Lightship survey

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Lightship Survey

A lightship survey is a form on experiment used to determine the lightship displacement of a vessel. With an rough estimate of the VCG (or, when performed in conjunction with an inclining experiment, which can determine the VCG), the LCG and TCG can also be determined.

Contents

Requirements

Lightship surveys are often required of flag states for passenger vessels in regular intervals - for example, Transport Canada's TP 10405 E (P.III, S.9) requires a lightship survey performed every 5 years. This is to ensure no significant changes have occurred in the vessel's weight (see ship constant) which may affect it's stability. If the lightship displacement varies by more than 2%, or the LCG location by more than 1%, the vessel will need to be inclined to check for any VCG rise.

Uses

Lightship surveys can be used during construction to monitor progress, or to evaluate ballast requirements to correct a heel or trim problem before being inclined. For sister vessels in a series which are equivalent in construction, a lightship survey can be used to verify simularity. If the vessel is close enough (within 1 or 2%), regulatory bodies may allow the vessels to share VCG's or even entire stability booklets.

Performing

For a lightship survey, all tanks need to first be sounded to determine their volume. A manifest of the non-lightship weights onboard must be conducted, with the individual weights mass and location noted. The draught marks are recorded as a check. Then, at numerous points along the vessel, the distance from the water to the moulded deck edge is measured and recorded (freeboard measurements).

Evaluating

The measured marks can be compared against a vessels linesplan and integrated to determine the vessels submersed volume. Care must be taken to include applicable appendages, and to ensure the correct trim and heel are used. With modern computer systems, it is usually sufficient to provide the list of data points collected, the software determining an appropriate waterplane for the calculations. For longer vessels, deflection (hogging or sagging of the hull) needs to be considered.

Once the as-measured displacement is determined, the heel and trim values can be used with the tank soundings to determine the quantity of fluids onboard and their locations. These can be subtracted from the measured displacement, along with the non-lightship weights from the aforementioned manifest, to determine the final vessels lightship information.

Caveats

For a lightship survey to be valid, it is important that care be taken in the measurement of values.

  • The draft marks trim line, and freeboard measurements, should be in close agreement.
  • No shifting weights should be present onboard the vessel, particularly personnel. This also implies calm waters (esp. if there are fluids in the tanks). On small vessels, a person or two on deck taking freeboards can be enough to heel/trim the vessel to the point were the measurements may not be valid.
  • Vessels with large freeboards are at risk of inaccurate measurements if the measuring device is affected by wind (can be mitigated to some extent by weighting the line).
  • For reference to the linesplan, the distance to the moulded line must be accurately determined. If the deck edge is not immediately accessible (to which the deck thickness must be subtracted), then adjustments must be made from the location of measurement. A measurement from the top of a bulwark will need to be taken outboard with a level. The distance from the level to the deck will need to be recorded as well. This distance will then have to be adjusted for camber and plate thickness to get the freeboard reading relative to the deck edge.
  • Freeboard readings also need to be accurately adjusted for the location of the baseline to determine equivalent draught. Accurate location of the measuring point is essential to ensure an accurate moulded depth at side, accounting for sheer, etc. The draught can be determined with the following relationship:
Moulded depth at side - freeboard reading to moulded deck at side = draught
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