Marine Operations EFFICIENT CARGO OPERATION
Reducing the cargo discharging period It is of the utmost importance to keep the cargo discharging period to a minimum and, therefore, minimize berth occupancy time. Oil terminals worldwide have set limits of berth occupancy time depending on the size of the oil tanker and the quantity of cargo. A violation of the maximum occupancy time reflects negatively on the ship’s reputation. Oil terminals may ban oil tankers which repeatedly violate the maximum berth occupancy time. The discharging rate can only be constrained by the oil terminal’s requirements. The ship’s equipment involved in cargo discharging must operate flawlessly without leakages, in automatic mode and at the rated output in order to discharge the cargo in the minimum possible time.
Cargo Oil Pumps Centrifugal pumps are to be run at their full nominal RPM during bulk discharge. A reduction in RPM leads to a rapid decrease in both the pumping rate and pumping efficiency. Lowering the RPM of the cargo pumps prolongs the discharging period and results in higher fuel consumption in the auxiliary boilers and diesel generators. The cargo oil discharge plan should be carefully prepared to allow for the oil terminal’s constraints (maximum flow rate
Ballast pump driven by electric motor at the rated rpm of the AC motor
and back pressure). Extra care is required when operating centrifugal pumps in parallel in order to avoid the overheating of any pump lagging behind the other two in terms of discharge pressure. Ensure that all pumps are operating at the same speed and discharge pressure. Please refer to Wavelength issue 66, article: “Efficient Operation of Centrifugal Cargo Oil Pumps”
Cargo pumps are driven by steam turbines and must be operated at the rated speed according to the pump’s particulars.
Automatic Unloading System In order to facilitate discharging operations, ships are fitted with an Automatic Unloading System (AUS) that extracts vapour and gas produced as a result of the liquid level lowering in the tank. The various components of the AUS should be properly inspected and maintained according to the Planned Maintenance System to ensure that it will operate satisfactorily. A malfunctioning or non-operating AUS will result in longer discharging periods and damage to the cargo pump due to dry running of the pump. Automatic Cargo Unloading System (AUS)
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Issue 69-September 2019
