Accidental limit states (ALS) are one of four types of limit states (described in Section 5.1), and they represent a condition in which a particular structural member or an entire structure fails to perform its designated function as a result of excessive structural damage, resulting from accidents such as unintended flooding, collisions, fires or explosions (Paik 2018, 2020). A range of adverse events may ensue if ALS are reached on a ship-shaped offshore installation, including severe injuries or loss of life among the crew and severe damage to and/or loss of property, with consequent substantial financial losses and environmental pollution.

Classification society rules or recommended practices dictate the types of steel to be used in the assembly of the hull structures of ship-shaped offshore installations. The steel must exhibit high levels of buckling and fatigue performance and be amenable to corrosion management. It is recommended that the proportion of reduced thickness, high strength steel be minimised and the proportion of ordinary steel (e.g., grade A) be maximised in a hull structure. However, structural members that require an ordinary steel plate with a thickness greater than 30 mm may instead be made from high strength steel to avoid the need for heavy welding and to simplify the construction. The greater resistance of high strength steel to corrosion reduces the need for repairs in dry dock, which is critical for the long on site life required in a ship-shaped offshore installation. Three grades of steel are used in the assembly of hulls for installations that will be in service at sub-zero temperatures: grade D steel for use at −20°C, grade E steel for use at −40°C and grade F steel for use at −60°C. Steel may be exposed to colder temperatures than it has been designed to withstand or even to cryogenic conditions as a result of the accidental release of liquefied gas (e.g., liquefied natural gas or hydrogen). This can cause a brittle fracture in structural steel and a subsequent catastrophic failure.

Criteria that are relevant to the safety engineering of ship-shaped offshore installations are influenced by ocean environmental conditions that affect the transit, operation, survival and decommissioning of these installations. The actions of ocean environmental conditions on ship-shaped offshore installations are different from their actions on trading ships. Particularly, the nature and operation of the former mean that the structures are substantially affected by the action of waves, winds and currents, whereas the latter are primarily affected by waves only. Thus, accurate and efficient modelling of ocean environmental conditions at the proposed sites of ship-shaped offshore installations is essential for safety engineering and long operational uptimes.

The performance of a structure and its components is described using limit state functions that separate desired from undesired states. The physical effects of exceeding a limit state may be reversible or irreversible. If the effects are reversible, the removal of the cause of the exceedance allows a structure to return to the desired state. If the effects are irreversible, a return to the desired state is not possible, and certain consequences, such as damage, may ensue depending on the nature of the limit state. These consequences may themselves be reversible or irreversible. For example, if consequential damage is limited, such as an undesired and localised permanent set, it may be repairable (e.g., by replacing the affected parts). Limit states are examined against different target safety levels, where the target to be attained for any particular type of limit state is a function of the consequences of and ease of recovery from that state.

Ultimate limit states (ULS) are one of four types of limit states (described in Section 5.1), and represent a condition in which a particular structural member or an entire structure fails to perform its designated function as a result of progressive collapse due to a loss of structural stiffness and strength caused by buckling, plasticity and fracture (Paik 2018, 2020). If ULS are reached on a ship-shaped offshore installation, catastrophic failures may occur, leading to human casualties, structural collapse and environmental damage.

A mooring system, thrusters, a dynamic positioning system or a combination of these elements is critical for the station-keeping of a floating structure in various environmental conditions involving wind, waves and current. Thus, position keeping and motion control are both required to enable the functional and operational requirements of ship-shaped offshore installations to be met. The analysis of the fluid dynamic behaviours of offshore installation structures above sea level, which are subjected to wind, involves a different set of complications from the analogous analysis of submerged parts, which are subjected to waves and current.

Various types of engineering structures have been developed over the course of human civilisation. One type is the ship-shaped offshore installation, which is a floating structural system located at sea. As a result of their multiple functionalities, these installations are widely used in the production, processing and storage of energy derived from marine sources and electrical power generation in a marine environment.

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.