General

Many offshore steel structures are designed as truss frameworks in which tubular members are used as the structural elements, such as in the jacket structure in Figure I.5. For a more detailed background, see, for example, Marshall (1992).

Waves and currents generate relatively small loads on tubular members due to their low drag coefficients. However, the intersections between different members that are connected to the same joint may be rather complex, and this may lead to relatively high local stresses at the hot spot areas with correspondingly short fatigue lives. Thus, in order to design structures that meet the required fatigue life, it is necessary to have adequate knowledge about the stress condition at tubular joints.

Tubular joints may be classified into the following groups:

1. Simple tubular joints

2. Overlapping joints

3. Tubular joints with internal ring stiffeners

4. Heavy stiffened tubular joints

5. Grout reinforced joints

6. Cast steel joints.

A simple tubular joint, as shown in Figure 8.1, is understood to mean a joint other than a circumferential girth weld between tubular members, and that is not stiffened by internal or external stiffeners. Furthermore, the braces are welded into the chord without overlapping each other. Overlapping joints are understood to be overlapping braces at the intersection to the chord. Overlapping joints may be used when there is difficulty in placing the tubular members within a specific area, leading to acceptable eccentricities in the joint. These connections can show rather high capacity with respect to the Ultimate Limit State and the Fatigue Limit State. However, they may be more complex to fabricate than simple tubular joints are, and therefore other solutions are often preferred. For example, one solution would be to increase the chord diameter to allow for a larger space for brace intersections, using internal ring stiffeners to achieve the required chord ring stiffness and capacity. In some joints, such as K-joints, where the axial force in one brace is to be transferred to a second brace, it may also be efficient to use longitudinal stiffeners internally in addition to ring stiffeners to reduce the hot spot stress. These joints may be categorized as heavy stiffened joints. Heavy stiffened joints in jacket structures were more frequently designed during the 1980s than today; see, for example, Callan et al. (1981).