K. Murali

Abstract: Extensive use of autonomous underwater vehicles (AUVs) in oceanographic applications necessitates investigation into the hydrodynamic forces acting over an AUV hull form operating under deeply submerged condition. This paper presents a towing tank-based experimental study on forces and moment on AUV hull form in the vertical plane. The AUV hull form considered in the present program is a 1:2 model of the standard hull form Afterbody1. The present measurements were carried out at typical speeds of autonomous underwater vehicles (0.4–1.4 m/s) by varying pitch angles (0–15°). The hydrodynamic forces and moment are measured by an internally mounted multi-component strain gauge type balance. The measurements were used to study variation of axial, normal, drag, lift and pitching moment coefficients with Reynolds number ( Re ) and angle of attack. The measurements have also been used to validate results obtained from a CFD code that uses Reynolds Average Navier–Stokes equations (ANSYS™ Fluent). The axial and normal force coefficients are increased by 18% and 195%; drag, lift and pitching moment coefficients are increased by 90%, 182% and 297% on AUV hull form at α =15° and Re v =3.65×10 5 . These results can give better idea for the efficient design of guidance and control systems for AUV.

Abstract: Conventional floating breakwaters are examined and the feasibility of developing a cage floating breakwater is explored. Earlier studies on floating breakwaters reveal that, to achieve a transmission coefficient (\iK\i\dt) less than 0.5, the breakwater width to wave length ratio (\IW/L\N) should necessarily be greater than 0.4 for most of the configurations. Recent studies on cost-effective floating breakwaters indicate that by fixing a row of pipes below the floating body, the \IW/L\N requirement can be reduced to 0.15 without any compromise in the performance. This concept has been adopted in developing a new configuration to serve as: (1) a floating breakwater; and (2) as a possible shallow water cage culture unit. Experiments were conducted to study the performance of the cage floating breakwater under wave and wave-current environment. The results on transmission and reflection coefficients are presented and compared with those reported in the literature. The variation of water surface oscillations and velocities within the cage, the effect of mooring line stiffness, and initial tension on transmission characteristics are also discussed.

Abstract: Minimum power requirement is one of the important design criteria for successful operation of underwater vehicles. CFD based prediction and estimation of power requirement is increasingly carried out in practice. However, reliable prediction depends on suitable turbulence models. This paper presents a comparative assessment of four low Reynolds number (low-Re) k- Iµ models for computation of hydrodynamic forces on underwater vehicle hull forms. The low-Re models are being considered more suitable for underwater axisymmetric bodies due to the following merits (i) they have no wall function approximations, (ii) they could compute low turbulence levels such as in the viscous sub-layer and (iii) they could account for the effect of damped turbulence. The low-Re models used in the present study are namely the models of Abe-Kondoh-Nagano (k- Iµ AKN), Chang-Hsieh-Chen (k- Iµ CHC), Launder-Sharma (k- Iµ LS), and Yang-Shih (k- Iµ YS). It has been found that the k- Iµ AKN low-Re model consistently provided superior performance in predicting the flow characteristics around underwater vehicle hull forms. Keywords: Axisymmetric bodies, autonomous underwater vehicle, CFD, damping functions, low Reynolds k- Iµ models, QUICK scheme doi: 10.3329/jname.v2i1.2029 Journal of Naval Architecture and Marine Engineering 2(1)(2005) 41-54

Abstract: The current system in the Strait of Singapore is fairly complicated and variable because it is under the influence of major currents driven by trade winds as well as the Asian monsoons. In addition to tidal forcing, circulation in the Strait is governed by a strong hydrodynamic pressure gradient which reverses direction semi-annually and coincides with seasonal monsoon changes. In this investigation, we examine the mechanism that controls the circulation in the Strait of Singapore by using a three-dimensional circulation model forced by hydrodynamic pressure gradient, tides and winds. Model results delineate in detail the characteristics of circulation and hydrodynamic structure in the strait. These results compare favourably with the available field measurements and agree, in general, with observations. It is shown that: (1) the low frequency net transport across the strait correlates well with the seasonal hydrodynamic pressure gradient, and (2) the tidal variation across the strait have a sign...

Abstract: Soft marine clay deposits are present in many coastal regions of the world and they are usually enriched with chlorides and occasionally with sulphates. The use of lime to improve the properties of soft clay deposits is not new. A test program was carried out to study the changes in the particle level of soil systems attributed to lime in a marine environment. The formation of various new reaction products due to soil-lime reactions were identified by X-Ray Diffraction (XRD) and the same was investigated using Scanning Electron Microscope (SEM). The test results indicate that the presence of chlorides encourage the efficiency of lime stabilization, but the presence of monovalent sulphates affect the improvement of the soil system due to the formation of the swelling compound, etttringite. It is suggested, that the lime injection technique can be conveniently used to improve the behavior of soft marine clay deposits.

Abstract: The evaluation of structural responses is key element in the design of ships and offshore structures. Fundamental to this is the determination of the design loads to support the Rule requirements and for application in direct calculations. To date, the current design philosophy for the prediction of motions and wave-induced loads has been driven by empirical or first-principles calculation procedures based on well-proven applications such as ship motion prediction programs. In recent years, the software, engineering and computer technology available to predict the design loads imposed on ships and offshore structures has improved dramatically. Notwithstanding, with the stepwise increase in the size and structural complexity of ships and floating offshore installations and the advances in the framework of Rules and Standards it has become necessary to utilise the latest technologies to assess the design loads on new designs. Along the lines of the recommendations from the International Ship and Offshore Structures Committee (ISSC) I.2 on Loads this paper reviews some of the recent advances in the assessment of loads for ships and offshore structures with the aim to draw the overall technological landscape available for further understanding, validation and implementation by the academic and industrial communities. Particular emphasis is attributed on methodologies applicable for the prediction of environmental and operational loads from waves, wind, current, ice, slamming, sloshing and operational factors. Consideration is also given to deterministic and statistical load predictions based on model experiments, full-scale measurements and theoretical methods.

Abstract: In August 2005, a team surveyed the effects of the December 2004 Indian Ocean tsunami on the southern coast of Oman. Runup and inundation were obtained at 41 sites, extending over a total of 750 km of shoreline. Measured runup ranged from 3.25 m in the vicinity of Salalah to a negligible value at one location on Masirah Island. In general, the largest values were found in the western part of the surveyed area. Significant incidents were documented in the port of Salalah, where a 285-m-long vessel broke its moorings and drifted inside and outside the port, and another ship struck the breakwater while attempting to enter the harbor. The general hazard to Oman from tsunamis may be greatest from the neighboring Makran subduction zone in western Pakistan.

Abstract: Swelling and shrinkage behaviour of marine clays impose foundation problems that may sometimes results in excessive settlements. Improving the behaviour of soft clays using lime or cement is not new. Recent studies reported the occurrence of high swelling and pavement failures in lime and cement stabilized clays containing sulphates. The above phenomena resulted in the formation of swelling minerals such as ettringite and thaumasite. In this paper, the earlier reported case histories of sulphate attack in lime and cement stabilized clays have been reviewed, and a list of precautionary measures to be adopted for controlling the ettringite formation have been suggested. An attempt has been made to investigate the application of deep lime mixing technique in sulphate enriched marine clays. Physico-chemical and engineering properties of the lime treated clays containing ettringite have been examined. Test results indicate that lime and cement stabilization techniques should be cautiously approached in sulphate enriched clays.

Abstract: The sea-surface microlayer (SML) represents the interfacial layer between the ocean and atmosphere and covers about 70% of the world's surface. Gel-like transparent exopolymer particles (TEP) in the SML were studied in oceanic and estuarine SML and subsurface water samples from South East Asia. The TEP enrichment factor, determined as the ratio of the TEP concentration in the SML to that in the corresponding subsurface water, was in the range of 0.39 to 2.43 (1.31 ± 0.52 mean ± standard deviation) and 0.29 to 9.72 (1.77 ± 3.03) in the oceanic and estuarine samples, respectively. Sulfate half-ester groups in the TEP showed a higher enrichment (3.29 ± 2.36) than the less strongly binding carboxyl groups (1.12 ± 0.71). Enrichment processes of TEP to the SML are discussed including diffusion to the SML, bubble scavenging and higher production rates of TEP in the SML than in subsurface waters. The results of a general enrichment of gel particles support the concept of a hydrated gelatinous interfacial layer with a complex matrix of dissolved organic matter rather than a more classical model of organized layers of “wet” and “dry” surfactants.