Pile Driving Underwater Noise Attenuation
Port of Anchorage Modernization Project Adds to Body of Knowledge
The AdBm Technologies prototype system in the tank at the University of Texas Applied Research Laboratory in Austin, where the system was designed, built, and initially tested before going to the North Sea for a test in the summer of 2014.
Photo by James N. Piper, ARL, University of Texas at Austin
The 2015 Alaska Association of Harbormasters and Port Administrators Conference took place in October at the Hotel Captain Cook in Anchorage. One of the presentations on the second day of the conference was about the Port of Anchorage Modernization Project, given by Lon Elledge, Program Manager for the CH2M Project Management Consultant team. The project will include replacing petroleum terminals 1 and 2; replacing cargo terminals 1 and 2; improving the seismic resilience of the port; incorporating more modern technology; and enhancing operational efficiencies. Elledge stated that the estimated cost, “for everything,” is an anticipated $485 million, with a timeline of nine years, five to six of which would be construction. According to the Project’s website, some construction is anticipated in 2016.
Port of Anchorage Modernization
The Port of Anchorage’s annual throughput averages 3.2 million to 3.5 million tons. For Southcentral, which holds the majority of Alaska’s population, about 74 percent of incoming freight and upwards of 90 percent of imported refined petroleum is waterborne, Steve Ribuffo, Port of Anchorage director, reported at the conference.
The Port of Anchorage is a vital part of the state’s transportation infrastructure, and while the project is expensive, it’s also necessary. “The oldest terminals were built in the 1960s, and the newest container terminal is still forty years old; seismically, they’re not current and suffer from severe corrosion,” Elledge said. “We also want to add backup power for the cranes in case there’s a power outage [so] during that time we can still handle cargo.”
Several aspects of the project are being designed specifically with longevity in mind. For instance, the Port’s container clearings currently in use are thirty-eight foot gauge; with the modernization, the Port plans to install “at least fifty-foot gauge cranes, which will give us a container range of fourteen containers across,” Elledge said; however, plans allow for future installation of one-hundred-foot gauge cranes without having to rebuild the wharf. Plans also call for a composite pile design, which have a seventy-five-year life.
One of the earliest aspects of the project is looking at pile driving technology. At the time of the conference, the Port was in the process of receiving permitting for a test pile program. The goal of this program is to gather site-specific data about the effectiveness of underwater noise mitigation methods.
Keeping Sea Life Safe
Why all the concern about underwater noise? Often when people think of “noise pollution” related to construction, it’s to make sure that human communities and persons aren’t unduly harassed or harmed. However, with pile driving, the concern is the potential that underwater noise has to harm various forms of sea life.
NOAA Fisheries has published “Interim Sound Threshold Guidance” for underwater noise creation, excluding tactical sonar and explosives. This guidance is “interim” since NOAA is still developing comprehensive guidance as to appropriate underwater sound levels. For now, there are two levels of underwater acoustic thresholds: Level A, underwater sound levels which may potentially injure an animal, and Level B, which may lead to “behavioral disruption.” For pinnipeds (seals, walruses, sea lions) the current Level A threshold is 190 decibels and for cetaceans (whales, dolphins) the Level A threshold is 180 decibels. Level B for both groups is 160 decibels for impulsive noise, such as with impact pile driving, or 120 decibels for non-pulse noise, such as vibratory pile driving or drilling. These are the levels of “harassment permitted under the Marine Mammal Protection Act,” according to the NOAA Fisheries’ website. The noise created by underwater pile driving often exceeds these levels. This means that if sea life is spotted within the construction site area, construction activities may grind to a halt as crews wait for the animals to move away, leading to inflated costs and schedule delays.
AdBm Technologies Resonators production units made of injection molded HDPE plastic. For the Alaska project at the Port of Anchorage, AdBm will be using their green and white models, the yellow ones shown are for a project in the North Sea. The parts have a specific gravity of less than one, so in the unexpected event any of them are broken or come loose from the system, the parts will float to the surface.
Photos Courtesy of AdBm Technologies
It was timely that, during the same conference, there was a presentation on new pile technology titled “Breakthrough in Mitigating Underwater Noise from Steel Piles” given by two members of Marine Construction Technologies, a public benefit corporation based in Washington state: Julie Hampden, environmental director, and Tim Dardis, a PhD candidate and one of the company’s co-founders.
Hampden explained a little of the corporation’s beginning: “The Washington Department of Transportation was coming up against these permitting bottlenecks, time and time again, for any pile driving work that they were doing in the water. It was becoming such a time constraint on many of their projects that they actually issued a competition and requested that engineers somehow figure out how to fix this problem and address the issue of underwater noise… Over the last six years it has been an ongoing effort to try to fix the problem from the ground up.”
The breakthrough, Dardis explained, is that current methods of underwater noise attenuation address noise in the water column, but don’t address noise that propagates through seafloor sediment. Even using other methods of mitigation under perfect conditions, Dardis says, “you get significant energy coming from the pile in the sediment… How do we block the sound in the dirt? The solution that we came up with is we need some kind of barrier that covers the entire pile.”
Instead of installing equipment around the pile, they created a new type of pile. Two pipes are mounted concentrically, one inside the other, and joined at the pile toe with flexible connections. When driving the pile, only the inner pipe is struck, and the connection at the toe drags down the outer pipe with the inner. Since the outer pipe itself isn’t being struck, the space between the two pipes (approximately two to three inches) and the outer pipe itself disrupt the ability of noise to spread, reducing noise along the entire length of the pile.
Dardis emphasized that throughout the development phase they consulted with construction companies, and they focused on ensuring that this new pile could be driven with standard industry equipment.
At the time of the conference, the corporation has performed one test, which took place in Puget Sound in October 2014. The first test involved three piles: a steel pile tested with and without a bubble curtain, one double wall pile, and one mandrel driven double wall pile. In the case of the mandrel driven double wall pile, the inner pipe, or mandrel, can be extracted after the driving process and reused as the inner pipe of other piles, reducing costs. The piles were thirty-inch OD pile, eighty feet long.
Their goal was to achieve noise reduction of twenty decibels, which they met. Overall the piles saw reduction of twenty-one to twenty-three decibels.
Applications in Anchorage
The Port’s test pile project’s object is to gather data, using underwater microphones, from three different tests, one base-line noise test and two methods of sound mitigation: bubble curtains and a resonator panel system that had recently been deployed in the North Sea.
As promising as the double-wall pile technology is, Port Engineer Tod Cowles of the Port of Anchorage says it will not be utilized in the test pile project taking place in March.
There are a few reasons, he says, why the double-walled pile won’t be part of the test. “We looked into them, but we don’t believe the double-wall system is constructible due to the diameter and length of the piles required on our project.” Additionally, the double-wall construction of the pile, essentially two pipes in one, increases the costs of the pile. Even considering the mandrel double-wall pile, where the inner pole can be removed and reused, Cowles says the pile doesn’t financially make sense when considering the scope of the Port of Anchorage project. In the test conducted in Puget Sound, the piles were eighty feet; the piles being driven for the Port of Anchorage Modernization project will be two hundred feet long, and excluding the ten pile driven during the pile test, about a thousand pile will need to be driven; extracting an inner pipe from each doesn’t pencil in.
At press time, permits for the test had still not be finalized, but they were expected to be granted by the end of 2015, allowing Kiewit Infrastructure West Co., the general contractor for the test, to perform the work in March.
Bubble curtains are almost exactly what one would expect them to be. There are various ways to engineer and deploy them, but all use a system whereby air bubbles are created underwater to surround a pile as it is being driven. Because of the difference in density between air and water, the bubble curtain is an effective barrier to sound propagating through water.
Cowles says the Port is particularly excited to see results from the resonator panel system: “It looks kind of like a venetian blind,” he says. Essentially, the system is a series of small cavities, or cells, with both rigid and elastic wall members. These cells have a single open end which faces down into the water, so air remains trapped inside. The result of this design is that energy is removed from a passing sound wave, attenuating the acoustic wave. One of the benefits of this design is that it is a rigid structure. Ocean currents can reduce the effectiveness of a bubble curtain, but do not have the same negative impact on a resonator panel system.
As the Port project moves forward, it will be invaluable to know which system provides the most effective noise attenuation. Quieter pile driving not only reduces potential harm to sea life, but it reduces the size of the area which must be monitored during construction. A smaller monitored area can mean fewer vessels with fewer marine mammal observers, which can be a huge benefit to project timelines and costs.
Tasha is an Associate Editor at Alaska Business Monthly.
This article first appeared in the January 2016 print edition of Alaska Business Monthly.