Bumblebees have large bodies and tiny wings but are much better at flying than you might think. They’re found thriving on mountain ranges far beyond sea level. The challenge with altitude is the higher you go, the thinner the air gets, making it much harder to generate lift forces. For a Bumblee, which already doesn’t have much to work with such a bulky body and tiny wings, you’d think they’d stay near sea level where the air is the thickest. But Bumblebees have proved they can handle a wide range of conditions. Controlled lab tests prove air thinner than the top of Mt Everest doesn’t really bother them and they keep on flying just fine. They’re incredibly insensitive to these harsh conditions.
Being insensitive to harsh conditions can be a significant advantage. It can even be a design objective in some instances. When it comes to floating systems, spar hulls are very insensitive to disturbances from the ocean. Spar floaters can remain rock-solid-stable in a wide range of ocean wave conditions. Forces from extreme storms can be substantial and make many floating systems heave and tilt substantially, but they don’t really bother spar floaters.
Why are spars so insensitive to ocean waves?
Though there’s no strict definition for what makes a spar, but you’ll know it when you see one: they are typically a slender vertical shape in the water. It’s most common to see a long vertical cylinder with a small diameter compared to the length. It’s their slenderness that makes them insensitive to the environmental effects of ocean waves.
Ocean waves can create immense forces. For a sizeable floating ship, the changing water surface from ocean waves is literally like the ground shaking beneath your feet – and it means the vessel will start heaving, pitching, and rolling. But the amount of motion depends on the nature of the floating hull that’s in the water, too. But on the other hand, a floater with a spar form factor will be very insensitive to ocean waves: it doesn’t move around much at all despite extreme ocean wave conditions.
What’s useful about this insensitivity is that it is a very stable and static platform
This lends itself to a lot of advantages depending on what the particular goal is. For example, an oceanographic buoy may have specific instruments whose measurements are disrupted by hull motion. In this case, minimizing the buoy’s motion is crucial to improve data quality.
But spars aren’t limited to small oceanographic buoys. They can also be a large platform, too, with people on board. They’re used as large working platforms in the oil and gas industry that make it easy and safe to do work in a wide range of ocean conditions.
The key to understanding their stability is in the link between buoyancy and the waterplane area
Spar floaters typically have a sizeable submerged volume to support the weight of the platform. In addition to this, because of how slender they are, there is always a tiny waterplane area. The waterplane area is merely the intersection of the water surface through the floater hull. When ocean waves roll past, this moves the intersection up and down and causes fluctuations in buoyancy. But as long as these fluctuations in buoyancy force are small, the disturbance is slight, and there’s not much discernable motion. Because a spar has such a large submerged volume relative to these changes at the waterline, there isn’t much change in buoyancy and resulting movement, even if ocean waves are large.
Are spar floaters always stable?
Though very stable, spar hulls can also resonate like all floating systems. Fortunately, the heave resonance period is very predictable for spar floaters. By carefully adjusting the design diameter and length, the resonance period can be adjusted far from typical ocean wave conditions. It’s normal for spar floaters to have very long heave resonance periods that coincide with extremely rare and long ocean swell. The heave natural period and motion in these conditions is still an important consideration, even though it always depends on the specifics of each spar floater.
What’s the downside to such insensitivity to ocean waves?
In the most towering waves, insensitivity can become a problem. If the platform isn’t moving in heave in spite of waves much taller than the platform, it can mean the entire system can submerge. However, this can be a significant advantage and survival strategy because it prevents an overload of the mooring system. But it does introduce new challenges to the system design, including hull strength or instrument depth rating. Submerging is also not an option for a working platform with people on board! But the solution to this is also considering how much reserve buoyancy the system has – in other words, building up the dry portion of the platform, so it’s likely to avoid submerging in the tallest wave conditions.
Let’s look at a few examples
JASCO Applied Sciences manufactures the ObserveBuoy Spar. This 6m long buoy is built for harsh environments with instruments carefully sealed within the hull. While a relatively small platform, other examples in the oceanographic space can be much larger.
The National Research Council of Italy (CNR) maintains a 50m long data buoy in the Mediterranean. It has been in operation for decades as a stable platform that facilitates air-sea interaction studies and can collect data in very rough conditions. While spar floaters are useful for oceanographic applications, they also show up in aquaculture systems, too.
Floating aid to navigation systems are crucial for marine safety. Yet many of them need to maintain upright and visible in fully exposed conditions. This 38m tall north cardinal spar buoy was installed off the west coast of Australia and was designed for survival in 11m significant wave height and an 18m maximum wave. Spar buoys also come in handy in helping aquaculture systems survive in extreme conditions, too.
TendOcean develops technology for the offshore seaweed aquaculture industry. Depending on the farm location, there can be substantial loads on the farm and mooring system from currents and waves. TendOcean uses 6m long spar floaters as a crucial element in the design of these systems. Their spar buoys keep the mooring system secure in operational conditions but have minimal mooring loads because they don’t try to fight to stay at the surface in extreme conditions. Spars are a key part of this resilient and stable system design. So far, these are relatively small examples compared to what can be found in offshore energy.
Equinor develops floating offshore wind systems. The Hywind Scotland offshore wind project consists of an array of five turbine systems using spar floaters. Each of the hulls has a displacement of 78m and provides a very stable platform for generating power in extreme wind and wave conditions.
It’s summary time
Spar floaters typically have long and slender form factors and sit vertically in the water. What’s notable about them is they are so insensitive to a wide range of ocean wave conditions. The form factor with a large submerged volume and small waterplane area means that ocean waves don’t create significant changes in buoyancy when they roll by. Spar floaters make a wonderfully stable platform for making measurements in the ocean or a stable working platform either for offshore wind or oil and gas systems. But each spar floater needs to be carefully designed as they do have a resonance period, and careful consideration needs to be made on what happens in the most extreme conditions when submergence is possible.
A spar hull form is only one of many types. But it offers a lot of advantages like insensitivity to extreme conditions. Much like the humble high-altitude bumble bee!
Most of the time, spar buoys are designed to be self-stable, meaning they stay upright on their own without any need for additional static mooring load. Any dynamic model of a self-stable buoy must account for linear and rotational motion. The RigidBody model in ProteusDS captures the effects of linear and rotational motion and dynamics and would be one way to model a spar buoy in an oceanographic mooring system. Check out this video tutorial on how to generate a self-stable buoy using the RigidBody model in ProteusDS on our YouTube channel: