Testing of a New Grouted Joint for Offshore Platforms

Abstract

Abstract A grouted joint which connects (or splices) two tubes together by an internal grouted pin has been subjected to both finite element analysis and quarter scale model tests. The results are reported in this paper. The joint was originally conceived for a multi-part, or "modular", jacket concept developed by Kvaerner. It was formed around a spliced K-joint joining the modules, and was to be made underwater during offshore assembly of the jacket. The joint is required to perform the same functions as a standard welded joint of a jacket (i.e to withstand static and cyclic loading throughout its life). When such a joint is designed to follow the same rules as those applicable to pile-to-sleeve connections, its static strength should not be influenced by the presence of the splice. However, no similar rules exist to assist in designing for good fatigue performance. The fatigue damage caused in a joint by cyclic loading is, amongst other parameters such as the wave regime, dependent on the Stress Concentration Factors (SCFs) applicable. The determination of SCFs was therefore the primary reason for performing the tests and analyses reported in this paper. Four scale models were tested, three of which simulated possible construction deficiencies such as lack-off it and the presence of grout voids. Although the primary object of the tests was the determination of suitable SCFs, the tests were extended to assess the ultimate loads the joints could withstand. Introduction The most common application of grouted connections in the Offshore industry is in making the subsea bond between piles and pile-sleeves. In recent years grouted connections have also been widely used to carry out repairs to damaged tubulars using split clamps and grouted annuli (Reference 1). This simple and well proven technique has been proposed to establish the subsea connection between the constituent parts of a multi-part, or modular jacket (Reference 2). The jacket concept is described in some detail in the next section. The spliced, grouted joint is a key component of the modular jacket, and has been the subject of several research projects forming part of the development. As the splice is constructed underwater the conditions are not conducive to obtaining tight fabrication tolerances which are achievable in the fabrication yard. Therefore, the design must ensure that the integrity of the joint is not compromised even if the assembly tolerances are outwith the established fabrication norms. Recognising this, a series of finite element analyses were commissioned whose aim was to determine the static behavior of the joint when assembly errors are present. The analyses concluded that the joint should be designed to resist compressive forces by a combination of shear in the grout annulus and direct bearing between the connecting elements through a conical grout sandwich. A general arrangement of such a joint is shown in Figure 1. In the case of fatigue performance of a grouted joint, it is generally recognised that the presence of the grout improves the chord stiffness and hence reduces the applicable SCFs (Reference 3). The grouted joint described in this paper has a discontinuity which means that the findings for standard grouted joints are not necessarily valid.