Static Strength of Tubular X-joints with Chord Fully Infilled with High Strength Grout.

Abstract

Circular hollow section (CHS) joints are widely used in offshore steel platforms (e.g. jacket and jack-up structures) due to their attractive structural properties. Cement grouts have been used on these steel jacket platforms in pile-to-sleeve connections, for strengthening or repair. Complete infilled grout of tubular members offers benefits for both the intact and especially the damaged members, without any increase in the environmental loading acting on the members. Infilled grout of a dented tubular member can re-instate its original strength or provide enhanced strength for it. A grouted tubular joint is the one in which the chord member is filled with cement grout materials. It has been recognized that the infilled grout in a chord member offers an efficient and cost-effective method to meet the strengthening or repair requirements for jacket structures. However, there is a lack of the guidance available in codes, guidance documents or the technical literature for this type of joints. Most of the researches in this subject have been conducted the individual joints commissioned by the Oil and Gas Companies with their geometries specific to the offshore platform joints requiring strengthening The objective of the present study is, therefore, to extend the understanding of the static behavior of fully grouted tubular joints and develop an effective joint failure model for the ultimate strength analyses of offshore structures. The whole study comprises two parts: the experimental investigations and the numerical simulations. Two series of experimental investigations have been conducted on the fully grouted X- joints under brace axial loading and in-plane bending respectively. A total number of 15 large scale tubular joints have been tested up to failure. Some unique characteristics of the fully grouted joints under the ultimate loads have been observed and evaluated. The test joint strengths have been compared with the predictions from the existing codes. Two new failure models have been proposed for fully grouted joints based on the main findings from the experimental investigations. The corresponding design equations have also been developed and verified by the available test data. After the experimental investigations, a consistent modeling procedure has been established for the FE analyses of the fully grouted joints. This procedure has been verified by the present and the previous test results. A continuum damage mechanics (CDM) approach have been proposed for the simulation of the crack initiations in the fully grouted joints and the validity of the approach has been verified by the test results. A parametric study has been carried out for the fully grouted X-joints under various loading conditions. The failure models and the design equations proposed have been further verified by the FE results. Besides, a set of formulations for the representation of the characteristic of the fully grouted joint stiffness have also been developed.