Methodology
If seismic acceleration parameters (S
s
, S
1
) for a probabilistic
earthquake in the Cushing area inclusive of the recent
seismic activity exist, they can be used for the analysis.
Furthermore, if the probabilistic earthquake exists, other
methods, such as time history or response spectrum
analysis, can also be used to predict the response. In the
absence of these parameters, and in order to expeditiously
frame the effects, the following approach is used.
At locations that do not have probabilistic response
spectra, but have peak ground acceleration (PGA) data, the
PGAs can be used to approximately estimate the S
s
and S
1
values for design per API 650 Appendix E.
2
The USGS has a
multitude of recording stations throughout the US where
recorded PGA data is represented using Shake Maps
(Figure 6). If PGAs at a station in the vicinity of tanks are
available from these shake maps, they can be used to
estimate the values of S
s
and S
1
. This approach is an
approximation and has limitations, but can be used as a
tool. There is some evidence in the literature
4
to indicate
that using PGAs directly can over predict the response. For
this analyses, the recorded PGA values are reduced by 33%.
Results
For analysis of the representative tank set, Matrix PDM
reviewed the PGAs recorded at stations around Cushing
during the 10 October 2015 (M4.3) earthquake. For this
earthquake, two stations were critical. One station
recorded a maximum PGA of 13% g at the airport nearest to
the tanks; another station at West 9
th
Street, five miles
away, recorded a maximum PGA of 60% g. These were the
highest PGAs recorded at these stations among the recent
earthquakes. This is logical as the earthquake epicentre was
in Cushing. These raw PGAs roughly approximate the
response and should be used with caution. Small scale
geological differences can significantly change the high
frequency acceleration amplitude and waveform character,
and cause local focusing and amplification. Per USGS, on
certain occasions, but not always, these PGAs are
corrected after the event.
A wide range of tank diameters in Cushing were
evaluated with regard to the seismic parameters computed
from the aforementioned PGAs.
3
The tank details used in
the evaluation were based on the minimum requirements
of API 650. The tanks performed well with the PGA
recorded (13% g) at the closest station. For all tank
diameters and sizes, the responses computed (lateral
stability, dynamic hoop tensile stresses, overturning
moment, shell buckling stress and sloshing) were well
within the design norms when subjected to the 13% g PGA.
This is supported by the fact that there were no reports of
any earthquake-related tank damage in Cushing.
A similar analysis was performed using the 60% g PGA
recorded at the West 9
th
Street station. There was a slight
exceedance over the design limit for hydrodynamic hoop
stress for larger tanks, if the highest PGA recorded (60% g)
was used. In addition, the smaller diameter tanks (<100 ft in
diameter) may require anchorage. Both of these design limit
exceedances were based on staying within API 650 factors
of safety with full tanks. Additional analysis was completed
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