Explosive removal of offshore structures – information synthesis report

Abstract

From the Conclusions: "The most commonly used technique for explosive cutting of piles and conductors is with bulk explosive charges. Other techniques such as ring charges, focusing charges, linear shaped charges, and cutting tape are used in some instances. Increased use of techniques that involve smaller charge sizes could reduce potential impacts on marine life.

The physical principles of underwater detonations and of the propagation of shock and sonic waves in the surrounding medium are well understood, but there are significant gaps in applying this knowledge to actual removal of offshore structures. It is recommended that a research effort be undertaken to model numerically blast propagation for a range of charge sizes consistent with structure removal practices through a variety of sediment types and for different deployment depths below the sediment surface. Experimental validation of model results should form an integral part of the study. Ultimately, forecasting of the acoustic levels distribution for planned operations would become possible from the type and location of the charge to be exploded, the depth of the water column, and the measured properties of the sediment in which the detonation occurs.

Environmental data concerning platform removals serve two main purposes: 1) to allow impacts to be estimated (e.g., numbers of animals that may be killed or injured during structure removals); and 2) to aid in developing or refining mitigation measures that prevent or reduce the likelihood of impacts. Impact estimation is the main goal with respect to marine fishes, whereas for marine turtles and marine mammals, mitigation is the main goal (i.e., predicting the areal extent of mortality and injury zones so that these can be monitored prior to detonation to prevent impacts).

There are important differences among marine fishes, marine turtles, and marine mammals with respect to the types and adequacy of data available. In general, marine fishes are the best-studied group, and marine turtles the least studied. Effects of underwater explosions on fishes have been studied and modeled extensively, and field studies have quantified fish kills associated with structure removals in the Gulf of Mexico. For marine turtles, there have been no laboratory studies of blast injury, only limited field observations and experiments, and no mechanistic models developed specifically to estimate impacts. For many years, the main data available for predicting both blast and auditory impacts on marine mammals were extrapolations from terrestrial mammal studies, but ongoing studies are underway that will aid in developing improved mortality, injury, and auditory impact criteria.

For predicting impacts on marine fishes, the empirical data from observations during actual structure removals would seem to be more useful than any attempt to calculate impacts based on experiments or mechanistic models. Since existing observations cover a limited geographic and water depth range, additional observations will be needed to better estimate future impacts.

Years of experience using the 3,000-ft (914-m) “safety range” monitored under the “generic consultation” suggest it has been effective in preventing most deaths and serious injuries of marine turtles and marine mammals. However, the empirical and theoretical basis for this specific number is weak. It is recommended that mortality/injury zones be calculated for marine turtles and marine mammals using standard sound level metrics and incorporating detonation characteristics appropriate for offshore structure removals. This would provide a firmer foundation for marine turtle and marine mammal “safety ranges.”

In all three groups, there is relatively little information about sublethal impacts, particularly on the auditory system. While mitigation measures appear to be effective in preventing death or injury of mammals and turtles, it is uncertain to what extent sublethal effects may be occurring beyond the safety range. Recent and ongoing studies may provide the basis for estimating auditory impacts in marine mammals, which are particularly important in the regulatory context of “harassment” under the MMPA. There is almost no information to estimate auditory impacts on marine turtles.

The time interval between shots may be an important mitigation measure. The amount of time for an animal to recover from a shock wave is unknown. Do closely spaced shock waves, each of which is insufficient to cause damage, cause damage in a proportion to the sum of the damage parameters or a sum of a time dependent percentage of damage parameters? The effect of short time intervals between blasts such as those used in Gulf of Mexico structure removals has not been addressed.

The Office of Naval Research (ONR) is conducting research on physical and auditory effects of blast parameters on marine mammals and detection of marine mammals with passive acoustic devices. Synergy between MMS and the ONR in these areas of research would be fruitful. The Department of the Navy, Naval Surface Warfare Center is measuring actual blast parameters in the sea and working on new and/or revisions to blast propagation models. Synergy with this group would also be fruitful.

To date, mitigation and monitoring requirements for structure removals in the Gulf of Mexico have focused on marine turtles, bottlenose dolphins, and spotted dolphins. As structures are removed in greater water depths, many more species of marine mammals are likely to be encountered, including one endangered species (sperm whale) and other deep-diving species (such as beaked whales) that pose challenges for detection. Passive acoustic monitoring could aid in the detection of sperm whales. Not enough is known about the vocal behavior of beaked whales and some other species to determine the probability of their being detected by passive acoustic monitoring.”