A new 30” pipe-line from an offshore field to an onshore Gas Plant was ready for commissioning. During commissioning a pig was propelled through the line with 100bar nitrogen. The ‘hydrocarbon’ gas that was in front of the pig was to pass through an orifice plate into an existing line (at lower pressure). Inevitably this would cause a temperature drop if the flow rate was too high. If the temperature had dropped below -40°C there was a danger of the carbon steel pipes failing.
Method & Objectives
- Measure pressure and temperature both upstream and downstream of an orifice plate installed on the sub-sea manifold.
- Display live pressure and temperature sub-sea for viewing by ROV
- Transmit data to surface via ROV umbilical
- Record pressure and temperature for later analysis
The data logging was provided by OEL 6000SD units which perfomed all the recording and transmission of data from MATRE sensors. Each 6000SD was connected to a pair of MATRE sensors to provide 100% redundancy. Data from each tapping point was displayed live on each 6000SD for viewing by the ROV, allowing a double check on accuracy of sensors.
Pressure and temperature measurement was by MATRE dual channel sensors fitted to each tapping point. The MATRE sensors have two pressure and two temperature measurement ports in a single tapping housing (rated to 30 year life). This design ensured the minimum of connections to either side of the orifice plate whilst providing 100% redundancy. Connection to the MATRE sensor was via TRONIC wet-mateable connectors which are standard units that are approved for use by most subsea installation companies.
As shown in the schematic above the ROV could connect to either of the 6000SD units via a TRONIC connector, with data then displayed on OEL provided note-books in the ROV control room.
The 6000SD and ROV switches were mounted on a single ‘template’ that allowed easy removal and installation by the ROV. The template was designed to provide optimum connection and disconnection.
All electrical connections and splices were proof tested to 150bar to ensure water-tight integrity and the TRONIC units were rated to 300bar.
The operation was successfully concluded with zero harm to the pipeline.