How to give your mooring design a quick reality check
It’s easy to spend a lot of money at the grocery store, but it can be jarring when it happens by accident. I remember one time at my local grocery store, a customer was checking out ahead of me when a WHUMP caught my attention. They dropped a bag of cashews on the conveyor belt. And it was really, really big.
The thing about cashews is that they are expensive, even in bulk. Rightly so, because like many nuts, they are very nutrient dense and costly to grow. But because they are so dense, it also doesn’t take much to get a hefty and expensive bag. There are always little spring scales in the bulk aisle to confirm how much you’re getting. But most people I see (me included!) usually hold the bag in their hand, guess the weight, and walk right by the scale. Even if you don’t use the spring scale, you can make a check against something you know the weight of that’s already in your cart. But if you aren’t paying too much attention and just going through the motions, it’s easy to get more than you expected.
I couldn’t take my eyes off this massive bag, inching toward the cashier on the conveyor belt. How much was it going to be? I could tell it was a surprise when the eye-watering price came up on the till, and the customer’s jaw dropped! It was a jarring surprise and a sign of a missed opportunity for a reality check.
Likewise, we all need a reality check from time to time, even in analysis and design, and moorings are no different. It’s easy to get lost in the details of the problem, and if you aren’t careful, you might be going through the motions of analysis without checking if things make sense. An unreasonable calculation can be a pretty expensive mistake if you’re unlucky. Fortunately, a reality check isn’t hard to do, and we’re going to cover a few elements of how to do this:
- Comparison with a simple calculation
- Get an outside perspective
- Check similar system design history
First, we’re going to cover a comparison with a simple calculation.
How simple is simple?
A simple calculation can mean many things, but ideally, you have something you can calculate with pencil and paper, but it doesn’t necessarily have to be that way. You might make a simple spreadsheet calculation or even use a different piece of software that helps you get a different perspective on the problem.
A different perspective is what you are looking for
By approaching the problem using a completely different method, you get insight into what to expect. You may need to make a lot of assumptions, but that’s okay because it gives you something to test your more sophisticated analysis model against. A sophisticated analysis model may have many inputs and settings, and it can be challenging and impractical to gauge how much they influence the results. A simple calculation helps you gauge where you are regarding the reality of the results you’re getting in a specific scenario.
For an oceanographic mooring, there are a couple of easy and quick checks you can do. The static tension at the top of the mooring is often easy to estimate. The assumption here ignores any effects from drag load or other environmental conditions – just the result of buoyancy and weight. For a subsurface mooring, the top tension should equal the net flotation of the main float. For a deepwater surface mooring, the top tension should be roughly equal to the wet weight of the components hanging off the buoy.
Let’s take a closer look at the SOFS mooring. There’s about 1.7km of wire rope hanging off the surface buoy. A quick summation of the ADCP frames, connector, and wire wet weights comes to about 10kN. This is what we’re seeing in both Oceanographic Designer and the static load calculated by ProteusDS.
Static conditions are fine, but what simple checks can you do when things get more complicated?
Dynamics are more challenging because the acceleration and inertia of the system come into play. But for oceanographic moorings, you can use a few simple ideas.
The buoy on most surface moorings typically has a maximum uplift or reserve buoyancy. In the worst-case scenario, if the buoy completely submerges, this gives an idea of the worst-case vertical load on the mooring. There can be more forces from drag and wave excitation, but the more dynamic the scenario, the more these forces move the buoy and mooring around and the less they transfer into mooring strain and tension. Even the buoyancy forces won’t instantly translate into mooring tension because some of these forces go into accelerating the buoy rather than stretching and lifting the mooring. Still, the maximum reserve buoyancy gives some basis for an upper limit of load to expect in a dynamic scenario.
Again, looking at the SOFS buoy, in extreme wave conditions with a lot of acceleration and drag loading on the system, the maximum loads are on the order of 30kN. The maximum reserve buoyancy for the buoy is 6000kg, which translates to almost 60kN. The reality check here is that if the mooring loads are getting to 60kN or even higher, there’s likely something dramatically wrong – it might mean there are significant shock loads in the system, or a setting in the analysis might be off – and a closer look at the system is warranted.
A simple calculation is a great way to get a quick comparison, but it’s not the only way to get a different perspective. This brings us to the second point on getting an outside perspective.
Uncertainty rears its head when you’re designing something on your own
Especially when you’re working on something new that you need more experience with. You might feel like you are going in circles looking at different configurations and trying to find something that makes sense. Getting an outside perspective from someone else can help break the cycle and help you find a way forward. It doesn’t mean that they’re confirming it’s going to work, but instead, they can share what they’ve seen work in the past. That alone can help build confidence that you’re on the right track.
Who can you ask for help?
There’s almost always someone who can shed some light on what you’re doing that’s closer than you think. The first place to look is within your own organization. Someone with design experience is obviously an ideal person to look for, but there’s others who can help you, too. Often, technical staff involved in inspecting, sourcing, or deploying equipment can give you an idea of what they’ve seen in their own experience.
Another resource to look for is equipment suppliers. They often see thousands of designs throughout their operations. They can provide some indication of whether they’ve seen a similar design before or what rope materials are most common in different applications, too. Finally, reaching out to others at similar organizations is always a possibility, too. Of course, competition is always a factor, but this varies depending on the nature of the problem and the industry you’re working in.
For example, the oceanographic mooring industry often has many collaborative and supportive people working together across organizations. The MTS Buoy Workshop is a good example of how many people in the industry come together to discuss and solve common problems. There are more ways to get a different perspective on your design that doesn’t necessarily rely on direct feedback from others. This brings us to the third and final point on checking similar systems.
A similar system doesn’t have to be identical to provide reassurance
What you need to look for are similar characteristics. Does the mooring use geometric or elastic compliance? Is it a taut mooring, or does it use a long wire section in the upper water column? Is the surface buoy the same kind you’re using, or at least close enough in shape and reserve buoyancy? What’s related to this is a little bit of knowledge on the track record of the design, too: it helps to know if the mooring was deployed or if it’s only a conceptual design that’s never been used before.
This can apply to your own moorings, and moorings your organization have deployed in the past, too. Knowing the buoy and mooring conceptual design has a successful track record of deployment helps give you some expectation that success is possible, at least in theory! Of course, every project is site specific – namely, the wind, waves, and currents may differ. But that’s also part of the design process you should go through for each project in evaluating the effects.
Generally, you also have some indication of where a mooring was deployed. Open ocean? Open ocean in a major current? Exposed but shallow coastal water? Coastal in heavy tidal current? Each of these give hints about what metocean conditions could be expected. Open ocean moorings may be subjected to extreme wave conditions. Ocean currents often have moderate flow speeds that covers a large portion of the water column. Coastal locations may be in shallow water, but strong tidal currents can be severe, too. So what are some examples?
There are many detailed sample moorings online
The NOAA Pacific Marine Environmental Laboratory posts some of its mooring designs online. You can see examples of taut and geometric compliant moorings designed that have been deployed in open ocean conditions here.
Del Mar Oceanographic provides guidance for mooring their wave-powered oceanographic profiler, the Wirewalker. Additional information can be found here.
What if I can’t find an existing design or anyone to help me look at what I’m working on?
Some organizations can provide a much more detailed review of the work you’re doing as a commercial service. This is often referred to as a Third Party Verification and can be a valuable way to reduce risk and build confidence that a design is going to work. This can be especially helpful when you are working under a shorter timeline. DSA Ocean has a consulting engineering team that can help you on a project you’re working on if it’s needed!
We covered a few topics on reality checks, and now it’s time to summarize. It’s easy to get lost in the details when working on a mooring design. A reality check helps get you the perspective you need to make sure you’re on the right track. The quickest way is to use a simple calculation. Often, this simple calculation is quick and easy to do and gives you a basis for comparison of what you get from a more sophisticated model. Also, look for others to provide you with an outside perspective. Ideally, there are other designers in your organization you can talk with, but many others have valuable knowledge, too, including people involved in deploying these systems. Equipment suppliers and mooring designers at other organizations can be helpful too. Finally, some idea of what designs have worked in real deployments is beneficial. There are many online resources from organizations like NOAA and the ProteusDS sample layout files. Of course, conditions will be site-specific, but it helps to know what’s worked in the past, and a general sense of where they were deployed.
A reality check doesn’t have to be hard. It can be quick to do and save you from a nasty surprise. Like avoiding a massively expensive bag of cashews at the grocery till!
Check out the ProteusDS Oceanographic Designer sample mooring files. These include sample mooring configurations of the Waverider and Wirewalker, too, mentioned earlier in this article. Download the files here and view and edit them with the free community features in ProteusDS.