Method for operating winch, and winch

1. A method for operating a winch comprising a winch drum for spooling a spoolable medium for mooring a vessel, the method comprising:

monitoring a tension of the spoolable medium between the vessel and a point of mooring;

in response to the monitored tension of the spoolable medium becoming higher than a first upper tension threshold or lower than a first lower tension threshold, initiating, either immediately or after a first predetermined delay, driving of the winch drum,

wherein, during the driving of the winch drum, when the monitored tension of the spoolable medium is higher than a predetermined tension set point, which has a value between the first upper tension threshold and the first lower tension threshold, the winch drum is driven in such a direction that the spoolable medium is spooled out,

wherein, during the driving of the winch drum, when the monitored tension of the spoolable medium is lower than the predetermined tension set point, the winch drum is driven in such a direction that the spoolable medium is spooled in, and

wherein, during the driving of the winch drum, at least when the monitored tension of the spoolable medium is between a second upper tension threshold, which has a value equal to or lower than the first upper tension threshold but higher than the predetermined tension set point, and a second lower tension threshold, which has a value equal to or higher than the first lower tension threshold but lower than the predetermined tension set point, an absolute value of the driving speed of the winch drum has a value that is proportional to an absolute value of a difference between the monitored tension of the spoolable medium and the predetermined tension set point; and

in response to the monitored tension of the spoolable medium being, during the driving of the winch drum, between the second upper tension threshold and the second lower tension threshold for a second predetermined delay, stopping the driving of the winch drum.

2. The method of claim 1 , wherein, during the driving of the winch drum, when the monitored tension of the spoolable medium is higher than the second upper tension threshold, or when the monitored tension of the spoolable medium is lower than the second lower tension threshold, the absolute value of the driving speed of the winch drum has a value that is a predetermined constant after the driving speed of the winch drum has reached said predetermined constant driving speed.

3. The method of claim 1 , wherein the value of the absolute value of the driving speed of the winch drum is determined according to the following equations:

when the monitored tension of the spoolable medium is between the second upper tension threshold and the predetermined tension set point:

| Ref|=AM _ ref *([| Rt−Sp |]/[| T 2 u−Sp |]), and

when the monitored tension of the spoolable medium is between the second lower tension threshold and the predetermined tension set point:

| Ref|=AM _ ref *([| Rt−Sp |]/[| T 2 l−Sp |]),

where:

Ref=the driving speed of the winch drum

AM_ref=a predetermined constant

Rt=the monitored tension of the spoolable medium

Sp=the predetermined tension set point

T 2 u =the second upper tension threshold

T 2 l =the second lower tension threshold.

4. The method of claim 1 , wherein the winch drum is driven by an electric motor.

5. The method of claim 4 , wherein the tension of the spoolable medium between the vessel and a point of mooring is monitored by monitoring a torque of the electric motor.

6. The method of claim 5 , wherein the torque of the electric motor is monitored by monitoring a current of the electric motor.

7. A winch comprising:

a winch drum for spooling a spoolable medium for mooring a vessel;

a winch drive for driving the winch drum; and

a controller configured to:

monitor a tension of the spoolable medium between the vessel and a point of mooring;

in response to the monitored tension of the spoolable medium becoming higher than a first upper tension threshold or lower than a first lower tension threshold, initiate, either immediately or after a first predetermined delay, driving of the winch drum,

wherein, during the driving of the winch drum, when the monitored tension of the spoolable medium is higher than a predetermined tension set point, which has a value between the first upper tension threshold and the first lower tension threshold, the winch drive is configured to drive the winch drum in such a direction that the spoolable medium is spooled out,

wherein, during the driving of the winch drum, when the monitored tension of the spoolable medium is lower than the predetermined tension set point, the winch drive is configured to drive the winch drum in such a direction that the spoolable medium is spooled in, and

wherein, during the driving of the winch drum, at least when the monitored tension of the spoolable medium is between a second upper tension threshold, which has a value equal to or lower than the first upper tension threshold but higher than the predetermined tension set point, and a second lower tension threshold, which has a value equal to or higher than the first lower tension threshold but lower than the predetermined tension set point, an absolute value of the driving speed of the winch drum is configured to have a value that is proportional to an absolute value of a difference between the monitored tension of the spoolable medium and the predetermined tension set point; and

in response to the monitored tension of the spoolable medium being, during the driving of the winch drum, between the second upper tension threshold and the second lower tension threshold for a second predetermined delay, stop the driving of the winch drum.

8. The winch of claim 7 , wherein, during the driving of the winch drum, when the monitored tension of the spoolable medium is higher than the second upper tension threshold or when the monitored tension of the spoolable medium is lower than the second lower tension threshold, the absolute value of the driving speed of the winch drum is configured to have a value, which is a predetermined constant after the driving speed of the winch drum has reached said predetermined constant driving speed.

9. The winch of claim 7 , wherein the winch drive is configured to determine the second value of the absolute value of the driving speed of the winch drum according to the following equations:

when the monitored tension of the spoolable medium is between the second upper tension threshold and the predetermined tension set point:

| Ref|=AM _ ref *([| Rt−Sp |]/[| T 2 u−Sp |]), and

when the monitored tension of the spoolable medium is between the second lower tension threshold and the predetermined tension set point:

| Ref|=AM _ ref *([| Rt−Sp |]/[| T 2 l−Sp |]),

where:

Ref=the driving speed of the winch drum

AM_ref=a predetermined constant

Rt=the monitored tension of the spoolable medium

Sp=the predetermined tension set point

T 2 u =the second upper tension threshold

T 2 l =the second lower tension threshold.

10. The winch of claim 7 , wherein the winch drive comprises an electric motor for driving the winch drum.

11. The winch of claim 10 , wherein the winch drive is configured to monitor the tension of the spoolable medium between the vessel and a point of mooring by monitoring a torque of the electric motor.

12. The winch of claim 11 , wherein the winch drive is configured to monitor the torque of the electric motor by monitoring a current of the electric motor.

13. The winch of claim 10 , wherein the electric motor is an AC motor or a DC motor.

14. The winch of claim 7 , wherein the spoolable medium is a cable, a rope, a wire or a chain.

15. A control system for a winch including a winch drum for spooling a spoolable medium for mooring a vessel and a winch drive for driving the winch drum, the control system comprising:

a controller for controlling the winch drive, the controller comprising a processor and a non-transitory computer readable medium storing instructions that, when executed by the processor, cause the controller to:

monitor a tension of the spoolable medium between the vessel and a point of mooring;

in response to the monitored tension of the spoolable medium becoming higher than a first upper tension threshold or lower than a first lower tension threshold, initiate, either immediately or after a first predetermined delay, driving of the winch drum, such that during the driving of the winch drum:

when the monitored tension of the spoolable medium is higher than a predetermined tension set point, which has a value between the first upper tension threshold and the first lower tension threshold, the winch drum is driven in such a direction that the spoolable medium is spooled out,

when the monitored tension of the spoolable medium is lower than the predetermined tension set point, the winch drum is driven in such a direction that the spoolable medium is spooled in, and

at least when the monitored tension of the spoolable medium is between a second upper tension threshold, which has a value equal to or lower than the first upper tension threshold but higher than the predetermined tension set point, and a second lower tension threshold, which has a value equal to or higher than the first lower tension threshold but lower than the predetermined tension set point, an absolute value of the driving speed of the winch drum has a value that is proportional to an absolute value of a difference between the monitored tension of the spoolable medium and the predetermined tension set point; and

in response to the monitored tension of the spoolable medium being, during the driving of the winch drum, between the second upper tension threshold and the second lower tension threshold for a second predetermined delay, stop the driving of the winch drum.