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

A method for replacing an insert unit (26) included in an insert valve assembly (22) belonging to a subsea production system for oil and gas using a manipulator (1) which

• by means of a hoisting cable (3) can be raised and lowered to the desired depth,
• by means of guide wires (4, 5), acting as guidelines passing through guide frames (6, 7) fixed to the manipulator, can be positioned at the desired location on the bottom of the sea,
• is mounted on a beam (8) with guide slots (9, 10), said guide slots being intended to receive guide posts (11, 12), which are placed on the bottom of the sea and which are provided with spacing sleeves (13, 14) to obtain the proper height above the bottom of the sea for the manipulator,
• has a built-in remotely operable telescopic arm (15, 16) provided with a hook (20) at the end of the arm,
• has a built-in carriage (18) movable on the telescopic arm, said carriage comprising a moment-producing tool (31) as well as aligning, retracting and fixing devices (32, 33, 34) for said carriage and said tool, and that the insert valve assembly with its insert unit is mounted on a valve tree (17) which
• has a lug (21) for receiving the hook of the telescopic arm, and
• has guide cones (35, 36, 37) for aligning, retracting and fixing devices mounted on the carriage, said insert unit comprising a nut (24) by which the insert unit is screwed to and unscrewed from, respectively, the rest of the insert valve assembly, and that the moment-producing tool is formed so as to engage and grapple the nut of the insert unit, said method comprising the sequential steps of
• lowering the manipulator down towards the bottom of the sea by means of the hoisting cable and guide wires and positioning via the beam, the guide slots, the guide posts and the spacing sleeves so that the tool present in the carriage arrives in front of that insert valve assembly whose insert unit is to be replaced,
• bringing the telescopic arm of the manipulator towards the valve tree so that the hook of the arm is secured to the lug on the valve tree,
• moving the carriage towards the valve tree,
• maneuvering the aligning, retracting and fixing devices in the carriage into engagement with the respective retraction cones on the valve tree,
• guiding the moment-producing tool towards the insert valve assembly so as to engage and grapple the nut of the insert unit,
• applying a releasing moment on the nut of the insert unit via the moment-producing tool,
• applying a moment for threading the insert unit out of the rest of the insert valve assembly via the moment-producing tool,
• detaching the aligning, retracting and fixing devices in the carriage from their respective retraction cones on the valve tree,
• moving the carriage with the insert unit back to the manipulator,
• replacing the disengaged insert unit by a new insert unit,
• moving the carriage with the insert unit towards the valve tree,
• maneuvering the aligning, retracting and fixing devices in the carriage into engagement with the respective retraction cones on the valve tree,
• maneuvering the moment-producing tool towards the remaining part of the insert valve assembly, the new insert unit being inserted into said part,
• applying a moment on the nut of the insert unit for threading in the insert unit via the moment-producing tool,
• applying a fastening moment on the nut of the insert unit via the moment-producing tool,
• withdrawing the moment-producing tool from the nut of the insert unit,
• detaching the aligning, retracting and fixing devices in the carriage from their respective retraction cones on the valve tree,
• moving the carriage back to the manipulator,
• disengaging the hook on the telescopic arm from the lug on the valve tree,
• moving the telescopic arm back to the manipulator, and
• raising the manipulator to the surface of the sea by means of the hoisting cable and the guide wires.