In-depth | TANKER TECHNOLOGY
would amount to 860tonnes. With white-coated cylinders, the evaporation would still be 520tonnes. The advantage of the white coating is obvious. Such cargo losses can be avoided by using a vapour recovery system (VRS). The environmental
impact of VOCs on
nature and on humans is detrimental in many aspects. Therefore the installation of a VRS is required by the IMO and the USCG. Systems based on re-absorption and/
or re-liquefaction of the VOCs exist. The VOCs are collected in one main line and transferred into one tank. The contents of this tank are circulated by a pump. The circulating oil drives a Venturi-ejector which transfers the bubbles of the VOC/N2 the
tank, where it is
-mixture into re-absorbed.
The question is whether VOCs of all cargo tanks can be re-absorbed in one tank.
Cargo oil and ballast water (B.W.) system Since the midship section of the cylinder tanker differs significantly from that of a double-hull tanker, the geometrical cross-section figure 4 and the General Arrangement plan, figure 5 present the arrangement of the main piping. Due to limited space at CL, the main pipes are stacked above the cylinder bottom and also above the main deck. The cargo oil-piping system is designed to provide independence for three grades of cargo. The capacity of the three, self-priming, centrifugal, cargo-oil pumps is 5000m³/h each. They can serve the other grades in case of a pump failure. The pump room is located below cylinder No.1 with a pump-drive room on a flat above. The electric pump drives are frequency- controlled. A stripping ring-line is located above
inner bottom. The stripping pump will transfer the residual cargo from the cylinders, grade-by-grade ashore. After stripping, standard tank washing follows. Then the stripping pump will transfer the tank washing slop to the slop tanks. After the time needed for draining wash water from oil sludge, the drained water will be transferred
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from the slop tanks by the stripping line to a bilge tank in the pump room and thereafter, via the de-oiling plant, to sea. The sludge in the slop tanks will be pumped ashore by a steel -enclosed sludge pump located at the slop tank. Two B.W. pumps of 3000m³/h
each pump B. W. from sea via two, redundant, treatment plants to the B. W. tanks. The B.W. ring-pipe system enables the exchange of B. W. between the tanks.
Conclusions The feasibility of a Vossnack Cylinder Tanker has been investigated, demonst rated,
and conf irmed.
Advantages of this concept are: 1 High safety against oil spillage in case of a grounding or a collision using a triple-hull structure. Low oil spillage, if at all, may be expected in case of a collision due to wide-side tank structure and the use of ductile, TM-rolled material for the cylinder shells; No bottom spillage occurs in case of grounding due to high, double-bottom and a cofferdam above;
2 No direct contact between the
shell and oily media due to the arrangement
of cofferdams. The
H.F.O. bunker is located at the CL between two entirely-independent main engine rooms and steering gear;
3 No direct contact occurs between the warm cargo oil and B.W. tanks and void spaces,
thus there is
less corrosion of the double-hull structure;
4 Long service-life of the vessel is expected due to the smooth, internal surface of the cylinders which can be cleaned easily. Thus, significantly- reduced corrosion and hence less maintenance cost of the cargo tanks will result;
5 Substantially-less corrosion by N2
-inertization and high-quality
coating; In the author’s opinion, the Vossnack
cylinder tanker represents the optimum tanker design from an ecological point of view. The initial cost is very high. This will be caused, as anticipated
from the beginning, by a substantially- higher, net steel weight. The high initial investment however will effectively be compensated, over time, by lower maintenance cost, primarily for less tank preservation during the vessels’ long service.
Further investigations Further Investigations should deal primarily with the following topics :
• Statistical oil spill calculations for collisions
impact energy
• Investigations of the rolling and heaving behaviour with dynamic
• Dynamic stability calculations FEM calculations of
cylinders, including the surrounding hull structure.
References [4]Hylke Vellinga, experienced the material
Nedlloyd,
tanker chief engineer of comments
on tanker
operation, letter to Vossnack, March 2004, unpublished
[5]Les Callows, Amtec Consultants Ltd., Bottomless Pitts
[6]D. Engel, S. Koller, Germanischer Lloyd, Corrosion Protection in Cargo Tanks of Crude Oil Tankers, Schiff und Hafen, 9 / 2006
[7]W.L. Nelson, Petroleum Refinery Engineering, 4th Ed., Mc Graw Hill 1969
[8]Tankers cross the Hellespont, Lloyd’s List, June 2001
Acknowledgements The author thankfully received basic information and advice regarding coating from A. Biermann of the Muehlhan Company. J. Bergmann from DNV Hamburg contributed useful information about vapour losses. The author thanks Prof. Dr. Ing. Hansheinrich Meierpeter for information on the recycling of volatile organic
compounds. Rolf
Jürgens deserves thanks for assisting the author in systems design. Thanks to Peter Hoes, Canada for improving the wording. NA
The Naval Architect April 2011
• Energy absorption calculations for collisions, based on the materials
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