To widen the understanding of the risks in relation to intact stability 36 intact stability incidents at sea have been analysed. These incidents does not represent a complete list of incidents and therefore not intended to be used for calculating probabilities or frequencies. The list is used to highlight the different types of conditions and different stability failure modes that lead to an intact stability incident, the often severe consequences that follow with an intact stability incident, and the large variations in the operational conditions.
The aim is to discuss qualitative aspects of intact stability risk. Most of the incidents described are serious accidents, i.e., leading to one or more fatality, damage to the vessel that interrupt the service or vessel lost. The 36 incidents add up to more than 408 fatalities. The incidents described in Table 2 can all most often be contributed to a combination of causes and for many of the accidents the cause is uncertain.
Many of the incidents (approximately 20 out of 36) are cases were the operational condition and ship state was not according to design. For example, vessels that are over loaded and/or operated in heavy weather with hatches open potentially in combination with forces from fishing gear. Cargo shift is also common. These conditions lead to a poor recoverability after large heel angles.
For cargo vessels the cargo and ship status is generally changed under controlled circumstances (often at port). There is a potential for a high level of internal and external control. Therefore, a high level of detail in the data on the ship status is possible. On the other hand, vessels such as fishing vessels are an example of an operation where the ship status is changed at sea dynamically without external control which lead to large uncertainties.
This difference in potential control over the ship’s loading condition produce different conditions for safety work, different reliability of the passive safety designed into the craft, and different reliability as well as different need for operational safety measures. However, knowledge on safe operations, based on knowledge about the vessel’s limitations and weaknesses (edge awareness) could increase the reliability of the crew decisions taken on-board in relation to intact stability especially for ships and vessels that relatively often operate beyond the operational conditions defined during the design. Therefore, operational safety measures can be an effective approach to reach acceptable levels of safety, especially for operations with large uncertainties.
It is here argued that the conditions for operational measures differs between ship types as a result of different types of operations and different conditions for implementing the measures on-board. Therefore, it is here proposed that there is an important distinction between a ship’s general likelihood for intact stability incidents such as large roll motions (vulnerability to intact stability failures) and if the ship at a specific situation will not, when it experience an intact stability incident, return to a safe mode (recoverability after intact stability failures). Vulnerability is then typically a result of ship design whereas recoverability can be a result of ship design as well as operational aspects such as decisions taken on-board in relation to loading or unclosed hatches.
The safety introduced by design measures can deteriorate by lower control of ship condition (large uncertainties) and the resulting operations outside the design conditions.
The second-generation intact stability rules mainly investigate the vulnerability to intact stability failure for ships operating within the operational conditions. However, the ships recoverability to intact stability failure as well as other life saving measures need to be included if the safety effects of high vulnerability to intact stability failure is to be assessed. It is still not identified that high vulnerability alone is enough to introduce a safety problem according to IMOs definitions of.
Ships with high recoverability and high vulnerability includes for example modern PCTC with high possible control and specialized hull forms (that lead to vulnerability to specific intact stability failure modes) and superstructures that can contribute to high recoverability after large heel angles. For such ships high-end on-board simulations can be an effective way of supporting the master’s decisions about routing as well as manoeuvres to avoid intact stability incidents. However, as mentioned above, such on-board operational guidance is not necessarily needed to meet IMO’s safety level ambitions according to the FSA and should if that is the case not be mandatory. The operational safety measures are motivated by the aim to increase effectiveness and quality of service, i.e. with the aim to reduce injuries to personnel and damages to cargo during the incident. Suitable operational measures for these ships need to be ship specific and supported by support tools, i.e., operational guidance. Therefore, the exchange of stability knowledge between the design phase and the development of stability management support systems should be facilitated by the IMO rules.
For ships with high control and standard configuration standard operational safety measures is enough.
For ships with low recoverability and moderate to high vulnerability the uncertainty in relation to the effectiveness of engineering solutions is high (because the conditions defined during design cannot be assumed to be valid). The effective approach is most likely found in making sure that risk drivers, such as open hatches and overloading, are reduced, especially in situations when the ship is more vulnerable to intact stability incidents. In such situations decisions support, such as operational guidance, can be ineffective as a result of the limited possibility to take in the information presented by such support. Identifying and tending to risk drivers is a work that has to be performed by the whole crew by strengthening risk knowledge and risk awareness on-board thru safety management. Operational safety measures are a precondition for safe operations for this type of ships. Specific knowledge and risk management could be the primary choice for safety assurance (compare with the UK Safety Case approach for the offshore industry and the risk based approach for the Norwegian offshore industry).
A wider understanding of the terms for operational measures is needed, especially in relation to a ship’s recoverability after intact stability incidents. They cannot be judged in the same way as passive engineering solutions for safety. Such a view takes away the strength of safety solutions in the ship operation. However, the acceptable level of uncertainty varies between types of ships and especially with the ship’s recoverability after stability incidents.
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