Ships – Building – Freighters
Patent
1993-11-03
1995-03-21
Basinger, Sherman
Ships
Building
Freighters
114211, B63B 2512
Patent
active
053986296
DESCRIPTION:
BRIEF SUMMARY
FIELD OF INVENTION
The present invention relates to a method for loading and discharging of tankers for transportation of crude mineral oil/petroleum products, in the following referred to as oil, a method for transportation of oil in tankers, as well as a pipe and valve arrangement on tankers for application of said methods.
PRIOR ART
Transportation of oil with tankers consists mainly of four operations; loading of oil into the cargo tanks at the supply location, transportation of oil from the supply location to the destination, discharging of oil from the cargo tanks of the tanker at the destination and the ballast trip, i.e. a trip in which the tanker is not transporting oil, from the destination back to the supply location.
Today's methods for loading and discharging of oil may result in losses of oil and substantial strains on the surroundings resulting from the fact that some of the oil transforms into gas which in turn is pushed out to the atmosphere. Upon starting the loading of oil into the cargo tanks of the tanker, the oil is supplied under high pressure to tanks with lower pressure. These tanks are prior to loading typically occupied by gases with a low pressure and low concentrations of hydrocarbons, in the following referred to as HC, and the oil will therefore start developing gas to reach a combination of pressure and HC-concentration giving saturation at the present temperature of the oil.
Upon discharging of oil from the cargo tanks the oil will leave the tanks by means of pumps via pipes at the bottom of the tanks. This results in movements in the oil, and will further lead to that the ullage space increases and that the pressure above the oil decreases. Consequently, the oil will release HC gas until the saturation pressure is reached. When the pressure drops below a certain value, valves at the top of the tank will open and inert gas or air is supplied. After this there exists a low pressure gas mixture above the oil. This interaction between pressure drop, de-gassing of the oil and introduction of inert gas or air will continue throughout all of the discharging, and upon completion of the discharging, relatively great amounts of HC gas may have been released to the tank atmosphere. The actual amount will depend on the vapor characteristics of the oil and the saturation pressure of the oil gas at the present temperature.
De-gassing of oil may also represent a problem during the transportation trip. Because of a possible expansion of the oil during transportation, the cargo tanks are typically loaded up to 98% of the cargo capacity. During transportation the movements of the tanker will propagate to the oil. The continuing movements at the surface of the oil together with the pressure fluctuations in the ullage space filled with gas, results in that the oil constantly may release gases to the free air outside of the tanker resulting from the fact that the gas pressure is higher than the setting pressure of the pressure/vacuum valves typically arranged at the top of the tanks.
Great discharges of oil resulting from a possible grounding of the tanker may represent another problem during the transportation trip. Prior to the grounding of a fully loaded tanker the pressure at the inside of the tank bottom will be higher than the pressure from the outside; there exists a hydrostatic pressure difference. Upon grounding there is formed a rupture in the tanker bottom, resulting in that oil is leaking out of the tanker until the pressure at the inside of the bottom equals the pressure at the outside; a hydrostatic balance has been established at the bottom. This balance will be established when the surface of the oil has been reduced to a level corresponding to the hydrostatic pressure difference existing at the tanker bottom prior to the grounding plus the outer pressure reduction resulting from the fact that the draft of the tanker is reduced when oil is leaking out of the cargo tanks. If the gas-filled ullage space is communicating with the atmosphere, there will exist an approximate bala
REFERENCES:
patent: 2049987 (1936-08-01), Willenborg
patent: 4144829 (1979-03-01), Conway
patent: 4233922 (1980-11-01), Conway
patent: 4292909 (1981-10-01), Conway
patent: 4446804 (1984-05-01), Kristiansen
patent: 5054526 (1991-10-01), Perkins
Basinger Sherman
Den Norske Stats Oljeselskap AS
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