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CONTENTS
Volume 12, Number 1, March 2022
 


Abstract
The majority of tubular joints commonly found in offshore jacket structures are multi-planar. Investigating the effect of loaded out-of-plane braces on the values of the stress concentration factor (SCF) in offshore tubular joints has been the objective of numerous research works. However, due to the diversity of joint types and loading conditions, a number of quite important cases still exist that have not been studied thoroughly. Among them are internally ring-stiffened two-planar TT-joints subjected to axial loading. In the present research, data extracted from the stress analysis of 243 finite element (FE) models, verified against available numerical and experimental data, was used to study the effects of geometrical parameters on the chord-side SCFs in two-planar tubular TT-joints reinforced with internal ring stiffeners subjected to two types of axial loading. Parametric FE study was followed by a set of nonlinear regression analyses to develop six new SCF parametric equations for the fatigue analysis and design of axially-loaded two-planar TT-joints reinforced with internal ring stiffeners.

Key Words
fatigue; internal ring stiffener; offshore jacket structure; stress concentration factor (SCF); two-planar tubular TT-joint

Address
Hamid Ahmadi: Faculty of Civil Engineering, University of Tabriz, Tabriz 5166616471, Iran;
Center of Excellence in Hydroinformatics, Faculty of Civil Engineering, University of Tabriz, Tabriz, Iran
Hossein Imani: Faculty of Civil Engineering, University of Tabriz, Tabriz 5166616471, Iran

Abstract
this paper, a pitch-type wave energy converter (WEC-rotor) is investigated in irregular wave conditions for the real sea testing at the west coast of Jeju Island, South Korea. The present research builds on and extends our previous work on regular waves to irregular waves. The hydrodynamic characteristics of the WEC-rotor are assessed by establishing a quasi-two-dimensional numerical wave tank using computational fluid dynamics by solving the Reynolds-averaged Navier–Stokes equation. The numerical solution is validated with physical experiments, and the comparison shows good agreement. Furthermore, the hydrodynamic performance of the WEC-rotor is explored by investigating the effect of the power take-off (PTO) loading torque by one-way and two-way systems, the wave height, the wave period, operational and high sea wave conditions. Irrespective of the sea wave conditions, the absorbed power is quadratic in nature with the one-way and two-way PTO loading systems. The power absorption increases with the wave height, and the increment is rapid and mild in the two-way and one-way PTO loading torques, respectively. The pitch response amplitude operator increases as the wave period increases until the maximum value and then decreases. For a fixed PTO loading, the power and efficiency are higher in the two-way PTO loading system than in the one-way PTO loading system at different wave periods.

Key Words
absorbed power; efficiency; numerical simulation; one-way and two-way PTO loading; wave energy converter

Address
Sunny Kumar Poguluri and Yoon Hyeok Bae: Department of Ocean System Engineering, Jeju National University, Jeju, South Korea
Dongeun Kim: Multidisciplinary Graduate School program for Wind Energy, Jeju National University, Jeju, South Korea

Abstract
Renewable energy such as wave energy has gained popularity as a means of reducing greenhouse gases. However, the high cost and lack of available sea space in some countries have hindered the deployment of wave energy converters (WEC) as alternative means of sustainable energy production. By combining WECs with infrastructures such as floating breakwaters or piers, the idea of electricity generated from WECs will be more appealing. This paper considers the integration of vertical raft-type WEC (commonly known as the vertical flap WEC) with floating breakwater as means to generate electricity and attenuate wave force in the tropical sea. An array of 25 WECs attached to a floating breakwater is considered where their performance and effect on the wave climate are presented. The effects of varying dimensions of the WEC and mooring system of the floating breakwater have on the energy generation are investigated. The integrated WECs and floating breakwater is subjected to both the regular and irregular waves in the tropical sea to assess the performance of the system. The result shows that the integrated vertical flap-floating breakwater system can generate a substantial amount of wave energy and at the same time attenuate the wave force effectively for the tropical sea when optimal dimensions of the WECs are used.

Key Words
flap WEC; integrate WEC and floating breakwater; raft-type WEC; regular wave; uni-directional irregular waves

Address
Zhi Yung Tay and Luke Lee: Engineering Cluster, Singapore Institute of Technology, 10 Dover Drive, Singapore 138683

Abstract
refined projection-based purely Lagrangian meshfree method is presented towards reliable numerical analysis of fluid flow interactions with saturated/unsaturated porous media of uniform/spatially-varying porosities. The governing equations are reformulated on the basis of two-phase mixture theory with incorporation of volume fraction. These principal equations of mixture are discretized in the context of Incompressible SPH (Smoothed Particle Hydrodynamics) method. Associated with the consideration of governing equations of mixture, a new term arises in the source term of PPE (Poisson Pressure Equation), resulting in modified source term. The linear and nonlinear force terms are included in momentum equation to represent the resistance from porous media. Volume increase of fluid particles are taken into consideration on account of the presence of porous media, and hence multi-resolution ISPH framework is also incorporated. The stability and accuracy of the proposed method are thoroughly examined by reproducing several numerical examples including the interactions between fluid flow and saturated/unsaturated porous media of uniform/spatially-varying porosities. The method shows continuous pressure field, smooth variations of particle volumes and regular distributions of particles at the interface between fluid and porous media.

Key Words
porous media; smoothed particle hydrodynamics; variable porosity; volume fraction

Address
Yuma Shimizu, Abbas Khayyer and Hitoshi Gotoh: Department of Civil and Earth Resources Engineering, Kyoto University,
Katsura Campus, Nishikyo-ku, Kyoto 615-8540, Japan

Abstract
An analysis for the computation of Fatigue Damage Index (FDI) under the effects of the various combination of the ocean loads like random waves, current, platform motion and VIV (Vortex Induced Vibration) for a certain design water depth is a critically important part of the analysis and design of the marine riser platform integrated system. Herein, a 'Computer Simulation Model (CSM)' is developed to combine the advantages of the frequency domain and time domain. A case study considering a steel catenary riser operating in 1000 m water depth has been conducted with semi-submersible. The riser is subjected to extreme environmental conditions and static and dynamic response analyses are performed and the Response Amplitude Operators (RAOs) of the offshore platform are computed with the frequency domain solution. Later the frequency domain results are integrated with time domain analysis system for the dynamic analysis in time domain. After that an extensive post processing is done to compute the FDI of the marine riser. In the present paper importance is given to the nature of the current profile and the VIV. At the end we have reported the detail results of the FDI comparison with VIV and without VIV under the linear current velocity and the FDI comparison with linear and power law current velocity with and without VIV. We have also reported the design recommendations for the marine riser in the regions where the higher fatigue damage is observed and the proposed CSM is implemented in industrially used standard soft solution systems (i.e., OrcaFlex*TM and Ansys AQWA**TM), Ms-Excel***TM, and C++ programming language using its object oriented features.

Key Words
computer simulation model; current velocity; deep water marine riser; fatigue damage; linear law; power law; random waves; vortex induced vibration

Address
Chirag A. Pallan and Rajiv Sharma: Design and Simulation Laboratory, Department of Ocean Engineering, IIT Madras,
Chennai (TN) - 600036, India


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