The Effect of Shallow Gas on the Performance of Pile Foundation for Offshore Jacket Platform in Malaysian Waters

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4. The Effect of Shallow Gas on the Performance of Pile Foundation for Offshore Jacket Platform in Malaysian Waters

ABSTRACT:  In 2023, gas bubbles were observed in the proximity of conductor and pile foundation of an offshore jacket platform, Platform BA. The objective of this study was to demonstrate the potential effect of shallow gas in soil to the pile axial and lateral response, and the offshore jacket platform performance. The presence of gas is linked to the development of excess pore pressure (XPP) in the soil. In turn, this affects the effective stresses in the soil and ultimately the soil response. First, axial capacity check was performed based on the soil response due to the presence of gas. Then, three different methods, namely, API, NGI-Petronas (NGI-P) and Jeanjean et al. (Jeanjean). The parameters from the Pile/Structure Interaction (PSI) assessments were then used in structural in-service condition analysis. Results from the axial capacity check indicates a reduction in axial capacity when considering the effect of XPP thus not meeting the required safety factors (SF). However, after considering ageing effects, Platform BA piles are still within code compliance. In terms of the lateral response, the NGI-P and Jeanjean methods simulate a stiffer response than API, leading to smaller lateral displacement at pile head. The magnitude of XPP in the gas-charged layer did not have a meaningful impact on the pile lateral response. This is attributed to the large depth of the gas-charged layer. In turn, the soil near seabed, which offers the largest lateral resistance, was mostly unaffected. Results from structural analyses indicate that the different P-Y approaches have minimal influence on the pile unity check for piles above and below mudline, up to 9% improvement on offshore jacket platform natural period and up to 7% improvement on offshore platform reserve strength ratio when adopting Jeanjean method.

Keywords: Shallow Gas; Excess pore pressure; Axial capacity Lateral response; Reserved strength ratio

5th International Symposium on Frontiers in Offshore Geotechnics (ISFOG2025)

1 - Site investigation strategies, geohazards and integrated studies