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Oil tanker cargo loading procedure

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 from a conventional oil terminal through the midship manifold . Modern oil tankers may equipped with the most advanced loading systems, combining a Bow Loading (BL) system and the ship's part of the Submerged Turret Loading (STL) 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 buoy is the turret connection (toranj) to the mooring and riser systems. The outer buoy hull can rotate freely with the vessel around the turret by means of internal turret bearings.
Single point mooring system for oil tanker
Fig: Single point mooring system for oil tanker

Oil is transferred through an in-line swivel via the loading manifold to the piping system of the vessel. Disconnected, the buoy will float in an equilibrium position ready for 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..
Submerged Turret Loading system for oil tanker
Fig:Submerged Turret Loading system

Liquid cargo is nowadays usually transferred using an articulated arm loading/discharge systems, and groups of arms are often found on a shore refineries or on offshore loading facilities. It connects to the tanker’s manifold usually located near the centre of the ship.

Loading oil cargo in a tanker ship require utmost diligence in planning and most careful consideration will need to be made for safe operation. Following are the basic procedures at various stages of loading oil cargo,

Line up of the Vent lines

Prior loading operation commence, cargo tanks IG inlet lines to the designated tanks shall be re-checked and confirmed in desired position.

The control of the key to the locking arrangements for cargo tank IG inlet valves shall be with the Chief Officer.

For tanks which 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 in accordance with 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 in accordance with the loading plan.

• Commence loading at 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’ and 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 case of heated cargo, confirmation of temperature of cargo 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. Close watch of the manifold back pressure 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 IG back pressure 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 shall 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 in flowing 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 back pressure, 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 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:

a) General

The vessel’s maximum loading rate and maximum venting capacity must be posted in the cargo control room giving details of the rates for homogenous(entire the vessel), Group-by-group and Tank-wise loadings.

Such information, based on calculations, shall assist the Master to determine how fast the ship can safely load a particular cargo at a particular facility, taking into account the vessel’s design parameters and the cargo 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 generation of gas when loading, the venting rate shall be taken as 125% of the oil loading rate.

Maximum loading rates are affected by a number of 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 arrangement and capacity of the ship’s cargo lines and gas venting systems: the vent line pressure should not exceed that indicated by the builder and 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 number of tanks 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 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.

Crude oil tanker STENA ARCTICA
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 Officer’s Standing order:

Chief Officer shall give his written instructions of cargo plan to 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.
Where possible, comparison of real & calculated draft & trim shall be carried out to give proactive warning of any unplanned or unobserved deviation from plan.

Trim and draft

Ensure the draft maintained, after allowing for tidal variation, is well within the limits of the height limitation of manifold / loading arms. 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 “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 be used for Topping Off along with 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 CCR tank gauges.

• The Chief Officer should be notified of any discrepancies when he is called for 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 deck complete with extra oil and hoses, in case of a failure in the valve operating system.

Topping Off Operation

• If at any time the Topping off operation gives significant cause for concern, 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, then the shore should be requested to further reduce the pumping rate.

• 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 make sure that 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.

• On confirming with the manufacturers, to prevent the possibility of the hydraulically operated cargo valve to “creep”, the control switch shall be left in the ‘closed’ position on non operational tanks when working / or after finished loading cargo.

• A warning notice to be posted in the cargo control room of all tankers, that have a neutral position on the valve remote control switch, to the effect that the valve is to be kept in the closed position on non operational 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 knowledge of 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 CCR that the valves are opened.

• After draining of all oil in pipe lines, close tank valves and vent valves. Ensure connection is depressurized and isolated from the internal cargo tank IG 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, vapour 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 IGS 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 individual 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 presence of free water.

* The vessel is to prepare the ullage report upon completion of gauging of cargo tanks. The surveyors Ullage report shall be verified for ullages and temperatures only. If available, copy of the surveyors document to be retained onboard.

* Closed method of dipping such cargo tanks shall be followed.

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