The results of several geotechnical studies performed for the Gulf of Suez Petroleum Company (GUPCO) in their Gulf of Suez July field are presented here. Soils at the site primarily consist of weak to moderate ly cemented carbonate sands and silts (carbonate content in excess of 90 percent). Pullout test results for 24-inch diameter pipe piles in this deposit illustrate that the current state-of-the-art practice in predicting soil pile response of driven pipe piles in calcareous deposits is not adequate.This study identifies the important geotechnical parameters and recommend at ions are provi ded to improve predi ct i ve capability in similar foundation environments. this study is gratefully acknowledged. Review and lelpful suggestions by Wayne B. Ingram and Michael W. )'Neill are also acknowledged. references 1. American Petroleum Institute, ARecommended Practice for Practice for Planning, Designing and Constructing Fixed Offshore Platforms, API RP 2A,

As the exploration and development for ot%shoreoil and gas reserves moves info the deepwater environment of the continents/ slope, a good model js necessaty to evaluate drivability of large-diameter, long pjles jn normally consolidated clays. Procedures are available to predict soil resistance to driving in stiff to hard overconsolidated clays of the North Sea and the Arabian Gulf Use of these procedures in normally consolidated clays grossly overpredicts soil resistance. Such gross overpredictions msutt in the mobilization of larger hammers or thicker pile wall than necessary for pile installation. A simp/e model k proposed to estimate soil resistance to driving. Case histories of recent deepwater pile installations are presented to illustrate the adequacy of the new procedure in deepwater normally consolidated clays. INTRODUCTION As shown on Figure 1, a pile drivability assessment requires two independent analyses. First, a series ofwave equation analyses is performed at different penetrations to estimate the driving resistance that a particular hammer-pile-soil system can overcome. The second step in the process is an estimation of soil resistance to driving that the pile is likely to encounter. These two independent analyses are then combined to predict the pile driving response for the particular hammer-pile-soil combination. The first analysis is very straightforward and is dependent on pile dimensions, hammer characteristics, and the load transfer (damping and quake) properties of the soil. Computation of the soil resistance to driving is analogous to the computation of the ultimate static capacity of piles, except that the static undrained properties of the soil are degraded to account for remolding as a result of pile driving. Available procedures to compute soil resistance to driving in clays were developed based on experience in stiff to hard overconsolidated clays of the North Sea and the Arabian Gulf (TooIan and Fox, 1977; Semple and GemeinhanX, 1981; Stevens, et al., 1982). These procedures grossly overpredict soil resistance in normally consolidated clays. Therefore, in the drivability assessment, the overprediction of soil resistance leads to selection of bigger hammers or thicker pile wall than is actually necessary. Before going into the mechanics of the soil-pileinteraction during driving, a few comments regarding pile driving experience in the normally consolidated clays of the deepwater are in otier. First, the soil resistance during continuous driving is low and increases slowly with depth. Based on hindcast analysis of the pile driving data, the dynamic resistance is typically between 20 and 40 percent of the ultimate static capacity. Another observation, which is even more important, is that the piles drive unplugged, i.e., the soil plug is generally within a few feet of the mudline. Soil resistance to driving is overpredicted when piles are considered to be plugged during driving in normally mnsolidated clay. In this paper, all discussions will be limited to the coringcase, that is, the piles will be considered to be unplugged during driving. EXISTING PROCEDURES In clay, for a coring pile with an internal shoe, Toolan and Fox (1977) computed soil resistance as the summation.

This paper will show on the basis of hindcast wave equation analysis that soil-pile set-up for large diameter driven pipe piles in clay is faster than what the current state-of-practice utilizes for design. Results of hindcast wave equation analyses utilizing observed blowcounts and hammer energy records from two deepwater sites, one in West Africa and the other one in the Gulf of Mexico, are presented to support this conclusion. For offshore structures, this increase in the rate of soil-pile set-up reduces foundation/anchor costs or reduces the waiting time until topsides can be set on the structure or mooring lines can be hooked up to anchors. Hindcast wave equation analyses were performed utilizing the observed blowcount and hammer energy information from two deepwater West Africa and Gulf of Mexico locations where pile driving was stopped and then restarted after a few days. This is a trial and error method of analysis where the soil resistance to driving (SRD) was varied until a good match was obtained between the observed blowcount at the startup of driving and the reported hammer energy using the GRLWEAP wave equation program. The set-up time for the piles ranged from about one to twelve (12) days and the piles ranged from 2134 mm to 2743 mm diameter open-ended pipe piles. The soils generally consisted of normally to slightly overconsolidated highly plastic clays. For the purpose of computing pile set-up, the ultimate pile capacity was computed using the API RP 2A (2000) guidelines. Results show that 60 to 80 percent of the ultimate pile capacity is mobilized in about 7 days, and the extrapolation of the set-up model suggests that the set-up is almost complete in about 60 days.

No abstract

No abstract