Draft survey

What is a Draft Survey ?

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A draft survey is a procedure used to determine the weight of cargo loaded onto or discharged from a vessel. This process relies on Archimedes' principle of flotation, which states that a floating object displaces a volume of water equal to its own weight. During a draft survey, the draft of the vessel (the amount of the ship submerged in the water) is measured to calculate the weight of the cargo. The draft survey process can be complex due to several factors. The ship’s shape affects its position in the water, and changes in draft can occur because the ship may bend (hogging or sagging) or due to weather conditions like wind and waves.

The concept of draft surveys and precise weight measurement has significant historical roots. In the nineteenth century, maritime trade was plagued by overloaded ships, sometimes known as "coffin ships," which caused countless shipwrecks. Samuel Plimsoll, a British MP, became a passionate advocate for maritime safety after witnessing these hazards. His efforts culminated in the Merchant Shipping Act of 1876, which established the Plimsoll Line—a visible line on ships that indicates safe loading levels.
This Draft survey, now part of international standards, ensures that ships retain proper buoyancy and avoid overloading. 
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Draft Survey Procedure

The draft survey involves measuring the draft at six standard points on the hull: forward, midships, and aft on both port and starboard sides. This thorough approach ensures accuracy in the measurements.

Before conducting a draft survey, verify the following:

1. Declared Constant: Check the declared constant and compare it with previous draft surveys to identify any anomalies or trends.
2. Approval of Data: Ensure all tables and the ship's hydrostatic data are approved by class.
3. Modifications: Note any modifications or refits, such as ballast water systems, that might affect the lightship or constant.
4. Hatch Covers and Cranes: Examine the condition of hatch covers and cranes, ensuring mooring is complete to avoid squat.
5. Ballast and Bunkering Operations: Ensure no ballast or bunkering operations occur during the draft survey.

1. Initial Inspection: Thoroughly examine the vessel's condition and verify all relevant documentation.
2. Pre-loading Measurements: Take precise measurements of the ship's draft at multiple points.
3. Density Determination: Measure the water density to account for variations that can affect calculations.
4. Loading/Unloading Monitoring: Closely monitor operations during the cargo transfer process to ensure accuracy.
5. Post-loading Measurements: After cargo transfer, conduct a draft survey to take another set of draft measurements.
6. Calculations: Using industry-standard formulas and specialised software, calculate the cargo weight.
7. Report Generation: Provide a comprehensive draft survey report detailing all measurements, calculations, and findings.

Calculating the Ballast
To achieve accurate ballast calculations, Ruswal Global will:

1. Cross-Check Ballast Tanks: Check and measure ballast tanks against their designed pipe heights.
2. Confirm Readings: Use water-finding paste to ensure readings are accurate.
3. Adjust for Trim: Avoid measuring empty double-bottom tanks at low trim; instead, take soundings with moderate trim by the stern.
4. Press Tanks: Ensure tanks are fully pressed to remove air cushions, preventing discrepancies in measurements.

Useful Alternative Methods for Conducting Draft Surveys
To enhance accuracy, Ruswal Global sometimes employs the following methods:

1. Camera with Zoom Function: A zoom camera helps observe and record draft marks, particularly useful at night. Playback recordings assist in verification.
2. Laser Level: A laser level, along with boards or rulers aligned to midship freeboard marks, serves as a quick alternative to manometers.
3. Small Camera on Heaving Line: For draft marks that are hard to reach, a small camera can be lowered on a heaving line to capture live readings, stabilising the camera if needed.

Draft Marks
Accurately reading draft marks is essential for a precise draft survey. Ruswal Global ensures:

1. Visual Inspection: Check that draft marks are clean and clearly visible before starting the survey. For night-time surveys, use high-zoom cameras or illuminated drones to capture readings effectively.
2. Preventing Parallax Errors: Align the line of sight with the waterline to avoid parallax errors that could lead to significant discrepancies in cargo weight calculations.
3. Six-Point Measurement: Measure at six points along the hull—forward, midships, and aft on both port and starboard sides. Comparing averages across these points helps detect any hogging or sagging effects.
4. Minimising Wave Impact: Consider using a wave dampening device to stabilise the water surface near the draft marks for more accurate readings.
5. Cross-Checking Measurements: Verify readings on both sides of the ship to identify any unusual flex conditions, ensuring additional measurements if necessary.

Key Definitions in Draft Surveying

• Lightship Weight: The ship's weight, including operational equipment but excluding crew, cargo, fuel, ballast, fresh water, and stores. It generally increases by about 0.2% per year due to factors like additional paint layers and equipment changes.
• Constant: The difference between the calculated lightship weight (from a draft survey without cargo) and the lightship weight recorded in the ship’s documents. This accounts for variables like crew weight, rust, added equipment, and sludge. Comparing this constant with historical data helps identify potential errors in cargo calculations.
  
  

Hogging and Sagging

• Hogging: Occurs when the bow and stern are heavier than the midship, causing the midship to rise. This results in the average of forward and aft drafts being greater than the midship draft.
• Sagging: Occurs when the midship is heavier than the bow and stern, causing the vessel to sag in the middle. This results in the midship draft being greater than the average of forward and aft drafts.

Factors Affecting Draft Survey Accuracy

• Adverse Weather Conditions: High waves or strong currents can impact draft readings.
• Vessel Trim and List: Uneven weight distribution can skew measurements.
• Water Density Variations: Changes in salinity or temperature affect buoyancy.
• Vessel Deformation: Hull stress can slightly alter the ship's shape.

Our experienced surveyors are trained to recognise and manage these challenges, ensuring the most precise results possible.

Additional Considerations

• Use calibrated equipment for all measurements.
• Account for trim and list when taking readings.
• Consider using a manometer for hard-to-reach areas.
• Document all readings thoroughly for reference.

By following these steps, Ruswal Global improves accuracy in draft surveys, leading to more reliable cargo weight calculations.

Specialised Equipment and Calculations Used to Conduct a Draft Survey
To reduce errors caused by wave interference, Ruswal Global uses:

1. Wave Dampening Tube: This device allows for accurate midship draft readings by minimising the impact of surface waves.
2. Manometer: A manometer measures water level differences across the ship’s beam. By laying a hose across the beam with rigid pipes at both ends, we accurately calculate draft differences.
3. Hydrometer: We use a calibrated hydrometer to measure dock water density at the forward, aft, and midship sections, deriving an average density for precise cargo weight calculations. Multiple samples ensure accuracy due to fluctuations in water density.

Draft Survey Calculations

Ruswal Global uses two key formulas to determine a vessel's weight:

1. Displacement: Measures the weight based on water displaced:
   Displacement=LBP×B×Tm×Cb×ρ\text{Displacement} = LBP \times B \times T_m \times C_b \times \rhoDisplacement=LBP×B×Tm​×Cb​×ρ
2. Tons Per Centimetre Immersion (TPC): Indicates the weight needed to sink the vessel by one centimetre:
   TPC=LBP×B×Cw×ρ×0.01\text{TPC} = LBP \times B \times C_w \times \rho \times 0.01TPC=LBP×B×Cw​×ρ×0.01

These calculations ensure accurate assessments of cargo weight and vessel stability.

Documentation and Evidence

Documenting the draft survey process with photos and videos, including drone footage, is vital. Ruswal Global ensures:

1. Recording Disagreements: All discrepancies are recorded and addressed promptly.
2. Video Evidence: Video recordings of draft readings provide visual proof and help resolve any issues.

Importance of Accurate Draft Surveys
Draft surveys are important in the marine industry for determining cargo quantities accurately. Ruswal Global’s adherence to best practices reduces disputes and ensures a transparent, reliable process. Accurate measurements and calculations are essential for maintaining cargo integrity and operational transparency.
For further assistance or inquiries, please contact Ruswal Global's expert marine survey team.

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Frequently Asked Questions: Draft Survey