Ship 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. Naval architecture is an engineering discipline dealing with the design construction and repair of marine vehicles Considerations are made as to the center of gravity and center of buoyancy of vessels and how they interact. The metacentric height (GM is the distance between the center of gravity of a ship and its metacenter
Contents |
History
Ship stability is a complicated aspect of naval architecture which has existed in some form or another for hundreds of years. A ship model basin may be defined as one of two separate yet related entities namely a physical basin or tank used to carry out hydrodynamic Newcastle University is a leading research intensive University located in Newcastle upon Tyne in the north-east of England. Naval architecture is an engineering discipline dealing with the design construction and repair of marine vehicles Historically, ship stability calculations for ships relied on rule-of-thumb calculations, often tied to a specific system of measurement. Some of these very old equations continue to be used in naval architecture books today, however the advent of the ship model basin allows much more complex analysis. A ship model basin may be defined as one of two separate yet related entities namely a physical basin or tank used to carry out hydrodynamic
Master shipbuilders of the past used a system of adaptive and variant design. Ships were often copied from one generation to the next with only minor changes being made, and by doing this, serious problems were not often encountered. Ships today still use the process of adaptation and variation that has been used for hundreds of years, however computational fluid dynamics, ship model testing and a better overall understanding of fluid and ship motions has allowed much more in-depth analysis. Computational fluid dynamics (CFD is one of the branches of Fluid mechanics that uses Numerical methods and algorithms to solve and analyze problems that involve
Transverse and longitudinal bulkheads were introduced in ironclad designs between 1860 and the 1880s, anti-collision bulkheads having been made compulsory in British steam merchant ships prior to 1860[1]. A bulkhead is an upright wall within the hull of a Ship. Other kinds of partition elements within a ship are decks and Deckheads Etymology Prior to this, a hull breach in any part of a vessel could flood the entire length of the ship. Transverse bulkheads, while expensive, usually increase the likelihood of ship survival in the event of damage. It does so by isolating seawater to smaller sections protected by bulkheads. Longitudinal bulkheads were intended to do the same, however, the damaged stability effects must be taken into account to eliminate excessive heeling. Today, most ships have means to equalize the water in sections port and starboard (counter flooding), which helps to limit the stresses experienced by the structure, and also alter the heel and/or trim of the ship.
Add-on Stability Systems
These systems are designed to reduce the effects of waves or wind gusts. They do not increase the stability of the vessel in a calm sea. The IMO International Convention on Load Lines does not mention active stability systems as a method of ensuring stability. The International Maritime Organization ( IMO) formerly known as the Inter-Governmental Maritime Consultative Organization ( IMCO) is a late 20th century The hull must be stable without active systems.
Passive Systems
Bilge Keel
A bilge keel is a long fin of metal, often in a "V" shape, welded along the length of the ship at the turn of the bilge. Bilge Keel is a strake, or small keel or blister running along much of the length of the hull. Bilge keels are employed in pairs (one for each side of the ship). A ship may have more than one bilge keel per side, but this is rare. Bilge keels increase the hydrodynamic resistance when a vessel rolls, thus limiting the amount of roll a vessel has to endure.
Outriggers
Outriggers may be employed on certain vessels to reduce rolling. An outrigger is a part of a boat's Rigging which is rigid and extends beyond the side or Gunwale of a boat Rolling is reduced either by the force required to submerge buoyant floats or by hydrodynamic foils. In some cases these outriggers may be of sufficient size to classify the vessel as a trimaran, however on other vessels they may simply be referred to as stabilizers. A trimaran is a Multihulled Boat consisting of a main hull ( vaka) and two smaller Outrigger hulls ( amas) attached
Antiroll Tanks
Antiroll tanks are tanks within the vessel fitted with baffles intended to slow the rate of water transfer from the port side of the tank to the starboard side. The tank is designed such that a larger amount of water is trapped on the higher side of the vessel. This is intended to have an effect completely opposite to that of the free surface effect. The free surface effect is one of several mechanisms where a craft can become unstable and roll-over ( Capsize)
Paravanes
Paravanes may be employed by slow moving vessels (such as fishing vessels) to increase stability. Paravane may refer to Operation Paravane, a World War II operation Paravane (water kite, a towed winged underwater object
Active Stability Systems
Many vessels are fitted with active stability systems. Active stability systems are defined by the need to input energy to the system in the form of a pump, hydraulic piston, or electric actuator. These systems include stabilizer fins attached to the side of the vessel, or tanks in which fluid is pumped around to counteract the motion of the vessel.
Stabilizer Fins
Active fin stabilizers are normally used to reduce the roll that a vessel experiences while under way. This article refers to the nautical term For other uses see Stabilizer. The fins extend beyond the hull of the vessel below the waterline, and alter their angle of attack depending upon heel angle of the vessel. They operate in a very similar way to airplane wings. Cruise ship frequently use this type of stabilizer system because the high cost of incorporating it into the vessel can be justified. A cruise ship or cruise liner is a Passenger ship used for pleasure voyages where the voyage itself and the ship's amenities are part of the experience Pleasure yachts down to 15M LOA will increasingly choose active fin stabilization as the cost/benefit ratios are perceived to improve. This system may have any of the following disadvantages:
- Because the fins may be retractable, they may take up valuable space in the engine compartment.
- When fins are not retractable, they constitute fixed appendages to the hull, possibly extending the beam or draft envelope; at a minimum, requiring attention for additional hull clearances.
- Altering the angle of attack requires the vessel to use fuel in supplying the power required to do so. However the power expended for fin motion may be offset by power recovered through more stable tracking on course. Power saved by following a more accurate course may be difficult to quantify.
- The fin and actuator mechanism is expensive to manufacture and fit into the vessel, especially when compared to a bilge keel. Bilge Keel is a strake, or small keel or blister running along much of the length of the hull.
While the typical "active fin" stabilizer will effectively counteract roll for ships under way, some active fin systems have been shown capable of reducing roll motion when vessels are not under way. Referred to as Stabilization while not under way or Stabilization at Rest, these systems work by moving fins of special design, with the requisite acceleration and impulse timing to create effective roll cancellation energy. Stabilization while not under way or Stabilization at Rest or "'ZeroSpeed Stabilization'" or "'OnAnchor Stabilization'" refers to the process of augmenting
Calculated stability conditions
When a hull is designed, stability calculations are performed for the intact and damaged states of the vessel. Stabilization while not under way or Stabilization at Rest or "'ZeroSpeed Stabilization'" or "'OnAnchor Stabilization'" refers to the process of augmenting Stability conditions (watercraft is the term used to describe the various standard loading configurations to which a Ship, Boat, or offshore platform may be subjected Ships are usually designed to slightly exceed the stability requirements (below), as they are usually tested for this by a classification society. In the Shipping industry classification societies are Non-governmental organizations or groups of professionals Ship surveyors and representatives of offices
Intact Stability
Intact stability calculations are relatively straightforward and involve taking all the centers of mass of objects on the vessel and the center of buoyancy of the hull. Cargo arrangements and loadings, crane operations, and the design sea states are usually taken into account.
Damaged Stability
Damaged stability calculations are much more complicated than intact stability. Finite element analysis is often employed because the areas and volumes can quickly become tedious and long to compute using other methods.
The loss of stability from flooding may be due in part to the free surface effect. Water accumulating in the hull usually drains to the bilges, lowering the centre of gravity and actually increasing the metacentric height. This assumes the ship remains completely stationary and upright. However, once the ship is inclined to any degree (a wave strikes it for example), the fluid in the bilge moves to the low side. This results in a list.
Stability is also lost due to flooding when, for example, an empty tank is holed and filled with seawater. The lost buoyancy of the tank results in that section of the ship lowers into the water slightly. This creates a list unless the tank is on the centerline of the vessel.
In stability calculations, when a tank is holed, its contents are assumed to be lost and replaced by seawater. If these contents are lighter than seawater, (light oil for example) then buoyancy is lost and the section lowers slightly in the water accordingly.
Required Stability
In order to be acceptable to classification societies such as the American Bureau of Shipping, Lloyd's Register of Ships, and Det Norske Veritas, the blueprints of the ship must be provided for independent review by the classification society. In the Shipping industry classification societies are Non-governmental organizations or groups of professionals Ship surveyors and representatives of offices The American Bureau of Shipping (ABS is a Classification society, with a mission to promote the security of life property and the natural environment primarily through The Lloyd's Register Group is a maritime Classification society and independent risk management organisation providing risk assessment and mitigation services Det Norske Veritas or DNV is a Norwegian company established in 1864 Calculations must also be provided which follow a structure outlined in the regulations for the country in which the ship intends to be flagged.
For U. S. flagged vessels, blueprints and stability calculations are checked against the U. S. Code of Federal Regulations (CFR) and SOLAS conventions. The Safety of Life at Sea (SOLAS is the most important treaty protecting the safety of merchant ships Ships are required to be stable in the conditions to which they are designed for, in both undamaged and damaged states. The extent of damage required to design for is included in the regulations. The assumed hole is calculated as fractions of the length and breadth of the vessel, and is to be placed in the area of the ship where it would cause the most damage to vessel stability.
In addition, U. S. Coast Guard rules apply to vessels operating in U. S. ports and in U. S. waters. Generally these Coast Guard rules concern a minimum metacentric height or a minimum righting moment. The metacentric height (GM is the distance between the center of gravity of a ship and its metacenter Because different countries may have different requirements for the minimum metacentric height, most ships are now fitted with stability computers that calculate this distance on the fly based on the cargo or crew loading. The metacentric height (GM is the distance between the center of gravity of a ship and its metacenter "Cargomax" is a popular computer program used for this task.
See also
- Stability conditions (watercraft)
- Stabilizer (ship)
- Free surface effect
- Metacentric height
- Stabilization at zero speed
References
- ^ From Warrior to Dreadnought DK Brown
- Title 46 U.S. Code of Federal Regulations
- ABS Rules for Building and Classing Steel Vessels 2007
- Overview of a few common Roll Attenuation Strategies
External links
- |Arcturus Marine TRAC - thrusters, digital stabilizers, integrated hydraulics
- Naiad Marine - manufacturers of roll stabilizers, stabilization at anchor systems, interceptors, bow and stern thrusters, integrated hydraulic systems
Dapyx Software network: MP3 Explorer | Ebook Manager | Zenithic