Gas explosion accidents have been recognized as a major hazard of offshore facilities in oil & gas industries. Due to the nature of offshore topside structures, even a single collapse of structural members or equipments may lead to enormous economic and environmental losses. Therefore, such potential hazards that cause the accidental collapse need to be evaluated closely. Gas explosion has been categorized as an important issue of the design of offshore structures regarding the severity of the accident. This paper presents practical considerations for the nonlinear dynamic structural analysis of offshore structures under blast loadings from gas explosion accidents. Numerical investigations including modeling of blast loads and idealization of structural materials and members have been conducted for the overall topside structures. As a design step for offshore structures under blast loadings, an applicable guidance on the finite element analysis (FEA) is described in this study.
Sloshing is the liquid movement in the container excited by the motion. The sloshing flow becomes more violent and results in intensive liquid impact when the excitation period gets closer to the resonance period of internal liquid. This sloshing impact is the most critical load component in the structural design of LNG Cargo Containment Systems (CCSs) of LNG carriers or FLNGs.
As the offshore oilfields shift into the deeper ocean of less infra, FPSOs become a popular solution. Many FPSOs operate in fields where they are expected to stay for decades without drydocking for maintenance. Currently one of the most active area of the FPSO installation is the west Africa offshore where the sea state is benign and the swell from the south is dominant and the water depth is deep. In the case of long operation in the benign site, the fatigue is rather important in the structural design point of view. This paper describes the first extensive practical application of the full stochastic fatigue analysis method to the design of the FPSO-hull interface design like topside supports, flare tower foundation, riser porches, caissons, crane pedestals, mooring foundations, helideck connections, deck penetrations and etc. Especially, topside support is highlighted in detail because of its potential inertia loading due to heavy weight and importance in fabrication stage. The full length of Kizomba ‘A’ FPSO being built in Hyundai Heavy Industries is modeled for the global analysis and the boundary conditions from the global model analysis are transferred for the local model analysis. Some discussions of screening analysis and hot spot stress calculation and roll damping evaluation are also presented.
By the combination of theoretical and empirical approach, the methodology for practical structural assessment of offshore structures for wave slap is proposed. It is developed for engineers in the sense that the precise design pressure is easily obtainable and quickly applicable in early and detail design stage. For impact load prediction, the Pressure-Impulse theory that was well developed and validated in coastal engineering field is applied. The impact pressures are classified into three types (traditional, sharp, and immersed slap) according to model tests and BP Schiehallion FPSO’s bow monitoring. The time histories of impact pressures for the classified impact types are generated with the pressure impulse predicted by the Pressure-Impulse theory. Nonlinear transient structural analyses are performed using the time series of impact pressures to obtain equivalent static pressure factors. Finally, the design pressure is determined by multiplying the maximum peak pressure by the equivalent static pressure factor. The results are validated through the comparison with model tests and dedicated reports.
The Hyundai FLNG has been developed in 2011 and designed to be installed at the North-western sea of Australia with turret mooring system. The Cargo Containment Systems (hereinafter, "CCS") of the FLNG is designed to withstand the sloshing impact loads for any LNG partially-filled condition in the site specific environmental condition. The GTT Mark III membrane type is adopted as a LNG CCS. Since the sloshing load is one of the most important design factors for this membrane type CCS, the longitudinally 2-rows containment systems are applied to the Hyundai FLNG.In this paper, the practical procedure was proposed to assess the structural safety of Hyundai FLNG CCS. Sloshing model tests were conducted to define the characteristics of sloshing impact pressure in forms of peak and rising time. To consider dynamic response of structure such as dynamic amplication factor (DAF), one-way transient FE analysis was performed against sloshing impact pressure. The Utilization Ratio (UR) was proposed as a quantified structural safety index. Each UR of the Hyundai FLNG CCS and existing LNGCs were calculated to compare their safety concerning operation and sailing condition respectively. Conclusively, the structural safety of the FLNG CCS was demonstrated through the comparison of UR between FLNG and existing LNGCs.
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