GB2030654A

GB2030654A - Marine Arm Control Systems

Info

Publication number: GB2030654A
Application number: GB7918140A
Authority: GB
Original assignee: NAT SUPPLY CO Ltd
Current assignee: NAT SUPPLY CO Ltd
Priority date: 1978-09-28
Filing date: 1979-05-24
Publication date: 1980-04-10
Also published as: GB2030654B

Abstract

The invention relates to a marine arm control system and in particular to an emergency control system which enables a marine arm to be retracted in an emergency at an acceptable rate even if the arm is one of a series controlled by a common hydraulic circuit. The system comprises a plurality of hydraulic actuators 16, 18 and 19 for controlling the relative orientation of elements of the arm, a hydraulic fluid distribution circuit for distributing pressurised hydraulic fluid to the actuators, hydraulic control valves 12, 13, 14 and 15 for controlling the supply of hydraulic fluid to respective actuators, and an emergency control means. The emergency control means comprises means for limiting the volume of hydraulic fluid which can flow through the actuators subsequent to the disconnection of the arm in an emergency to thereby limit the lift of the arm. <IMAGE>

Description

SPECIFICATION Marine Arm Control Systems The present invention relates to marine arm control systems, and in particular to emergency control systems which are operative to disconnect a marine arm from a tanker to which it is connected and to lift the disconnected arm clear of the tanker.

Marine arm control systems are known in which a plurality of hydraulic actuators control the relative orientation of elements of the arm, for example a first actuator controls the angle in the vertical plane between a fixed vertical riser and an inboard arm, a second actuator controls the angle in the vertical plane between the inboard and an outboard arm, and a third actuator controls pivotting of the arms about a vertical axis through the riser. Suitable control of the actuators enables the end of the outboard arm to be manouvered into position adjacent a tanker flange to which it is desired to connect the arm. Once connected to the tanker flange, "free wheel" hydraulic circuits are connected across the actuators so that the arm is free to move as the tanker moves with the tide or settles in the water as it is loaded for example.

It is known to provide emergency control systems which when actuated automatically disconnect the arm from the tanker flange.

Subsequent to disconnection in an emergency however the arm must be lifted clear of the tanker. Heretofore this requirement has caused problems.

If the marine arm is counterbalanced so as to be "tail-heavy", i.e. so that its natural tendency is to swing away from the tanker towards a vertical "stowed" position, release of the arm in an emergency allows the arm to swing away out of control as a result of the free wheel circuits connected across the actuators. This problem can be overcome by adapting the emergency control system such that immediately after emergency release the arm is retracted automatically under full hydraulic control by the first and second actuators. Unfortunately where there is a series of marine arms all connected to the same tanker and all controlled by the same system, although it is relatively easy to simultaneously release all the arms they do not all retract simultaneously and unacceptable delays can occur before all the arms are retracted.This delay is due to the fact that the capacity of the hydraulic system to deliver pressurised fluid is limited and the capacity of the individual actuators is considerable. Thus the arm which due to its position is easiest to retract is retracted first. When this arm is fully retracted, the next easiest to retract arm is retracted and so on until all the arms are retracted.

One solution to the problem is to increase the capacity of the hydraulic circuit to deliver pressurised fluid. Unfortunately this would considerably increase capital costs.

It is an object of the present invention to obviate or mitigate the above problems by providing a marine arm which can be retracted in an emergency at an acceptable rate even if the arm is one of a series controlled by a common hydraulic circuit.

According to the present invention, there is provided a control system for a marine arm, comprising a plurality of hydraulic actuators for controlling the relative orientation of elements of the arm, a hydraulic fluid distribution circuit for distributing pressurised hydraulic fluid to the actuators, hydraulic control valves for controlling the supply of hydraulic fluid to respective actuators, and emergency control means for disconnecting the arm from a tanker and lifting the arm to a retracted position in the event of an emergency, characterised in that the emerency control means comprises means for limiting the volume'of hydraulic fluid which can flow through the actuators subsequent to the disconnection of the arm in an emergency to thereby limit the lift of the arm.

In the case of "tail-heavy" arms, limiting the lift of the arm so that the arm is just clear of the tanker greatly reduces the distance moved by the arm and prevents the arm from swinging out of control. In the case of a series of "powered lift" arms all controlled by the same hydraulic system, the individual arms will still tend to rise sequentially rather than simultaneously but the limit set on the movement of the arms limits the stroke of the actuators and thus limits the demands on the hydraulic fluid supply so that all the arms are lifted within an acceptable period.

Preferably the emergency control means comprises means for connecting a flow limiting device in series with the actuators. For example, where a free wheel circuit is provided to allow movement of the arm with a tanker which it is connected, the flow limiting device may comprise means for blocking the free wheel circuit, a bypass circuit for bypassing the blockage in the free wheel circuit, and means for limiting the volume of hydraulic fluid which can flow through the bypass circuit. The means for limiting the volume of fluid which can flow through the bypass circuit may comprise a piston and cylinder arrangement in parallel with the said blocking means. An adjustable stop may be provided to enable adjustment of the maximum stroke of the piston and cylinder arrangement and thus to adjust the limit imposed upon the lift of the arm in an emergency.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a schematic diagram illustrating the basic characteristics of a marine arm; and Figure 2 is a hydraulic circuit diagram of a marine arm control system embodying the present invention.

Referring to Fig. 1, the outline of a marine arm in its fully retracted position is shown in full line and the outline of the arm in an extended position is shown in broken line. The marine arm comprises a riser 1 fixed to a dockside 2, an inboard arm 3 pivotal relative to the riser 1 about horizontal and vertical axes and an outboard arm 4 pivotal relative to the inboard arm 3 about a horizontal axis. The outboard arm 4 supports a connector 5 at its end, which connector 5 is in use connected to a tanker flange.

Hydraulic piston and cylinder actuators (not shown in Fig. 1) control the relative orientation of the marine arm elements 1, 3 and 4. When the connector 5 is linked to a tanker flange however the actuators must not exert any force on the arm as the arm must move with the tanker. In an emergency, a hydraulic device (not shown) provided on the connector 5 disconnects the arm from the tanker flange. The arm is then retracted either under the influence of counterweights illustrated schematically at 6 in Fig. T or under the control of the actuators.

Referring to Fig. 2, the illustrated hydraulic circuit controls two marine arms such as that schematically illustrated in Fig. 1. Control is effected by manipulation of three control valves 7, 8 and 9 via arm selectors 10, 11 either one of which may be actuated to connect the valves 7, 8 and 9 to the three actuators of a respective arm.

Only the arm control actuating pistons and cylinders of the arm connected to selector 10 are illustrated for the sake of simplicity.

Each selector 10, 11 comprises a control valve 12 the actuation of which switches valves 13, 14 and 1 5 together. The valve 1 3 is connected directly to a piston and cylinder actuator 1 6 which controls movement of the inboard arm about a vertical axis. The valves 14 and 1 5 are connected via valve 1 7 to actuators 1 8 and 1 9 which respectively control relative movement between the riser and inboard arm and relative movement between the inboard and outboard arms. The valve 17 is bypassed by piston and cylinder arrangements 20 and 21. Flow control valves.and pressure relief valves for example which in accordance with normal practice would be included in the circuit have been omitted in the interests of clarity.

The circuit is shown in the condition in which the arm is connected to a tanker flange. The valves 13, 14 and 1 5 are arranged to complete a free wheel circuit linking the two sides of each of the piston and cylinder actuators 16, 1 8 and 1 9 and thus the pistons of the actuators move with the movement of the arm and tanker. When it is desired to retract the arm, the connector 5 (Fig. 1) is released and valve 12 is actuated, thereby connecting the actuators 16, 1 8 and 1 9 to the valves 7, 8 and 9. The valves 7, 8 and 9 can then be manipulated to deliver pressurised hydraulic fluid from source 22 to the actuators 16, 18 and 1 9 to move the arm to its fully retracted position.

In an emergency, the connector 5 is released and valve 1 7 is actuated to block the free wheel circuits provided via valves 13, 14 and 1 5. The counterbalanced arm swings towards its retracted position under the influence of its own weight and the pistons of actuators 1 8 and 1 9 move accordingly. Hydraulic fluid flows as a result into the cylinders of arrangements 20, 21 with the result that the pistons thereof rise. As little or no load is exerted on the pistons, the arm behaves as if the normal free wheel circuit was open until the pistons of arrangements 20, 21 contact respective adjustable stops 23, 24. Further movement of the actuator pistons and therefore of the arm is prevented. Thereafter the valve 1 7 can be reset and control of the arm can be reassumed from valves 7, 8 and 9 as desired.

The arrangements 20, 21 can be reset by extending the arm as far as possible under control of the valves 7, 8 and 9 prior to resetting valve 1 7. Aiternatively a small load may be applied to the pistons of the arrangements 20, 21 so that they are reset automatically after resetting of valve 17 by flow of hydraulic fluid in the loop including valve 17.

Powered emergency retraction of the arms can be achieved by appropriate modification of the hydraulic circuit upstream of the selectors 10, 11 and by ensuring that in an emergency both selectors 10, 11 connect the actuators 18, 1 9 to the hydraulic supply simultaneously.

It will be appreciated that more than two arms can be controlled in accordance with the present invention and that alternative flow limiting devices to the piston and cylinder arrangements 20, 21 could be used.

Claims

1. A control system for a marine arm comprising a plurality of hydraulic actuators for controlling the relative orientation of elements of the arm, a hydraulic fluid distribution circuit for distributing pressurised hydraulic fluid to the actuators, hydraulic control valves for controlling the supply of hydraulic fluid to respective actuators, and emergency control means for disconnecting the arm from a tanker and lifting the arm to a retracted position in the event of an emergency, characterised in that the emergency control means comprises means for limiting the volume of hydraulic fluid which can flow through the actuators subsequent to the disconnection of the arm in an emergency to thereby limit the lift of the arm.

2. A control system as claimed in claim 1 in which the emergency control means comprises means for connecting a flow limiting device in series with the actuators.

3. A control system as claimed in claim 2 in which the flow limiting device comprises means for blocking part of the distribution circuit, a bypass circuit for bypassing the blockage, and means for limiting the volume of hydraulic fluid which can flow through the bypass circuit.

4. A control system as claimed in claim 3 in which the means for limiting the volume of hydraulic fluid which can flow through the bypass circuit comprises a piston and cylinder arrangement connected in parallel with the said blocking means.

5. A control circuit as claimed in claim 4 in which an adjustable stop is provided to enable adjustment of the maximum stroke of the piston and cylinder arrangement and thus to adjust the limit imposed upon the lift of the arm in an emergency.

6. A control circuit as hereinbefore described with reference to the accompanying drawings.