Evaluating the Carrying Capacity of Floating Driven Tubular Piles for Hydraulic Engineering

Abstract

In past decades, the world-wide tendency in marine hydraulic engineering has been erection, in the continental shelf, of offshore structures for recovery of gas and oil. Of the total of offshore platforms erected over the world, about 80% are provided with pile anchorage [1]. For erecting the pile foundation of platforms, steel tubular piles — hollow thin-walled tubes of diameter larger than 0.6 m (most commonly, 1.2 – 2.0 m) and hold length of piles down to 60 m or more — are used. The carrying capacity of tubular piles is mainly provided by the forces of friction operative over the lateral surface; by conventional terminology, these piles are called floating (or friction) piles. In designing a pile foundation, the following parameters should be specified: pile diameter, pile hold length (the depth to which a pile is driven into the ground), pile wall thickness, type of termination of the end of pile, spacing between the piles, number of piles and pile pattern, connection between structural elements, strength of materials, mode of erection, etc. To determine the adequate characteristics of a pile foundation, a step-by-step feasibility analysis is conducted with allowance for the joint operation of the “structure – foundation” system. Feasibility analysis of the reliable “platform – pile foundation” connection during erection and service should involve the following characteristics: (i) carrying capacity of the foundation ground; (ii) strength of the material of piles and other structural foundation components; (iii) settling of piles and pile foundations; deformation of poles (displacement, angle of twist) jointly with the foundation ground subjected to design loads; (iv) softening of the loose ground subjected to dynamic loading. By the existing regulations, the strength analysis of pile foundations should be conducted in conformance with SNiP (Standard Norms and Rules) 2.02.03–85 “Pile foundations” [2]. Still, SNiP regulations are mainly intended for designing pile foundations in civil engineering and industrial construction and do not reflect in full measure the specific features of tubular piles that are used for erecting structures on the continental shelf [3 – 5]. As a rule, the main factor that determines the potential use of an individual pile of particular type in constructing a pile foundation is the pile’s carrying capacity. Here we omit details of the carrying capacity analysis of axially loaded tubular piles; we merely wish to address some problems that arise in carrying out strength analysis using the method of [2]. According to this method, the carrying capacity of an axially loaded pile is estimated as the sum of resistance forces operative over the outer lateral surface of the tubular pile and at its lower end (that is, over the net cross-section of the pile — without the ground core taken into consideration). This estimate is valid for the conditions of pile penetration using a leading borehole; however, it is a conservative estimate of the carrying capacity for a pile penetrated without leading hole. The resistance data in [2] for laterally and terminally (at the lower end) loaded piles were obtained from processing a large amount of experimental material — mainly for spud reinforced concrete piles of relatively short length commonly used in the construction of civic and industrial buildings [3]. The carrying capacity analysis of long steel tubular piles, typically used in the construction of hydraulic facilities on the continental shelf, in most cases meets with problems in conforming with standard requirements of [2]. Thus, the SNiP rated values for lateral and terminal resistances are available for hold lengths of 35 m only, whereas the actual length of tubular piles is usually greater than this. It should be noted that Provisional Building Regulations VSN 41.88 [6] and VSN 51.3–85 [7] allow the use of SNiP methods for hold lengths greater than 35 m. It is recommended that rated values for the limit length of 35 m can also be used for greater hold lengths. In the world practice of designing foundations using tubular piles, normative documents of the American Petroleum Institute (API) [8] and Norvegian norms Det Norske Veritas (DNV) [9] have gained wide acceptance. The API and DNV recommendations on evaluating the carrying capacity of ground-driven piles are fundamentally different from the SNiP requirements. The former are based on the use of mechanical characteristics of the ground (internal friction angle o, undrained strength cu), whereas design resistance by the SNiP method is specified depending on physical characteristics of the ground (particle size and compactness for sandy grounds, and liquidity index IL and porosity index for clays). A comparison of the carrying capacity of a single pile under different ground conditions as estimated by SNiP and API techniques has revealed significant discrepancies in both