Many tankers now load from oilfields at sea. To do this, they moor up (usually by
the bow) to a gantry, buoy, or turret (toranj). Tankers on the North Sea-run
(often called shuttle tankers) have been specially designed to load at the bow
from a single point mooring at sea.
Crude oil can be loaded into a tanker from a variety of offshore facilities or
a conventional oil terminal through the midship manifold. Modern oil tankers may be equipped with the most advanced loading systems, combining a Bow Loading (B.L.) system
and the ship's part of the Submerged Turret Loading (S.T.L.) system.
The basis of the Submerged Turret Loading system is the buoy moored to the
seabed. The buoy is pulled into and secured in a mating cone in the bottom of the
vessel and thus connecting the mooring system. Internal in the float is the turret
connection (toranj) to the mooring and riser systems. The outer buoy hull can
rotate freely with the vessel around the turret using internal turret bearings.
Oil is transferred through an in-line swivel via the loading manifold to the piping
system of the vessel. The disconnected buoy will float in an equilibrium position
ready for a new connection.
The Floating turret system enables the vessel to be easily moored at the bow and
oil transferred conventionally to the midship manifold.
Floating production, storage, and offloading systems (FPSO) can offer significant
advantages over fixed production platforms, particularly in remote offshore
locations where deep water, strong ocean currents, and harsh weather conditions
may occur, or where export pipelines are difficult to install or uneconomic to run.
Liquid cargo is nowadays usually transferred using an articulated arm
loading/discharge system, and groups of arms are often found onshore
refineries or offshore loading facilities. It connects to the tanker's manifold
usually located near the center of the ship.
Loading oil cargo in a tanker ship requires utmost diligence in planning, and most careful consideration will need to be made for safe operation. Following are the necessary procedures at various stages of loading oil cargo,
Fig:Submerged Turret Loading system
Loading oil cargo
The primary consideration for restricting the maximum rate at which a vessel may load is to prevent the excessive build-up of pressure within the cargo system. This is governed by several factors, including the diameter of the smallest section of pipe in the order, material from which pipeline is constructed, angular frequency of bends, and the capacity of the venting system, etc.
The closing loading system, whereby vapors are exhausted through either vent risers or high-velocity vents, must always be used.
The rate of loading must be adjusted to consider the number of tanks or holds open at any one time.
The maximum loading rate mustn't be exceeded to avoid over pressurization and undue stress on the pipeline system.
Consideration of the rate of loading must also be taken into account given electrostatic hazards when handling static accumulator oils where an electric charge can build up utilizing flow or turbulence. Generally speaking, controlling the electrostatic generation restricts the flow rate in the initial stages of loading until all splashing and unrest in the tank have ceased. Full details of such precautions are contained in ISGOTT.
An offshore installation
Besides, it is also essential to allow gas from the ullage space to be vented and to dissipate to below L.E.L. at a safe distance from the outlet point.
It is also important that the cargo being loaded is correctly distributed. It is not permissible to load at the maximum loading rate into one tank or into tanks served by only one section of the gas line so that all the gas being given off by the cargo is expelled through a single outlet.
One person must be delegated to keep watch within sight of the manifold area throughout loading. If an incident at the manifold occurs, such as a burst pipe or failure of the manifold connection, the agreed to shut down procedure must be put into action first, and the Emergency Alarms sounded. The gangway watchman may perform this duty.
If the person delegated to watch the manifold area is a rating, he must be instructed in the course of action to be taken in the event of an emergency before commencing his watch.
Tank hatches must not be opened, or ullage plugs left open during loading.
At the time of loading, the atmospheric conditions are the main factors in the successful dispersal of the gas at a safe distance from the gas line outlet. Hydrocarbon gases are, on average 1.5 times heavier than air and in still air or near calm conditions will fall to deck level and may accumulate there; in these conditions, the Master may consider stopping loading operations, if gas concentrations are approaching hazardous levels, until conditions improve.
Similarly, if the existing atmospheric conditions cause the gas to flow towards the accommodation, and the gas is detected inside the accommodation, loading must be stopped immediately, and the Emergency Alarms sounded. The Master must ensure all precautions are taken to prevent vapors from entering the accommodation. This must include having the air conditioning on recirculation.
The Emergency Organisation is then to take the appropriate action to isolate the electrical supply of the accommodation from the main switchboard, and clear the accommodation of gas.
The accommodation must be kept under positive pressure to prevent the entry of Hydrocarbon Vapours the operation of sanitary and galley extraction fans will cause a vacuum. Therefore the air conditioning system intakes must not be kept fully closed. The accommodation air conditioning system should be maintained on partial recirculation during cargo operation.
Fig: Single point mooring system for oil tanker
Appropriate oil cargo sampling is one of the critical jobs for crew onboard to prevent cargo contamination and avoid future claims. A comprehensive plan should be made on cargo sampling focusing on safety precautions before sampling, sampling equipment, and on the critical locations on sampling onboard a vessel.
Line up of the Vent lines
Prior loading operation commences, cargo tanks I.G. inlet lines to the designated tanks shall be re-checked and confirmed in the desired position.
The control of the key to the locking arrangements for cargo tank I.G. inlet valves shall be with the Chief Officer.
For tanks that are required to be isolated by vapor (as per the Charterer's instructions), the individual I.G. pressure shall be monitored Every 4 hrs.
Safety Confirmations and Clearance:
Once the Chief Officer is satisfied that all preparations have been made following the cargo oil loading plan and the shore facility representative has confirmed that the facility is ready to load cargo, he may order the opening of the designated manifold valves and loading operation to commence following the loading plan.
Commence loading at a reduced rate (to avoid static generation), watching the manifold back pressure at all times.
The first loading tank shall be documented in the 'Tanker Cargo Log Book,' The number should be restricted to a minimum.
Ullage confirmation shall be carried out to confirm cargo oil flowing as planned into the designated cargo tank.
In the case of heated cargo, confirmation of cargo temperature is as per agreed value and within the Charterer's instruction. Also, the loaded cargo temperature shall be within the vessel's design criteria (of valve/tank coating limitations)
Only after receiving reports of all safety checks confirmed from all stations of deck/pump room watch, and the chief officer may open other loading tanks and carefully increase the loading rate. A close watch of the manifold backpressure shall be maintained until completion of settling down of final maximum agreed loading rate.
Close communication to be kept with shore side, until all parameters have stabilized.
Loading cargo tanks I.G. backpressure shall be adjusted to maintain slight positive, at all times. The same shall be monitored for any change.
Deck Watch and Personnel Arrangement
The deck watch should check for oil leaks in the cargo area throughout the cargo oil loading operation. At the beginning of the operations, confirm that no oil leaks from piping joints and that no oil inflowing into tanks other than the tank being loaded.
Keep continuous monitoring of the Oil Level of the loading tanks, until settling down of shore flow rate.
Also, monitor other tanks (unused) for any change in the level. After reaching the desired full loading rate and confirmation reports have been received from all stations at deck/pump room watch (including the cargo piping and sea surface around the vessel), the Chief Officer may dismiss the off duty crew and revert to the routine Watch Schedule. During loading operations, monitor the manifold backpressure, especially when changing over the valves/tanks.
Leakage Monitoring System
Cargo leakage, however, small shall be paid attention to- at an early stage of operations.
Leakages from the piping system, joints, and valves shall be monitored.
Tanks not being loaded shall be monitored to ensure that no oil is flowing into tanks other than the loading tanks.
During loading operations, watch oil loading pressure all the time, and monitor portions where oil is likely to leak. Excessive vibrations on piping systems must be attended to immediately.
Cargo Loading Rates:
The vessel's maximum loading rate and maximum venting capacity must be posted in the cargo control room. It gives details of the standards for homogenous(entire the ship), Group-by-group, and Tank-wise loadings.
Such information, based on calculations, shall assist the Master in determining how fast the ship can safely load a particular cargo at a specific facility, taking into account the vessel's design parameters and the shipment involved.
The Chief Officer should indicate, in the loading plan, rates required at stages throughout the operation.
b) Theoretical Rates
The maximum flow rate into any single tanks shall be less than the maximum venting capacity (SOLAS).
To allow for the generation of gas when loading, the venting rate shall be taken as 125% of the oil loading rate.
Maximum loading rates are affected by several factors:
Diameter of Manifold valve / line.
A cross section of the Pipe [m2] x Instant Flow Rate 7[m/sec] x 3,600[sec] = Reference Max. Loading Rate
Number of tanks being loaded at any one time.
Gas venting capacity main system.
Secondary gas venting capacity.
c) Setting Loading Rates
The initial and maximum loading rates, topping off rates and normal stopping times should be considered, having regard to -
The nature of the cargo to be handled;
The ship's cargo lines and gas venting systems' arrangement and capacity: the vent line pressure should not exceed that indicated by the builder. It must be closely monitored at terminals where loading rates are known to be high.
Builder's maximum vent pressure may be based on a rate for loading all tanks simultaneously; rates must be reduced accordingly for a smaller tank being loaded.
The ability and competence of the vessel's staff.
The loading rate should also be governed by the age, condition and reliability of the vessel's pipeline system and the gauging system.
Precautions to avoid the accumulation of static electricity.
Any other flow control limitations.
De-Ballasting of Segregated Ballast:
Obtain the Berth (Loading) Master's permission before starting to de-ballast the segregated ballast tanks.
In principle, de-ballasting operations should commence, after starting of cargo operations.
De-ballast, as per the cargo plan to achieve ample trim, especially towards the completion of de-ballasting operations.
Such period should be planned well before the level in cargo tanks are near Topping-off ullages.
Fig: 117,100 dwt crude oil tanker STENA ARCTICA
Recording during operations in Tanker Log Book:
Following items shall be recorded in Tanker Cargo Log Book hourly.
Loading Quantity (Rate) to compare it with that of the terminal side
Regular ship/shore comparisons of loaded cargo figures shall be carried out and changes in difference to be investigated / reported.
If the Duty Deck Officer cannot account for the variation of rate then he must call the Chief Officer immediately.
Manifold Pressure / Temperature,
Draft & Trim
Monitor of levels in tanks not being discharged
The Stress and Stability of the vessel
Tank pressure
SBM / FSO position monitoring shall be carried throughout the operations. The crew on watch shall be briefed as to the danger limits for the bearing and distance of the SBM / Hawser to be reported.
Chief Officers Standing order:
Chief Officer shall give his written instructions of the cargo plan to the duty officer.
Stress monitoring and print-outs of intermediate conditions shall be recorded during regular cargo operation. Loading computer shall be updated hourly for conditions on board.
A comparison of real & calculated draft & trim shall be carried out to give proactive warning of any unplanned or unobserved deviation from the plan.
Trim and draft
Ensure the draft maintained, after allowing for tidal variation, is well within the limits of manifold/loading arms' height limitation. The vessel shall always be maintained well within the operating limits (envelope) of the shore arms.
Topping Off
Before commencement of Topping off operations, arrange adequate personnel for the operations.
As the number of remaining tanks are reducing with progressing of Topping off operations, lower the loading rate down to have sufficient time to cope with the Final Loading Topping off.
Confirm the 'Check items when Topping-Off', as per the "Tanker Loading Checklist" to record the results.
Preparation for Topping Off:
The Chief officer should indicate, in the loading plan, the method he wishes to use for Topping Off and the maximum permitted topping off rate(s).
The Chief Officer should indicate to the Duty Deck Officer when he wishes to be called for Topping Off.
The Duty Deck Officer should calculate when the Topping Off operation will begin and advise the shore terminal well in advance.
Well, before topping off, the Duty Deck Officer should have the deck watch verify and compare the portable gauges with the fixed cargo tank gauge. He shall complete the "Cargo Tank Level Gauge Check Record at Loading Ports" of the tanks to confirm the accuracy of the C.C.R. tank gauges.
The Chief Officer should be notified of any discrepancies when he is called for a Topping Off operation.
On the assumption that the tank to be topped off is not the final tank and that there are other tank valves opened for the grade being loaded, the valve should be operated when there is sufficient ullage remaining in the tank, to ensure that the valve will close as required.
When Topping Off tanks, there must be enough personnel available to monitor the operation and provide assistance.
It should be remembered that this is a critical point in a loading operation.
A portable hydraulic pump must be readily available on the deck complete with extra oil and hoses, in case of a failure in the valve operating system.
Topping Off Operation
If the Topping off operation gives significant cause for concern at any time, such as equipment malfunction, STOP LOADING!!
Then take the necessary time to get things settled down again before resuming.
After slowing down the loading rate for Topping off, it should be checked that the loading rate is reduced as requested.
If the loading rate is still too high, the shore should be requested to reduce the pumping rate further.
It is essential that all the vessel's valves are not shut against a flow of oil.
To avoid this, not less than a pre-determined minimum number of valves must be open during periods of maximum flow rate and specified in the loading plan.
Care must be taken when topping off tank(s) to ensure there are sufficient other valves open.
When the first tank has been topped off, the deck watch shifts to the next tank as directed by the Chief Officer and the process is repeated.
When the final tank is to be topped off, this valve should not be closed against the flow of oil.
Slack or empty tanks should be monitored to ensure that the set ullage does not change.
Care must be taken not to operate the tank valve controls by mistake, and if possible, a system of marking the valves to remain closed should be arranged.
Care must be taken to make sure that valves are shut properly, and the levels of tanks already topped off must be monitored to make sure there has been no change in the final ullage.
After confirming with the manufacturers for preventing the possibility of the hydraulically operated cargo valve to "creep," the control switch shall be left in the 'closed' position on nonoperational tanks when working / or after finished loading cargo.
A warning notice to be posted in the cargo control room of all tankers, that has a neutral position on the valve remote control switch, to the effect that the valve is kept in the closed position on nonoperational tanks when working / or after finished loading cargo.
Completion of Loading
Close the manifold gate valves after confirming the completion of transferring oil from the terminal.
Once cargo operations have ceased the Mast riser or other venting system in use should be closed to reduce the loss of light ends to the atmosphere, but the tank pressure should be closely monitored to ensure that the system is not over-pressurized.
Drain hoses and arms at the manifold. All manifold drain valves are to be operated under the Chief Officer; the duty deck officer must be stationed at the manifold and ensure that the correct valves are opened before confirming to the Chief Officer in the C.C.R. that the valves are opened.
After draining all oil in pipelines, close tank valves, and vent valves.
Ensure the connection is depressurized and isolated from the internal cargo tank I.G. pressure
All cargo in deck cargo lines should be dropped by gravity into a designated tank or tanks. Lines should not be dropped back to the pump room.
In parallel with draining work, measure the temperature and ullage in each tank to work out the loaded quantity.
On completion of gauging and sampling, all ullage ports, vapor locks, and any other tank openings should be confirmed closed.
Care should be taken to ensure that cargo lines do not become over-pressurized due to high ambient temperatures
The I.G.S. recorder shall be switched on to record and monitor the cargo tanks' pressure. It shall be suitably marked for details of Voyage Number, date, and time of turning on and corresponding present pressure.
This record shall be in continuous operation until the final discharge port.
Tank Gauging / Survey upon completion of Loading operations:
* Ullage Report
The following would need to be considered when carrying out accurate cargo measurements.
Case-1 Line Volume: If NOT included in the ship's tank measurement tables and more than one grade loaded;
* Loaded qty of 1st grade= Loaded Tanks Qty of 1st grade + ALL Lines (used for loading) Qty
* Loaded qty of 2nd, 3rd grades= Loaded Tanks Qty of 2nd, 3rd grades only
Case-2 Line Volume: If INCLUDED in the ship's tank measurement tables and more than one grade loaded;
* Loaded qty of 1st grade= Loaded Tanks Qty of 1st grade + Empty Tanks only: Lines (used) Qty (A')
* Loaded qty of 2nd, 3rd grades= Loaded Tanks Qty only of 2nd, 3rd grade above qty (A')
* The cargo tanks are to be gauged in the presence of the attending Surveyor / Loading master to confirm final ullages, temperatures, and free water presence.
* The vessel is to prepare the ullage report upon completion of the gauging of cargo tanks.
The surveyor's Ullage report shall be verified for ullages and temperatures only.
If an available copy of the surveyor documents to be retained on board, the Closed method of dipping such cargo tanks shall be followed.
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