TECHNICAL FEATURE Some Knowledge Gaps in Marine Foundation Practice A pplications of driven piles as diverse as offshore oil and gas platforms, large bridges, ports and harbors, and the off- shore wind industry are often seen to share similar design and construction issues. Here we explore the need to investigate some recent issues and challenges that have arisen in practice. Designing marine and offshore struc- tures is a complex process with many interfaces between different engineering disciplines, from structural design issues to fabrication and installation details. However, one of the greatest sources of uncertainty can be attributed to the geo- technical design of the supporting foundations. Determining an appropriate soil reaction (both ultimate capacity and stiffness) is complicated by the fact that soil is a naturally occurring, non-linear, aniso- tropic, non-homogenous material, which is both non-linear and viscous when loaded. As a result, offshore foundations are often designed using a series of simplifying assumptions. The accuracy of these empirical approaches varies in different foundation designs and can impact the constructability as well as the underlying factors of safety. It is clear that there is significant room for improvements in marine foundation design to ensure that there is a consistent reference for factors of safety and reliability across every offshore or marine structure, while adopting effi- cient engineering designs. The scope of this paper presents a focus on driven pile foundations and the adequacy of present practice. Several technical challenges are highlighted here that reflect current and future issues facing the industry. Challenge 1: Foundation Codes for Offshore Axial Pile Design The method of securing offshore structures to the seabed has been a crucial con- sideration for the success of offshore oil/gas developments over the past 60 years. Piled foundations often represent the most suitable method of transferring large structural loads to competent bearing 10 15 20 25 0 5 ICP-05 UWA-05 Fugro-05 NGI-05 API-2007 Pile Design Method Figure 1: Pile lengths predicted according to the API design code strata. Chow (1997) reported that over 90% of oil and gas platforms in the North Sea are founded on open ended steel piles. The rapidly developing wind energy sector has also recently led to further expansion in the offshore driven pile industry. However, offshore pile design is still heavily reliant on empirical design methods, which have not been developed for the specific loading conditions or geometries encountered offshore (Jardine and Chow, 2007). The most commonly adopted offshore pile design guidelines are those specified by the American Petroleum Institute (API), which were formally introduced into practice in 1969. These guidelines suggest a design approach for piles in cohesive soils based on the traditional “alpha” method, which relates the unit skin friction directly to the undrained shear strength. The caveats of this method are well documented by a number of researchers, one of which is the difficulty in obtaining representative undrained shear strength values (Lehane, 1992). Similarly, an empirical approach relating the unit shaft friction to the vertical effective stress has remained the API industry standard design approach for piles in sand, and similar caveats apply, with the design issues for piles in sand described in detail by Gavin et al, 2011. For piles in sand, there have recently been significant strides in developing CPT (cone pene- tration testing) based solutions for predicting pile capacities. These are now included in the commentary of the main API text, where any of five methods (ICP, UWA, NGI, Fugro and API) can be employed. However, despite a similar underlying framework these methods can give drastically different results. Com- parison of the pile penetrations required to resist a 10 MN (225 kip) design load (assuming a 2.5 m [8 ft] diameter pile installed in dense sand) is shown in Figure 1, where pile lengths range from 4 m to 22 m (13 ft to 72 ft). Clearly, there is considerable scope for improvements in these prediction methods. AUTHORS: Paul Doherty, Senior Geotechnical Engineer, Gavin & Doherty Geosolutions Robert F. Stevens, P.E., Senior Technical Manager, Fugro-McClelland Marine Geosciences, Inc. Gerry Houlahan, P.E., Chairman DFI Marine Foundation Committee Moffatt & Nichol DEEP FOUNDATIONS • JULY/AUG 2012 • 51 10 MN Design Load Pile Lengths (m)
