(55bc) Investigating the Influence of 3D Model Completeness on Offshore Explosion Studies

The petroleum exploration and production are activities known to present a significant accident risk of various natures, some of which may have major consequences, such as fire and explosion scenarios, being further intensified in the case of offshore maritime operations due to the remote location and high operating pressures involved in the production, separation, and compression processes, for example. Moreover, given the limited physical space, offshore oil and gas producing units, such as FPSOs (Floating, Production, Storage, and Offloading Units) usually have a high congestion and confinement, which can increase the consequences in case of unintended releases followed by ignition (fire and explosion scenarios).

Thus, several risk analyses are carried out during the design phase of the units to seek, among other objective, safer and more efficient operations and to minimize the consequences of potential accidental scenarios through the appropriate sizing of equipment, structures, and safety systems. One of the safety studies developed during the design of an oil rig is the Explosion Risk Analysis (commonly referred to as ERA).

The ERA is carried out to enable the decision-making process with regards to the design of structures, equipment and piping against the probabilistic explosion loads. In other words, this assessment provides input to design an FPSO capable of withstanding the loads of the explosion scenarios envisaged in this analysis and in accordance with the previously defined acceptance criterion. The ERA must be individualized for each unit, since the accidental loads generated by explosion are influenced by their unique characteristics, such as meteorological conditions, physical and chemical properties of the various fluids in the unit, process conditions, level of congestion and confinement of the unit, among other relevant factors.

The levels of congestion and confinement of each unit is related to the quantity, shape and arrangement of the equipment, structures, and pipes in the different modules, which have significant influence in the overpressure generated during an explosion event. Thus, explosions in less congested and/or confined environments usually generate lower overpressures than in highly congested environments.

During the detailed phase of FPSO projects, three-dimensional geometric models (3D model) are created, in which the shape and arrangement of equipment, pipes and structures are detailed, as support for different design disciplines and as input data for safety studies. The above-mentioned elements are defined and detailed throughout the detailing and evolution of the project of an FPSO, which includes its physical construction. Therefore, it is expected that the degree of congestion of the 3D model of a unit in the design phase will increase along the progress and evolution of the project. It is expected that, at the end of the detailed design phase of the project, the 3D model of the FPSO will present the most assertive degree of congestion and like the as-built condition. However, waiting for the availability of the as-built model is not feasible to carry out safety studies (explosion study, for example), because when these are available, the project will have already defined the accidental loading necessary for the design of structures, equipment, and safety functions. To carry out the safety studies in the early stages of the detailed design in a timely manner for the implementation of the recommendations, the incomplete model is used (for example, the 30% complete model) followed by the important assumption of the anticipated congestion.

The use of robust approaches and assumptions related to the anticipated congestion is necessary for the inclusion of the so-called small pieces. The aim is to predict equipment, structures and pipes that have not yet been included in the 3D model and, therefore, represent the expected congestion level in the final geometric model (or as built). This approach must be followed to obtain reliable explosion results, which can be considered representative of the reality of the future installation.

The purpose of this work is to review and evaluate the impact of the 3D geometry model completeness on the probabilistic explosion load calculated during the detailed design phase, considering the different possible strategies to deal with the uncertainties in the 3D model’s degree of congestion throughout the different phases of the project, as well as the limitations relevant to the strategies adopted. It is further written to propose a guidance to the industry and regulators on how to handle the congestion factor during the early stages of the detailed design phase and the expected correlation with the as-built condition.

The work presented herein is developed based on the international technical references, papers from recognized journals, industries’ best practice and DNV’s calculations in analogous projects.