• Ocean Systems Engineering
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• v.4 no.3
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• pp.215-241
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• 2014

Steel catenary riser (SCR) is a popular/economical solution for the oil/gas production in deep and ultra-deep water. The behavioral characteristics of SCR have a high correlation with the motion of floating production facility at its survival and operational environments. When large motions of surface floaters occur, such as FPSO in 100-yr storm case, they can cause unacceptable negative tension on SCR near TDZ (touch down zone) and the corresponding elastic deflection can be large due to local dynamic buckling. The generation, propagation, and decay of the elastic wave are also affected by SCR and seabed soil interaction effects. The temporary local dynamic buckling vanishes with the recovery of tension on SCR with the upheaval motion of surface floater. Unlike larger-scale, an-order-of-magnitude longer period global buckling driven by heat and pressure variations in subsea pipelines, the sub-critical local dynamic buckling of SCR is motion-driven and short cycled, which, however, can lead to permanent structural damage when the resulting stress is greatly amplified beyond the elastic limit. The phenomenon is extensively investigated in this paper by using the vessel-mooring-riser coupled dynamic analysis program. It is found that the moment of large downward heave motion at the farthest-horizontal-offset position is the most dangerous for the local dynamic buckling.

• Journal of Korea Water Resources Association
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• v.32 no.4
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• pp.457-467
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• 1999

No complete methods for estimating soil loss, included by rain storms, from a small watershed are available yet, and the best recommended method is to use measured data from the watershed. When no measured data is available from the watershed, empirical models for estimating the soil loss, such as the Universal Soil Loss Equation(USLE), is well recommended in practice. For using this equation, it is necessary to estimated the rainfall erosivity, commonly expressed as R, of the watershed. In this study, first we collected data of the probable rainfalls with the return periods of 2, 5, 10, 20, 30, 50, 80, 100, 200, and 500-yr and with the duration hours of 0.5, 1, 2, 3, 6, 12, and 24-hr. Using this data, we calculated the design values for R for the return period of 24-hr at each major rainfall-measuring station nationwide. Then we constructed the iso-erodent map of Korea for each return period of the 24-hr design storm. This study shows that the 24-hr duration iso-erodent map of the 5-yr return period is very similar to the annual average iso-erodent map of Korea. This study also shows that the 24-hr duration R-values of a certain return period can be estimated by multiplying certain parameters, obtained from this study, to the 24-hr duration R-values for the 5-yr return period or the annual average R-values. Finally, the R-values of the design storm with the 24-hr return period obtained from this study can be used for designing the settling basins at small watersheds.

• Coupled systems mechanics
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• v.2 no.1
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• pp.111-126
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• 2013

The structural design requirements of an offshore platform subjected to wave induced forces and moments in the jacket can play a major role in the design of the offshore structures. For an economic and reliable design; good estimation of wave loadings are essential. A nonlinear response analysis of a fixed offshore platform under structural and wave loading is presented, the structure is discretized using the finite element method, wave plus current kinematics (velocity and acceleration fields) are generated using 5th order Stokes wave theory, the wave force acting on the member is calculated using Morison's equation. Hydrodynamic loading on horizontal and vertical tubular members and the dynamic response of fixed offshore structure together with the distribution of displacement, axial force and bending moment along the leg are investigated for regular and extreme conditions, where the structure should keep production capability in conditions of the 1-yr return period wave and must be able to survive the 100-yr return period storm conditions. The result of the study shows that the nonlinear response investigation is quite crucial for safe design and operation of offshore platform.

• Ocean Systems Engineering
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• v.6 no.3
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• pp.265-287
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• 2016

The change of the global performance of a turret-moored FPSO (Floating Production Storage Offloading) with DP (Dynamic Positioning) control is simulated, analyzed, and compared for two different internal turret location cases; bow and midship. Both collinear and non-collinear 100-yr GOM (Gulf of Mexico) storm environments and three cases (mooring-only, with DP position control, with DP position+heading control) are considered. The horizontal trajectory, 6DOF (degree of freedom) motions, fairlead mooring and riser tension, and fuel consumptions are compared. The PID (Proportional-Integral-Derivative) controller based on LQR (linear quadratic regulator) theory and the thrust-allocation algorithm which is based on the penalty optimization theory are implemented in the fully-coupled time-domain hull-mooring-riser-DP simulation program. Both in collinear and non-collinear 100-yr WWC (wind-wave-current) environments, the advantage of mid-ship turret is demonstrated by the significant reduction in heave at the turret location due to the minimal coupling with pitch mode, which is beneficial to mooring and riser design. However, in the non-collinear WWC environment, the mid-turret case exhibits unfavorable weathervaning characteristics, which can be reduced by employing DP position and heading controls as demonstrated in the present case studies. The present study also reveals the plausible cause of the failure of mid-turret Gryphon Alpha FPSO in milder environment than its survival condition.

• Journal of Soil and Groundwater Environment
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• v.16 no.6
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• pp.34-45
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• 2011

For the Hancheon drainage area in Jeju island, a groundwater flow model using Visual MODFLOW was developed to simulate artificial recharge through injection wells installed in the Hancheon reservoir. The model was used to analyze changes of the groundwater level and the water budget due to the artificial recharge. The model assumed that $2{\times}10^6m^3$ of storm water would recharge annually through the injection wells during the rainy season. The transient simulation results showed that the water level rose by 39.6 m at the nearest monitoring well and by 0.26 m at the well located 7 km downstream from the injection wells demonstrating a large extent of the affected area by the artificial recharge. It also shown that, at the time when the recharge ended in the 5th year, the water level increased by 81 m at the artificial reservoir and the radius of influence was about 2.1 km downstream toward the coast. The residence time of recharged groundwater was estimated to be no less than 5 years. The model also illustrated that 15 years of artificial recharge could increase the average linear velocity of groundwater up to 1540 m/yr, which showed 100 m/yr higher than before. Increase of groundwater storage due to artificial recharge was calculated to be $2.4{\times}10^6$ and $4.3{\times}10^6m^3$ at the end of the 5th and 10th years of artificial recharge, respectively. The rate of storage increase was gradually diminished afterwards, and storage increase of $5.0{\times}10^6m^3$ was retained after 15 years of artificial recharge. Conclusively, the artificial recharge system could augment $5.0{\times}10^6m^3$ of additional groundwater resources in the Hancheon area.

• Journal of Environmental Science International
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• v.21 no.10
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• pp.1171-1179
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• 2012

Headwaters initiate material export to downstream environments. A nested headwater study examined the flux of dissolved constituents and water from a perennial stream and four ephemeral/intermittent streams in the Upper Gulf Coastal Plain of Mississippi. Water was collected during storm and baseflow conditions. Multiple linear regression was used to model constituent concentration and calculate flux. Event was the major source of water discharged from the ephemeral and intermittent streams however, baseflow was the major source for water discharged by the perennial stream during events. The perennial stream had an area weighted average yields of 10.1, 0.01, 1.03, 0.65 kg/ha/yr of DON (dissolved organic nitrogen), $NO_3^-$-N, $NH_4^+$-N and $PO_4^{-3}$, respectively while large variabilities existed between the ephemeral and intermittent streams. These findings highlight the importance of headwaters in protecting the low order drainage basins as a key to water quality within perennial streams.

• International Journal of Naval Architecture and Ocean Engineering
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• v.1 no.1
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• pp.13-19
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• 2009

The primary objective of this paper is to investigate the dynamic stability and survivability of a four-column classic TLP (tension-leg platform) under less-than-extreme storm conditions where one or more tendons have been lost due to damage or disconnect. The transient responses of the platform and tendon tensions at the moment of disconnection are particularly underscored. The numerical simulation is based on the BE-FE hybrid hull-tendon-riser coupled dynamic analysis in time domain. Compared to the common industry practice of checking the system without a failed tendon in the beginning, the maximum tension on the neighboring tendon can be significantly increased at the moment of disconnection due to the snap-like transient effects, which can lead to unexpected failure of the total system. It is also found that the transient effects can be reduced with the presence of TTRs (top-tensioned risers) with pneumatic tensioners. It is also seen that the TLP cannot survive in the 100-yr hurricane condition after losing one tendon.

• Ocean Systems Engineering
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• v.13 no.3
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• pp.287-312
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• 2023

The global performance of a 15 MW floating offshore wind turbine, a newly designed semisubmersible floating foundation with multiple heave plates by CNOOC, is investigated with two independent turbine-floater-mooring coupled dynamic analysis programs CHARM3D-FAST and OrcaFlex. The semisubmersible platform hosts IEA 15 MW reference wind turbine modulated for VolturnUS-S and hybrid type (chain-wire-chain with clumps) 3×2 mooring lines targeting the water depth of 100 m. The numerical free-decay simulation results are compared with physical experiments with 1:64 scaled model in 3D wave basin, from which appropriate drag coefficients for heave plates were estimated. The tuned numerical simulation tools were then used for the feasibility and global performance analysis of the FOWT considering the 50-yr-storm condition and maximum operational condition. The effect of tower flexibility was investigated by comparing tower-base fore-aft bending moment and nacelle translational accelerations. It is found that the tower-base bending moment and nacelle accelerations can be appreciably increased due to the tower flexibility.