ABSTRACT

A centrifuge test program was designed to explore the influence of repeated cyclic loading on axial capacity of offshore wind turbine structure (OWTS) foundations. The program focused on low amplitude high-cycle (e.g., 10,000 to 100,000 cycles) loads meant to emulate conditions imposed on an operating OWTS. The program was focused in an area of limited previous test data to:

  1. evaluate the feasibility of using a small centrifuge for this application;

  2. develop an initial data set from scaled centrifuge testing of piles subjected to one-way (tension) and two-way (tension and compression) cyclic axial loading;

  3. evaluate the data set within an interaction diagram framework relating the average static axial load and applied cyclic axial loads to tested maximum pile load and applied number of cycles; and

  4. provide guidance on implications of the findings and recommended next steps.

Observations on the degradation with load demand and number of loading cycles to failure are presented. Preliminary analysis on the soil-pile degradation pattern with depth and cyclic loading is also presented through soil-shaft stiffnesses computed for discretized pile elements.

INTRODUCTION

Understanding the influence of the soil-pile interface is critical for reliable operations of OWTS. The foundation and support structure for an OWTS must satisfy a number of criteria, including limits on stiffness, displacement, tilt, and other characteristics that can influence the behavior of the system. These requirements are particularly challenging for OWTS due to their complex dynamic loading conditions. The influence of cyclic loading on multi-piled support structures represents a major gap in the current state of the practice for offshore wind. These structures involve unique design challenges due to:

  1. the potential for complete load reversal within the piles due to the relatively low gravity loads in the system;

  2. the potential for support structures and foundations to be loaded near peak demand during normal operation (i.e., the operating wind load may generate mudline overturning moment close to or in some cases greater than during the design storm condition); and

  3. the substantial number of load cycles that occur due to the almost continuous operation of the rotor.

Keywords:
cyclic load, degradation, rpr, displacement, cyclic loading, axial load, model pile, foundation, loading, load packet
Subjects:
Offshore Facilities and Subsea Systems, Facilities and Construction Project Management, Platform design, Offshore projects planning and execution
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2017. International Society of Offshore and Polar Engineers
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