Explosive offshore structure removal noise measurements

The underwater structures that support wind turbines and oil drilling rigs must eventually be removed. Explosive severing is a commonly used removal method in which charges are inserted into a pile and placed below the seafloor to sever the pile. The open-water source level of an explosive charge is readily determined from its composition and weight (Urick 1986). However, the sediment and pile absorb much of the explosion’s energy. A recent study (BSEE project M13PX00068) measured explosive removals in the Gulf of Mexico. Peak pressure, impulse and energy flux density metrics were measured with a 12-element, two-dimensional array spanning 90 ft vertically and at distances out to 200 ft. Peak amplitudes, compared with theoretical open-water predictions, were reduced from 76% (75 lb charges) to 54% (200 lb charges). Measured results were also compared to predictions from the UnderwaterCalculator (Dzwilewski and Fenton 2003) that included pile and sediment attenuation effects. It accurately predicted the propagation from the current study and somewhat overestimated propagation for earlier data collected in shallower water with smaller charges. These data suggest that the model can be improved. Nevertheless the model is further validated for its primary purpose of estimating ranges to designated safety thresholds.