ASTM F1321-92(2008)
(Guide)Standard Guide for Conducting a Stability Test (Lightweight Survey and Inclining Experiment) to Determine the Light Ship Displacement and Centers of Gravity of a Vessel
Standard Guide for Conducting a Stability Test (Lightweight Survey and Inclining Experiment) to Determine the Light Ship Displacement and Centers of Gravity of a Vessel
SIGNIFICANCE AND USE
From the light ship characteristics one is able to calculate the stability characteristics of the vessel for all conditions of loading and thereby determine whether the vessel satisfies the applicable stability criteria. Accurate results from a stability test may in some cases determine the future survival of the vessel and its crew, so the accuracy with which the test is conducted cannot be overemphasized. The condition of the vessel and the environment during the test is rarely ideal and consequently, the stability test is infrequently conducted exactly as planned. If the vessel is not 100 % complete and the weather is not perfect, there ends up being water or shipyard trash in a tank that was supposed to be clean and dry and so forth, then the person in charge must make immediate decisions as to the acceptability of variances from the plan. A complete understanding of the principles behind the stability test and a knowledge of the factors that affect the results is necessary.
SCOPE
1.1 This guide covers the determination of a vessel's light ship characteristics. The stability test can be considered to be two separate tasks; the lightweight survey and the inclining experiment. The stability test is required for most vessels upon their completion and after major conversions. It is normally conducted inshore in calm weather conditions and usually requires the vessel be taken out of service to prepare for and conduct the stability test. The three light ship characteristics determined from the stability test for conventional (symmetrical) ships are displacement (“displ”), longitudinal center of gravity (“LCG”), and the vertical center of gravity (“KG”). The transverse center of gravity (“TCG”) may also be determined for mobile offshore drilling units (MODUs) and other vessels which are asymmetrical about the centerline or whose internal arrangement or outfitting is such that an inherent list may develop from off-center weight. Because of their nature, other special considerations not specifically addressed in this guide may be necessary for some MODUs.
1.2 This standard does not purport to address the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: F1321 − 92(Reapproved 2008) An American National Standard
Standard Guide for
Conducting a Stability Test (Lightweight Survey and
Inclining Experiment) to Determine the Light Ship
Displacement and Centers of Gravity of a Vessel
This standard is issued under the fixed designation F1321; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the Department of Defense.
INTRODUCTION
This guide provides the marine industry with a basic understanding of the various aspects of a
stability test. It contains procedures for conducting a stability test to ensure that valid results are
obtained with maximum precision at a minimal cost to owners, shipyards, and the government. This
guide is not intended to instruct a person in the actual calculation of the light ship displacement and
centersofgravity,butrathertobeaguidetothenecessaryprocedurestobefollowedtogatheraccurate
dataforuseinthecalculationofthelightshipcharacteristics.Acompleteunderstandingofthecorrect
procedures used to perform a stability test is imperative to ensure that the test is conducted properly
and so that results can be examined for accuracy as the inclining experiment is conducted. It is
recommended that these procedures be used on all vessels and marine craft.
1. Scope of the user of this standard to establish appropriate safety and
health practices and determine the applicability of regulatory
1.1 This guide covers the determination of a vessel’s light
limitations prior to use.
ship characteristics. The stability test can be considered to be
two separate tasks; the lightweight survey and the inclining
2. Terminology
experiment.The stability test is required for most vessels upon
2.1 Definitions:
their completion and after major conversions. It is normally
2.1.1 inclining experiment—involves moving a series of
conducted inshore in calm weather conditions and usually
known weights, normally in the transverse direction, and then
requires the vessel be taken out of service to prepare for and
measuringtheresultingchangeintheequilibriumheelangleof
conduct the stability test. The three light ship characteristics
the vessel. By using this information and applying basic naval
determined from the stability test for conventional (symmetri-
architecture principles, the vessel’s vertical center of gravity
cal) ships are displacement (“displ”), longitudinal center of
KG is determined.
gravity(“LCG”),andtheverticalcenterofgravity(“KG”).The
2.1.2 light ship—a vessel in the light ship condition (“Con-
transverse center of gravity (“TCG”) may also be determined
dition I”) is a vessel complete in all respects, but without
for mobile offshore drilling units (MODUs) and other vessels
consumables, stores, cargo, crew and effects, and without any
which are asymmetrical about the centerline or whose internal
liquids on board except that machinery fluids, such as lubri-
arrangement or outfitting is such that an inherent list may
cants and hydraulics, are at operating levels.
develop from off-center weight. Because of their nature, other
special considerations not specifically addressed in this guide 2.1.3 lightweight survey—this task involves taking an audit
may be necessary for some MODUs.
ofallitemswhichmustbeadded,deducted,orrelocatedonthe
vessel at the time of the stability test so that the observed
1.2 This standard does not purport to address the safety
condition of the vessel can be adjusted to the light ship
concerns, if any, associated with its use. It is the responsibility
condition. The weight, longitudinal, transverse, and vertical
location of each item must be accurately determined and
recorded. Using this information, the static waterline of the
This guide is under the jurisdiction of ASTM Committee F25 on Ships and
Marine Technology and is the direct responsibility of Subcommittee F25.01 on
ship at the time of the stability test as determined from
Structures.
measuring the freeboard or verified draft marks of the vessel,
Current edition approved Nov. 1, 2008. Published December 2008. Originally
thevessel’shydrostaticdata,andtheseawaterdensity;thelight
approved in 1990. Last previous edition approved in 2004 as F1321–92 (2004).
DOI: 10.1520/F1321-92R08. ship displacement and longitudinal center of gravity can be
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F1321 − 92 (2008)
obtained. The transverse center of gravity may also be would result from a significant change in the waterplane area
calculated, if necessary. during heeling, is not introduced into the stability calculations.
4.2 Metacentric Height—The vertical distance between the
3. Significance and Use
center of gravity (“G”) and M is called the metacentric height
3.1 From the light ship characteristics one is able to calcu- (“GM”). At small angles of heel, GM is equal to the initial
late the stability characteristics of the vessel for all conditions
slope of the righting arm (“GZ”) curve and is calculated using
of loading and thereby determine whether the vessel satisfies
the relationship, GZ = GM sin θ. GM is a measure of vessel
theapplicablestabilitycriteria.Accurateresultsfromastability
stability that can be calculated during an inclining experiment.
test may in some cases determine the future survival of the
As shown in Fig. 2, moving a weight (“W”) across the deck a
vessel and its crew, so the accuracy with which the test is
distance (“x”) will cause a shift in the overall center of gravity
conducted cannot be overemphasized. The condition of the
(G–G`) of the vessel equal to (W)(x)/displ and parallel to the
vessel and the environment during the test is rarely ideal and
movementofW.Thevesselwillheelovertoanewequilibrium
consequently, the stability test is infrequently conducted ex-
heel angle where the new center of buoyancy, B', will once
actly as planned. If the vessel is not 100% complete and the
againbedirectlyunderthenewcenterofgravity(G`).Because
weather is not perfect, there ends up being water or shipyard
the angle of inclination during the inclining experiment is
trash in a tank that was supposed to be clean and dry and so small,theshiftinGcanbeapproximatedbyGMtan θandthen
forth, then the person in charge must make immediate deci-
equated to (W)(x)/displ . Rearranging this equation slightly
sions as to the acceptability of variances from the plan. A results in the following equation:
complete understanding of the principles behind the stability
~W!~x!
GM 5 (1)
test and a knowledge of the factors that affect the results is
displ tan θ
~ !~ !
necessary.
SinceGManddisplremainconstantthroughouttheinclining
4. Theory experiment the ratio ( W)(x)/tan θ will be a constant. By
carefully planning a series of weight movements, a plot of
4.1 The Metacenter—(See Fig. 1). The transverse metacen-
tangents is made at the appropriate moments. The ratio is
ter (“M”) is based on the hull form of a vessel and is the point
measured as the slope of the best represented straight line
around which the vessel’s center of buoyancy (“B”) swings for
drawn through the plotted points as shown in Fig. 3, where
small angles of inclination (0° to 4° unless there are abrupt
three angle indicating devices have been used. This line does
changes in the shape of the hull).The location of B is fixed for
not necessarily pass through the origin or any other particular
any draft, trim, and heel, but it shifts appreciably as heel
point, for no single point is more significant than any other
increases. The location of B shifts off the centerline for small
point. A linear regression analysis is often used to fit the
anglesofinclination(“θ”),butitsheightabovethemoldedkeel
straight line.
(“K”) will stay essentially the same. The location of M, on the
4.3 Calculating the Height of the Center of Gravity Above
other hand, is essentially fixed over a range of heeling angles
the Keel—KM is known for the draft and trim of the vessel
up to about 4°, as the ship is inclined at constant displacement
during the stability test. The metacentric height, GM,as
and trim. The height of M above K, known as “KM”, is often
calculated above, is determined from the inclining experiment.
plotted versus draft as one of the vessel’s curves of form.As a
The difference between the height KM and the distance GM is
general “rule of thumb,” if the difference from the design trim
ofthevesselislessthan1%ofitslength,the KMcanbetaken the height of the center of gravity above the keel, KG. See Fig.
4.
directly from either the vessel’s curves of form or hydrostatic
tables. Because KM varies with trim, the KM must be com-
4.4 Measuring the Angle of Inclination— (See Fig. 5.) Each
puted using the trim of the ship at the time of the stability test
time an inclining weight, W, is shifted a distance, x, the vessel
whenthedifferencefromthedesigntrimofthevesselisgreater
will settle to some equilibrium heel angle, θ. To measure this
than 1% of its length. Caution should be exercised when
angle, θ, accurately, pendulums or other precise instruments
applying the “rule of thumb” to ensure that excessive error, as
FIG. 1 Movement of the Center of Buoyancy FIG. 2 Metacentric Height
F1321 − 92 (2008)
batten and (“Z”) is the distance the wire deflects from the
reference position at the point along the pendulum length
where transverse deflections are measured. Tangent θ is then
calculated:
tan θ 5Z/Y (2)
Plotting all of the readings for each of the pendulums during
the inclining experiment aids in the discovery of bad readings.
Since ( W)(x)/tan θ should be constant, the plotted line should
be straight. Deviations from a straight line are an indication
that there were other moments acting on the vessel during the
inclining. These other moments must be identified, the cause
corrected, and the weight movements repeated until a straight
line is achieved. Figs. 6-9 illustrate examples of how to detect
some of these other moments during the inclining and a
recommended solution for each case. For simplicity, only the
average of the readings is shown on the inclining plots.
4.5 Free Surface—During the stability test, the inclining of
thevesselshouldresultsolelyfromthemovingoftheinclining
weights. It should not be inhibited or exaggerated by unknown
moments or the shifting of liquids on board. However, some
liquids will be aboard the vessel in slack tanks so a discussion
of “free surface” is appropriate.
4.5.1 Standing Water on Deck—Decks should be free of
FIG. 3 A Typical Incline Plot
water.Watertrappedondeckmayshiftandpocketinafashion
similar to liquids in a tank.
4.5.2 Tankage During the Inclining—If there are liquids on
board the vessel when it is inclined, whether in the bilges or in
the tanks, it will shift to the low side when the vessel heels.
This shift of liquids will exaggerate the heel of the vessel.
Unless the exact weight and distance of liquid shifted can be
precisely calculated, the GM from Eq 1 will be in error. Free
FIG. 4 Relationship betweenGM,KM, andKG
FIG. 5 Measuring the Angle of Inclination
areusedonthevessel.Whenpendulumsareused,thetwosides
NOTE 1—Recheck all tanks and voids and pump out as necessary; redo
of the triangle defined by the pendulum are measured. (“Y”) is
all weight movements and recheck freeboard and draft readings.
the length of the pendulum wire from the pivot point to the FIG. 6 Excessive Free Liquids
F1321 − 92 (2008)
NOTE 1—Redo Weight Movements 1 and 5.
NOTE 1—Take water soundings and check lines; redo Weight Move-
ments 2 and 3. FIG. 9 Gusty Wind From Port Side
FIG. 7 Vessel Touching Bottom or Restrained by Mooring Lines
rectangular, trapezoidal, and so forth) when viewed from
above, so that the free surface moment of the liquid can be
accurately determined. The free surface moment of the liquid
in a tank with parallel vertical sides can be readily calculated
by the equation:
M 5 lb /12Q (3)
fs
where:
M = free surface moment, ft-Ltons
fs
l = length of tank, ft,
b = breadth of tank, ft,
Q = specific volume of liquid in tank (ft /ton), and
(See Annex A3 for liquid conversions or measure Q
directly with a hydrometer.)
Lton = long ton of 2240 lbs.
Free surface correction is independent of the height of the
tank in the ship, location of the tank, and direction of heel.
4.5.3 As the width of the tank increases, the value of free
surface moment increases by the third power. The distance
available for the liquid to shift is the predominant factor. This
is why even the smallest amount of liquid in the bottom of a
wide tank or bilge is normally unacceptable and should be
FIG. 8 Steady Wind From Port Side Came Up After Initial Zero
removed before the inclining experiment. Insignificant
Point Taken (Plot Acceptable)
amounts of liquids in V-shaped tanks or voids (for example, a
chainlockerinthebow),wherethepotentialshiftisnegligible,
mayremainifremovaloftheliquidwouldbedifficultorwould
surfaceshouldbeminimizedbyemptyingthetankscompletely
cause extensive delays.
and making sure all bilges are dry or by completely filling the
tanks so that no shift of liquid is possible. The latter method is
5. Preparations for the Stability Test
not the optimum because air pockets are difficult to remove
frombetweenstructuralmembersofatank,andtheweightand 5.1 General Condition of the Vessel—Avessel should be as
centeroftheliquidinafulltankmustbeaccuratelydetermined complete as possible at the time of the stability test. Schedule
toadjustthelightshipvaluesaccordingly.Whentanksmustbe the test to minimize the disruption in the vessel’s delivery date
left slack, it is desirable that the sides of the tanks be parallel or its operational commitments. The amount and type of work
vertical planes and the tanks be regular in shape (that is, left to be completed (weights to be added) affects the accuracy
F1321 − 92 (2008)
of the light ship characteristics, so good judgment must be
used. If the weight or center of gravity of an item to be added
cannot be determined with confidence, it is best to conduct the
stability test after the item is added. Temporary material, tool
boxes,staging,trash,sand,debris,andsoforthonboardshould
be reduced to absolute minimum during the stability test.
5.2 Tankage—Include the anticipated liquid loading for the
test in the planning for the test. Preferably, all tanks should be
empty and clean or complete
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