CHEMICAL TANKERS: CARRIAGE AND SAMPLING OF CARGOES OF BENZENE

Published: August 1, 2013

Benzene is an organic chemical compound classed as a hydrocarbon. A constituent of crude oil, benzene is a basic petrochemical product usually extracted from mineral oils. The main use of benzene is as a solvent and as a constituent part of gasoline where its high octane characteristics are beneficial. Benzene is known for its carcinogenic properties. Stringent specification requirements are applied to benzene

The club was recently involved in an incident involving benzene which highlighted various issues relating to sampling and testing.  The entered vessel, a chemical tanker, carried several parcels of Benzene from Kuwait to various ports in the Netherlands. The vessel was equipped with pump stack sampling equipment in the form of an outlet valve to which a poly vinyl chloride (PVC) sampling hose was connected as well as a dedicated hermetically sealed sampling system. In Kuwait, the cargo was loaded through the common manifold connection and in accordance with usual practice, on the orders of the chief officer, a pumpman took cargo samples from the ship’s manifold sampling outlet.  Additionally, first foot and final samples of the relevant cargo tanks were taken while circulating of the cargo on the pump stack and by taking samples with the PVC sampling hose.  Following analysis of the samples the content of chlorides in the benzene was reported as varying between nil and 1.5 mg/kg (this was within the maximum specification of 3 mg/kg). Samples were retained on board and the vessel sailed.

Upon arrival at the first discharge port the Chief Officer ordered the pumpman to carry out a pre-discharge sampling of all the tanks using the same sampling hose and in the same manner as was done in Kuwait ie. sampling while circulating the cargo on the pump stack.  The test results revealed the cargo was clearly off specification with a high content of organic chlorides present. First test showed 48 mg/kg and second test showed 16 mg/kg.  The local correspondent was notified as soon as these discrepancies were discovered.  Surprisingly, similar results were shown from the re-testing of the samples taken in Kuwait even though these samples were initially on specification when tested in Kuwait. This substantiated suspicions of contamination.

Due to the level of organic chlorides in the benzene the terminal refused to accept the cargo and the vessel was asked to vacate the berth.  At this stage a specialist surveyor was appointed to attend.  The vessel sailed to a vacant berth nearby where joint sampling was arranged. On this occasion the samples were taken utilising the vessel’s dedicated hermetically sealed sampling system and, to the surprise of the parties, no organic chlorides were found i.e. the cargo was perfectly within specification.

At the second discharge port a similar joint sampling was arranged and the ship’s dedicated hermetically sealed sampling system was again used. The benzene was perfectly within specifications.

The vessel now returned to the first discharge port to discharge the parcel initially rejected.  Again, the surveyor participated in the joint sampling using the ship’s hermetically sealed system and he also used a brought-in hermetically sealed absolute bottom sampling system. The cargo was within specifications and was subsequently discharged.

Following investigation by the surveyor appointed by the Association the source of the confusion became clear.  The pump stack sampling arrangement on the vessel was positioned at a high level above the weather deck and so, for practical reasons and to make sampling easier, the crew on the vessel had rigged up a PVC hose extension to the pump stack sampling gear.  When this PVC hose was exposed to benzene, which is a solvent, the inner reinforcement of the hose dissolved causing an accumulation of organic chlorides in the samples taken via the PVC hose.  As the hose was only used for sampling, the increased level of organic chlorides was present only in the samples and did not in any way give a representative view of the condition of the cargo.  Accordingly, sampling of the cargo done using the ships dedicated hermetically sealed sampling equipment and the external bottom sampling appliance brought in for the occasion showed that the cargo was consistent with the specifications.

The observant reader will have noticed that the initial sampling carried out in Kuwait using the pump stack sampling gear (including the PVC sampling hose) did not show excessive presence of organic chlorides.  The surveyor found that the testing carried out on the loading samples in Kuwait responded only to inorganic chlorides i.e. chlorides originating from seawater and thus not to organic chlorides. The testing carried our in Kuwait was thus incorrect.

With hindsight one might say that the correlation between the pump stack sampling and the results appear obvious. However, the combination of inappropriate testing carried out in Kuwait as well as the seemingly correct operation of the sampling equipment by the crew – it was not until later the PVC hose modification itself was discovered – led to an increasingly stressful situation as the supposed contamination would have resulted in substantial depreciation in the value of the cargo.

It should also be noted that, due to the minor nature of the discrete modification to the sampling gear, it went unnoticed by the shipping company’s own operational audits, vetting inspections and external cargo surveyors.

The lesson to be learned from this incident is that on chemical tankers, the crew must only modify or change the cargo sampling gear after due consultation with the ship’s technical managers.

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