Unit-5-Draught-Surveying

112 Pages·11.912 MB·English

by  International Institute of Maritime Surveying

HNC and HND in
Marine Surveying
UNIT 5
Draught
Surveying
UNIT 5 |
Draught Surveying
Version 1
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Draught Surveying
CONTENTS
page
1. INTRODUCTION ............................................................................................................................. 5
How is the weight of a ships cargo determined
What is draught surveying?
2. LAWS OF FLOATATION ............................................................................................................. 7
Archimedes Principle
Density
Reading the Hydrometer
Sampling the surrounding water
3. DRAUGHTS ......................................................................................................................................... 16
Draught Marks
Reading the draught marks
Ships without midships marks
Plimsoll Line
Readings with Waves and swell
Draught Reading Equipment
Manometer
4. HYDROSTATIC PARTICULARS (STABILITY BOOK) .............................................. 29
The Stability Book
Example Hydrostatic pages
Reading hydrostatics
5. CORRECTING THE DRAUGHTS ........................................................................................... 37
Perpendicular Corrections
Perpendicular Correction
6. HULL DEFORMATION (HOG AND SAG) ........................................................................ 45
Hog and Sag
Two Thirds mean draught
3/4 mean Draught
Deflection Correction
7. CORRECTIONS FOR TRIM AND DENSITY ................................................................... 55
First Trim Correction, (Layer Correction) Sometimes called “A” correction
Second Trim Correction, (Nemoto’s Correction) Sometimes called “B” correction
List Correction
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Applying the trim corrections
Alternative Hydrostatic Information
Trim Factors
Combined Trimmed Hydrostatics
Final Net weight
“Constant”
8. BALLAST, FRESH WATER, FUEL, AND OTHERS ...................................................... 72
Variable Quantities
Sounding of Ballast and Fresh Water Tanks
Calculating Volumes
Weight of the ballast
Refractometers
Sounding Problems
Fresh Water
Fuel and Oil
Stores, Spares, Swimming pool, anchors and cables etc.
Ship Construction
9. PRACTICAL PROCEDURE ......................................................................................................... 92
APPENDIX 1 ............................................................................................................................................. 95
A ten-step draft survey
APPENDIX 2 ............................................................................................................................................. 101
Simple Wave Damping Tube
APPENDIX 3 ............................................................................................................................................. 102
Density Sampler
APPENDIX 4 ............................................................................................................................................. 105
Survey Report sheets
Draught Survey Certificate
Sounding Record
APPENDIX 5 ............................................................................................................................................. 107
Recommended further reading
APPENDIX 6 ............................................................................................................................................. 108
Additional information
Position of Lcf
Position of sounding pipes
Ballast table from base line
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Chapter 1. INTRODUCTION
1.1 Learning Outcome
At the end of this module students should be able to perform and calculate a draught survey. However
there is no substitute for experience in some of the operations needed to obtain the draughts and data
required for the calculation. It is strongly recommended that students spend as much time as possible
with an experienced surveyor in order to become familiar with the practical aspects of reading draughts,
sounding tanks, understanding the large variety of hydrostatic particulars and many other aspects of
the procedure.
NOTE – This module has been written for students who have not had seagoing experience
and are not familiar with ships and thus ex seafarers may find some of the content a bit
obvious. However it is hoped that they will still gain from carrying out the study.
How is the weight of a ships cargo determined
Many years ago, if it was necessary, the cargo on a ship was weighed using a balance of some description.
This is a basic device that is used to find the weight of something by “balancing” it with a known weight
at the opposite end of a balance beam that is resting on a knife-edge support. However, often cargo was
just loaded and declared as a package or carton containing “x” number of items and the weight was not
absolutely necessary.
Times have changed and nowadays cargo is mainly shipped in bulk, although there are exceptions, the main
one of which are containers and these are weighed on a weighbridge.
The modern system of weighing is based on the load cell, a device that converts force or strain on it into
an electrical signal. This signal will vary in direct relation to the strain (or weight) that it is subjected to.
The variation in the signal is translated into a weight readout.
However, bulk cargos need a different approach. This is not to say that bulk cargoes cannot be weighed
on a weighbridge but it would be a slow process to weigh and load a shipment of 40,000 tonnes of a
commodity in 25 or 50 tonne lots.
All parties involved in the venture want the vessel loaded as quickly as possible and alternative methods
of weighing the cargo are required. The most common ones in use today are belt weighers and draught
surveys.
The load cell is utilised in dynamic weighing systems commonly known as belt weighers or scales. These
are in line devices that are inserted into, or mounted on, the transfer belts often used to move bulk cargos
from stockpiles to the vessels holds via a bulk loader. The in line belt weigher assesses the weight of cargo
by continuously monitoring the weight passing on the belt and producing an average weight per time
interval which is translated into the weight of cargo loaded.
However there is also a large demand for the weight of cargo to be determined by calculating the weight
of the ship before and after loading by means of a draught survey. In some ports this may be the only way.
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What is draught surveying?
Draught surveying is a commercially acceptable form of weighing. It is based on Archimedes principle,
which states that anything that floats will displace an amount of the liquid it is floating in equal to its
own weight.
Briefly the weight of the ship is determined both before and after loading and allowances made for
differences in ballast water and other changeable items. The difference between these two net weights is
the weight of the cargo.
In order to do this the depth that the ship is floating at is assessed from the “draught marks” and then
the vessels stability book is consulted to obtain the hydrostatic particulars such as the “displacement” and
other necessary data.
Several corrections are required and the quantities of ballast and other consumable stores need to be
assessed so as to obtain the net weights as in the following diagram.
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Chapter 2. LAWS OF FLOATATION
Learning Outcome
At the end of this chapter you will understand what Archimedes Principle is and how it affects the
calculation of the weight of a ship. You should also understand what density is and how it is used to
determine the weight of a floating body.
Archimedes Principle
Archimedes Principle states that when a body is wholly or partially immersed in a fluid it appears to suffer
a loss in mass equal to the mass of fluid it displaces.
M ass is the amount of matter that a body contains and is expressed in kilograms and tonnes. The weight of
a body is its mass multiplied by the force due to gravity (9.81m/s2). However for the purposes of draught
surveying the two can be considered to be the same, as the effects of variations in the force of gravity can be
ignored and weight can therefore be assumed to be the same as mass.
The mass (weight) of a cubic meter of fresh water is 1000kgs.
If a solid block of volume 1m3 and mass 4,000kgs is immersed in fresh water it will appear to suffer a
loss in mass of 1000kgs. This can be verified by suspending it from a spring balance, which would indicate
a mass of 3000kgs. As the mass of the block has not changed there must be a supporting force acting
upwards that, in this case, is 1000kgs. This is known as the buoyancy force. The volume of water displaced
by the block is obviously 1m3, as this is the volume of the block and, as stated above, 1m3 of fresh water
has a mass of 1000kgs, which is the buoyancy force. We can see then, that the buoyancy force is equal to
the mass of water displaced.
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If we now take the same solid block and hollow it out until its mass is reduced to 500kgs and then
immerse it in the same fresh water it will now float. This is because it still has the same volume of 1m3 but
its mass is now only 500kgs.
If the block is completely immersed, the buoyancy force will still be 1000kgs as before, because the volume
of water displaced is still the same at 1m3. However the mass acting downwards is now only 500kgs
and, once released, the block will rise until the buoyancy force acting upwards is equal to the mass acting
downwards. This will be when the block is in equilibrium at a point when the underwater volume is equal
to 0.5m3, which is half the depth of the block and the point at which the mass of water displaced is equal
to 500kgs. A spring balance will now indicate zero mass or weight indicating that the block is floating.
From the above we can see that the weight of a ship can be calculated from the underwater volume, if it is
known, and the density of the liquid in which it is floating.
Why volume? Well in the above explanation of Archimedes Principle the block was immersed in fresh
water.
However, had it been salt water the volume of the part of the block that was underwater would have been
less as the density of salt water is greater than that of fresh water, meaning for equal volumes the salt
water is heavier, and thus a lesser volume of it would need to have been displaced for the block to float.
In order calculate this volume we need to know how deep the ship is floating in the water as the deeper
the “draught”, as it is called, the greater the weight of the ship. We also need to know the density of the
water that the ship is floating in and this needs to be measured at the same time as the draughts are read.
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Density
Density is Mass per unit volume at a certain temperature. As already stated mass can be considered the
same as weight as far as draught surveying is concerned.
Therefore the weight of the block above is its underwater volume multiplied by the density of the liquid
that it is floating in. Conversely as the weight of the block is 500kgs (0.5 tonnes) and its underwater
volume 0.5m3 then the density of the water it is floating in is 1.000 t/m3 (Weight / volume) The density
of a substance can be determined by weighing a unit volume, which in the case of the metric system is a
cubic metre.
Weight in Air
If a quantity of liquid - for example, fresh water or sea water - is weighed on a balance or on a weighbridge
against the equivalent of brass weights, then the atmosphere will exercise an upward thrust upon the
water much greater than the upward thrust exercised on the smaller volume of brass weights. This
‘air buoyancy correction’ amounts to 0.0011 grams per ml of product. This correction represents the
difference between the air buoyancy effect on the liquid and that on the brass weights. The air buoyancy
effect is in fact the same as the buoyancy force for a body immersed in a liquid as Archimedes explained,
only this time the fluid is air, which has a density of 0.00122t/m3.
It is commercial practice to make no allowance for air buoyancy so that commercial weights are normally
weights in air. Weight in air per unit volume is known as apparent density.
Weight in vacuum
If a quantity of liquid - for example, fresh water or seawater - is weighed on a laboratory balance or
weighbridge and a correction made for the effect of air buoyancy, then for most practical purposes the
resulting ‘weight in vacuo’ is equivalent to mass. Mass per unit volume is known as density, or ‘true
density’, and is usually expressed in g/ml or kg/ml3.
Measuring the density
Density is measured using an instrument called a hydrometer. This should be one designed for draught
survey purposes and as such will be calibrated to read density in air (apparent density) because the ship is
in air and cannot be placed in a vacuum.
In draught survey work the hydrometer may be regarded as the apparatus used to weigh cargoes of up
to 200,000 tonnes or even more. In these circumstances it should be obvious that great care should be
exercised in choosing the right type of instrument and in checking its accuracy. The time has long passed
when a brass load line hydrometer, possibly corroded and maybe deformed, is acceptable for draught
survey purposes.
Hydrometers for draught survey purposes should be made of glass as they can be certified by an official
standardising body. Metal hydrometers cannot usually be certified because they may corrode or deform
after they have left the manufacturers’ premises.
The reading of a hydrometer is affected by the surface tension of the liquid in which it is used. High
surface tension liquids will tend to pull the instrument more deeply into the liquid than will liquids of low
surface tension. In practical terms if a hydrometer calibrated for use in petroleum is used in seawater, then
the hydrometer reading will be too low by an amount depending on the size and shape of the instrument.
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Hydrometers used for draught survey purposes should have some form of certificate. The hydrometer scale
should be numbered so that the number may be quoted in the certificate. Certificates are of two kinds:-
1 A certificate of conformity, which is issued by the manufacturer and which certifies that the
instrument is accurate to within a specified maximum limit. Usually +/- 0.0005 kg/m3
2 A correction certificate issued either by the manufacturer or by some official body such as the
British Standards Institution, giving corrections to be applied to the scale readings at, for example,
four or five different points of the scale.
The Zeal Draught Survey Hydrometer has become internationally recognised and surveyors are
strongly recommended to use one for their work. This instrument is graduated to read apparent density
and the scale is of sufficient length to be marked every 0.0005kgs/m3 from 0.990 to 1.040.
An instrument with official correction certificates should be available in the office so that replacement
hydrometers can be checked for any errors or in the case of a dispute.
Although the use of a hydrometer such as the Zeal draught survey hydrometer will avoid many errors and
misunderstandings, which occurred in the past, this does not mean that other types of hydrometer cannot
be used, but they must be acceptably accurate and the necessary corrections applied.
Such hydrometers should be made of glass, should be adjusted for the correct surface tension, should
cover the range of about 0.990 to 1.040 and should preferably have a scale length of minimum 125-mm.
Reading the Hydrometer
The instructions issued with a Zeal Draught Survey Hydrometer are included here and should be self
explanatory. Please note the instruction regarding the actual water level and not the top of the meniscus.
Also note that temperature corrections should not be made.
Why not make corrections for temperature?
A draught survey hydrometer is made of glass and will be designed to be most accurate at a particular
temperature. If it is used in water of a higher temperature it will expand slightly and float higher in the
water indicating a greater density.
At the same time the ship will also expand and its underwater volume will therefore increase. As its
weight has not changed it will float slightly higher in the water also to compensate. The cubical expansion
of glass is 0.0000250C and that of steel is 0.0000330C. These two expansion rates are very close together
and the effect of the expansion of both the ship and the hydrometer is to cancel each other out.
Example
A ship of 60,000m3 displacement (volume) is floating in water, which is at a temperature of 15oC.
A hydrometer designed for 150C is reading 1.015 kg/l
Then the weight of the ship is 60,000 x 1.0150 = 60,900 metric tonnes.
Now the same ship at the same volume of displacement is moved to water at 30oC
The total weight of the ship remains unchanged at 60,900 tonnes.
When the glass hydrometer is placed in the water at 300C it will expand and float slightly higher in the water.
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