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CONTENTS
Volume 10, Number 2, June 2020
 

Abstract
The objective of this study was to evaluate the downtime cost of side-by-side offloading operations in Malaysian waters. With the help of a numerical time domain tool, the structure and cable response of moored FPSO vessel was simulated for heading and beam sea-states under irregular waves. The weather downtime was assessed by comparing the response under operational wave condition with the pre defined industrial safe offloading criteria. Additionally, two cases of cable failure were simulated for each sea-state. The novel study on downtime cost was presented for three different location of Malaysia subcontinent for which the location specific wave scatter diagram facilitated to estimate the probability of occurrence of operational wave condition. It was concluded that an unpredictable increment in wave height by 0.5 m can significantly impact the production cost.

Key Words
floating bodies; hydrodynamics; irregular waves; mooring; downtime; offloading, mooring

Address
M.S. Patel, M.S. Liew, Zahiraniza Mustaffa and
Abdurrasheed Said: Department of Civil and Environmental Engineering, Universiti Teknologi PETRONAS, Perak, Malaysia
Andrew Whyte: Department of Civil and Environmental Engineering, Curtin University, Perth, Australia

Abstract
Considering the huge demand of several types of subsea equipment, as Christmas Trees, PLEMs (Pipeline End Manifolds), PLETs (Pipeline End Terminations) and manifolds for instance, a critical phase is its installation, especially when the equipment goes down through the water, crossing the splash zone. In this phase, the equipment is subject to slamming loads, which can induce impulsive loads in the installation wires and lead to their rupture. Slamming loads assessment formulation can be found in many references, like the Recommended Practice RP-N103 from DNV-GL (2011), a useful guide to evaluate installation loads. Regarding to the slamming loads, RP-N103 adopt some simplifying assumptions, as considering small dimensions for the equipment in relation to wave length, in order to estimate the slamming coefficient CS used in load estimation. In this article, an experimental investigation based on typical subsea structure dimensions was performed to assess the slamming coefficient evaluation, considering a more specific scenario in terms of application, and some reduction of the slamming coefficient is achieved for higher velocities, with positive impact on operability.

Key Words
slamming; subsea engineering; model testing; subsea equipment installation

Address
Allan C de Oliveira and Rafael G Pestana: Petrobras R&D Center, Petrobras, Av Horacio de Macedo, 950, Cidade Universitaria, Brazil

Abstract
Cyclone Heat Potential (CHP) is an essential parameter for accurate prediction of the intensity of tropical cyclones. The variability of the heat storage in the near-surface layers and the vertical stratification near the surface due to large fresh water inputs create challenges in predicting the intraseasonal and interannual evolution of monsoons and tropical cyclones in the Bay of Bengal. This paper for the first time presents the D26- referenced cyclone heat potential observed in the Bay of Bengal during the period 2012-17 based on the in-situ data collected from 5.5 million demanding offshore instrument-hours of operation in the Ocean Moored Buoy Network for Northern Indian Ocean (OMNI) buoy network by the National Institute of Ocean Technology. It is observed that the CHP in the Bay of Bengal varied from 0-220 kJ/cm during various seasons. From the moored buoy observations, a CHP of ~ 90 kJ/cm with the D26 isotherm of minimum 100m is favorable for the intensification of the post-monsoon tropical cyclones. The responses of the D26 thermal structure during major tropical cyclone events in the Bay of Bengal are also presented.

Key Words
Bay of Bengal; Cyclone Heat Potential; Tropical cyclones; OMNI Buoy

Address
G. Vengatesan, R. Venkatesan,
N. Vedachalam and Jossia K. Joseph: National Institute of Ocean Technology, Ministry of Earth Sciences, Chennai, India
P. Shanmugam: Indian Institute of Technology, Chennai, India

Abstract
A systematic numerical comparative study of the performance of semicircular and rectangular submerged breakwaters interacting with solitary waves is the basis of this paper. To accomplish this task, Nwogu\'s extended Boussinesq model equations are employed to simulate the interaction of the wave with breakwaters. The finite difference technique has been used to discretize the spatial terms while a fourth-order predictor-corrector method is employed for time discretization in our numerical model. The proposed computational scheme uses a staggered-grid system where the first-order spatial derivatives have been discretized with fourth-order accuracy. For validation purposes, five test cases are considered and numerical results have been successfully compared with the existing analytical and experimental results. The performances of the rectangular and semicircular breakwaters have been examined in terms of the wave reflection, transmission, and dissipation coefficients (RTD coefficients) denoted by K_R, K_T, K_D. The latter coefficient K_D emerges due to the non-energy conserving K_R and K_T,. Our computational results and graphical illustrations show that the rectangular breakwater has higher reflection coefficients than semicircular breakwater for a fixed crest height, but as the wave height increases, the two reflection coefficients approach each other. On the other hand, the rectangular breakwater has larger dissipation coefficients compared to that of the semicircular breakwater and the difference between them increases as the height of the crest increases. However, the transmission coefficient for the semicircular breakwater is greater than that of the rectangular breakwater and the difference in their transmission coefficients increases with the crest height. Quantitatively, for rectangular breakwaters the reflection coefficients K_R are 5-15% higher while the diffusion coefficients K_D are 3-23% higher than that for the semicircular breakwaters, respectively. The transmission coefficients K_T for rectangular breakwater shows the better performance up to 2.47% than that for the semicircular breakwaters. Based on our computational results, one may conclude that the rectangular breakwater has a better overall performance than the semicircular breakwater. Although the model equations are non-dissipative, the non-energy conserving transmission and reflection coefficients due to wave-breakwater interactions lead to dissipation type contribution.

Key Words
Nwogu\'s extended Boussinesq equations; semicircular breakwater; rectangular breakwater; RTD coefficients; performance; finite difference method

Address
Mohammad Barzegar: Department of Physical & Environmental Sciences, Texas A&M University, Corpus Christi,
6300 Ocean Drive, Corpus Christi, TX 78412
D. Palaniappan: Department of Mathematics & Statistics, Texas A&M University, Corpus Christi,
6300 Ocean Drive, Corpus Christi, TX 78412



Abstract
This paper deals with the problem of the global stabilization for a class of tension leg platform (TLP) nonlinear control systems. Problem and objective: Based on the relaxed method, the chaotic system can be stabilized by regulating appropriately the parameters of dither. Scope and method: If the frequency of dither is high enough, the trajectory of the closed-loop dithered chaotic system and that of its corresponding model the closed-loop fuzzy relaxed system can be made as close as desired. Results and conclusion: The behavior of the closed-loop dithered chaotic system can be rigorously predicted by establishing that of the closed-loop fuzzy relaxed system.

Key Words
intelligent control function; chaotic relaxed form; automated design

Address
C.-Y.J. Chen: Department of Civil Engineering, Universidade de Brasília, Brasília, 70910-900 Distrito Federal, Brazil
Chia-Yen Hsieh: Special Education, National Kaohsiung Normal University, Kaohsiung
Aiden Smith: Faculty of Engineering, Universidad del Norte, Barranquilla, AA 1569 Atlantico, Colombia
Dariush Alako: Department of Natural Science, Bost University, Lashkargah Helmand, Afghanistan Afghanistan
Lallit Pandey: Applied Math Department, St. George\'s University, Grenada
Tim Chen: AI Lab, Faculty of Information Technology, Ton Duc Thang University, Ho Chi Minh City, Vietnam


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