Setting the value of an operational parameter of a well

1. A method of setting a value of an operational parameter of a well, the method comprising:

providing a measure related to an actual value of the parameter;

setting a maximum limit for the measure;

setting a minimum limit for the measure;

setting a demanded value for the parameter; and

automatically overriding the demanded value if it is such that it would result in the measure exceeding the maximum limit or being below the minimum limit to produce a value for the demanded value for the parameter which results in the measure not exceeding the maximum limit and not being below the minimum limit,

wherein automatically overriding the demanded value comprises:

comparing the measure with the maximum limit and producing a first value for the parameter from a maximum limit error between the measure and the maximum limit, wherein the first value increases as the demanded value increases so that, if the demanded value would result in the measure being at or above the maximum limit, the first value would result in the measure being at the maximum limit;

producing a new first value by comparing the demanded value and the first value and selecting the lower of the demanded value and the first value;

comparing the measure with the minimum limit and producing a second value for the parameter from a minimum limit error between the measure and the minimum limit, wherein the second value decreases as the demanded value decreases so that, if the demanded value would result in the measure being at or below the minimum limit, the second value would result in the measure being at the minimum limit; and

setting the value for the demanded value for the parameter as the higher of the first new value and the second value.

2. The method according to claim 1 , wherein:

the first value is produced by multiplying the maximum limit error by a constant factor to result in a proportional maximum limit error that is added to a dynamically lagged version of the actual demanded value.

3. The method according to claim 2 , further comprising wherein the second value is produced by multiplying the minimum limit error by a constant factor to result in a proportional minimum limit error that is added to a dynamically lagged version of the actual demanded value.

4. The method according to claim 1 , wherein the operational parameter is a parameter of an actuatable member.

5. The method according to claim 4 , wherein the member comprises a choke.

6. The method according to claim 4 , wherein the measure related to the actual value of the parameter is fluid pressure at the member, the parameter being a position of the member.

7. The method according to claim 1 , wherein the well is a hydrocarbon production or injection well.

8. The method according to claim 1 , wherein the second value is produced by multiplying the minimum limit error by a constant factor to result in a proportional minimum limit error that is added to a dynamically lagged version of the actual demanded value.

9. A control system of a well, for setting the value of an operational parameter of the well, the system comprising:

a sensor configured to provide a measure related to the actual value of the parameter,

the control system being configured to:

set a maximum limit for the measure;

set a minimum limit for the measure;

set a demanded value for the parameter; and

automatically override the demanded value if it is such that it would result in the measure exceeding the maximum limit or being below the minimum limit to produce a value for the demanded value of the parameter which results in the measure not exceeding the maximum limit and not being below the minimum limit,

wherein automatically overriding the demanded value comprises:

comparing the measure with the maximum limit and producing a first value for the parameter from a maximum limit error between the measure and the maximum limit, wherein the first value increases as the demanded value increases so that, if the demanded value would result in the measure being at or above the maximum limit, the first value would result in the measure being at the maximum limit;

producing a new first value by comparing the demanded value and the first value and selecting the lower of the demanded value and the first value;

comparing the measure with the minimum limit and producing a second value for the parameter from a minimum limit error between the measure and the minimum limit, wherein the second value decreases as the demanded value decreases so that, if the demanded value would result in the measure being at or below the minimum limit, the second value would result in the measure being at the minimum limit; and

setting the value for the demanded value for the parameter as the higher of the first new value and the second value.

10. The control system according to claim 9 being further configured to: produce the first value by multiplying the maximum limit error by a constant factor to

result in a proportional maximum limit error that is added to a dynamically lagged version of the actual demanded value; and

produce the second value by multiplying the minimum limit error by a constant factor to result in a proportional minimum limit error that is added to a dynamically lagged version of the actual demanded value.

11. The control system according to claim 9 , wherein the operational parameter is a parameter of an actuatable member.

12. The control system according to claim 11 , wherein the member comprises a choke.

13. The control system according to claim 11 , wherein the measure related to the actual value of the parameter is fluid pressure at the member, the parameter being a position of the member.

14. The control system according to claim 9 , wherein the well is a hydrocarbon production or injection well.

15. The method according to claim 9 , wherein the second value is produced by multiplying the minimum limit error by a constant factor to result in a proportional minimum limit error that is added to a dynamically lagged version of the actual demanded value.

16. A method of setting the value of an operational parameter of a well, the method comprising:

providing a measure related to the actual value of the parameter;

setting a maximum limit for the measure;

setting a minimum limit for the measure;

setting a demanded value for the parameter; and

automatically overriding the demanded value if it is such that it would result in the measure exceeding the maximum limit or being below the minimum limit,

wherein automatically overriding the demanded value comprises:

comparing the measure with the maximum limit and producing a first value for the parameter from a maximum limit error between the measure and the maximum limit, wherein the first value increases as the demanded value increases so that, if the demanded value would result in the measure being at or above the maximum limit, the first value would result in the measure being at the maximum limit;

producing a new first value by comparing the demanded value and the first value and selecting the lower of the demanded value and the first value;

comparing the measure with the minimum limit and producing a second value for the parameter from a minimum limit error between the measure and the minimum limit, wherein the second value decreases as the demanded value decreases so that, if the demanded value would result in the measure being at or below the minimum limit, the second value would result in the measure being at the minimum limit; and

setting the value for the demanded value for the parameter as the higher of the first new value and the second value.

17. The method of claim 16 , wherein if the demanded value results in the measure exceeding the maximum limit or being below the minimum limit, automatically overriding the demanded value comprises:

maintaining the actual measured value of the parameter.

18. The method of claim 16 , wherein if the demanded value results in the measure exceeding the maximum limit or being below the minimum limit, automatically overriding the demanded value comprises:

not adjusting the actual measured value of the parameter based upon the demanded value.