The Applicability of the Ras Tanajib Pile Capacity Method to Long Offshore Piles

Abstract

Abstract The applieabfity of the Ras Tanajib pile capacity method to piles driven to penetrations greater than 30 m in the very dense sands of the Safaniya Field of the Arabian Gulf is investigated. Comparisons of observed and predicted blow counts, and measured and computed soil resistances to driving are presented. Blow counts are predicted using procedures recommended by Stevens, Wiltsie, and Turton (1982). The measured soil resistance to driving is the Case- Goble bearing capacity computed using a damping coefficient of 0.20. The computed soil resistance to driving is the lower and upper bound values computed for a plugged pile. CAPWAP analyses are performed at final penetration to confirm pile capacities computed using the Ras Tanajib method. Introduction We platforms were recently installed as part of the GOSP- 4 complex in the Safaniya Field of the Arabian Gulf. Design penetrations were selected from pile capacities computed using the Ras Tanajib method, and ranged from 48 to 69 m for the six-pile Accommodation Platform, 59 to 77 m for the eight-pile Tie-In Platform, and 35 to 84 m for the twelve-pile Production Platform. The Flare Bridge Support and Flare Platforms were lightly loaded structures. Previously, the maximum driven pile penetrations in the Safaniya Field were only 28 and 30 m for 36- and 42-in. (0.91- and 1.07-m) -diameter piles, respectively. Pile refusal has occurred at shallow penetrations due to rock layers and cemented zones. Since 1979, of the 41 piles that have refused, 19 were accepted based on the results of pile monitoring, 10 were accepted with a grout plug, 6 were redriven after drilling a pilot hole, one was redriven after removing the soil plug and 5 drilled and grouted insert piles were installed. The average penetration for the piles driven to refusal is 19 m The Ras Tmusjib Method Piles driven to shallow penetrations in the Safaniya Field had soil resistances to driving that were as much as 10 times greater than the computed static pile capacity. Because discrepancies of this magnitude were observed, a total of 26 pullout tests were performed in 1982 at eight offshore siteson 20- to 30-in. (0.51- to 0.76-m) -diameter open-ended pipe piles driven to penetrations of 11 to 22 m. Tensile capacities were 2.5 to 6 times greater than the static pile capacity computed using the API method. Wiltsie, Stevens, and Vines (1984) provide additional information. Load tests were performed in 1983 on two 18-m long, 24-ii. (0.61- m) -diameter open-ended pipe piles driven in the very dense sands at Ras Tanajib, A new procedure was developed to compute pile capacity in very dense sands. Helfrich, Wiltsie, COL and A1-Shafei (1985) and A1-Shafei, COL and Helfrich (1994) describe these load tests in more detail. The engineering design parameters adopted by Saudi Aramco for designing piles driven into medium-dense to very dense sand are presented on Table 1. The very dense, over consolidated sand and silty sand typically have cone sleeve resistances greater than 400 kpa.