When Water Ballast Tank was discharged, an oily sheen was observed on the sea surface
A double-hulled oil tanker with segregated ballast tanks (SBT) was standing by off-limits at a loading port in good weather. In accordance with the pre-arrival schedule and loading plan, extra ballast was being pumped out. When 2S Water Ballast Tank (WBT) was discharged, an oily sheen was observed on the sea surface. Deballasting was immediately stopped and investigations detected an oil layer (innage) of about 15 cm on top of the ballast water in the tank.
Shore management was informed, the oily mixture from tank 2S WBT was skimmed off with a portable salvage pump and transferred to 3S cargo oil tank (COT) and from there, to the slop tank. Thereafter, No. 2S WBT was superficially washed and gas freed to make the tank safe for human entry. After complying with all safety procedures, the inspection team entered the tank.
They discovered that during the previous loaded voyage, oil from the adjacent cargo tank had leaked into the ballast tank through a crack on a weld seam approximately 3.5 metres below the deckhead, at the intersection of the longitudinal bulkhead and first stringer flat.
Corrective actions
1. With approval from shore management and under close monitoring of tank atmosphere, temporary repairs were carried out by drilling crack-arrestor holes, jamming a retaining steel plate between adjacent brackets and casting a cement box over the crack;
2. Risk assessment was carried out for a revised loading condition with reduced quantity of oil in the adjacent cargo tank so as to load to a level below the crack in the weld seam;
3. On the loaded passage, regular atmosphere and content checks were carried out on all ballast tanks. No leakage or ingress of hydrocarbon vapours was observed in any tank;
4. Thorough cleaning of the contaminated ballast tank was considered impractical, so environmentally-friendly tank cleaning chemicals were added to the remaining ballast in the tank to aid in dispersing the traces of oil and in cleaning the tank internally;
5. The remaining ballast was carefully decanted out at sea in compliance with MARPOL regulations and the final residue was transferred to the slop tank;
6. On completion of discharging operations in North Europe, the vessel proceeded to a reputable ship repair yard for permanent repairs which were executed safely and without any incidents.
Root cause
Poor standards and improper workmanship / quality control on the part of the shipbuilder.
Preventative actions
Fleet circular issued to all vessels with instructions to:
1. Discuss this incident at their next safety meeting;
2. Ensure that on ballast voyages the atmosphere of all ballast tanks is regularly monitored at intervals as per company SMS using fixed or portable gas detection equipment;
3. Ensure that the water in ballast tanks is uncontaminated prior to discharge, by sighting of the surface and sample drawn from each tank;
4. Maintain continuous visual check overside while ballast is being discharged overboard;
5. Carry out regular ballast tank inspections in accordance with company Planned Maintenance System (PMS), informing the technical superintendent of any abnormality;
6. In consultation with shore management, confirm the thickness measurement of plating at any suspect location using the onboard ultrasonic gauging instrument and compare the readings with the original scantlings / records from the previous close-up survey.
Source: Mars/Nautical Institute
