Method for maintenance of valves included in a subsea production system for oil and gas

摘要

The present invention relates to a method for replacing an insert unit included in an insert valve assembly belonging to a subsea production system for gas and oil. In carrying out the method, a manipulator (1) is brought by means of a sheave (2), a hoisting cable (3) and guide wires (4, 5) down to a predetermined location on the bottom of the sea. The manipulator comprises a telescopic arm (15, 16) which is then moved towards and fixed by means of a hook (20) and a lug (21) to a valve tree (17) which supports a valve (22), the insert unit (26) of which is to be unscrewed from the respective valve housing and be replaced. A carriage (18) located in the manipulator and comprising retracting, aligning and fixing devices (32, 33, 34) is brought with the aid of the telescopic arm and a rail, located on the top side of the carriage, towards the valve tree and is fixed to retraction cones (35, 36, 37) mounted on the valve tree. This allows a moment producing tool (31), mounted in the carriage, to grapple that castellated nut (24) which holds together the insert unit and valve housing of the valve. Thereafter, a releasing moment is applied on the castellated nut, whereby the castellated nut with the insert unit is unscrewed from the valve housing and the valve tree. The carriage with the insert unit is separated from the valve tree and is brought back via the telescopic arm to the manipulator. The insert unit of the valve is replaced by a new insert unit, either by a new insert unit stored in the manipulator and with the aid of gripping arms present in the manipulator, or by raising the manipulator to the surface where replacement can be carried out, whereafter the carriage is again moved towards and fixed to the valve tree. This also allows the moment producing tool to move the new insert unit with its castellated nut towards the valve housing, whereupon a tightening moment can be applied.