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CONTENTS
Volume 7, Number 1, March 2017
 

Abstract
This paper presents the numerical simulation results for the dynamic responses of two types of submerged floating tunnels (SFT) under wave and/or seismic excitations. Time domain simulations are conducted by the commercial program OrcaFlex (OF) and in-house CHARM3D program (CP). The dynamic performances of a short/rigid/free-end SFT section with vertical and inclined mooring lines are evaluated. The SFT numerical models were validated against Oh et al.´s (2013) model test results under regular wave conditions. Then the numerical models were further applied to the cases of irregular waves or seismic motions. The main results presented are SFT surge/heave motions and mooring tensions. The general trends and magnitudes obtained by the two different software packages reasonably agree to each other along with experimental results. When seabed seismic motions are applied to the SFT system, the dynamic responses of SFTs are small but dynamic mooring tension can significantly be amplified. In particular, horizontal earthquakes greatly increase the dynamic tension of the inclined mooring system, while vertical earthquakes cause similar effect on vertical mooring system.

Key Words
SFT (Submerged Floating Tunnel); dynamic responses; coupled dynamics; irregular waves; earthquake; line tensions; vertical/inclined mooring

Address
Department of Ocean Engineering, Texas A&M University, 199 Spence St, College Station, Texas, USA

Abstract
In real sea environments, excessive dynamic axial tension variations can be exerted on the top-tensioned risers (TTRs) and lead to structural integrity issues. The traditional riser-tension-variation analysis, however, by using parametric formulation is only conditionally valid under certain strict limits and potentially underestimates the total magnitudes of tension variations. This phenomenon is especially important for the long stroke tensioner in dry-tree semisubmersible with larger global heave motion and longer stroke. In this paper, the hydro-pneumatic tensioner (HPT) is modeled in detailed component-level which includes a set of hydraulic and pneumatic components. The viscous fluid frictional effect in the HPT is considered. The main objectives are (i) to develop a detailed tension variation model of the HPT; (ii) to identify the deviations between the conventional parametric formulation and component-level formulation; (iii) to numerically analyze the tension variation of long stroke tensioner in a dry-tree semisubmersible (DTS). The results demonstrate the necessity of component-level formulation for long stroke tensioner in the development of DTS.

Key Words
top-tension riser; hydro-pneumatic tensioner; tension variations; dry-tree semisubmersible

Address
Hooi-Siang Kang: Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, Johor Bahru, Johor, Malaysia

Moo-Hyun Kim: Department of Ocean Engineering, Texas A&M University, College Station, Texas, USA

Shankar S. Bhat Aramanadka: Sabah Shell Petroleum Co. Ltd., Kuala Lumpur, Malaysia

Abstract
Fishes detect various sensory stimuli, which may be used to direct their behavior. Especially, the visual and water flow detection information are critical for locating prey, predators, and school formation. In this study, we examined the specific role of these two different type of stimulation (vision and vibration) during the obstacle avoidance behavior of carp, Cyprinus carpio. When a visual obstacle was presented, the carp efficiently turned and swam away in the opposite direction. In contrast, vibration stimulation of the left or right side with a vibrator did not induce strong turning behavior. The vibrator only regulated the direction of turning when presented in combination with the visual obstacle. Our results provide first evidence on the innate capacity that dynamically coordinates visual and vibration signals in fish and give insights on the novel modulation method of fish behavior without training.

Key Words
fish behavior; behavior control; carp (Cyprinus carpio); obstacle avoidance; virtual stimulation

Address
Cheol-Hu Kim, Dae-Gun Kim, Phill-Seung Lee: Department of Mechanical Engineering Korea Advanced Institute of Science and Technology 373-1 Guseong-dong, Yuseong-gu, Daejeon 34141, Republic of Korea

Daesoo Kim: Department of Biological Sciences Korea Advanced Institute of Science and Technology 373-1 Guseong-dong, Yuseong-gu, Daejeon 34141, Republic of Korea

Abstract
Ship shaped FPSO (Floating Production, Storage and Offloading) units are the most commonly used floating production units to extract hydrocarbons from reservoirs under the seabed. These structures are usually much larger than general cargo ships and have their natural frequency outside the wave frequency range. This results in the response to first order wave forces acting on the hull to be negligible. However, second order difference frequency forces start to significantly impact the motions of the structure. When the difference frequency between wave components matches the roll natural frequency, the structure experiences a significant roll motion which is also termed as second order roll. This paper describes the theory and numerical implementation behind the calculation of second order forces and motions of any general floating structure subjected to waves. The numerical implementation is validated in zero speed case against the commercial code OrcaFlex. The paper also describes in detail the popular approximations used to simplify the computation of second order forces and provides a discussion on the limitations of each approximation.

Key Words
second order roll; FPSO roll; newman approximation; potential theory; OrcaFlex; KVLCC2; Quadratic Transfer Function (QTF)

Address
Department of Ocean Engineering, Texas A&M University, College Station, TX - 77840, USA


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