Challenges in Structural Engineering Design and Analysis of Offshore Plants under Probabilistic Vapor Cloud Explosion Loads

Hydrocarbon gas explosion is one of the critical hazards resulting in huge environmental impact as well as loss of valuable assets and lives as observed from the historical disasters in the oil and gas industry. In response to these events, stronger international rules and regulations have been made to ensure safety of these structures. Hence, considerable effort has been devoted to quantify accidental design loads for flammable gas based on probabilistic approaches which requires extensive computational fluid dynamic simulations. Also, demand for 3D nonlinear dynamic finite element structural simulations has increased significantly with rapid progress in computer performance. Some of the major issues and difficulties in structural design and evaluation for probabilistic design loads are discussed in this paper. Uncertainties in explosion hazard analysis which cause large variations in probabilistic explosion responses are reviewed. Some gaps between provisions for design load estimation based on probabilistic approaches versus current structural design and analysis schemes are compared. Finally, it is concluded that there is an urgent need for reliable guidelines for risk-based structural design and simulation for probabilistic explosion design loads. INTRODUCTION Offshore safety and environmental standards have been dedicatedly developed after historical accidents. Provisions for offshore safety has been formed which underlie the safety rules and regulations of the day based on probabilistic approaches since Piper Alpha explosion disaster still holding the record for the largest number of death (167) within the shortest time (22 minutes) [1] in 1988 at North Sea. Whereas such project based probabilistic/risk-based regulatory preparations have been widely adopted among European Countries and UK, US adhered to prescriptive regulations until the Deepwater Horizon oil spill disaster at Gulf of Mexico in 2010 [2]. Although there are significant concerns of instituting the probabilistic regulatory system in US [3], it is inevitable to cope with the rapid changes in the safety culture. Yet, it is critical to pay attention to what is necessary to rectify in the current risk-based offshore safety provisions developed mainly by UK and Norway. Issues in quantifying probabilistic design explosion load and its application to structural design and analysis for offshore facilities are investigated in this paper. OFFSHORE DESIGN PROCEDURE Offshore projects are linked to a series of design phases and can be described by the typical three design phases as follows: 1) Phase I : Conceptual Design 2) Phase II : Front End Engineering Design 3) Phase III: Detailed Design During the conceptual design phase, feasibility studies of the project with the most successful options for the plant type are done. Also, for the conceptual plant design, quantification safety studies on the identified hazardous events are carried out for all critical elements of the plant such as main structures, equipment, piping, etc. and personnel. After assessing the successful plant option in conceptual design phase, all relevant technical requirements of the company standards, classification society guidelines and practices, and international and national standards as well as the authority rules and regulations are thoroughly assessed during the front-end engineering design (FEED) phase. Hazard identification (HAZID) and quantitative risk assessment (QRA) for critical hazards are updated in FEED Proceedings of the ASME 2016 35th International Conference on Ocean, Offshore and Arctic Engineering OMAE2016 June 19-24, 2016, Busan, South Korea