Evaluation of Seismic behavior of damaged offshore jacket platform under the environmental condition

Most of the fixed offshore platforms in the Persian Gulf have survived more than ۲۵-year design life and have suffered from significant damages in this period. Seismic acceleration modifications and changes in seismic criteria of API-۲EQ-۲۰۱۴ increase the importance of seismic assessment of the offshore platforms in the Persian Gulf. This paper presents a case study for modeling and evaluating the seismic behavior of an existing damaged fixed offshore platform in the Persian Gulf with consideration of actual structural damages as per provided subsea inspection reports and comparing with the intact condition of the platform to obtain the effect of assessment initiators like; actual damages and increased spectral acceleration as per API۲EQ ۲۰۱۴ in the structural integrity of the fixed offshore platforms under the seismic loads in Persian gulf. Following the actual jacket inspection reports, Excessive corrosion, flooding of some members, marine growth, and anode wastage are the significant damages on this platform. Spectral nonlinear and static-dynamic analysis with SACS۱۲.۰۰ software considering the pile-soil interaction in the three following scenarios have been performed to verify structural seismic assessment. The first scenario contains a damaged platform with a lighter topside, the second scenario is a damaged platform with a heavier topside, and the third one includes the intact platform with initial design assumptions and criteria. The evaluation of the structure in three parts of the jacket members, joints, and piles has been done under the ALE & ELE earthquake levels. According to the results, jacket legs have the significant effect on the structural seismic strength. In abnormal level earthquake, the first plasticization occurs in the deck legs which are connected to the topside and the piles below the seabed. The comparison of the RSR values ​​indicates that the initial assumption in platform design criteria has been stringent and uneconomical in the past. Also, the actual presented damages do not have much effect on the seismic strength of the structure. A Comparison of the Joint and member capacity illustrates a more significant impact of uniform corrosion on joint capacity than member strength. Finally; buckling in the deck legs at the splash zone and yielding in the Piles near the sea bed causes the global collapse of the structure.

Keywords:

assessment , seismic , jacket , fixed platform , offshore , offshore platforms