Discussion. Geotechnical Planning of Piled Foundations for Offshore Platforms.

Abstract

Since the Forties Field FB Platform, my company has been involved in the design and installation of the following jackets in the British sector of the North Sea; Forties FD; Brent A; Claymore; Heather and Ninian. The Forties Field was underlain by normally consolidated clays, sands and at depth by overconsolidated clay. The other four fields are further north and are underlain by very hard clays and dense sands. No major pile installation problems were encountered at these northern sites. It was reassuring to find that the design methods used for planning pile installations work just as well for preconsolidated sands and clays as they do for normally consolidated soils.Professor T. J. Poskitt, Queen Mary College, London My comments are on the use of the wave equation in driveability analyses and on the problem of plugging.71. By any standards the piles used offshore are extremely large. Load testing is clearly out of the question and it is therefore natural for engineers to seek ways in which they can relate the soil resistance at the time of driving (SRD) to the static capacity. This is proving to be an extremely difficult job because the SRD is essentially a dynamic phenomenon which has to be studied by the wave equation. In addition pore pressure changes and thixotropy in the remoulded soil will produce time dependent changes which are difficult to predict analytically. These are areas of piling behaviour which are not fully understood and research is necessary in order to gain a better understanding of the mechanisms involved.72. The Authors give a comprehensive description of the use of the wave equation in drivability studies. In principle the wave equation is a powerful tool which will enable the designer to take into account the many different factors known to affect piles. However, in my experience considerable skill is needed if it is to give meaningful answers. Parameters such as quake,, damping factors etc., are as important as shear strength. This is not always sufficiently well appreciated by engineers and must have led to many disillusioned users of the wave equation. It is my opinion that where the wave equation is being used intensively as the Authors suggest in 0 7(e)-(g), the determination of the constants for use in the wave equation should have the same priority as is attached to other soil tests.

Performance of CPT-based methods to assess monopile driveability in North Sea sands

“…and is arguably the most challenging aspect (Prendergast and Gavin, 2016;Wu et al, 2018). Traditional 92 approaches for predicting SRD such as Stevens et al (1982), Toolan and Fox (1977), and Semple and 93 Gemeinhardt (1981) are largely empirical and therefore, extrapolation to pile geometries and soil 94 conditions outside of the dataset on which they are based is highly questionable. The application of such 95 methods in the extreme hard tills or very dense sands (where the CPT end resistance qc value is typically 96 in the range 30 -100 MPa) in the North Sea and for the large diameter monopiles supporting wind 97 turbines should be assessed.…”

“…They 111 found this approach gave consistent predictions of driving resistance for four, open-ended pipe piles 112 driven with diameters ranging from 0.356m to 2m at three sites, in Japan, USA and the Netherlands. 113 Byrne et al (2012) examined the ability of commonly employed pile driveability models (Semple and 114 Gemeinhardt, 1981; Stevens et al, 1982;Toolan and Fox, 1977) to predict the installation response of 115 a 0.762m diameter skirt pile and a 4.2m diameter monopile installed in dense North Sea sand. Somewhat 116 surprisingly the models provided poorest predictions of the installation response of the 0.762m pile with 117 the range of measured to predicted blow-counts varying from 30% to 180% at the final penetration 118 depth of 34m.…”

Stress History Approach To Analysis Of Soil Resistance To Pile Driving

“…(4) effects are available in the literature (Housel, 1950;Seed and Reese, 1957;Eide et al, 1961;Stermac et al, 1969;Flaate, 1972;McClelland and Lipscomb, 1972;Fox et Some data on set-up FP=0.5(OCR)0.3. .…”

Bayesian Update of Load Capacity for a Large Steel Piling in a Stratified Soil Profile

“…The soil pile resistance mobilized during pile driving differs from the pile capacity estimated for long term conditions, although the calculations are carried out quite similarly. The resistance mobilized during driving, known in the literature as SRD (soil resistance to driving), is that mobilized during pile penetration from the hammer blows (Toolan & Fox, 1977;Stevens et al, 1982;Semple & Gemeinhardt, 1981).…”