7.6.9.3 Water Balance
g) Failure to follow standard test methods.
7.6.9.2 Sediment and Water (S&W)
h) Improper sample handling and/or mixing.
Typically, an increase in free water volumes on the vessel during a voyage may indicate that water has settled out. However, this could indicate incorrect measurement, sampling, or the possible introduction of water into the cargo. Free water may be from internal or external sources, such as shore tanks, pipelines, sea water, steam lines, or vessel compartments. Analysis of samples, if available, will help to determine its source. Free water quantities in the first vessel tanks loaded may indicate the presence of water in either the shore or vessel lines.
A difference between reported S&W at load port and discharge port will give a shore-to-shore NSV gain or loss unless this is associated with a similar change in FW. Inconsistent S&W results can occur for any of the following reasons. a) The non-homogeneity of product may result in samples that contain more or less water than the whole cargo.
Auto sampler records may be available so that performance throughout the movement can be verified against relevant standards (API MPMS Ch 8.2 or ISO 3171). Manual samples obtained from shore tanks and vessels may be used for reference comparison purposes. An automatic flow proportional sampler is the preferred method for collecting a representative sample from a cargo at loading and discharge. Various types of auto sampler are in use (portable, fixed inline probe, fast loop, fast loop with pumped mixer). Verify if the product was susceptible to layering (vertical separation of lighter/heavier components or settling of water in layers, or even horizontal stratification in very viscous products) when the spot or zone samples were taken. Density (degrees API/kg/m3) and water content measurements on the individual zone or spot samples may confirm layering such that running samples or upper/middle/lower samples may not be sufficient to determining correct parameters to apply to the full cargo.
b) Different methods of sampling, e.g. manual sampling versus automatic inline sampling.
c) The use of incorrect sampling method or procedure.
7.6.10 Additional Factors
7.6.10.1 Temperature
d) Inability to obtain representative samples with closed or restricted equipment. e) Different methods of laboratory analysis, e.g. Karl Fischer titration, water by distillation, centrifuge, etc. f) Settling of S&W can decrease the entrained S&W content of the vessel composite at a discharge port as compared to load port. Likewise, mixing resulting from turbulence during pumping may increase S&W as FW becomes entrained and thus part of S&W.
Temperature measurements are critical in standard volume calculations, and a careful review of temperature measurements is recommended as part of any reconciliation. Temperature changes in a short period of time may indicate a measurement problem unless the oil is being aggressively heated. Temperatures can be examined volumetrically, and the resulting theoretical values compared to the actual temperatures reported. Small differences in temperatures can result in significant volume changes.
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