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.

API RP2A in its 14th edition (1984) recommends the use of an earth pressure coefficient of 0.8 for computing the tensile skin friction of an open ended pile driven into silica sands.In view of the impact of this recommendation on the design of North Sea piles driven into sands, a study was made of an expanded data base on the pull-out capacity of piles driven into sands. The expanded data base includes test data on relatively long North Sea conductors and in all 24 tension tests considered of reasonably high quality were analysed.The new data base together with observations of pile behaviour in sand has enabled the authors to develop two procedures for designing piles in sand.These two procedures differ from API in allowing skin friction to reduce at a given level as a pile is driven deeper. Comparisons of predicted and measured capacities of the test piles were made and compared to API (1984) predictions.It is concluded that the API Method is not reliable for loose or very dense~and sites. The two new methods provide much better predictions in these soil conditions.-33

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