This paper reviews the research and debates that have led to substantial changes being made in 2007 to the API-RP2A recommendations for assessing offshore driven pile axial capacity. The reasons for the conventional Main Text method's large scatter, strong skewing and significant biases are explored, with particular emphasis on piles driven in sand. Recent alternative design frameworks are reviewed critically and conclusions are drawn regarding their practical application. Comments are also made on predicting load-displacement behaviour, assessing the impact of load cycling, group interaction effects and aspects of foundation disturbance by drilling.
The technologies associated with the manufacture and installation of offshore piles are relatively mature; very large piles may now be driven routinely in a wide range of water depths and geotechnical settings. However, the understanding of the ground's reaction to driven pile installation and loading has lagged behind the impressive developments made by the offshore construction industry, as design approaches are still in an imperfect state of evolution. Severe problems have arisen during pile installation in some major projects1. Considerable mismatches have been found in other cases where it has proved possible to check Industry-standard design expectations by static tests on large offshore scale piles2, 3, 4.
Research in several centres has emphasised the scientific weaknesses of the industry-standard American Petroleum Institute (API) RP2A5 methodologies, which have remained practically unchanged between 1993 and 2007. While most practitioners have continued to use the conventional methods, alternative geotechnical design frameworks have been proposed that have been applied comprehensively in some sectors6. Vigorous debate has taken place over several years, prompted by industry-sponsored reports, academic papers, conference proceedings and meetings of the relevant API/International Organization for Standardization (ISO) review panels. Important changes are included in the 2007 API-RP2A recommendations for piles driven in sand that will also affect the ISO documents and industrial practice. However, progress is being made cautiously and further evolution of design practice can be expected.
This paper offers one perspective on some of the issues raised in the recent debates, referring to background research and highlighting physical aspects of pile behaviour that are important to practice. Particular emphasis is placedon the question of axial capacity, as this is arguably the most important issue and has proved to be the focus for most discussion. Consideration is also given to the assessment of load displacement behaviour. While movement prediction and control is emphasised more strongly in onshore foundation projects, as reflected in the recent review by Mandolini et al.7, offshore engineers may become more concerned with making better fatigue life predictions for critical structures. Assessing limits for acceptable displacements also could become more important as a means of monitoring platform safety in critical cases.
It is useful to consider at the outset the ranges of pile sizes specified by offshore engineers. Piles with diameters of ~5m have been driven for offshore wind turbines. Smaller diameters of 3 to 4m have been specified for such structures in the North Sea, where piles with diameters of up to 2.5m