Cetaceans (whales, dolphins and porpoises) rely heavily on sound for communication, foraging, predator avoidance, orientation, and navigation. Noise generated by offshore construction work, such as piling during wind-farm construction and conductor hammering during exploration-drilling operations, has the potential to cause behavioural changes, masking of communication signals or, in extreme cases, a temporary loss of hearing in marine mammals. Numerous countries have issued individual standards for offshore noise monitoring before, during and after construction, but few standards specify actual noise thresholds, due to the complexity of underwater environments. Underwater noise measurements were taken from an offshore support vessel, stationed at distances of 750 m, 1 km, and 2 km away from a drilling-rig conductor hammering site in the North Sea. Results were then compared with the only official threshold value, which was issued by the German Federal Environment Agency (UBA). Sound Pressure Level (SPL) at various measurement locations, and beyond was predicted. The Sound Exposure Level for conductor hammering noise was monitored in real time, and did not reach 160 dB re 1 μPa at a distance of 750 m, in accordance with the UBA. Given the known behaviour of porpoises around offshore installations, it is unlikely that animals were exposed to levels of sound that might be potentially detrimental in the single and brief 2 h period that conductor hammering occurred.