JANUARY 2001: DESCRIPTION OF THE EQUIPMENT
The overall system is rather similar to the one experimented with at Nyos in 1995. Special attention has been paid to the instrumentation (see below), both for the purpose
of technical surveillance of the two-phase flow instabilities, and for the collection some more fundamental parameters.
The polyethylene bars which constitute the column were electrically soldered on the spot. The pipe external and internal diameters are 180 mm and 145 mm, respectively. The depth of the inlet removal mouth is as close as possible to the bottom of the lake, say, around -203 m.
It is to be noticed that the dissolved gas concentration considerably increased at 200 m depth since the last experiment in 1995 ( 9 lgas/lliquid as compared to 5.5 lgas/lliquid ): the self-sustained fountain is at a much higher energetic level. The gas release rate has largely overcome the natural CO2 recharge (from 2 to 5 Mm3/year).
Monitoring of the diphasic stream
liquid speed 1 (-100 m)
liquid speed 2 and 3 (-140 m)
internal pressure 1 (-100 m)
internal pressure 2 (-140 m)
differential pressure at the inlet aperture
conductivity (-203 m)
temperature (-203 m)
depth of the column foot(-203 m)
5 temperature sensors from 10cm to 40m
temperature logger at the bottom of the lake
5 temperatures in the sediment
battery 1 and 2
pressure bottle 1 and 2
The surveillance principle of the degassing column is based on a safety criteria two levels. 1st level: an automatic control platform, set up on the instrumentation raft, continuously monitors the flow rate most significant parameters (fluid speed, internal pressure, depth of the pipe inlet mouth). An internal software analyses the collected data in real time and the CPU board is programmed in order to take the decision to automatically stop the self-siphon (opening the lateral valve) in case of major the detection of anomalies (red alert). In case a minor anomaly appears during the data analysis (orange alert), the monitoring platform automatically asks for a remote connexion with the PC control stations (located in Chambery, Yaounde or elsewhere) through the Inmarsat satellite system. After careful examination of the likely origin of the anomaly, an operator could take the decision to order, though the satellite connexion, the activation of the various valves in order to stop the self-siphon. During normal functioning, the collected data are stored in the CPU memory. At any time, an operator in France or in Yaounde can take the decision to retrieve a proper selected data set via Inmarsat.
The separator device was designed in order to continuously monitor the parameters characteristics of the water which flows through the inlet mouth of the column.
A small pipe (1 cm diameter) is attached to the bottom of the column and a self siphon is triggered in this pipe so that the water is permanently driven inside the separator drum.
The following parameters are measured :
* Gas flow rate
* Liquid flow rate
Using those two values, the dissolved gas concentration can be precisely inferred.
* Conductivity of the degassed water (2 inductive sensors)
* Temperature of the water (2 sensors)
* Temperature of the gas
The above characteristic parameters allow us to determine from which layer of the lake the removed water has come. It is an invaluable piece of information to monitor the water de-stratification created by the self-siphon.
Investigation of the bottom of the lake using a radon sensor : Open the pdf