Improved Understanding of Foundation Strength for Piled Offshore Structures

Abstract

Abstract Within the oil and gas industry major inconsistencies have been found in the correlation of predicted pile response using design code prescribed methods and the measured response from installed on-line monitoring systems. This paper presents the first phase of a study conducted to determine whether a finite element continuum model which uses an elasto-plastic constitutive soil model can better determine the measured/observed response of piled offshore platforms. The first phase of the study entailed a comparison of the measured natural period of an existing platform to the natural period determined numerically using a dynamic eigenvalue analysis with nonlinear soil springs. Sensitivity studies were also conducted to identify how modelled parameters such as topsides mass, conductor mass and marine growth affected the calculated natural period of an existing platform. Single pile models were also developed for detailed finite element (FE) analyses using the nonlinear springs (API soil springs) and a continuum soil model with the soil properties defined using the Mohr-Coulomb plasticity constitutive soil model. The results of the analyses have confirmed industry observations that, even accounting for the various parameters considered in the sensitivity studies, the design code derived soil springs show poor correlation to the measured response of the jacket. The dominant reason for this is the lateral stiffness (p-y) of the API derived soil springs which required an amplification factor in the range of 8-10 to obtain a suitable correlation to the measured jacket response. The continuum model demonstrated a significantly improved correlation between the numerically derived pile response and the measured data. The numerical representation of the soil-pile interaction described in this paper provides a more realistic determination of the structural response of the modelled platform. The results obtained have major implications for both the design and assessment of offshore structures. An increased lateral soil stiffness will largely eliminate the foundation failures predicted by conventional ultimate strength assessments and improve correlation to observed post hurricane structural performance. In the case of platform risk assessments, this improved representation of the soil-pile interaction will reduce the industry observed excessive conservatism in the prediction of failure probabilities of existing offshore facilities. In extreme cases, platforms have had to be strengthened or even removed based on predictions of foundation failures that were inconsistent with measured (OLM systems) and observed performance (hurricane survivals). Based on the results obtained from the first phase of this study, further work is planned to validate the FE continuum approach single pile model and finally provide a numerical solution for pile-soil interaction which can be easily adopted by practicing engineers in the oil and gas industry.