Despite substantial preventive efforts, severe accidents continue to occur on engineering structures, resulting in catastrophic effects on personnel, assets and the environment. These accidents are caused by volatile, uncertain, complex and ambiguous (VUCA) environmental and operational conditions. The types of hazards associated with engineering structures, including ship-shaped offshore installations, are (Paik 2020)

Fatigue cracking damage is a primary reason why aging structures require expensive repair work. In this context, fatigue limit states (FLS) are equally relevant among the four types of limit states (described in Section 5.1). FLS describe conditions in which a particular structural member or an entire structure fails to perform its designated function because of the initiation and growth of cracking damage (Paik 2018, 2020). FLS are associated with structural details that are vulnerable to stress concentration and crack damage accumulation under repeated loading. Cracks also form as a result of defects that are generated during the fabrication of a structure, and may remain undetected and increase in size. Under further cyclic loading or monotonic extreme loading, such cracks and defects grow with time, as shown in Figure 6.1. Large cracks may lead to the progressive or catastrophic failure of a structure in association with ultimate limit states (described in Chapter 7), and thus FLS design and engineering, coupled with close-up survey and maintenance strategies, is needed to obtain crack-tolerant structures.

Ship-shaped nuclear power plants may be subjected to aircraft impacts from terrorist attacks or accidents. Even in such a hazard scenario, the catastrophic consequences of casualties, property damage and environmental pollution must be prevented or minimised (Paik 2020).

Ship-shaped offshore installations deteriorate over time. This deterioration leads to significant problems in terms of safety, health and the environment and may require substantial financial expenditure to remedy. Moreover, age-related deterioration has reportedly been a factor in many failures (including total losses) of ships and offshore structures.

Site-specific wave-induced hull girder loads must be calculated to enable the ultimate limit state (ULS) engineering of ship-shaped offshore installations (described in Chapters 7 and 15). Unlike trading ships which are associated with sea states of the twnenty-five-year unrestricted service condition in the North Atlantic Ocean, wave-induced hull girder loads of ship-shaped offshore installations are defined in association with survival conditions of most probable extreme waves for a one-hundred-year return period as far as they always remain on site. However, ship-shaped offshore installations with single-point or turret mooring systems can be disconnected if extreme environmental loads are imminent, sailed to sheltered areas and then returned to restart operation when the weather calms (described in Section 9.4). Also, environmental conditions in some regions may be fully benign accommodating spread mooring systems. In this case, their wave-induced hull girder loads may be defined in association with benign conditions which represent similar environments to those of trading ships but reflecting site-specific metocean data.

Both human bodies and engineering structures must receive regular and proper care through ongoing health monitoring, periodic condition assessments and predictions of likely future health conditions (shown in Figure 15.1) (Paik 2020). Refined and sophisticated technologies are used to minimise errors in human judgement during these processes, although simple and rapidly effective tools remain useful.

The liquid storage tanks of a ship-shaped offshore installation are periodically loaded and unloaded, and the tanks are in motion. Consequently, sloshing occurs in the tanks owing to resonance between the natural sloshing period of a partially filled liquid tank and the roll or pitch period of the offshore installation itself. Notably, a larger and wider tank has a longer natural period, which increases the risk of sloshing impacts that may result in structural damage. Increased non-impact pressures may also be created by sloshing. Thus, engineering approaches are required to mitigate the effects of sloshing.