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How to differentiate the value of ship design changes with operability analysis
Leaf insects have spectacular camouflage. It’s an extreme survival strategy: these bugs have their entire bodies shaped to look identical to leaves. So, is it worth it?
Absolutely! It’s incredibly common for predators to overlook them as they blend so well into trees and bushes. But it’s more effective than you might think. In fact, it’s such good camouflage that leaf insects can’t even tell each other apart from real leaves. This leads to them accidentally nibbling on each other from time to time. Fortunately, their bodies are particularly thin on the edges, so this doesn’t seem to hurt or bother them. And as a bonus, they look even more like real leaves with wear and tear.
As a survival strategy, it works well for leaf insects that even they can’t tell each other apart from the real thing. However, depending on the situation, being unable to tell things apart can be a major disadvantage.
When it comes to design, we need ways to tell things apart and sense the differences. These differentiations then show us how design changes make an improvement. In the world of ship design, an operability analysis is one way to make a crucial comparison. It’s so crucial because, when done correctly, it can show you how valuable a design change may be. In this article, we’re going to cover just what an operability analysis is and how it helps the vessel design process.
What is operability analysis and why is it important?
An operability analysis is a calculation process that shows you how much you can use your ship in a typical year. What’s so important about an operability analysis is that it shows you how much you can get out of your design. You can then use it to gauge how valuable changes to a ship design might be, too. So, in one case, if your ability to work on a ship is limited by a certain rolling angle, this will tell you how often you can expect to use your vessel for work in a typical year.
How do you compute an operability analysis?
An operability analysis goes beyond an individual ship motion prediction in a particular sea state. It has to factor in seakeeping predictions across a range of sea state intensities. Also key is data on the probabilities of those sea states, which depend on where the ship will be during its operational life. Finally, there needs to be a set of operability limits. These limits depend on the specific use of the vessel. When these limits are exceeded in a particular sea state, it means the ship can’t be used whenever that sea state occurs. But what are these limits, exactly?
The limits are always related to factors caused by motion in waves
As mentioned earlier, a limit might be a particular roll angle. Or it might be a local vertical or lateral acceleration value at a specific location on the ship. Other commonly used factors that gauge the effect of ship motion on people include Motion Sickness Incidence (MSI), and Effective Gravity Angle (EGA), which shows the combined effect of different motions at a specific location.
Depending on the intent of the ship’s use, there may be different magnitudes and types of limits. For example, military vessels tend to have higher limits for acceleration and rolling angle than a yacht or research vessel—it depends on what the ship is most suitable and intended for. Regardless of the number, magnitude, and type of limits, they are crucial ingredients to an operability analysis.
An operability analysis tells you how much you can get from your ship.
When you get a picture of all the sea state intensities the ship can operate in, the probability of occurrence of these sea states makes a link to how much time you can use the vessel each year. In other words, it’s a calculation showing you how valuable the ship is. For an application like an offshore supply vessel, this will correlate to how much business you can do each year. So as an absolute value, it can help you discern the financial impact of a particular vessel on your business. But there’s another way an operability analysis can help in design, too.
An operability analysis can show the value of design changes, too
In the early stages of design, an operability analysis is another way to measure the value of design changes. It can quickly make a business case for augmenting a ship design with technologies like fin stabilizers or anti-roll tanks. After all, if anti-roll technologies reduce ship motion below limiting roll or acceleration values, it’s going to result in more days you can do work offshore. However, there are many more parameters to explore in the ship design space.
Ship motion is strongly affected by vertical CG location. An operability analysis in the early stages of design could give valuable feedback on what range of vertical CG produces the most useful and valuable ship for a client.
This isn’t limited to new builds, either. Ships are modified throughout their lives as new equipment is added, or the hull is extended, changing CG location, among other crucial parameters. An operability analysis shows how to ensure these changes benefit the vessel’s operability. But this kind of analysis isn’t limited to helping naval architects in the design stage. It can also help owners ensure they get what they are paying for.
Operability analysis may be a contract requirement
A minimum operability threshold can be helpful for owners, too. The ability to show how a design meets a particular set of operability criteria is a systematic way to give everyone confidence that the owner will get what they need from the ship design.
How can you be sure of the operability thresholds to use?
Making up your own operability thresholds is not easy. It requires a careful understanding of the intent to use the vessel, and where it will be through its lifetime. The limits can make a big difference to the operability level. But regardless of their values, whatever thresholds are picked, the operability approach provides a systematic way to make comparisons of ship designs. Fortunately, standards like ISO 22834 give you concrete criteria to use and apply for specific kinds of ship design.
Example time
We covered a lot of fundamentals of operability analysis, and now it’s time for an example. In this example, we’re going to look at an operability analysis of a 50m Generic Yacht. This uses operability limits and conditions specified by the ISO 22834 standard, which produces a motion-affected comfort rating for a large yacht. The analysis factors in long-term probability sea state data for two regions commonly used by large yachts: the Mediterranean and the Caribbean. The data is provided in the ISO 22834 standard, and generally, the longer period waves are less common than shorter waves.
The operability limits are 2-degree EGA and 10% MSI. Both of these values are functions of the local vertical and lateral accelerations to be calculated at five different areas on the yacht. The values are then calculated across a range of sea state conditions from 4 to 12 s waves, at a 0 kn and 12 kn forward speed, all in a bow quartering condition.
Whenever the ship motion is too much such that the EGA or MSI exceeds the limits in a particular sea state, the operability value reduces. This reduction is based on the probability value of the sea state – so if it passes all sea state conditions, it’s 100% operable, if it fails all conditions, it’s 0% operable. According to ISO 22834, the operability value then corresponds to a star rating system, with a maximum of 5 stars for the highest comfort rating.
In the specific case of the 50m Generic Yacht with no fin stabilizers, we used the ProteusDS Ship Motion toolbox to compute the ISO 22834 comfort rating. The resulting operability evaluated for the nominal design was 51%. A quick sensitivity study was conducted to show the change in operability and motion affected comfort when the vertical center of gravity was raised by 10%. A recalculation of the operability value was then increased to 60%, an increase of almost 18 percentage points. This highlights how you can get quantifiable feedback from design changes from an operability analysis.
It’s summary time
We covered a few aspects of operability analysis, and now it’s time to summarize. An operability analysis shows what you can get out of a floating system. In the context of ship design, it often indicates how much you can use it each year. This allows a designer to make a value-based comparison between design changes – whether for new builds or for significant retrofits. Ship owners can also use these as contract requirements to help get a quantitative way to get confidence their vessels will do what they expect. The operability analysis needs several key elements: a ship motion prediction model, wave probability data for specific areas, and operability limits. The operability limits are specific for each application, and need careful attention when selecting them.
Operability is a different way of thinking about ship motion prediction and incorporating it into the design process. But it’s a helpful way to see the differences of what’s right in front of you. Much better to tell the differences, unlike a leaf insect who doesn’t always know if they’re taking a bite out of food or their neighbour.
Next step
In the example, we showed an operability analysis applied to a 50m generic yacht. The result of the analysis is a comfort rating directly related to the operability of a large yacht. Learn more about how you can do this analysis yourself and incorporate it in the ship design process using the ProteusDS Ship Motion toolset here.
