There are increasing numbers of ageing offshore structures in the water of South China Sea which come along with various types underwater structure anomaly, such as joint crack, member flooding, shallow gas and subsidence. This phenomenon brings an immediate attention to the Asset Owner to rectify the anomalies through various Strengthening, Modification and Repair (SMR) options. However, execution of SMR option is often a very costly operation. Optimum mitigation measure shall be established through proper structural re-assessment work with in-depth understanding of structural failure mechanism.
The present paper will focus on the established re-assessment methodologies in evaluating the structural global strength requirement. The re-assessment methodologies formed a step-wise evaluation process encompassing determining Risk Level Assessment, Design Level Assessment and Global Ultimate Strength Assessment (GUSA)
The risk level assessment considered all factors influences the likelihood of failure and consequence of failure of the structure to be determined to enable the establishment of its Risk Level. Next, the type of analysis which required to be performed under Design Level Assessment is similar to those analyses undertaken during Detailed Design Phase i.e. static in-place analysis, fatigue analysis, and dynamic analysis. Output generated from design level assessment to be feed into Global Ultimate Level Assessment (GUSA) if the structure fail to comply with the design level requirement.
GUSA comprises of three (3) integrated analyses which are Non-linear Plastic Collapse Analysis, Member Importance Analysis and Structural Reliability Analysis. These analyses will assist in quantifying the ultimate strength of the structure through Reserve Strength Ratio (RSR) and Probability of Failure (Pof). The outcome from the analysis can efficiently assist in understanding the structure failure mechanism and correctly defining type of mitigation required.
A few case studies is presented to give an insight on how this assessment is undertaken to demonstrate fitness for purpose of the structure and define the optimum mitigation measure.
