PRESSURE-IMPULSE DIAGRAM OF THE FLNG TANKS UNDER SLOSHING LOADS

Sloshing impact loads can cause severe structural damage to cargo tanks in liquefied natural gas floating production storage offloading units (LNG-FPSOs or FLNGs). Studies of sloshing can be classified into two types, namely, hydrodynamics-related and structural mechanics-related studies. This study is a sequel to the authors’ previous studies (Paik et al. 2015; Lee et al. 2015), but is more related to issues of structural mechanics. In this study, a new method for probabilistic sloshing assessment, which has been previously developed by the authors, is briefly explained. The nonlinear impact structural response characteristics under sloshing impact loads are examined by a nonlinear finite element ANSYS/LS-DYNA method. An iso-damage curve, representing a pressure-impulse diagram, is derived for the self-supporting prismatic-shape IMO B type LNG cargo containment system of a hypothetical FLNG. The insights developed from this work can be useful for the damage-tolerant design of cargo tanks in FLNGs.

Research on Damage Assessment of Buried Standard and Carbon-Fibre-Reinforced Polymer Petroleum Pipeline Subjected to Shallow Buried Blast Loading in Soil

Buried petroleum pipelines may encounter threats from blast loading due to terrorist attacks, accidental explosions, and artificial blasting during in-progress construction. Carbon-fibre-reinforced polymer (CFRP) is often used for the repair and reinforcement of buried petroleum pipelines. It is meaningful and necessary to distinguish the different responses and establish an effective damage assessment method for standard petroleum pipelines and CFRP-supported petroleum pipelines buried in soil under blast loading. In this study, under fixed end constraints, experimental analysis and numerical simulations were combined to assess the damage of a standard petroleum pipeline and a CFRP petroleum pipeline buried in soil under blast loading. The results showed that, for a scaled distance of 0.19 m/kg1/3, plastic deformation occurred on the surfaces of the two pipelines facing the explosive. The antiexplosion performance of the CFRP pipeline was better than that of the standard pipeline, and the CFRP sheets had a positive effect on the protection of the buried petroleum pipeline during the buried blast loading. Furthermore, based on pressure-impulse damage theory and with consideration of the feasibility under real circumstances, two pressure-impulse damage evaluation curves for standard and CFRP pipelines facing explosive loads were established separately based on a new critical ratio of the dent depth and length. Finally, based on the two pressure-impulse damage evaluation curves and the new critical ratio, two pressure-impulse damage criteria for these two buried petroleum pipelines were defined. Moreover, with the two pressure-impulse damage evaluation curves, mathematical formulae for the two different buried petroleum pipelines were established to generate pressure-impulse diagrams. With the established formulae, the damage to the standard buried pipeline and the CFRP pipeline could be evaluated effectively. Damage to other similar standard pipelines or CFRP pipelines buried in soil with different design parameters due to shallow buried blast loading could also be evaluated using this method.

New Hydraulic High-Pressure Impulse Generator for Long-Duration Impulse Tests

Water hammer wave is widely applied to test hydraulic components in various areas. A new hydraulic high-pressure impulse generator is presented in this paper in order to provide the standard water hammer wave for long-term usage. A combination of a sleeve and a rotary spool was used to build the impulse generator, and a booster piston was applied to amplify the output pressure. Mathematical models were established using commercial software, and a prototype and a test rig were built based on the simulation results. The experimental results for both single wave and repeated periods show the feasibility of the new design and indicate that the new hydraulic high-pressure impulse generator can be used for long-time impulse tests.

Research on Damage Assessment of Concrete-Filled Steel Tubular Column Subjected to Near-Field Blast Loading

Concrete-filled steel tubular (CFST) columns are widely used in engineering structures, and they have many different cross section types. Among these, normal solid sections and concrete-filled double-skin steel tubular sections are often used. Although many studies have been conducted on CFST columns with these two section types, no studies have been conducted on their damage assessment under blast loading. In this study, experimental analysis and a numerical simulation method were integrated to evaluate the responses and assess the damage of two concrete-filled steel tubular (CFST) columns with different cross sections subjected to near-field blast loading. The results showed that for a scaled distance of 0.14 m/kg1/3, plastic bending deformation occurred on the surfaces of the two CFST columns facing the explosive. The antiexplosion performance of the normal solid-section (NSS) CFST column was better than that of the concrete-filled double-skin steel tubular (CFDST) column. The explosion centre was set at the same height as the middle of column, and the distributions of the peak pressure values of the two columns were similar: the peak pressures at the middle points of the columns were the greatest, and the peak pressures at the bottom were higher than those at the top. With the analysis of the duration of the positive pressure, the damage at the middle was the most severe when subjected to blast loading. Using pressure-impulse damage theory and the validated numerical simulations, two pressure-impulse damage evaluation curves for NSS and CFDST columns were established separately by analysing the experimental and simulation data. Finally, based on the two pressure-impulse damage evaluation curves, the two pressure-impulse damage criteria for these two different fixed-end CFST columns were defined based on the deflection of the surfaces facing the explosives. Furthermore, the mathematical formulae for the two different column types were established to generate pressure-impulse diagrams. With the established formulae, the damage of the CFST columns with these two cross section types can be evaluated. Damage to other similar CFST columns with different cross section types due to near-field blast loading can also be evaluated by this method.

Extreme Wave Loads on Monopile Substructures: Precomputed Kinematics Coupled With the Pressure Impulse Slamming Load Model

Monopiles are nowadays the preferred substructure type for bottom-fixed offshore wind turbines at shallow to intermediate water depths. At these locations, the large waves that contribute to extreme loads are strongly nonlinear. Therefore they are not easily reproduced via the simple engineering models who are commonly used in the offshore industry. In the current approach, we develop a design pattern which improves this standard methodology. To retain nonlinearity in the force computations, we have precomputed a number of wave realizations by means of a potential fully-nonlinear code (OceanWave3D), for a wide span of nondimensional water depths and significant wave heights. The designer can then extract a wave kinematics time series from the precomputed set, scale it by the Froude law, and couple it with a suitable force model to compute loads. To complete the picture, slamming loads are calculated by means of the so-called pressure impulse model, recently developed at DTU. Rather than computing the time series of the slamming load, the model uses a few parameters, all except one determinable from the incident wave to calculate the pressure impulse. First comparisons with experimental results, obtained in the framework of the DeRisk project, are promising. The force and the wave elevation statistics from the precomputed simulations are in good agreement with the experiments. Some discrepancies are present, due to an imperfect scaling and to the differences in the physical and numerical domains. The computed loads from the slamming model match the experimental ones quite closely, when the wave celerity is extracted as the ratio between the time gradient and the x-wise space gradient of the surface elevation.

Observed airblast variability and model error from repeatable explosive field trials

Explosive field trials have been conducted to measure the peak incident pressure, impulse and time of positive phase duration following the detonation of 15 different masses of the Plastic Explosive No #4. A novel aspect of these field trials was the repeatability of tests. Eight pressure gauges collected data during each blast, and at each scaled distance. In all, 4 blasts were conducted for each scaled distance (i.e. up to 32 measurements recorded for each scaled distance) – 60 blasts were fired in total. Consequently, this repeatability of testing allowed the mean and variance of blast pressure–time histories to be quantified, with a view to better characterise the variability of a blast itself and model error variability. This article describes the explosive field trials, and the statistical analysis of blast load variability and model error for peak incident pressure, impulse and time of positive phase duration. It was found that the mean model error is close to unity with a coefficient of variation of up to 0.15 for pressure and 0.21 for impulse. The lognormal probability distribution best fits the model error data. The probabilistic models derived from these tests can be used for a variety of structural engineering applications, such as calculating reliability-based design load or partial safety factors for explosive blast loading, and estimating the probability of damage and casualties for infrastructure subject to explosive blast loading. This is illustrated for a terrorist explosive scenario involving a spherical free-air burst, where the damage modes of interest are breaching and spalling of a concrete slab. It was found that the variability of charge mass, range and model error have a significant effect on reliability-based design.