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Palmer A.,National University of Singapore | Wei B.,National University of Singapore | Hien P.L.,National University of Singapore | Thow Y.K.,Keppel Offshore and Marine Technology Center
Proceedings of the International Conference on Port and Ocean Engineering under Arctic Conditions, POAC | Year: 2015

Ice jamming occurs when ice fills the space between the legs of a multi-legged structure. The jammed ice often has sufficient strength and size not easily to clear away from the legs, and the legs and the jammed ice may then act as a single structural unit. There are many analogous phenomena in the flow of fragmented solids through gaps. Relationships between ice jamming, leg dimension and leg-leg spacing are discussed in ISO 19906. The choice of leg diameter and leg spacing is an important design decision that affects the commercial value of the structure. The primary objective of the present study is to develop a deeper understanding of ice jamming events, in order better to formulate design loads on a multi-leg structure.

Tan K.H.,National University of Singapore | Kong D.,Keppel Offshore and Marine Technology Center
American Concrete Institute, ACI Special Publication | Year: 2010

This paper presents a simple direct method to determine the external tendon configuration required for a desired increase in load-carrying capacity of continuous beams. The tendon layout is selected based on the concept of equivalent loads, but need not be concordant. By considering the collapse mechanism of the beam, the increase in load-carrying capacity can be related directly to the tendon force. It is shown that the increase in load-carrying capacity is partly due to an increase in the force in the compression zone arising from the horizontal component of the prestressing force, and partly due to the upward components of the prestressing force. The method was verified with a test program on six two-span continuous beams, in which the tendon profile and loading pattern were varied. Comparison of the test results and those available in the literature showed that the proposed method gives a reasonably conservative design. A simplified method based on the direct balancing of increased loads is also proposed.

Zhang M.Q.,Keppel Offshore and Marine Technology Center | Beer M.,University of Liverpool | Koh C.G.,National University of Singapore
Journal of Engineering Mechanics | Year: 2012

Modeling errors, represented as uncertainty associated with the parameters of a mathematical model, inevitably exist in the process of constructing a theoretical model of real structures and limit the practical application of system identification. They are usually represented either in a deterministic manner or in a probabilistic way. However, if the available information is uncertain but of a nonprobabilistic nature, as it may emerge from a lack of knowledge about the sources and characteristics of model uncertainties, a third type of approach may be useful. Presented in this paperis an approach to treat modeling errors with the aid of intervals, resulting in bounded values for the identified parameters. Compared with the traditional identification procedures where model-based forward dynamic analysis is often involved, computing bounded time history responses from a computational model with interval parameters is avoided. Two required submatrices are firstly extracted from identified state-space models by applying a subspace identification method to the measurements, and then interval analysis is performed upon these two matrices to estimate the bounded uncertainty in the identified parameters. The effectiveness of the proposed methodology is evaluated through numerical simulationof a linear multipleedegree-of-freedom (MDOF) system when modeling errors in the mass and damping parameters are taken into account. The results show the ability of the proposed method to maintain sharp enclosures of the identified stiffness parameters. © 2012 American Society of Civil Engineers.

Wu J.,Shanghai University of Engineering Science | Liu X.,Queensland University of Technology | Xu H.,Keppel Offshore and Marine Technology Center | Du H.,National University of Singapore
Advances in Materials Science and Engineering | Year: 2016

The industry has embraced self-compacting concrete (SCC) to overcome deficiencies related to consolidation, improve productivity, and enhance safety and quality. Due to the large deformation at the flowing process of SCC, an enhanced Lagrangian particle-based method, Smoothed Particles Hydrodynamics (SPH) method, though first developed to study astrophysics problems, with its exceptional advantages in solving problems involving fragmentation, coalescence, and violent free surface deformation, is developed in this study to simulate the flow of SCC as a non-Newtonian fluid to achieve stable results with satisfactory convergence properties. Navier-Stokes equations and incompressible mass conservation equations are solved as basics. Cross rheological model is used to simulate the shear stress and strain relationship of SCC. Mirror particle method is used for wall boundaries. The improved SPH method is tested by a typical 2D slump flow problem and also applied to L-box test. The capability and results obtained from this method are discussed. © 2016 Jun Wu et al.

Yap K.T.,Keppel Offshore and Marine Technology Center | Yap K.T.,National University of Singapore | Palmer A.C.,National University of Singapore
Proceedings of the International Conference on Port and Ocean Engineering under Arctic Conditions, POAC | Year: 2013

Ice-induced vibrations of offshore structures have been a serious issue for engineers since the discovery of the problem as early as in the late 1960's. Despite considerable research efforts and experiences since then, the problem is not completely understood. In order to further investigate this problem, a collaboration had been initiated between National University of Singapore (NUS) and Dalian University of Technology (DUT) to conduct a model test study on ice-induced vibrations, utilizing existing test facilities at DUT. A model jacket structure with variable indentor sizes and saline ice sheets of variable ice thicknesses was used in the current study. The current study successfully replicated various ice-induced vibration modes ranging from low to high indentation velocities. The test parameters are varied in such a way that the characteristics of the various vibration modes can be traced in a consistent manner, allowing a closer look at the transition between distinct vibration modes which up to now have had to be limited to qualitative descriptions of these modes. The results show that the vibration modes leading to, and departing from the lock-in mode can be described by a combination of the ratio of average structure vibration velocity amplitude to ice velocity, and another ratio representing the variability of the structure vibration displacement amplitudes. This provides a basis for characterizing the lock-in vibration mode. Finally, it is shown that dimensionless parameters can be relevant in describing the lock-in phenomenon and in relating its occurrences across small-scale and full-scale structures.

Jiang S.-C.,National University of Singapore | Bai W.,National University of Singapore | Choudhary A.,Keppel Offshore and Marine Technology Center | Hussain A.,Keppel Offshore and Marine Technology Center | Xu W.,Keppel Offshore and Marine Technology Center
Proceedings of the International Offshore and Polar Engineering Conference | Year: 2016

A general four-cylinder semi-submersible platform is considered for optimizing the steady heave motion and mean drift force. The commercial software, HydroStar, is adopted for analyzing the hydrodynamic behavior of the semi-submersible. The optimizations of different types of proposed semi-submersibles are carried out, including the column shape, center column and pontoon shape. According to the optimization mentioned above, the influence of them on heave motion and mean drift force are investigated. © Copyright 2016 by the International Society of Offshore and Polar Engineers (ISOPE).

Bienen B.,University of Western Australia | Gaudin C.,University of Western Australia | Cassidy M.J.,University of Western Australia | Rausch L.,FH Darmstadt | Purwana O.A.,Keppel Offshore and Marine Technology Center
International Journal of Offshore and Polar Engineering | Year: 2012

This paper establishes the undrained capacity of a circular skirted mat under uniaxial horizontal and moment loading, respectively, and presents the combined vertical, horizontal and moment (VHM) capacity envelopes for a novel concept for foundations that combines a skirted mat with a suction caisson. This foundation concept enables self-installation and preloading of the footing. Specifically, this research explores the effect of the central caisson on the failure mechanisms and the resulting VHM capacity through finite element analysis. The results demonstrate that the central caisson more than doubles the horizontal capacity while moderately increasing the capacity in the vertical and moment loading directions. © The International Society of Offshore and Polar Engineers.

Celik S.U.,Fatih University | Bozkurt A.,Fatih University | Hosseini S.S.,Keppel Offshore and Marine Technology Center | Hosseini S.S.,Tarbiat Modares University
Progress in Polymer Science | Year: 2012

Fuel cells are gaining increasing attention as a clean and promising technology for energy conversion. One of the key benefits of fuel cells compared to other methods is the direct energy conversion that enables the achievement of high efficiency. The electrolyte membrane is the most essential parts of a fuel cell unit, and consequently has been the subject of considerable research and development. Among the various types of proton conducting electrolytes examined for fuel cell applications, polymer electrolyte membranes (PEMs) are regarded as viable candidates since they enable operation of the cells at desirably low temperatures. This review describes recent progress in the design and development of high performance proton conducting PEMs, including the analysis of the design requirements and strategies for development of advanced PEMs for operation in anhydrous conditions. Some of the most widely used types of azole heterocycles are introduced and compared, particularly in terms of their performance characteristics in polyacids containing different functional groups. In addition, the latest research studies and progress in the field of azole-containing and azole-functionalized electrolyte systems are discussed and reviewed. © 2011 Elsevier Ltd.

Gaudin C.,University of Western Australia | Mohr H.,University of Western Australia | Cassidy M.J.,University of Western Australia | Bienen B.,University of Western Australia | Purwana O.A.,Keppel Offshore and Marine Technology Center
Proceedings of the International Offshore and Polar Engineering Conference | Year: 2011

To enable self-installation and suction-induced preloading of subsea foundations a novel hybrid foundation is being considered. The concept combines a skirted mat and one or more suction caissons with a skirt length that extends significantly beyond the mat skirt. This results in a significant contribution of the caisson to the overall foundation capacity. Centrifuge experiments have been undertaken to investigate the capacity of the hybrid foundation under combined vertical (V), horizontal (H) and moment (M) loading, relevant to offshore conditions. Tests were performed on hybrid foundations featuring a caisson with varying diameters and skirt lengths. The paper present results of horizontal translation tests, which are interpreted as failure envelopes and compared to existing envelopes for a skirted mat. The contribution of the central caisson to the change in shape and size of the mat failure envelopes are also quantified. Copyright © 2011 by the International Society of Offshore and Polar Engineers (ISOPE).

Perry M.J.,Keppel Offshore and Marine Technology Center | Mobbs C.B.,LeTourneau Technologies
Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE | Year: 2011

The Drag-Inertia method is one of the most popular time domain methods for estimating the contribution of dynamics to the response of a Jackup. The method calculates the Dynamic Amplification Factor (DAF) using the simulated response of the Jackup to a short, usually one hour, random storm. The Drag-Inertia method has been subject to some criticism as it produces results which are, in some cases, less conservative than other methods. Despite this, we feel the benefits of the Drag-Inertia method; efficient and repeatable assessment, are essential to a site assessment. Other methods, such as the Winterstein method, require much longer storm durations, while the variability in DAF results is considerably more. In this study, we propose that factoring the DAFs produced by the Drag-Inertia method is a practical way to improve conservatism, bringing the DAFs in line with the mean DAF results produced by the Winterstein method. The factor is proposed based on simulations of 18 different Jackup configurations and eight different wave conditions. The simple factor is a function of period ratio, Tn/Tp, allowing for efficient and repeatable factoring of DAF values without additional simulation time. When the factor is applied to the Drag-Inertia DAF values the resulting DAFs are within 0.05 of the mean Winterstein DAF in 61% of cases and in only 7% of cases is the Winterstein DAF more than 0.10 above the factored Drag-Inertia result. Copyright © 2011 by ASME.

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