rosette
Chemical
- May 29, 2006
- 1
Reference "bean" thread481-127012
Are there any experienced oil production operators familiar with the operation and failure scenarios of production chokes at the top of risers?
I am trying to consider the worst case relief cases which might occur downstream system of a choke failing open at the top of an offshore oil production riser during start-up.
The Topside choke in question may during normal production be almost fully open with relatively little pressure drop as most of the pressure loss from the well is taken across subsea choke valves at each wellhead. Unless this choke is throttled to increase the pressure in the riser and the subsea chokes opened transfering control of production to the Topside choke rather than the subsea chokes, it is unlikely that failure open during operation would result in excessive production flows and possible overpressurisation of the downstream system if for any reason the choke control fails open.
If the valve was closed to stop production from the riser it is unlikely that the valve could be initially open at start-up and it is assumed that normal start-up procedure would begin with the valve closed.
During a shutdown of a few hours it is possible if one of the subsea well valves were not completely closed the pressure in the riser would increase and gas would tend to migrate to the top section of the riser. If then there is a failure of the controls on the "hand control" choke valve
that caused it to be suddenly opened then there could be a large flow of gas to the downstream system possibly causing overpressure and resulting in a 'worst case relief" scenario. Another possibility might be a sudden failure of the valve trim as the riser ESDV is opened and the choke experiences the packed riser pressure. I have heard an operator suggesting this possibility but I do not know how realistic it is.
Determination of the worst case relief will depend on essentially two factors
(i) the maximum initial pressure upstream of the choke when the failure occurs
(ii) the maximum CV across the valve that might realistically occur when the failure occurs
Can anyone help rationalise what is realistic or should I assume a scenario in which the failure occurs when there is maximum gas packed riser pressure upstream and the choke CV is equivalent to the CV of the choke without trim (assuming internal trim to have disintegrated!)?
It has been suggested that if the valve is designed to "fail in position" this might help but not knowing enough about the choke design and operation I remain unconvinced.
Are there any experienced oil production operators familiar with the operation and failure scenarios of production chokes at the top of risers?
I am trying to consider the worst case relief cases which might occur downstream system of a choke failing open at the top of an offshore oil production riser during start-up.
The Topside choke in question may during normal production be almost fully open with relatively little pressure drop as most of the pressure loss from the well is taken across subsea choke valves at each wellhead. Unless this choke is throttled to increase the pressure in the riser and the subsea chokes opened transfering control of production to the Topside choke rather than the subsea chokes, it is unlikely that failure open during operation would result in excessive production flows and possible overpressurisation of the downstream system if for any reason the choke control fails open.
If the valve was closed to stop production from the riser it is unlikely that the valve could be initially open at start-up and it is assumed that normal start-up procedure would begin with the valve closed.
During a shutdown of a few hours it is possible if one of the subsea well valves were not completely closed the pressure in the riser would increase and gas would tend to migrate to the top section of the riser. If then there is a failure of the controls on the "hand control" choke valve
that caused it to be suddenly opened then there could be a large flow of gas to the downstream system possibly causing overpressure and resulting in a 'worst case relief" scenario. Another possibility might be a sudden failure of the valve trim as the riser ESDV is opened and the choke experiences the packed riser pressure. I have heard an operator suggesting this possibility but I do not know how realistic it is.
Determination of the worst case relief will depend on essentially two factors
(i) the maximum initial pressure upstream of the choke when the failure occurs
(ii) the maximum CV across the valve that might realistically occur when the failure occurs
Can anyone help rationalise what is realistic or should I assume a scenario in which the failure occurs when there is maximum gas packed riser pressure upstream and the choke CV is equivalent to the CV of the choke without trim (assuming internal trim to have disintegrated!)?
It has been suggested that if the valve is designed to "fail in position" this might help but not knowing enough about the choke design and operation I remain unconvinced.
