API RP-2A has existed in versions for both load and resistance factor (LRFD) and working stress (WSD) design methods. However, the partial factors recommended for foundation design are based on calibrations carried out using data and practice applicable to the Gulf of Mexico and U.S. waters. A study has therefore been carried out as Joint Industry Project to investigate the applicability of the LRFD method for pile design for foundations in the North Sea.
The first stage of the study was an assessment of the effect of the LRFD methods of design on required pile length for various types of platform and soil conditions, using the pile design methods recommended on API RP-2A. in some cases the pile lengths required by the LRFD method are very similar to those required by the WSD method, however, in other cases, particularly for tension piles in sand, use of the LRFD method leads to a requirement for considerably longer piles.
The second stage was an assessment of the partial factors that would be required to give the same reliability for the LRFD method as are currently obtained using the WSD method. This was achieved by assessing the reliability of various pile design methods, including those recommended by API RP-2A and other more recently developed methods. The results of this study were used to assess the current reliability of a representative selection of existing North Sea foundations and also their reliability if designed using the LRFD method.
It is concluded that it is not possible to select a unique set of partial factors to achieve the same reliability for the LRFD method as are currently achieved for the WSD method. This is primarily because some loading conditions are very different from those for which the methods were fitted, The fit between the methods is made worse by the fact that current API methods of pile design, particularly for sands, have low reliabilities.
Since 1991, API RP-2A has existed inversions for both load and resistance factor (LRFD) and working stress (WSD) design methods. However, the partial factors recommended for foundation design are based on calibrations carried out using data and practice applicable to the Gulf of Mexico and US waters. A study has therefore been carried out as a Joint Industry Project to investigate the applicability of the LRFD method for pile design for foundations in the North Sea.
The paper first presents a simple idealisation of foundation design to illustrate that the LRFD formulation could lead to considerably longer piles than the WSD formulation when environmental loads are high compared with gravity loads, particularly for tension piles in sands. The results of a more detailed study are then described, in which this was investigated in detail for various types of platform and soil conditions. In some cases the required pile lengths were found to be very similar to those required by the WSN method. As anticipated however, in other cases, particularly for tension piles in sand, use of the LRFD method leads to a requirement for considerably longer piles.
