DESCRIZIONE
There is disclosed a downhole circulation tool (128) comprising: a housing (210, 212, 214) having an axially extending delivery bore 222 for conveying a drilling mud flow therethrough, the housing having a circulation port (332) for discharging drilling mud; and a valve member (268) rotatably disposed within the housing, the valve member (268) comprising a through-flow channel (223) and a circulation channel (328). The valve member (268) is rotatable between a through-flow position in which the through-flow channel (223) is arranged to convey drilling mud flow from an upstream portion of the delivery bore (222) to the downstream portion of the delivery bore (222), and a circulation position in which the circulation channel (328) is arranged to convey drilling mud from the upstream portion of the delivery bore (222) to the circulation port (332) to discharge drilling mud from the housing. There is also disclosed a downhole tool (128) comprising a unidirectional drive mechanism, an actuation element (510) for a downhole tool, and a method of operating a downhole tool.