Abstract

The paper reports a previously unpublished tension load test to failure on a 38.lm long, 660mm diameter steel pipe conductor driven in predominantly dense sands at the Leman BD site in the Southern North Sea. The case history is used, with other pile test data, to evaluate and calibrate new design methods that have been developed from recent research. As an example, the findings are applied to a study into the axial capacity of piles driven at the nearby Leman AD H site.

The Lemon soil conditions are described, and the driving records and test result are presented. The new design methods are then summarised and applied to analyse the conductor's capacity. A comparison with other pile capacity prediction methods is made and the results are used, in conjunction with other data to demonstrate that a modified form of the new method can give accurate predictions for offshore pipe piles.

Applying these approaches the Leman AD 11 platform's foundations indicates axial capacities greater than expected previously. Increases in compressive and tensile capacities of at least 15 and 50Y0, respectively, are predicted. Research is continuing to fizrther refine the new methods for calculating axial capacity.

Introduction

Shell UK Exploration and Production operate groups of offshore jacket structures in the southern North Se% some of which were installed more than 25 years ago. To help assess the effects of a possible reconfiguration of loads applied to the Leman AD 1[ structure, a study was made of the axial capacity of its foundation piles.

Leman AD 11 has piles that were driven in predominantly dense sands, for which standard axial capacity calculation methods are unlikely to offer accurate predictions. Reviews such as those by Focht and O'Neill, Briaud and Tucke, Jardine and Christoulas, Tang, Randolph Chowb or Gavin and Lehane show that conventional design methods for piles driven in sand are relatively unreliable.

New approaches were considered necessary and the study was shaped to:

  1. . Interpret the ground conditions and operational soil parameters at Leman AD 11on the basis of existing site investigation data,

  2. Assess data from a conductor pull-out test performed at the nearby Leman BD site in 1969 and the recent field tests on driven open-ended pipe piles at Dunkirk described by Chow

  3. Use the above to test the reliability of a new Imperial College ([C) method for calculating the shaft resistance of open-ended driven piles in dense North Sea sands and

  4. . Apply the new methods, taking account of the recent research concerning base capacity described by Hight et alg and Chowb.

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