Hydraulic control of borehole tool deployment

A drilling installation comprising:

• an elongate drill string extending longitudinally along a borehole, the drill string having a housing that defines a longitudinally extending internal bore configured to convey drilling fluid under pressure;
• a drill string tool forming part of the drill string and configured to be disposable between an activated condition and a deactivated condition;
• a control mechanism coupled to the drill string tool and configured to allow operator-controlled switching of the drill string tool by control of drilling fluid pressure conditions, the control mechanism comprising:
• a valve body within the housing, the valve body defining a valve port in fluid communication with the internal bore and with a hydraulic deployment mechanism of the drill string tool;
• a valve closing element configured for switching between an open condition in which the internal bore is in fluid communication with the activation volume, via the valve port, and a closed condition in which the closing element substantially prevents fluid flow through the valve port;
• a switch ram coupled to the valve closing element and configured for hydraulically driven movement in an activation direction in response to predefined above-threshold downhole drilling fluid conditions, to switch the valve closing element from the open condition to the closed condition;
• and
• a switch regulator coupled to the switch ram and configured to regulate switching of the valve closing element from the closed condition to the open condition by providing regulated hydraulic resistance to movement by the switch ram in the activation direction.

A method of controlling a drill string tool coupled in a drill string within a borehole, the drill string defining an internal bore to convey drilling fluid under pressure, the method comprising:

• incorporating in the drill string a control mechanism for the drill string tool, the control mechanism comprising
• a valve body within the housing, the valve body defining a valve port that provides fluid communication between with the internal bore and a hydraulic deployment mechanism of the drill string tool;
• a valve closing element configured for switching between an open condition in which the internal bore is in fluid communication with the activation volume, via the valve port, and a closed condition in which the closing element substantially prevents fluid flow through the valve port;
• a switch ram coupled to the valve closing element and configured for hydraulically driven movement in an activation direction in response to predefined above-threshold downhole drilling fluid conditions, to switch the valve closing element from the open condition to the closed condition;
• and
• a switch regulator coupled to the switch ram and configured to regulate switching of the valve closing element from the closed condition to the open condition by providing regulated hydraulic resistance to movement by the switch ram in the activation direction;
• and
• controlling downhole drilling fluid conditions from a surface control system, to cause the predefined above-threshold downhole drilling fluid conditions, thereby switching the valve closing element to the open condition and causing deployment the drill string tool.