Passive-Fire-Protection Optimization in Offshore Topside Structures
Applying sufficient passive fire protection (PFP) on topside structural-steel members is critical. Simplified and conservative approaches are available to estimate the extent and amount of PFP necessary. The main concern with simplified approaches is that they can lead to overapplication of the PFP, resulting in substantial increase in topside weight. These methods also may result in underestimation of the required amount of PFP, which can compromise structural integrity. This paper presents a risk-based approach for PFP-scheme development.
The structural-integrity assessment and fire-response analysis of offshore platforms in fire focus on providing safe escape routes for personnel for a specified period of time and minimizing the probability of damage and fracture in primary structural steel, hydrocarbon-equipment supports, secondary steel along the primary escape routes, and pressurized hydrocarbon pipes and vessels.
In order to achieve this, PFP is used extensively in offshore topside structures. A PFP material is a good thermal insulator, and application of PFP on steel components of the topside structure results in a delay of the heat transfer to the protected members. Therefore, material degradation and thermal expansion are postponed in protected members. Consequently, structural integrity of the escape routes and safety of hydrocarbon pipes and vessels can be retained for a specified period of time, depending on the rating of the PFP material (properties and thickness). On the other hand, if PFP is used excessively, a considerable amount of cost will be imposed related to PFP material, installation, inspection, and maintenance and a considerable amount of dead load will be added to the structure. Therefore, investigations on the optimal application (e.g., choosing the right places—members, joints, vessels, and pipes—along with the right material in the appropriate thickness) in offshore platforms can result in cost-effective use of PFP while still achieving the purpose of the PFP application.
Existing PFP-Optimization Approaches
Global Fire (Individual-Member Failure). The global fire analysis method for identifying PFP application on a topside structure requires analysis of the whole structure when entirely exposed to the most severe fire loading. The PFP scheme is then developed on the basis of the structure’s response. This method is very conservative and results in an excessive amount of required PFP to resist the fire loading.
Redundancy Analysis. In the redundancy-analysis method, in a repetitive process, a topside structure is analyzed against operational loads with one or several members of the structure being removed in each repetition. In this method, no fire loading is defined on the structure. In each repetition, failed members that result in progressive collapse of the structure are identified. These critical members are then selected for developing a PFP scheme.
The redundancy-analysis method may result in an unconservative amount of required PFP. First, topside structures are designed to be highly redundant to withstand different loading conditions during construction, installation, and operation. Therefore, it is likely that the topside structure can survive the operational loading with some members removed. Second, in an actual fire, by the time a member or several members fail because of fire exposure, the strength of remaining members (yield strength and Young’s modulus) is already degraded. Therefore, the capacity of the topside structure may be overestimated in the redundancy analysis.
Conventional PFP-Optimization Methods Using Ductility-Level Analysis. A ductility-level analysis (DLA) is a nonlinear, progressive collapse time history analysis. It allows redistribution of structural loads from overused members and can indicate failure of the structure after which no further load distribution is possible. Ductility-level fire-response analysis accounts for material and geometric nonlinear behavior of steel.
Risk-Based Approach for PFP Optimization
Nonlinear structural-response analysis is best suited for fire response assessment and PFP optimization because it serves to minimize conservatism and identify failure mechanisms with good precision. PFP optimization based on DLAs and risk assessment (e.g., individuals, asset, environment) may reduce PFP requirements significantly when compared with results obtained in conjunction with traditional linear screening analyses or based only on deterministic DLA.
Unified Risk-Based Approach for Accidental Loadings. The unified risk-based approach is a method to quantify the risk of accidental loads such as fire, blast, ship impact, and dropped objects, in terms of both probability/frequency and consequence. It takes into account the consequence of human loss as well as asset loss. In the frequency analysis, technical-safety and process-safety inputs such as hazard curves, frequency-exceedance curves, and return periods are used to determine how frequently an event is expected to happen. Outputs of the frequency analysis can include a fragility curve, damage-frequency matrix, and individual-risk matrix. In the consequence analysis, severity levels resulting from an event are determined on the basis of the risk associated with that event. On the basis of the frequency and consequence analyses, a risk ranking is identified by use of a risk matrix. If the risk is not acceptable, mitigation, repair, or strengthening options may be considered.
Risk-Based Fire-Response Analysis and PFP Optimization. In a risk-based fire-structural-response analysis and PFP optimization using DLA, the fireresponse analysis starts with an identification of a target zone of interest. For the identified target zone, time-dependent exceedance-frequency curves are developed by a technical safety team. Then, for each event frequency, a time-dependent fire load, which can be regarded as an individual fire case, is developed for a heat-transfer-simulation exercise. Thermal-stress analysis is performed for each fire case, on the basis of heat-transfer-simulation results, according to DLA. On the basis of the consequence-analysis results for each fire case, damage probabilities for each damage level are computed.
Damage probabilities for each case, along with the associated fire-event frequencies, are used to develop a damage-frequency matrix. Occupant-presence probability, occupant vulnerability, and the damage-frequency matrix are then combined to form the individual risk matrix for all damage levels. Using two variables, such as individual risk for different damage levels and different consequence-severity levels, risk is identified on the risk matrix. If the risk is found to be not tolerable, then an optimal PFP scheme is developed in an iterative process to shift the risk to a tolerable region in the risk matrix.
Examples of optimized PFP schemes developed for a topside structure using both conventional and risk-based methods are presented in Fig. 1. In both cases, the structural integrity of the topside structure is maintained against accidental fire. Additionally, the excess amount of the PFP is removed while, at the same time, eliminating or minimizing the associated risks. Members with PFP are highlighted in the figures. It can be seen that using the risk-based approach can result in a significant savings on the required amount of PFP. This will further decrease the weight of the structure, construction costs, and maintenance of the PFP.
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04 June 2019
03 June 2019