해양공정 개념설계 단계에서 위험성 비용을 고려한 경제성 평가 방법론 연구
- 해양공정 개념설계 단계에서 위험성 비용을 고려한 경제성 평가 방법론 연구
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- Offshore installations have distinctive characters from onshore industrial facilities in that they are unique, safety-critical, independent and subject to varying operating conditions. Each offshore plant is unique in that it is fixed in a site and designed so as to meet the site-specific conditions and the feed fluid characteristics. The environmental conditions and the product specification are different for every installation. Safety-criticality is another intrinsic feature of offshore installations. It stems from the hazards due to treating the flammable hydrocarbon materials in the limited and congested space. A list of common hazards in offshore plants includes fire, explosion, and dropped objects during crane operations. The offshore installations should be able to protect themselves and provide people onboard to escape safely in case of emergency. Being independent implies that the offshore installations should produce such required utility as electric power, cooling medium and instrument air for themselves. The varying condition is another challenge over the life cycle ranging from 20 to 30 years. The well fluid composition and pressure change with production. This means that offshore installations should control a feed with properties which change over the life cycle of them.
These features of offshore installations represent important challenges of economic evaluation of offshore process at their conceptual design stage. The objective of the conceptual design for offshore installations should choose a process that is the best both in performance and safety. One of the selection criteria is the life cycle cost (LCC). This criterion claims that the optimum solution should have the minimum LCC. Conventional LCC methodologies are well developed to address the capital expenditure (CAPEX) and the operation expenditure (OPEX). Considering the intrinsic features of offshore plants aforementioned, a meaningful LCC analysis for the offshore plants should include not only the CAPEX and the OPEX, but also the expenditure of the production loss due to the equipment failure and that of the mishap caused by undesirable events.
This study proposed a new life cycle cost methodology with the risk expenditure taken into account for comparative evaluation of offshore process options at their conceptual design stage. The risk expenditure consisted of the failure risk expenditure and the accident risk expenditure. The former accounted for the production loss and the maintenance expense due to equipment failures. The costs connected to the equipment failure risk are resulting from the production loss and the corrective maintenance. The production availability is the measure of the production loss. It is defined as the ratio of actual production rate to planned production rate over a specified period of time. It is an important indicator of the capability of the target plant since it depicts the gap between the production demand and the actual production rate. The corrective maintenance is defined by the maintenance work which involves the repair or replacement of failed components. The cost for the corrective maintenance is related to the maintenance hours, the repair or replacement cost of the component, and the failure rate of the component. The accident risk expenditure reflected the asset damage and the fatality worth caused by such disastrous accidents as fire and explosion. Fire and explosion are inherent risk to offshore installation handling flammable gas within a congested space. For the accident risk expenditure, the QRA (Quantitative risk assessment) is used to estimate risk levels and to evaluate risk reduction measures. The accident risk is measured by the combination of the frequency and consequence of the accidents.
It was demonstrated that the new life cycle cost methodology was capable of playing the role of a process selection basis in choosing the best of the liquefaction process options including the power generation systems for a floating LNG production facility. There are a number of conceivable options for the LNG liquefaction, among which the most plausible candidates are the mixed refrigerant cycle and the nitrogen expansion cycle. In this study, the two cycles are taken into comparison and their LCC with risk expenditure is evaluated. The mixed refrigerant typically consists of a mixture of methane, ethane, propane and nitrogen which allows for consistency in the temperature differential across the cryogenic heat exchangers. For the nitrogen expansion cycles, the feed gas enters the heat exchanger and is cooled by the nitrogen refrigerant. Expansion through the expanders causes the refrigerant to sufficiently cool down for condensation of the feed gas. Unlike in the mixed refrigerant cycle, the refrigerant in this cycle never experience phases change.
Without the risk expenditure, a simple economic comparison apparently favored the mixed refrigerant cycle which had the better efficiency. The new methodology with the risk expenditure, however, indicated that the nitrogen expansion cycle driven by steam turbines should be the optimum choice, mainly due to its better availability and safety. The case study illustrated that the most important aspects of the choice of the offshore process were related to availability and safety, even though the other processes were favored for the performance evaluation. Consequently, the optimum selection of the offshore process options should take into account the LCC with the risk expenditure.
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