Seakeeping
Encyclopedia
Seakeeping ability is a measure of how well-suited a watercraft
is to conditions when underway. A ship
or boat
which has good seakeeping ability is said to be very seaworthy and is able to operate effectively even in high sea state
s.
Clearly, a drillship
and a ferry
have different missions and operate in different environments. The performance criteria will be different as well. Both may be considered seaworthy, although for different reasons based on different criteria.
s called Response Amplitude Operators
(RAO). For a floating structure they will need to be calculated for all six motions
and for all relative wave headings.
Ship motions are important for determining dynamic loading on the crew, passengers, ship system components, secured cargo, and structural elements. Excessive ship motions may hinder the vessel's ability to complete its mission such as the deployment and recovery of small boats or aircraft. A measure of an individual's ability to complete a specific task while onboard a moving ship is the Motion Induced Interruptions (MII). It gives an indication of the number of events in which a standing person will look for support in order to maintain balance. MII is measured in occurrences per hour.
Ship motions have physiological effects on ship passengers and crew. The magnitudes and accelerations of ship motions, (particularly heave, roll and pitch
) have adverse effects on passengers and shipboard personnel. Sea sickness will have negative effects on the ability of crew to accomplish tasks and maintain alertness and will obviously distress passengers. An important metric in evaluating sea sickness is the Motion Sickness Incidence (MSI). The most important study on MSI was published in Aerospace Medicine by O'Hanlon and McCauley in 1974, which established common subjective thresholds of MSI tolerance. MSI is measured in percentage of people who experience sea sickness during a given amount of exposure time. A commonly accepted limit of MSI is 20% occurrence of sea sickness over a four hour exposure period. A small amount of people are very susceptible to sea sickness and become ill even in the slightest conditions, while other people rarely get sea sick despite severe conditions. It has also been shown that most people acclimate to ship motions within a period of about four days, but some never acclimate at all.
Seakeeping directly impacts the design of a vessel. Ship motions are considered when determining the principal dimensions of the ship and in developing the general arrangements of the ship's internal spaces. For example, in most vessels the far forward parts of the ship experience the worst ship motions and are commonly unacceptable for berthing passengers or crew. In exceptional cases where ship motions pose a threat to crew, structure or machinery, or when ship motions interfere with the ability of the ship to accomplish its mission, then the design must be modified so that ship motions are reduced.
Watercraft
A watercraft is a vessel or craft designed to move across or through water. The name is derived from the term "craft" which was used to describe all types of water going vessels...
is to conditions when underway. A ship
Ship
Since the end of the age of sail a ship has been any large buoyant marine vessel. Ships are generally distinguished from boats based on size and cargo or passenger capacity. Ships are used on lakes, seas, and rivers for a variety of activities, such as the transport of people or goods, fishing,...
or boat
Boat
A boat is a watercraft of any size designed to float or plane, to provide passage across water. Usually this water will be inland or in protected coastal areas. However, boats such as the whaleboat were designed to be operated from a ship in an offshore environment. In naval terms, a boat is a...
which has good seakeeping ability is said to be very seaworthy and is able to operate effectively even in high sea state
Sea state
In oceanography, a sea state is the general condition of the free surface on a large body of water—with respect to wind waves and swell—at a certain location and moment. A sea state is characterized by statistics, including the wave height, period, and power spectrum. The sea state varies with...
s.
Measure of seakeeping ability
In 1976, St. Denis suggested four principal terms needed to describe a seakeeping performance. These are:- Mission: what the ship is intended to accomplish. The role of the ship while at sea.
- Environment: the conditions under which the ship is operating. This can be described as sea stateSea stateIn oceanography, a sea state is the general condition of the free surface on a large body of water—with respect to wind waves and swell—at a certain location and moment. A sea state is characterized by statistics, including the wave height, period, and power spectrum. The sea state varies with...
, wind speed, geographic region or some combination thereof. - Ship responses: the response of the ship to the environmental conditions. The responses are a function of the environment and the vessel characteristics.
- Seakeeping performance criteria: the established limits for the ship's responses. These are based on the ship motionsShip motionsShip motions are defined by the six degrees of freedom that a ship, boat or any other craft can experience.- Translation :HeaveSwaySurge-Vertical axis:Vertical axis, or yaw axis — an axis drawn from top to bottom, and perpendicular to the other two axes...
and the accelerations experienced, and include comfort criteria such as noise, vibration and sea sickness, performance based values such as involuntary speed reduction, and observable phenomena such as bow immersion.
Clearly, a drillship
Drillship
A drillship is a maritime vessel that has been fitted with drilling apparatus. It is most often used for exploratory offshore drilling of new oil or gas wells in deep water or for scientific drilling. The drillship can also be used as a platform to carry out well maintenance or completion work such...
and a ferry
Ferry
A ferry is a form of transportation, usually a boat, but sometimes a ship, used to carry primarily passengers, and sometimes vehicles and cargo as well, across a body of water. Most ferries operate on regular, frequent, return services...
have different missions and operate in different environments. The performance criteria will be different as well. Both may be considered seaworthy, although for different reasons based on different criteria.
Background
In ship design it is important to pre-determine the behavior of the ship or floating structure when it is subjected to waves. This can be calculated, found through physical model testing and ultimately measured on board the vessel. Calculations can be performed analytically for simple shapes like rectangular barges, but need to be calculated by computer for any realistic shaped ship. The results of some of these calculations or model tests are transfer functionTransfer function
A transfer function is a mathematical representation, in terms of spatial or temporal frequency, of the relation between the input and output of a linear time-invariant system. With optical imaging devices, for example, it is the Fourier transform of the point spread function i.e...
s called Response Amplitude Operators
Response amplitude operator
In the field of ship design and design of other floating structures, a response amplitude operator is an engineering statistic, or set of such statistics, that are used to determine the likely behaviour of a ship when operating at sea...
(RAO). For a floating structure they will need to be calculated for all six motions
Ship motions
Ship motions are defined by the six degrees of freedom that a ship, boat or any other craft can experience.- Translation :HeaveSwaySurge-Vertical axis:Vertical axis, or yaw axis — an axis drawn from top to bottom, and perpendicular to the other two axes...
and for all relative wave headings.
Ship motions are important for determining dynamic loading on the crew, passengers, ship system components, secured cargo, and structural elements. Excessive ship motions may hinder the vessel's ability to complete its mission such as the deployment and recovery of small boats or aircraft. A measure of an individual's ability to complete a specific task while onboard a moving ship is the Motion Induced Interruptions (MII). It gives an indication of the number of events in which a standing person will look for support in order to maintain balance. MII is measured in occurrences per hour.
Ship motions have physiological effects on ship passengers and crew. The magnitudes and accelerations of ship motions, (particularly heave, roll and pitch
Ship motions
Ship motions are defined by the six degrees of freedom that a ship, boat or any other craft can experience.- Translation :HeaveSwaySurge-Vertical axis:Vertical axis, or yaw axis — an axis drawn from top to bottom, and perpendicular to the other two axes...
) have adverse effects on passengers and shipboard personnel. Sea sickness will have negative effects on the ability of crew to accomplish tasks and maintain alertness and will obviously distress passengers. An important metric in evaluating sea sickness is the Motion Sickness Incidence (MSI). The most important study on MSI was published in Aerospace Medicine by O'Hanlon and McCauley in 1974, which established common subjective thresholds of MSI tolerance. MSI is measured in percentage of people who experience sea sickness during a given amount of exposure time. A commonly accepted limit of MSI is 20% occurrence of sea sickness over a four hour exposure period. A small amount of people are very susceptible to sea sickness and become ill even in the slightest conditions, while other people rarely get sea sick despite severe conditions. It has also been shown that most people acclimate to ship motions within a period of about four days, but some never acclimate at all.
Seakeeping directly impacts the design of a vessel. Ship motions are considered when determining the principal dimensions of the ship and in developing the general arrangements of the ship's internal spaces. For example, in most vessels the far forward parts of the ship experience the worst ship motions and are commonly unacceptable for berthing passengers or crew. In exceptional cases where ship motions pose a threat to crew, structure or machinery, or when ship motions interfere with the ability of the ship to accomplish its mission, then the design must be modified so that ship motions are reduced.
Factors affecting seakeeping
A number of factors affect seakeeping or more correctly the ship responses.- Size : A larger ship will generally have lower motions than a smaller one. This is because the relative size of the waves is lower.
- DisplacementDisplacement (fluid)In fluid mechanics, displacement occurs when an object is immersed in a fluid, pushing it out of the way and taking its place. The volume of the fluid displaced can then be measured, as in the illustration, and from this the volume of the immersed object can be deduced .An object that sinks...
: A heavier ship will generally have lower motions than a lighter one. Given that the wave energy is the same for each vessel and provides the exciting force, the one with the greater mass will have the lower accelerations. - StabilityShip stabilityShip stability is an area of naval architecture and ship design that deals with how a ship behaves at sea, both in still water and in waves. Stability calculations focus on the center of gravity and center of buoyancy of vessels and on how these interact....
: A stable ship will tend to follow the wave profile closer than a less stable one. This means that a more stable ship will generally have higher accelerations but lower amplitudes of motion. - FreeboardFreeboard (nautical)In sailing and boating, freeboardmeans the distance from the waterline to the upper deck level, measured at the lowest point of sheer where water can enter the boat or ship...
: The greater a vessel's freeboard the less likely it is to immerse the deck. Deck immersion is often a seakeeping criterion, as it affects mission capability in a number of ships.