1. No category

SME’s Annual Meeting 2012
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
–1–
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
0.3
2.0 times
3.0 times
0.1
1.3 times
1.4 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.
the fastest and only move the ground horizontally in an inplane 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.
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
Figure 1.
Surface waves (Geological Department Michigan Tech., 2011)
–2–
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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)
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
.
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
–3–
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
–4–
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:
S MS = Fa ⋅ S S = 0.8
–5–
S M 1 = FV ⋅ S1 = 0.26
And finally the design spectral response
acceleration parameters, SDS and SD1, can be determined,
following the formulas specified in IBC 2009:
S DS =
2
⋅ S MS = 0.53
3
S D1 =
2
⋅ S M 1 = 0.17
3
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.
Figure 6.
Figure 7.
USA contour map for S1, extracted from IBC 2009
Site class definitions according to IBC 2009
Site coefficients Fa and Fv according to IBC 2009
–6–
SME’s Annual Meeting 2012
Fullfilling requirements
Stiffeners
Physical explanation for the effect of collapsing
structures
Lateral support beam
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.
–7–
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.
SME’s Annual Meeting 2012
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.
Figure 9.
GMD assembled on sole plates
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 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
! " !#$$#
"
#
!
–8–
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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 12.
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.
Figure 11.
Development of the current of the three
phases over time in case of commutation loss
–9–
GMD air gap torque in case of
commutation loss
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 triaxial 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.
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
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
– 10 –
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.
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_a
rticle_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/.
Acknowledgements
The authors kindly thank Christian Horstmann,
ABB mill drives development, for his contribution to the
seismic structural resistance for the GMD design.
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.
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