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CONTENTS
Volume 3, Number 2, June 2013
 

Abstract
Suction anchors are widely adopted in mooring systems. However there are still challenges in predicting the failure mode and ultimate pullout capacity of the anchor. Previously published methods for predicting the inclined pullout capacity of suction anchors are mainly based on experimental data or the FEM analysis. In the present work, an analytical method that is capable of predicting the failure mode and ultimate pullout capacity of the suction anchor in clay under inclined loading is developed. This method is based on a rational mechanical model for suction anchors and the knowledge of the mechanism that the anchor fails in seabed soils. In order to examine the analytical model, the failure angle and pullout capacity of suction anchors from FEM simulation, numerical solution and laboratory tests in uniform and linear cohesive soils are employed to compare with the theoretical predictions and the agreement is satisfactory. An analytical method that can evaluate the optimal position of the attachment point is also proposed in the present study. The present work proves that the failure mode and pullout capacity of suction anchors can be reasonably determined by the developed analytical method.

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

Address
Haixiao Liu, Chen Wang and Yanbing Zhao : School of Civil Engineering, Tianjin University, Tianjin 300072, China

Abstract
For the optimization design and strength evaluation of the umbilical cable, the calculation of cross section stress is of great importance and very time consuming. To calculate the cross section stress under combined tension and bending loads, a new integrated analytical model of umbilical cable is presented in this paper. Based on the Hook\'s law, the axial strain of helical components serves as the tensile stress. Considering the effects of friction between helical components, the bending stress is divided into elastic bending stress and friction stress. For the former, the elastic bending stress, the curvature of helical components is deduced; and for the latter, the shear stress before and after the slipping of helical components is determined. This new analytical model is validated by the experimental results of an umbilical cable. Further, this model is applied to estimate the extreme strength and fatigue life of the umbilical cable used in South China Sea.

Key Words
un-bonded umbilical; stress analysis model; axial stiffness; bending stiffness

Address
Xiqia Chen, Shixiao Fu, Leijian Song, Qian Zhong and Xiaoping Huang : State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Min Hang, Shanghai, China

Abstract
This study is devoted to the optimal design of compressed bars under axial compressive forces and exposed to a corrosive environment. The initial volume of the structure is taken as an optimality parameter. Gutman – Zainullin\'s exponential stress corrosion model is adopted for analysis. Analytical and numerical results are derived for optimal variation of the cross-sectional area of the bar along its axis.

Key Words
corrosion; optimization; stability

Address
Mark M. Fridman : 1National Metallurgical Academy of Ukraine (NMetAofU), 7/29 Yuzhniy Avenue, UA-50026,
Krivoy Rog, Ukraine
Isaac Elishakoff: 2Department of Ocean and Mechanical Engineering, Florida Atlantic University, Boca Raton,
FL33431-0991, USA

Abstract
Moonpools are vertical wells in a floating body used onboard many types of vessels like cable-laying vessels and offshore support vessels. Moonpool gives passage to underwater activities for different types of ships as per their mission requirements. It is observed that inside a moonpool considerable relative motions may occur, depending on shape, depth of the moonpool and on the frequency range of the waves to which the ship is exposed. The vessel responses are entirely different in zero and non-zero Froude number. Former situation is paid attention in this study as the mission requirement of the platform is to be in the particular location for long period of operation. It is well known that there are two modes of responses depending on the shape of the moonpool viz., piston mode for square shape and sloshing mode for rectangular shapes with different aspect ratios of opening like 1:1.5 and 1:2 ratios. Circular shaped moonpool is also tested for measuring the responses. The vessel moored using heavy lines are modeled and tested in the wave basin. The moored lines are provided with pre-tension and the dynamic tensions on the lines are measured. The different modes of oscillations of water column are measured using wave gauge and the vessel response at a particular situation is determined. RAOs determined for various situations provide better insight to the designer. The experiments done in the wave basin may also be compared with a software package meant for handling moored floating bodies.

Key Words
moonpool; vessel response; piston mode; sloshing mode; pretension; dynamic tension

Address
Sharanabasappa C. Sajjan and S. Surendran : Indian Institute of Technology, Madras, Chennai 600 036, India

Abstract
Tension leg platforms (TLP\'s) are highly nonlinear due to large structural displacements and fluid motion-structure interaction. Therefore, the nonlinear dynamic response of TLP\'s under hydrodynamic wave loading is necessary to determine their deformations and dynamic characteristics. In this paper, a numerical study using modified Morison Equation was carried out in the time domain to investigate the influence of nonlinearities due to hydrodynamic forces and the coupling effect between all degrees of freedom on the dynamic behavior of a TLP. The stiffness of the TLP was derived from a combination of hydrostatic restoring forces and restoring forces due to cables and the nonlinear equations of motion were solved utilizing Newmark\'s beta integration scheme. The effect of wave characteristics was considered.

Key Words
compliant structures; tension leg platforms; hydrodynamic wave forces

Address
A.M. Abou-Rayan : Department of Civil Engineering, Northern Border University, Saudi Arabia to KSA
Amr R. El-gamal: Department of Civil Engineering Technology, Benha University, Egypt


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