Part VII Combination Carriers - TP 4295 E

Transport Publication TP 4295 E

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The basic principles of inerting are exactly the same for a combination carrier as for a tanker; however, differences in the design and operation of these vessels make necessary the relevant considerations outlined below.

Slack Holds

48. It is particularly important for combination carriers to have their holds inerted because, whenever a hold in an OBO carrier (which could extend the full breadth of the ship) is partially filled with clean or oily ballast, water agitation of this ballast can occur at small angles of roll, resulting in the generation of static electricity; the agitation, sometimes referred to as ‘sloshing’, can happen whenever the ullage above the liquid level of the hold is more than 10 per cent of the depth of the hold, measured from the underside of the deck (see Figure 18 for remedy condition).

figure 18

Figure 18 Cargo tank ullage effect on generation of static electricity ^


49. To ensure that leakage of tank gas, particularly through the hatch centreline joints, is eliminated or minimized, it is essential that the hatch covers be inspected frequently to determine the state of their seals, their alignment, etc; when the hatch covers have been opened, particularly after the ship has been carrying a dry bulk cargo, the seals and trackways should be inspected and cleaned to remove any foreign matter.

Ballast and Void Spaces

50. The cargo holds of combination carriers are adjacent to ballast and void spaces; leakages may occur in pipelines or ducts in these spaces, or from a fracture in the boundary plating; oil, inert gas and hydrocarbon gas may leak into the ballast and void spaces; consequently gas pockets may form and cause difficulty with gas-freeing because of the considerable steelwork, acting as stiffening, that is characteristic of these spaces; personnel should be alerted to this hazard.

Inert Gas Distribution System

51. Due to the special construction of combination carriers, the vent line from the cargo hatchway coaming is situated very close to the level of the cargo surface; in many cases, the inert gas main line passing along the main deck may be below the oil level in the hold; during rough weather, oil or water may enter these lines, completely blocking the opening, and thus prevent an adequate supply of inert gas during either tank cleaning or discharge; vent lines should therefore have drains fitted at their lowest point, which should always be checked before any operation takes place within the cargo hold.

Application when Carrying Oil

52. When combination carriers are engaged exclusively in the carriage of oil, the inert gas system should be utilized in the manner described in Part (V).

Application when Carrying Cargoes Other than Oil

53. (1) A combination carrier transporting a cargo other than oil should be considered a tanker, unless the requirements in subsection 53(8) are complied with.

(2) When cargoes other than oil are intended to be carried, it is essential that all holds/cargo tanks other than slop tanks referred to in subsection 53(6) and subsection 53(7) be emptied of oil and oil residues, cleaned and ventilated to such a degree that the tanks are completely gas free and internally inspected; the pumproom, cargo pumps, pipelines, duct keel and other void spaces are to be checked to ensure that they are free of oil and hydrocarbon gas.

(3) Holds required to carry cargo other than oil should be isolated from the inert gas main and oil cargo pipeline by means of blanks; these blanks should remain in position at all times when cargoes other than oil are being handled or carried.

(4) During the loading and discharging of solid cargoes, and throughout the intervening periods, all holds/cargo tanks [other than the slop tanks referred to in subsection 53(6) and subsection 53(7)], cargo pumprooms, cofferdams, duct keels and other adjacent void spaces should be kept in a gas-free condition and checked periodically at intervals of not more than two days to ensure that:

  1. no hydrocarbon gas has been generated or leaked from slop tanks referred to in subsection 53(6) and subsection 53(7); if concentrations of more than 20 per cent of the lower flammable limit are detected, the compartments concerned should be ventilated;
  2. no oxygen deficiency exists that could be attributable to leakage of inert gas from another compartment.

(5) As an alternative to subsection 53(4), empty cargo tanks may be re-inerted in accordance with section 30, provided they are subsequently maintained in the inert condition and at a minimum pressure of 100 mm water gauge at all times, and provided that they are checked periodically, at intervals of not more than two days, to ensure that any generation of hydrocarbon gas does not exceed one per cent by volume; if such a concentration is detected, the compartments concerned should be purged in accordance with section 39.

(6) Slops should be contained in a properly constituted slop tank and should be

  1. discharged ashore, and the slop tanks cleaned and ventilated to such a degree that they are completely gas free and then inerted; or
  2. retained on board for not more than one voyage when, unless the vessel reverts to carrying oil, the slop tank should be treated as in subsection 53(5).
If slops are retained on board for more than one voyage because reception facilities for oily residues are not available, the slop tank should be treated as in subsection 53(5) and, in addition, a report be forwarded to the Board.

(7) Slop tanks that have not been discharged should comply not only with the requirements of subsection 53(6), but also with the Fire Protection, Detection and Extinguishing Equipment Regulations, subsection 11(3), Schedule VII; the latter requires that they be isolated from other tanks by blank flanges, which will remain in position at all times when cargoes other than oil are being carried, except as provided for in this Standard; reference is also made to subsection 53(3); on combination carriers where there are also empty cargo tanks that are not required to be isolated from the inert gas main, the arrangement for isolating the slop tanks from these tanks should be such as to:

  1. prevent the passage of hydrocarbon gas from the slop tanks to the empty tanks; and
  2. facilitate monitoring of the pressure in slop tanks and in any empty cargo tanks, and, if necessary topping it up, if the latter are being kept in the inert condition as referred to in subsection 53(5)

Figure 19 shows suggested arrangement.

figure 19

Figure 19 Inert gas by-pass arrangement for topping up cargo tanks ^

in addition, all cargo pipelines to or from the slop tanks should be blanked off.

(8) Instead of complying with the requirements in subsections 53(2) to 53(7), a combination carrier may operate as a bulk carrier without having to use its inert gas system if either

  1. it has never carried a cargo of oil; or
  2. after its last cargo of oil, all its cargo tanks, including slop tanks, the pumproom, cargo pumps, pipelines, cofferdams, duct keel and other void spaces are emptied of oil and oil residues, cleaned and completely gas-freed, and the tanks and void spaces internally inspected to that effect; in addition, the monitoring required in subsection 53(4) should be continued until it has been established that generation of hydrocarbon gas has ceased.

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