ASTM F1321-92
(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
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 problems, 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
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 (ϵ) indicates an editorial change since the last revision or reapproval.
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), and the vertical center of gravity (KG). The
architecture principles, the vessel’s vertical center of gravity
transverse center of gravity (TCG) may also be determined for
(KG) is determined.
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
This guide is under the jurisdiction of ASTM Committee F25 on Ships and
vessel at the time of the stability test so that the observed
Marine Technology and is the direct responsibility of Subcommittee F25.01 on
condition of the vessel can be adjusted to the light ship
Structures.
condition. The weight, longitudinal, transverse, and vertical
Current edition approved Dec. 15, 1992. Published February 1993. Originally
published as F1321–90. Last previous edition F1321–91. location of each item must be accurately determined and
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
F 1321–92
recorded. Using this information, the static waterline of the trim of the vessel is greater than 1% of its length. Caution
ship at the time of the stability test as determined from should be exercised when applying the “1% rule of thumb” to
measuring the freeboard or verified draft marks of the vessel, ensure that excessive error, as would result from a significant
thevessel’shydrostaticdata,andtheseawaterdensity;thelight changeinthewaterplaneareaduringheeling,isnotintroduced
ship displacement and longitudinal center of gravity can be into the stability calculations.
obtained. The transverse center of gravity may also be calcu- 4.2 Metacentric Height—The vertical distance between the
lated, if necessary. center of gravity (G) and M is called the metacentric height
(GM).At small angles of heel, GM is equal to the initial slope
3. Significance and Use
of the righting arm (GZ) curve and is calculated using the
3.1 From the light ship characteristics one is able to calcu-
relationship, GZ = GM sin θ. GM is a measure of vessel
late the stability characteristics of the vessel for all conditions stability that can be calculated during an inclining experiment.
of loading and thereby determine whether the vessel satisfies
As shown in Fig. 2, moving a weight (W) across the deck a
theapplicablestabilitycriteria.Accurateresultsfromastability distance (x) will cause a shift in the overall center of gravity
test may in some cases determine the future survival of the
(G–G`) of the vessel equal to (W)(x)/displ and parallel to the
vessel and its crew, so the accuracy with which the test is movementof W.Thevesselwillheelovertoanewequilibrium
conducted cannot be overemphasized. The condition of the heel angle where the center of buoyancy (B`) will once again
vessel and the environment during the test is rarely ideal and be directly under the center of gravity (G`). Because the angle
consequently, the stability test is infrequently conducted ex- ofinclinationduringtheincliningexperimentissmall,theshift
actly as planned. If the vessel is not 100% complete and the in G can be approximated by GM tan θ and then equated to
weather is not perfect, there ends up being water or shipyard (W)(x)/displ. Rearranging this equation slightly results in the
trash in a tank that was supposed to be clean and dry and so following equation:
forth, then the person in charge must make immediate deci-
~W!~x!
GM 5 (1)
sions as to the acceptability of variances from the plan. A
~displ!~tan u!
complete understanding of the principles behind the stability
SinceGManddisplremainconstantthroughouttheinclining
test and a knowledge of the factors that affect the results is
experiment the ratio (W)(x)/tan θ will be a constant. By
necessary.
carefully planning a series of weight movements, a plot of
tangents is made at the appropriate moments. The ratio is
4. Theory
measured as the slope of the best represented straight line
4.1 The Metacenter—(See Fig. 1). The transverse metacen-
drawn through the plotted points as shown in Fig. 3, where
ter (M) is based on the hull form of a vessel and is the point
three angle indicating devices have been used. This line does
around which the vessel’s center of buoyancy ( B) swings for
not necessarily pass through the origin or any other particular
small angles of inclination (0 to 4° unless there are abrupt
point, for no single point is more significant than any other
changes in the shape of the hull).The location of B is fixed for
point. A linear regression analysis is often used to fit the
any draft, trim, and heel, but it shifts appreciably as heel
straight line.
increases. The location of B shifts off the centerline for small
4.3 Calculating the Height of the Center of Gravity Above
angles of inclination, but its height above the molded keel (K)
the Keel—KM is known for the draft and trim of the vessel
will stay essentially the same. The location of M, on the other
during the stability test. The metacentric height (GM), as
hand, is essentially fixed over a range of heeling angles up to
calculated above, is determined from the inclining experiment.
about 4°, as the ship is inclined at constant displacement and
The difference between the height KM and the distance GM is
trim. The height of M above K, known as KM, is often plotted
theheightofthecenterofgravityabovethekeel(KG).SeeFig.
versus draft as one of the vessel’s curves of form. If the
4.
differencefromthedesigntrimofthevesselislessthan1%of
4.4 Measuring the Angle of Inclination—(SeeFig.5.)Each
itslength,the KMcanbetakendirectlyfromeitherthevessel’s
timeanincliningweight(W)isshiftedadistance(x),thevessel
curves of form or hydrostatic tables. Because KM varies with
trim,the KMmustbecomputedusingthetrimoftheshipatthe
time of the stability test when the difference from the design
FIG. 1 Movement of the Center of Buoyancy FIG. 2 Metacentric Height
F 1321–92
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 trans-
verse deflections are measured. Tangent θ 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 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.
FIG. 3 A Typical Incline Plot
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.
Unless the exact weight and distance of liquid shifted can be
FIG. 4 Relationship between GM, KM, and KG
FIG. 5 Measuring the Angle of Inclination
will settle to some equilibrium heel angle, θ. To measure this
NOTE—Recheckalltanksandvoidsandpumpoutasnecessary;redoall
angle (θ) 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
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
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
oftheliquidinatankwithparallelverticalsidescanbereadily
calculated by the formula:
Freesurface ~ft2tons!5 lb /12Q (3)
where
l = length of tank, ft,
b = breadth of tank, ft, and
Q = specific volume of liquid in tank (ft /ton).
(See Annex A3 for fuel oil conversions or measure Q
directly with a hydrometer.)
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
removed before the inclining experiment. Insignificant
FIG. 8 Steady Wind From Port Side Came Up After Initial Zero
Point Taken (Plot Acceptable) amounts of liquids in V-shaped tanks or voids (for example, a
chainlockerinthebow),wherethepotentialshiftisnegligible,
mayremainifremovaloftheliquidwouldbedifficultorwould
precisely calculated, the GM from formula (1) will be in error.
cause extensive delays.
Free surface should be minimized by emptying the tanks
completelyandmakingsureallbilgesaredryorbycompletely
5. Preparations for the Stability Test
filling the tanks so that no shift of liquid is possible. The latter
method is not the optimum because air pockets are difficult to 5.1 General Condition of the Vessel—Avesselshouldbeas
remove from between structural members of a tank, and the complete as possible at the time of the stability test. Schedule
weightandcenteroftheliquidinafulltankmustbeaccurately the test to minimize the disruption in the vessel’s delivery date
determined to adjust the light ship values accordingly. When or its operational commitments. The amount and type of work
tanks must be left slack, it is desirable that the sides of the left to be completed (weights to be added) affects the accuracy
F 1321–92
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 surface effect can be
accurately determined.
5.2.1 Slack Tanks:
5.2.1.1 The number of slack tanks should normally be
FIG. 10 Tank Containing Entrapped Air
limitedtoonepairofportandstarboardtanksoronecenterli
...
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