In-depth | TANKER TECHNOLOGY


Systems on the Vossnack Cylinder Tanker


Helge Kringel, a Bremen-based naval architect looks at the systems for the Vossnack Cylinder Tanker in the second and final part of the story on the triple-skinned tanker design.


L


ightweight could be a significant disadvantage of the Vossnack design,


but the safety and


preservability of the cargo is significantly improved with the cylinders offering a third protective barrier. Additionally the coatings on the cylinders could help to maintain the temperature inside the cargo tanks minimising the evaporation losses that can occur onboard crude oil tankers.


Coatings The following types of steel preparation and coating are anticipated:


• Outer surface of the cylinders: SA 2.5, 4x100 μ Zn Si, sealer, 2x 2K PU


double-hull, the cost of its coating probably will not be higher, because the area of the double-hull ship requiring a higher-quality coating is larger.


• Inner surface of the cylinders: SA 2.5, 3x100 μ EP


• Double Bottom: SA 2, 2x175 μ EP To increase sunlight reflection and


to reduce the evaporation of valuable, gaseous, cargo components,


the outer


surface of the cylinders above main deck should be finished with a white, matte coating. Te success of this measure has been proven by the Hellespont ULCCs, [8] which were delivered in 2002. The necessity of heating coils and


insulation for the cylinders above the main deck depends on the service routes of the vessel. Te crude oil is embarked at 50-70°C . Due to the distance of 500mm between


cylinders and the side structure and the 1m- cofferdam below the cylinder bottom, there is no direct contact between the warm crude oil and the void side spaces and double-bottom, hence less corrosion in the ballast tanks may be expected [6], and furthermore, the cargo cools down more slowly. Though the coating area of the


cylinder tanker is approximately 150,000m² greater than that of a


20


Inertisation Vossnack, referring to [4], proposed to abandon any inertization by double- scrubbed flue gases due to their residual sulphur compounds, like H2S, SO2


Inspection and maintenance feature The cylinders must be fully accessible internal ly for inspection and maintenance. The author proposes two aluminium inspection and maintenance rigs per ship. These will be lowered by a floating crane into the tanks during maintenance via the 2m tank hatches. The rig consists of a centre column


of 0.85m diameter, internally equipped with an emergency escape ladder. A ring-type lift shall surround the column which can be lowered to equi-distant stations and locked there. The lift is equipped with a foldable walkway jig, rotary through 360degs designed for two workers with equipment.


distillates. On the bottom, underneath the cargo sludge deposits, sulphuric compounds and elemental sulphur enhance corrosion. Consequently all deposits have to be removed by tank washing before the tank coating starts to fail, however less tank washing may be necessary for a cylinder tanker. Pure N2


-Inertization implies cost


and problems, of course. One reverse- osmosis membrane separator can supply 1250kg /h of 95-98% super-dry N2


of O2 of volatile hydro-carbons, N2 This means that O2


. The remaining 2-5% consists mainly . To avoid an explosive gas mixture , and O2


in the void space during unloading, the O2


-content must never exceed 8%. has to be limited


to between 5% and 8%. The total gas pressure in the void space must also be kept below the overpressure setting of the P/V-valve which is 0.2bar while the vessel is being discharged by three cargo pumps, each having a capacity of 5000 m³/h at 15.5° (60°F). The rules require an hourly N2-supply of 19000m³/h. The previously-described membrane separator is capable of delivering 21800m³/h of N2


at a partial pressure , H2SO4 , and the formation of


elemental S. H2S can be the main cause of super-rust with material losses of up to 3.08mm / year. The CO2


-content of


flue gases creates tonnes of carbon acid which remain in the fully-inertized tanks after cargo discharge [6]. In


[4], Vossnack’s concept of


N2-Inertization was welcomed, and omission of the tank washing system was proposed. The author doubts that this is recommendable. Sulphur- reducing bacteria in many crude-oils, create micro-climates in the ullage region and on the tank bottom [5], [6]. Under this humid ceiling H2S dissolves and forms sulphur crystals under waxy


Vapour emission and recovery In the ullage space, the sum of partial pressures of volatile organic compounds (VOC), N2


,and O2


of 0.05bar. GL requires two 100% power-redundant compressors, each of 360kW.


, especially in warm


regions, is substantially higher than the P / V-valve setting. The mixture of gases would evaporate into the atmosphere, if not recovered. The evaporation of VOC, which is valuable cargo, during a transit from the Arabian Gulf to Japan and, assuming brown-coated cylinders above and including the main deck,


The Naval Architect April 2011


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