ASTM GUIDE for conducting stability tests _inclining experiment_ - W004aa

Inclining Experiment & Lightweight Survey, SWA-002-05-P04-W004 Attachment A - Revision 1 Page 1 of 30 Designation: F 1321 − 90 Standard Guide for Conducting a Stability Test (Lightweight Survey and Inclining Experiment) to Determine the Light Ship Displacement and Centers of Gravity of a Vessel1 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 or revision, the year of last revision. A number in parenthesis indicates the year of last reapproval. A superscript _____________ indicates as editorial change since the last revision or reapproval. INTRODUCTION This guide provides the marine industry with a basic understanding of various aspects of a stability test. It contains procedure for conducting a stability test in order 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 centers of gravity, but rather to be a guide to the necessary procedures to be followed to gather accurate data for use in the calculation of the light ship characteristics. A complete understanding of the correct procedures used to perform a stability test is imperative in order 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 vessel and marine craft. 1. 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 outlining 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. __________ 1This grade is under the jurisdiction of ASTM Commerce F-25 on Ship- building and is the direct responsibility of Subcommittee F25.04 on Hull Structure. Current edition approved Oct. 26, 1990. Published January 1991. 1.2 This standard does not purport to address the safety problems 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. 2. Terminology 2.1 Definitions: 2.1.1 inclining experience—involves moving a series of known weights, normally in the transverse direction, and then measuring the resulting change in the equilibrium heel angle of the vessel. By using this information and applying basic naval architecture principles, the vessel's vertical center of gravity (KG) is determined. 2.1.2 light ship—a vessel in the light ship condition (Condition 1) is a vessel complete in all respects, but without consumables, stores, cargo, crew and effects, and without any liquids on board except that machinery fluids, such as lubricants and hydraulics, are at operating levels. 2.1.3 lightweights survey—this task involves taking an audit of all items which must be added, deducted, or relocated on the vessel at the time of the stability test so that the observed condition of the vessel can be adjusted to the light ship condition. The weight, longitudinal, transverse and vertical location of each 删除的内容: 30 删除的内容: 1 F 1321 Inclining Experiment & Lightweight Survey, SWA-002-05-P04-W004 Attachment A - Revision 1 Page 2 of 30 is equal to the initial slope of the righting arm (GZ) curve and is calculated using the relationship, GZ = GM and sin θ. GM is a measure of vessel stability that can be calculated during an inclining experiment. As shown in Fig. 2, moving a weight (W) across the deck a distance (x) will cause a shift in the overall center of gravity (G−G') of the vessel equal to (W)(X)/disp. and parallel to the movement of W. The vessel will heel over to a new equilibrium heel angle where the center of buoyancy (B') will once again be directly under the center of gravity (G'). Because the angle of inclination during the inclining experiment is small, the shift in G can be approximated by GM tan θ and then equated to (W)(x)/disp. Rearranging this equation slightly results in the following equation: GM W x= ( )( ) (disp)(tan θ) (1) Since GM and displ. remain constant throughout the inclining experiment the ratio (W)(x)/tan θ will be a Fig. 1 Movement of the Center of Buoyancy item must be accurately determined and recorded. Using this information, the static waterline of the ship at the time of the stability test as determined from measuring the freeboard or verified draft marks of the vessel, the vessel's hydrostatic data, and the sea water density; the light ship displacement and longitudinal center of gravity can be obtained. The transverse center of gravity may also be calculated, if necessary. 3. Significance and Use 3.1 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 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 isn't 100% complete, the weather isn't perfect, there ends up being water or shipyard trash in a tank that was supposed to be clean and dry, etc. then the person in charge must made immediately 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 which affect the results is necessary. 4. Theory 4.1 The Metacenter—(See Fig. 1). The transverse metacenter (M) is based on the hull form of a vessel and is the point around which the vessel's center of buoyancy (B) swings for small angles of inclination (0 to 4" unless there are abrupt changes in the shape of the hull). Since the position of B is a fixed point above the molded keel (K) for any given draft and trim. M is also a fixed value above K for any given draft and trim. The height of M above K, known as KM, is often plotted versus draft as one of the vessel's curves of form. If the difference from the design trim of the vessel is less than 1% of its length, the KM can be taken directly from either the vessel's curves of form or hydrostatic tables. Because KM varies with the 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 vessel is greater than 1% of its length. Caution should be exercised when applying the "1% rule of thumb" to ensure that excessive error, as would result from a significant change in the waterplane area during heeling, is not introduced into the stability calculations. 4.2 Metacentric Heights—The vertical distance between the center of gravity (G) and M is called the metacentric height (GM). At small angles of heel, GM Fig. 3 Metacentric Height 删除的内容: 30 删除的内容: 1 F 1321 Inclining Experiment & Lightweight Survey, SWA-002-05-P04-W004 Attachment A - Revision 1 Page 3 of 30 FIG. 4 Relationship between GM, KM, and KG constant. By carefully planning a series of weight movements a plot of tangents is made at the appropriate moments. The ratio is measured as the slope of the best represented straight line drawn through the plotted points as shown in Fig. 3, where three angle indicating devices have been used. This line does not necessarily pass through the origin or any other particular point, for no single point is more significant than any other point. A linear regression analysis is often used to fit the straight line. 4.3 Calculating the Height of the Center of Gravity Above the Keel—KM is known for the draft and trim of the vessel during the stability test. The metacentric height (GM), as calculated above, is determined from the inclining experiment. The difference between the height KM and the distance GM is the height of the center of gravity above the keel (KG). See Fig. 4. 4.4 Measuring the Angle of Inclination—(See Fig. 5.) Each time an inclining weight (W) is shifted a distance (x), the vessel will settle to some equilibrium 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. Figure 6 through 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 the vessel should result solely from the moving of the inclining 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 Tan θ Fig. 3 A Typical Incline Plot heel angle, θ. In order to accurately measure this angle (θ), pendulums are used, the two sides of the triangle defined by the pendulum are measured. Y is the length 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 = Z/Y (2) FIG. 5 Measuring the Angle of Inclination 删除的内容: 30 删除的内容: 1 F 1321 Inclining Experiment & Lightweight Survey, SWA-002-05-P04-W004 Attachment A - Revision 1 Page 4 of 30 Tan θ (Z/Y) FIG. 8 Steady Wind From Port Side Came Up After Initial Zero Point Taken (Plot Acceptable) Tan θ (Z/Y) NOTE—Re-crack at tanks and _________ and pump out as necessary: Re___________ at weight movement and re-crack ______________________ FIG. 6 Excessive Free Liquids 4.5.1 Standing Water on Deck—Decks should be free of water. Water trapped on deck may shift and pocket in a fashion 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 Tan θ (Z/Y) NOTE—Re-co weight movements 1 and 5. FIG. 9 Gusty Wind From Port Side Tan θ (Z/Y) NOTE— FIG. 7 Vessel Touching Bottom or _________ by Mooring Lines 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 formula (1) will be in error. Free surface should be minimized by emptying the tanks completely and making sure all bilges are dry, or 删除的内容: 30 删除的内容: 1 F 1321 Inclining Experiment & Lightweight Survey, SWA-002-05-P04-W004 Attachment A - Revision 1 Page 5 of 30 by completely filing the tanks so that no shift of liquid is possible The latter method is not the optimum because air pockets are difficult to remove from between structural members of a tank, and the weight and center of the liquid in a full tank must be accurately determined in order to adjust the light ship values accordingly. When tanks must be 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, etc.) 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 formula Free surface (ft-tons) = lb3/12Q (3) Where: l = length b = breadth of tank, ft. and Q = specific volume of liquid in tank (ft3/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 the heel. 4.5.3 As the width of the tank increases, the value of free surface moment increased 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 prior to the inclining experiment. Insignificant amounts of liquids in V-shaped tanks or voids (for example, a chain locker in the bow), where the potential shift is negligible, may remain if removal of the liquid would be difficult or would cause extensive delays. 5. Preparations for the Stability Test 5.1 General Condition of the Vessel—A vessel should be complete as possible at the time of the stability test. Schedule the test to minimize the disruption in the vessel's delivery date or its operational commitments. The amount and type of work left to be complete (weights to be added) affects the accuracy of the light ship characteristics, so good judgement must be used. If the weight or center of gravity of and 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, etc. on board should 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 tanks to a minimum. The viscosity of the fluid and the shape of 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 limited to one pair of port and starboard tanks or one centerline tank of the following: (a) Fresh water reserve feed tanks, (b) Fuel/diesel oil storage tanks, (c) Fuel/diesel oil storage tanks (d) Lube oil tanks (e) Sanitary tanks, or (f) Potable water tanks. 5.2.1.2 To void pocketing, slack tanks should normally be of regular (that is, rectangular, trapezoidal, etc.) cross section and be 20 to 80% full if they are deep tanks and 40 to 60% full if they are double bottom tanks. These levels ensure that the rate of shifting of liquid remains constant throughout the heel angles of the stability test. If the trim changes as the vessel is inclined, then consideration must also be given to longitudinal pocketing. Slack tanks containing liquids of sufficient viscosity to prevent free movement of the liquids, as the vessel is inclined (such as Bunker C at low temperature), should be avoided since the free surface can not be calculated accurately. A free surface correction for such tanks should not be used unless the tanks are heated to reduce viscosity. Communication between tanks should never be allowed. Cross connections, including those via manifolds, should be closed. Equal liquid levels in slack tank pairs can be a warning sign of open cross connections. A bilge, ballast, and fuel oil piping plan can be referred to, when checking for cross-connection closures. 5.2.2 Pressed Up Tanks—Pressed up means completely full with no voids caused by trim or inadequate venting. Anything less than 100% full, for example, the 98% condition regarded as full for operational purposes, is not acceptable. The vessel should be rolled from side to side to eliminate entrapped air before taking the final sounding. Special care should be taken when pressing fuel oil tanks to prevent accidental pollution. An example of a tank that would appear "pressed up," but actually contained entrapped air is shown in Fig. 10. 5.2.3 Empty Tanks—It is generally not sufficient to simply pump tanks until suction is lost. Enter the tank after pumping to determine if final stripping with portable pumps or by hand is necessary. The exceptions are very narrow tanks or tanks where there is a sharp deadrise, since free surface would be negligible. Since all empty tanks must be inspected, all manholes must be open and the tanks well ventilated and certified as safe for entry. A safe 删除的内容: 30 删除的内容: 1 F 1321 Inclining Experiment & Lightweight Survey, SWA-002-05-P04-W004 Attachment A - Revision 1 Page 6 of 30 testing device should be on hand to test for sufficient oxygen and minimum toxic levels. 5.3 Mooring Arrangements—The importance of good mooring arrangements cannot be overemphasized. The arrangement selection will be dependent upon many factors. Among the most important are depth of water, wind, and current effects. Whenever possible the vessel should be moored in a quiet, sheltered area free of extraneous forces such as propeller wash from passing tugs, or sudden discharges from shore side pumps. The depth of water under the hull should be sufficient to ensure that the hull will be entirely free of the bottom. The tide conditions and the trim of the vessel during the test must be considered. Prior to the test, measure the depth of water and record it as may locations as necessary to ensure the vessel will not contact the bottom. If marginal, conduct the test during high tide to move the vessel to deeper water. 5.3.1 The vessel should be held by lines at the bow and the stern, attached to temporary pad eyes installed as close as possible to the centerline of the vessel and as near the waterline as practical. If temporary pad eyes are not feasible then lines can be secured to bollards or cleats, or both, on the deck. This arrangement requires that the lines be slackened when the ship is heeled away from the dock. The preferred arrangement is with t he vessel lying in a slip where it can be moored, as shown in Fig. 11. In this case, the lines can be kept taut to hold the vessel in place, yet allowing unrestricted heeling. Note, however, that wind or current or both, may cause a superimposed heeling moment to act on the vessel throughout the test. For steady conditions this will not affect the results. Gusty wind or uniformly varying wind or current, or both, will cause these superimposed heeling moments to change, which may require additional test points to obtain a valid test. The need for additional test points to obtain a valid test. The need for additional test points can be determined by plotting te