Abstract

Fatigue damage is an important consideration in the design of Tension Leg Platforms (TLP) foundations. The portion of fatigue damage from installation dominates the total damage of the pile foundation. Driving fatigue damage can be affected by factors such as soil resistance to driving (SRD) and hammer efficiency. A parametric study, utilizing data of Magnolia TLP to assess the effect that SRD and hammer efficiency had upon driving fatigue damage was performed.

The results provided a database and guidelines for the field engineer to control the pile driving fatigue damage by adjusting hammer efficiency, if the field pile-driving blow counts were different from that predicted. The study proved to be important in reducing pile driving fatigue damage during pile installation.

Introduction

For pile fatigue design, the combined in-place cumulative and installation damage are considered. The in-place fatigue damage is caused by environmental forces, such as wind, waves and current. The installation fatigue damage is basically generated by hammer blows from pile driving. Our study indicated that approximately 70% to 90% of the total fatigue damage is generated during pile driving. Therefore, how to reduce the driving fatigue damage is an important consideration in pile fatigue design and field driving operation. However, due to the potential for variation in the soil conditions, it is sometimes hard to accurately predict the soil resistance to driving (SRD) and thus the expected corresponding blow counts. During pile installation, the real blow count per foot could be higher or lower than that of predicted or higher at certain penetrations and lower at other penetrations. When variations occur (especially when blow counts are higher than predicted), the field engineer is left with trying to minimize fatigue damage by optimizing the driving plan.

The goal of this study had two primary questions which utilized data from the Magnolia TLP that was installed in Gulf of Mexico during early 2004 to assess the effects that SRD and hammer efficiency had upon fatigue damage occurring during installation. These included:

  • The effects of hammer energy to driving fatigue damage in order to determine whether to use high energy, low blow count, or low energy and high blow count to reduce fatigue damage, and

  • The effects of soil resistance to driving (SRD) so that if the SRD is higher than that predicted, how much more fatigue damage will be added to the pile?

The results of the study were used as a guideline during the field pile driving operations to minimize the driving fatigue damage, and it has generated a field method (program) to determine the fatigue damage for each pile during the pile driving operations. If necessary, an alternative driving plan can be generated by following the guideline in case the actual pile driving resistance is notably different from predictions with higher hammer energy and/or blow counts being required.

Description of the Study Methodology

For pile driving fatigue assessment, the fatigue damage at each girth welds under the impact loads of each hammer blow must be first calculated.

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