Oil tankers cargo and ballast systems functional requirement
Oil tankers cargo and ballast pipelines and pumps used for the loading and discharging of the cargo and ballast need some careful observation. The oil cargo heating system and crude oil washing (COW) system, where fitted, is also equally important.
The operator of the cargo and ballast systems should have access to up-to-date drawings and information on the systems as fitted to the ship. The ship should be provided with an Operations Manual describing how the systems should be operated.
The cargo system is one of the primary locations where loss of cargo containment may occur and care should be taken not to over-pressurise sections of the system or to subject it to shock loads.
Operation of the cargo and ballast systems should only be carried out by personnel who have been trained in the correct operation of the pumps and the associated systems, as described in the Operations Manual.
Cargo and ballast system integrity
The cargo and ballast systems are subjected to many conditions that may ultimately lead to failure resulting in loss of containment. These include the following:
Preferential corrosion is found where internal coatings have failed and the corrosion is concentrated at a small location. This localised corrosion may be accelerated when the water is allowed to lie in the bottom of pipelines, in association with sulphurous products from cargo or inert gas, or if electrolytic corrosion cells are set up when pipeline connections are not securely bonded.
The presence of any latent defect in the cargo system will usually reveal itself when the system is pressurised during the discharge operation. It is good practice to pressure test cargo lines on a periodic basis, depending on the trade of the vessel. Although these pressure tests may provide an indication of the systems condition at the time of the test, they should not be considered a substitute for regular external inspection of the pipeline system and periodic internal inspections, particularly at known failure points, such as pump discharge bends and stub pipe connections.
The presence of any latent defect in the ballast system will usually reveal itself when the system is being used during the deballasting operation. The inability to fully discharge or drain ballast tanks may result in stability problems on double bottom or double hull vessels and, in some instances, could result in the vessel being in an overloaded condition.
SOLAS requires a ships master to be provided with information on maximum permissible loading rates for each cargo and ballast tank and, where tanks have a combined venting system, for each group of cargo or ballast tanks. This requirement is aimed at ensuring that tanks are not over or under pressurised by exceeding the capacity of the venting system, including any installed secondary venting arrangements.
Other considerations will also need to be taken into account when determining maximum loading rates for oil tankers and include precautions against static electricity hazards.
Venting capacity is based on the maximum volume of cargo entering a tank, plus a 25% margin to account for gas evolution (vapour growth).
When loading cargoes having a very high vapour pressure, this factor could be exceeded and the allowance of 25% is likely to be insufficient. Actions to consider to ensure that the capacity of the venting system is not exceeded include a close monitoring of vapour line pressures on inerted vessels and limiting loading rates on non-inerted vessels throughout the loading period. It should be noted that the vapour growth increases when the liquid levels in the tank are above 80%.
On inerted vessels, close attention should be given to monitoring inert gas system pressures, particularly when topping-off during loading operations or on commencing crude oil washing during discharge operations.
When calculating loading rates, a maximum venting line velocity of 36 metres per second should be considered. This flow rate should be calculated for each diameter of line used.
The volume throughputs may be aggregated where a common vent riser is used, but the maximum flow rate should not be exceeded anywhere within the system.
Flow rates in loading lines
Depending upon the trade of the tanker, a number of loading rates need to be determined for each cargo tank. These loading rates will be dependent on the maximum flow rates in the cargo lines for different products and loading operations. In general, the following flow rates may need to be calculated for each section of the cargo system.
Where a number of tanks are loaded through a common manifold, the maximum loading rate may be determined by the flow rate through the manifold or drop lines. For this reason, it is important that a constant check is kept on the number of cargo tank valves that are open simultaneously and that a suitable loading rate is determined for the particular loading operation.
Offshore floating hose supplied to OCIMF Standards and having a nominal diameter of less than 400 mm is suitable for continuous operation at a flow velocity of 21 m/s. Offshore floating hoses having a diameter greater than 400 mm are suitable for continuous operations at flow velocity of 15 m/s. However, the maximum loading rate may be controlled by the size of the ship’s loading line inboard of where the hose is connected.
Rate of rise of liquid in the cargo tank
Some small tanks, such as slop tanks, may have larger filling or suction valves than their size would normally require due to certain operations that they may be used for, such as recirculatory crude oil washing. In such instances, the limiting factors of the venting flow rate and the liquid line flow rate may not be suitable for assessing maximum loading rates. It is then necessary to also consider the rate of rise of the liquid in the tank if risks associated with over-filling are to be addressed.
To exercise control over the rate of liquid rise in any cargo tank, it may be appropriate to set the loading rate so as to limit the rate of rise of liquid in a cargo tank to a maximum of 150 mm/minute.
Loading rates for ballast tanks
Loading rates for ballast tanks should be determined in the same manner as for cargo tanks, taking into account the size of vent outlets using a vent velocity of 36 metres/second. Liquid filling rates can be calculated using a pipeline flow rate of 12 metres/second and a similar rate of rise of liquid of 150 mm/minute should also be considered.
Problems with plastic piping
Many ships are today fitted with both ballast and IG system pipes manufactured from approved types of Plastics and Glass Reinforced Plastics (GRP). Since this piping is said to be impervious to corrosion, it is often implied that there are no particular maintenance problems associated with its use on board vessels. However, during inspections, it is often found that certain major problems do occur, which can prove harmful for the ship and its crew.
Pipes in double bottom and side tanks are normally of continuous lengths, secured by metal hoops to the frames of the tank. Over time, the strength of these hoops is degraded by corrosion which can allow the pipe to shift under the pressure of water passing through it, or from a hydraulic shock caused when a valve is open. This can crack the pipes and cause problems at flanges that are required to pass through tank bulkheads.
It is therefore recommended that crewmembers pay particular attention to these pipes when inspecting ballast spaces in the normal course of planned maintenance and cleaning. Any damaged securing hoops should be replaced as soon as possible.
Where plastic pipes are used at tank suction bells or in wells of tanks for suction, they are less prone to erosion caused by the suction turbulence and, in fact, the steel of the tank is more liable to be eroded heavily under these suction bells, which can lead to serious holes forming in shell plating. In these areas it is recommended that the plate's thickness is checked on a regular basis to avoid such problems.
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