Abstract: A hollow body is provided with a seal by which it may be sealed within a nipple profile included in a tubing string. The body allows a wireline to be passed through it while maintaining a substantial seal. During running-in lower collet fingers engage a stop ring mounted on the body. When the plug becomes seated in the nipple profile by the weight of a logging tool also suspended on the wireline, fingers are disengaged by inward movement of external collet fingers and rollers. Consequent downward movement of the fingers allows the rope socket to push them outwardly and allows the logging tool to be lowered. An equalizing sleeve can be pulled up by the socket head from the sealing position to a raised position which allows pressure equalization above and below the plug to facilitate withdrawal thereof. Profiles on the fingers act to trap the socket head should the wireline pull out or break.

Abstract: A downhole instrumentation system comprises one or more transducers coupled in a DC current circuit between the star point of an electrical submersible pump and the chassis. A sensing circuit including a DC power supply and an ammeter is arranged to monitor the current drawn. The system includes active electronic components which provide a sequence of signals that may include reference and measuring signals or multiplexed signals from the transducers.

Abstract: A pressure-actuated head for detonating a gun for perforating a well bore is suspended on a tubing string for lowering down the well. The head includes a detonator pin biased towards a detonator but restrained until detonation is required. Detonation is effected by causing or allowing a pressure differential to develop across a piston. Fluid communication between chambers on respective sides of the piston is via a pressure control system which allows the pressure differential to develop over a period of time.