The U.S. National Research Council's Marine Board predicts that, between 1990 and 2000, 100-130 oil production platforms will be removed around the world.
Through his research at the Santa Barbara Museum of Natural History in Santa Barbara, Calif., Dr. Charles D. Woodhouse has developed a set of simple guide-lines that can prevent injury to marine mammals during such activities.
In an unpublished study, Woodhouse reports that the information base regarding animal deaths and injuries caused by underwater explosions and other activities is probably sufficient to assess the impacts related to the abandonment and removal of offshore structures.
Although his work addresses Southern California specifically, the techniques he suggests can apply to other regions.
AFFECTED SPECIES
There is broad diversity in the sea mammal species living along the U.S. West Coast. The list can be narrowed, however, to only a few species of greatest concern, says Woodhouse.
Oil platforms and other structures in place today off the coast of Southern California are in relatively shallow waters. Animals that occupy more oceanic environments, therefore, are not likely to be affected by offshore operations.
The mammals with the greatest probability of being affected by abandonment and removal activities are seal lions, to a lesser extent harbor seals, and gray, humpback, and minke whales.
EXPLOSIVES
The physics of shock waves produced by the underwater detonation of high-energy explosives are well-documented. The effects of these waves on marine mammals, however, are not.
Shock waves attenuate relatively rapidly underwater. The work of Yelverton, et al., indicates that the determination of the safe distance from an underwater charge depends on three factors: target animal, depth of detonation, and charge weight.1
Other factors influencing the calculated safe range include the nature of the sea floor (hard vs. soft), the presence of an ice cover, and the water depth.
If a great enough pressure wave is generated, the shock waves can damage living organisms. In mammals, the primary sites of damage are the respiratory system and gas filled viscera, says Woodhouse.
Other sublethal effects include a startle response and hearing loss, including echo-location abilities for some animals.
EXPERIENCE
Geraci and St. Aubin recommend several techniques when using explosives:
- Use the Yelverton formula to calculate ranges at which various levels of shock wave damage are likely.
- Assess behavioral patterns of the species occupying, using, or migrating through the potential blast area.
- Use techniques to reduce damage, such as modifying the charges and their placement to focus most of the energy toward accomplishing the desired task (for example, bury the charges in the sea floor).
- Detonate a small charge before the main blast to frighten away any animals that are too close.
The use of explosives to remove platforms in the Gulf of Mexico has provided some useful data, says Woodhouse. In this work, sea turtles were placed in test cages at various distances from the explosion, revealing a variety of sublethal effects (and sometimes none).
In addition, Woodhouse observed the use of explosives off California to form a shallow sea floor trench in which to lay a pipeline. In this case, there were sea lions, harbor seals, and bottlenose dolphins within 1 mile of the activity.
Preliminary work taught the engineers how to place the explosives so that their energy was directed accurately. Sea lions apparently were attracted to the sound or at least took advantage of the resulting supply of dead fish after an explosion.
No mammal deaths were attributed to this exercise, which continued for several weeks. The seals and dolphins continued to inhabit or pass through the area throughout the activity.
ABANDONMENT VS.
REMOVAL
The potential effects of abandonment and removal activities on protected species are summarized in Table 1.
Platforms, buoys, booms, and other structures create an artificial habitat. For many of these animals, prey are associated with the structures. Seal lions also regularly use these structures to rest, seek refuge from predators, and thermo-regulate after extended periods in the water.
Platform removal, therefore, brings with it the loss of a habitat for many sea mammals. Aside from this, the effects of abandonment and removal are probably similar to those of construction, says Woodhouse.
There is also the possibility of so called reverse impacts. Curious marine mammals often are attracted to divers, explosions, and other noises.
In 1992, Woodhouse observed the movements of gray whales through the northwestern portion of the Santa Barbara Channel (Fig. 1). An underwater pipeline project was being conducted in the channel at the time. In this case, the area of disturbance was relatively localized, but cut right across the migratory path.
Fig. 1 shows the paths taken by the whales during the project, indicating no disruptance to the whales' movement. In fact, calculations indicate that about 7% of the whales had moved north through the project area before the project ended.
During the 3 month study, no overt incidents occurred with marine mammals. To qualify this observation, Woodhouse notes that every effort was made to alert boat operators to the presence of the whales.
MITIGATION
Because abandonment and removal activities are localized, it is unlikely that species of special concern will be affected in a significant portion of their range, barring a catastrophe. The habitat created by the structures, however, along with the typical number of individuals that occupy a single structure at any time, are important considerations.
Woodhouse's mitigation concepts, drawn from literature and his research, are:
- Understand the nature of the marine mammals in the region of activity.
- Avoid peak migration periods.
- Avoid peak periods of reproductive activity.
- Use training sessions.
- Use monitors.
- Adapt techniques to minimize damage from explosions.
Training sessions involving all members of the work force can heighten the project personnel's awareness of the potential conflicts that may occur with the animals. Workers need to know what to expect, in terms of the kinds of mammals they may encounter and how to deal with them.
Monitors are vital as a first line of detection for mammals entering a project area, says Woodhouse, particularly at the time of detonation. Monitors should have the ability to move around in and beyond the project area, on a vessel dedicated for this purpose.
This enables the monitors to detect animals in the area more easily. Regular contact between the monitors and other personnel also is important.
Effective placement of the charges is important, as is the use of Yelverton's equation to determine a safe distance. Another consideration, says Woodhouse, is to confine the use of explosives to daylight hours. The monitors can function more effectively when visibility is good.
REFERENCES
- Yelverton, J.T., Richmond, D.R., Fletcher, E.R. and Jones, R.K., 'Safe distances from underwater explosions for mammals and birds," Defense Nuclear Agency, Department of Defense, Tech. Rep. DNA 3114T, p. 64.
- Geraci, J.R., and St. Aubin, D.J., "Offshore petroleum resource development and marine mammals: a review and research recommendations," Mar. Fish. Rev., November 1980, pp. 1 12.
Copyright 1994 Oil & Gas Journal. All Rights Reserved.
