Assessment of Existing Structures and Repairs - Offshore Structures: Design, Construction and Maintenance - page 493

7.8.2 Environmental Load Effect
The catastrophic structural failure of the platform, an unmanned satellite plat-
form, most likely was related to the peak of the storm preceding failure by
two days, when onshore maximum wind gust speeds higher than 50 knots
(25.7 m/s) were recorded at the nearest onshore location
The mode of failure was structural overload primarily caused by severe
wave loading from the north and northwest (N-NW) impacting the platform
over a prolonged period.
The failure characteristics were similar to the pushover collapse analyses
performed 5 years before failure and another study for a similar platform
1 year before failure.
There was no evidence of vessel impact, a seismic event or another possible
source of loading identified as a hazard to this platform, with the possible
exception of an operated support vessel tying up to the boat landing to ride
out a storm.
While the vessel tie-up option cannot be completely discounted, it is very
unlikely because this platform is near a natural sheltered location and the bol-
lards show no signs of damage.
The storm conditions recorded for 2 days before structural failure were typi-
cal of storms recorded around 3-4 times per year for the 7 years before failure.
Based on the onshore gusts measured, the estimated maximum wave height and
maximum 1-hour average wind speed during the storm are considered to be
roughly wave height (H max ) equal to 6.5 - 7.5 m and wind speed (Ws) equal to

19.5 m/s, respectively.
A storm 2 months before failure was more severe and more typical of storms
recorded around once every 2 years. Estimated maximum wave height and
maximum 1-hour average wind speed during this storm were considered to
be around H max = 6.7

-

8.7 m and Ws = 21 m/s, respectively.
Other severe storms that happened 3, 4 and 7 years before failure had similar
peak gusts and duration.

7.8.3 Structure Assessment
The platform s subsea jacket was considered to have been in a degraded condi-

tion prior to catastrophic collapse.
The collapse analysis of the platform was performed using the SACS model
with no joint failure modeled and the estimated minimum reserve strength ratio
(RSR), which is equal to the load at collapse divided by load under 100-year
environmental conditions, was found to be equal to 1.25 at collapse. The push-
over analysis revealed an RSR equal to 1.13 at first failure due to pile yield for
example increasing environmental load 25% greater than maximum wave

'