Gas leak during removal of temperature sensor from thermo well

During a recent diver inspection, repair and maintenance programme undertaken by one of our members, one of the identified tasks was the replacement of a defective temperature sensor mounted in a thermo well (a pressure retaining pocket) on a production tree. When the diver started to unscrew the sensor from the thermo well, gas started to escape from the threads. During the completion of the task it became apparent that the thermo well around the temperature sensor was no longer in place or was defective, the sensor was open to the tree internal pressure and the diver was unscrewing a collar behind which was a direct communication path into the tree production pipework.

Both the onshore project team and those at the offshore worksite had extensive experience of this type of operation and the hazard had been identified beforehand, with control measures identified and implemented to reduce the risk to a level that was as low as reasonably practicable.

The company involved has provided the following description of the task which was being carried out.

The replacement of a defective pressure or temperature sensor in a subsea tree or manifold is a very common diver maintenance task. Unlike a pressure sensor which, by design, must be in contact with the internal pressure being monitored, a temperature sensor does not necessarily need to be in the pipeline and is commonly mounted in a pressure-retaining thermo well.

The thermo well is best described as a tube sealed at the bottom and open at the top, welded or bolted to the pipe wall and extending through the wall into the bore of the line to be monitored. Because it is a sealed tube sitting in the bore of the line, the outside of the thermo well is subjected to line pressure but the inside remains at ambient pressure. The temperature sensor, which is screwed into the centre of the thermo well, can monitor the temperature of the line (by heat conduction) but is protected from the effects of the line pressure. It is common for the (threaded) connection between the temperature sensor and the thermo well to be a pressure retaining connection often rated far in excess of the tree internal pressure.

Unlike a pressure sensor which, prior to removal, will typically be isolated from the pipeline internal pressure by double block and bleed valves the integrity of which can be tested, the ‘isolation’ of the temperature sensor from the pipeline pressure is totally dependent on the integrity of the thermo well tubing.

In service the thermo well can be exposed to corrosive fluids, abrasive sand, vibration and turbulent fluid flow all of which can affect the integrity of the thermo well and allow direct pressure communication between the outside and the inside of the tubing.

Because of the pressure-retaining nature of the temperature sensor/thermo well connection it is possible there will be no visible external indication that the thermo well is defective until the temperature sensor/thermo well connection is broken by the diver, at this point the diver may be exposed to the full pressure in the line.

Conclusion

Modern thermo wells are generally very reliable and are appropriately designed, selected and specified for the environment in which they are used. It would, however, be incorrect to suggest that they cannot fail.

Thermo wells are primarily installed to protect the delicate temperature sensor probe from the pressure, flow and contents in the line rather than to allow the removal of the sensor with the pipeline under pressure. If the thermo well is not defective then the removal and replacement of the temperature sensor is not an intrusive task. Because the Temperature Sensor to thermo well connection is normally pressure retaining there may be no way of ensuring the integrity of the thermo well until the Temperature Sensor is unscrewed from it – releasing the contents of the pipeline into the environment.

However, it is good practice to assume that there may be a hazard caused by retained pressure when working on a thermo well.

The company has noted the following aspects to be considered when planning, risk assessing and carrying out the removal of a temperature sensor from a thermo well:

• Never assume that a thermo well is pressure-retaining – always approach the task in the knowledge that on more than one occasion divers have unscrewed a temperature sensor to find communication though into the pipeline;
• Find out the history of the worksite – How old is the field? Have there been any reported thermo well failures in the past? Is the product particularly corrosive, abrasive or turbulent? Have any temperature sensors previously been changed?
• Consider the design of the temperature sensor – How does it attach to the thermo well? Is it pressure-retaining or non pressure-retaining? Can the unit be released in a controlled manner?
• Identify the pressure and the contents of the pipeline. Check that the pressures and volumes of pipeline contents around the thermo well have been minimised as far as practical;
• Consider that the temperature sensor may be faulty because it has been exposed to the pressure, flow, abrasion or corrosiveness of the pipeline contents;
• The risk assessment should consider the hazard of ‘uncontrolled release of retained pressure’ in addition to the usually-addressed ‘contact with live electricity’ when considering the task of temperature sensor removal from a thermo well;
• Ensure that the dive plan and diver briefing highlight the possibility that there may be pressure behind the temperature sensor should the thermo well be defective;
• Ensure that the replacement temperature sensor is deployed and ready to be installed, thereby minimising the time that the thermo well is open;
• Ensure that the divers position themselves out of the ‘line of fire’ when slackening the temperature sensor;
• Ensure that the divers slacken the connection slightly and break the seal, to check for gas or fluid release before completely removing the temperature sensor;
• If practical use a ‘whip check’ or otherwise physically restrain the potential movement of the temperature sensor during the slackening of the connection.