Measurement of liquefied gas on sea-going vessels

Brodovi za prijevoz ukapljenih plinova posebna su vrsta tankera čiji tekući teret čini određena vrsta plinova ili mješavina plinova. Teret se dovodi u tekuće agregatno stanje, odnosno ukapljuje, i u njemu se održava za vrijeme operacija ukrcaja i iskrcaja kao i skladištenja tijekom prijevoza morem. Naglim razvojem petrokemijske industrije, a naročito intenzivnijim iskorištavanjem prirodnog plina, koji je energent s najbržim porastom potrošnje u svijetu, pojavio se problem transporta plina do potrošača. Visoki troškovi izgradnje, održavanja i korištenja kopnenih i podmorskih plinovoda, kojima se plin transportira u plinovitom stanju pod velikim tlakom, uvjetovali su graničnu udaljenost iznad koje je prijevoz plina u ukapljenom stanju brodovima ekonomski isplativ. Prijevoz ukapljenih plinova brodovima samo je jedan dio lanca poslovanja s ukapljenim plinovima koji obuhvaća ukapljivanje i skladištenje kod proizvođača, prijevoz brodovima te prihvat, skladištenje, isparavanje i isporuku potrošačima. U ovom radu objašnjen je cjelokupan proces prijevoza i mjerenja ukapljenog plina na brodovima. U navedenom prijevoznom lancu vrlo je važno točno odrediti količine tereta preuzetog na ukrcaju, i izvršiti utvrđivanje točne količine tereta u brodskim tankovima. Nakon raspreme ili dokovanja broda, teretni tankovi napunjeni su s inertnim plinom ili dušikom u plinovitom stanju. Ako je pročišćavanje (Purging) izvršeno s inertnim plinom, tankovi tereta moraju se zagrijavati (Gassing up) i ohladiti kada brod stigne na terminal za ukrcaj tereta. Tijekom zagrijavanja inertni plin u tankovima za teret zamjenjuje se sa zagrijanim LNG evaporatom (parom). Temeljeno na praktičnom iskustvu na mjerenjima utvrđivanja količine tereta na LNG brodovima, provedenim tijekom mnogih operacija prevoženja ukapljenih plinova, utvrđivanje količina ukapljenog plina treba biti temeljeno na brodskim mjerenjima. Precizno mjerenje tereta na temelju brodskih značajki ovisi o preciznost kalibracije tankova i baždarenju povezanih mjernih uređaja za razinu, temperaturu i tlak u svrhu dosljednog provođenja određene metodologije mjerenja.Vessels for the transport of Liquefied Gases are a special type of tanker whose liquid cargo is made up of certain types of gases or mixtures of gases. The cargo is supplied in the liquid state, ie is liquefied, and is held therein during the operation of loading and unloading and storage during the sea transport. Due to rapid development of the petrochemical industry, and particularly intense leveraging natural gas, which is an energy source with the fastest increase in consumption in the world, there was a problem of transport of gas to consumers. The high costs of construction, maintenance and use of land and subsea gas pipelines, in which gas is transported under high pressures, have caused the boundary distance over which the transport of gas in liquefied state to ships is economically payable. Liquefied gas transport is only one part of the liquefied gas chain, which involves the liquefaction and storage at the producers, the transport by ships, and the acceptance, storage, evaporation and delivery to consumers. This paper explains the entire process of transport and measurement of liquefied gas on ships. In mentioned transport chain, it is very important to precisely determine the quantities of cargo loaded on board, and to determine the exact amount of cargo in the ship's tanks. After loading or unloading the cargo tanks are filled with inert gas or nitrogen in the gaseous state. If purging is performed with inert gas, the cargo tanks must be gassing up and cool when the ship arrives at the cargo terminal. During gassing up, the inert gas in the cargo tanks is replaced with heated LNG evaporator. Based on the practical experience of the measurements of the quantity determination on LNG ships observed during a number of Liquefied Gas shipping operations, the quantity determination of liquefied gas should be based upon ship measurements. The key to accurate cargo measurement based upon ship's figures depends on the precision of the tank calibration and calibration of associated level, temperature and pressure measuring devices in addition to the use of consistent methodology