Kwangpil Chang

A study on availability and safety of new propulsion systems for LNG carriers

Abstract This study investigated the availability and safety concerns of the conventional and prospective propulsion systems for LNG carriers: • dual-fuel steam turbine mechanical (DFSM) propulsion, • dual-fuel diesel electric (DFDE) propulsion, • dual-fuel gas turbine electric (DFGE) propulsion, • dual-fuel diesel mechanical (DFDM) propulsion, and • diesel mechanical propulsion with reliquefaction (SFDM+R). The two prospective candidates, the DFDM and DFGE, exhibited the availabilities of design and emergency propulsion loads as high as the newly adopted DFDE and SFDM+R, while the DFSM demonstrated the highest. All the propulsion systems achieved a satisfactory level of the availability for the BOG utilization. The newly introduced dual-fuel systems of DFDE, DFDM, and DFGE accompanied new hazards due to their need for pressurized fuel gas supply. The failure modes caused by these hazards were identified, and feasible safe guards were suggested. The hazards of fire and explosion stemming from flammable gas leak were considered to be acceptably mitigated by the safety requirements from the current industrial standards and classification society.

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.

Safety and Environmental Challenges in Developing a Closed-Circuit LNG Regasification System for Offshore Natural Gas Export

This study examined the safety and environmental issues encountered in the course of development of a closed-circuit LNG regasification system to be installed on offshore LNG terminals or regasification shuttles. Restriction on sea water usage as the heating medium intrinsically determined that the developed system should be based on a closed circuit assisted by steam-generating boilers. Care were taken in establishing the operation and control philosophy, not to generate any residual steam that needed inevitably to be condensed with sea water or to be vented to the atmosphere. One of the most critical hazards was the huge amount of blowdown gas in case of emergency shutdown. It was found that the discharge through a vent mast still left a considerable risk due to the gas dispersion around the facility. Instead, a restricted blowdown to the LNG containment system or a combustion unit resulted in a less risk with a prolonged duration. Rigorous studies were performed to quantify the risks and to anticipate the system behavior. The leak and fire risk analysis indicated that the system had significantly low frequency of fire and explosion compared with usual offshore facilities. However, any guidelines were not available to assess the risk quantitatively. The process dynamic simulation suggested how the system should be shut down. This study showed that several critical challenges were solved successfully and the risk level of the closed-circuit regasification system was acceptable.Copyright

A Study on Reliability-Based Improvement of Reliquefaction System for LNG Carriers

The objectives of the study are to quantify the reliability of a boil-off gas (BOG) reliquefaction system for liquefied natural gas (LNG) carriers and to verify design improvements based on the reliability analysis. The system is broken into subsystems and then further into components. Failure rates are collected from generic references, primarily from the OREDA handbook. The reliabilities of the subsystems are estimated, and a reliability block diagram for the whole system is established. The subsystems are classified into three ranks according to their reliability: the high reliability rank comprises the subsystems ‘BOG preparation system,’ ‘Seawater intake system,’ and ‘Buffer N2 reservoir system’, the medium reliability rank the subsystem ‘BOG liquefaction system’, and the low reliability rank the subsystems ‘BOG compression system,’ ‘N2 cooling system’, and ‘Buffer N2 generation system.’ The reliability and availability are estimated for various process configurations where some of the low reliability category have a standby. The ‘bare’ system without any redundancy fails to attain an availability higher than 0.96. Addition of redundancy to one of the least reliable three commonly results in an increase in reliability improvement, to around 0.97. If all of the three subsystems have standby units, the system reliability improves to 0.99. It is recommended that maintenance efforts should be concentrated on the rotating machines that caused the subsystems to have a low reliability.Copyright

Reliability and Availability Study on Critical Safety Systems of an Offshore Drilling Platform

The main purpose of the study is to ensure whether the safety systems of an offshore drilling platform maintain their integrity and perform their duty under probable accident scenarios. This assurance is completed via reliability and availability analysis of the systems, verifying the design outcomes and recommending design and operational changes. The scope is limited to determining the reliability and availability for continuous systems, and only the reliability, often represented probability of failure on demand (PFD), for stand-by systems. Qualitative analysis is to be carried out to identify each failure mode and the sequence of events associated with it for the systems within the scope. Based on the result of qualitative analysis, failure scenarios are logically constructed to comprise basic events and failure effects. The failure data is taken from generic references such as the offshore reliability data handbook (OREDA). In the final analysis, the values of availability for the all systems are quantified to be above 99%. It is considered that the analysis results are broadly acceptable for general safety requirements.Copyright