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Title
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Cargo vapour concentrations on board chemical tankers in the non-cargo area during normal operations
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Author
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Abstract
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| Chemical tankers transport a wide variety of chemical products over sea. On the modern parcel tankers, up to 50 different cargoes can be transported on the same voyage and request special attention towards compatibility and segregation of the products. But the characteristics of the various products might also be a danger for the ship and its crew, related to flammability and toxicity. Prevention of contamination of the cargo is another major concern of a ship’s officer. It is very unusual that the same cargo is loaded consecutively in the same tank, this in contrast to the transportation of crude oil. Considering the number of tanks and parcels on board modern chemical tankers, tank cleaning and gas-freeing operations are frequently occurring events, both in port and at sea. It is common for several cargo operations to happen at the same time. The discharging of product A is going on while the loading of product B is underway and also the cleaning of another tank that previously contained product C, not forgetting possible ballast operations and sampling of tanks with surveyors. IMO circular 1095§11 [1], which refers to possible danger for the crew, was the prime motive for this thesis. This circular states verbatim that during cargo loading, tank cleaning and gas-freeing operations on board a tanker carrying liquid bulk containing benzene, the crew is exposed to the largest risk of exposure to vapours from the products carried, both whilst in the accommodation and on open deck. IMO circular 1220 [2] discusses the same cargo operations, stating “Experience has shown that it has proven impossible to keep the measured vapour concentrations below acceptable levels”. The presence of cargo vapours in the accommodation gives particular cause for concern and is something we investigated in more detail. To date, Marpol Annex VI [3], regulation 15, is the only regulation related to the emission by ships of volatile organic compounds. This regulation requires only for the loading of the vessel a vapour emission collecting system. There are no regulations for the other cargo operations. At first we performed an on-board measurement program where we quantitatively analysed the presence of 7 aromatic hydrocarbons, namely Benzene, Toluene, Ethylbenzene, m+p-xylene, o-xylene (BTEX), 1,3,5 trimethylbenzene (1,3,5-TMB) and 1,2,4 trimethylbenzene (1,2,4-TMB) . With the use of the Radi¨ello® passive samplers we analysed the presence of cargo vapours on board 3 sister ships during their multi-stop journeys and normal cargo operations. At every port of call, cargo was loaded and/or discharged. The following voyages were studied: (1) ship A handling BTEX related cargo, (2) ship B with cargo that was not BTEX related, (3) ship C with the focus on transport of benzene, and (4) ship C where BTEX pollutant sources other than the cargo sources were identified. From these various journeys we found confirmation of the IMO statement related to the tank cleaning and gas-freeing operations of tanks with liquids in bulk containing benzene. We compared our results with the European Union threshold-limit values, time-weighted vi average. Only for benzene we exceeded this threshold during tank cleaning and tank ventilating. We compared the position of the accommodation ventilation inlet with the engine room ventilation inlet regarding possible pollution with cargo vapours. Our results further indicated that the difference in absolute concentration between the cargo area and the non-cargo area is less pronounced than expected. Furthermore, the pollutant concentrations inside the engine room were relatively high during all 4 investigations. This might be an indication of insufficient or inefficient ventilation or the fact that other pollution sources being present in the engine room. Further research is needed to identify the exact cause for the increased concentrations. Secondly, we performed wind tunnel simulations in order to obtain a complete picture of the pollution gradients around the ship and their evolution for different wind orientations and wind velocities. In the first wind tunnel experiment we used the method of smoke visualization to ascertain the streamlines around the vessel. Via the cargo tank outlet we released an amount of smoke that was visualized by means of a laser plane and recorded with a CCD camera. Here we developed our own method of image treatment in order to make even the smallest smoke concentrations visible. In the second wind tunnel experiment we used a tracer gas in order to calculate K-values as a measure for dispersion. The tracer gas was released from the tank manhole and the tank high velocity pressurevacuum valve (PV-valve). One of the goals of this experiment was to compare both outlets regarding the resulting concentration of cargo vapours inside the accommodation and engine room. The PV-valve gave the best results for all different wind conditions, except for the situations with a wind direction from the bow. This situation must be avoided and therefore the ship’s course and/or speed must be changed to obtain a different relative wind direction when the PV-valve is in use. Finally we set up a CFD simulation of the situation where for different wind velocities headwind situations were analysed. In the resulting images we focused on the streamlines of the cargo vapours when using the PV-valve compared to the use of the manhole. These images were a confirmation of previous results, namely that (1) for a wind direction near the bow, the use of the PV-valve as tank outlet will significantly increase the cargo vapour concentrations in and around the accommodation and (2) the position of the ventilation inlet of the accommodation is the best option. Thanks to its elevation it proves a far better position than the engine room ventilation inlet. |
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Language
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English
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Publication
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Antwerp
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Hogere Zeevaartschool
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2019
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Volume/pages
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141 p.
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Note
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Baere, De, K. [Supervisor]
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Lamoen, D. [Supervisor]
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Full text (open access)
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