Countermeasures against various stresses :
It is the shipowner’s responsibility to ensure that his vessel is built to a standard
high enough to withstand all the stresses she may be expected to
encounter. By their very nature ships are called upon to carry heavy loads,
and considerable thought and experience is required to load heavy weights
without causing structural damage to the vessel.
Cargo ship unstable in a seaway
Heavy weights tend to cause a downward deflection of the deck area supporting
the load .This subsequently produces stresses, with
consequent inward and outward deflections of supporting bulkheads,
depending on the position of initial loading.These stresses are generally of a
localised nature, in the neighbourhood of built-in structures such as windlasses,
accommodation blocks etc. and increased scantlings are the norm to
prevent excessive distortion.The shipping of heavy seas may add to the load
and aggravate the situation, causing unacceptable, excessive distortions.
Another form of stress comes from the water surrounding the ship,
which exerts considerable pressure over the bottom and side areas of the
shell plating .The pressure will increase with depth of immersion,
i.e. the pressure on the bottom shell plates will exceed that on the
side shell plates.
Water pressure does not maintain a constant value, and will vary when the
vessel is in a seaway, especially when a heavy swell is present. Fluctuations in
water pressure tend to cause an ‘in and out’ movement of the shell plating,
with more noticeable effects at the extreme ends of the vessel.The effect of
water pressure is usually more prominent at the fore end of the vessel than
the after end.The general effect is accentuated by the pitching motion of the
vessel and is termed ‘panting’.
Panting beams, which are substantial metal
beams running from port to starboard, in the forepart of the vessel.They
are positioned forward of the collision bulkhead to resist the in and out
motion of the shell plating either side of the fore and aft line. Situated at
various deck levels, panting beams form a combination with panting
stringers on either side in the forepart of the vessel.
Ships shearing stress
A third form of stress is shearing stress in a material, which tends to
move one part of the material relative to another.
The two forces of gravity and buoyancy acting in opposition causes
the shearing stress to be experienced at various points in ship. Shearing
forces are undesirable within a ship in any shape or form, and prudent
loading, together with careful ballast distribution, can reduce them.
Values of stresses incurred during the loading period may be mathematically
worked out and then plotted to show the areas of stress by graph.
It is worth noting that the mathematical calculations are lengthy, and
always leave the possibility for error.‘Stress finders’ and computerised loadicators
have reduced the risk of errors. There are several on the market,
generally custom made for individual vessels, and they provide the operator
with such items of information as:
Bending moment.
Shear stress at critical points.
Mean draught.
Trim of the vessel.
GM final, after loading.
Deadweight.
The information supplied depends on the make and type of software
employed.
What is hogging stress
Hogging stress
The length of the vessel may be considered to act like a long girder
pivoted on a wave about its centre. In this position the fore and after ends
of the vessel will bend downwards, causing compression forces in the keel
area and tension forces at the upper deck level.
The condition is brought about by increased buoyancy forces being created
at and around the midships point of the vessel. Increased gravitational force, due to the metal structure of the vessel acting vertically downward,
occurs at the extremities of the ship.When both forces exist at the same
time, e.g. as the vessel is pivoted by a wave midships, a ‘hogging’ condition
is present.
This can be accentuated in a vessel of an all-aft design, where the additional
weight of the machinery space would produce high loading in the
aft part of the vessel.The condition may also be unnecessarily increased by
‘bad’ cargo loading in the fore and after parts of the vessel, leaving the
midships area comparatively lightly loaded.
What is sagging stress
sagging stress
‘Sagging’ is the direct opposite of hogging. When a vessel is supported at
bow and stern by wave crests, she will tend to sag in the middle. High buoyancy
forces occur at the extremities of the ship. High gravitational forces,
from the weight of the ship’s structure, act vertically down about the midship’s
point, in opposition to the buoyancy forces. In comparison with the
condition of hogging, the vessel has a tendency to bend in the opposite
direction .
Incorrect loading of the vessel or design characteristics may accentuate
the condition of sagging.Watchkeeping officers should be aware of the frequency
of the waves and the likelihood of this condition developing and, if
necessary, take action to relieve any sagging or hogging conditions by altering
the ship’s course.
Due consideration at the time of loading, with regard to weight distribution
may alleviate either hogging or sagging.With shipbuilding producing
larger and longer ships either condition is most undesirable, as the
prospect of breaking the ship’s back in a heavy seaway or swell becomes a
frightening reality. Prudent ballast arrangements, together with increased
scantlings at the time of building, coupled with efficient ship and cargo
loading, will help minimise any structural damage at a later stage due to
hogging or sagging.
Ropes are made of short fibres that are spun into yarns,
which are then made into flat or twisted strands.
And the strands are spun or braided to make the finished
rope . More .....
Although natural fibre ropes are still widely used throughout the marine industry, they have been superseded by synthetic fibres for a great many
purposes. Not only do the majority of synthetic ropes have greater strength
than their natural fibre counterparts, but they are more easily obtainable
and at present considerably cheaper. More .....
All natural fibre rope is manufactured from manilla, sisal, hemp, coir, cotton
or flax fibres.The process of manufacture consists of twisting the fibres
into yarns and turning the yarns in an opposite direction to establish the
strands. More .....
The lay of rope is a term used to describe the nature of the twist that produces the complete rope .The most common form of rope at sea is known as ‘hawser laid rope’ comprising three strands laid up right- or lefthanded. More .....
Heavy weights tend to cause a downward deflection of the deck area supporting the load .This subsequently produces stresses, with
consequent inward and outward deflections of supporting bulkheads,
depending on the position of initial loading . More .....
Prior approaching an area for anchoring ships master should investigate fully a suitable anchoring position and conduct a planned approach including speed reduction in ample time and orienting the ships head prior anchoring to same as similar sized vessels around or stem the tide or wind whichever is stronger . Final decision to be made on method of anchoring to be used , the number of shackles , the depth of water, expected weather and holding ground. . More .....
Hatch covers are used to close off the hatch opening and make it
watertight. Wooden hatch covers, consisting of beams and boards over
the opening and covered with tarpaulins, were once used but are no
longer fitted. Steel hatch covers, comprising a number of linked steel
covers, are now fitted universally. Various designs exist for particular
applications, but most offer simple and quick opening and closing,
which speed up the cargo handling operation.. More .....
The access shall be separate from the hatchway opening, and shall be by a stairway if possible. A fixed ladder, or a line of fixed rungs, shall have no point where they fill a reverse slope
. More .....
Washing is always carried out after the compartment has been swept. Drying time for washed compartments must be allowed for, before loading the next cargo; this time will vary with the climate, but two to three days must be expected. More .....
The vessel's structure, crane, derrick or other lifting device and the supporting structure should be of sufficient strength to withstand the loads
that will be imposed when operating at its maximum load moment . More .....
When there is any suspicion that any appliance or item of
equipment may have been subjected to excessive loads, exceeding the Safe
Working Load (SWL), or subjected to treatment likely to cause damage, it
should be taken out of service until it can be subjected to a thorough
examination by a competent person. More .....
Derricks for lifting cargo on board is required to be of adequate strength and stability for each load, having regard in particular to the stress induced at its mounting or fixing points , securely anchored, adequately ballasted or counterbalanced and supported by outriggers
as necessary to ensure its stability when lifting. More .....
Deck cranes have a number of advantages, the rigging
time being negligible, and the crane is able to pick up and land permitted
loads anywhere within its working radius. The safe working loads of cranes
is generally of the order of 10 to 15 tonnes and larger cranes are available
capable of lifts from 30 to 40 tonnes.. More .....
Paint consists of pigment dispersed in a liquid referred to as the ‘vehicle’.
When spread out thinly the vehicle changes in time to an adherent dry film.
The drying may take place through one of the following processes.. More .....
It is often assumed that all paint coatings prevent attack on the metal
covered simply by excluding the corrosive agency, whether air or water. This
is often the main and sometimes the only form of protection; however there
are many paints which afford protection even though they present a porous
surface or contain various discontinuities. . More .....
classification societies publish rules and regulations which are principally concerned with the strength of the ship, the provision of adequate equipment, and the reliability of the machinery . More .....
To maintain the assigned class all steel ships are required to be surveyed and examined by the Society’s
surveyors at regular periods.
The major hull items to be examined at these surveys only are discussed
here.. More .....
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