ABNORMAL LEVEL EARTHQUAKE STUDY USING ISO 19901-2

Malaysia has over 200 offshore jacket steel platforms which are the main source of oil and
gas. All these platforms have been designed for gravity loads and environmental loads
(wind, wave and current). The recent seismic activity in the region is the motivation for this
work. This work examines the ISO procedure for evaluation of seismic forces on jacket
platforms. Platforms located in seismic areas are designed for ultimate limit state and
abnormal limit state (ALS). The ALE design event is the Abnormal Level earthquake (ALE).
The ALE analysis of a structure is done using non-linear static Push-over method in three
directions including two orthogonal and one diagonal loading. ALE analysis results show
that the ultimate reserve capacity in east-west direction was the lowest among the three.
The structure exhibited a seismic reserve capacity factor of 6.67 for this load case, which is
178% greater than that assumed in design (based on ISO code, reserve capacity factor is
2.4). Thus, the initially estimated ISO spectral acceleration for the Extreme Level
Earthquake (ELE) is found to be conservative. The more accurate estimate of the seismic
reserve capacity factor obtained from this analysis can be used for mitigating the ELE
accelerations for member design and code check in ELE analysis. The ELE seismic
acceleration reduction ratio is determined as 39.4% and it can be used for members design
justification, if required.