense the direction of the mooring arm base attachment in relation to the vessel, so that the vessel's propellers, thrusters, and steering can be used to reduce the effect of the environmental forces.

17. A vessel as claimed in claim 14 in which the hawser direction sensing means monitors the movements of the hawser as the mooring head is hauled from the sea bed into the reception chamber, and enables the vessel's propellers, thrusters, and steering to assist in manoeuvering the vessel advantageously.

18. A vessel as claimed in any of claims 14 to 17 which has means for receiving fluid from the mooring arm head ports through the reception chamber and transferring the fluid through manifolds to storage tanks built into the vessel's hull.

19. A vessel as claimed in any of claims 14 to 18 in which the reception chamber is built into the underside of the hull of the vessel and is located within the middle third of the overall length of the vessel, and forward of the longitudinal centre point.

20. A combination comprising: a) a vessel floating on the surface of the sea equipped with a reception chamber and having means for locating, lifting, and securing the head of a mooring arm lying on the sea bed, and, b) a seabed installation consisting of: a hollow base unit which can act as a fluid storage reservoir, an articulated mooring arm flexibly and swivelly attached to the base unit composed of a number of rigid sections flexibly connected to give the head of the mooring arm flexibility about three axes and ability to move vertically and horizontally within the confines of a circle defined by the geometry and weight of the mooring arm, it's point of attachment to the base unit, and the height and attitude of the mooring arm head in relation to the base unit, the mooring arm being hollow and capable of conveying fluid between the base unit and the vessel, and the mooring arm head having means for attaching a lifting hawser, and an annular slip ring for cooperation with the reception chamber gate seal.

21. A combination as defined in claim 20 wherein the reception chamber comprises additionally: a bottom gate for locating the lower part of the mooring arm head during fluid transfer operations, an annular stopping ring for locating the upper part of the mooring arm head during fluid transfer operations, an inflatable gate seal to stop fluid communication between the reception chamber and the sea during fluid transfer operations, and, hawser direction sensing means.

22. A combination as defined in claim 20 or 21 wherein the hollow base unit is installed in a shallow depression excavated out of the sea bed, such that when not in use the base unit and the mooring arm are below the general sea bed level and are in no danger from deep keels.

23. A combination as defined in any of claims 20 to 22 whereby fluids can be quickly transferred between the sea bed and a surface vessel.