Pile Design And Installation Features Of The Thistle Platform
Authors: Young Alan G. , Sullivan Richard A. , Rybicki Chester A. 
Published: December 2007

The massive Thistle platform situated in 161.5 m of water about 210 km northeast of the Shetland Islands is supported by 42 piles carrying ultimate axial loads ranging from 35 to 52 MN. Installation of the pile foundations to penetrations of 140 m below seafloor involved detailed preconstruction planning after soil conditions were determined from planning after soil conditions were determined from geotechnical investigations.

The 31.8 Gg steel jacket has a 76-m by 76-m deck weighing over 25.0 Gg and is designed to withstand a 100-year design wave of 28.5 m. The 42 foundation piles are comprised of 12-pile groups supporting the two large flotation legs and of 9-pile groups for the two battered legs. Eight of the piles in a 12-pile group are two-stage piles consisting of a 1.37 m-diameter pipe pile driven to 30-m penetration with a 42-in. pipe pile grouted into a 1.21 m hole extending below the driven section to 140-m penetration. The other four piles in the group consist of 1.37 m-diameter pipe piles driven to 30-m penetration to increase the lateral load capacity of penetration to increase the lateral load capacity of the group. The 9-pile groups consist of 1.37 m-diameter piles driven to 30-m penetration with 1.21 m grouted piles extending to 90-m penetration.

The paper describes the scope of the geotechnical and geophysical investigations undertaken to explore soil stratigraphy and to determine pertinent physical soil properties for design of the driven and grouted sections of the foundation piles. Pile drivability by the wave equation analysis is presented and compared with actual pile driving performance. The paper also describes the construction techniques employed in drilling the 1.21 m-diameter holes and grouting the insert piles as deep as 140 m beneath the seafloor. Finally, the paper compares the predicted mud mat penetration with the penetration observed when the penetration with the penetration observed when the steel jacket was set on the seafloor.

Introduction

The Thistle Field "A" Platform was placed on the seafloor on August 10, 1976 in 161.5 m of water at a location in Block 211/18 of the United Kingdom sector of the North Sea as shown on Figure 1. The steel tower-type structure was towed to its location about 210 km northeast of the Shetland Islands as it was held afloat by the two 9-m-diameter flotation legs compartmentalized to allow a slow controlled placement on the seafloor. The structure was temporarily supported after placement on the seafloor by 12.8-m-diameter mud mats placed beneath each of the four legs. After placement, a modular construction deck was erected on the template and included all equipment necessary to install both the driven and drilled-and-grouted steel piles thus avoiding the use of a derrick barge for foundation installation. Upon completion of the pile installation, the construction deck was removed and replaced with a 4-level permanent deck having all equipment and facilities necessary to drill the 60 planned wells.

The initial portion of this paper describes the design features of the Thistle -A" Platform. It is followed by a description of the scope of the geotechnical and geophysical investigations undertaken to explore soil stratigraphy and to determine pertinent physical soil properties for design of the driven and physical soil properties for design of the driven and grouted sections of the foundation piles. Pile drivability is predicted by the wave equation analysis and compared with actual pile driving performance. Design and installation of two-stage driven and grouted piles to support axial and lateral loads are described; results of analyses for pile groups are presented. Predicted and observed penetrations of presented. Predicted and observed penetrations of mud mats to initially support the structure are compared.

DESIGN DETAILS

The 31.8 Gg steel tower has a 76-m by 76-m permanent deck weighing over 27.4 Gg. The permanent deck weighing over 27.4 Gg. The 185-m-tall structure was designed by CJB-Earl and Wright, Ltd. to withstand a 100 year design wave of 28.5 m and maximum winds of 200 km/h. As illustrated on Figure 2, the two large legs used for flotation have a diameter of 9 m. The 3-m-diameter small legs are battered at 7.5 degrees. The 42 foundation piles are comprised of 12-pile groups supporting the two large flotation legs and of 9-pile groups for the two battered legs as illustrated on Figure 3.

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