Civil Engineering Reference
In-Depth Information
time. In addition, a pushover study using the SACS model was performed on
this platform 5 years before failure and the reserve strength ratio was suitable.
Failure is usually caused by decreased strength and an increase in the
affected load at the same time.
In this case, it was observed that the topsides were inclined 5
toward
the south, indicating a significant horizontal force most likely applied well
above the base of the jacket structure. The leg members above the pile stabbing
points were cracked in a manner consistent with a horizontal shear-type action.
The subsea survey indicated multiple joint failures on the east and west
faces, with bright steel on most visible failure surfaces and lack of corrosion
andmarinegrowthonmostexposedpilesurfaces where joint failure led to
pull-out of the leg. Thus, these surfaces had only recently been exposed to
seawater.
Failure was shown on the leg by shear cracks with rotation, which revealed
that the leg had been under applied loads that caused shear stress with torsion
effect. In addition, the ROV subsea inspection that was performed after failure
showed a punching shear and detachment of brace member from the leg.
°
to 10
°
7.8.1 Strength Reduction
Four braces near the base of the jacket were reported to be flooded in previous
subsea surveys, potentially indicating that some cyclic fatigue action had led to
through-thickness cracking of the members, but it appears that the joint failure
happened some time before complete structural failure, because the cracks had
discolored surfaces and were covered by marine growth. It is considered unli-
kely that these flooded members led directly to the subsequent structural failure.
The tubular steel used offshore at the time would have had a high carbon
equivalent value and consequently would be more brittle and less ductile
than other offshore steel rolled sections.
Visual inspection of the welding was satisfactory, with no gross defects
detected. No obvious weld fabrication problems were identified on this platform
in particular or on any platform in the fleet in general.
In general, the K-braced system of a jacket with no joint-cans has little scope
for redistributing load following initial joint failure.
Joint plasticity would normally be associated with structural overload but
there was little evidence of this from subsea ROV inspections post-collapse.
This may be due to the more brittle steel employed in the late 1960s/early
1970s when this platform was fabricated, and/or the relatively thin steel
sections, because the member thicknesses are around 7
-
10 mm, and/or other
factors discussed below.
In most cases, the cause of failure is a combination of many factors
converging at the same time.
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