Beom Seon Jang

Study on the Probabilistic Scenario Based Fire Risk Analysis of FPSO Topside Module

Many offshore platforms are normally exposed to the flammable oil & gas circumstances; hence hydrocarbon fire is a big threat to the offshore platform. Fire accidents on the offshore platform usually occupy a major part of the total risks. Nevertheless, existing methods of fire risk analysis used in the industry are still not intact enough to advisably evaluate the risks related to fire accident especially for supporting structures of process modules. For this reason a new procedure of fire risk analysis is established in this paper to overcome those deficiencies in the existing methods. The new method is constructed through a scenario based probabilistic approach.In the new FRA (Fire Risk Analysis) procedure, the key parameters used for constructing fire scenarios are cautiously considered and discussed in case of causing a conservative judgment. Also, dynamic effect of transient fire accident is properly considered by applying an effective method called ‘snapshot’ during the CFD fire simulation. Furthermore, for investigating the structural failure under the fire, thermal elasto-plastic structural analysis is included in the new FRA.In the former FRAs, the traditional exceedance curve with certain physical variable is used in the frequency analysis to combine the results of fire simulation and fire frequency calculation. The main purpose of using exceedance curve is for deciding a DAL (Design Accidental Load), which is used for structure design against fire accident or even determining of the proper risk mitigation measures e.g. PFP (Passive Fire Protection). However, different from former FRAs, structure response analysis is carried out in the new procedure and the results are taken into consideration through a newly proposed concept i.e. structural cumulative failure frequency. The purpose of proposing cumulative failure frequency in the new FRA can be summarized as following two aspects. One is to avoid the difficulties of applying the identified DAL to the structure analysis in the former FRAs, and other one is to determine the optimized PFP application area for topside structure. For showing the details of new FRA procedure a demo is also demonstrated in this paper.Copyright

Study on Slamming Pressure on the Bottom of Upper Hull of Semi-Submersible Type Floater

Normally design bottom slamming pressure of semi-submersible type floater is determined in a simple way using a relative speed obtained from an air-gap analysis. However, there are few studies on a robust background on the estimation of the design pressure. In this study, to investigate the slamming pressure on the bottom of semi-submersible rig, a simplified deformable stiffened plate with 0° dead rise angle is simulated by using nonlinear FEM software, LS-DYNA which can take the influence of Fluid-Structure Interaction (FSI) into account. To simulate the slamming condition of semi-rig in a similar way, water hitting the structure with fixed boundary conditions is proposed. Various parametric studies are carried out to examine the effect of structural flexibility, coupling stiffness, mesh size, velocity, stiffener size, and air cushion effect. To further investigate the effect of horizontal velocity, the model is expanded and simulated with various horizontal water velocities. Pressure response on the plate by the coupling of fluid and structure is studied and the hydroelastic effect of each parameter is discussed.Copyright