A Prestressed, High-Strength Grouted Connection for Offshore Construction

Abstract

ABSTRACT This paper concerns newly developed prestressed grounted connections for steel tubu1ars in offshore applications. A mechanical prestressing method has been devised and shown to give an increase in ultimate bond strength of 6 or more times that of a plain bonded connection. The method was developed by small scale testing using models with varying degrees of prestress. The test information was appreciably augmented by the results of a parametric study carried out using a new non-linear finite element program. The information obtained from the small scale tests and numerical study was used to design a series of large scale models. During the testing of these large models an alternative method of prestressing using expansive grout was also shown to produce satisfactory results. This test series confirmed that the results of the small scale tests and computer studies could be safely extrapolated to full scale connections. Based on these studies a new comprehensive equation is proposed for designers which accounts more accurately for pile length/diameter ratio than existing design rules and which for the first time predicts the increase in bond strength in prestressed connections. INTRODUCTION Grouted connections are used in forming the main structural connections between steel jacket structures and their foundation piles, and in repairs to damaged tubu1ars. Both plain and shear keyed connections have been adopted in practice. Plain connections are known to have very unreliable and low bond strengths, since they are very sensitive to shrinkage and imperfections in the tubu1ars. To overcome these problems shear keyed connections, using weld beads, have in recent years become the more common solution. Billington and his co-workers (1,2) have carried out extensive testing of both types of connection and have presented an empirical design formula. An alternative to the use of shear keys for strength enhancement, is the use of a radial prestress in the grout annulus, which will increase the frictional bond strength. The radial prestress may be generated either by employing a mechanical system of active stressing, or a passive chemical system, using expansive grout. In both cases, the tubu1ars are being used in the most efficient manner, since both hoop and axial strength are utilised, and the annulus provides an ideal environment for the triaxial confinement of the grout. A new mechanical system of stressing, based on the high pressure grouting, has been developed at Imperial College, the general details of which are shown in Figure 1. The validity of the prestressing concept was the first established by small scale model testing, the details of which are discussed below. In parallel with the test program, a nonlinear numerical model was developed and calibrated to the available experimental data. It was then used to perform a large number of analyses to quantify the effect of geometric configuration and applied prestress on the ultimate bond strength of grouted connections.