Crude Oil Washing Of Cargo Tanks
The advent of the very large crude carrier (VLCC) not only brought a new dimension in cargo handling, it also brought a new dimension to tank washing. Water washing of cargo tanks of these large vessels requires tremendous volumes of water. For this reason the industry cast about for means to decrease the amount of water involved in tank washing. Water that must be returned to the sea.
A first measure was to develop a recirculating system which used two slop tanks. Upon completion of washing, the slop tank contents are allowed to settle for at least 24 hours, and the water is drained to the sea following usual load-on-top procedures.
Following the recommendations regarding cargo tank atmosphere control made by the API-ICS Task Group, many operators (Exxon included) fitted inert gas systems on their vessels to maintain tank atmosphere non-flammable during tank washing.
The introduction of inerting as a safety measure on VLCCs not only provided a safe atmosphere for water washing, but also opened the way to development of alternatives to water washing, which could give rise to opportunities for a safer, more efficient, and possibly pollution-free operation.
A more recent development in the ongoing effort to minimize pollution of the seas by oil is crude oil washing of the cargo tanks.
Exxon's initial experiments with waterless tank washing started in 1971. The first test was washing a tank truck with naphtha. This test led to further work, ultimately arriving at the point where we subsequently washed cargo tanks of a crude carrying ship with naphtha and with crude oils.
The use of naphtha presents serious logistical problems and can result in major delays to a vessel during discharge of cargo. As an alternative to naphtha, we looked at using the crude cargo itself as a washing medium. It may seem surprising that crude oil, which one considers to be a dirty material, would be suitable as a tank washing medium. However, most crude oils have a sizeable fraction of light components which have solvent properties.
We believe that this solvent effect contributes to a large degree to the efficiency of the operation.
We have now progressed to a position where we are cleaning all or part of the tanks in our VLCCs with crude cargo. From our discussions with shipowners and other industry members, we believe that the move to crude washing is gathering momentum.
The interior of a cargo tank would have sludge in varying amounts covering all horizontal surfaces. In addition, there would be a measurable film of oil — referred to as clingage — on the vertical surfaces. Water washing removes most of this material, but some remains — particularly trapped on the bottom by structural members — and this must be removed manually.
Crude washing in a normal single-terminal discharge is carried out concurrently with discharging and requires the use of about half the capacity of one cargo pump.
*Mr. Kirk, marine technical associate.
Tanker Department, Exxon International Company, 220 Park Avenue, Florham Park, N.J. 07932, presented the paper condensed here before a recent meeting of The Institute of Marine Engineers, Eastern U.S.A.
Branch, held in the New York Times Building in New York City.
Usual eductor stripping capacity is in the order of 600/800 tons per hour, which effectively limits the number of fixed-inplace machines that can be used simultaneously to five or six when bottom washing, depending on the nozzle size. Machine cycle time is in the order of one to one and a half hours, since up to 10 machines may be required to effectively wash a large center cargo tank, washing time can reach three hours per tank.
In the usual case, pressure at the ship's rail is less than 100 psi at full discharge rate. Therefore, it is necessary to isolate the pump used for crude washing, reducing the vessel's effective discharge rate by at least 25 percent.
The overall average discharge rate is about 1,000 cubic meters per hour below the average rate maintained when the vessel was not crude washing. Crude washing was carried out for nine hours, during which time some 64,000 cubic meters of cargo were discharged at a reduced rate. This cargo could have been discharged at 10,000 cubic meters per hour in 6.3 hours, so the net delay amounts to 2.7 hours in this case.
At the present value of VLCC time, this is a negligible price to pay for the benefits obtained.
When two port discharges are scheduled, it is frequently possible to reduce the delays in discharging by carrying out the top wash portion of the crude washing cycle during the transit between ports, leaving only the half hour bottom wash part of the cycle to be carried out during discharge.
Various industry sources,- as well as our own in-house data, indicates that under fairly optimum conditions about 1,500 to 2,000 cubic meters of cargo remain aboard a VLCC at the end of discharge if nothing is done in the way of crude washing.
While some authorities may differ with how this figure is broken down, our best estimate of the breakdown is: 3/400 tons trapped in pipelines, pumps and valves, 1,000/1,400 tons of clingage on bulkheads and structure and trapped on horizontal surfaces, and 200 tons of sludge and unpumpable residue on horizontal surfaces.
On average, retained slops on arrival at the loading point amount to 1,200 tons on vessels which have not crude washed during the previous discharge.
We have identified on recent voyages where crude washing of about one-third of the tanks was carried out during the previous discharge with significant reduction in the retained slops, Table 1.
Cargo-tank sludge samples have been analyzed and while the results show a wide scatter, the mean oil content of some 60 samples was 45-percent recoverable hydrocarbon. On the basis that one-third of the tanks are crude washed each voyage, and 200 tons of sludge are laid down, some 60 tons of sludge are returned to solution with the crude. At least half of this is recoverable oil — 30 tons—at $70 per ton, or $2,100 worth of crude is delivered each voyage that otherwise would be lost.
Table 2 outlines the economic considerations of crude washing.
These are secondary considerations, but this is a case where what started solely as a pollutionprevention measure turned out to have substantial economic incentives.
If we assume a 24-month drydock cycle and no crude washing or manual desludging of cargo tanks until the vessel is proceeding to drydock, and no water washing of tanks other than the cargo/clean-ballast tanks, the figures in Table 2 assume economic significance.
Not only would this material be lost to the cargo owner, but usually the ship operator has to pay for its removal and its disposal ashore. We have experienced costs in the neighborhood of $30-$100,000 for this operation.
If the vessel crude washes its cargo/clean-ballast tanks plus one-third of the remaining cargoonly tanks each voyage, about 560 tons of additional cargo is discharged, one-third of the sludge and clingage. On a normal loadon- top voyage, this material would remain aboard in the slop tanks after water washing, but it would be dead freight on the loaded passage. While this is relatively insignificant at today's depressed market, at a more normal market of World Scale 40, it is worth $1,600 per voyage or $9,600 per year.
Another important credit to the vessel operator is the saving in time in preparing a vessel for drydock. If the entire vessel is crude washed at the last discharge before drydock, a delay of about 12 hours in discharge time will be incurred. However, the vessel can be readied for drydock, safe for men and fire, in about two and one-half days instead of eight days with usual procedures. This can be significant in the case of a North European discharge followed by a North European or Mediterranean drydocking.
There are other less tangible credits. Fuel saving due to less water washing on the voyage.
Overtime credits due to reduced labor involved in tank cleaning and tank entry is another. One that operators report is probably significant — reduction in corrosion of cargo-tank internals due to reduction in water washing.
Operating debits are some increase in hydrocarbons lost to the atmosphere.
The average delay in discharge of 2.7 hours is worth $3,300 per year at WS 30. Lastly, there is the investment for outfitting the vessel for crude washing. Since all Exxon's VLCCs were already partially outfitted with fixed-inplace tank cleaning equipment, our economics are based on the incremental cost to completely outfit a vessel. Roughly speaking, the cost of completely outfitting a vessel is about $600,000. Our incremental costs are in the order of $250,000 per vessel.
The economics outlined are those of an integrated oil company.
The economic attractiveness to the independent owner is less. However, the underlying principle behind crude washing was protection of the environment, and the fact that it is an economically sound procedure was secondary to our decision to proceed.
There have been some questions regarding the hydrocarbon content of the atmosphere within cargo tanks which have been crude washed as compared to the hydrocarbon content of a tank atmosphere after a normal discharge.
There is a small amount of data on this, and we are currently engaged in a program to obtain additional data. In general, the data we have indicates a moderate increase in hydrocarbon levels in the tank atmosphere during crude washing. If a crude-washed tank is ballasted in port, we consider that while some increase in atmospheric pollution over previous operations would occur, it would not be severe.
The basic concepts used by Exxon are the same with other operators but minor procedural differences in tank atmosphere control, operation of inert gas systems and tank washing procedures may be found. For example, Exxon vessels are fitted with four different inert gas systems, and several different types of tank cleaning machines (almost all manufactured by Butterworth Systems Inc.), so there is some variation even in our own operation.
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