Delft University of Technology
Faculty Mechanical, Maritime and Materials Engineering Transport Technology
S.H. van Leeuwen Registration of stock control at Gate. Designing a registration system for the stock control of an LNG terminal Masters thesis, Report 2006.TIL.7090, Transport Engineering and Logistics / Transport, Infrastructure and Logistics.
Gate terminal
In April 2005, a joint venture between N.V. Nederlandse Gasunie and Royal Vopak exists, named 'Gate' (Gas Access to Europe). This joint venture will set up the first free entrance Liquefied Natural Gas (LNG) import terminal in the North-west section of the Maasvlakte at Rotterdam. The terminal will be operational in 2010.
Characteristics of LNG
LNG is natural gas that is converted into its liquid state by cooling it at atmospheric pressure to approximately -160 °C. LNG is odorless, non-toxic and non-corrosive and is not kept under pressure. LNG is flammable only if it comes into contact with an ignition source in its regasified state and the percentage of the gas in the air is between 5% and 15%. The volume of LNG is approximately 600 times less than that of gaseous natural gas. In the form of LNG, natural gas is easy to transport in large volumes over large distances.
Motive for research
The free entrance-principle means that during a short period in the supply chain, Gate terminal B.V. is the custodian of the LNG. The ownership of the LNG can vary along the supply chain, but it is never Gates' property, and therefore the terminal has to know whose property it is, at any time in any circumstance. For these purposes, suitable stock control is vital. It is the first LNG terminal that works according to this principle; at existing LNG terminals, customers have separate tanks, or the LNG is property of the terminal operator. Because the Gate terminal is the first free-entrance terminal, there is no suitable registration system for stock control available yet.
Besides the free entrance-principle, LNG has chemical properties that have to be taken into account.
Different qualities of LNG are mixed into the same tank, as customers share tanks at Gate. These different qualities all have different Wobbe-indexes (this index number is calculated from the caloric value of the LNG, this is explained in section 2.2.1), and mixing these different qualities results in one (assumed to be) homogeneous new quality of LNG. Gate terminal B.V. is struggling with this phenomenon, since the chemical characteristics of the LNG have changed and it becomes difficult to return the same product to the customers. Since this phenomenon is new, there is no data available on the impact of the mixed LNG.
Main research question
The free entrance principle and the chemical properties of LNG formed the motive of the research that is being executed; and of which the outcome is presented in this report. The main research question that will be answered in this report is:
How to design a continuous registration system for stock control for an independent LNG terminal, with the Gate terminal as reference, which provides insight into the stock position in the terminal processes at any time, and can be linked to the invoicing of customers?
In order to answer this question, the following subjects where analyzed in this research project: Future customers of Gate terminal B.V.
It is expected that suppliers from countries like Algeria, Qatar, Nigeria, Egypt Libya and Oman will deliver LNG to the Gate terminal. The future customers of Gate terminal B.V. are downstream market players (distribution and trade companies) and large scale consumers like industries. The main requirements of the future customers towards Gate terminal B.V. are: being able to have variable sendout (including swing), variable inventory and variable inflow; to arrive with variable LNG qualities, and all should be expressed in terms of volume.
Unit of measurement
Gate terminal B.V. decided to choose volume as the leading unit of measurement, mainly since the throughput of the terminal is expressed in this unit of measurement. The customer will receive an invoice that will contain the amount of throughput in cubic meters of LNG, cubic meters of gas and the converted energy in the unit of measurement that they desire.
Terminal processes and metering
Figure A: Terminal processes, metering and changing characteristics of LNG at the Gate terminal Unloading of the ship
When the LNG carriers arrive at the terminal, the volume is being measured by making use of the tanker dip method; the quality is being measured by a gas chromatograph (GC). The Wobbe index is derived from the density and Gross Caloric Value that comes from the GC (accuracy of +/- 1%). These play an important role in the registration system for stock control.
LNG to unloading line and recirculation line
The LNG is being pumped through the above mentioned lines to the storage tank. All lines in the terminal have flow meters. These are not used for the registration system for stock control since it is not necessary and because the measurement is not accurate enough. The amount of LNG that is present in the lines is part of the dead stock of the terminal, since it is needed to keep the terminal cold.
LNG storage tank
The LNG is pumped in the storage tanks in a way that the effect of 'roll over' is minimized. There is temperature-, pressure- and level- metering present in the tank, but this is not used for the registration system for stock control since it is not accurate enough; the metering serves to support the balanced operations (overall check).
Boil off gas (BOG)
The BOG is not metered separately. For the registration system for stock control the BOG does not have a particular function. The BOG to recondenser is metered with the total outflow, the BOG during unloading of ship is metered at the jetty by the tanker dip method, and BOG to vent/ flare only plays a role during emergency; then it is metered apart from the registration system for stock control (not accurate!).
Recondenser / absorber
In the recondenser the metering of the LNG flow is not accurate enough to use for stock control. The amount of Nitrogen that is added to the recondenser is controlled by the QMI, and plays a role in the registration system for stock control.
Pressurizing of LNG
During this process only the pressure is metered and it plays no role in the registration system for stock control. Vaporizers + Natural gas sendout to GTS
Between the vaporizers and the sendout, the natural gas is being metered by dual flow meters (ultrasonic type) which are run in parallel and trigger a shut down if calibrations appear to drift. The quality of the natural gas is being measured by gas chromatographs which are very precise (+/- 1%). The Wobbe index is derived from the density and Gross Caloric Value that comes from the GC. These play an important role in the registration system for stock control.
Invoicing
The customer is being charged by making use of the pricing model of Gate; which contains two parts:
Basic service: charges the customer on number of LNG cargoes, maximum ship size and annual throughput. The basic service charge is determined for 20 years. The customer is able to adjust it yearly and pays monthly.
Additional service charge: charges all additional services that the customer has used. These are: inventory option, flexibility option and deviation ship size. The Nitrogen is charged separately from the pricing model and can have a large impact on the invoice, depending on the Wi of the customer and the mean Wi in the tank (as result of mixing).
TPS-model
The registration system for stock control and the pricing model of Gate terminal B.V. are linked by the Terminal Processes Simulation model (TPS-model), which is an Excel based model that simulates the terminal processes and links the output of these to the pricing model (which is included in the TPS-model) by making use of an algorithm.
Sub models: Mixing LNG and adding Nitrogen
how LNG will mix in the storage tank, how much Nitrogen is needed to meet the specifications of GTS (Wi = 54) and simplifies the relation between the calculation of the mean Wobbe index when mixing, and the relation between the Wobbe index and Nitrogen.
The findings of the sub model are the following:
When different types of LNG mix, the Wi- value of the newly formed LNG is almost equal to the arithmetic mean. For the model in section 4.2, the arithmetic mean is sufficiently accurate.
The relation between the Wi and Nitrogen adding is linear when the gas/liquid ratio is constant. Since the g/l ratio differs per LNG type, and the calculation of the g/l ratio is complex, a band width is assigned between g/l = 550 and g/l = 600. All 17 LNG types considered in this analysis have a 95% probability that the estimation error is between 43,5 and 39,8. For the TPS-model the g/l ratio of 600 is chosen in order to be on the safe site; Gate terminal B.V. rather wants a bit too much Nitrogen, instead of having to stop the sendout to wait for more Nitrogen. The formulae for Nitrogen adding are:
For Wi - limit = 54,0: y = 1970*Wi - 106.410 For Wi - limit = 56,0: y = 1993*Wi - 111.608
Where y is the amount of required Nitrogen [m3 liquid], and Wi is the Wobbe index of the LNG that has to converted to the Wi-limit [MJ/m3(n)]. These formulae are based on a ship size of 145.000 m3 LNG and only hold when the relative density is within the range of 0.571-0.671 kg/m3 (n). The other assumptions are stated in the conclusions of appendix C2 of the report.
Conclusions
The registration system for stock control that is designed in the TPS-model is a simplification of the reality, but is shows how a registration system for stock control can be designed that meets all requirements of Gate terminal B.V. and its customers, including the ability to update the stock level almost
continuously (approximately every 10 minutes, depends on processing time of the metering). The registration system for stock control that is introduced in this report is applicable to other LNG receiving terminals that have the free-entrance principle; the Gate terminal is used in this report as a reference terminal, but there are no indications that another similar terminal would operate differently.
Recommendations
The TPS-model is build in MS Excel, and does not take any stochastic events into account, since these are not required in order to design a registration system for the stock control of the terminal. It would be possible to implement these features in the TPS-model (enlarges the sensitivity of the model), but other simulation tools are specially designed for these purposes, and the recommendation is to use those.
The gas/liquid ratio and the Gross Caloric Value are assumed to be constant in the TPS-model. As indicated, this is done because there is no data available on the topic of mixing LNG. In case Gate terminal B.V. wants to have a more realistic model, the recommendation is to do more research on the chemical properties of LNG and its associated parameters.
The next step for Gate terminal B.V. would be to implement the basis of the registration system for the stock control as described in this report into a software system. The recommendation is to find existing software that is already working at other similar terminals. When these software packages are found, the functional requirements of Gate terminal B.V. should be compared to the functionality of that system. Based on that comparison, Gate terminal B.V. can decide to develop new software or to use existing software that needs application for the Gate terminal.
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