A 35,000 DWT oil tanker, loaded homogeneously with low sulphur waxy residue cargo was unloading in north European Port. Two hours after commencing the discharge, an oil spray was found on the deck, from slop tank P/V valve. The slop tank which was loaded to 98% was de-bottomed and unloading re-commenced. 15mts after commencement of unloading, the terminal requested to stop discharging due to oil sheen found on the water around the vessel.Vessel had loaded 98% of LSWR in her forward and aft tanks.
LSWR has a pour point of 48ºC. after making all fast, the vessel started discharging with 1 wing tanks. This resulted in rapidly increasing trim. Heating was increased at the commencement of discharge, due to the pour point of the cargo. The IG branch line to the wing tanks were blocked with LSWR. This made the IG pressure in slop tank not getting neutralized by the fall in pressure in the wing tanks.
The Vessel then discharged her slops and brought the ullage to more than 4.0m. the IG line was full of oil vapours which entered the deck seal through the manual and N/R valve. These oil vapours formed an oil sheen on the wet type deck seal and this escaped through the deck seal overboard, polluting the waters around the vessel.
Lesson Learnt :
1)After the oil spray from the slop P/V valve, a thorough examination and evaluation should have been carried out.
2)De-bottom all filled up tanks before trim increases.
3)Heating of cargo should be controlled, increase in heating of LSWR, before they reach stripping level should be considered.
4)Pre-arrival tests should not be just exercising the P/V valves, but also physical checks for vapours coming out should be checked.
5)Enough rest should be given to the duty officers, to avoid fatigue, so that the critical operations can be carried out without errors.