The technical effort will focus on discovering and validating by proof-of-concept testing a reliable cost-effective method or methods and equipment to significantly reduce lateral noise in the marine environment from seismic activities and operations. This research project was conducted in two parts.
Part 1 of this project contained six tasks:
Perform a brief literature investigation) of technologies for marine acoustic wave propagation and methods of noise reduction. Develop or assemble a previously developed analytical model of the physics of marine acoustic wave production and propagation. This method will be essential in performing analyses of the candidate concepts for noise reduction. Use Invention MachineTM, the Scientific Method (analysis, synthesis, and hypothesis) and other inventive methods including old fashioned discovery to identify candidate methods and equipment to reduce lateral noise for further evaluation. We have unique skills and experience in this task. Use engineering analysis and engineering judgment to rank order the potential performance and potential reliability of the candidate lateral noise abatement techniques. Task 2 above will provide the analysis methods for estimating performance, and simple Failure Modes and Effects Analysis (FMEA) methods can be performed to rank the relative reliability of candidate concepts. Propose simple proof-of-concept testing methods to determine the best candidate method and equipment in terms of performance and reliability. This might involve borrowing a production air gun to be fired in a water pit at a remote location. If proof of concept testing validates at least qualitatively a concept, further testing of a more rigorous nature would need to be performed to determine quantitative proof. Perform proof-of-concept tests - if funds exist - after the above work,
Three concepts were evaluated.
Attenuate lateral noise with air bubble curtains, like has been shown in the literature, or with some special bubble curtain material, acting as a more solid (like a curtain) barrier. Make arrays more directional, and thus narrow the cone of sound. Change the structure of the airguns to reduce high frequency sound (noise) while maintaining the strong source signal needed for exploration
In April 2009, BOEMRE initiated Part 2 of this research project to address the conclusions and recommendations contained in the final report from Part 1.
The objective of Part 2 of this research project was to conduct a more rigorous 3D acoustic analysis of the preferred configuration of bubble curtain design to include shallow-water seafloor effects. The effect of sound attenuation within the bubble curtain was also included in order to achieve the most realistic conditions possible before engaging in physical testing.
JASCO Applied Sciences was commissioned to perform a 3D computer modeling study to investigate the effectiveness of bubble curtains at reducing underwater sound pressure levels received to the sides of airgun arrays operated in typical ocean environments.
Conclusions and Recommendations from Part 1:
Of all of the concepts for lateral noise reduction considered, the simple air bubble curtain, produced by a streaming manifold on either side of the marine seismic exploration vessel, outboard of the airgun arrays was found to be the most practical.
Contrasted with deployment of more massive structural barrier systems, deployment of air manifolds simple inexpensive hoses streaming on either side of the vessel - was relatively simple, and the operational reliability of such manifolds should be relatively high.
Fluid dynamics analyses and design work on the air bubble curtain system provided key sizing information for the manifolds, nozzles, air pressure and compressor horsepower requirements, and bubble parameters needed for the acoustic analysis.
We have found that a practical limit on the manifold is about 20 m. Deeper depths require excessive amounts of horsepower, and the manifold diameter becomes too large to be practically deployed and recovered.
Preliminary two-dimensional acoustic analysis results showed that that deploying an air bubble curtain outboard of marine seismic vessels to reduce lateral noise could possibly achieve a noise reduction of 20 dB or more. This preliminary conclusion needed analytical confirmation using three-dimensional analysis before much larger sums of money would be spent on full scale offshore testing.
BOEMRE concurred with the recommendations that determined the bubble curtain concept should be considered further and that the towed air hoses (producing the bubbles) should be allowed to drape down to the seafloor by proper weight/buoyancy adjustment since the Arctic Beaufort and Chuckchi Seas are relatively shallow.
BOEMRE has accepted the final project report for Part 1.
A scientific paper: Methods To Reduce Lateral Noise Propagation from Seismic Exploratory Vessels was presented at the 28th International Conference on Ocean, Offshore and Arctic Engineering (OMAE 2009).
Conclusions from the JASCO Analysis
Model results showed generally poor performance of the air curtains at reducing sound levels except at short distances from the source where direct path sound propagation was directly shielded by the curtains. In most cases the model predicted little difference between scenarios with and without curtains in place.
It appears that sound reflects internally between the curtains with only moderate losses until it escapes beneath. In some circumstances, especially in deep water conditions, the scenarios with curtains in place produced higher sound levels than without. This behavior is attributed to two effects: (1) the curtains steer energy down to the deeper receivers, and (2) the curtains selectively attenuate the surface reflected paths more strongly than the corresponding direct paths. The surface reflected paths normally destructively interfere with the direct paths, but their attenuation decreases the interference thereby increasing the overall received sound levels.
The bottom line from this work is that the bubble curtain would not be a viable noise attenuator as proposed. It is possible that the nozzles for bubble production could be dragged on the seafloor at the shallow water depth (50 m) in order to capture the noise within the bubble curtains, but the power to produce effective bubbles would be exorbitant, and the risk of entanglement of the weighted nozzles would be high.
Conclusions and Recommendations from Part 2:
We do not advise further development of the bubble curtain for lateral noise attenuation because little noise, if any would be attenuated. Because the bubble curtain solution will not be viable, future research should be directed to the following two areas:
Making arrays more directional - This is more the responsibility of the geophysical operator and its oil company client. We considered towing a parabolic reflector to be deployed over the towed arrays. If such a reflector could be deployed successfully, the arrays can be focused. But deploying such a broad structure, and towing it behind the vessel, along with the arrays and streamer cables, might be a very risky effort due to potential collisions and entanglements.
Changing the structural shape of airguns to add lateral noise reducers without affecting the required source signal would mean that the airgun manufacturers, Bolt and Sercel, would have to develop and test totally new products - and such developments would be outside the scope and funding of this relatively small research project.
BOEMRE has accepted the final project report for Part 2.