Grocery stores are places of scientific discovery. It’s true: in recent years, biologists wanted to know just what was in a package of store-bought dried mushrooms. Taking an extremely close look, they sequenced the DNA of each mushroom and were stunned at what they found. They were mushrooms all right – but what was surprising was that they found three entirely new species of porcini mushrooms. A mushroom might look like any other, but a closer look reveals surprising differences.
The closer you look, the more often you find these surprising differences. In ship design, one hull of a specific ship size and class may look like many others. But a few invisible changes can make all the difference in how they respond in the ocean.
Seakeeping analysis is a prediction of how a ship will move in a particular sea state
Any floating vessel moves in a linear (heave, surge, sway) and rotational (roll, pitch, and yaw) fashion in different amounts based on many factors. Essentially, the amount of movement depends on the ship’s forward speed and orientation to the prevailing sea, the size of the waves, and the range of wave period.
Ship motion is a crucial factor of seaworthiness
Seaworthiness is how safe and easy it is to live and work on a ship in different sea conditions. In the worst-case extreme seas, you might think the hull flipping over or capsizing is the biggest concern. It certainly is, but many more things can go wrong before that extreme condition would happen.
Seakeeping analysis reveals deck accelerations and motion at any point on the ship in a particular sea state. These accelerations and how severe they are affects comfort and safety. The larger the accelerations, the more difficult it is to work and operate the ship properly. Ship motions also drive sea sickness for people on board.
Health and safety are not the only factors
There is also the possibility of damage to the ship as large accelerations may damage specific equipment. It’s also possible to damage or lose cargo at sea.
In large pitch motions, massive slamming forces are likely from the impact of the hull on the water. Large pitch motions may also mean the bow is partially submerged and water can wash over the deck. The weight and pressure of water on the deck from these kinds of events are considerable. These are much more extreme scenarios that can create tremendous forces and dangerous stresses on the hull structure.
Seakeeping analysis is possible through a few methods
Physical tests involve a scale ship model in a wave basin. But test tank facilities are expensive and physical models take a lot of time and cost to create. Numerical tests are possible through a variety of commercial software tools.
One of the most common approaches is a numerical one based on potential flow theory. These tools calculate how a specific hull form interacts with an ocean wave field. There are a few limitations to what is possible with this approach but it is a robust technique that has been used for decades. ShipMo3D is an example of a software program that is purpose-built for seakeeping analysis.
Most often, seakeeping analysis is part of new ship design
The shape of the hull and the configuration of equipment and cargo are all factors that play into how the ship will move in certain sea conditions. Once the vessel is constructed, there are many reasons to check the ship’s motions. Depending on the vessel configuration and possible sea state intensity in an upcoming voyage, a specific seakeeping analysis may be necessary to evaluate risks.
Even though two ship hulls may look similar, there are many factors in addition to the hull shape that goes into a ship motion analysis. The load out of the hull can affect the inertia and shift the center of mass. The inertia and location of the center of mass alone have a significant influence on the ship’s motions. All these details need to be carefully considered for an accurate assessment.
Do we always have to complete a seakeeping analysis?
Seakeeping analysis is one of many tools used in the design process. Applying it depends on the risk involved and how much concern there is for ship performance. Naval architects, engineers, and ship designers build a tremendous amount of experience working with certain styles of ships and often understand how similar vessels will respond.
In this way, seakeeping analysis is a complementary tool to experience. But at the same time, many details affect a ship’s seakeeping ability. Changing only one of many parameters on a vessel can make a surprising change in the motion response. Tools like ShipMo3D can quantify those differences. But whether a seakeeping analysis is necessary or not depends on the risks involved and consequences of damage. There may also be a regulatory requirement for specific analysis depending on a particular jurisdiction.
It’s time for an example
A ShipMo3D model of a generic frigate is shown in the picture below. The wet and dry hull mesh is shown in yellow and green, respectively, and highlights the draft for the specified vessel weight. Hull appendages like bilge keels, skegs, and rudders appear in red that round out the details necessary for the ship motion evaluation.
A software tool like ShipMo3D calculates the hydrodynamics and then resulting motions of a vessel like this in a variety of sea conditions. So how much can these parameters change? To illustrate these changes, we increased the roll inertia of the generic frigate by 15% and recalculated the performance metrics in ShipMo3D.
In a pure roll-resonance condition, the generic frigate with larger roll inertia has a 20% larger amplitude. But roll resonance from a pure sinusoidal ocean wave is not always the most common sea state. What are the differences in motion in a more realistic sea?
A more realistic sea might look like a short-crested sea state with a spectrum of different wave frequencies. We picked a short-crested sea state five condition with a spectrum peak period at ten seconds to investigate. The two generic frigate models were set to a 10-knot forward speed in beam sea condition. This time, the generic frigate with larger roll inertia reduced the peak roll accelerations by 10%. In this specific case, the added inertia moved the natural roll period farther away from the spectrum peak period, helping reduce roll activity.
A short time history of roll motion in a short-crested sea state in the figure below gives an idea of the output of a seakeeping program. There are more detailed outputs that show statistics like maximum motions, accelerations and probabilities for motion sickness and interrupted work – all important factors to consider for safety at sea. This is only one specific sea state and ship condition that illustrates how one key parameter can make a significant difference – in spite of all the physical similarities of hull shape and appendages.
Seakeeping analysis is all about predicting how a particular ship will move in a specific sea state. The ship hull form and specific sea conditions all factor into this analysis. A host of information on the ship’s motions feeds into indicators like interruptions of work to seasickness. There are also many indicators for large loads, the potential for damage to the hull, or cargo loss.
Ships of a particular type may look one and the same, like mushrooms at the grocery store. Seakeeping analysis will help you take a deeper dive and reveal how each one differs from the other.
We talked a lot about what seakeeping is and how it helps better understand ships. A ship motion response amplitude operator (RAO) is a fundamental parameter in seakeeping analysis. But what is an RAO and how does it work? Read more about RAOs here.
- Ryan Nicoll, Co-founder and chief technology officer, DSA Ocean