Combination Carriers

Combination Carriers

This Chapter sets out safety measures to be taken on combination carriers in addition to those necessary for conventional tankers. For this purpose a combination carrier is a tanker designed to carry oil or solid cargoes in bulk, and is one of two main types, an Oil/Bulk/Ore ship or an Oil/Ore ship. Other types of combination carriers, which may for example carry liquefied gas and petroleum, or containers and general cargo, are not covered. Due account must be taken of any special local regulations.

GENERAL INSTRUCTIONS

Petroleum and dry hulk cargoes must not be carried simultaneously.

Attention should be paid to the gas contents of wing tanks when the vessel is discharging bulk ore. Similarly, it should be realized that damaged bulkheads might lead to flammable gas mixtures in ore holds.

TYPES OF COPABINATION CARRIERS

Oil/Bulk/ore (OBO)

The OBO ship is capable of carrying its full deadweight when trading as an ore carrier with cargoes of heavy ore concentrates. This type of ship is also designed to carry other types of dry bulk cargo such as grain and coal.

Holds are usually arranged to extend the full breadth of the ship, with upper and lower hopper tanks and double bottom tanks. In some cases holds may have wing tanks, Oil or dry bulk cargo is carried in the holds. Oil may in addition be carried in one or more sets of upper hopper tanks, and where there are wing tanks they may also be used. Normally wing tanks for the carriage of oily slops are fitted aft of the cargo holds. Permanent ballast may be carried in top and bottom hopper tanks and in double bottom tanks.

Conventional bulk carrier hatches, normally of the side rolling type, are fitted with a special sealing arrangement.

Cargo and ballast pipelines are typically installed in a duct keel or in two pipe tunnels located either side of the centerline and separated by a double bottom tank.

Oil/Ore (0/0)

These ships are designed to carry their full deadweight when trading as tankers and also when carrying heavy ore concentrates. They are not usually designed to carry light bulk cargoes. Heavy ore concentrates are carried only in the center holds. Oil cargo may be carried in both center holds and cargo wing tanks.

Holds are constructed so as to extend approximately one half of the total breadth of the ship. Conventional wing tanks incorporate the main strengthening sections, allowing smooth sides in the center holds. Holds are always constructed with double bottom spaces beneath them. Hatches are generally single piece side rolling with a sealing arrangement similar to that on OBO ships.

Cargo pipelines are usually installed in the wing tanks, whilst ballast-pipelines are typically installed in the double bottom tanks. Where cargo pipelines pass through permanent ballast tanks, the possibility of pollution caused by pipeline failure should be borne in mind.

VOID SPACES, DUCT KEELS AND PIPE TUNNELS

Between cargo holds there may be a void space, through which various piping systems can pass and access be gained to tank valves and double bottom tanks.

A single duct keel may be fitted along the centerline. On some ships two duct keels are fitted, one on either side of the centerline.

Some duct keels and pipe tunnels may be fitted with wheeled trolleys on rails to permit easier access to personnel and equipment. These spaces may be fitted with fixed lighting, fixed washing systems and a fixed gas monitoring system.

Because of their restricted natural ventilation these spaces may be oxygen deficient. Furthermore, they are adjacent to cargo holds and ballast tanks, so both hydrocarbon vapor and inert gas may leak into them. The precautions for entry into enclosed spaces given in Chapter 11 should therefore be strictly applied. The rescue of an unconscious or injured person from these confined spaces may be extremely difficult.

SLACK HOLDS IN COMBINATION CARRIERS

General

Because of the broad beam and size of the holds, the very large free surface in slack holds (i.e. holds not filled to within the coaming) permits substantial movement of liquid, which can result in both loss of stability and 'sloshing'.

Loss of Stability

Particular care should be taken when loading or discharging liquid cargo from combination carriers and when handling ballast on such ships to ensure that the total free surface effect of cargo and ballast tanks is kept within safe limits, otherwise a sudden, and possibly violent, change of list could occur.

In compliance with government requirements all combination carriers are supplied with stability data and loading and unloading instructions. These instructions should be carefully studied and followed. Generally, these instructions will specify a maximum number of cargo holds or tanks, which may be slack at any one time. Sometimes it may be necessary to adjust the quantity of cat-go to be loaded in order to avoid slack holds. Where double bottom ballast tanks extend across the whole width of the vessel, the free surface effect of water in these tanks will be as great as that of full cargo holds and account must be taken of this fact.

Some combination carriers have a valve interlocking system, which limits the number of tanks, which may be loaded or discharged simultaneously. Such systems may fail or can be bypassed, and it is recommended that a conspicuous notice is displayed at the cargo control station warning of the danger of free surface effect and stating the maximum number of holds that can safely be slack at any one time.

Before arriving in port, a plan should be prepared for the anticipated loading or discharging sequence, bearing in mind the free surface effect and distribution of all cargo, fuel and ballast at all stages of the operation.

Terminal operators should appreciate that combination carriers may be subject to loading rate limitations and to specific discharge procedures. These arise from the danger of hatch seals leaking if placed under excessive pressure, as well as from the free surface effects.

If a loss of stability becomes evident during loading or discharge, all cargo, ballast and bunker operations must cease and a plan is prepared for restoring positive stability. If the vessel is at a terminal the terminal representative should agree this plan and it might be necessary or prudent to disconnect the loading arms or hoses.

The specific action required to restore stability will be determined by the vessel's detailed stability information in relation to a particular condition.

In general the following principles will apply:

The vertical center of gravity must be lowered in the most effective way.

Where slack double bottom ballast tanks exist these should be filled, starting with those on the low side, followed by those on the high side.

If the pressing-up of slack double bottom tanks is insufficient to regain stability, it may be necessary to consider filling empty double bottom ballast tanks. It must be recognized that this will initially result in a further loss of stability caused by the additional free surface effect; this, however, will soon be corrected by the effect of the added mass below the vessel's original center of gravity.

No attempt should be made to correct a list by filling compartments on the high side, as this is likely to result in a violent change of list to the opposite side.

The restraint provided by moorings should be considered. To attempt to control a list by adjusting mooring rope tension could be dangerous and is therefore not recommended

On completion of loading, the number of slack holds should be at a minimum and in any event not more than that specified in the stability information book.

'Sloshing'

'Sloshing' is the movement of liquid within a hold when the vessel is rolling or pitching.

It can give rise to:

Structural damage caused by the slamming effect of the liquid against the ship's side or bulkheads.

An electrostatically charged mist in the ullage space in holds partially filled with mixture of oil and water, such as dirty ballast or retained tank washings. This can even occur with only a slight rolling motion.

In order to eliminate these problems, slack holds should be avoided whenever possible. This may be difficult when loaded with an oil cargo, but may be more readily achieved when vessel is in ballast

5 LONGITUDINAL STRESS

Consideration should be given to the distribution of the weights along the ship, taking account of the ship's longitudinal strength.

VENTING OF CARGO HOLDS

Venting Systems

The vent lines from the cargo holds may lead either to individual vent outlets, to a main gas line venting system which expels the hydrocarbon vapor through a riser at a safe height above the deck, or to an inert gas pipeline system.

Blockage of Vent Lines

Owing to the movement of liquid within the cargo hold in rough sea conditions, the possibility of liquid entering the vent line is greater than on a conventional tanker. Various trap systems may be incorporated, such as a U-bend or a7 special valve, but the possibility of a blockage should always be suspected after a rough voyage. A blockage may also occur if the vessel has been in very hot weather which has caused the cargo 'to expand above the gas line outlet.

Drains are normally fitted in each gas line and these should be routinely checked before commencing cargo operations in order to ensure that the cargo hold is able to 'breathe'.

These drains may become blocked, particularly during the carriage of high pour point cargoes, and gas lines should be blown through with inert gas to ensure 'they are clear.

Venting During Carriage of Dry Bulk Cargoes

During the carriage of dry bulk cargoes, the holds should be sealed from the main oil cargo pumping and gas venting systems and alternative venting systems utilized as required. Wing tanks should be maintained either in a gas free or an inert condition (see Section 10).

HATCH COVERS

Sealing

The hatches of combination carriers have a much more onerous duty to perform these ships are carrying liquid cargo than when carrying dry bulk cargo as they are required to remain gas and liquid tight at all times, even when the sharp is working in a seaway.

Regular attention should be paid to the closing devices, for example by adjusting them evenly and by lubrication of screw threads.

When closing hatch covers, the closing devices should be evenly and progressively pulled down in the correct sequence in accordance with the manufacturer's instructions.

In ships fitted with inert gas or fixed high-capacity gas freeing systems, a positive test of the of the sealing arrangements can be carried out by pressurizing the holds and applying a soapy solution to the sealing arrangements. Any leakage is readily detectable and should be rectified by further adjustment of the closing devices in the affected area. (See Section 9.3.2(f) for advice on the use of a fixed gas freeing system.)

The cover joints should be examined for gas leakage when the compartment is loaded with liquid cargo and any gas or liquid leaks which cannot be stopped by adjusting the closing devices should be marked or noted so that the jointing material can be examined when the opportunity arises and the joint made good. Additional sealing by means of tape or compound may be necessary.

If the ship is fitted with an inert gas system, the gas tightness of the hatch covers will affect the frequency with which the inert gas pressure needs to be topped up.

Most combination carriers use synthetic rubber for the hatch seals, and this material should be examined whenever a suitable opportunity occurs. It is also advisable to have on board a reasonable stock of jointing material of the correct size so that the repairs can be carried out at sea.

Rubbing in a Seaway

The hatch covers on combination carriers generally work when a ship is in a seaway and it is thus possible for the steel hatch cover to rub on the steel coaming. Investigations have shown that this is unlikely to provide a source of ignition. However, the joints between the hatch cover and hatch coaming should be cleaned before closing the hatch, especially after a dry bulk cargo has been carried. After donning appropriate personnel protective equipment, a compressed air hose with a suitable nozzle might be used for this purpose.

Foreign Matter in Runways

It is important to keep hatch cover runways clear of foreign matter in order to ease the opening and closing of hatches. After donning appropriate personnel protective equipment and an initial clearing with a brush, runways can be cleaned using a compressed air hose or washdeck hose. -

OPENINGS INTO CARGO HOLDS

Owing to the height of hatch coamings, which are partially filled on completion of loading a liquid cargo, all maindeck openings into cargo holds may have to withstand a positive pressure. It is essential therefore that all seals and gaskets on tank cleaning covers, access hatches, trimming hatches etc. provide an oil tight and gas tight seal. The seats should be cleaned to ensure a proper seal and all securing bolts should be hardened down prior to loading a bulk liquid cargo.

TANK WASHING

Any tank washing should be carried out in accordance with the, guidelines given in Chapter 9.

Cargo holds should not be used as slop tanks during cleaning because of risk of sloshing. Holds containing dirty ballast should not be discharged when the ship is rolling or pitching. Hatch covers should not be opened until hold is gas free. All closing devices should be kept secured to prevent movement of the hatch covers.

When cargoes other than oil, are intended to be carried it is essential that all holds and cargo tanks other than slop tanks are emptied of oil and oil residues and cleaned and ventilated to such a degree that the tanks are completely gas free. They should then be inspected to confirm this condition. The pumproom, cargo pumps, pipelines, duct keel and other void spaces should be checked to ensure that they are free of oil and hydrocarbon gas.

Most dry bulk ports require a gas free certificate to be issued in respect of a combination carrier presenting to load or discharge dry bulk cargo. Such certificates normally relate to holds holds and other spaces but will not confirm that pumps and pipelines are free from oil and/or hydrocarbon gas.

CARRIAGE OF SLOPS WHEN TRADING AS DRY BULK CARRIER

In addition to compliance with the requirements set out in Section 9, every effort should be Made to ensure that before a combination carrier is to be operated as a dry bulk carrier, any oil contained in the slop tanks is discharged ashore. After discharging the slops, the empty tanks should be cleaned and either gas freed or inerted prior to loading any dry bulk cargo.

If, however, slops cannot be discharged and have to remain on board the following precautions should be taken:

All slops must be collected in the slop tank specially designated for this purpose.

Blanking plates or other approved means of closure must be fitted in all pipelines, including common vent lines, leading to or from the slop tank to ensure that the contents   and atmosphere of the slop tank are isolated from other compartments.

The slop tank should be purged with inert gas and a positive pressure maintained within the tank at all times.

Carbon dioxide must never be used in liquid form to provide inert gas to the ullage space of the dirty slop tank because of the risk of generating an electrostatic charge.

Unless the tank is fully inerted, the slops should be handled in such a way as to avoid a -free fall of slops -into the receiving tank, as this may cause a build up of an electrostatic charge.

Unless the vessel reverts to carrying oil, oil slops should not be contained on board for more than one voyage. If, however, it is impossible to remove the slops because of the lack of shore reception facilities for oily residues, the slop tank should be treated as indicated above and a report forwarded to the owner and the appropriate administration.

LEAKAGE INTO BALLAST TANKS ON COMBINATION

CARRIERS

On combination carriers, a serious problem occurs if there is leakage of oil from the cargo holds into the permanent ballast tanks.

The known weak structural points are as follows:

On vessels with vertically corrugated transverse bulkheads, cracks may occur in the welded seams between these bulkheads and the upper hopper tanks. On vessels where, the upper hopper tanks and the lower hopper tanks are connected by a trunkway or a pipe, the contamination would affect the lower hopper tank in addition to the area around the actual fracture. On vessels where the upper hopper, tank is connected to the lower hopper tank by means of a pipe, it may be advisable to install a valve in the drop line to confine oil contamination to the upper hopper tank.

In double-hulled vessels, leaks may be found in the upper welded seams of the longitudinal bulkhead between ballast tank and cargo tank abutting the sloped deckhead of the cargo tank.

INERT GAS SYSTEM

Inert gas systems will generally be fitted on combination carriers for use when trading as oil tankers. They should be operated is described in Chapter 10.

An additional problem on combination carriers is that owing to the size of the hatch cover there is a greater chance of petroleum gas or inert gas leaks than on conventional tankers, particularly in view of the tendency of covers to work in a seaway. Adequate checks should be made to locate leaks and reduce the loss of inert gas (see Section 7.1).

TESTING OF CARGO TANKS AND ENCLOSED SPACES ON DRY BULK VOYAGES

Before loading a dry bulk cargo, all spaces, which have previously contained oil should be cleaned, gas freed and inspected internally. Once ail tank cleaning has been completed, daily checks for petroleum gas should be made in all empty cargo holds, empty cargo tanks and empty double bottom and ballast tanks, as well as in pumprooms, pipeducts, cofferdams, stool tanks and similar void spaces. If no petroleum gas has been detected after 14 days, the frequency of the readings may be reduced to every two days, unless the ship passes through areas with higher sea or air temperatures, in which case the daily chocks should continue.

If the next voyage is to continue in dry bulk cargo the readings on that voyage need be taken only every three days.

If petroleum gas is detected during any dry cargo voyage, the space should be ventilated with air. If the petroleum gas cannot be controlled by ventilation, the space should be inerted and remain so until it can be cleaned again.

CARGO CHANGE-OVER CHECK LISTS

The following check lists are of a general nature and each ship should develop its own comprehensive checklists using these as a guide.

Oil to Dry Bulk Cargo

Wash cargo holds and tanks including access trunks.

Flush all main Suctions into cargo holds and tanks and strip dry.

Gas free all cargo holds and tanks.

Hose off, blow through, disconnect and stow heating coils as required. Plug securing sockets as necessary.

Complete hand hosing and digging of holds and sumps to the requirements of the next cargo.

Drain cargo holds and suction wells.

Blank off main suctions to holds as necessary. Ensure stripping. discharge line to after hold is securely blanked.

Ensure sounding pipes to bilge wells are open and clear of obstructions.

Fit main and stripping suction recess doors as necessary. Also fit-heating coil connecting pipe recess doors.

Wash cargo pipeline system thoroughly, including pumps, decks lines, bottom line, and pumproom.

Ensure gauging system, where fitted, is stowed or blanked as necessary to manufacturers recommendations.

Drain, vent and prove gas free all gas lines and risers.

Blank off gas line to holds as necessary.

Set venting system to 'the requirements of the next cargo.

Check hatch covers sealing arrangements and closing devices.

Check ballast tanks, void spaces, cofferdams and pumprooms for flammable gas. Ventilate as necessary and prove gas free.

If slops are retained, ensure designated pipeline segregations are fitted, slop tanks are fully inerted and the relevant venting system adopted as necessary.

Dry Bulk Cargo to Oil

Sweep holds clean and lift cargo remains out of hold for disposal.

Wash cargo remains off bulkheads with a high-pressure water jet, stripping slowly to remove water, leaving solid residues.

Remove solid residues from the tank top and sumps, and prove that the stripping suction is clear.

Remove suction doors and attach securely to stowage positions.

Close off sounding pipes to sumps as required.

Remove blanks from main cargo suctions and stripping discharge to after hold.

Lower, secure in place, connect -arid prove tight heating coils as necessary.

Remove requisite blanks from gauging system and render fully operational.

Wash all stripping lines thoroughly to remove solid residues.

Open, clean and check all strainers in cargo systems.

Check and clean hatch cover sealing arrangements, trackways etc.

Close hatches and check sealing and bolting down arrangements.

Remove blanks from gas lines as necessary.

Set venting system for next cargo.

Prove all valves and non-return valves in cargo system operational.

Inert holds prior to loading where applicable. During inertirig prove tightness of hatch covers, clearing covers, access hatches and all openings auto cargo spaces.

15 DISCHARGE OF DRY BULK CARGO

During the discharge of dry bulk cargo it may be necessary to ballast one or more holds to reduce the air draught of the ship. This is unlikely to introduce hazards if the pipeline system his been well washed. However if a pump or pipeline has not been adequately washed, the ballasting operation may discharge residual oil into the hold. Atmospheric tests in the hod (in accordance with Section 2.8.4) should therefore be made before any hot work is carried out in, adjacent to, or above a ballasted hold.