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Cargo sampling & manifold arrangement for liquefied gas carriers

Why Sampling ?
: Cargo is normally sampled by shippers’ or receivers’ personnel, or by authorised petroleum inspectors. The responsible officer must, however, be present when sampling is carried out to ensure that samples are taken from correct sampling points and that this is performed in a correct and safe manner. He is to also make a proper record of the samples taken as these may be of considerable value subsequently. A good rule is to request samples to be taken from the liquid shore connections at the start of loading to safeguard against possible contamination of shore transfer lines.

The following precautions are to be observed when sampling cargo liquid or vapour:-

Liquid samples

(a) The sample container must be completely clean and compatible with the cargo to be sampled and is to be able to withstand the extremes of temperature and pressure anticipated.

(b) Sample containers must be purged of air by pure nitrogen before use with flammable cargoes.

(c) If the sample is to be representative its container has to be purged thoroughly with cargo from the sampling connection. Sufficient cargo must be passed through the container to cool it down to liquid temperature. If the cargo is a mixture (which is often the case) the most volatile components will evaporate more rapidly than the heavier fractions as the container is cooled down; this will leave the sample with a higher concentration of the heavy fraction than present in the cargo, and it will therefore be unrepresentative.

To counteract this, sample containers are to be turned with the vent valve downwards during cool down, to drain off the liquid that first collects. For the same reason, samples from the bottom of cargo tanks at the beginning of, or just after, loading may not be representative. It is recommended that the cargo is circulated using the cargo pump, if possible, before taking bottom samples.

(d) It is imperative that sufficient ullage or vapour space is left in the sample container to allow for the liquid expansion that will occur when the temperature increases to ambient. Ullage is obtained by holding the full sample container upright after disconnecting it from the sample connection and draining some liquid by opening the bottom valve for a moment.

(e) Unless the sample container is free of cargo vapour, it should not be stored in an unventilated space.

(f) Gloves, goggles and protective clothing must be worn when sampling cold cargoes.

(g) If the cargo is toxic, a suitable respirator, or preferably self-contained breathing apparatus, must be worn. If sampling in an enclosed space, a respirator is unsuitable, due to the possibility of asphyxiation, then breathing apparatus is necessary.

(h) If electrical equipment is used when taking samples this is to be of the certified-safe type.

Vapour samples

(a) Appropriate precautions as stated above paragraph (a), (b), (c), (f), (g) and (h) are to be observed when sampling cargo vapour or inert gas.

(b) Plastic sample bags are sometimes used for collecting vapour samples. These must be handled carefully, never used for liquid samples and always purged after use.

Manifold arrangement

(1) The manifold area should be clear of obstructions which could interfere with the automatic release of a hard arm.

(2) Pressure gauges should be regularly checked during the discharge for manifold valve leakage.

(3) All flange connections should be fully bolted.

(4) This includes any line which is being used for, or might become pressurized during, cargo operations on both sides of the vessel.

(5) The manifold valves and lines should be clearly marked as to whether they are liquid or vapour.

(6) The manifolds fitted with drain lines and purge points should be valved & capped.

Related Information:

Custody Transfer Measurement (CTM) System

Records of the calibration of key cargo instrumentation, including temperature and pressure gauges

The high level alarm system

Gas analyzing equipment

Safety equipment

Decontamination showers and an eye-wash

Gas cargo containment systems - primary barrier (the cargo tank),secondary barrier, thermal insulation and more

Discussion prior to cargo transfer in liquefied gas carrier

Safety checklist for gas carrier

Tanker Cargo Operations Logbook

Connecting Bonding Cable

Safety checklist for gas carrier

Reactivity of liquefied gas cargo and safety guideline

Procedure for loading Liquefied Gas Cargoes

Procedure for Cargo Conditioning in Liquefied Gas Carriers

Procedure for segregation of Liquefied Gas Cargoes

Procedure for Stripping Liquefied Gas Cargoes

Procedure for Changing Liquefied Gas Cargoes

Displacing Atmosphere with Inert Gas (Inerting)

Maintaining tank pressure during a laden voyage

Liquefied gas carrier safety training

Tackling fire onboard liquefied gas carrier ship

LNG spill risk during marine transportation and hazards associated

Inerting of Cargo Tanks prior loading LNG cargo

Gassing-up requirement for cargo tanks

Preparation for Cargo Transfer

Procedure for discussion prior cargo transfer

Defining various gas carrier types

Fuel flexibility of LNG ships

LNG ship spillage risk

LNG shipment

Initial Cool Down of cargo tanks

Leaks on the Cargo System, Continuous Flow - how to prevent

LNG tank leaks and immediate action by gas carriers

Leaks from a Loading Arm due to Tidal or Current Effects

Minor or major leaks from LNG tanks

Procedures for LNG cargo discharging

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