Abstract

Soil Resistance to Driving (SRD) has been recognized as a consideration to be studied for the purpose of offshore pile foundation driveability assessment. Since its establishment in 1982 and until now, Robert Stevens approach has been widely utilized for the SRD prediction in fixed offshore structures located in different soil types including the carbonate sand.

During the engineering of one of the fixed platforms in offshore Egypt (Helal platform), adopting Stevens carbonate sand model in 102 m penetration pile, the driveability analysis revealed the need to a hammer with extreme high energy and weight, where the real behavior during installation was different. As a result of this experience, a bench mark study was conducted for eight platforms founded on carbonate sand in different locations in offshore Egypt with available pile field driving reports. Wave equation driveability analyses were carried out based on Stevens SRD model (Stevens' four cases) by GRL-Weap 2010 (Offshore version) software. Then the results were compared to the real driving records for those locations.

In the eight locations, the results uncovered the fact that the Stevens based predicted blow counts are much higher than the recorded ones. And it was found that in most of the cases the analyses output showed potential premature refusal whilst the real records showed that the target penetrations were achieved smoothly. Furthermore, in most of the cases actual driving hammer energy was found dramatically less than the hammer energy required by the Stevens analysis which in turn leads to a substantial over sizing in the pile section, the hammer and the installation cost. Bearing in mind what is recommended by the API-RP2A to reduce the limiting values of the carbonate sand static resistance, a series of trials was conducted to evolve a new carbonate sand model from Stevens' considering reduced soil resistance values. Eventually, a new model was reached that showed great consistency between the predicted SRD and the actual recorded ones.

This paper elaborates how effective is implementing the new approach to the offshore installation industry from the engineering perspective, as it optimizes the pile section and makeup. This is in addition to the construction and management perspectives, as it could significantly reduce the installation time and cost on one hand and relax the requirement of qualified hammers by widening the range of hammers selection on the other hand.

Although the proposed method are found to be suitable for the selected locations considered in this study, it should be noted that the results of the back-analyses in the current study may be affected by the inherent uncertainty pertaining to the adoption of default hammer properties or applied hammer energy necessitated by the unavailability of instrumented data. Hence, the applicability of the proposed method at other location with similar soil conditions should be further verified using site specific instrumented pile monitoring data.

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