Tracking Seafloor Motion: Scientific Breakthroughs, Offshore Applications
Liquid Robotics — August 18, 2017
With advances in sensor technology and autonomous systems, scientists are now able to measure seafloor motion down to millimeter-scale accuracy through two miles of seawater. This emerging scientific field of seafloor geodesy is not only opening up research opportunities, but also commercial applications in areas like offshore Oil and Gas.
Ryan Carlon, our Science & Research lead; and Sean Halpin, our Oil & Gas lead, sat down to discuss the scientific and commercial applications and implications.
Ryan Carlon: For most earthquakes in the United States, the fault lines that cause them are on land, like the well-known San Andreas Fault. On land, it’s a lot easier for scientists to go out and place sensors to measure a spot that’s moving.
Dr. David Chadwell of Scripps Institute of Oceanography is working with the National Science Foundation (NSF) and the United States Geological Survey (USGS), and they’re interested in the Cascadia Subduction Zone. The Cascadia Subduction Zone stretches from the northern end of California all the way up into Canada. It veers offshore in the Pacific Northwest, and the water is deep—we’re talking thousands of meters. What’s happening is that the offshore plate is pushing underneath the onshore plate, creating the Cascades mountain range up the coast.
The goal of Dr. Chadwell’s work is to better understand how these tectonic plates are actually interacting and moving, down to centimeter resolution. Eventually if we learn enough about it, then we can have a better idea of when a major event is more likely to occur. Right now, the best we can do is to look at how often earthquakes have happened in the past, and that gives a good idea of how often it’s supposed to happen in the future.
Orphan Tsunami
Hundreds of years ago, the Pacific Northwest was only populated by Native Americans, and there was no written documentation of seismic activity. But there was evidence of an abrupt land subsidence, with a forest of trees suddenly killed by saltwater. Meanwhile, 5,000 miles across the Pacific, Japanese records showed a tsunami without an earthquake—what they called an orphan tsunami. Scientists were able to match that tsunami with its parent earthquake, ten hours apart in January of 1700. There is a great article in The New Yorker that gives more color on this, it’s really a fascinating story.
For the Cascadian Subduction Zone, scientists have calculated the earthquake recurrence interval to be 243 years. It has been 317 years since that last Cascadian earthquake, and millions of people have settled in the impact zone, which is why the USGS has so much interest in the science Chadwell is advancing here.
Seafloor Geodesy Projects in Chile and Singapore
Since Dr. Chadwell began this work, other researchers have taken on similar seafloor geodesy projects in other earthquake-intense parts of the world. Scientists from GEOMAR Helmholtz Centre for Ocean Research Kiel are working off the northern coast of Chile, and scientists from Earth Observatory of Singapore are studying the Mentawai Gap near Indonesia. In each case, Wave Gliders offer the ability to collect the data in real-time, which would be prohibitively expensive with boats.
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Fun fact: Dr. Chadwell’s original grant included a diesel-powered buoy. However, after he realized the advantages that the Wave Glider’s mobility and longevity could offer, he successfully petitioned to use a Wave Glider instead.
Sean Halpin: Dr. Chadwell is well known in the oil industry. When an offshore energy customer of ours found out what Dr. Chadwell was doing, he was particularly interested in the ability to measure motion on the seafloor down to sub-centimeter resolution in greenfield and brownfield developments.
And there are more applications this development opens up.
Think about characterizing the active movement of a slope during geohazard studies, measuring and monitoring slope stability. A lot of oil and gas finds are in the deep ocean, close to the continental slope. The seabed adjacent to or under some deep-water developments can be quite steep and often there is evidence of mass wasting in these areas. Until now, the way to identify and mitigate this hazard has been to take sediment samples, age date them to figure out the last time the slope moved. Then they work to figure out the likelihood of it moving again, ultimately determining the risk of placing infrastructure on or near the hazard based on historical data and geologic models.
Now, instead of an educated guess, we can give people the tools to track the actual movement of the seafloor. That’s pretty amazing. And because the Wave Glider is not very expensive to own or operate, and you’re not sending a boat out there, it’s no longer prohibitively expensive to get this data.
Pipeline “Walking”
Another way to apply this capability is to measure the movement of infrastructure. With a pair of reference sensors on the seafloor, and a pair of sensors on the pipe, we can see how the pipe is actually moving. This is important in fields where pipeline movement (or “Walk”) is possible.
Increasingly, in deep-water developments you have very hot oil coming out of the ground at very high pressures, going through a pipeline in a really cold ocean. Metal is going to do what metal always does when exposed to hot and cold things under pressure—it expands and contracts. This axial stress translates into horizontal movement called pipeline walking (or lateral buckling). Often pipelines can be engineered to compensate for this, but estimating the amount of “walk” is somewhat of a black art. How do companies figure out if the pipeline has buckled out of specification?
Your choices are limited when you want to figure out how the pipeline is moving. You can send a boat with a crew of 30 people out for $100,000 a day, then launch an ROV to visually observe and measure the walk. You don’t do it very often because it can cost you $1-$2 million just to get to location. It’s not very cost-effective. You can monitor it with data loggers, but you don’t really know what’s going on until you come back and download the data. Or you can do it real time with Wave Gliders and determine movement immediately. This is what we’ve done with some deep-water pipelines.
Economics of Using Wave Gliders
Ryan: Adding an unmanned system changes the whole cost structure of getting real-time data. Before he started using Wave Gliders to collect the data, Chadwell would go out on a ship with a transducer mounted to the ship hull, but it was just really, really expensive.
Sean: Absolutely. For that one pipeline, maybe you save $40 million dollars over the field life because you don’t have to hire an ROV and boat to go out all the time.
Ryan: And the economic impact is even bigger when we consider what a few extra minutes of tsunami warning can mean. It may not seem like a lot, but it has all these impacts you don’t really think about. You can alert fire stations and other first responders. You can stop trains and subways, close bridges or other infrastructure that may be moving. Hospitals can stop surgeries and fire up the backup generators. Every second counts. Ultimately this work is about a bigger humanitarian goal.
Sean: Knowing where something is on the seafloor to a very high accuracy is pretty powerful. It’s pretty incredible that in 3,000 meters of water you can tell where something is to 2 centimeters (less than an inch) or less. Far and away the importance of this development is to humanity— especially if it can help predict earthquakes, tsunamis, and to better understand Earth. The work that Chadwell and others are doing is on the path to saving lives, that’s really powerful.
