Offshore structures are large platforms that provide the necessary facilities and equipment for exploration and production at sea. Generally, these structures are designed to withstand environmental loads such as waves, currents, wind, earthquakes and daily operational forces. Inspection and maintenance procedures must be performed in an effective way to reduce the risk of fatigue and failure of these structures. This thesis will discuss international and national codes & regulations concerning inspections and maintenance of offshore structures, as well as the different types of underwater inspections and maintenance that must be carried out at sea. In addition, we will develop a strategy to be followed for an effective underwater inspection, allowing a better understanding of the risk levels during the expected service life of the structure. Finally, we will discuss the risks faced by divers, the access limitations and the role of automation in this sector. The aim of this thesis is to study different inspection and maintenance systems in order to finally develop an efficient methodology that will keep offshore structures safely out of dry-dock.