This paper documents the use of the Imperial College Pile (ICP) design methods to obtain axial capacity of driven steel pipe piles for nine platforms in the UK North Sea. The diameter, wall thickness and length of the installed piles are based on these methods. The ICP methods have been used in preference to those given by API because they give more reliable values. The paper summarises site soil conditions, compares API and ICP capacities, discusses pile instrumentation data from installations and provides guidance on suitable factors of safety for use with the ICP methods.
The pile capacity methods recommended by API have been successfully used to design foundations for offshore structures since the 1960s. The methods have recognised shortcomings and, in order to address these, the methods have been continuously improved and updated with the changes being documented in successive revisions to the published recommended practice, a process that continues to this day.
In the early 1980s Shell U.K. Exploration and Production (Shell Expro) identified a need to improve design methods for its offshore structures, especially those relating to pile capacity in dense sand. To that end Shell Expro commissioned a major review of end bearing, for which a summary has been published1, and commenced sponsorship of a joint industry research project at Imperial College into shaft capacity that lasted ten years and culminated in the publication of a design guide2.
As a result of this work Shell U.K. Limited (Shell) has not used API RP2A3 pile design methods for platform foundations since 1996, when the Imperial College Pile (ICP) design methods2 were first published. The reason for the change in design practice was because the new methods are based on improved soil models that give better predictability of pile load test results and they have been incorporated into the Company's Standards. The new methods allow Shell to demonstrate the safety of its designs through structural reliability assessments. In certain situations the new methods also result in smaller, more cost effective foundations.
This paper summarises the soil conditions and foundations for nine North Sea platforms. It compares axial capacities derived by ICP4 and API RP2A3 methods with results from pile instrumentation that monitored pile-driving stresses during foundation installation. A procedure to assess foundation reliability is also described and target reliabilities are suggested. The results of reliability assessments for the platforms are given and the minimum resistance factors needed to meet the stated targets are tabulated. Shell has also used the ICP design methods to reanalyse the foundations of a number of existing platforms when updating structural assessments or considering additional topside loads, however, these platforms are not covered in this paper.
Seven of the platforms are installed in the UK Southern North Sea (SNS), with the other two being installed in the UK Central North Sea (CNS). The platforms vary from 5 to 30m in plan dimensions and stand in 25 to 120m of water. They are all founded on driven steel pipe piles with diameters