On 16 March 2016, in connection with a well wash during completion of a drilling operation at Visund, a well-control incident occurred involving inflow from the reservoir (a well kick). In advance of the incident, Statoil had assumed that barriers, notably in the form of a cemented 7" liner, had been verified. Shortly after the washing process was completed and the well was filled with seawater, the drill string was raised and a volume increase was observed. The well was shut in using a valve on the BOP. Pressure was observed in the well, which finally stabilised at 84 bar.
While trying to kill the well, it was discovered that both kelly cock valves below the top drive had jammed, one in the closed position, which prevented the use of normal kill procedures. Alternative kill methods were assessed, while attempts were made to operate the valves.
The installation was depressurised and production at Visund was shut down. No injuries to persons, material damage or emissions to the external environment were registered as a result of the inflow into the well.
What made this incident special is that normal well-control methods for killing the well were prevented by a jammed valve below the top drive, and that there was originally assumed to be a verified barrier in the well in the form of a confirmed inflow test. In very slightly different circumstances, the well kick might have led to a complicated and long-lasting kill operation with the potential for escalation of risk.
The investigation has identified non-conformities relating to the design of well barriers, verification of well barriers, classification of safety-critical equipment, the maintenance programme for kelly cock valves and the configuration of the sea-bed BOP.
Furthermore, improvement points were identified in respect of well barriers, the risk register in DOP documents, and training and drills.
Statoil has been given a deadline of 18 August 2016 to report on how the non-conformities and improvement points will be dealt with.
The Visund field
The Visund oil and gas field, in blocks 34/8 and 34/7, is 22 kilometres northeast of the Gullfaks field in the Tampen area. Oil production began in 1999 and gas production in 2005. The field has been developed using the Visund FPDU, a semi-submersible drilling, processing and accommodation platform, in a water depth varying between 270 and 380 metres on the west slope of the Norwegian Trench. The field was subsequently developed with further wells from well templates on the sea bed, called Visund Sør and Visund Nord, respectively. The wells from the main field and the wells from Visund Nord are tied to the Visund A platform by flexible risers. Statoil is the operator of the Visund field.