g) Use of different or improper water paste.
NOTE: Refer to Marpol 73/78 International Convention for the Prevention of Pollution from Ships, and HM40 “Guidelines for the crude oil washing of ships’ tanks and the heating of crude oil being transported by sea”.
If additional water is reflected in a larger gross measurement on the vessel after loading and then at discharge, then there may not have been any apparent net product loss. Differences in the free water quantity could be due to the following: a) Introduction of water into the cargo from vessel pipelines, loading/ discharge lines (particularly under sea lines or floating hoses), or shore lines during the loading or discharge operations. Water in the shore line between an automatic sampler and the vessel on loading may not be accounted for. b) Mixing of FW with cargo as it is pumped. This will reduce the FW volume while increasing the S&W volume. However, total water should remain the same. c) Settling out of S&W, which will increase the FW content at the discharge port, compared to the load port.
h) FW volumes on the vessel not properly corrected for wedge or trim conditions.
i) Different sea conditions when measuring the FW on the vessel at the load port and discharge port, e.g. rough seas versus calm seas. j) Changes in trim and/or list from loading port to discharge port. Depending on gauge point locations a wedge of FW may not be detected under certain conditions. k) Tank bottom deformation or sediment in tanks affecting FW measurement. l) The datum plate height above the tank bottom preventing quantification of water below the datum plate. This is a particular problem with cone bottom tanks where gauging points are typically offset to one side of the tank. m) Shut down or malfunction of the automatic sampler during a part of the loading or discharge or improper cleaning and operation. n) Ballast water entering the vessel’s cargo tanks or lines. Refer to “API MPMS Chapter 17.3, Guidelines for Identification of the Source of Free Waters Associated with Marine Petroleum Cargo Movements”, for a possible explanation of the origin of excessive FW.
7.6.8 Slops
Slops are a readily identifiable source of gain/losses in cargo outturns when compared with BOL volumes and should be taken into consideration in the reconciliation. Slops discharged with the cargo may have been either commingled with the cargo (loaded on top) or segregated from the cargo in a separate tank.
7.6.9 Water Determination
7.6.9.1 Free Water
Sales are based on GSV or NSV and therefore accurate water measurements are critical.
When investigating possible losses, a water balance should be carried out between each measurement point. In cases where large amounts of water are found, analysis may determine the source of the water (estuary, formation water, sea water, etc.). Care should be taken that all aspects are checked as there is often confusion between fresh ballast water and formation water. Now that segregated ballast is almost universal, ballast water should not normally be found in cargo tanks.
d) Insufficient time allowed for water to settle.
e) Different measurement methods, e.g. separate S&W and FW measurements versus total S&W of homogenized samples from an in-line sampler. f) Different methods of detecting FW, e.g. water paste versus electronic interface detector, especially for crude oils containing emulsified water.
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