• International Journal of Naval Architecture and Ocean Engineering
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• v.11 no.2
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• pp.923-938
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• 2019

Surge motion of top-end platform induced by periodic wave makes marine flexible riser encounter equivalent sheared-oscillatory flow, under which the Vortex-induced Vibration (VIV) response will be more complicated than pure sheared flow or oscillatory flow cases. Based on a time domain force-decomposition model, the VIV response characteristics under sheared-oscillatory flows are investigated numerically in this paper. Firstly, the adopted numerical model is validated well against laboratory experiments under sheared flow and oscillatory flow. Then, 20 sheared-oscillatory flow cases with different oscillation periods and top maximum current velocities are designed and simulated. Under long and short oscillation period cases, the structural response presents several similar features owing to the instantaneous sheared flow profile at each moment, but it also has some different patterns because of the differently varying flow field. Finally, the effects and essential mechanism of oscillation period and top maximum current velocity on VIV response are discussed systematically.

• International Journal of Naval Architecture and Ocean Engineering
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• v.11 no.1
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• pp.521-529
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• 2019

Interlocked armor layers of unbonded flexible risers may crush when risers are being launched. In order to predict the behavior of interlocked armor layers, they are usually simplified as rings with geometric and contact nonlinearity ignored in the open-literature. However, the equivalent thickness of the interlocked armor layer has not been addressed yet. In the present paper, a geometric coefficient ${\gamma}$ is introduced to the equivalent stiffness method, and a linear relationship between ${\gamma}$ and geometric parameters of interlocked armor layers is validated by analytical and finite element models. Radial stiffness and equivalent thickness of interlocked armor layers are compared with experiments and different equivalent methods, which show that the present method has a higher accuracy. Furthermore, hoop stress distribution of interlocked armor layer under crushing is predicted, which indicates the interlocked armor layer can be divided into two compression and two expansion zones by four symmetrically distributed singular points.

• Journal of Ocean Engineering and Technology
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• v.13 no.3B
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• pp.83-90
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• 1999

본 논문에서는 금후 널리 사용되리라고 예상되는 장대 유연한 해양구조계의 동적 응답에 대한 능동제어를 다루었다. 제어기법으로는 optimal, adaptive 제어와 우주구조물의 진동제어에 사용되는 LAC/HAC 제어를 도입하여 simulation과 모형실험을 수행하였다. 기존의 발표된 최적제어 실험결과는 만족스럽지 못하여 다시 수행할 필요성이 있었다. 그 결과, 해양구조계의 위치 및 탄성변형이 타당한 제어력 범위내에서 제어가 되어 본 연구에서 제안한 제어시스템의 유효성이 입증되었다. 또한 제어기법간의 비교를 통하여 장대 유연한 해양구조계에 대한 최적의 제어기법에 대하여 조사를 하였는데, 초기 시스템 파라미터의 추정에 오차가 있는 경우 적응제어 알고리즘의 장점을 확인할 수가 있었다.

• Journal of Ocean Engineering and Technology
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• v.26 no.6
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• pp.1-6
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• 2012

The purpose of this paper is to present a rational method for analyzing, designing, or evaluating the spread mooring systems used with floating drilling units. This paper presents a validated model to calculate the catenary static configuration. A semi-resolution approach is presented in this paper that is capable of predicting the static performance of a caisson mooring system. The solution is derived as a function of only three parameters, which can be solved numerically by implementing different kinds of boundary conditions. The efficiency and accuracy of the method permit quick parametric studies for the optimal selection of the system particle, which is undoubtedly useful for a preliminary design. A number of numerical examples demonstrate the validity of the adopted approach. The paper contains a complete description of the test cases and reports the results in such a way that it can provide a "benchmark" test for users and programmers of computer codes for flexible riser analysis.