Soil Resistance and Deflection Analysis of Cyclic Laterally Loaded Offshore Well Conductors for Fatigue Evaluations

Abstract

Offshore conductors are an integral part of the oil and gas well and they support the motions from the riser, which are usually subjected to environmental loads. These structures are designed to meet requirements for Ultimate Limit States (ULS), Serviceability Limit States (SLS), and Fatigue Limit States (FLS), design conditions. This ensures that if the foundation experiences cyclic loads, during its project span, it can withstand the fatigue damage that accumulates without failure. Fatigue failure in offshore structural components, can result from accumulation of fatigue stresses, with magnitudes considerably below the ultimate design stresses. Improved accuracy in characterizing the cyclic lateral soil response is critical in fatigue assessment of well conductors and piles subjected to dynamic fatigue loads. In this study, a phenomenological and bounding surface plasticity cyclic P-y model was discussed. Key features of the models include nonlinear load-displacement behavior with stiffness degradation during cyclic loading. These models provide full description of soil resistance during lateral loading, including an initial short-excursion monotonic loading stage, a transient stage of progressive degradation in stiffness from the first excursion, and a steady-state stage involving minimal changes in soil stiffness after a large number of load cycles. The model input parameters were obtained from calibrations to data derived from centrifuge tests on model conductors subjected to harmonic lateral loads. These models are able to simulate random load sequences which is useful in fatigue analysis. Fatigue damage in well conductors and piles arises from changes in axial and bending stresses, with the latter being more dependent on lateral soil response. These models are evaluated based on their ability to accurately predict bending moments when the spring model is used in conjunction with a laterally loaded soil-structure interaction model. These models successfully predict the maximum change in cyclic bending moment (change in moment during a load reversal) and the location of the maximum cyclic moment along the conductor depth approximately within a 20% range. This range is evaluated by comparing the model computations/predictions to the test data from the centrifuge program and validated within the test displacement range up to 0.15D (15 % pile diameter). The current form of the presented models do not consider consolidation effects, which may significantly affect long-term loading predictions used in fatigue life assessments.