Analysis of performance and reliability of offshore pile foundation systems based on hurricane loading

Abstract

Jacket platforms are fixed base offshore structures used to produce oil and gas in relatively shallow waters worldwide. Their pile foundation systems seemed to perform better than what they were designed for during severe hurricanes. This observation has led to a common belief in the offshore oil and gas industry that foundation design is overly conservative. The objective of this research is to provide information to help improve the state of practice in designing and assessing jacket pile foundations to achieve a consistent level of performance and reliability. A platform database consisting of 31 structures was compiled and 13 foundation systems were analyzed using a simplified foundation collapse model, supplemented by a 3-D structural model. The predicted performance for most of the 13 platform foundations is consistent with their observed performance. These cases do not preclude potential conservatism in foundation design because only a small number of platform foundations were analyzed and only one of them actually failed. The potential failure mechanism of a foundation system is an important consideration for its performance in the post-hurricane assessment. Structural factors can be more important than geotechnical factors on foundation system capacity. Prominent structural factors include the presence of well conductors and jacket leg stubs, yield stress of piles and conductors, axial flexibility of piles, rigidity and strength of jackets, and robustness of foundation systems. These factors affect foundation system capacity in a synergistic manner. Sand layers play an important role in the performance of three platform foundations exhibiting the largest discrepancy between predicted and observed performance. Site-specific soil borings are not available in these cases. Higher spatial variability in pile capacity can be expected in alluvial or fluviatile geology with interbedded sands and clays. The uncertainties in base shear and overturning moment in the load are approximately the same and they are slightly higher than the uncertainty in the overturning capacity of a 3-pile foundation system. The uncertainty in the overturning capacity of this foundation system is higher than the uncertainty in shear capacity. These uncertainties affect the reliability of this foundation system.