Design of Offshore Structures for Accidental Load Conditions

Abstract This paper summarizes recent draft Guidance Notes [1] developed by the ABS (American Bureau of Shipping) which addresses the process of identifying and assessing the effects of structural loads arising from accidental events. An essential element in the determination of accidental loads in this Guidance Note is the use of risk-based assessment techniques. This is different from the traditional approach to accident-induced structural loads which uses prescriptive criteria primarily derived from experience and studies done for similar situations. For at least the last decade, there has been greater recognition of the use of risk-based procedures to replace or support the prescriptive criteria. With the use of risk-based hazard evaluation criteria, this Guidance Note is meant to provide an overview of the approach that can be used to identify and assess the effects of accidental structural loads arising from four hazards; i.e., dropped objects, vessel collision, fire and blast. The given methodologies should be adapted to accidental structural loads arising from other hazards as specified in the classification criteria of a particular type of offshore installation or mobile offshore drilling unit (MODU). Introduction Accidents are defined as unintended events that arise during the course of installing, operating, or decommissioning an offshore oil and gas facility. The purpose of assessing accidental loadings is to understand the extent of initial damage and verify that the accident does not escalate in terms of personnel health and safety, environmental concerns, or facility damage (i.e., financial consequences). Escalation of the accident occurs when the local failure causes a chain of additional cascading events. For instance, a fire may impinge on a primary structural member resulting in its failure. While this may prove to be a financial consequence to the facility in terms of the member repair, an escalation of additional members failing may raise the event to include a subsequent global collapse with potential loss of life or significant environmental release. Unlike design loadings where elastic behavior is expected, the accidental loading assessments may consider the structure well into the plastic (inelastic) regime. The sheer magnitude of the accident makes requiring an elastic response impractical in most cases given the low likelihood of the event occurring. Naturally, the acceptance criteria for the events will need to be modified to reflect the acceptability of these high strains and large deflections during the event provided the damage is such that the event does not escalate. ABS (American Bureau of Shipping) has developed Guidance Notes [1] outlining the process for evaluation of accidental loadings for the oil and gas industry that utilizes standard risk assessment methods incorporating traditional hazard identification methods and existing assessment methods. First, a preliminary risk assessment examines the potential for accidental loading based on evaluating the risk exposure that includes both the likelihood of the accident and its consequence. A detailed accidental hazard risk assessment is then performed. The events identified during the preliminary risk assessment are categorized so that low level risk and extremely low likelihood events are removed from further consideration based on the owner specified risk tolerance. A detailed assessment is then performed on the remaining accidents with the intention to either conclude sufficient structural capacity or operational constraints exist to reduce the risk (i.e., advanced analysis reduces the level of conservatism present in the initial risk assessment), or highlight activities that can successfully mitigate the risk exposure. This document provides a brief overview of the Guidance Notes, highlighting key activities and concerns that should be considered when assessing the risk exposure for a facility to accidental loading.