Browsing by Subject "P-y curves"

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Design of large diameter monopiles for offshore wind turbines in clay
(2016-08) Senanayake, Asitha Indun Madusanka Joshua; Gilbert, Robert B. (Robert Bruce), 1965-; Wang, Shin-Tower; Cox, Brady; Manuel, Lance; Murff, James D
Offshore wind power has great potential as a clean and renewable energy source that is capable of reducing our reliance on fossil fuels. The main drawback of offshore wind power is its comparatively high capital cost. One area in which this cost can be reduced is by optimizing the design of these structures. More efficient foundation designs is key in this regard. The p-y method is extensively used for the design and analysis of laterally loaded piles due to its simplicity and versatility. Matlock (1970) or the API RP 2GEO (2011) “soft” clay p-y model is the guideline of choice for normally consolidated to moderately overconsolidated clays. However, this p-y model is not yet verified for piles with very large diameters and low aspect ratios. Design of wind turbine monopiles is governed by serviceability limits such as the natural frequency of the structure and the accumulated tilt under long-term low-amplitude cyclic loads, but these guidelines have not been verified for serviceability limit state designs. The main objectives of this study were to: (a) assess the ability of the Matlock (1970) p-y model to accurately model the behavior of laterally loaded piles at both small and large displacements, (b) investigate the effect of gapping on the backside of laterally loaded piles and develop a theoretical framework to quantify its effect and predict its occurrence, (c) re-examine the derivation of lateral bearing capacity factors (N p ) used in published p-y models, (d) evaluate the effect of large numbers of small-amplitude cyclic load on the stiffness and the post-cyclic ultimate capacity of laterally loaded piles, (e) assess the ability of the Matlock (1970) p-y model to adequately account for pile diameter effects, (f) assess the ability of the Matlock (1970) p-y model to accurately predict the behavior of a pile in a variety of undrained shear strength versus depth profiles, (g) assess the ability of published p-y models to accurately predict the natural frequency of wind turbine structures. The methodology consisted of analyzing field tests, laboratory model tests (1-g and centrifuge), and numerical modeling. An extensive database of field tests and laboratory centrifuge tests was compiled. This data was then supplemented by a series of 1-g model tests in a variety of clay test beds (normally consolidated to heavily overconsolidated, kaolinite and Gulf of Mexico clay) carried out at The University of Texas at Austin and 3-d finite-elements models using Abaqus carried out by Ensoft Inc. The following conclusions were drawn from this study: (a) Matlock (1970) p-y model underestimates the lateral soil resistance on piles in normally consolidated and overconsolidated clays, regardless of pile diameter or aspect ratio, (b) the effect of gapping plays an important role in determining the pile response as it can lead to a loss of capacity and a reduction in stiffness, (c) lateral bearing capacity factors used in the Matlock (1970) model are too low, (d) the degradation in the stiffness of the pile response, when subjected to cyclic loading, was limited to approximately 30% and occurred within the first 100 cycles, (e) the method of normalizing used in the Matlock (1970) model successfully accounts for pile diameter effects, (f) estimates of the natural frequency of wind turbine structure based on the API RP 2GEO (2011) p-y model are lower than those based on the Matlock (1970) and Jeanjean (2009) p-y models.
Design of laterally loaded monopile foundations in sand for offshore wind turbines
(2017-05) Dasgupta, Udit Shankar; Gilbert, Robert B. (Robert Bruce), 1965-
Globally renewable energy is gaining popularity with the largest scope in offshore wind energy. Although USA is one of the leaders in the onshore wind industry, it is yet to tap into the abundance of coastline it has available for this offshore wind development. High costs of installation have been a deterrent, with the foundation being a major part of that cost. Among the various options for offshore foundations, the most popular and at the forefront of feasibility in construction are the monopiles. The aim of this research is to see how well the p-y method specified in the American Petroleum Institute (API) design standard predicts the response of these laterally loaded piles in sands. This thesis also investigates possible approaches to modify the codes to better predict the response and improve the efficiency of the design of the monopiles. The p-y approach was developed for analyzing laterally loaded long slender piles which are widely used for offshore oil and gas platforms. These piles tend to bend and fail through formation of flexural plastic hinges. The large diameter relatively short monopiles for wind turbine applications are relatively stiffer and behave differently. They tend to rotate and/or translate instead of bending, evidenced by sizeable movements of the tip of the pile. Additionally, there is a serviceability criteria of the wind turbines to operate at low displacements i.e. low strain levels. This research focuses on the small strain response of the piles. The main behavior investigated with the long- slender pile load tests historically has been the response at large strain levels i.e. failure. The empirically calibrated API method may not be valid for such short-rigid piles and need to be verified before they can be used with confidence. A database of laterally loaded piles in sands is compiled and presented. Specifically the impact of the initial stiffness modulus (k) on the response was investigated using the finite difference code LPILE. The Embedded Length to Diameter Ratio (L/D) is an important factor of design that influences the response. Based on the few field tests chosen for analysis from the database, it is observed that the API code overestimates the initial stiffness of the pile for piles with lower L/D ratios for monotonic loading. For cyclic loading, the LPILE analysis does not lead to much change in initial stiffness from the monotonic simulation, but the actual measurements suggest an increase in stiffness. There is an added effect of base shear which might be an important force that is taken into consideration for short rigid piles, that the current code does not specify. Higher stresses locked into the soil due to pile driving might lead to higher base shear. Finally, since the problem is one of small strain, feasibility of future in-situ seismic testing is looked at by collection of sands from Mustang Island, TX, where the original field load tests were conducted that led to the specifications in the API code.