Methodology used to calculate technical capacity

The role of the transmission system is to enable transmission of natural gas from the source (production fields, entry interconnections, underground gas storage, LNG terminal) to the location of its consumption (plants of end gas buyer directly connected to the transmission system, distribution systems to which other end buyers are connected to) and enable gas transit to other countries via interconnection exits. By the liberalisation of gas market in Europe, connecting national gas markets and developing new supply routes (e.g. LNG terminal on the island of Krk) the need for transit of gas via territory of the Republic of Croatia (hereinafter: Croatia) increases, so technical capacity of transmission system entries and transmission system exits (hereinafter: entry/exit) are adjusted to the relevant requirements.
The term technical capacity is defined by Regulation (EC) no. 715/2009 of the European Parliament and of the Council of 13 July 2009 on conditions for access to the natural gas transmission networks: technical capacity means the maximum firm capacity that the transmission system operator can offer to the network users, taking account of system integrity and the operational requirements of the transmission network.

Procedure of determining technical capacity of transmission system entries and transmission system exits
The aim and purpose of determining and updating technical capacity of the transmission system entry/exit is to harmonise technical capacities with the requirements of the Croatian natural gas market and markets of neighbouring countries taking into account technical possibilities of the transmission system. Procedure of determining technical capacity is an important phase in the procedure of planning transmission system development and providing sufficient capacities of the Croatian transmission system for reliable and secure supply of natural gas to all end buyers in Croatia and for the transit of natural gas for the needs of other national markets.
Technical capacity determined by this procedure is a maximum firm capacity that the transmission system operator Plinacro can offer to the transmission system users at an individual transmission system entry/exit and the users can contract it and use it. Technical capacity of transmission system exit depends directly on technical capacity of transmission system entry and it cannot be higher than the associated connecting capacity determined in energy consent.
The procedure of determining and updating technical capacity is carried out at least once a year and that is two months before the deadline for submitting annual capacity booking and exceptionally when the expressed demand for capacities at transmission system entries/exits exceeds technical capacity.
Procedure of determining and updating data on technical capacity for entries/exits which may be offered to users of gas transmission service as firm capacity contains following main phases:

1. Collection and analysis of data on demand for capacity use
   • at transmission system exits within Croatia
   • at transmission system entries from the network of production pipelines  
   • at interconnections
2. Preparation of network models for hydraulic calculations/simulations
3. Preparation of characteristic scenarios and implementation of hydraulic analyses
4. Interpretation of calculation results and defining the amount of technical capacity 

 Collection and analysis of data necessary for capacity use 
Having in mind fluctuations in the consumption of natural gas in Croatia, changes of supply routes and dynamics at the gas market in Croatia and neighbouring countries as well as demand for using Croatian transmission system capacities it is necessary to periodically collect data from energy entities used when determining technical capacity.
Data used in the procedure of determining technical capacity are the following:

1. Exits from the transmission system which are entries into the distribution systems

• Peak daily gas consumption determined by measuring flow at an individual exit over the previous ten years,
• Data on development plans of distribution system capacities and projection of the total annual gas amount planned to be off-taken from the transmission system for the ten-year period, which distribution system operators deliver each year to the transmission system operators in compliance with the Gas Market Act for the purpose of drawing up the ten-year plan of transmission system development.

To avoid possibility that due to possible insufficient estimate of the distribution system operator insufficient technical capacity is determined, the higher of the above stated figures is taken as the technical capacity.

2. Exits from the transmission system for end buyers directly connected to the transmission system

In compliance with the Gas Market Act each year end buyers of gas, directly connected to the transmission system, deliver projections of the total annual gas volume which they intend to off-take from the transmission system as well as information on the planned maximum capacities of natural gas off-take at the relevant connections (exits from the transmission system) for the ten-year period.
By comparing data received in such manner with actually used capacity at the relevant exit, that is, with the peak daily gas consumption, determined by flow metering at an individual exit over the previous ten years, technical capacity is determined by applying the “higher figure rule”.

3. Transmission system entries from the network of production fields

The amount of technical capacity is determined based on the data on the natural gas production plan in Croatia for a ten-year period, which is submitted every year by a natural gas producer in compliance with the Gas Market Act.
The amounts of announced new entries into the transmission system, which should become active in the coming period, are also taken into consideration. In connection with the above, it should be mentioned that many years exploitation and depletion of production fields show a natural decline in the production, that is, the current gas flow that the production plants can achieve, so it is unrealistic to retain the amounts of technical capacity at the entries from production at the levels achieved in the past.

4. Entry/exit at interconnections

Technical capacity of interconnections is determined by bilateral cooperation with transmission system operators with which there is a joint interconnection and it is determined in the joint interconnection agreement, in accordance with the Commission Regulation (EU) 2015/703 dtd 30 April 2015 establishing a network code on interoperability and data exchange rules.
 
Preparation of network models for hydraulic calculations/simulations
Hydraulic calculations check whether the transmission system can meet the gas transmission needs arising from the amount of flow at entries/exits determined in the earlier stages of the process.
For the stated hydraulic calculations, a specialised computer software package is used for simulation of the processes in gas networks, in static and dynamic flow scenarios. The SIMONE software package is highly standardised and offers a choice of various features and modelling depths, all of which can be controlled by the same graphical user interface. When using a computer programme for hydraulic simulations as two sets of data necessary for carrying out simulations, a distinction should be made between networks and scenarios.
A network is a description of technical characteristics of the main components of the transmission system (gas pipelines, valves, regulation valves, compressor plants) and their mutual connection. The basic parameters for each individual segment of the gas pipeline network are inner pipe diameter, length, the wall roughness and altitude of characteristic points. For simulations, a network model corresponding to the actual state of construction of the transmission system is always used, with possibly adding new network elements that will be active in the period for which technical capacities are determined.
A Scenario is a set of data on expected gas flow levels at the transmission system entries/exits, set parameters of the expected or allowed gas pressure, flow rate of a medium, temperature etc. and the data on the mode of operation of active network elements (regulation valves and compressor plants).
The static simulation scenario implies constant set conditions and parameters (entry/exit flows, set pressures, flow and pressure regulation settings), with equal amount of the total gas flow at the transmission system entry and exit.
The result of the simulation is either a successful or an unsuccessful simulation. An unsuccessful simulation means that the network with the current characteristics cannot satisfy the parameters of flow and pressure set by the scenario. If the simulation is successful, it can be concluded that the network meets the set gas transmission scenario with its characteristics, and the simulation results in the amounts of the capacity or pressures set in the scenario as unknown quantities or values required as a result of the calculation.
 
Preparation of characteristic scenarios and implementation of hydraulic analyses
Considering the number of transmission system entries/exits and the fluctuations in gas consumption over the year, it is possible to set a large number of scenarios with different combinations of gas flow parameters. In order to reduce the number of required simulations, characteristic least favourable gas transmission scenarios are selected. The least favourable transmission scenarios for the Croatian network are the scenarios with the highest expected gas consumption of distribution systems in winter conditions having at the same time the highest consumption of all end buyers directly connected to the transmission system, as well as the transmission scenarios with the lowest expected gas consumption, but with a high level of gas imports, off-take of gas from the LNG terminal, storage and transit. The highest expected consumption scenarios typically check sufficiency of entry capacities and network characteristics for meeting gas consumption in Croatia and determine potentials for gas transit and export in such conditions. Scenarios of the least expected gas consumption are characterised by high flow rates in individual parts of the network, and consequently higher gradient of pressure decrease along the gas pipeline which can also be a limiting factor, despite the lower level of gas consumption.
If the relevant network can meet characteristic scenarios with diametrically opposite settings and parameters, it can be concluded that transmission scenarios with less demanding requirements are also feasible.
 
Interpretation of the calculation results and determining the amount of capacity for announcement and booking
By the described procedure the transmission system operator verifies the amounts of technical capacities that will be available to transmission system users for booking and use, which means that transmission system users are guaranteed the availability of capacities in all conditions during the year regardless of the gas consumption level or various combination of using supply routes. By entering technical capacities in this manner determined in the SUKAP information system, they become valid parameters in the capacity booking and allocation procedure and they are also publicly available via the existing mechanisms of public announcement of data on the web page of the transmission system operator.