script:-Sea Keeping Script

7/28/2019 script:-Sea Keeping Script

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5.8 Seakeeping

5.81CONTENTS

1.Introduction2.Measure of sea keeping ability3.Ship motions4.Factors affecting sea keeping5.Sea keeping criteria6.Overall sea keeping performance7.Improving Sea keeping Performance8.Stabilization9.Summary


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INTRODUCTION

Seakeeping ability is a measure of how well-suited a watercraft is to conditionswhen underway. A ship has good sea keeping ability is said to be very seaworthy

and is able to operate effectively even in high sea states.

In their broadest sense the terms sea keeping and seaworthiness cover allthose features of a vessel which influence its ability to remain at sea in

all conditions, for which it has been designed, and carry out its intended

mission. They should, therefore, embrace stability, strength, maneuverabilityand endurance as well as the motions of the ship and related

phenomena. In this chapter only those aspects of a ships performancedirectly attributable to the action of the waves are considered. Other

aspects are discussed in later chapters.

Considered as a rigid body, a ship has six degrees of freedom. They

are the three rotations of roll (or heel ), pitching (or trim) and yaw, togetherwith the three translations of heave, surge and sway. For a stable ship

the motions of roll, pitch and heave are oscillatory and these are the

three motions dealt with here. The other three degrees of freedom willbe excited in a seaway but are of lesser importance. As the ship is flexible

other degrees of freedom will be excited but these are dealt with understrength and vibration.


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Measure of seakeeping ability

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 bedescribed as sea state, 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.

Sea keeping performance criteria: the established limits for the ship's responses.

These are based on the ship motions and the accelerations experienced, and includecomfort criteria such as noise, vibration and Sea sickness a, performance based

values such as involuntary speed reduction, and observable phenomena such asbow immersion.

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.


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SHIP MOTIONS

Translation

Heave: is the linear vertical (up/down) motion

Sway: is the linear lateral (side-to-side)motion

Surge: is the linear longitudinal (front/back) motion

Rotation motions

]Roll : is the rotation of a vessel about its longitudinal (front/back) axis

Pitch : is the rotation of a vessel about its transverse (side-to-side) axis

Yaw: is the rotation of a vessel about its vertical axis


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Factors affecting seakeeping.

Size: A larger ship will generally have lower motions than a smaller one. This isbecause the relative size of the waves is lower.

Displacement: A heavier ship will generally have lower motions than a lighterone. Given that the wave energy is the same for each vessel and provides theexciting force, the one with the greater mass will have the lower accelerations.

Stability: A stable ship will tend to follow the wave profile closer than a less stableone. This means that a more stable ship will generally have higher accelerationsbut lower amplitudes of motion.

Freeboard: The greater a vessel's freeboard the less likely it is to immerse the

deck. Deck immersion is often a sea keeping criterion, as it affects missioncapability in a number of ships.

Human factor: Often the most critical factors in sea keeping especially in smallvessels are the experience and skills of the crew in extreme situations. Allied to

these are avoiding seasickness, getting sufficient sleep, food and drink and stayinginjury free for the duration of an extreme event.


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Seakeeping criteria

Slamming

Slamming is the impact of the bottom structure of a ship onto the sea surface. It is

mainly observed while sailing in waves, when the bow raises from the water andsubsequently impacts on it. Slamming induces extremely high loads to ship

structures and is taken under consideration when designing ships.

Slamming is a high frequency transient vibration in response to the

impact of waves on the hull, occurring at irregular intervals. The most

vulnerable area is the ships outer bottom between about 10 and 25 percent of the length from the bow. The impact may cause physical damageand can accelerate fatigue failure in this area. For this reason this area

of the outer bottom should be given special attention during survey.

Slamming is relatively local and often in a big ship, those on a bridgewell aft may not be aware of its severity. Because the duration of the slam

is only of the order of of a second, it does not perceptibly modify

the bodily motion of the ship but the ensuing vibration can last for30 seconds. A prudent master will reduce speed when slamming badly.

This speed reduction leads to less severe slamming or avoids it altogether.

Often a change of direction helps. Lightly loaded cargo ships are particularlyliable to slam with their relatively full form and shallow draught

forward, and enforced speed reductions may be as high as 40 per cent.Slamming is less likely in high speed ships because of their finer form.

Slamming is likely when the relative velocity between the hull and water

surface is large and when the bow is re-entering the water with a significant

length of bottom roughly parallel to the sea surface. It is amplifiedif the bottom has a low rise of floor. The pressure acting in a slam can be

shown to be proportional to the square of the velocity of impact and

inversely proportional to the square of the tangent of the deadrise angle.


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Wetness

By wetness is meant the shipping of heavy spray or green seas over theship. The bow area is the region most likely to be affected and is assumedin what follows. It may limit a ships speed and the designer needs some

way of assessing the conditions under which it will occur and how severeit will be. To some degree wetness is subjective and it certainly depends

upon the wind speed and direction as well as the wave system. In the pastit was often studied by running models in waves but it is now usually

assessed by calculating the relative motion of the bow and the local seasurface. The assumption made is that the probability of deck wetness is

the same as that of the relative motion exceeding the local freeboard.The greater the difference, the wetter the ship is likely to be.

Direct model study of such phenomena can, of course, be made by

running the model in a representative wave train over a longish periodalthough spray does not scale accurately. Tests in regular waves can

assist in a simple slamming investigation in which two designs are

directly compared. It is now usual to assess slamming by calculating the

relative motion of the bow and the local sea surface. The assumptionmade is that the probability of deck wetness is the same as that of the

relative motion exceeding the local freeboard. The greater the difference,the wetter the ship is likely to be.

Increased freeboard, say by increasing sheer forward is one means of

reducing wetness. At sea the master can reduce wetness by reducingspeed and, usually, changing the ships heading relative to the predominant

waves. Good round down on the deck will help clear water quickly.A bulwark can be used to increase the effective freeboard but in that

case adequate freeing ports are needed to prevent water becomingtrapped on the deck. The size of freeing ports to be fitted is laid down

in international regulations. The designer would avoid siting otherthan very robust equipment in the area where green seas are likely.

Any vents would face aft and water traps provided.


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Propeller emergence

The probability of the propeller emerging from the water, as the resultof ship motions, can be assessed in a similar way to wetness. That is, by

calculating the motion of the ship aft relative to the local sea surface.If the propeller does emerge, even partially, it will be less effective in

driving the ship. It will tend to race and cause more vibration.

Human performance

It is a common experience that ship motions can cause nausea and thensickness. This discomfort can itself make people less efficient and make

them less willing to work. Motions can make tasks physically more difficultto accomplish. Thus the movement of weights around the ship, say

when replenishing a warship at sea, is made more difficult. Also tasksrequiring careful alignment of two elements may become impossible

without some mechanical aid. Over and above this the motions, and the

drugs taken to alleviate the symptoms of motion sickness, may adverselyaffect a persons mental dexterity.

In broad terms the effects of motion on human behaviour dependupon the acceleration experienced and its period. The effect is most

marked at frequencies between about 0.15 to 0.2 Hz. The designer can

help by locating important activities in areas of lesser motion, by aligning

the operator position with the ships principal axes, providing an externalvisual frame of reference and providing good air quality free of odors.


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OVERALL SEAKEEPING PERFORMANCE

The most common cause of large amplitude rolling, as shown by linear

theory, is the closeness of the wave encounter frequency to the shipsnatural roll frequency. Large roll angles can also be experienced due to

the fact that a ships effective metacentric height varies as it passesthrough waves. These are non-linear effects. One case is when waves are

slowly overtaking a ship with largish water plane area aft (for instanceships with a transom stern). Relatively large transverse stability variations

can occur and roll angles of 40 degrees amplitude, or more, canrapidly build up. Secondly severe rolling can occur when the dominant

encounter frequency is close to half the natural period of roll.An overall assessment of sea keeping performance is difficult because

of the many different sea conditions a ship may meet and the differentresponses that may limit the ships ability to carry out its function. A number

of authorities have tried to obtain a single figure of merit but this is

difficult. The approach is to take the ships typical operating pattern overa period long enough to cover all significant activities. From this isdeduced:

(1) The probability of meeting various sea conditions, using statisticson wave conditions in various areas of the world

(2) The ship speed and direction in these seas.(3) The probability of the ship being in various conditions, deep or

light load.


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Improving Seakeeping Performance

Siting critical activities in less-affected areas of the ship. Placing

helicopter operations aft in frigates and placing only very rugged

equipment forward on the forecastle.

rerouting of ships to avoid the worst sea conditions.

providing local stabilization for certain equipments such as radder.

Skeg for straight line stability

STABILIZATION

A ships rolling motions can be reduced by fitting a stabilization system.

In principle pitch motions can be improved in the same way but inpractice this is very difficult. An exception is the fitting of some form of

pitch stabilizer between the two hulls of a catamaran which is relativelyshorter than a conventional displacement ship. In this section attention

is focused on roll stabilization. The systems may bepassive oractive.

Bilge keels

Of the passive systems, bilge keels are the most popular

and are fitted to the great majority of ships. They are effectively plates

projecting from the turn of bilge and extending over the middle half

to two-thirds of the ships length. To avoid damage they do not normallyprotrude beyond the ships side or keel lines, but they need to

penetrate the boundary layer around the hull. They cause a body ofwater to move with the ship and create turbulence thus dampening the

motion and causing an increase in period and reduction in amplitude.


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Although relatively small in dimension the bilge keels have large

levers about the rolling axis and the forces on them produce a large

moment opposing the rolling. They can produce a reduction in rollamplitude of more than a third. Their effect is generally enhanced by

ahead speed. They are aligned with the flow of water past the hull instill water to reduce their drag in that state. When the ship is rolling

the drag will increase and slow the ship a little.


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Passive tanks

These use the movement of water in specially designed tanks to opposethe rolling motion. The tank is U-shaped and water moves from one side

to the other and then back as the ship inclines first one way and then

the other. Because of the throttling effect of the relatively narrow lowerlimb of the U joining the two sides of the tank, the movement of

water can be made to lag behind the ship movements. By adjusting the

throttling, that is by tuning the tank, a lag approaching 90 can beachieved. Unfortunately the tank can only be tuned for one frequency

of motion. This is chosen to be the ships natural period of roll as thisis the period at which really large motions can occur. The tank will stabilize

the ship at zero speed but the effect of the tanks free surface onstability must be allowed for.

Active fins

This is the most common of the active systems. One or more pairs of

stabilizing fins are fitted. They are caused to move by an actuating systemin response to signals based on a gyroscopic measurement of roll

motions. They are relatively small although projecting out further than

the bilge keels. The whole fin may move or one part may be fixed and

the after section move. A flap on the trailing edge may be used toenhance the lift force generated. The fins may permanently protrude

from the bilge or may, at the expense of some complication, beretractable,


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The lift force on the fin is proportional to the square of the ships

speed. At low speed they will have little effect although the control system

can adjust the amplitude of the fin movement to take account of speed,using larger fin angles at low speed.


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Active tanks

This is similar in principle to the passive tank system but the movementof water is controlled by pumps or by the air pressure above the water

surface. The tanks either side of the ship may be connected by a lowerlimb or two separate tanks can be used. Figure shows a system in

which the air pressure above the water on the two sides is controlled to

tune the system. The air duct contains valves operated by a roll sensingdevice. The system can be tuned for more than one frequency. As with

the passive system it can stabilize at zero ship speed. It does not requireany projections outside the hull.

The capacity of the stabilization system is usually quoted in terms of

the steady heel angle it can produce with the ship underway in stillwater. This is then checked during trials. It is possible to use modern

theories to specify performance in waves but this would be difficult to

check contractually


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SUMMARY

It has been shown that a ships motions in irregular ocean waves can be

synthesized from its motions in regular waves. The energy spectrum has

been shown to be a powerful tool in the study of motions as it was in thestudy of waves. Factors limiting a ships sea keeping capabilities, including

the degradation of human performance, have been discussed and ithas been seen how they can be combined to give an overall assessment

of the probability that a ship will be able to undertake its intended mission.

Means of limiting motions by stabilization have been outlined.

It has only been possible to deal with the subject in an elementaryway.

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