When moorings overload wave buoys (and sink data quality)
Marine Iguanas sneeze out salt crystals. It’s a fantastic adaptation for them because they have free reign over immense algae fields in the sea they can use for food. But not just any lizard can swim in the sea – in fact, they’re the only lizard species that can. The problem with swimming in the ocean is that it leads to a significant intake of salt water and elevated salt levels in the bloodstream. Other animals can’t handle this imbalance, and their systems will crash. But the Marine Iguana has a unique adaptation that helps them: an organ that filters out excessive salt from their bodies in the form of salt crystals that the Iguana then sneezes out. This adaptation means they avoid getting overloaded.
Similarly, in the world of wave buoy mooring design, you need to find a way to avoid getting overloaded. Wave buoys can get overloaded easily by their moorings in certain conditions. This crashes the system because it leads to buoy submergence, and once you have buoy submergence, you have all kinds of headaches like holes in data and uncertainty in data quality. So, how can you figure out when your mooring is overloading a wave buoy?
What we’re going to cover three scenarios:
- combined currents and waves
- steady high winds
First, we’re going to review evaluating the effects of currents.
Too much drag from currents on the entire system can lead to buoy submergence.
It’s easy to fixate on the drag on the wave measurement buoy because it’s the primary purpose of the deployment. Often, it’s the largest individual component in the mooring system as well, compared to connectors and line diameter. Intuitively, it is the largest source of drag on the system. But it’s not the only source of drag.
The drag load from currents on a mooring line can be surprisingly high. Though the line diameter may be small, you have to remember that drag acts on the entire length of the mooring. So, the greater the water depth and line involved, the greater the total drag load. But why is an evaluation of the mooring response in current a separate consideration?
The worst-case current profile rarely occurs with maximum winds and waves
While wind and waves may impact local flows, most often, completely separate processes drive the current profile and intensity, such as tidal effects or large-scale prevailing ocean currents. The main point is to focus on what the maximum current conditions may be separately to gauge the impact on submergence.
However, the worst-case scenario that causes buoy submergence may not be from the profile with maximum surface current. Remember that the overall drag on the mooring is essential, too, and a lower magnitude current profile with a more uniform shape could lead to more submergence.
Yet, current alone isn’t the only consideration when checking buoy submergence. This brings us to the next point on checking combined wind and waves.
Wave buoys are light enough to ride ocean waves until they can’t
What stops them is when the mooring reaction loads creep up in and prevent the buoy from freely following the wave surface. So when does this happen?
Drag loads creeps in again. But unlike before, when considering maximum current, it’s not only about steady drag forces – it’s about the resistance to mooring motion induced by waves. Remember that there’s drag on the entire buoy and mooring system that is a function of relative speed – so it resists movement through the water column. Larger waves can induce significant mooring motion, which increases resistive drag, and the reaction load on the buoy. Of course, any prevailing current condition in this scenario adds to the total drag, too. This combination of a moderate current and large enough mooring motion in waves can result in regular submergence of the buoy.
So far, we have talked about currents and waves, but have yet to consider wind. This brings us to the third and final point on checking wave buoy submergence conditions in extreme wind conditions.
Steady high winds can cause submergence by driving local waves and currents
Extreme winds can generate surface currents, and though generally small, they add to the effect of any prevailing currents, increasing steady drag load on the buoy. But what’s more problematic are wind-driven waves.
All ocean waves are generated by wind action on the surface.
The waves generated grow with wind speed, the amount of time the wind blows consistently, and the distance on the water surface over which the wind acts, otherwise known as fetch distance. The challenge here is that short-duration, intense wind storms can produce relatively small but steep wave conditions. Steep, short period waves mean higher accelerations. If the acceleration is extreme enough, the buoy can struggle to follow waves and start to submerge as waves overtop the buoy.
But how much submergence is tolerable?
Many wave buoys are designed to tolerate some level of overtopping and submergence in the water. But there are other important implications for data quality and connectivity. Data quality starts to suffer if wave buoys are not measuring the full wave height. If the data is also broadcast in real-time, connectivity will be interrupted as the antenna is submerged.
Each mooring deployment will have site-specific metocean characteristics and requirements about what is acceptable for data quality. Yet the most important part of the mooring design process is finding a way to characterize what kind of submergence to expect and see if there is a way to adjust the mooring to find an acceptable balance. Even if you can’t adapt the mooring design, it’s useful to characterize how much submergence to expect so you can have an expectation of what data quality is possible.
Let’s look at a few examples of what these conditions might look like.
We used ProteusDS to evaluate the Datawell Waverider 200m mooring in several current, wind and current, and wind conditions to establish the amount of submergence. As this is based on the 200m template mooring without any site-specific metocean conditions, we’ll review some possible scenarios and find indicators for the threshold of submergence.
Example: extreme current load case
Using a linear shear current profile, we ran through a range of scenarios with increasing surface current speed in a static analysis. The Waverider stays at the water surface in 1.5m/s, but submerges at 2m/s current.
Example: combined waves and current load case
In this scenario, we used a moderate current profile of 0.5m/s linear shear as representative of moderate water depth flow in a coastal environment. We then checked the buoy submergence characteristics in a range of wave spectrums with increasing significant wave height. The wave spectrum peak period was maintained at 9 seconds, representing a relatively intense coastal storm condition.
The buoy has no submergence in 3m waves. In 5m waves, there are only minor submergence and overtopping events less than 0.01% of the time. However, in 7m waves, there are an increasing number of submergence events, and the buoy is submerged approximately 4% of the time. In the most extreme sea state condition of 9m, 9s waves, the buoy submergence is approximately 30% of the time.
Example: extreme sustained wind load case
In this scenario, we also used a moderate current profile of 0.5m/s linear shear. An intense local wind storm may have extreme sustained winds. In an unlimited fetch, unlimited duration scenario, a sustained heavy wind of 50 kts can produce waves of 2m significant wave height and 4.5s peak period. These are very short period and steep waves that risk submergence more through overtopping and swamping the buoy. In this scenario, the Waverider buoy is submerged approximately 8% of the time.
We covered several facets on checking for buoy submergence and now it’s time to summarize. Submergence is a key consideration in the mooring design of wave measurement buoys and it often happens when the mooring overloads the buoy. Submergence can happen separately from processes dominated by current, waves with an associated current, and wind with an associated wave and current condition. It’s too conservative to combine all the extreme wind, wave, and current conditions as that’s unlikely to happen in reality at the same time. But checking each condition separately in separate load cases will help you characterize the buoy submergence and adapt the mooring design as much as possible, if needed. Adapting is always the ideal outcome, much like the marine Iguana that can thrive in the marine environment!
You can see the workflow in ProteusDS and the Oceanographic toolset looks like to evaluating mooring performance, including deflection, dynamic loads, and submergence, in this video tutorial from our YouTube channel here:
There are lots of interesting videos of Marine Iguanas online, and the one below shows how they sneeze out salt crystals. Hopefully they will learn to sneeze into their elbows soon!