Daejun Chang

Risk-based determination of design pressure of LNG fuel storage tanks based on dynamic process simulation combined with Monte Carlo method

This study proposes a new methodology that combines dynamic process simulation (DPS) and Monte Carlo simulation (MCS) to determine the design pressure of fuel storage tanks on LNG-fueled ships. Because the pressure of such tanks varies with time, DPS is employed to predict the pressure profile. Though equipment failure and subsequent repair affect transient pressure development, it is difficult to implement these features directly in the process simulation due to the randomness of the failure. To predict the pressure behavior realistically, MCS is combined with DPS. In MCS, discrete events are generated to create a lifetime scenario for a system. The combination of MCS with long-term DPS reveals the frequency of the exceedance pressure. The exceedance curve of the pressure provides risk-based information for determining the design pressure based on risk acceptance criteria, which may vary with different points of view.

An economic evaluation of operating expenditures for LNG fuel gas supply systems onboard ocean-going ships considering availability

This paper proposes a methodology for economic evaluations of liquefied natural gas fuel gas supply (LNG–FGS) systems. To evaluate the economics of various LNG–FGS systems, expected operating expenditures during long-term voyages were compared, including the loss from venting boil-off gas (BOG) to prevent overpressurisation of the fuel tank, penalties for venting BOG, expenses for consuming marine diesel oil when LNG–FGS systems are unavailable, and expenses for electricity consumption. A secondary objective was to compare the economics of various LNG–FGS systems for large-sized ships. An analysis demonstrated that the comparative operational expenditures vary considerably within various design options. Three LNG–FGS systems were compared. As a case study, an Aframax tanker (a highly popular crude carrier) was chosen, and the actual voyage scenario between Ulsan and Antwerp was used for calculating expenses. The results indicated that group FP-P (fully pressurised-type storage tank with cryogenic pump) provides a profit of approximately 7 MUS

Fuzzy-based HAZOP study for process industry.

per 25 yrs compared to group UP-P (unpressurised-type storage tank with cryogenic pump); conversely, group FP-P incurs a loss of approximately 5.4 MUS

Application of Monte Carlo Simulation for the Estimation of Production Availability in Offshore Installations

per 25 yrs compared to group FP-N (fully pressurised-type storage tank with vapour-generating unit). Nevertheless, group FP-N is valuable for small-sized tanks because the vapour-generating unit requires high energy consumption during an extended initial time to raise pressure. In conclusion, group FP-P is the most economical system if it can compensate for approximately 5.4 MUS

Absorption refrigeration system utilising engine exhaust gas for bulk gas carriers

in capital expenditure and maintenance costs. The proposed methodology reflects the availability of necessary equipment and the results of process dynamic simulation in an actual voyage scenario.

Design of a prismatic pressure vessel with internal X-beam structures for application in ships

This study proposed a fuzzy-based HAZOP for analyzing process hazards. Fuzzy theory was used to express uncertain states. This theory was found to be a useful approach to overcome the inherent uncertainty in HAZOP analyses. Fuzzy logic sharply contrasted with classical logic and provided diverse risk values according to its membership degree. Appropriate process parameters and guidewords were selected to describe the frequency and consequence of an accident. Fuzzy modeling calculated risks based on the relationship between the variables of an accident. The modeling was based on the mean expected value, trapezoidal fuzzy number, IF-THEN rules, and the center of gravity method. A cryogenic LNG (liquefied natural gas) testing facility was the objective process for the fuzzy-based and conventional HAZOPs. The most significant index is the frequency to determine risks. The comparison results showed that the fuzzy-based HAZOP provides better sophisticated risks than the conventional HAZOP. The fuzzy risk matrix presents the significance of risks, negligible risks, and necessity of risk reduction.

Safety systems design of VOC recovery process based on HAZOP and LOPA

The purpose of this chapter is to show the practical application of the Monte Carlo simulation method in the evaluation of the production availability of offshore facilities, accounting for realistic aspects of system behavior. A Monte Carlo simulation model is developed for a case study to demonstrate the effect of maintenance strategies on the production availability, e.g., by comparing the system performance under different preventive maintenance tasks.

Effect of Ortho-Para Conversion on Economics of Liquid Hydrogen Tanker with Pressure Cargo Tanks

This study demonstrated the feasibility of an absorption refrigeration system (ARS) that is capable of utilising the engine exhaust gas of bulk gas carriers as the heat source and generating cooling source either for reliquefaction of the boil-off gas (BOG) or for heating, ventilating and air conditioning. The ARS employed an NH3/H2O mixture as the working fluid. The process was designed and simulated using the properties of the exhaust gas from the commercially available engines. The coefficient of performance increased with the temperature of the generated cooling source. A decrease in the engine load resulted in a decrease in the exhaust flow rate and, consequently, in a decrease in the cooling capacity. Although the exhaust temperature reached its minimum around the normal continuous rating, the dependence of the cooling capacity on the engine load was not significantly affected. A case study for a liquefied natural gas carrier verified that the refrigeration system afforded a sufficient cooling capacity to satisfy both the required work and the temperature level needed for reliquefaction of the BOG.

Design of Pile-Guide Mooring System for Offshore LNG Bunkering Terminal : A Case Study for Singapore Port

ABSTRACT This research proposed a prismatic pressure vessel for gas-fuel storage on a gas-fuelled ship. The prismatic pressure vessel consisted of flat-plated shells, stiffeners, and beam structures. According to the design principle of pressure vessels, the beam structures were designed to receive tensile stresses generated from the internal pressure. International regulations and a finite element analysis were considered for the design, strength calculations, fatigue assessments, plastic collapse, and thermal effects. A case study in which the prismatic pressure vessel was applied as a gas-fuel tank to provide ship propulsion on a crude-oil tanker was performed. Nine per cent nickel steel was used to be applicable for cryogenic service. Several design loadings were considered to evaluate the design feasibility of the pressure vessel. The computational analysis results indicated that the prismatic pressure vessel satisfied the design regulations and should be applicable for use as a gas-fuel storage tank for ships.

Analysis and optimization of cascade Rankine cycle for liquefied natural gas cold energy recovery

Crude tankers emit volatile organic compounds (VOCs) that are highly flammable and hazardous to human health and the environment. In light of these hazards, an international law has been enforced to regulate obligatory implementation of VOC management plans on board crude oil tankers. For the sake of safety, a risk assessment is performed to obtain information needed to make decisions about the safe design of a VOC recovery process. A hazard and operability analysis (HAZOP) is conducted to identify plausible hazardous scenarios. A layer of protection analysis (LOPA) is subsequently conducted to overcome the qualitative nature of HAZOP. A risk ranking method is used to select HAZOP results for use as inputs to LOPA for a more quantitative assessment. The initiating causes and existing protection layers are identified. The frequencies of the initiating causes and the probability of failure on demand (PFD) of each independent protection layers (IPL) are estimated quantitatively. The current mitigation is estimated by multiplying the initiating cause likelihoods by the PFDs for the applicable IPLs and adjustment values. The estimated current mitigation is then compared to a risk acceptance criterion to make recommendations for design improvements to further reduce risks to an acceptable level.