General

It is important that designers work in close cooperation with fabricators to achieve structures that are suitable for welding. Simple fabrication and the possibility of access for welding and non-destructive testing (NDT) during fabrication should be planned in order to realize structures that fulfill specified requirements. In order to weld braces to chords without an overlap, there should be a gap between the braces not less than 50 mm, and the brace angle should not be below 30° to allow for proper welding at the heel area (see Figure 8.1). Seam welds and girth welds in tubular sections are usually placed outside the main hot spot areas in tubular joints. Recommendations regarding this can be found in design and fabrication standards such as ISO 19902 (2007), API RP 2A (2014), and NORSOK M-101 (2011).

Selection of Material

Material should be selected to meet requirements in material and fabrication standards, in addition to requirements for yield strength with respect to design for the Ultimate Limit State, as presented in Section I.4. Where the Fatigue Limit State governs the design, using material with a very high-yield strength is not recommended, as the fatigue strength of most steels does not particularly depend on the yield strength when it is welded, as explained in Section 4.4. It is important to use materials with documented properties showing good weldability. Where significant stresses from in-service loads or from fabrication are going to be transferred in the thickness direction, it is important to use material with documented through-thickness properties. It is also important to use material with sufficient ductility and fracture toughness for the lowest in-service temperature that can be expected during the lifetime of the structure. Thus, the grade of steel to be used in different structural parts is generally related to service temperature and thickness. Fracture toughness becomes reduced after welding and this should be accounted for when material is selected. For marine structures it is important to consider the actual environment, potential corrosion, and corrosion protection for material selected for a specific project.

Welding

Welding is a process in which notch-like imperfections and deviations in geometry from nominal are difficult to avoid completely.