Structures in the oil and gas context refers to the means of support of the pipeline or pipework so that it can function correctly. This can vary from a simple saddle support to the construction of a major offshore process platform.
The simpler constructions are no major problem, provided that a few simple precautions are carried out, particularly in the way in which the pipe is attached to the support.
However the fabrication of large offshore structures can be quite complex. The materials are high strength, micro-alloyed steels, they may be clad with monel, or similar, in the splash zone where the platform legs are subjected to increased attach by the waves. They are frequently very thick and the nodes where the legs and cross-bracings join may be quite complex. The ability to weld these complicated joints without welding defects, whilst maintaining the correct metallurgical structure and reducing the joint distortion to a minimum, requires skill.
The most common standard for construction of major structures is AWS D1.1. Other specifications exist but these tend to be for less onerous duties.