by Dr. Jun Yang, Civil Engineering
Feb 20, 2016
Large-diameter steel pipe piles used as foundations for offshore structures (photos by Bilfinger Berger & Vattenfall)
The world's ever increasing demand for energy poses many challenges to the design and construction of offshore structures, such as oil and gas platforms, wind turbines and cross-sea bridges, most of which are founded on steel pipe piles (see photos above). For the offshore geotechnical engineers worldwide, how to reliably and economically design these piles is a big challenge as the pile's loading capacity is largely associated with the installation of the pile into seabed, during which a column of soil tends to form inside as soil enters the pile from the pile tip – a phenomenon known as soil plugging. The degree of soil plugging is affected by many factors, leading to very complicated load transfer mechanism. The most popular method of analysis for offshore piles is that recommended by the American Petroleum Institute, known as the API method, with its new edition issued in 2006. However, there has been considerable concern with the reliability and rationality of this method since it is highly empirical and does not distinguish between plugged and unplugged piles. Researchers and engineers have devoted great efforts to developing more reliable methods for analysis and design. Notably known are the ICP method developed at the Imperial College London and the ICP-based UWA method improved by researchers well known in offshore geotechnical engineering at the University of Western Australia.
More recently, Dr. Jun Yang of the Department of Civil Engineering and his co-worker have developed a new method for analysis and design of open-ended pipe piles, and they named it as the HKU method. Unlike other current methods, the HKU method properly takes into consideration the mechanisms of the annulus and plug resistance mobilization based on theoretical analysis and experimental observation. Evaluation of the applicability of the HKU method has been conducted by Dr. Yang and his co-worker against a number of well-documented field tests including two fully instrumented large-diameter offshore piles in Tokyo Bay, Japan, and it has been shown that the HKU method has an attractive capability and advantage. A technical paper on HKU’s work was submitted in 2011 to the premier ASCE Journal of Geotechnical and Geoenvironmental Engineering and received comment from peer reviewers that “It deals with an important, still unsolved engineering problem that is very timely”. The paper was published in Volume 138 of the journal in 2012.
In a recent study, researchers at the University of Stuttgart in Germany conducted a structured overview of these methods as well as the EAP-EAU method widely used in Germany and presented their results at the 40th Annual Conference on Deep Foundations held in Oakland, California in October 2015. They further reported a case study on the predictive performance of these methods against measurement from a high-quality load test on a 40-m long, steel pipe pile installed at Eemshaven, The Netherlands. Their study showed that the HKU method is the most accurate among the methods considered (“accurately correspond with the measurement” as stated in their paper; refer to the figure below which is reproduced from their paper). A rational and accurate method for design is not only of academic interest but particularly of practical significance. It may allow steel pipe piles to support heavier structures on top than that is currently possible using the standard methods and thereby allow significant cost savings.
The HKU method tested to be most accurate (from Moormann, Labenski & Aschrafi, 2015)