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9 Samundra Spirit | APR 2012 ISSUE 17 KNOWLEDGE

Know your ShipBitumen/ Asphalt Carriers

Structure and FacilityBitumen is a mixture of organic liq-uids that are highly viscous, black,sticky, entirely soluble in carbon di-sulphide, and composed primarily ofhighly condensed polycyclic aromatichydrocarbons. Bitumen is the resid-ual (bottom) fraction obtained byfractional distillation of crude oil andalso known as Tar in some countries.It is mainly used for paving roadsand for general waterproofing prod-ucts, including the use of bitumen inthe production of roofing felt and forsealing flat roofs. It is transported inbulk by sea on specialized Bitumenor Asphalt Carriers, and on land byspecial rail cars which are equippedto handle it, but in some cases it isblocked and shipped as solid.

Construction

In view of its nature, loading temperature for Bi-tumen is normally about 140C, carriage about150C and discharge temperature 150-160C.These high temperatures are necessary for thecargo to ow.

Bitumen ships are specially built to handle thisproduct at high temperatures. Not only are theships equipped with double bottoms, the tanksare designed and insulated on all sides to re-tain the high temperatures to prevent thermalloss. The heating coils are rather more elabo-rate than those in the normal ship. In additionto the coils covering the bottom of the tanks,additional layers of coils are provided at differ-

ent levels in the tanks. Piping and pumps in thepump-room as well as deck are provided withlagging, and the deck lines are often providedwith a stream trace in addition to this. Mostgrades of bitumen are handled at temperaturesthat exceed 120C and below this temperaturethey become hard to handle.

Cargo Tanks DesignEssentially, the vessel has a construction likethat of a bulk carrier. The hold spaces are ar-ranged segregated from the side shell by bal-last tanks. An independent steel asphalt tankis mounted on the tank supports (hard blocks)

xed on the tank top of double bottom in thehold. Tank Supports are provided at the front,rear, both side, and top, in addition to the bot-tom. The tank support on the top has suf cientclearance from the hull and suf cient strengthand structure to respond to thermal expansion

and to shifting when the ship rolls and pitches.

The tank is covered on the outside with rockwool (thickness: 200 mm) as insulation andthis rock wool is in turn covered with a thinstainless steel plate. Both suction and deliverypiping pass through the tank. This is becausepreheating the piping before loading the cargois convenient, and during loaded passage, thedelivery pipe is heated to the same tempera-ture as the cargo.

The pipes for pump suction and delivery passthrough the rear wall of the asphalt tank, sostainless steel thermal expansion joints are in-stalled on the outside of the through part. Asfor the loading pipe, air vent pipe, etc., whichare mounted on the top of the tank, the trunkdeck through part is a loose hole and expan-sion joint are tted to the pipe ends so that nostress will be applied directly to the tank duringits expansion, contraction or swinging in roughweather conditions.

Cargo PumpsJust like in an ordinary oil tanker, Asphalt carrierhas a pump room wherein it houses the cargopumps. In view of the thick viscous nature ofcargo the pump is of gear type and is con-structed of one set of gears, a shaft supportedby the bearings, casing, side covers, etc. Thisgear pump is of the positive displacement type.

It picks up oil between each of the gearteeth on the suction side and by the actionof the gears turning, continuously carries the

oil along inside of the casing around to thedischarge side. Bearings are inside of thepacking and are lubricated by the oil handled.

After lubricating the bearings, lubricating oilreturns to the suction side through the oil wayprovided in side cover. It is driven by dieselengine located at the engine room equipped withan enclosed reduction gear box of helical gearstype. The output shaft is connected through thecargo pump via engine room and pump roombulkhead by a stuf ng box. It is equipped withan engaged/ disengaged coupling, which ismade of specially vulcanized shock-absorbingrubber between the inner and the outer wheelstogether with a pneumatic-clutch. Because ofthe couplings exibility, it absorbs torsionalvibrations from the driver to prevent these frombeing transmitted to the driven unit, which isthe cargo pump.

A typical sized bitumen carrier has two pumpsprovided of individual capacity of 400 cubicmeters/ hour.

Venkateswara Rao Katta (left)Fleet ManagerESM, Singaporewith inputs fromCapt. Paterno B. Olasiman (right)Master, M.T New Glory

Heating coil with bell mouth, Note the greyasphaltene material, which prevents heattransfer

The midship cross-section of the vessel across a web space and common tank space.

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Samundra Spirit | APR 2012 ISSUE 17 10KNOWLEDGE

Thermal Oil Boiler Due to the high temperatures involved in as-phalt tankers, steam is not suitable as a heat-ing media. To maintain the high temperaturerange, the tanks have hot oil (thermal) heatingcoils.

Heating medium is thermal oil. The main dif-ference between thermal oil boiler and steamboiler is that thermal oil boiler is mainly moni-tored on the temperature of the thermal oil andsecondly the pressure, whereas the steamboiler is mainly on pressure and secondly ofthe temperature of the steam generated.

Typically the thermal oil boiler is designed tohave a heat output of 990,000 kcal/h. The sev-eral cylindrical tubes are arranged with suitableoverlaps with both ends of these tubes (inlet

and outlet) welded with headers. The burner isplaced at top side of the boiler body. Fuel oilwill be burnt by burner at combustion cham-ber. Exhaust gas will be discharged from boilerfunnel after passing through the heating tubes.Thermal oil will be circulated by Boiler Side Cir-culating Pump.

Key Issues in carriage of bitumenThere are a number of key aspects in storingand handling bitumen as given below:1. Condensation in empty tanks is extremely

dangerous. Dangerous amount of steam

can be created when hot liquid Asphalt isintroduced into a tank that has been emptyfor a while and accumulated moisture from

condensation. To circumvent this, cargotanks are pre-heated prior to arrival at loadport .

2. Extreme caution should be exercised whenloading, unloading and sampling, due tothe high temperatures involved. Failureof cargo tank insulation may create somehot spots on the vessels weather deck anddirect contact of hot surfaces can causesevere burns e.g. handling manual tankvalves etc.

3.Protective equipment must be made avail-able and used by all personnel involved incargo operations. Suitable skin protectionsubstances shall be readily available andpersonnel shall be instructed in its use.

4. Use catwalks, screens, barrier guards andshields to protect from steam, hot asphaltor other hot surfaces.

5. Bitumen being a high density cargo, due con-sideration should be accorded to the abovewhilst planning for the cargo. The density ofthe grade being loaded must be comparedwith the permissible load density of the car-

go tanks and appropriate handling precau-tions must be in place. The cargo should beloaded at a very slow rate initially to prevent

high velocity impact of cargo ow on thetank structures, the operation of the valvesand pumps must be gradual to avoid pres-sure surges from being set up in the lines.

During the loading of the high-temperature car-go, the tank swells, giving a tight t to the hardblocks between the hull and the tank; however,after discharging of cargo has been completed,as the tank starts to cool and shrink, some playmay appear in the blocks. Care should be tak-

en to prevent the shifting of the tank. In roughweather, it is necessary to drive iron wedges inthe gaps of the blocks at the front and back ofthe tank to minimize swinging.

After repeated loading and discharging, thehard blocks beneath the tank become com-pressed, allowing the tank to subside, whichsubjects the cargo oil pipe, expansion joint atthe through part, etc. to unreasonable force.The amount of deviation of the tank should bemeasure periodically. There are several otherprecautions to be taken while handling thiscargo due to the high viscosity and high melt-ing point thus the high handling temperature.

Thermal Boiler Top Portion