HOW TO ENSURE SEISMIC REQUIREMENTS FOR SAG … · HOW TO ENSURE SEISMIC REQUIREMENTS FOR SAG AND...

SME’s Annual Meeting 2012

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HOW TO ENSURE SEISMIC REQUIREMENTS FOR SAG AND BALL MILLS

Dr. Axel Fuerst, ABB Switzerland, Baden, CH

David Casado, ABB Switzerland, Baden, CH

Reinhold A. Errath, ABB Switzerland, Baden, CH

Abstract

The recent seismic events have shown that earthquakes have an immense destructive power. Chile was hit in February 2010 with a seismic intensity of 8.8 on the Richter scale, and had loss of about 700 killed persons, while Haiti was hit in January 2010 with 7.0 killing about 220’000 people.

The event in Haiti showed that the seismic events can lead to a disaster if no preventive measures are taken. On the other side, the seismic event in Chile has hit a well-prepared country with moderate consequences. The earthquake with 6.6 Richter in Japan in March 2011, have further shown how severe the consequences can be if critical equipment is involved. Fortunately, the equipment in a mine has not the potential hazard as a nuclear power plant. Anyway, we are diligent and it is obvious that the seismic risk may lead to a real disaster even if the probability is small.

The GMD is an important component in the grinding circuit of a mine. This paper maps out earthquake-prone areas including expected earthquake magnitude calculated according to relevant standards, focusing mainly on the building code IBC 2009. An overview is provided on how this standard is applied to mining equipment to minimize damage and aid large mills in operation to successfully withstand seismic effects. The differences in stresses and forces on the GMD and foundation are shown based on a turning and stopped mill simulation during an earthquake. The results demonstrate the need to maintain active and operating earthquake

detection systems. Aside from detecting an earthquake’s magnitude, the sensor information can also be used to reduce the harmful effects that an earthquake has on mining equipment. As follows the earthquake requirements are defined. The last part of this paper explains how to fullfill these requirements.

Defining requirements

Global Seismic Hazards

The brittle tectonic plates float on the relatively ductile asthenosphere (isostasy). The plates are in constant motion due to the thermal convection of the asthenosphere. They reach a relative speed of up to 10 cm per year. Friction between the plates leads to stress concentrations. The sudden release of these stress concentrations is what is perceived as earthquakes on the surface.

The magnitude of such a seismic event is commonly described with the Richter scale. The scale applies a base 10-logarithm for magnitude calculations; therefore each number on the scale represents a tenfold amplitude increase over the previous number. A magnitude 5.3 Richter earthquake is considered moderate, while a 6.3 magnitude earthquake is regarded as powerful, since its amplitude is the tenfold of the 5.3 Richter quake and releases 32 times more energy, see ��.


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Table 1. Change of Magnitude versus ground motion and energy

Magnitude Ground Motion Energy

1 10 times 32 times

0.5 3.2 times 5.5 times



The energy level is what best indicates the destructive power of an earthquake. The Chilean earthquake (February 2011), with 8.8 Richter had a 61.2 times higher energy level than the event in Haiti with 7.0 Richter (January 2010).

When the accumulated tension under the earth crust is suddenly released, the strain energy causes a tremor or so-called seismic wave, which moves through the earth (Earthquake Terminology, 2011). The point vertically above this wave is called the epicenter. There are two types of energy waves, body and surface waves. This paper will focus on surface waves as they cause the damage perceived by humans.

An earthquake generates a shock wave in the earth’s elastic crust, which then generates Rayleigh and Love waves on the surface, see ��. Love waves, are

the fastest and only move the ground horizontally in an in-plane motion. The movement of a Rayleigh wave can be best compared to a wave in the ocean.

Due to its rolling movement it shakes the ground in an upward-downward and side-to-side motion always in direction of the wave movement. Thus being responsible for most tremors felt during a seismic event.

Figure 1. Surface waves (Geological Department Michigan Tech., 2011)


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Figure 2. Acceleration measurements from Antofagasta earthquake, 13 June 2005 (Cruz, E.F., 2009)

When combined at the earth’s surface the two wave types create a visible three-dimensional motion. The plot in �� shows the acceleration measurement taken during the Antofagasta earthquake in 2005. The following types of acceleration were detected:

• Vertical direction (U-P)

• North-South(N-S)

• East-West (E-W)

Response Spectra, Amplitude and Frequency

Earthquakes are usually measured in multiples of the earth acceleration 1g (=9.81m/sec²). The performance level of earthquake motion is represented by response spectra from anticipated strong ground motions. Based on soil constitution and structural design, different damping percentages are applied.

The equipment basically behaves like a single degree of freedom oscillator. To apply the response spectra to a GMD, the eigenfrequency of the structure has

to be determined first. For example, with a first eigenfrequency of 16Hz at 5% damping, the acceleration introduced by the earthquake is 1.5g, cf. square label as seen in ��.

Figure 3. Typical response spectra, acceleration in units of g versus frequency in Hertz in the regime of a

moderate seismic performance level


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Design codes. IBC 2009

To prevent structural failures during seismic events, standardized codes are created, typically on a national basis. Structural failures in civil constructions can be divided into structural inadequacy, foundation failure or a combination of both.

This confirms that one and the same seismic event can selectively destroy some structures while leaving others intact. It demonstrates that some structures are more capable in resisting seismic loads than others. Local conditions, such as type of soil, are a major factor. A common design code helps to prevent this. The Uniform Building Code (UBC) (Uniform Building Code, 1997) was established in the USA as early as 1927 with the intention to promote public safety and provide standardized requirements for safe construction. In 2000 the new International Building Code (IBC) (Woodson, R.D., 2009) replaced the UBC. IBC is updated regularly.

The IBC includes a fire, structural, earthquake and plumbing code. Compared to UBC, IBC has improved ground motion parameters and contour maps for spectral response quantities that replace zone maps. The mitigation of losses from natural hazards is based on statistical data and applied in form of probability groups of occupancy. The philosophy of IBC is to design for “collapse prevention” in a 2475-year earthquake, which corresponds to 2 percent probability of being exceeded in 50 years. The UBC philosophy is to design for “life

safety” in a 475-year earthquake, which represents a 10 percent probability of being exceeded in 50 years. The problem with this UBC design earthquake was that it did not provide adequate protection for infrequent but very large seismic events. It should be noted that the 2475-year earthquake in the Eastern U.S. is on the order of 4 to 5 times as strong as the 475-year earthquake. On the other hand, the difference between “collapse prevention” and “life safety” is that in the first case the structure remains standing, but only barely, while in the second case, the structure remains stable and has significant reserve capacity. In that way, design ground motions in IBC are set to 2/3 of MCE (Maximum Considered Earthquake – 2475 year event).

According to IBC, next data is necessary to completely define the earthquake:

• Occupancy category

• Mapped (MCE) acceleration parameters Ss and S1

• Site class

• Site coefficients

• Long-period transition period TL

�� clarifies the process to calculate SDS and SS1, the final earthquake design spectral response acceleration parameters and the associated design response spectrum:


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Figure 4. Flow process for calculating seismic design parameters according to IBC 2009

As follows, an example will show the procedure to calculate the design spectral response acceleration parameters SDS and SD1. The input parameters SS and S1 are extracted from the IBC maps (see �� ) or are determined directly after a geotechnical investigation on site. For this example let us suppose SS=1.00 and S1=0.33. Site class is defined according to Table 1613.5.2, IBC 2009 (see ��). In this example we suppose that the soil is basically hard rock, so then site class is A. Fa and Fv are calculated according to Table 1613.5.3 in IBC 2009 (see ��), entering with SS, S1 and site class. For the present case Fa=0.8 and Fv=0.8.

Next step is to calculate the spectral response acceleration parameters adjusted for site class effects, SMS and SM1:

8.0=⋅= SaMS SFS

26.011 =⋅= SFS VM

And finally the design spectral response acceleration parameters, SDS and SD1, can be determined, following the formulas specified in IBC 2009:

53.032 =⋅= MSDS SS

17.032

11 =⋅= MD SS

Having calculate SDS and SD1, it is possible to build the design response spectrum which completely defined the design loads, see ��.


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Figure 5. USA contour map for S1, extracted from IBC 2009

Figure 6. Site class definitions according to IBC 2009

Figure 7. Site coefficients Fa and Fv according to IBC 2009


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Fullfilling requirements

Physical explanation for the effect of collapsing structures

A seismic event consists of an energy wave with an amplitude and dynamic content (see �� ). The destructive action has two effects:

• The peak amplitude may lead to a stress load in the structure exceeding the stress limit of the material. This may cause a collapse due to rupture or buckling.

• As the seismic event arrives in waves, it is a dynamic incident that lasts seconds to minutes, cf. �� . If the frequency of the waves meets the eigenfrequency of the structure then resonance starts. In the case of resonance the energy introduced at each cycle is accumulated and the vibrations stack up. The longer the excitation lasts and the better the frequencies match, the more the vibration increases.

The structural damage applies to building and machine structures as well as their foundations. Steel is able to bear a pressure load that is tenfold greater than what concrete can bear. The effect on the tensile load is roughly a hundredfold. Additionally, in case of many vibration cycles, steel has a superior low-cycle fatigue behavior. Generally the foundation design requires more effort than the seismic design of the GMD, simply due to the fact that the motor structure is made of steel. It is known from site experience that foundations get damaged before the GMD and mill structures.

Design and construction of GMDs approach to withstand harmful effects To reach a sound design ABB takes into account precisely what is described in the previous chapter. The equipment is built in a way that ensures peak stresses to be well below the material limits on one hand, while on the other hand the base structure is built quite stiff to elevate the eigenfrequencies well above the excitation frequencies. To construct a seismic proof GMD, the internal structure receives additional stiffeners. For high horizontal loads sometimes a support beam is required and the sole plate foundation anchors need to be reinforced.

Stiffeners

Lateral support beam

Reinforced fixation

Stiffeners

Lateral support beam

Reinforced fixation

Figure 8. Basic GMD structure with reinforcements

Calculation methods

Large mining equipment and especially GMDs are engineered products. The basic design is adapted to the specific needs of each individual project. The key parameters are power, speed, torque, site altitude, seismic requirements, dimensions and transport requirements. Due to the colossal dimensions, lead-time and high costs, no prototypes for testing are built. The design verification is based on manufacturer experiences and simulations. Today simulations are performed with Finite Element (FEM) analysis, performed by the manufacturers and experienced consultants. The FEM studies include structural analysis to find and eliminate stress concentrations as well as studies of the dynamic behavior. The simulations are verified employing measurements taken during operation, special investigations and during the manufacturing process.

Comparison of loads to the structure during a seismic event for a GMD with and without protection

The previous chapters explained how GMD reinforcements can prevent serious damage caused by a seismic event. This passive protection can be improved by adding an active protection. The heart of the active protection is a seismic sensor that detects up-coming seismic loads and switches the GMD off as soon as an event is detected. The load to the structure during an earthquake can be drastically reduced, as the following chapters will explain. A strong seismic event often goes along with short circuits of the power supply. If both events occur together, the GMD structure reaches its limit.


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Since presenting a full stress analysis in this paper is not possible, only the loads from the GMD to the sole plates are presented. This analysis also explains the influence of the force to the foundation.

In case of an earthquake – resulting forces on running GMD

When the GMD is in operation it is exposed to a rich profile of loads with different origins such as dead weight, torque, thermal load, magnetic pull at the centre and unbalanced magnetic pull (UMP). For the following observation, let us concentrate on the most important: weight, torque and UMP.

Figure 9. GMD assembled on sole plates

Figure 10. Support definition of the GMD for

simulation

During a worst case earthquake scenario, the full horizontal acceleration such as the following example of 0.5g horizontal and 0.3g vertical are applied. Additionally there is a high chance of having a short circuit in the system, with a line-to-line short circuit (LL) being the worst case. Further chapters will explain what a commutation failure of the cycloconverter, due to sudden power cut and a consequential short circuit, means for the system.

Table 2. Reaction forces to foundation at rated operation with earthquake and LL short circuit

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�� shows that the highest load contribution is originated by the short circuit with 2177.8 kN. The second largest contribution is the seismic load with 969.7 kN. The highest-loaded support 3 bears 4162 kN. Certainly, the whole structure and foundations are loaded accordingly.

In case of an earthquake - Forces when the GMD is stopped

If a seismic sensor is installed and operational, it can detect events and trip the GMD in case of extreme amplitude increases. In that case the GMD can be switched-off and the loads torque, unbalanced magnetic pull and line-to-line short circuit do not apply anymore. The highest load is now only caused by the seismic event at 970 kN. The highest-loaded support 3 now bears only 1572 compared to 4162 kN. This is a substantial improvement. Consequently, the entire structure and foundations are loaded much less.

In case of an earthquake - forces when the GMD is running and having a commutation failure due to sudden power cut

It is not unusual for earthquakes with a high magnitude to suddenly cut the voltage from the overhead power lines. As a result, the cycloconverter, belonging to the family of commutating network converters, can loose commutation. This can create a condition similar to a short circuit between the running GMD, the cycloconverter and the converter transformer. The short circuit cannot be stopped until the GMD has stopped. What happens under such conditions, is that the low ripple constant torque, produced by the GMD under normal conditions, changes to a sinus oscillating torque with a frequency that depends on mill speed, number of poles and excitation. The following graphs document the resulting steps of the sudden commutation loss.

Figure 11. Development of the current of the three phases over time in case of commutation loss

Figure 12. GMD air gap torque in case of commutation loss

The graphs in �� and �� show the development in case of a loss of commutation over time. The y-axis shows the amplitude relative to the nominal values. �� depicts the three cycloconverter currents feeding the GMD. It can be noted that the current peek is some fivefold of the nominal current. The resulting torque at the GMD air gap, c.f. �� , has a fast, oscillating and highly damping behavior with a peak of five times the nominal torque. The changeover from maximum negative peak to maximum positive peak happens in less than 50 msec. Since the inertia of the involved masses - stator and rotor - is immense, the resulting force to the mechanical structure is smaller than the air gap torque. The structure simply cannot follow the quick change due to its inertia.

Figure 13. Stator vibration due to a short circuit (Diaz, R.; Errath, R.A.; 2004)

The dynamic torque analysis shows components 3.09 p.u at 5Hz, 1.98 p.u. at 10Hz and 0.5 p.u at 15Hz compared to the nominal torque. Under normal operating conditions the GMD is failure-proof, switching off safely and in a controlled manner when any disruption occurs. In case of an earthquake, however, it is very likely to have a power shortage without a defined switching off cycle. In


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such cases the commutation can be lost with the above mentioned effects.

Active protection

Monitoring of earthquakes and action taken

To reduce risks it is strongly advised to switch off the GMD in a controlled manner once the earthquake magnitude reaches a certain level. This can prevent loss of commutation. If the motor is stopped without loss of commutation, no harm will occur to the mechanical part of the GMD system and mill. Therefore, earthquake sensors should be installed onsite.

Propagation of earthquake intensity and placement of tri-axial sensor

Since a seismic event detector is installed at concentrator plants, the actual positioning of the detector, in terms of distance from the equipment to be protected, is less relevant. The speed of a Rayleigh wave is some 3km/sec. The most suitable place in terms of infrastructure and accessibility for the detection equipment can be chosen as one sees fit.

Figure 14. Tri-axial sensor with orientation

definitions mounted on a concrete block

The sensor housing has to be mounted with one of its axis in upright position. Usually both other axis are directed towards the N/S axis and the W/E axis. For the purpose of this installation the two horizontal sensors will be directed axially and in a radius towards the axis of the mill bodies.

The ideal sensor base is a concrete structure with high density, such as the foundation of the incoming transformer.

Special consideration needs to be given to the proximity of the sensor to the main building. Ideally this

should be 30 to 200m and not further that 500m from the ABB E-house where the equipment will be connected. The area should be free of mechanical vibration coming from process equipment.

The seismic event detector (SED) will be installed in the E-house and powered with UPS. During a power cut, the recorder will remain operable and active over a few days time from a charged battery. A computer equipped with Geophysical Data System (GeoDAS) software can be connected to the SED and used for general tasks, like state of health monitoring: GeoDAS performs permanent or periodical monitoring of instrument status.

Actions taken when an earthquake is detected

To minimize damage, the GMD will be automatically disconnected as soon as a certain level of earthquake magnitude is exceeded. The default level is approximately half the acceleration in g for which the equipment was designed.

The seismic activity monitoring and protection system must be checked and tested regularly. The experience gained in the last decade with this kind of equipment is disappointing. Maintenance staff often does not check and calibrate these sensors regularly. If checks are not performed regularly, sensors that become inoperable with time can go unnoticed and not serve their intended function. A test function is available to check the status of x, y and z accelerometer sensors. By sending a test pulse, the sensor feedback can and should be compared with results from previous maintenance to detect a time or level difference.

Conclusion and discussion

This paper shows that, while mining equipment is often exposed to seismic load, it can be protected from failures by means of reinforcement and tuning of the dynamic behavior as form of passive protection. Design codes, like the IBC standard, give engineers options to build safe structures. For a GMD, a critical component within the production chain, it is very advantageous to add active protection to drastically reduce the load to the structures and foundations. During a strong earthquake the power supply to the motor system may fail. This may then lead to a short circuit adding even larger and additional loads to the structure.

The active seismic protection system was already foreseen or installed in the past, but often left


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unmaintained. In addition the calibration and trip values of the sensor were most likely underestimated in the past. The intention of this paper was to explain the physical processes of such an active protection system, thus creating more common awareness and revealing the advantages of this system for mining operations. This paper shall serve as a starting point for further discussion and exchange between all stakeholders.

Acknowledgements

The authors kindly thank Christian Horstmann, ABB mill drives development, for his contribution to the seismic structural resistance for the GMD design.

References

1. Baska, D.A. “A Geotechnical Engineer’s Transition from the UBC to the IBC”. Retrieved May 21, 2011 from www.calgeo.org/downloads/UBC-IBC.pdf

2. Cruz, E.F (2009, July) “Present Status of Earthquake Preparedness Activities in Chile”. Presented at UNESCO IPRED ITU Workshop: Make the People a Part of the Solution, Istanbul Technical University, Istanbul.

3. Diaz, R., Errath, R.A. (2004) “Desplazamiento horizontal del estator de un gearless mill drive! Producto de una falla eléctrica”, Presented at the International Mining Plant Maintenance Meeting 2004 (MAPLA) Iquique, Chile.

4. Dowty, S., Ghosh, S.E. and Ghosh, S. K (2002) “IBC Structural Provisions: A Better Alternative”. Building Standards May-June, 12-14.

5. Earthquake Terminology “The mechanics of tectonics”. Magazine on European Research. 11/2004 Vol. 43, Retrieved May 21, 2011 from http://ec.europa.eu/research/rtdinfo/43/01/print_article_1668_en.html.

6. Geological Department Michigan Tech “What are seismic waves?” Retrieved May 21, 2011 from http://www.geo.mtu.edu/UPSeis/waves.html.

7. Global Seismic Hazard Map. Retrieved May 21, 2011 from http://www.seismo.ethz.ch/static/GSHAP/.

8. National Geophysical Data Center “Leaning Apartment Houses in Niigata, Japan”. Retrieved May 27, 2011 from http://www.ngdc.noaa.gov/nndc/struts/results?eq_1=1&t=101634&s=0&d=4&d=44.

9. Uniform Building Code. International Conference of Building Officials, Whittier, CA, 1997

10. US Geological Survey “The Richter Magnitude Scale: The Severity of an Earthquake”. Retrieved May 27, 2011 from http://earthquake.usgs.gov/learn/topics/richter.php.

11. Woodson, R. D. (2009) “2009 International Building Code Need To Know”. McGraw Hill: New York.

12. International Code Council, “International Building Code”, 2009

13. “ASCE 7-10 Minimum Design Loads for Buildings and Other Structures”

HOW TO ENSURE SEISMIC REQUIREMENTS FOR SAG … · HOW TO ENSURE SEISMIC REQUIREMENTS FOR SAG AND...

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