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CONTENTS
Volume 5, Number 4, December 2015
 

Abstract
The numerical simulation of wave slamming on a 3D platform deck was investigated using a coupled Level-Set and Volume-of-Fluid (CLSVOF) method for overset grid system incorporated into the Finite-Analytic Navier-Stokes (FANS) method. The predicted slamming impact forces were compared with the corresponding experimental data. The comparisons showed that the CLSVOF method is capable of accurately predicting the slamming impact and capturing the violent free surface flow including wave slamming, wave inundation and wave recession. Moreover, the capability of the present CLSVOF method for overset grid system is a prominent feature to handle the prediction of wave slamming on offshore structure.

Key Words
computational fluid dynamics; CLSVOF; overset grid; slamming; fluid-structure interaction

Address
Yucheng Zhao and Hamn-Ching Chen: Zachry Department of Civil Engineering, Texas A&M University, College Station, TX 77843-3136, USA
Xiaochuan Yu: School of Naval Architecture & Marine Engineering, College of Engineering, The University of New Orleans, New Orleans, LA 70148, USA

Abstract
Internal solitary waves occur due to density stratification and are nonlinear in nature. These waves have been observed in many parts of the world including the South China Sea, Andaman Sea and Sulu Sea. Their effect on floating systems has been an emerging field of interest and recent offshore developments in the South China Sea where several offshore oil and gas discoveries are located have confirmed adverse effects including large platform motions and riser system damage. A valid numerical model conforming to the physics of internal waves is implemented in this paper and the effect on a spar platform is studied. The physics of internal waves is modeled by the Korteweg-de Vries (KdV) equation, which has a general solution involving Jacobian elliptical functions. The effects of vertical density stratification are captured by solving the Taylor Goldstein equation. Fully coupled time domain analyses are conducted to estimate the effect of internal waves on a typical truss spar, which is configured to South China Sea development requirements and environmental conditions. The hull, moorings and risers are considered as an integrated system and the platform global motions are analyzed. The study could be useful for future guidance and development of offshore systems in the South China Sea and other areas where the internal wave phenomenon is prominent.

Key Words
internal waves; KdV equation; coupled analysis; spar platforms; mooring tension; offset

Address
Nishu V. Kurup, Shan Shi, Lei Jiang: Offshore Dynamics, Inc. 16225 Park Ten Place Drive, Suite 500, Houston, TX, 77084, USA
M.H. Kim: Department of Ocean/Civil Engineering, Texas A&M University, College Station, TX, 77843, USA

Abstract
Suction anchors are widely adopted and play an important role in mooring systems. However, how to reliably predict the failure mode and ultimate pullout capacity of the anchor in sand, especially by an easy-to-use theoretical method, is still a great challenge. Existing methods for predicting the inclined pullout capacity of suction anchors in sand are mainly based on experiments or finite element analysis. In the present work, based on a rational mechanical model for suction anchors and the failure mechanism of the anchor in the seabed, an analytical model is developed which can predict the failure mode and ultimate pullout capacity of suction anchors in sand under inclined loading. Detailed parametric analysis is performed to explore the effects of different parameters on the failure mode and ultimate pullout capacity of the anchor. To examine the present model, the results from experiments and finite element analysis are employed to compare with the theoretical predictions, and a general agreement is obtained. An analytical method that can evaluate the optimal position of the attachment point is also proposed in the present study. The present work demonstrates that the failure mode and pullout capacity of suction anchors in sand can be easily and reasonably predicted by the theoretical model, which might be a useful supplement to the experimental and numerical methods in analyzing the behavior of suction anchors.

Key Words
suction anchor; failure mode; pullout capacity; inclined loading; analytical model; sand

Address
Haixiao Liu: State Key Laboratory of Hydraulic Engineering Simulation and Safety, Tianjin University, Tianjin 300072, China;
Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration, Shanghai Jiao Tong University, Shanghai 200240, China
Jinsong Peng and Yanbing Zhao: State Key Laboratory of Hydraulic Engineering Simulation and Safety,
Tianjin University, Tianjin 300072, China



Abstract
The present investigations introduce the shell-finite element discretization for the dynamics of slender marine pipelines. A long catenary pipeline, corresponding to a particular Steel Catenary Riser (SCR), is investigated under long-standing cyclic loading. The long structure is divided into smaller tubular parts which are discretized with 8-node planar shell elements. The transient analysis of each part is carried out by the implicit time integration scheme, within a Finite Elements (FE) solver. The time varying external loads and boundary conditions on each part are the results of a prior solution of an integrated line-dynamics model. The celebrated FE approximation can produce a more detailed stress distribution along the structural surface than the simplistic \"line-dynamics\" approach.

Key Words
offshore applications; SCR; sagbend; nonlinear dynamics; planar shells

Address
Stefanos A. Katifeoglou and Ioannis K. Chatjigeorgiou:School of Naval Architects, Division of Marine Structures, National Technical University of Athens, 9 Heroon Polytechniou Ave, GR157-73, Zografos Campus, Athens, Greece

Abstract
Submerged breakwaters are used to prevent shore line erosion and sediment transportation. One of their advantages is low visual impact. In this paper, the effects of discontinuous submerged breakwaters over water surface elevation was numerically studied considering the extended Boussinesq equations as governing equations using MIKE21 software. The result of discontinuous breakwater was compared with a beach without breakwater. The results showed that the gap dramatically effects on surface elevation from shore line to offshore. It is also evident from results that with approaching the center of the gap, fluctuation of surface elevation is generated. It is because of passing longshore currents towards offshore through the gap which leads to an increase in sediment transportation rate. Nevertheless, transferring water mass from breakwater gap results in powerful rip currents leading to high changes on longshore wave profile.

Key Words
submerged breakwater; Boussinesq equations; wave-breakwater interaction; rip currents

Address
Mohammad J. Ketabdari, Mohammad Barzegar Paiin lamouki and Alireza Moghaddasi: Department of Maritime Engineering, Amirkabir University of Technology, Hafez Avenue, Tehran, Iran


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