GB2189911A

GB2189911A - Marine position control apparatus

Info

Publication number: GB2189911A
Application number: GB08710146A
Authority: GB
Inventors: Michael Ruggier
Original assignee: Individual
Current assignee: Individual
Priority date: 1986-04-29
Filing date: 1987-04-29
Publication date: 1987-11-04
Anticipated expiration: 2007-04-29
Also published as: NO871768D0; GB8610415D0; NO871768L; GB8710146D0; GB2189911B

Abstract

The apparatus can be used between a marine vessel 2 moored to another, relatively stationary vessel 1 e.g. two tankers moored together for the transfer of oil or gas from a production/storage tanker to a shuttle tanker; or can be used to moor a vessel 2 to a stationary structure (10), e.g. a supply vessel to a production platform, or for mooring together two vessels side-by-side while stationary or in motion for transfer of goods, oil or gas, or personnel between the vessels. The control apparatus comprises a linkage common to said vessel 2 and to the other vessel 1, or structure (10). The linkage comprises a connector 6 such a hauser under tension or a rigid link pivotally attached to said vessel 2, and pivotally attached to the other vessel 1, or structure 10. Sensors are provided for measuring angular variations o and theta of the disposition of the connector 6 relative to a reference position and a computer analyses the sensed variations. The computer controls operation of the propulsion means (7-9) of the moored vessel to steer that vessel such as to return the disposition of the connector 6 to the reference position. <IMAGE>

Description

SPECIFICATION Motion control apparatus This invention relates to motion control apparatus for a marine vessel moored to another, relatively stationary, vessel or to a marine structure, especially but not exclusively two tankers moored together for the transfer of oil or gas from a production/storge tanker to a shuttle tanker moored to the production taker.

It is known to transfer oil or gas from a storage tanker, moored by the bow to a loading platform or buoy to a shuttle tanker moored behind the storage tanker, but weather and sea state conditions limit such transfer to relatively calm waters, (i.e. with significant wave heights of less than 4.5m) in order to keep the tension in the morring connector within a safe limit.

It is an object of this invention to provide apparatus which can be used in hostile environments such as the North Sea.

It is also an object of the invention to provide apparatus which can be used to moor a vessel to a stationary structure, e.g. a supply vessel to a production platform and for mooring together two vessels side-by-side while stationary or in motion for transfer of goods, oil or gas, or personnel between the vessels.

According to the present invention there is provided a motion control apparatus for a marine vessel moored to another relatively stationary vessel or a marine structure, comprising a linkage system common to said vessel and to the other vessel or structure, said linkage system comprising connector means pivotally attached to said vessel and pivotally attached to the other vessel or structure, said connector or the points of connection to the connector having a predetermined reference position, sensor means for measuring angular variations of the disposition of the connector or points of connection to the connector relative to the reference position, computer means to analyse the sensed variations and means controlled by the computer means for operation of the propulsion of the moored vessel to steer that vessel such as to return the position of the connector or points of connection of the connector substantially to the reference position.

Preferably the linkage system is in the form of a single connector.

According to another aspect of the invention there is provided a marine vessel equipped with a motion control apparatus as defined in the two preceding paragraphs, mounted on said vessel, sensor means and computer means being adapted to operate the propulsion of said vessel and the connector being adapted for connection to another vessel, or to a marine structure.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 illustrates two marine vessels moored together in line in accordance with the invention; Figure 2a is a plan view and Fig. 2b is an elevation showing more detail of the apparatus according to the invention with the two vessels in line astern.

Figure 3 is a plan view showing the apparatus with the rear vessel off line; Figure 4 illustrates parameters for calculating the relative displacement and orientation of the two vessels; Figures 5 and 6 illustrate another use of the apparatus; Figures 7 to 9 illustrate a further use of the apparatus; and Figure 10 illustrates two marine vessels moored together bow to stern in accordance with a second embodiment of the invention.

Referring firstly to Figs. 1 to 4 of the drawings, there is shown two tankers 1, 2 moored together in line astern, the leading tanker 1 being moored by the bow to the sea bed via a buoy 3 or kept on location by the use of dynamic positioning. The lead tanker is loaded with oil or gas supplied from a platform or subsea facility, not shown. The rear tanker 2 is a shuttle tanker which in this embodiment is moored to the stern of the storage tanker 1 only for as long as it takes to receive a load of oil or gas from the storage tanker via a loading hose 5 at the stern of the storage tanker 1.

To enable the two vessels to be safely moored together in adverse weather and sea conditions, the vessels are connected together by a connector 6 which, when the vessels are correctly positioned relative to each other, is in a 'reference' position. In this embodiment, with the vessels moored in line astern, the reference position of the connector lies on the centre line of the both vessels. Sensor means is provided to sense alterations from the reference mode and a computer processes the sensed data and operates the shuttle tanker's propulsion (bow thruster 7, stern thruster 8 and/or main stern propulsion 9) to return the linkage to the reference mode, as hereinafter described to within a desired accuracy, e.g. +5m. In this embodiment, the computer is programmed to reposition the shuttle tanker 2 in line astern of the storage tanker 1.

The linkage comprises a connector 6 mounted on the centre lines of both vessels 1, 2 via pivots 4 so that when the shuttle tanker 2 is directly astern of the storage tanker 1, it is in its reference mode as best shown in Fig. 2 where the connector is attached to the stern of the storage tanker 1 and the other end is attached to the bow of the shuttle tanker 2 and is parallel to the vertical plane through the centre lines of the vessels.

The connector 6 may be a hawser, energy absorbing rigid connector or the like to moor the vessels together. The connector could, however, be independent of the mooring system and used solely for position measurement purposes.

If relative low frequency motion between the vessels occurs so that the shuttle swings to one side or the other, the angular disposition of the connector will change, i.e. the angle between the connector and the centre line of each tanker.

In Fig. 3 the angle 6 is the clockwise angle in the horizontal plane between the vertical planes containing the centre lines of the shuttle tanker and the connector while anti-clockwise angles have negative values. Angle 0 is the anti-clockwise angle in the horizontal plane between the vertical planes containing the centre lines of the lead tanker and the connector while clockwise angles have negative values. The data for the angle 0 that is measured on the lead tanker can be transmitted to the shuttle tanker by means of a telemetry unit that transmits the data by radio means or via a cable attached to the loading hose. When the vessels 1, 2 are in line astern, i.e. when the connector is in the reference position, angles sb and 6 are zero.Otherwise the two angles can be measured and the relative orientation and displacement of the two vessels with respect to each other can be calculated.

The following distances, which are fixed for a particular shuttle tanker, will have to be known (Fig. 4).

a-Distance beteeen the pivot 4 of the connector to the vessel and the vessel's centre of gravity, (m).

t1-Distance between the bow tunnel thruster 7 and the vessel's centre of gravity, (m).

t2-Distance between the stern tunnel thruster 8 and the vessel's centre of gravity, (m).

1Approximate length of connector when under tension (m) (I) Additionally, the following continuously varying input data will have to be continually measured.

6-Clockwise angle from axis of shuttle tanker 2 to the connector (degrees) (if anti-clockwise, 6 is negative).

HAnti-clockwise angle from axis of lead tanker 1 to the connector (degrees) (if clockwise, 0 is negative).

connector tension (Newtons).

Once these values are known, they can be inserted into a set of equations as illustrated hereinafter to give the distances that the bow and the stern through tunnel thrusters must move the vessel (shuttle) to get it in line and directly behind the production/storage tanker. These distances, using an appropriate control law, are used to generate the required thruster commands to reduce their values and get the tankers in line.

It is, however, necessary to remove the high frequency wave motion signals from the measurements of 6 and 4 before they can be used in the equations. Notch filters can be used to do this.

Let Q = OF + FF d1 =I sin 6 d2 =a sin Q NOTE: Subscript F represents the filtered signal.

D,AT=Lateral distance to port that the tanker is required to move (m) =d, +d2 Therefore let DBOw=Distance that bow tunnel thruster must move vessel to port (m) (To starboard if negative) DLATti sin n (m) DsTN=Distance that stern through tunnel thruster must move vessel to port (m) (To starboard if negative) DLAT+t2 sin fl (m) Angles 0 and 6 can each be measured by an angle measurement device such as a potentiometer or shaft encoder incorporated within pivots at each end of the connector or by additional linkage devices that are attached to the end sections of the connector.

Connector tension, is an essential operational parameter. During periods of bad weather, the tension level and peak values are key parameters for continuous monitoring of mooring system performance and, under extreme conditions are the deciding factor in whether to abort loading and to unmoor. The tension is measured using a load cell incorporated within the linkage mechanism.

The tension in the connector can then be automatically kept within a desired range by applying the stern main propulsion of the shuttle tanker either in the forward or the reverse mode.

If the connector consists of a hawser, its length could be measured by using a number of techniques although the simplest is probably that worked out from the average hawser tension over a small period of time. Using load/extension data for the particular hawser, the present hawser line length, i.e. distance between the two connection points, can be found by adding the hawser extension when subjected to the average measured load to the hawser length in the unloaded condition.

It is worth mentioning that the position measurements, 6 and 4), are the most important and that the Motion Control system could not function if one of these failed. On the other hand, hawser length measurements are not of great importance and high accuracies are not required, (i.e. a constant length can be assumed).

The apparatus can also be used as illustrated in Figs. 5 and 6 for mooring a vessel to a single-point-mooring terminal or to a fixed or compliant marine structure, such as a supply vessel 2, moored to an off-shore platform 10 for the transfer of cargo or personnel. In such a case, the supply vessel, would be moored stern-on to the platform with the connector pivotally secured at one end to the platform and the other end pivotally attached to the vessel.As before, the two angles 6 and 9 will be measured and used to determine the location of the vessel relative to the platform in order for the corresponding corrective action by the thrusters on the vessel to be taken. 6 is the horizontal clockwise angle from the longitudinal axis of the vessel to the connector and 0 is the horizontal anti-clockwise angle from the perpendicular to the side of the platform to the connector.

Another use of the apparatus as illustrated in Figs. 7 to 9 is for mooring two moving or stationary vessels together side by side for the transfer of cargo, oil or gas or personnel. The connector would extend between the sides of the vessels, and as before the two angles 6 and 0 will be measured and used to determine the relative positions of the vessels in order for the corresponding corrective action to be taken by the thrusters on one of the vessels. 6 is the clockwise angle from the perpendicular to the longitudinal axis on one vessel and 0 is the anticlockwise angle from the perpendicular to the longitudinal axis on the other vessel.

In a second embodiment, illustrated in Fig. 10, two vessels 1, 2 are moored bow to stern via the connector 6, but in this embodiment the reference position is related to the points of connection 4, of the connector 6 to each vessel, these points being, in this example in line astern of the leading vessel, storage tanker 1. Thus, the rear vessel shuttle tanker 2, can 'weather vane' but its bow connection 4 to the connector 6 is constantly repositioned as required via the sensing means on computer controlled propulsion means 7-9 to retain the connector 6 in line astern of the storage tanker 1.

A similar control is applicable to a vessel moored stern on to a marine structure as previously described with reference to Figs. 5 and 6, the vessel being constantly repositioned as necessary.

to return the stern connection 4 of the connector 6 to its correct position relative to the structure to retain the connector 6 in its reference position.

Referring again to Fig. 10, when the connector 6 is in its reference position 0 is zero and the longitudinal distance between the points of connection 4 is the distance I (desired).

When the bow of the shuttle tanker 2 veers from the reference position an imaginary triangle is formed comprising the connector 6, the imaginary connector I (desired) in line astern and an imaginary line X connecting them to complete the triangle.

Angle o is the clockwise angle from the centre line of the shuttle tanker to the line X. o is negative if anticlockwise from the centre line of the shuttle tanker to line X.

Knowing angles 0 and 6 distance X is calculated by the equation: X= \/(l desired cos 0)2+(1 sin 0)2 where I is the length of the connector and the angle a relative to the centre line of the shuttle taker 2 is calculated from the equation: a=0+0e+b Where

(+90 where 4)0 And b: (-90" where 0 > 0 The required thrust to move the bow of the shuttle tanker a distance X in the direction a relative to a shuttle tanker is obtained from one or more azimuthing thrusters and/or a bow tunnel thruster in conjunction with the main stern propulsion.

Claims

1. A motion control apparatus for a marine vessel moored to another relatively stationary vessel or marine structure, comprising a linkage system common to said vessel and to the other vessel or structure, said linkage system comprising connector means pivotally attached to said vessel and pivotally attached to the other vessel or structure, said connector or the points of connection to the connector having a predetermined reference position, sensor means for measuring angular variations of the disposition of the connector or points of connection to the connector relative to the reference position, computer means to analyse the sensed variations and means controlled by the computer means for operation of the propulsion of the moored vessel to steer that vessel such as to return the position of the connector or points of connection of the connector substantially to the reference position.

2. Apparatus as claimed in claim 1, in which the linkage system includes a single connector.

3. Apparatus as claimed in claim 2, in which the connector moors the vessel to the other vessel or structure.

4. Apparatus as claimed in claim 2, in which the connector is used solely for position measurement purposes.

5. Apparatus as claimed in any one of claims 1 to 4, in which the sensor means for measuring angular variations of the disposition of the connector comprises an angle measurement device incorporated with pivots at each end of the connector.

6. Apparatus as claimed in any one of claims 1 to 4, in which the sensor means for measuring angular variations of the disposition of the connector comprises additional linkage devices attached to end sections of the connector.

7. Apparatus as claimed in any one of the preceding claims, in which the distances that the bow and or stern of the moored vessel must travel to return the connector to the reference position determine the required thruster commands.

8. Apparatus as claimed in any one of the preceding claims, in which a load cell is incorporated within the linkage mechanism to measure connector tension.

9. Apparatus as claimed in any one of the preceding claims, in which notch filters are used to remove high frequency wave motion signals from the measurement of the angles of variation of the connector from the reference position.

10. A marine vessel equipped with a motion control apparatus as claimed in any one of the preceding claims, mounted on said vessel, sensor means and computer means being adapted to operate the propulsion of said vessel and the connector being adapted for connection to another vessel, or to a marine structure.