At around 0.15 a.m. on a Sunday, the technicians at a combined cycle gas turbine power plant undergoing construction detected an oil leak when rinsing the shaft of a turbine with oil. 8.1m³ of oil spilled on the ground under the building’s retention system and was collected by the building’s drainage system through to the retention tank equipped with lift pumps. This retention tank was normally connected to the process water basin. However, due to painting work underway in this basin, a temporary connection had been in place for the past several days to discharge the water from the retention tank into the stormwater tank. The night shift did not know that the temporary connection was no longer in place, because roadworks were in progress. The oily water therefore overflowed onto the ground in an area of compacted sand. The overflow of oil was detected only on Monday due to the absence of activity in the zone during the weekend. All the ongoing operations were stopped, and a specialist firm pumped the product on the ground and in the stormwater tank.

The aim of the operation was to clean the oil lines of any solid particles. It was stopped in order to replace the filters, which involved opening the drainage valve and the filter. After replacing the filters, the filter was closed again. Following this operation, the drainage valve was left in open position. During restarting of the circuit for a period of 20-30 min., the oil was entrained directly into the building’s retention system. Moreover, the pumping system was equipped with an oil detector and ought to have switched off the pumps and sent an alarm to the control room. However, during the construction phases and due to the nature of the effluents which could clog it with dirt and sludge, the detector had been disabled. Lastly, the technicians had not been informed that the building’s tank was no longer connected to the process water basin and that a temporary pipe had been installed to connect it to the stormwater tank.

The operator:

• organised the operation of excavation of the contaminated earth by a specialist firm;
• put in place a covering on the area to stop any rainwater runoff;
• cleaned and degreased the floor of the building;
• cleaned the contaminated networks and the stormwater tank;
• performed a study of soil contamination by random survey and produced a soil management plan based on the results;
• revised its procedures and instructions in the works phase and in degraded situations.