Re-assessment of P-Y curves for soft clays from centrifuge testing and finite element modeling

Abstract

The paper focuses on the results of centrifuge testing on a laterally loaded conductor. The conductor was installed to simulate jetted conditions and lateral transfer curves (so called P-Y curves) were generated for monotonic and cyclic conditions. The influence of a previous set of cyclic loads on the subsequent behavior of the pile when subjected to additional cyclic loads was investigated. The paper focuses on the determination of P-Y curves tangent and secant moduli since there are the key input parameters for structural software in determining the lateral displacements of the conductor and therefore the cyclic fatigue-generating stresses in the conductor. Results show that the P-Y curves as recommended by API RP2A (2000) can be conservative, with moduli and ultimate unit pressures less than what test data and theoretical modeling suggest. The centrifuge tests validated theoretical curves derived via Finite Element Analyses (FEA) and described by Templeton (2009). Equations are proposed to generate the backbone P-Y curves and to calculate the secant modulus to the P-Y curves to analyze the cyclic loading of interest to this paper.

Recent Advances in Soil Response Modeling for Well Conductor Fatigue Analysis and Development of New Approaches

“…Jeanjean [2] investigated the lateral soil response of normally consolidated to lightly over-consolidated clays through a series of physical experiments complemented by FE analysis [3] and concluded that lateral soil resistance under a monotonic pushover load (i.e. the monotonic backbone p-y) in soft clays are both stiffer and stronger than those estimated by the API guidelines [1].…”

“…Table 1 summarizes the critical (minimum) fatigue life predications normalized to that measured in centrifuge Test 4 and Figure 2 shows the corresponding profiles for the first two motions. For test details reference should be made to the original paper [2]. The FE analyses were done using the forces measured in the centrifuge tests (i.e.…”

“…Then FE and fatigue analyses were carried to investigate performance of the backbone curves (API [1] and Jeanjean [2]) in predicting the fatigue life observed in the centrifuge tests. The cyclic loads comprised random motions of sea state characteristics exerted symmetrically about the model conductor central axis and at an offset.…”

“…In most software programs the tangent stiffness along the back bone curve was used in the fatigue assessment. Jeanjean [2], however, discusses two problems with this approach. Firstly the API lateral soil springs were shown both numerically and experimentally to be too soft at the small displacements required for fatigue assessments and, secondly the proper characterization for fatigue fundamentally should not be based on the backbone response.…”

Effect of foundation modelling on the fatigue lifetime of a monopile-based offshore wind turbine

“…However, the p-y curves were developed for long and slender jacket piles with large length-to-diameter ratios, significantly different from typical monopile geometries. Several studies have shown the limitations of the p-y curve approach (Doherty and Gavin, 2011;Lesny, 2010;Jeanjean, 2009;Hearn and Edgers, 2010), and alternatives to the American Petroleum Institute (API) formulation have been proposed, such as py curves extracted from FE analysis of the soil-foundation system. Despite these curves being able to capture the pile stiffness more accurately, the same extracted p-y curve is often used in the simulation tools for loading, unloading, and reloading, which means that they neglect effects such as permanent deformations and soil damping.…”