AXIAL COMPRESSION BEARING CAPACITY OF DRIVEN OFFSHORE PILES IN THE PERSIAN GULF – A CASE STUDY

Cone penetration test (CPT) is broadly employed in the design of offshore piles. The reliability, high-quality results and continuous recording of soil resistance in depthare the CPT advantages which result in excellent performance of CPT rather than the other in situ tests. Moreover, the shapes of CPT and pile as well as the failuremechanisms developed during penetration are similar. These significant characteristics have motivated many researchers to propose direct estimation methods of pilebearing capacity using CPT data. Herein, the axial compression bearing capacity of an offshore steel-pipe pile driven in marine clay deposits of the Persian Gulf-SouthPars field is estimated using three property-based static analysis methods including API (2011), FBV (Fugro, 1996) and NGI (2005) as well as ten popular direct CPTbased methods including Aoki & Velloso (1975), Penpile (1975), Shmertmann (1978), de Ruiter & Beringen (European/Dutch, 1979), Tumay & Fakhroo (Cone-m, 1981), Bustamanate & Gianeseli (LCPC/LCP, 1982), Price & Wardle (1982), Eslami & Fellenius (Unicone, 1997), Jardine et al. (ICP, 2005), and Niazi & Mayne (Enhanced Unicone, 2015). The question is why these methods have been selected A majority of offshore piles worldwide has been designed based on API standard as the most common design code for offshore structures. In addition, the selected CPT-based methods in this study can be classified in two groups. The first group contains the more commonly used CPT-based methods which were mainlydeveloped before the year 2000 and the second consists of the more recently developed CPT-based methods which were included in the commentary of the new 22nd Editionof the API RP 2A Recommendation. It is mentioned that the direct CPT methods developed based on older types of mechanical CPT equipment with no pore pressuremeasurement, apply total stress values. The total stress approaches govern the short-term behavior of piles capacity, whereas property-based static analysis methodsapply effective stress values, and govern the long-term behavior of piles capacity. Determining the pile capacity in clay necessitates using CPTu sounding with pore pressuremeasurements. Details of the above CPT-based methods have been given in [2] and hence not reproduced here due to space constraints. For verification purposes, the resultsof the aforementioned methods are compared against the PDA records obtained from dynamic load tests conducted at various pile depths during jacket installation. The paperpresents the predictive performance of the above thirteen methods. The considered pile is a tubular steel pile with around 90 m length, 1.52 m diameter, and 50.88 mm wallthickness. Water depth at the location of jacket structure is nearly 75 m. Soil and CPT data relevant to the location of corresponding offshore structure are shown in Figure 1.The profiles have a general trend of increasing linearly with depth; however, the values fluctuate in some occasional cohesionless granular lenses. In the South Parsfield, the clayey soil is very soft to soft at above 20 m depth, stiff at 20–70 m depth, and very stiff to hard beyond 70 m depth. This layering pattern is dominant; it means thatno considerable variation is seen in the entire field [1]. 2. Results and Discussion The calculated curves of skin friction, end and ultimate compression bearing capacities obtained from different methods are presented in Figure 2. The results have been generated through a series of Spreadsheets which were developed and precisely verified by the authors. It is seenthat the results demonstrate a very wide range of variation in the predicted capacities. The methods yield skin friction, end and ultimate bearing capacities between 13400-72600MN, 4100-16200 MN and 19600-78600 MN, respectively. The calculations confirm that the end bearing contributes very little to the total ultimate bearing capacity. 2. Results and Discussion The calculated curves of skin friction, end and ultimate compression bearing capacities obtained from different methods are presented in Figure 2. The results have beengenerated through a series of Spreadsheets which were developed and precisely verified by the authors. It is seen that the results demonstrate a very wide range of variationin the predicted capacities. The methods yield skin friction, end and ultimate bearing capacities between 13400-72600 MN, 4100-16200 MN and 19600-78600 MN, respectively.The calculations confirm that the end bearing contributes very little to the total ultimate bearing capacity. 3. References [1] Ebrahimian, B., Movahed, V., and Nazari, A., Soilcharacterisation of South Pars field, Persian Gulf . Environmental Geotechnics, 1(2), 2014, pp. 96-107. [2] Niazi, F. S., and Mayne, P. W., Cone penetration test baseddirect methods for evaluating static axial capacity of single piles . Geotechnical and Geological Engineering, 31(4), 2013, pp. 979- 1009.