What Does Seakeeping Of Vessels Mean?
As the saying goes, “Calm seas make for a bad helmsman.” Therefore, sea conditions are rarely calm.
Waves of varying intensities, turbulence, currents, ripples, eddies, tidal fluctuations, severe weather, and the sea surface itself are all factors that create various disturbances.
Therefore, for ships, in addition to key standards such as hull strength and propulsion efficiency, maximising the ship’s efficiency in coping with these ocean conditions is a major challenge in the design process.
As the name suggests, “maintaining ship stability at sea” refers to the ship’s performance and response under various ocean conditions.
Stability is a widely discussed aspect of a ship’s response to its surrounding environment and loads, but it also encompasses other factors.
Seakeeping, also known as sea-state stability, measures a ship’s ability to navigate various ocean conditions. When we talk about “efficiency,” we are actually defining the ship’s motion and dynamics.
This efficiency does not consider structural factors, drag and thrust conditions, or static stability, as these are independent factors within their respective fields of study.
Description of Ship Motion
Ships rarely remain perfectly upright and balanced at all times. Seakeeping focuses only on the ship’s motion in a vertical plane (perpendicular to the sea surface).
Three possible motions along this plane, which can occur individually or simultaneously, are:
(1) Roll
(2) Lift
(3) Heel.
In short, roll is the transverse reciprocating motion of the ship about its longitudinal axis (from bow to stern). When the starboard side sinks and the port side rises, i.e., viewed longitudinally from bow to stern (+X direction), the ship rotates clockwise, and the roll angle is positive.
Heel is the purely vertical up-and-down motion of the ship relative to the sea surface. When this motion is downward (+Z), i.e., the ship sinks, the ship has a positive heel.
Roll is the longitudinal reciprocating motion of the ship about its transverse axis. In this case, the bow and stern move up and down in opposite directions (bow up, stern down, and vice versa).
When the bow moves upward, i.e., upward relative to sea level or along the -Z direction (and thus stern downward), rolling is positive. The following diagram illustrates the correct interpretation.
Motions considered in the synchronous horizontal plane (parallel to sea level) belong to another area of study, namely maneuverability, which is no less important than maintaining balance at sea.
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The three main motions in this area are: (1) thrust, (2) roll, and (3) heel. Thrust refers to the forward and backward (forward and backward) motion of the ship, i.e., the motion along the reference x-axis; positive thrust represents the bow direction.
Roll refers to the transverse motion of the ship along the y-axis; generally, roll to starboard is considered positive.
Yaw refers to the rotational motion of the ship about its vertical axis of rotation (ideally, this axis passes through the longitudinal center of buoyancy and is perpendicular to the longitudinal plane).
Yaw is positive when the bow moves to starboard, i.e., rotates clockwise along the Y+ direction, and vice versa. All of these are illustrated in the following diagram.
Understanding the conceptual framework is crucial when discussing the motion and dynamics of ships at sea.
Specifically, when discussing the stable motion of ships at sea, the best frame of reference for analyzing vertical plane dynamics (roll, pitch, and pitch) is the ship’s stable frame of reference relative to land, i.e., the ship’s stable frame of reference at sea.
The best analogy is: a person or point on an imaginary, undisturbed platform (with no motion values in the Y and Z axes) moving alongside the ship, parallel to the ship at the same forward speed and direction without any deviation; that is, the observer is in a relatively linear and stable state relative to the passing boat. Therefore, as illustrated, this frame of reference is considered to represent the ship’s safety perspective most accurately.
Ship Safety Concepts
Ship safety assessments and design considerations revolve around three main objectives or basic design pillars:
(1) Habitability
(2) Operability
(3) Survivability
Habitability is closely related to human well-being and cargo safety, encompassing passenger comfort and health. Irregular and unpredictable voyages can severely impact passengers and crew, causing seasickness and other health problems such as motion sickness, thus affecting their work.
Therefore, this aspect is considered a fundamental element in the design philosophy of any vessel. Furthermore, habitability varies depending on the vessel’s purpose. Passenger ships or cruise ships clearly place greater emphasis on this aspect compared to cargo ships.
Operability refers to the efficiency necessary for the ship’s equipment, machinery, and other systems to ensure the smooth and uninterrupted operation of the vessel.
Safety refers to the integrity of the vessel, including impacts that could compromise the structural integrity of the crew and cargo, thereby preventing personal injury or cargo damage.
Therefore, in moderate or very severe sea states, impacts may include any one or a combination of the following:
- Movements such as heave, tilt, and roll, as previously described.
- Combined loads from waves acting on the hull beams.
- Localised loads acting on various structural components.
- Irregular changes in hull pressure caused by external conditions.
- Acceleration loads acting on the ship.
- Deck dampness and green water problems caused by flooding.
- Bow impact.
- Propeller protrusion.
- High wave drag problems.
- Flooding of machinery and engine room.
- Effects of wave impact on vulnerable areas.
Ship Stability Maintenance Analysis
Before conducting any form of ship stability maintenance analysis, the entire field can be simplified into a three-part problem or system. The following diagram illustrates this:
This is analogous to an electronic filter that receives the input signal, filters it, and produces the desired output signal wave. The input signal can be considered as external disturbances occurring in the marine environment, such as waves encountered by the ship.
The current sea state is the direct determinant of these inputs. The ship’s characteristics, or its responsiveness, represent the system’s ability to absorb or withstand external disturbances, and this ability depends on the ship’s design, type, and the disturbance’s type and intensity. The ship’s response motion to these disturbances represents the output.
Therefore, designers of every vessel consider the following key aspects:
(1) Estimate and visualise all environmental conditions the vessel may encounter.
(2) Determine the vessel’s potential seaworthiness response characteristics based on the interaction between the vessel’s inherent characteristics and the likelihood and type of impending disturbances.
(3) Evaluate the required seaworthiness standards for the vessel under specific conditions and optimise performance without compromising maximum operational requirements.
The following flowchart illustrates the best representation of the entire seaworthiness analysis domain.
Marine and Environmental Conditions Assessment
In the first step of seaworthiness analysis, the overall marine conditions relevant to each vessel must be considered.
The following clearly illustrates the current marine conditions required for our analysis:
Wave Spectrum: The wave spectrum, or wave spectrum set, is the collection of waves under study, composed of superimposed random waves with different wavelengths, amplitudes, and periods. All these parameters are equally important because, in theory, they provide complete, real-time data on the marine state.
Significant Wave Height: The average of the wave height values in the first third of the selected wave spectrum.
Medium wave period, denoted by the typical period (T0): The most common wave period among all wave periods in the spectrum.
Frequency: This is also a key aspect of spectral analysis. High-frequency waves cause rapid ship movement, and vice versa. Furthermore, assuming that the frequency of any given wave motion matches the ship’s inherent structural frequency can cause resonance, leading to violent ship motion.
- Spectral variability or total area
- Root mean square (RMS) value of maximum wave height
- Wind speed and direction
- Real-time meteorological data
- Wave direction and azimuth
Angular propagation function, a complex multidimensional numerical method for measuring wave energy patterns and distribution (beyond the scope of this paper)
The best way to describe the spectrum is as the wave’s dissipated energy curve, or the relationship between wave spectral intensity and frequency for a given wave pattern.
Accurately determining the spectrum is a complex and challenging task. However, to simplify calculations, a normalised spectrum based on standard sea states is often used as a reference.
For example, the ITTC or Pearson-Moscowitz wind scales are used to describe stable and well-developed sea states over a wide area, while the JONSWAP wind scales are used to describe less developed but more stable sea states.
Today, specialized software is also used to simulate relevant sea states, which can reproduce them in real time.
The sea state discussed defines the current calm state of a specific study area. The Beaufort scale is currently the most reliable and widely used method for selecting sea states, and it is also the recognized standard.
Typically, the scale ranges from 0 to 12 (the highest), indicating the severity of the relevant sea state. Therefore, it is clear that the higher the sea state, the more severe the environmental conditions; consequently, the corresponding parameters used to maintain ship stability will be adversely affected.
Ship Parameters
In addition to the surrounding sea state, the characteristics that maintain ship stability depend primarily on various ship parameters. From the perspective of preserving ship stability at sea, the following parameters are crucial:
- Ship dimensions (length, beam, draft)
- Displacement and load distribution
- Appropriate ship stability and its required values
- Centre of buoyancy and longitudinal buoyancy (LCB and LCF)
- Hyster shape and transverse curvature
- Buoyancy limit (and reserve buoyancy)
- Ship speed
Bow and stern type
Specific features designed to maintain ship stability at sea, such as anti-roll (roll drag gauges, fins, anti-roll chambers) or anti-roll (roll resistance)
Tonnage
The effects of these parameters can vary. For example, ships with a lower draft-to-weight ratio (B/T) are more prone to capsizing than those with a higher draft ratio.
Similarly, shallower draft ships are more likely to have exposed keels and experience swaying and rolling in rough seas.
For example, during lifting operations, if the ship’s center of buoyancy is not aligned with the center of buoyancy below it, a coupling moment will be generated, causing the ship to roll. We will discuss these aspects in detail later in this article.
Ship Stability Response
After identifying the types of disturbances that may be encountered and assessing the ship’s inherent characteristics, the next task in the ship stability design hierarchy is to define the range of responses the ship, as a system, will take to these disturbances. Finally, we connect the above to estimate the ship’s response based on its dynamic characteristics.
The question now is: how do we predict and quantify the ship’s response to a given disturbance? The answer lies in a commonly used numerical operator called the Response Amplitude Operator (RAO).
This is a numerical transfer function used to determine how given ocean conditions affect the motion of a particular ship.
This operator varies with the type of motion across the six degrees of freedom and therefore depends strictly on the intensity and physical characteristics of the incident disturbance, as well as on the ship’s type, size, and class.
This numerical factor determines the dynamic characteristics of the output motion after the disturbance (input) passes through the ship, acting as a “filter,” as shown in the figure above.
This representative image shows the relationship between a typical ship response (roll, heel) and its RAO (response operator), as well as the frequency of the encountered wave family.
Starting from the initial point of the curve, the ship exhibits uniform motion because it simply moves up and down in step with the incident wave, which has a low frequency (and therefore a long wavelength), making it easier for the ship to follow its shape.
However, as the wavelength increases, the frequency increases, and the Resonant Occurrence Area (RAO) also increases. The ship’s motion becomes more violent.
At the wave crest, the encounter frequency matches the ship’s natural frequency, resulting in resonance, as previously observed.
At this point, the resonant frequency (RAO) rises sharply to an extremely high value, causing violent motion of the ship, which then gradually returns to stability. The wave spreads into many small wavelets with excessively high frequencies (very short wavelengths).
These superimposed wavelets interfere with each other, cancelling each other out, ultimately negating the net effect on the ship. The ship then attempts to regain equilibrium, and the resonant frequency (RAO) approaches zero. Therefore, the ship’s response coefficient is proportional to the measured RAO.
Mathematically, the definition of RAO is complex, involving multiple physical components and other factors that will not be discussed here.
However, there are several methods to determine RAO. These methods can be based on existing ship data, experimental data (e.g., pool tests), or digital simulations using fundamental principles or software.
Design principles and standards for marine environmental protection and improvement: The natural factors constituting the marine environment are immutable.
Ship Stability at Sea and Optimization Design Concepts and Standards
The environmental factors affecting a ship’s operation cannot be changed. Therefore, to improve a ship’s stability at sea and enhance its performance, marine engineers can only optimize the ship’s design.
The above lists all factors affecting ship performance. Therefore, based on the ship’s specific details, services, routes, and other aspects, the potential impact of design optimization can be studied.
When discussing optimized designs for desired sea stability characteristics, we also need to clarify the highest objectives and mission requirements for each design option.
For example, container ships must be designed to withstand high wave resistance because it extends their valuable transit time and reduces significant rolling, and they also carry a large number of containers on their open decks.
Similarly, passenger ships must carefully consider passenger safety and comfort. Warships must also consider many aspects, such as the safety of weapons, systems, and other equipment (such as aircraft, helicopters, and tanks), as well as the supplies carried on board. A ship’s stability at sea depends on its own conditions, but some critical factors can be discussed more broadly.
Horse size is one of the most critical factors. The larger the hull relative to waves, the stronger its resistance to strong motion, and vice versa. Therefore, small vessels like fishing boats or small yachts are rarely seen on long-distance ocean voyages. Length, beam, and draft all affect a vessel’s response to various disturbances.
Displacement is another key factor. In fact, the larger a vessel’s displacement, the greater its moment of inertia, thereby having a greater physical impact on these incoming disturbances. Therefore, due to the reduced excitation force caused by waves, acceleration and the coefficient of motion also decrease.
Appropriate stability and its specific coefficients are crucial to a vessel’s stability at sea. The better the vessel’s longitudinal and lateral stability, the better its stability at sea. However, there are exceptions. We’ve all heard of “rigid” and “light” vessels. Rigid vessels have a high center of gravity, resulting in a short turning period, meaning they tend to stop quickly when tilted by external disturbances. This short turning period leads to rapid rolling, affecting the vessel’s seaworthiness and habitability.
Conversely, auxiliary vessels have a low center of gravity, resulting in a more extended turning period. They take a long time to return to their original vertical position and are prone to capsizing. Therefore, designers must carefully consider all aspects before making such improvements. Buoyancy limits and reserve buoyancy: While some vessels require a higher draft, care must be taken to ensure the draft is within reasonable limits. Lower free buoyancy limits (and lower reserve buoyancy due to excessive draft) can create a risk of the deck being submerged in large waves. Conversely, a shallow draft (and therefore a high free buoyancy limit) can cause the keel to overhang, resulting in bow or stern impacts.
Horse shape is the most controversial aspect. While hull shape varies depending on the vessel type, seaworthiness can be improved in a specific design by optimising the hull shape. A fuller or steeper hull shape (with a higher beam/draft ratio and a lower length-to-beam ratio) produces greater seaworthiness compared to a more slender or wedge-shaped hull.
A larger beam also exacerbates wave disturbance. Conversely, a more slender hull shape (such as a V-hull) may increase the risk of rolling, short-distance capsizing, and side impacts and similarly, increasing the length while reducing the mass coefficient (displacement/[length × beam × draft]) results in greater lift. However, increasing the length relative to the draft itself (length-to-height ratio), rather than increasing the mass coefficient, results in relatively less lift.
Furthermore, a fuller bow shoulder and larger forewater deflectors can improve a ship’s sea stability and reduce problems such as hull beam overhangs. Therefore, there is currently no definitive framework for achieving “optimal ship sea stability.” Improved designs can be conceived through experimentation, optimising all these factors, and prioritising requirements.
The shape of the ship’s bow (depending on the ship type) also significantly influences its design. For example, a protruding bow is said to greatly mitigate the destructive effects of falling wave chains by disrupting their impact on the hull.
Container ships with more optimised hull shapes
There are many standards for assessing a ship’s water stability performance. International regulatory bodies and bodies such as the International Maritime Organization (IMO) and the International Technical Committee on Transport (ITTC) have been developing guidelines and recommendations based on increasing demands, improvements in analysis and testing technologies, and accident scenarios.
Within the IMO, the Ship Design and Construction Subcommittee (SDC) regularly reviews this issue. The IMO recently made significant improvements to the requirements for high-speed vessels in its High Speed Vessel Code (HSC Code).
There are many key standards for assessing a ship’s water stability performance. The Bales Water Stability Performance Index is one of them. This index considers various ship parameters and rates the ship’s stability on a scale of 1 (lowest) to 10 (highest). However, it is generally considered primarily applicable to warships.
Water stability performance remains one of the most complex aspects of ship design. Perfect water stability performance is unattainable. However, over time and with accumulated experience, design advancements often make ships safer and more reliable.
Frequently Asked Questions
1. What is seaworthiness?
Seaworthiness measures a ship’s ability to adapt to the marine environment. A ship is seaworthy if it can operate efficiently and ensure the safety of its crew, even on the high seas and in harsh conditions.
2. What is a seaworthiness analysis?
A seaworthiness analysis predicts a ship’s behavior in a specific marine environment. Every ship undergoes linear motion (roll, pitch, yaw, sway, or turn). However, its motion depends on many factors, all of which are considered in a seaworthiness analysis.
3. How is a ship’s acceleration calculated?
The Pythagorean theorem can be used: a = (av² × at² × al²)½
This formula provides the total acceleration, which (a) can be multiplied by the ship’s mass.
4. What are the three fundamental motions of a ship?
These three motions occur along three axes. The rotational motions of a ship, such as roll, heave, and heave, are caused by the resultant forces acting on different parts of the ship. For example, rotation about the vertical Z-axis is called roll.
5. Who determines whether a ship is seaworthy?
The shipowner is responsible for verifying the seaworthiness of their vessels or fleet. This requires taking the necessary measures and conducting studies, investigations, and inspections to ensure the safety of personnel and cargo on board. If any problems occur at sea, the shipowner will be liable and may face fines and penalties.
