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Survey Documentation - Special Considerations

Because of the wide variation in physical properties of all crudes world-wide, special consideration must be given to the properties of the particular crude in question when an attempt is made to gauge quantities on board marine tank vessels. Heavy fuel oils and even some clean products, may also have characteristics that require special consideration when performing measurement activities.  In addition, conditions existing at the time of measurement sometimes require special attention for the task at hand to be properly performed.  Some of those considerations are addressed in this section

HIGH-VISCOSITY AND HIGH-POUR-POINT CARGOES

High-viscosity and high-pour-point cargoes can present specific problems with respect to the gauging of free water.  In this case, when the presence of free water is suspected, the vessel's tanks should be sampled for free water, using a bottom or thief sampler.

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If examination of the sample clearly indicates the existence of free water, further steps should include the following:

  1. Advise principals.

  2. Advise vessel.

  3. Take additional samples for testing and retention.

  4. Document steps taken and findings.

When a high-viscosity or high-pour-point crude can be cut with water-indicating paste, at least one minute should be allowed for the paste to react with the oil/water emulsion.

HEATED CARGOES

Heating of cargoes may cause unusual temperature stratification.  In such cases, additional temperatures may be required to get a good average temperature of the cargo.

Heated cargoes sometimes present problems with respect to the type of water-indicating paste being used. Some pastes do not withstand higher temperatures and are either dissolved or washed off before readings can be taken.

MEASUREMENT ON BOARD ROLLING MARINE TANK VESSELS

During offshore operations or lightering, or when the vessel is at an exposed berth, cargo may be in motion within the vessel's tanks.  In such situations, at least five successive gauge readings should be taken, the highest and lowest readings dropped and the remaining three averaged and recorded.  The successive gauges should be taken as quickly as is practical and a description and extent of the adverse measurement conditions recorded.

SPIKED CRUDES

Some crudes are injected with light hydrocarbons such as butanes, liquefied petroleum gas, or condensate. Special care should be taken with these crudes because of potential vapor buildup, especially after long voyages.  In addition, in-transit vapor loss may be higher than usual.

HIGH RVP CARGOES

High RVP cargoes such as gasoline and other light products, some very light crude oils, and condensates all have potential problems as those indicated in 'Spiked Crudes'.  As such, special care should be used when taking open hatch measurements, especially on hot days.

SOLIDIFIED OBQ/ROB

 If the OBQ/ROB quantities are solidified (non-liquid), a trim and/or list correction may still be applied if the material is in contact with all four bulkheads and the angle of solidification can be determined.  If the solidified material is not in contact with all bulkheads, a wedge calculation may be done.  However, wedge calculations and/or wedge tables can only be used to calculate the volume of the solid OBQ/ROB if it can be determined that the solid has taken, and is in, the shape of a wedge.  It must be noted that such determinations usually involve taking measurements at more than one place in the tank and that it may not always be possible to do so because of physical restrictions presented by the tank or because of operating conditions.  In all circumstances, the cargo documents should include the vessel's trim and list and should note that material was solidified. Solids and small quantities of liquid for which temperatures cannot be obtained may be assumed to be at standard temperature. For calculations of small quantities see API MPMS Chapter 17.4.

OUT-OF-TRIM VESSELS

When a marine tank vessel is out of trim, oil and free water may not be measurable at the usual gauge points. Although the oil or water should be found in the general direction of the trim or list, it may be trapped in segregated areas of the tank.  In these circumstances, more extensive methods of liquid determination may be employed if safety and operational conditions permit.  In such circumstances, the cargo documents should include the vessel's trim and list, as well as any other pertinent facts.

STATIC ACCUMULATOR CARGOES

Some cargoes have a tendency to accumulate a static charge during the loading or discharge process and need a relaxation time for the charge to dissipate before measurement equipment can be safely introduced into the tank.  Most clean products are static accumulators while most black oils are not.  In addition, static inhibitors may be added to some cargoes to reduce the risk of static charge.  To determine which cargoes are accumulators and for special considerations to be taken during the measurement and sampling of them, refer to ISGOTT for full details.   

Sections of text taken from API - MPMS Chapter 17 Marine Measurement

                                                                             Precautionary Notes                                     Data Collection 

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