Our colleague, captain LE CALVEZ, gave a lecture to air spotters cadets on training course with CEDRE, in order to explain them the origin of the oily effluents onboard ships, the means and rules of processing and storage of these products. This lecture was also given during the seminar on maritime pollution fighting by ships (AGIS), organized in Brest by the Gendarmerie Maritime for European lawyers, police officers, customs officers, and gendarmes. |
The fuel used in vessels propulsion engines is a product from end of distillation of crude oil, directly unusable. This fuel requires purification by centrifugation, in order to eliminate water and abrasive particles of heavy metals (vanadium, nickel) sediments and others. This operation has to be carried out by the vessel and generates approximately 1% of residues which have to be either unloaded periodically, or incinerated progressively, but in no case dumped overboard. The logical alternative to make the poor sailor's life easier would be to carry out this operation industrially ashore, at refining exit, but unfortunately, this residue generated has a commercial value in some countries of south-Saharan Africa and in China. It concerns very little our developed countries, which prefer to delegate to the vessels the management of these undesirable goods. Some figures to give an idea of the vessels consumption :
Here is the B&W engine, 7 cylinders in line, of a 300.000 tons oil tanker , on the construction and test bench in a Japan shipyard, with the following characteristics :
|
|
|
|
photo P. Le Calvez |
PPM : Part Per Million, figure without dimension, which uses numbers of same nature (weight or volume). 1 ppm = 1 g per ton or 1 cm3 per m3 and not grams per cubic meter or cm3 per ton. |
Propeller shaft sealing
It happens periodically to all vessels some worries with this sealing system for propeller shaft, due to fouling with nets for example, the ship has a continuous leak of a small quantity of oil.
To repair afloat this damage, requires the sealing replacement, bring out from the water the propeller shaft hub, and thus the unloading of the ship combined with a ballast movement to trim by the head. Oil coolers Leaks caused by corrosion or constraint movement of the ship's structure on the sea water oil coolers may be the source of continuous or episodic lubricant leakages by this mechanism. |
The operation of the seawater valves of ballast water tanks is usually hydraulic. On some ships without duct-keel, pipings and valves actuators are located directly in the ballast water tanks and may perspire, even leak episodically or continuously some oil, due to corrosion, loose connections, or constraints caused by the localized deformation of the ship's structure normal work. Some vessels may be equipped with immersed ballast pumps hydraulically driven (FRAMO standard) with same risks of potential leakages. Damages being able to cause an unforeseeable and uncontrolled dumpings to the sea, of transmission lubricant, during full ballast water draining, even congregated to the sludge mud. Situation and event which may happen to all vessels (and even to good crews). |
AFCAN photograph |
AFCAN photograph |
The discovery last year, with a media coverage, of carbonaceous blackish releases on the beaches of the south western part of France, considered at the beginning as pollution by hydrocarbon, made me think about a voyage on a gas transport vessel, from Gijon in Spain towards the North Sea, during which we had to wash the decks and superstructures after a rather long call in this port with inopportune loading, by the wind, of a remarkable quantity of coal dust from the next quay. This operation generated a beautiful blackish trail on calm sea. |
P. Le Calvez photograph
|
Having experience of crude oil transport for the last 35 years, the VLCC with double-hull is, today, very ecological operationally compared to old structures with simple hull and ballasting operations in the cargo capacities, Clean Ballast Tanks (C.B.T. ) and segregated ballast tanks (S.B.T. ). The sea water transfers in cargo capacities of double-hull vessels are unusual and exceptional, the contamination possibilities of ballast tanks by pipings damages impossible because there is no passage of ballast pipings in the cargo and conversely. Moreover, the various crude oils are liquid ores more or less viscous according to the origin, and loaded at temperatures between 30 and 45°C. They are transported throughout the planet during 2 to 5 weeks, and sometimes discharged in winter zones. The products, cooled down by the heat exchange of the hull in contact with the ocean, become thicker, adhere to bulkheads and structures, settles on the handrails, and sometimes are difficult to pump at low temperature. This phenomenon always applies to single hull existing vessels. |
P. Le Calvez photograph
|
The handrails and structures of the double hull vessels are mainly localized in the ballast water tanks sections.The plane surface areas of the cargo capacities, bulkheads and floors, do not offer any more hiding places to the viscous elements, and a washing of the bottoms with light crude incomparably minimizes the residues remaining onboard at the end of the discharge. Moreover the empty double-hull limits the heat exchange with the sea during the voyage,vessel loaded. For example: Loaded at 42°C in Saudi Arabia, the product is delivered at more than 30°C in Saint-John, Canada, in February after 35 days at sea by the Cape of Good Hope, for 10 to 15°C on a voyage with a single hull. The disadvantage of these vessels is, at sea, the very difficult paintings maintenance of ballast water tanks, because of the access, of containment, of moisture, of heat, of mud accumulation, and the paintings drying time, especially if they were not perfectly succesful during construction. These paintings of ballast water tanks are applied at the shipyard at 98% in open air before assembling the various elements. After the assembly, only are remaining the weldings to be covered, this in good conditions. To note: The stop periodicity for the careenage of these types of vessels is only about ten days every 5 years. |