Hydrostatic Effects on the Capacity of DT Tubular Joints
Authors: Connally L.M. , Zettlemoyer N. 
Published: October 2008

An investigation into the influence of hydrostatic pressure on the ultimate capacity of tubular joints is presented. Finite element studies of cross (DT) joints subjected to axial compression and in-plane bending are used to assess the effect that varying amounts of external pressure have on joint capacity. The matrix of geometries evaluated include variations in branch to chord diameter ratio (?), chord radius to thickness ratio (?), and branch to chord thickness ratio (?). Variations in yield strength were also considered, but the same value was always assumed for both the branch and chord.

Findings can be divided into two categories. First, longitudinal loads resulting from capped end pressures, as deduced from consideration of isolated members or a frame analysis, may be treated merely as additional axial loads. For deeper water, the magnitude of the branch axial loads from hydrostatic pressure can be substantial and, thus can increase the joint strength utilization ratio. The effect of capped end forces on the chord can be estimated from the normal chord stress effect term, Qf, of the joint capacity equations. However, the impact is expected to be trivial in most instances.

The second category of results concerns the effect of hydrostatic radial pressure alone on joint capacity. The analyses show that the capacity can be reduced by as much as 25%, depending on joint parameters and the type of longitudinal load in the branch. Parameter ? is the most important one in relating nondimensionalized branch load and nondimensionalized hydrostatic pressure.

Based on a limited number of T-joint cases studied, it appears that the DT-joint interaction equation adjustments, suggested in this study, are a conservative representation T-joint behavior. Also, it seems likely that capacities of other joint configurations will be affected by hydrostatic pressure and should be investigated.

INTRODUCTION

The offshore oil industry is exploring in progressively deeper water in order to locate economic oil reserves. The deeper waters confront the designers of offshore structures with challenging problems. The member and joint dimensions for shallow water structures are designed according to the expected loading and component capacities, which are provided in various design equations. For deep water structures, an important additional source of loading can be hydrostatic pressure. Overall member behavior in the presence of hydrostatic pressure has received some attention latelyl. In fact, the draft version of API RP2A-LRFD2 reflects these recent studies. However, the effect of hydrostatic pressure on joint capacity has received little or no attention to date.

The intent of this analysis has been to provide some design guidance by determining correction factors that can be used to adjust existing joint interaction equations to account for the effect of hydrostatic pressure on joint capacity. The following are interaction equation provided by Hoadley3 and the API2 for tubular joints.

Mathematical equation (Available in full paper)

The out-of-plane bending terms are not included in Eq. 1 or 2. The results presented below will permit the designer to replace the PU and MU terms in the above equations by new terms that include the effect of hydrostatic pressure.