PLANNING METHOD FOR GAS STORAGE DEVELOPED

Laszlo Tihanyi
University of Miskolc
Hungary

A method has been developed that permits natural-gas storage requirements to be determined.

The method makes possible the analysis of the load-duration curve and enables determination of the most advantageous combination of the different types of storage facilities.

It also makes possible analysis of the weather effects, especially those due to degree-day changes.

GROWING ROLE

Underground storage can play a strategic role when a country has no resources of its own and needs to buy natural gas from another country. Gas transport by large-capacity pipeline can be reduced or cease for various reasons.

In such interruptions, the strategic stock in underground storage facilities ensures supply to consumers (Bolleli, 1985).1

In Hungary, imported natural gas has been continuously growing. In the last decade, large-capacity pipelines have been installed between Hungary and the former Soviet Union to the extent that at present the only possibility for Hungary is to buy natural gas from countries in the C.I.S.

The Hungarian government tries hard to diversify the purchases, but Eastern Europe has no real gas market. Therefore, underground storage plays a major role in the supply and is increasingly valuable.

Underground storage ensures gas supply during peak demand periods, serving mainly seasonal demand. Required storage capacity is determined by the following:

Annual consumption
Weather-sensitive ratio of the annual consumption
Weather conditions, primarily annual degree-days
Gas rate supplied into the pipeline system during the heating season.

The planning process of the required storage capacity needs to involve, on the one hand, the enlargement of the available storage capacity year to year, according to the parameters listed.

On the other hand, every year the gas volume for injection must be determined taking into account the probable severity of the winter to follow.

Presented here is a planning method for determining both the storage capacity and required working gas.'

DEGREE-DAY INDEX

The degree-day index relating to the heating season gives an appropriate description of the severity of a heating season. It lends itself particularly well to estimates of heating demands of a group of customers.

Heating use is determined by multiplying the number of degree-days in a time period by a constant. The value of the constant can be determined from the relationship between the daily gas consumption and the average temperature data.

For the present purposes, the average temperatures recorded in Budapest in the 50 years between 1920 and 1970 were used (Vida, 1991).2 Because Hungary is a relatively small country, the average temperatures and degree days of Budapest are valid for the entire country (Table 1).

The duration curve of these average values can be seen in Fig. 1.

The straight line illustrates the base temperature, which is a threshold level. As the daily average temperature falls below this base, the heating demand increases approximately in proportion to the difference between the base temperature and the outdoor temperature.

In Hungary the base temperature is 16 C. (61 F.), according to the studies determining the relationship between weather parameters and load. The intersection of the base temperature line and the average temperature duration curve yields the time period of the heating season.

In Fig. 1, this value is 236 days.

After the intersection point, the temperature duration curve is unnecessary for the method described here.

WEATHER AND LOAD

Fig. 2 illustrates a load-duration curve for a given year's weather pattern, a diagram of daily demand against number of days.

The area below the curve is proportional to the annual gas consumption which can be divided into two components. One is the weather-sensitive component of the demand (A1); the other the base load (A2) which is constant during the peak demand period.

The weather-sensitive load depends on the degree-days. The shape of this area resembles that of the temperature-duration curve in Fig. 1.

After the heating season days, a slow decrease in the daily load is assumed because in the summer, consumption slows or stops because of holidays or maintenance at the gas consumers.

The degree-days method is widely used for load estimation, although it ignores the effects of weather factors other than temperature.

The basic equation of the method is shown in Equation 1 in the accompanying equations box.

This equation can be expressed in the form of Equation 2.

The value of the constant in the weather-sensitive component gives the change in the daily load while the average daily temperature changes by 1'. The equation shows that the load does not relate to all days of the year but only to a shorter period which differs from the entire year by the number of correction days.

It does not mean that in summer there are days without gas supply, but it permits simplification in the mathematical model.

The daily load in the heating period is the sum of the temperature-sensitive load component and the base load (Equation 3).

If the daily average temperature equals the seasonal minimum temperature, the formula gives the peak-day demand.

Using the following definition for the weather-sensitive ratio R = Qws/Qa, the peak load can be expressed in the different form shown in Equation 4.

The average summer daily load is smaller than the base load, but the actual value decreases linearly starting from the base load, according to Fig. 2 (Equation 5 or 6).

This formula expresses that the gas amount given by the product qb(365-nhs - nc) must be equal to the term qsad(365nc).

The average summer daily load, therefore, depends not only on the daily base load, but on the number of correction days as well. In the model, the slope of the summer load curve can be modified by the number of correction days.

Supposing that the gas rate supplied into the pipeline system is constant during the peak load period, the load-duration curve can be supplemented by the line of the gas rate supplied.

Because of maintenance of the production facilities in the summer, the gas rate supplied is calculated in the same way as the daily base load (Equation 7).

The average summer daily gas rate supplied can be determined from Equation 8.

From the difference of the two lines, the required amount of the storage gas can be determined. The difference between the peak load and the gas rate supplied is the required maximum send-out capacity of the storage facilities.

After subtraction of Equations 4 and 7, the final equation is expressed as shown in Equation 9.

APPLYING THE METHOD

Fig. 3 illustrates an application of the method.

With the basic assumption that the annual consumption is 1 bcf, three cases were studied with weather-sensitive ratios of 20%, 50%, and 80%, respectively. The results can be seen in Table 2.

In Case 1, the seasonal load variation is low, which means that the gas is used mainly for industrial purposes.
The load pattern in Case 2 is mixed; the base load and the weather-sensitive components of annual consumption are equal.
Case 3 represents commercial usage.

Fig. 3 shows that the durations of the injection and withdrawal periods are independent of the weather-sensitive ratio of the annual consumption.

In all three cases, the time period is 155 days when the consumption is greater than the rate supplied. Gas therefore needs to be withdrawn from storage. In the rest of the year, gas must be injected into the storage facilities.

If the gas rate supplied into the pipeline system is constant during the heating season, the "rules of thumb" for the storage planning process are the following:

The required amount of working gas is 45% of the weather-sensitive component.
The peak withdrawal rate is 1.8% of the working gas.

With the method presented here, an existing system can be analyzed and the required capacities determined.

On the other hand, if the increment of the gas consumption [See Formula] is known, the necessary capacity-increment can be calculated.

The method makes the analysis of the load-duration curve possible and enables the most advantageous combination of the different types of storage facilities to be determined. It also makes possible analysis of the weather effects, especially those due to degree-day changes.

REFERENCES

Bolelli, Y., et al., "Minerbio: a giant natural gas storage field, 16th World Gas Conference, Munich, 1985.
Gaztechnikai kezikonyv (Natural Gas Handbook), M. Vida (ed.), Muszaki Konyvkiado, Budapest, 1991.