Episode 41: Chase Hogoboom, T.J. Perrotti, Composite Energy Technologies Inc.

In the wake of the loss of the OceanGate Titan submersible, there was much discussion and speculation about the materials used to manufacture the vehicle. Many pointed to the use of carbon-fiber as a potential source of failure, yet there has been a good deal of success in the use of the material for undersea applications including pressure vessels and unmanned submersibles. 

In this episode of CW Talks, we sit down with two representatives from Composite Energy Technologies Inc., a Bristol, RI based company that manufactures a wide range of composite structures from racing sailboats to architectural designs to theme park ride vehicles to defense applications including deep-sea submersibles.  Chase Hogoboom, president and TJ Perrotti, senior Naval architect and chief design engineer discuss their experience working with the U.S. Navy on submersible designs and the rigorous testing that goes into the use of carbon fiber for undersea applications. 

---

 

Transcript of Composite Energy Technologies interview with CW Talks 

Scott Francis (SF): So first, give us some back ground about CTE. What are some of your capabilities and the markets that you work in?

Chase Hogoboom (CH): Sure, well, we’re a small business in Bristol, Rhode Island, we have about 50 employees. About 10 of those are designers and engineers, specifically composite engineering, and the balance of our team, our production technicians, most of which are composite technicians. But we do have folks who are fairly capable in integrating equipment such as propulsion and mechanical control systems.

Our background, our legacy and heritage is in racing sailboats, and using aerospace technologies to help folks win Grand Prix yacht races. We started adopting prepreg carbon fiber for this application in the 80s, and had some good successes in the 90s with America’s Cup and other Grand Prix events.

In 2010, we shifted focus quite a bit and started stepping away from racing sailboats, direct-to-consumer entertainment type equipment to really honing in on two different areas of focus. One is in themed attractions, which consists of ride vehicles and show action equipment — generally stuff that moves around and needs to have very high factor of safety, as well as reliability. But then, also, a big part of our business is serving that defense community, specifically underwater applications.

So our area of focus is generally around the 6,000-meter deep domain. So we call that as full ocean depth, although there are some areas that are a little bit deeper, but 95% of the ocean is 6,000 meters or shallower. So that’s where we focus.

The solutions we develop are primarily unmanned, or autonomous. They carry payload, generally for sensing and information gathering. That’s a big core of our business. In the past five or so years, a lot of our  defense programs have been focused on specifically carbon fiber pressure vessels for what we call full ocean depth or 6,000 meters.

SF: In the wake of OceanGate, there was a lot of speculation about what might have caused the implosion — and a fair amount of speculation was placed on Titan’s use of carbon fiber yet, as you’ve mentioned, carbon fiber has a history of use and high-performance applications, from race cars to space applications. Can you talk a bit about what’s unique about these undersea applications that you’ve been working on?

T.J. Perrotti (TP): Yeah, that’s a great question. I think the dominant difference with dealing with deep oceans is the incredible pressures that are exerted. And just to put that in perspective, here on the surface of the Earth, we deal with basically one atmospheric pressure about 15 pounds per square inch. For craft that go up into space where you’re essentially operating in a vacuum, you are dealing with a differential, again of about 15 pounds per square inch.

If you’re going down to 6,000 meters at the depths of the ocean, you’re dealing with something like 9,000 pounds per square inch of pressure exerted on that object. So roughly 600 times the magnitude of pressure exerted. And so a good portion of the challenge to develop pressurized vessels that go down to the ocean is simply dealing with that intense pressure distributed over the surface area, which of course, is an enormous amount of force.

In addition to that, we have to understand the reliability of the materials chosen to create that structure. Can they handle repeated fatigue cycles? Do we understand how the mechanical properties of the material are maintained at depth in cold waters under pressure as things compress because of the pressure? Or those mechanical properties characterized with testing on the surface and labs? Or do we need to do more stringent testing to qualify what happens to mechanical properties at depth. So those are some of the key challenges that we face — enormous pressure, enormous force, and then understanding how the choice of materials reacts and may change when subjected to those sorts of intense pressures.

SF: So let’s get a little deeper into some of the specifics of your work with underwater, carbon fiber pressure vessels  — actually taking some of these vessels to the point of implosion. Can you talk about some of the specifics of the work; some of the variables that you were looking at when you would conduct tests. How were the tests were performed and what were some of the findings and results?

TP: We were contracted through the Office of Naval Research to help explore the possibility of applying carbon fiber towards deep sea pressure vessels. The Navy has various craft that have to take electronic components down to ocean depths in a dry environment. And so they put the electronic components in pressure vessels, so basically a can that maintains atmosphere pressure on the inside, when subjected to large pressure on the outside. Traditionally, those types of pressure vessels are made out of materials like titanium. They can be very expensive, it’s hard to procure the large billets of solid block material needed to machine those cylinders.

Carbon fiber, as you know, and as your audience knows, is a very strong, but particularly very lightweight material, about a third of the density of titanium. And so there is interest from the Navy in exploring materials that are lighter in density, so that the craft tasked to take that payload down to ocean depth has a lighter payload to carry.

And so that was the interest as we began to dip our toe in the deep end of the pool, so to speak. We had some assumptions about the mechanical properties of carbon fiber and how they might apply to the construction and verification and function of a carbon fiber pressure vessel. But again, we didn’t want to make the assumption that those quantified mechanical properties on the surface maintain themselves at ocean depth.

And so we set forth to do a very stringent systematic series of scaled pressure vessels, designed, fabricated and tested in a controlled, wet, pressurized environment. In this case, we’ve used the pressure chambers at Woods Hole Oceanographic Institute extensively, and have developed a family of scaled pressure vessels that we’ve designed, built and in some cases taken directly to implosion intentionally, to see if the predicted strengths and failure mode of the pressure vessel was consistent with what was evidenced in the pressure chamber.

And in other cases, we’ve taken things shy of implosion and fatigued them, meaning that we take them down to some depth, hold them for a certain amount of time, and then take them back to surface pressure, and then repeat that over and over and over a certain finite number of times and then take that to implosion. And we will compare it to a counterpart that was taken directly to implosion in order to see if there’s evidence of the influence of fatigue cycles.

Through the work that we’ve done, we’ve explored the results that we’ve obtained through the systematic series and then compare those results through reverse engineering to the assumptions that we had going into the work that we did, often cases finding that things are different at the bottom of the sea, but building a database of quantified empirical data that we can now go back to our design calculations and our design simulations and refine those calculations to design the next set of pressure vessels.

CH: I just want to re-emphasize the importance of simulating — actually performing these implosions  — in real-world type application. Many composite engineers do coupon testing, where you’re using data from actual samples that are created with the intention that you’re understanding the properties of the material as it’s going to be manufactured, not just taking the information off the data sheet, and then using that as your basis for your engineering calculations.

That’s what we did as our starting point. But then once we started building the tubes and bringing them to implosion, we very quickly learned that our typical approach and our preliminary assumptions were off. As TJ mentioned, we then started, essentially reverse engineering into material properties, through these implosion tests.

SF: Can you talk a bit about the factors that go into consideration when you’re engineering these kinds of vessels?

There are well-accepted engineering calculations for designing pressure vessels that take into account the size of the vessel, the wall thickness, the proportions — like the length to diameter ratio, the diameter to wall thickness ratio, etc. — that for traditional materials, would give you a fairly clear assessment of what kind of stresses and then therefore what kind of factors of safety to consider with traditional materials like metals.

As you and your readers know, carbon fiber is not an isotropic material, it has fiber orientation. As we’re building these pressure vessel tubes, we build and tailor a blend of varying fiber orientations throughout the wall thickness of the tube. That adds a complexity of variables that we can introduce to tailor a tube laminate a fiber orientation schedule, to the tube. And then we can explore and tailor how that laminate schedule would be best optimized to particular tube environment, meaning the depth and the proportions of the tube. So as part of our systematic series, where we looked at pressure vessels, we explored the mode of failure, which is closely related to the depth at shallow depths, meaning let’s say under 1000 meters or so, wall thickness can be thinner, because you don’t have the magnitude of pressure as you would at 6000 meters. But your mode of failure is typically one of flexural deflection within the tube. And so you will fail the tube through deflection, before you fail the tube in the physical strength of material. As you go deeper in depth, you’ve crossed through a transition phase, where it now becomes more of a strength driven criteria that affects the implosion of the tube, you have plenty of stiffness, because you typically are increasing wall thickness in proportion to the pressure. But now at the time of failure, it’s an ultimate fracture failure of the tube. Through that characteristic change in depth s wall thickness grows for the proportion of the tube, we explored pretty stringently, how to tailor a choice of fiber orientation for those tubes. And so that has a big influence on again for a proportions of what the customer is looking for, of how we tailor the wall thickness, the fiber orientation across the range of depth.

SF: One of the other things that was speculated a lot about with regard to Titan was the different materials and that were bonded together. Carbon fiber to titanium, for example. Can you talk about the design considerations go into intersections where the materials meet?

TP: Great question. So interestingly, when people think about the ocean, it’s a general conception, or in this case, a misconception, that water is an incompressible thing, right, you can’t squish water. Well, that’s not really true. When you go down deep in the ocean, at very deep pressures, water will actually compress, and other things will compress. So carbon fiber compresses, titanium compresses, epoxies that are used to bond different types of materials together compress. And so as you consider structures that are made of an amalgamation of different materials, you have to consider what’s happening to the very small scale, but quantify nonetheless, change in physical dimensions of those interfaced materials as they’re subjected to pressure and they're subjected to compression through the pressure when they are physically bonded directly to a neighbor of a different material.

And so that’s a big part of what we’ve looked at is quantifying the compression of materials. We use a term called bulk modulus, which is essentially a mathematical representation of how something compresses under load and we have to design to that. We’ve done considerable testing, with the Navy to test sample coupons of carbon fiber bonded to other pieces of carbon fiber with traditional epoxies. And we’ve tested those in labs at ambient pressure. And then we’ve taken identical samples, and we’ve put them in wet pressure chambers, brought them to ocean depth, let them sit there for considerable time, meaning days and weeks of time. And then we’ve taken those coupon samples and tested them and compared them to the baseline counterparts, again, quantifying what happens not only with the with the raw material, in this case, carbon fiber, but what happens importantly, with the bonds that may be used to bond carbon fiber to a neighboring piece of carbon fiber or carbon fiber to titanium or something along those lines. So it can be done. But one needs to be very careful about the tolerances, the clearances between parts, what types of adhesives are being used between components. So we’ve we've looked at fairly carefully when we’re trying to quantify and assess parts apart bond.

SF: So I think, kind of like to wrap some of this up by just talking a little bit about you know, there's there's plenty of speculation still about bear the failures with Titan were and I think it remains to be seen whether we'll really find out the answers to what happened as people analyze the remnants and try to piece things together. But from, from your point of view, what are maybe lessons that can be learned or things that you think about when you're engineering your own vessels?

CH: Well, I appreciate you that recognized recognizing that a lot of things could have gone wrong with the Titan example. You know, there’s a culture that existed obviously, before the thing went in the water, there’s engineering, there’s fabrication, there’s maintenance. I mean, not all types of different things. And when you’re dealing with such extreme pressures, it’s 360 degrees of unforgiving environment. And there’s a lot of things that need to go right at exactly the right time for no mistakes to happen. Prior to this, the industry I think is upheld a pretty outstanding record in obtaining that spotless, bill of health. People do follow protocol. They file follow good design practices, they test, they validate, they are peer reviewed. It’s a very open community, that undersea community. There’s a lot of sharing of information that happens pretty openly. And so this is certainly an anomaly.

And like we’ve said here, we’ve had great success with carbon fiber pressure vessels, even at much deeper depths than that Titanic.

TJ: Again, we and our customers are very keen to build vessels that can take equipment down to ocean depths. It’s in the case of our customers, very important to the mission and goals that they have. But we collectively, ourselves and our customers are very conscious that we want to mitigate risks at all costs. And although there’s a goal to get to the finish line, meaning to have operational craft in the field, there’s also a very conscious philosophy to mitigate risks throughout the program.

As we’ve developed craft and test vessels, and operational vessels, at each step of the developmental program, we’ve always put the safety of the people around the integrity of the equipment first. We have not shortcut things, we have not jumped to preconceived ideas or conclusions. We’ve followed a stringent path of hypothesis, design, test, build validation, and then go back to engineering and cross check design based upon that evidence and information. That’s been our philosophy all the way through. And as Chase said, in the undersea world, that’s not unique to CET, there certainly have been many successful crafts that have explored all sectors of the ocean.

SF: So how many successful vessels have you created that actually in service ones?

CH: Well, everything and service has had no issues and including one item that has seen over 6,000 hours at depth, which is quite considerable. We’ve manufactured and tested and imploded on purpose, far more items than  articles that we have in service. So if you if you look at this statistic of items that have broken versus items that happened, the items that have broken, far outweigh those that have not, however, it was intentional. The ones that are in service and being operated right now have performed very well.

SF: So as we think about designing carbon fiber pressure pressure vessels for these types of applications, what are some best practices that you would suggest or your company follows.

So we’ve talked in this dialogue about the magnitude of forces and pressures, and we’ve talked about the mechanical properties that we’re trying to achieve with carbon fiber. But another critical part of getting something out in a real world environment is the manufacturing process. With any manufacturing process, there’s handwork involved, there’s machine work involved. And those have an influence on the quality and the integrity of the part and ultimately, the mechanical properties associated with it.

We’ve spent a lot of time exploring different ways to build relatively thick walled types of carbon fiber pressure vessels that can handle these loads. We’ve conceived some innovative ways to apply carbon fiber, to cure carbon fiber, to tool the manuals that are used to create the carbon fiber to make sure that we’re hitting tolerances of diameter, variation and wall thickness and such. And so a good part of what goes into the development is not just an understanding of the forces and the mechanical properties of the material, but the methodologies that are used on the shop floor to actually achieve tolerances that provide high integrity, low void, very tight control of roundness of the tube, things along those lines, so that we can hit the thresholds of strength and factor of safety that we’re trying to achieve.

Clearly, there’s a lot of speculative information out there in the in the past several weeks. You know, part of that, though, is that there’s been a lot of discussion in internet world that carbon fiber is a great material in tension, but it’s a very risky material in compression. From our perspective, we think that that’s a mischaracterization of carbon. Carbon fiber is a material that has mechanical properties that vary with direction of load, magnitude of load, fiber orientation, ply orientation, etc. So our point of view is that carbon fiber could be very well adapted to different types of loading, including both tension and compression as long as the design of whatever structure it is and the testing the validation of whatever structure is, understands that mechanical properties of the material, of joining different subcomponents of structures together, needs to be assessed in accordance with whether the load is tension or compression or a variance across some some spectrum across a part.

So, we’ve been a little disheartened that the talk out there in internet land is kind of trying to drive a stake into carbon fiber from a compressive standpoint, but we think that’s a mischaracterization. In fact, the work that we’ve done through these pressure vessels through the stringent testing is to understand how to design and build to handle things that are under compressive loads. And we think that carbon fiber is a lightweight, strong, repeatable, reliable material that can be applied to achieve successful goals.