• Structural Engineering and Mechanics
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• v.57 no.3
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• pp.403-423
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• 2016

The challenges involved with fatigue damage assessment of steel catenary riser (SCR) in the touchdown zone (TDZ) are primarily due to the non-linear behaviour of the SCR-seabed interaction, considerable uncertainty in SCR-seabed interaction modelling and geotechnical parameters. The issue of fatigue damage induced by the cyclic movements of the SCR with the seabed has acquired prominence with the touch down point (TDP) interaction in the TDZ. Therefore, the SCR-seabed response is critical for reliable estimation of fatigue life in the TDZ. Various design approaches pertaining to the lateral pipe-soil resistance model are discussed. These techniques have been applied in the finite element model that can be used to analyse the lateral SCR-seabed interaction under hydrodynamic loading. This study investigates the sensitivity of fatigue performance to geotechnical parameters through a parametric study. In this study, global analyses are performed to assess the influence of vertical linear seabed springs, the lateral seabed model and the non-linear seabed model, including trench evolution into seabed, seabed normalised stiffness, re-penetration offset parameter and soil suction resistance ratio, on the fatigue life of SCRs in the TDZ.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.26 no.5
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• pp.300-313
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• 2014

Most analytical solutions for wave-induced soil response have been mainly developed to investigate the influence of the progressive and standing waves on the seabed response in an infinite seabed. This paper presents a new analytical solution to the governing equations considering the wave-induced soil response for the partial standing wave fields with arbitrary reflectivity in a porous seabed of finite thickness, using the effective stress based on Biot's theory (Biot, 1941) and elastic foundation coupled with linear wave theory. The newly developed solution for wave-seabed interaction in seabed of finite depth has wide applicability as an analytical solutions because it can be easily extended to the previous analytical solutions by varying water depth and reflection ratio. For more realistic wave field, the partial standing waves caused by the breakwaters with arbitrary reflectivity are considered. The analytical solutions was verified by comparing with the previous results for a seabed of infinite thickness under the two-dimensional progressive and standing wave fields derived by Yamamoto et al.(1978) and Tsai & Lee(1994). Based on the analytical solutions derived in this study, the influence of water depth and wave period on the characteristics of the seabed response for the progressive, standing and partial standing wave fields in a seabed of finite thickness were carefully examined. The analytical solution shows that the soil response (including pore pressure, shear stress, horizontal and vertical effective stresses) for a seabed of finite thickness is quite different in an infinite seabed. In particular, this study also found that the wave-induced seabed response under the partial wave conditions was reduced compared with the standing wave fields, and depends on the reflection coefficient.

• Ocean Systems Engineering
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• v.4 no.4
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• pp.279-291
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• 2014

Smart flanges are used for pipeline and riser repair in subsea. In a typical case in the gas export pipeline project, the end cap bolts of a 4inch smart flange were broken during operation, and in turn leakage occurred. This work presents the detail of three dimensional finite element analysis of the smart flange to support the observed end cap bolts failure. From finite element analysis it turns out that in the presence of external bending moment, an uneven contact distribution is present between seal and end cap, which in turn changes the uniform load distribution on bolts and threaten the integrity of bolts. On the other hand, 3D finite element analysis of interaction between pipeline and seabed is presented by means of Abaqus to explore the distribution of bending moment along the pipeline route. It is found that lateral buckling occurs in the pipeline which introduces large bending moment.

• Ocean Systems Engineering
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• v.5 no.1
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• pp.31-40
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• 2015

Three dimensional (3D) non-linear finite element analysis of offshore pipeline is investigated in this work with the help of general purpose software Abaqus. The general algorithm for the finite element approach is introduced. The 3D seabed mesh, limited to a corridor along the pipeline, is extracted from survey data via Fledermous software. Moreover soil bearing capacity and coefficient of frictions, obtained from the field survey report, and are introduced into the finite element model through the interaction module. For a case of study, a 32inch pipeline with API 5L X65 material grade subjected to high pressure and high temperature loading is investigated in more details.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.26 no.1
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• pp.49-64
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• 2014

Seabed beneath and near the coastal structures may undergo large excess pore water pressure composed of oscillatory and residual components in the case of long durations of high wave loading. This excess pore water pressure may reduce effective stress and, consequently, the seabed may liquefy. If the liquefaction occurs in the seabed, the structure may sink, overturn, and eventually fail. Especially, the seabed liquefaction behavior beneath a gravity-based structure under wave loading should be evaluated and considered for design purpose. In this study, to evaluate the liquefaction potential on the seabed, numerical analysis was conducted using 2-dimensional numerical wave tank. The 2-dimensional numerical wave tank was expanded to account for irregular wave fields, and to calculate the dynamic wave pressure and water particle velocity acting on the seabed and the surface boundary of the structure. The simulation results of the wave pressure and the shear stress induced by water particle velocity were used as inputs to a FLIP(Finite element analysis LIquefaction Program). Then, the FLIP evaluated the time and spatial variations in excess pore water pressure, effective stress and liquefaction potential in the seabed. Additionally, the deformation of the seabed and the displacement of the structure as a function of time were quantitatively evaluated. From the analysis, when the shear stress was considered, the liquefaction at the seabed in front of the structure was identified. Since the liquefied seabed particles have no resistance force, scour can possibly occur on the seabed. Therefore, the strength decrease of the seabed at the front of the structure due to high wave loading for the longer period of time such as a storm can increase the structural motion and consequently influence the stability of the structure.

• Journal of Ocean Engineering and Technology
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• v.31 no.3
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• pp.219-226
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• 2017

In this study, the impact characteristics of subsea pipelines that were installed in various soil types and burial depths were evaluated by a numerical method. An impact scenario replicated a dropped ship anchor that fell vertically and impacted an installed subsea pipeline. In order to calculate the impact force through terminal velocity, FLUENT, a computational fluid dynamic program and MDM (Moving Deforming Mesh) technique were applied. Next, a dynamic finite element program, ANSYS Explicit Dynamics, was used for impact analysis between the anchor and pipeline (or, subsea if they were buried). Three soil types were considered: loose sand, dense sand and soft clay by applying the Mohr-coulomb model to the seabed. The buried depth was assumed to be 0 m, 1 m and 2 m. In conclusion, a subsea pipeline was the most stable when buried in dense sand at a depth of 2 m to prevent impact damage.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.26 no.6
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• pp.367-380
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• 2014

Tsunami take away life, wash houses away and bring devastation to social infrastructures such as breakwaters, bridges and ports. The targeted coastal structure object in this study can be damaged mainly by the tsunami force together with foundation ground failure due to scouring and liquefaction. The increase of excess pore water pressure composed of oscillatory and residual components may reduce effective stress and, consequently, the seabed may liquefy. If liquefaction occurs in the seabed, the structure may sink, overturn, and eventually increase the failure potential. In this study, the solitary wave was generated using 2D-NIT(Two-Dimensional Numerical Irregular wave Tank) model, and the dynamic wave pressure acting on the seabed and the estimated surface boundary of the vertical revetment. Simulation results were used as an input data in a finite element computer program(FLIP) for elasto-plastic seabed response. The time and spatial variations in excess pore water pressure, effective stress, seabed deformation, structure displacement and liquefaction potential in the seabed were estimated. From the results of the analysis, the stability of the vertical revetment was evaluated.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.27 no.1
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• pp.63-77
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• 2015

Tsunami take away life, wash houses away and bring devastation to social infrastructures such as breakwaters, bridges and ports. The coastal structure targeted object in this study can be damaged mainly by the wave pressure together with foundation ground failure due to scouring and liquefaction. The increase of excess pore water pressure composed of oscillatory and residual components may reduce effective stress and, consequently, the seabed may liquefy. If liquefaction occurs in the seabed, the structure may sink, overturn, and eventually increase the failure potential. In this study, the bore was generated using the water level difference, its propagation and interaction with a vertical revetment analyzed by applying 2D-NIT(Two-Dimensional Numerical Irregular wave Tank) model, and the dynamic wave pressure acting on the seabed and the surface boundary of the vertical revetment estimated by this model. Simulation results were used as input data in a finite element computer program(FLIP) for elasto-plastic seabed response. The time and spatial variations in excess pore water pressure ratio, effective stress path, seabed deformation, structure displacement and liquefaction potential in the seabed were estimated. From the results of the analysis, the stability of the vertical revetment was evaluated.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.27 no.2
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• pp.118-134
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• 2015

An analytical solution of dynamic responses for seabed in shallow, finite and infinite thicknesses has been developed under flow and standing wave coexisting field at a constant water depth condition. To do this, based on the Biot's consolidation theory, the seabed is assumed as a porous elastic media with the assumptions that pore fluid is compressible and Darcy law governs the flow. The developed analytical solution is compared with the previous results and is verified. Using the analytical solution the deformation, pore pressure, effective and shear stresses of seabed are examined under various given values of flow velocity, incident wave period and seabed thickness. From this study, it is confirmed that the seabed response is quite different depending on consideration of flow, which causes changing period and length of incident and reflection waves.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2009.04a
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• pp.416-422
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• 2009

Offshore wind turbine are subjected to more various loads than general land structures and the stability of structures is supported by the piles driven deeply in the subsoil. So it is more important for offshore structures to consider seabed soil-structure interaction than land structures. And the response of a fixed offshore structure supported by pile foundations is affected by resist dynamics lateral loading due to wave forces and ocean environmental loads. In this study, offshore wind tower response are calculated in the time domain using a finite element package(ANSYS 11.0). Several parameters affecting the vibration characteristics of the natural frequency and mode shape and the tower response have been investigated.