ASTM F1321-92(2004)
(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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An American National Standard
Designation:F 1321–92 (Reapproved 2004)
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 F 1321; 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 (e) 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 1.2 This standard does not purport to address the safety
concerns, if any, associated with its use. It is the responsibility
1.1 This guide covers the determination of a vessel’s light
of the user of this standard to establish appropriate safety and
ship characteristics. The stability test can be considered to be
health practices and determine the applicability of regulatory
two separate tasks; the lightweight survey and the inclining
limitations prior to use.
experiment.The stability test is required for most vessels upon
their completion and after major conversions. It is normally
2. Terminology
conducted inshore in calm weather conditions and usually
2.1 Definitions:
requires the vessel be taken out of service to prepare for and
2.1.1 inclining experiment—involves moving a series of
conduct the stability test. The three light ship characteristics
known weights, normally in the transverse direction, and then
determined from the stability test for conventional (symmetri-
measuringtheresultingchangeintheequilibriumheelangleof
cal) ships are displacement (“displ”), longitudinal center of
the vessel. By using this information and applying basic naval
gravity(“LCG”),andtheverticalcenterofgravity(“KG”).The
architecture principles, the vessel’s vertical center of gravity
transverse center of gravity (“TCG”) may also be determined
KG is determined.
for mobile offshore drilling units (MODUs) and other vessels
2.1.2 light ship—a vessel in the light ship condition (“Con-
which are asymmetrical about the centerline or whose internal
dition I”) is a vessel complete in all respects, but without
arrangement or outfitting is such that an inherent list may
consumables, stores, cargo, crew and effects, and without any
develop from off-center weight. Because of their nature, other
liquids on board except that machinery fluids, such as lubri-
special considerations not specifically addressed in this guide
cants and hydraulics, are at operating levels.
may be necessary for some MODUs.
2.1.3 lightweight survey—thistaskinvolvestakinganaudit
ofallitemswhichmustbeadded,deducted,orrelocatedonthe
vessel at the time of the stability test so that the observed
This guide is under the jurisdiction of ASTM Committee F25 on Ships and
condition of the vessel can be adjusted to the light ship
Marine Technology and is the direct responsibility of Subcommittee F25.01 on
condition. The weight, longitudinal, transverse, and vertical
Structures.
location of each item must be accurately determined and
CurrenteditionapprovedJuly1,2004.PublishedJuly2004.Originallyapproved
in 1990. Last previous edition approved in 1992 as F1321–92. recorded. Using this information, the static waterline of the
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
F 1321–92 (2004)
ship at the time of the stability test as determined from vessel is greater than 1% of its length. Caution should be
measuring the freeboard or verified draft marks of the vessel, exercised when applying the “rule of thumb” to ensure that
thevessel’shydrostaticdata,andtheseawaterdensity;thelight excessiveerror,aswouldresultfromasignificantchangeinthe
ship displacement and longitudinal center of gravity can be waterplane area during heeling, is not introduced into the
obtained. The transverse center of gravity may also be calcu- stability calculations.
lated, if necessary. 4.2 Metacentric Height—The vertical distance between the
center of gravity (“G”) and M is called the metacentric height
3. Significance and Use
(“GM”). At small angles of heel, GM is equal to the initial
3.1 From the light ship characteristics one is able to calcu- slope of the righting arm (“GZ”) curve and is calculated using
late the stability characteristics of the vessel for all conditions
the relationship, GZ = GM sin u. GM is a measure of vessel
of loading and thereby determine whether the vessel satisfies stability that can be calculated during an inclining experiment.
theapplicablestabilitycriteria.Accurateresultsfromastability
As shown in Fig. 2, moving a weight (“W”) across the deck a
test may in some cases determine the future survival of the distance (“x”) will cause a shift in the overall center of gravity
vessel and its crew, so the accuracy with which the test is
(G–G8) of the vessel equal to (W)(x)/displ and parallel to the
conducted cannot be overemphasized. The condition of the movementof W.Thevesselwillheelovertoanewequilibrium
vessel and the environment during the test is rarely ideal and heel angle where the new center of buoyancy, B8, will once
consequently, the stability test is infrequently conducted ex- againbedirectlyunderthenewcenterofgravity(G8).Because
actly as planned. If the vessel is not 100% complete and the the angle of inclination during the inclining experiment is
weather is not perfect, there ends up being water or shipyard small,theshiftinGcanbeapproximatedbyGMtan uandthen
trash in a tank that was supposed to be clean and dry and so equated to (W)(x)/displ. Rearranging this equation slightly
forth, then the person in charge must make immediate deci- results in the following equation:
sions as to the acceptability of variances from the plan. A
~W!~x!
GM 5 (1)
complete understanding of the principles behind the stability
~displ!~tan u!
test and a knowledge of the factors that affect the results is
SinceGManddisplremainconstantthroughouttheinclining
necessary.
experiment the ratio (W)(x)/tan u will be a constant. By
carefully planning a series of weight movements, a plot of
4. Theory
tangents is made at the appropriate moments. The ratio is
4.1 The Metacenter—(See Fig. 1). The transverse meta-
measured as the slope of the best represented straight line
center (“M”) is based on the hull form of a vessel and is the
drawn through the plotted points as shown in Fig. 3, where
point around which the vessel’s center of buoyancy (“B”)
three angle indicating devices have been used. This line does
swingsforsmallanglesofinclination(0°to4°unlessthereare
not necessarily pass through the origin or any other particular
abrupt changes in the shape of the hull). The location of B is
point, for no single point is more significant than any other
fixed for any draft, trim, and heel, but it shifts appreciably as
point. A linear regression analysis is often used to fit the
heel increases. The location of B shifts off the centerline for
straight line.
small angles of inclination (“u”), but its height above the
4.3 Calculating the Height of the Center of Gravity Above
molded keel (“K”) will stay essentially the same. The location
the Keel—KM is known for the draft and trim of the vessel
of M, on the other hand, is essentially fixed over a range of
during the stability test. The metacentric height, GM,as
heelinganglesuptoabout4°,astheshipisinclinedatconstant
calculated above, is determined from the inclining experiment.
displacement and trim. The height of M above K, known as
The difference between the height KM and the distance GM is
“KM”,isoftenplottedversusdraftasoneofthevessel’scurves
the height of the center of gravity above the keel, KG. See Fig.
ofform.Asageneral“ruleofthumb,”ifthedifferencefromthe
4.
design trim of the vessel is less than 1% of its length, the KM
4.4 Measuring the Angle of Inclination—(SeeFig.5.)Each
canbetakendirectlyfromeitherthevessel’scurvesofformor
time an inclining weight, W, is shifted a distance, x, the vessel
hydrostatic tables. Because KM varies with trim, the KM must
be computed using the trim of the ship at the time of the
stability test when the difference from the design trim of the
FIG. 1 Movement of the Center of Buoyancy FIG. 2 Metacentric Height
F 1321–92 (2004)
of the triangle defined by the pendulum are measured. (“Y”) is
the length of the pendulum wire from the pivot point to the
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 u is then
calculated:
tanu5 Z/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 u 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
FIG. 3 A Typical Incline Plot
of “free surface” is appropriate.
4.5.1 Standing Water on Deck—Decks should be free of
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.
FIG. 4 Relationship betweenGM,KM, andKG
FIG. 5 Measuring the Angle of Inclination
will settle to some equilibrium heel angle, u. To measure this
NOTE—Recheckalltanksandvoidsandpumpoutasnecessary;redoall
angle, u, accurately, pendulums or other precise instruments
weight movements and recheck freeboard and draft readings.
areusedonthevessel.Whenpendulumsareused,thetwosides FIG. 6 Excessive Free Liquids
F 1321–92 (2004)
NOTE—Redo Weight Movements 1 and 5.
NOTE—Take water soundings and check lines; redoWeight Movements
2 and 3. FIG. 9 Gusty Wind From Port Side
FIG. 7 Vessel Touching Bottom or Restrained by Mooring Lines
left slack, it is desirable that the sides of the tanks be parallel
vertical planes and the tanks be regular in shape (that is,
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
(SeeAnnexA3 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
FIG. 8 Steady Wind From Port Side Came Up After Initial Zero is why even the smallest amount of liquid in the bottom of a
Point Taken (Plot Acceptable)
wide tank or bilge is normally unacceptable and should be
removed before the inclining experiment. Insignificant
Unless the exact weight and distance of liquid shifted can be
amounts of liquids in V-shaped tanks or voids (for example, a
precisely calculated, the GM from Eq 1 will be in error. Free chainlockerinthebow),wherethepotentialshiftisnegligible,
surfaceshouldbeminimizedbyemptyingthetankscompletely
mayremainifremovaloftheliquidwouldbedifficultorwould
and making sure all bilges are dry or by completely filling the cause extensive delays.
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—Avesselshouldbeas
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
F 1321–92 (2004)
or its operational commitments. The amount and type of work
left to be completed (weights to be added) affects the accuracy
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 completely full. Keep the number of slack
tankstoaminimum.Theviscosityofthefluidandtheshapeof
the tank should be such that the free
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