Design criteria for the Brooklyn Union gas storage caverns at JFK airport, New York

To cite this paper: Int. J. Rock Mech. & Min. Sci. 34:3-4, paper No. 179. Copyright © 1997 Elsevier Science Ltd

Copyright © 1997 Elsevier Science Ltd

Int. J. Rock Mech. & Min. Sci. Vol. 34, No. 3-4, 1997 ISSN 0148-9062

To cite this paper: Int. J. RockMech. &Min. Sci. 34:3-4, Paper No. 179

DESIGN CRITERIA FOR THE B R O O K L Y N UNION GAS STORAGE CAVERNS AT JFK AIRPORT, N E W Y O R K

U. E. L i n d b l o m

Department of Geotechnical Engineering Chalmers University of Technology S-412 96 Gothenburg, Sweden

A B S T R A C T

In the planning of a major underground natural gas storage project in the New York area recently, the author developed criteria to ascertain the full operation integrity of the caverns. These criteria, which primarily addressed rock stability and gas containment, are based on experimental research and theoretical work, and are outlined in the paper.

The containment criteria address questions like the layout of the caverns and the permeability distribution of the rock mass. These factors determine the level of the break-through pressure of individual caverns and groups of caverns. Contrary to what is commonly believed, this pressure could be as low as 50% of the abundant, hydrostatic groundwater pressure in the rock mass.

The paper also provides design criteria for water curtains, either operating at normal hydrostatic pressure or for overpressurized curtains, used to increase gas storage capacity.

To obtain stability criteria for the cavern roof and walls, account must be taken of the primary stress field in the host rock and of the gas pressure in the cavern. With the high horizontal to vertical stress ratio, occurring even at great depth at the site, there is a risk that a state of tension develops in the cavern walls in combination with the gas pressure. Compressive roof stresses may, on the other hand, approach the compressive strength of the rock mass at depletion of the gas pressure.

C o p y r i g h t © 1997 E l s e v i e r Sc ience Ltd

K E Y W O R D S

R o c k Caverns • Natura l Gas • R o c k M e c h a n i c s • Des ign • Gas C o n t a i n m e n t • R o c k Support

I N T R O D U C T I O N

Brooklyn Union, the fifth largest natural gas distribution company in the United States, has a service territory which covers a 187 square mile area in New York City. The market base served is predominately residential heating and cooking. Heating load accounts for nearly 90% of the annual sales (Reges 1996).

Of reasons of supply reliability, Brooklyn Union might need to add storage capacity to its gas distribution system in the future. A targeted volume of such a storage is approximately 700 MMSCF (about 20 million standard cubic meters of gas). Of several reasons, the possibility of storing the gas below ground near the consumption area is of interest. Since the geology at depth in the New York area

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is composed of crystalline, hard rocks, storage can be achieved in excavated caverns. A site near the JFK international airport was selected for study.

A geotechnical feasibility study was performed 1988-90, including five drillholes and geologic, hydrogeologic as well as rock mechanical evaluation of the site, assuming a preliminary cavern location at 750 m, see Figure 1. The Author was given the task by Brooklyn Union to control the gas containment criteria, the effect of a water curtain and the stability criteria for the caverns. A summary of this work is given in the paper.

GAS C O N T A I N M E N T

Leakage Criteria for Unlined Caverns

A simple precondition for tightness of any rock opening containing compressed gas is that a downward vertical hydraulic gradient greater than unity persists in the rock over the opening. Thus, by assuming gradually increased gas pressure, the inflow of water to the cavern will decrease. At one point, the inflow at the highest point of the roof will cease; this gas pressure is called the "critical pressure", since any further increase will cause gas leak in the roof area. The discussion above presumes saturated rock fracture network and negligible capillarity which is normally the case in deep crystalline rock.

At gas breakthrough, the pressure curve has a vertical tangent just above the cavern roof, and the groundwater becomes stagnant, Liang, Lindblom 1994. The critical gas pressure determines the storage capacity of the caverns and is thus a very important design criterion. The magnitude of the critical gas pressure is dependent on a number of factors, the most obvious one being the location depth of the caverns (below the groundwater surface). Other important factors, affecting the critical gas pressure include

• rock permeability distribution with depth

• number of rock caverns and spacing between them

• cavern cross-section (size and shape)

The gas storage capacity of the cavern arrangement at the JFK site shown in Figure 1 was analysed with respect to location depth, permeability distribution and effects of a water curtain.

Constant Rock Permeability with Depth

Figure 2 is a summary diagram which displays the critical gas pressure of the four-cavern gas storage as a function of the location depth. For example, the proposed caverns at 750 m depth would have a critical gas pressure of only about 5.6 MPa, although the groundwater pressure at this depth is 7.5 MPa. A single cavern would have a critical gas pressure of about 7.0 MPa.

As for rock masses with constant permeability, gas break-through of caverns in rock masses displaying decreasing permeability with depth is associated with a vertical tangent of the pressure diagram at the cavern roof. However, the vertical tangent (= zero pressure gradient) develops at a higher gas pressure in the case of b > 0. This improves the storage capacity of the caverns.

Figure 4 shows the pore pressure distribution around the caverns at the critical gas pressure, assuming b = l .

Permeability decreasing with depth

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The critical gas pressures denoted with solid lines in Figure 2 were obtained under the assumption that the permeability of the rock mass is constant. However, there is evidence from experience and in the literature that the permeability of most rock masses reduces with depth. The main reasons for this appears to be that the rock joints are fewer and more compressed at depth.

Permeability reduction with depth (D) may be formulated mathematically as suggested by Carlson, Olsson 1986,

k =aoD -b

where a and b are site specific constants, defining the magnitude of the permeability and its trend to decrease with depth. Application of this relation to a number of sites in crystalline rock led Carlson, Olsson 1986 to suggest that a reasonable range of variation of the parameter b is between 1 and 2. The higher the b parameter, the greater the permeability decrease with depth.

For gas storage specifically, a high b-value of the rock mass is warranted since this will improve the containment capacity of the rock caverns. This effect is shown for a four cavern gas storage on Figure 2, where the broken line represents a b-value of 1.

Hydraulic well testing at the site revealed clear permeability decrease with depth, but was unconclusive as regards, the magnitude of factor b. Further critical gas pressure calculations were therefore based on the lower limit value of b = 1, according to Carlson, Olsson 1986. One calculation with b = 2 was made for a four cavern location at 750 m. Table 1 displays the results of the calculations. The table also shows the critical gas pressure at constant permeability (b =0). The adopted permeability and hydraulic head distributions at gas break-through for the cavern layout at 750 m depth are displayed in Figure 3.

EFFECTS OF A WATER CURTAIN

Layout and Influence of Water Curtains

A water curtain consists of a series of drillholes for injection of water. When a water curtain is installed above the roof of the gas storage caverns, and water is injected into it, the distribution of groundwater pressures changes from the natural one. This will increase the critical gas pressure of the caverns and increase the storage capacity. An optimum location of caverns with a water curtain is more shallow than for caverns without a curtain.

An activated water curtain is a water source from which water flows to the storage caverns and to the environment. Water losses and pumping costs are essential cost items during the operational stage of the storage.

Containment Criteria

Figure 5 shows a four-cavern gas storage at 400 m depth, equipped with a water curtain and operated at 8.00 MPa. The figure displays the distribution of groundwater pressure on a vertical axis through one of the central caverns. Break-through occurs when the tangent of the pressure diagram becomes vertical in the cavern floor. As the gas pressure increases, the inflow of water gradually decreases. At the critical pressure of 7.17 MPa, inflow stops at the cavern floor (but continues in the roof). Further gas pressure increase leads to leakage downwards-outwards from the floor area. If the gas pressure is higher than the water curtain pressure, gas can escape from the storage cavern in all directions.

Figure 6 shows the groundwater isobars in a cross-section through the caverns, at the critical gas pressure

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7.17 MPa at 400 m depth.

More likely than the situation in Figure 5 will be that the water curtain pressure is continuously maintained slightly above the gas pressure in the caverns. This way of operating a gas storage is favorable in terms of water consumption in the curtain and reduces water inflow to the caverns.

Figure 7 summarizes the containment criteria, i.e. the maximum gas pressure that can be used in the caverns at various location depths and water curtain pressures. As discussed by Lindblom, S6der 1990, there exist rock mechanical limitations to the maximum water curtain pressure. Such restrictions to the pressures are defined by the upper, broken line in Figure 7.

R O C K STABILITY

Elastoplastic Analyses and Rock Parameters

Due to the considerable size and location depth of caverns used for gas storage, very high stress levels will develop in the surrounding rock. Based on core analyses, rock classification and laboratory tests, the rock mass strength parameters were estimated at c=2,7 MPa (cohesion) and q0 = 46 ° (friction angle). Notwithstanding the high rock quality at the JFK site, rock stresses beyond the rock mass strength were predicted for locations deeper than about 500 m.

An elastoplastic rock mechanical analysis was performed which allowed the rock elements in the plastic regime continue to deform at constant or reduced stress. Thereby, load is transmitted to the neighbouring elements. Some of these may then also reach a state of failure and attain a plastic behaviour. The elastoplastic boundary migrates outwards until equilibrium is reached.

Rock Stress

The most significant factor for the stability of the caverns at the JFK site and for the support requirements is clearly the rock foliation and structure. The cavern orientation was selected to about N-S of this reason.

Field measurements at the site revealed high horizontal rock stress, about twice the overburden pressure. The lowest stress was found to be slightly above the overburden pressure. Considering the cavern orientation, the influence of both horizontal stress components and the uncertainty involved in the determination of magnitude and strike of the major horizontal stress all stability analyses were based on the following rock stresses on the model boundary

vertical = equal to the overburden (gravity)

horizontal = twice the overburden

Cross-section

In the light of the high stresses prevailing at the JFK site, a "barrel-shaped" cross-section was proposed, as shown in Figure 8. Such a cross-section reduces the plastic zones and minimizes the risk of creation of radial fractures in the abutment area when excavation advances downwards.

Stability of the Cavern Roof and Walls

Numerical, elastoplastic analyses were performed with FLAC for various cavern depths. It follows from the analyses that the roof (and floor) areas become highly compressed and that this stress concentration

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increases with depth. Even in the walls the failure mode is normally shear due to compression, although the stress levels here are much lower.

The plastic zones around the caverns for four depth locations are shown in Figure 9.

The compressive rock stresses for the most shallow location at 450 m are near the strength of the rock mass. The plastic zones are small in this case, Figure 9 A.

At increased location depth, the plastic zones gradually become larger. This, of course, increases the need for support of the cavern roof and walls to prevent instability; this becomes evident in a study of Figure 9 B-D.

The cross-section (size and shape) of the cavern is obviously favorable, since it reduces effectively the development of tensile stresses in the wall area of the caverns.

Support Requirements It is important to use a rock support system which is deformable and allows some of the rock deformations to take place as the support reaction develops.

All exposed rock surfaces (except the floor) should be covered with fibre reinforced shotcrete. To safeguard the structural interaction between the shotcrete and the bolts, fibre reinforced shotcrete should be used. Systematic bolting with 4,5 to 8,5 m long untensioned dowels were recommended for the JFK site. The rock bolts should have bearing plates and nuts.

Table 2 summarizes the general support requirements for the caverns at JFK. When passing pegmatite zones in the gneissic formation, the support density may have to be increased locally.

As depth increases, the support requirements increase quite drastically. The corresponding costs increase must be compared to the containment criteria, allowing higher storage capacity at greater depth.

FIGURES

ISSN 0148-9062

To cite this paper: Int. J. Rock Mech. & Mm. Sci. 34:3-4, paper No. 179. Copyright © 1997 Elsevier Science Ltd

Paper 179, Figure 1.

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Figure 1. Preliminary location of the storage caverns at JFK.

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To cite this paper: Int. J. Rock Mech. & Mm. Sci. 34:3-4, paper No. 179. Copyr ight © 1997 Elsevier Science Ltd


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TABLES

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Paper 179, TABLE 1.

TABLE 1 CRITICAL GAS PRESSURE AS A FUNCTION OF LOCATION DEPTH AND

VARIATION OF PERMEABILITY WITH DEPTH.

. m , l

Cavern location depth JD) m

Permeability decrease Critical gas pressure MPa

Percent of hydrostatic % factor with depth (b) ,

0 1 0

2.60 3.76

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600 "" 4.00 67 1 5.10 85

750 0 5.60 75 1 6.66 89 2 6.92 92

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900 0 6.30 70 i 8.I3 90

Paper 179, TABLE 2.

TABLE 2 SUPPORT REQUIREMENTS FOR THE JFK SITE.

Depth of location

(!I'1).. 450 600 750 9O0

Roof

c(m) L(mj i .8 4.5 1.8 4.5 1.5 4.5 1.2 4.5

Dowels Upper wall

c L 1 . 8 '

1.8 1.5 1.2

Lower wall

c L 6.5 1,8 3.5 7.5 1.8 3.5 8.5 1.8 3.5 8.5 1.8 3.5

Roof

(cm) 5f+4 5f+4

• 5f+4 8f+4

Shotcrete Upper t Lower wall* walt (cm) (cm) 5f+4 4 5f+4 4 5f+4 4 8f+4 4

* f denotes fibre-reinforced shoterete

References

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References

Carlsson A., Olsson T. 1986. Characterization of deep-seated rock masses by means ofborehole investigations. Swedish State Power Board, Report 5:1.

Liang J., Lindblom U.E. 1994. Critical pressure for gas storage in unlined rock caverns. International Journal o f R o c k Mechanics an Mining Sciences, 31:4, 377-381.

Lindblom U.E., S6der C-O. 1990. Unlined natural gas storage. Allowable gas pressure with respect to shear failure in the rock mass. Swedish State Power Board, FUD Report 1990/12, Stockholm.

Reges M. 1996. An LCD's Use of Storage for Hub Operations. Proc. 1996 A.G.A. Operating Section, Operations Conference, Montreal.

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Design criteria for the Brooklyn Union gas storage caverns at JFK airport, New York