Ultimate Strength of Tubular Joints
Authors: Pan R.B. , Plummer F.B. , Kuang J.G. 
Published: December 2007

Tubular-joint ultimate-strength equations that are helpful for offshore platform design are presented. The formulas are based on data from 346 platform design are presented. The formulas are based on data from 346 joint tests. They are easy to use and cover more joint configurations than do present design formulas.

Introduction
The sizing of tubular joints for ultimate strength is an important step in the design of offshore platforms. This process is performed once platform members have been designed to withstand operational and severe environmental loading conditions. Even though nominal member stresses may be at reasonable levels, the complex behavior of tubular intersections can result in highstress amplification that can lead to failure. In many cases, the walls of joint cans must be thickened or member diameters must be increased to provide adequate strength.
The procedure for designing tubular joints must be simple and should lend itself to automation to handle the large number of joints in most platforms. Considerable computer output must be screened to determine maximum applied loads at each joint. Predicted joint strength must be calculated for each member intersection and compared with the applied loads. Joints with deficient capacity are then sized again.
At present, empirical expressions represent the state of the art for predicting the ultimate strength of tubular joints. They are based on axially loaded laboratory joint tests. Very little ultimate-strength data exist for bending or combined axial and bending loads. Consequently, bending and combined loading effects are usually accounted for in an heuristic manner. Analytical methods have not been successful because of the geometric, computational, and analytical complexities involved. The finite-element method probably could be applied but would be too costly, time-consuming, and complicated for practical joint design. To be acceptable, a finite-element model would have to include nonlinear material behavior, a fracture criterion, and, possibly, nonlinear geometric terms to account for local buckling. Thus, for the short term, the empirical approach seems most practical. practical.This paper presents a new set of joint-strength equations that are helpful for platform design. The expressions are based on more data and cover more joint configurations than present procedures. The formulas are presented along with some comparisons of the new presented along with some comparisons of the new predictions with those of current methods. An example predictions with those of current methods. An example problem is also given to illustrate how the equations are problem is also given to illustrate how the equations are used in joint design.

Tubular-Joint Failure Behavior
Part of the problem in analytically predicting Part of the problem in analytically predicting tubular-joint strength is that many competing failure modes are possible in even the simplest of joints. Thus, it is possible in even the simplest of joints. Thus, it is helpful for designers to have some insight into failure mechanisms to produce both safe and efficient designs. In the following sections we review a number of potential failure modes for simple T-, X-, and K-joints to point out trends in joint behavior and important design considerations. For purposes of this discussion, the chord and branch members are as defined in Fig. 1 (Type 1). The chord generally has a large diameter and larger wall thickness than the branches. The branch members are coped and welded to the chord wall to form the joint.
JPT
P. 449