This paper deals with the design and installation of the foundation piles for the Skiff platform in the UK southern North Sea. The structure is novel because the well conductors are the primary platform foundation. The conductors were installed by drill-drive methods, for which conventional Soil Resistance to Driving)SRD) prediction method are not considered to apply. The paper shows that the Imperial College Pile design methods can be used to (1) obtain an SRD that gives a good prediction of blowcounts, and (2) provide a reliable estimate of the conductors? eventual static axial capacity. The paper compares predictions with actual installation data.
The Skiff field is part of the Shell U.K. Limited (Shell) Sole Pit development in Block 48/20 of the UK sector, southern North Sea (Figure 1). The Skiff platform is an innovative slim-line design that was installed with a jack-up rig in April 2000 immediately before the wells were frilled (Figure 2). The jacket structure was placed on the seabed and tied back to the rig whiles the piled foundations were installed, and the topsides were place prior to drilling the wells.
The main foundation elements for the platform are the well conductor, installed through the jacket legs. The conductors were installed using drill-drive techniques and axial pile capacity was based on the Imperial College Pile(ICP)1 methods rather than the current American Petroleum Institute (API) RP2A recommendation2.
As this was the first foundation of this type to be installed by Shell, extensive pile monitoring was carried out during installation and selected blowcounts were analysed by Heerema using the TNOWAVE computer program to match the driving stress waves.
Figure 1: Vicinity map(available in full paper)
A geophysical survey carried out at the location revealed the water depth to be 26m and the soil profile to consist of some 75m of Holocene and Quaternary soils over Tertiary mudstone. Site investigation boreholes were drilled at the location to a maximum penetration of 78.2m below the seafloor. The boreholes confirmed the predominance of granular soils in the top 75m except for two minor clay layers above 11m penetration and a 3m thick layer below 31 penetration.
A generalised soil stratigraphy and related design soil parameters, based on the site investigation data, are given in Tables 1. The design cone penetration point resistance qc? profile is show in Figure 3. Over-consolidation ratios (OCR) for the clay layers were found to be between five and ten, and this information was used to determine sand relative density, Dr? using the procedures of Lunne and Christoffersen3. Dr was assessed to vary from 80 to 65% with depth.
The platform's plan dimensions were dictated by the specification that is must be installed by a jack-up rig with the piles driven with an impact hammer hung from the jack-up rig's draw-woks. This meant all the piles had to be reached by skidding the drilling contilever without the need to reposition the whole rig. The pile diameter was fixed to that of a standard well conductor