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Volume 8, Number 2, June 2018

In case of conventional shallow-draft semisubmersibles, unacceptably large riser stroke was the restricting factor for dry-tree-riser-semisubmersible development. Many attempts to address this issue have focused on using larger draft and size with extra heave-damping plates, which results in a huge cost increase. The objective of this paper is to investigate an alternative solution by improving riser systems through the implementation of a magneto-rheological damper (MR Damper) so that it can be used with moderate-size/draft semisubmersibles. In this regard, MR-damper riser systems and connections are numerically modeled so that they can couple with hull-mooring time-domain simulations. The simulation results show that the moderate-size semisubmersible with MR damper system can be used with conventional dry-tree pneumatic tensioners by effectively reducing stroke-distance even in the most severe (1000-yr) storm environments. Furthermore, the damping level of the MR damper can be controlled to best fit target cases by changing input electric currents. The reduction in stroke allows smaller topside deck spacing, which in turn leads to smaller deck and hull. As the penalty of reducing riser stroke by MR damper, the force on the MR-damper can significantly be increased, which requires applying optimal electric currents.

Key Words
semisubmersible; dry-tree unit; TTR (top-tensioned riser); MR (magneto-rheological) damper; pneumatic tensioner; riser-stroke limit; active control; deepwater

Zaid Zainuddin, Moo-Hyun Kim and Heon-Yong Kang: Department of Ocean Engineering, Texas A&M University, College Station, Texas, USA
Shankar Bhat: Department of Civil, Structures and Offshore, Sabah Shell Petroleum Co. Ltd., Kuala Lumpur, Malaysia

The assessment of the potential for the design of marine renewable energy systems is reviewed and the current situation for marine renewable energy is promising. The most studied forms of marine renewable energy are ocean wind energy, ocean wave energy and tidal energy. Wind turbine generators include mostly horizontal axis type and vertical axis type. But also more exotic ideas such as a kite design. Wave energy devices consist of designs converting wave oscillations in electric power via a power take off equipment. Such equipment can take multiple forms to be more efficient. Nevertheless, the technology alone cannot be the only step towards marine renewable energy. Many other steps must be overcome: policy, environment, manpower as well as consumption habits. After reviewing the current conditions of marine renewable energy development, the authors analyzed the key factors for developing a strong marine renewable energy industry and pointed out the huge potential of marine renewable energy.

Key Words
marine renewable energy; renewable; wind; wave; generator; competitiveness

Maxime Duthoit: LNG & Renewables Product Line, SBM Offshore, 11 Avenue Albert II, 98000, Monaco
Jeffrey Falzarano: Department of Ocean Engineering, Texas A&M University, 727 Ross St, College Station,
Texas 77840, USA

RTI (Rayleigh-Taylor instability) is investigated by a multi-liquid MPS (Moving Particle Semi-implicit) method for both viscous and inviscid flows for various density differences, initial-disturbance amplitudes, viscosities, and surface tensions. The MPS simulation can be continued up to the late stage of high nonlinearity with complicated patterns and its initial developments agree well with the linear theoretical results. According to the relevant linear theory, the difference between inviscid and viscous fluids is the rising velocity at which upward-mushroom-like RTI flow with vortex formation is generated. However, with the developed MPS program, significant differences in both growing patters and developing speeds are observed. Also, more dispersion can be observed in the inviscid case. With larger Atwood (AT) number, stronger RTI flows are developed earlier, as expected, with higher potential-energy differences. With larger initial disturbances, quite different patterns of RTI-development are observed compared to the small-initial-disturbance case. If AT number is small, the surface tension tends to delay and suppress the RTI development when it is sufficiently large. Interestingly, at high AT number, the RTI-suppressions by increased surface tension become less effective.

Key Words
RTI (Rayleigh-Taylor instability); MPS (Moving Particle Semi-implicit) simulation; Atwood number; viscous vs. inviscid; initial disturbance; surface tension; RTI speed/pattern; mushroom-like RTI flows; comparison to linear theory

Kyung Sung Kim: School of Naval Architecture and Ocean Engineering, Tongmyong University,428 Sinseon-Ro, Nam-Gu, Busan, Republic of Korea
Moo Hyun Kim: Department of Ocean Engineering, Texas A&M University,3136 TAMU, College Station, Texas, USA

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