A method for calculating the empty weight and the center of gravity of a ship in a dock

Abstract

This invention provides a method for calculating the empty ship weight and center of gravity of a vessel within a dry dock, relating to the field of ship hydrostatic and stability parameter measurement technology. After the vessel is checked to be in an empty state, water is injected into the dry dock and the mooring lines are gradually loosened, allowing the vessel to detach from the dock piers and float freely. Personnel remaining on board for testing are weighed and their positions recorded. The draft on both sides (bow, midships, and stern) is manually read, and water samples are collected from the dock to measure water density. Based on the corrected draft, trim, and the vessel's hydrostatic data, the test state's displacement weight and longitudinal position of the center of buoyancy are determined. Further corrections are made based on the weight and longitudinal position of the test personnel, any temporary additions, and any missing loads, yielding the empty ship weight and longitudinal center of gravity. When lateral movement of personnel creates a identifiable change in trim, the weighed personnel move between two known lateral positions, and the vertical center of gravity is determined by combining the draft changes before and after the movement with the vessel's stability data.

Description

Technical Field

[0001] This invention relates to the field of hydrostatic and stability parameter measurement technology for ships, and in particular to a method for calculating the empty weight and center of gravity of a ship in a dry dock. Background Technology

[0002] The lightship weight and center of gravity position of a vessel are crucial foundational data for ship stability calculations, load factor verification, and delivery documentation. Currently, vessel weight and center of gravity measurements are typically conducted after launch or departure from dry dock, with measurements organized according to test requirements. This type of operation has specific requirements regarding the test waters, mooring conditions, and environmental conditions, and is easily affected by wind, waves, currents, tides, and the vessel's departure point from dry dock, resulting in extensive on-site preparation and organization.

[0003] After a ship is filled with water in the dock and detached from the dock piers, it is already in a free-floating state. If the mooring lines and other shore connections do not significantly constrain the hull, the ship's displacement status at that time can be determined by the draft on both sides of the bow, midship and stern, combined with the dock water density and the ship's hydrostatic data.

[0004] However, the actual condition of a ship during buoyancy in dry dock often differs from the prescribed empty ship condition. Test personnel, temporary items that cannot be removed immediately, and equipment not yet installed but included in the empty ship condition all affect the actual displacement weight and longitudinal weight distribution. Directly using displacement data during buoyancy can easily lead to deviations in the obtained empty ship weight and longitudinal center of gravity from the prescribed empty ship condition.

[0005] Therefore, there is a need for a method that can utilize the ship's free-floating state in the dock to obtain the ship's weight and center of gravity under a specified empty state by recording and correcting for draft, water density, personnel, temporary weight increases and missing weight. Summary of the Invention

[0006] The present invention aims to solve the problem that the calculation of the empty ship weight and center of gravity depends on the test conditions after leaving the dock and requires a lot of on-site preparation. It also solves the problem that the personnel, temporary items and missing equipment during the floating measurement in the dock affect the actual measurement results and make it inconvenient to directly convert them to the specified empty ship state.

[0007] The objective of this invention is achieved as follows: A method for calculating the empty weight and center of gravity of a ship in a dry dock includes the following steps: (1) Check the equipment, spare parts and temporary items on board according to the empty ship status list. Record the weight and longitudinal position of the added weight that is still on board during the test but is not in the empty ship status, and the missing weight that is in the empty ship status but has not been loaded during the test. Fill the dock with water and loosen the mooring lines as the water level rises until the ship leaves the dock and can float freely. (2) Weigh the test personnel who remain on the ship during the draft measurement, determine their fixed positions and record the longitudinal position of the positions, and keep the personnel from leaving their corresponding positions until the same group of draft readings is completed; (3) After the ship is in a stable floating state, manually read the water draft readings on the bow, midship and stern sides, and correct the readings to the bow perpendicular, midship and stern perpendicular according to the actual position of the water draft; while reading the draft, collect dock water samples and measure the water density. (4) Based on the corrected draft of the bow, midships, and stern and the longitudinal trim of the ship, find the corresponding displacement data and longitudinal position of the center of buoyancy in the ship's hydrostatic data, and make corresponding corrections to the displacement data according to the measured dock water density to obtain the ship's displacement weight under the test conditions. (5) Based on the displacement weight under the test conditions, deduct the increased weight of the test personnel and other personnel who are not in the empty ship condition, and add the missing weight of the empty ship condition to obtain the empty ship weight. (6) Determine the longitudinal weight moment of the test state based on the displacement weight of the test state and the longitudinal position of the buoyancy center corresponding to the floating state, and then make adjustments according to the longitudinal positions of the test personnel, other added weights and missing weights, and determine the longitudinal center of gravity of the empty ship accordingly.

[0008] Preferably, the longitudinal position of the added weight and missing weight mentioned in step (1) is recorded using the existing rib position, station number or longitudinal reference used by the shipyard for weight center of gravity management.

[0009] Preferably, the fixed positions in step (2) are set on the deck, passageway or other locations where personnel can remain still, with the positions referenced by the ribs or fixed structures on the ship; without affecting on-site safety, the distribution of personnel on both sides of the ship's centerline is not excessively biased towards the same side.

[0010] Preferably, the six water level readings in step (3) are completed under the same floating state; if the readings cannot meet the ship test outline or inspection requirements due to water surface swaying, personnel movement, cable tension or liquid transfer on the ship, the readings shall be taken again after the influencing factors are eliminated and the ship is stabilized again.

[0011] Preferably, in step (3), the density of the dock water is read from the on-site water sample using a hydrometer; when there is a mixture of fresh water and seawater in the dock or significant differences in readings at different locations, samples are taken near the bow, midship and stern of the ship for verification, and the density value used for drainage correction is determined according to the ship's test outline.

[0012] Preferably, before formally reading the draft, pumping, liquid transfer and hoisting operations that may change the ship's buoyancy should be stopped, and mooring lines, shore-connected hoses, temporary cables and other components connected to the shore should be inspected to ensure that they do not impose additional constraints on the hull that would affect the reading of the draft.

[0013] Preferably, in step (4), the average draft and trim are determined by adjusting the draft to the forward perpendicular, midships and stern perpendicular, and then the corresponding displacement data and longitudinal position of the center of buoyancy are retrieved from the ship's hydrostatic table, hydrostatic curve or similar recognized hydrostatic data.

[0014] Preferably, when it is necessary to calculate the vertical center of gravity, several personnel are selected from the test personnel who have completed the weighing as moving loads, and the personnel are moved from a first transverse station on one side of the ship to a second transverse station, and the transverse distance between the two stations can be determined; the drafts of the port and starboard sides are read before and after the movement, and the vertical center of gravity is determined based on the personnel weight, the transverse movement distance, and the heel situation reflected by the change in draft, combined with the ship's hydrostatic and stability data.

[0015] Preferably, the first and second lateral positions are based on the ship's centerline, deck fixing components, or marked position lines as lateral position references; the draft is read after the ship's sway has subsided to a stable reading after personnel have moved.

[0016] Preferably, the liquid tank state remains unchanged when the vertical center of gravity is calculated; if there is the influence of the free liquid surface, the liquid tank state is corrected accordingly according to the liquid tank state recorded during the test, and the reverse movement or repeated movement can be carried out in accordance with the requirements of the ship's test outline to verify the results.

[0017] Compared with the prior art, the beneficial effects of the present invention are: This invention provides a method for calculating the empty weight and center of gravity of a ship in a dry dock. This invention has the following advantages: (1) This invention utilizes the free-floating state of the ship after it leaves the dock to read the draft of the bow, midship and stern on both sides, and to measure the dock water density, thereby reducing the dependence on the external test water area and test nodes after the ship leaves the dock.

[0018] (2) By pre-recording the test personnel, the temporary increase in weight and the missing weight, the present invention deducts or adds the corresponding amount of water displacement weight in the test state, thereby obtaining the empty ship weight in the specified empty ship state, avoiding the direct use of the on-site floating state as the empty ship state.

[0019] (3) The present invention determines the longitudinal center of gravity of an empty ship by recording the longitudinal position of each corrected weight and combining it with the longitudinal position of the center of buoyancy corresponding to the current floating state, and making corresponding corrections to the longitudinal weight torque.

[0020] (4) This invention mainly utilizes the existing draft gauge, hydrometer, personnel weighing records and hydrostatic data of the ship to complete the measurement and data processing, without the need to set up a weighing support, hydraulic lifting device or large weighing equipment on the bottom of the ship. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the ship in this invention floating freely in the dock and being manually measured; Figure 2 This is a schematic diagram showing the draft measurement positions at the bow, midships, and stern on both port and starboard sides of the ship, as well as the fixed positions of the test personnel in this invention. Figure 3 This is a schematic diagram illustrating the calculation of the vertical center of gravity using the lateral repositioning of personnel in the weighed test, as described in this invention. Figure 4 This is a schematic diagram illustrating the calculation process for the empty ship's weight and longitudinal center of gravity in this invention.

[0022] in: 1. Dock; 2. Ship; 3. Dock water; 4. Mooring line; 5. Survey boat; 6. Hydrometer; 7. Draft measurement positions on the port and starboard sides at the bow; 8. Draft measurement positions on the port and starboard sides amidships; 9. Draft measurement positions on the port and starboard sides at the stern; 10. Test personnel. Detailed Implementation

[0023] To better understand the technical solution of the present invention, a detailed description will be provided below in conjunction with relevant illustrations. It should be understood that the specific embodiments described below are not intended to limit the specific implementation of the technical solution of the present invention, but are merely possible implementations of the technical solution of the present invention. It should be noted that the descriptions of the positional relationships of the components herein, such as component A being located above component B, are based on the relative positions of the components in the illustrations and are not intended to limit the actual positional relationships of the components. Example 1

[0024] See Figure 1-4 , Figure 4 A schematic diagram illustrating the calculation process of a method for calculating the empty weight and center of gravity of a ship in a dry dock according to Embodiment 1 is provided. As shown in the figure, the method for calculating the empty weight and center of gravity of a ship in a dry dock according to Embodiment 1 includes the following: Once the vessel reaches a floating state in the dry dock, the personnel responsible for weight and center of gravity management will first check it against the empty ship status list. Fixed equipment, instruments, and spare parts that should normally be on board will remain in their designated locations. Construction tools, temporary materials, and other movable items will be removed first. Items that cannot be moved from the site will be listed separately in an additional weight list, recording their name, weight, and longitudinal position. Equipment that has not yet been installed but should be included in the empty ship status will be listed in a missing weight list, recording the equipment weight and its designed installation location. The longitudinal position can directly use the rib position or benchmark used in the shipyard's existing weight and center of gravity ledger; it is not necessary to re-measure a new coordinate system.

[0025] After the dock begins to take on water, the personnel on duty will gradually loosen the mooring lines as the water level rises to avoid sudden tautness. Once the vessel has detached from the dock piers, check whether the hull can float normally with the water surface, and suspend any lifting, pumping, or liquid transfer operations that may change the vessel's attitude. If shore-connected hoses and cables still need to be retained, sufficient slack should be left to prevent them from becoming a source of tension that affects the draft.

[0026] Personnel required to remain on board for the test must be weighed before the official readings. A convenient standing position can be selected near the deck passageway, marked by adjacent ribs, bulkheads, or other fixed structures. Recording only requires matching the personnel number, weight, and longitudinal position. The seating arrangement does not need to be mechanically perfectly symmetrical, but all personnel should be avoided from concentrating on the same side; personnel remaining on board should not move around unnecessarily until a set of six o'clock draft readings has been completed.

[0027] After confirming that the water surface and hull condition are suitable for reading, the shipowner, surveyor, and shipyard surveyors will take the water level readings sequentially by boat along the port and bow, midship, and stern. Each set of data should be completed without any change in the ship's condition. If significant water surface movement, cable tension, or personnel movement occurs during the reading process, causing the readings to fail to meet the requirements of the ship's test syllabus, then that set of data will not be used directly and will be reread after the ship has stabilized.

[0028] After obtaining the draft readings, correct the readings at the actual draft positions to the forward, midship, and stern perpendiculars according to the ship's draft layout diagram. Determine the average draft and trim based on the corrected draft at the bow, midship, and stern. Simultaneously, obtain dock water samples near the hull and read the density using a hydrometer. If the dock water is determined to have significant stratification or be a mixture of fresh and seawater, take samples near the bow, midship, and stern for verification, and use the corresponding density values ​​according to the test plan.

[0029] Then, use the ship's hydrostatic gauge or hydrostatic curve to retrieve the displacement data and longitudinal position of the center of buoyancy corresponding to the current draft and trim. If the reference water density used in the hydrostatic data differs from the measured dock water density, a density correction must be made. The displacement weight obtained at this point includes the weight of personnel remaining on board and any additional weight added on-site, therefore, it must be corrected item by item according to the previously recorded list: deduct personnel and temporary additional weight, and add any weight that should be included in the empty ship condition due to missing equipment. The result after correction is taken as the empty ship weight.

[0030] The longitudinal center of gravity is processed using the same batch of weight position records. The displacement weight under test conditions and the corresponding longitudinal position of the center of buoyancy under that floating state can be used to determine the total longitudinal weight moment under test conditions; then, personnel and temporary additional weight are deducted according to the actual longitudinal position, and the missing weight is added according to the design installation position. After this processing, the longitudinal weight moment corresponding to the corrected empty ship weight is obtained, and the longitudinal center of gravity of the empty ship is determined by the two.

[0031] When repeated measurements are required, the difference in readings is not set as a uniform value in this method. The actual standard adopted is determined by the ship's test outline, classification society requirements, or reading requirements confirmed by the chief surveyor. If the remeasurement result exceeds the requirement, priority should be given to checking personnel positioning, cable condition, surface disturbance, tank condition, and whether the records are correct before deciding whether to remeasure. Example 2

[0032] Based on Example 1, the difference between Example 2 and Example 1 is that when the project still requires vertical center of gravity data, it can be carried out based on the test state drainage data already obtained in Example 1.

[0033] Select a number of weighed personnel to form a mobile personnel group. The first and second lateral positions can be determined using the centerline on the deck, hatch coaming, longitudinal bulkhead, or pre-marked position lines. The lateral distance between the two positions should be clear from the ship's dimensional data or on-site measurements.

[0034] The moving personnel group first remained stationary at the first station, and the draft on both port and starboard sides was read as the initial state before movement. They then moved to the second station along the prescribed route, while the other personnel remained in their original positions. Once the ship's swaying had subsided to a point where readings could be taken stably, the draft on both port and starboard sides was read again. The ship's heel response was determined by the total weight of the moving personnel, the lateral movement distance, and the change in draft. Initial stability parameters were obtained using the conversion relationships typically employed in stability tests for this type of ship. Finally, the vertical center of gravity was determined by combining this with the hydrostatic data of the current floating state.

[0035] This embodiment 2 applies to vessels where lateral movement of personnel can cause a identifiable change in draft. For vessels with a large displacement where personnel movement is insufficient to generate reliable readings, this step is not mandatory; instead, only the empty ship weight and longitudinal center of gravity calculations can be performed, or the vertical center of gravity can be obtained according to other approved test protocols for the vessel.

[0036] The liquid tank condition should remain unchanged during the test. If there is a free surface, make corresponding corrections according to the liquid tank condition recorded in the test; whether reverse movement or repeated movement is required shall be determined by the ship's test outline and inspection requirements.

[0037] The above are merely specific application examples of the present invention and do not constitute any limitation on the scope of protection of the present invention. All technical solutions formed by equivalent transformations or substitutions fall within the scope of protection of the present invention.

Claims

1. A method for calculating the empty weight and center of gravity of a ship in a dry dock, characterized in that, Includes the following steps: S1. Check the equipment, spare parts and temporary items on board according to the empty ship status list. Record the weight and longitudinal position of any added weight that is still on board during the test but is not in an empty ship status, and any missing weight that is in an empty ship status but has not yet been loaded during the test. Fill the dock with water and loosen the mooring lines as the water level rises until the ship leaves the dock pier and can float freely. S2. Weigh the test personnel who remain on the ship during the draft measurement, determine each person's fixed position and record the longitudinal position of the position, and keep the personnel from leaving the corresponding position until the same group of draft readings is completed; S3. After the ship is in a stable floating state, manually read the water draft readings on the bow, midship and stern sides, and correct the readings to the bow perpendicular, midship and stern perpendicular according to the actual position of the water drafts; while reading the draft, collect dock water samples and measure the water density. S4. Based on the corrected draft of the bow, midships, and stern and the longitudinal trim of the ship, find the displacement data and longitudinal position of the center of buoyancy corresponding to the current floating state in the ship's hydrostatic data, and make corresponding corrections to the displacement data according to the measured dock water density to obtain the ship's displacement weight under the test state. S5. Based on the displacement weight under the test conditions, deduct the increased weight of the test personnel and other personnel who are not in the empty ship state, and add the missing load weight that is in the empty ship state to obtain the empty ship weight. S6. Determine the longitudinal weight moment of the test state based on the displacement weight and the longitudinal position of the buoyancy center corresponding to the floating state. Then, make adjustments according to the longitudinal positions of the test personnel, other added weights, and missing weights to determine the longitudinal center of gravity of the empty ship.

2. The method for calculating the empty weight and center of gravity of a ship in a dry dock according to claim 1, characterized in that, The longitudinal positions of the added weight and missing weight mentioned in step S1 are recorded using the existing rib positions, station numbers, or longitudinal references used by the shipyard for weight and center of gravity management.

3. The method for calculating the empty weight and center of gravity of a ship in a dry dock according to claim 1, characterized in that, In step S2, the fixed positions are set on the deck, passageway, or other locations where personnel can remain still. The positions are referenced to the ribs or fixed structures on the ship. Without affecting on-site safety, the distribution of personnel on both sides of the ship's centerline is not excessively biased towards the same side.

4. The method for calculating the empty weight and center of gravity of a ship in a dry dock according to claim 1, characterized in that, The six draft readings in step S3 are completed under the same floating state. If the readings cannot meet the ship's test outline or inspection requirements due to water surface swaying, personnel movement, cable tension, or liquid transfer on board, the readings shall be taken again after the influencing factors are eliminated and the ship stabilizes again.

5. The method for calculating the empty weight and center of gravity of a ship in a dry dock according to claim 1, characterized in that, In step S3, the dock water density is measured by a hydrometer on the water sample. When there is a mixture of fresh water and seawater in the dock or significant differences in readings at different locations, samples are taken near the bow, midships and stern for verification, and the density value used for drainage correction is determined according to the ship's test outline.

6. The method for calculating the empty weight and center of gravity of a ship in a dry dock according to claim 1, characterized in that, Before officially reading the draft, stop any pumping, liquid transfer, and hoisting operations that may change the vessel's buoyancy, and check mooring lines, shore hoses, temporary cables, and other components connected to the shore to ensure they do not impose additional constraints on the hull that could affect the draft reading.

7. The method for calculating the empty weight and center of gravity of a ship in a dry dock according to claim 1, characterized in that, In step S4, the mean draft and trim are determined by adjusting the draft to the forward, midship, and stern perpendiculars. Then, the corresponding displacement data and longitudinal position of the center of buoyancy are retrieved from the ship's hydrostatic tables, hydrostatic curves, or other approved hydrostatic data.

8. The method for calculating the empty weight and center of gravity of a ship in a dry dock according to claim 1, characterized in that, When it is necessary to calculate the vertical center of gravity, select several personnel from the test personnel who have completed the weighing as moving loads, and move the personnel from the first transverse station on one side of the ship to the second transverse station. The transverse distance between the two stations can be determined. Before and after the movement, read the draft on both the port and starboard sides respectively. Based on the personnel weight, the transverse movement distance, and the heel situation reflected by the change in draft, combined with the ship's hydrostatic and stability data, determine the vertical center of gravity.

9. A method for calculating the empty weight and center of gravity of a ship in a dry dock according to claim 8, characterized in that, The first and second lateral positions are based on the ship's centerline, deck fixed components, or marked position lines as lateral position references; the draft is read after the ship's swaying has subsided to a stable reading after personnel have moved.

10. A method for calculating the empty weight and center of gravity of a ship in a dry dock according to claim 8 or 9, characterized in that, When calculating the vertical center of gravity, the liquid tank state remains unchanged; when there is the influence of the free liquid surface, the liquid tank state recorded during the test is corrected accordingly, and the reverse movement or repeated movement can be carried out in accordance with the requirements of the ship's test outline to verify the results.

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