Electromagnetic Modelling of Canadian Forces Auxiliary...

Defence R&D Canada – Atlantic

DEFENCE DÉFENSE&

Electromagnetic Modelling of Canadian

Forces Auxiliary Vessel (CFAV) Quest:

Phase I Report

John Wallace and Ken MacKayMartec Limited

Martec LimitedSuite 400, 1888 Brunswick St.Halifax, NSB3J 3J8

Martec Technical Report: TR-05-26, Rev 1

Contract Number: W7707-042694/001/HAL

Contract Scientific Authority: Zahir Daya, Ph.D., Marius Birsan, Ph.D.

The scientific or technical validity of this Contract Report is entirely the responsibility of the contractorand the contents do not necessarily have the approval or endorsement of Defence R&D Canada.

Contract Report

DRDC Atlantic CR 2005-083

June 2005

Copy No.________

Defence Research andDevelopment Canada

Recherche et développementpour la défense Canada

This page intentionally left blank.

Electromagnetic Modelling of Canadian Forces Auxiliary Vessel (CFAV) Quest: Phase I Report

John Wallace and Ken MacKay Martec Limited 1888 Brunswick Street Suite 400 Halifax, Nova Scotia B3J 3J8

Martec Technical Report: TR-05-26, Rev 1 Contract Number: W7707-042694/001/HAL Contract Scientific Authority: Zahir Daya, Ph.D., Marius Birsan, Ph.D.

Contract Report disclaimer The scientific or technical validity of this Contract Report is entirely the responsibility of the contractor and the contents do not necessarily have the approval or endorsement of Defence R&D Canada.

Defence R&D Canada – AtlanticContract Report DRDC Atlantic CR 2005-083 June 2005

Abstract The underwater electromagnetic signatures of steel-hulled vessels with active signature reduction measures can be accurately modelled by commercially developed software packages. Static magnetic signatures can be computed with FLUX3D which is a sophisticated finite element code developed by Cedrat Groupe, Grenoble, France. Underwater electric potential can be suitably modelled using boundary element codes such as BEASY, developed by BEASY Group, Southampton, UK. DRDC has a requirement to demonstrate magnetic and electric signature prediction for the research vessel CFAV Quest. This report describes the work by Martec Limited to develop underwater electric potential and static magnetic models of CFAV Quest. A static magnetic model of CFAV Quest has been produced for FLUX3D using the structural finite-element model of CFAV Quest. The model accounts for both the structural and non-structural components of the ship. Using this model, the Magnetic Anomaly Signature of CFAV Quest was predicted for several scenarios. An underwater electric potential model of CFAV Quest was created for prediction of the underwater electric potential using BEASY. The model was used to predict the underwater electric potential for two different Impressed Current Cathodic Protection (ICCP) configurations at various ICCP current output levels. The predicted results indicate that the electric potential varies linearly with ICCP current for a given configuration and the potential decreases rapidly with distance.

Résumé Les signatures électromagnétiques sous-marines des navires à coque d'acier faisant l’objet de mesures de réduction de la signature active peuvent être modélisées exactement à l’aide de progiciels commerciaux. Les signatures magnétiques statiques peuvent être calculées avec FLUX3D, code à éléments finis perfectionné développé par le Groupe Cedrat, de Grenoble, en France. Le potentiel électrique sous-marin peut être adéquatement modélisé à l’aide de codes à éléments finis comme BEASY, développé par le Groupe BEASY, de Southampton, R.-U. RDDC doit faire la démonstration de modèles pour la prédiction de signatures magnétiques et électriques du navire de recherche NAFC Quest. Le rapport décrit les travaux de Martec Limited visant à développer des modèles de potentiel électrique sous-marin et des modèles magnétiques statiques applicables au NAFC Quest. Un modèle magnétique statique du NAFC Quest a été produit pour FLUX3D à l’aide du modèle structural à éléments finis du NAFC Quest. Il s’applique aux composants structuraux et non structuraux du navire. Ce modèle a permis de prédire la signature d’anomalie magnétique du NAFC Quest pour divers scénarios.

DRDC Atlantic CR 2005-083 i

Un modèle de potentiel électrique sous-marin du NAFC Quest a été créé pour la prédiction du potentiel électrique sous-marin à l’aide de BEASY. Le modèle a permis de prédire le potentiel électrique sous-marin pour deux différentes configurations ICCP (protection cathodique par courant imposé) à divers niveaux de courant de sortie ICCP. Les résultats prédits indiquent que le potentiel électrique varie linéairement avec le courant ICCP pour une configuration donnée et qu’il diminue rapidement avec la distance.

ii DRDC Atlantic CR 2005-083

Executive summary

Introduction

The underwater electromagnetic signatures of steel-hulled vessels with active signature reduction measures can be accurately modelled by commercially developed software packages. Static magnetic signatures can be computed with FLUX3D which is a sophisticated finite element code developed by Cedrat Groupe, Grenoble, France. Underwater electric potential can be suitably modelled using boundary element codes such as BEASY, developed by BEASY Group, Southampton, UK. DRDC has a requirement to demonstrate magnetic and electric signature prediction for the research vessel CFAV Quest. Martec Limited has extensive experience with both software packages and was the prime contractor in developing the models used by DRDC in the study of underwater electric and magnetic signatures for a steel hulled naval vessel. Martec offers a project team, which has extensive qualifications and experience to develop the models for the CFAV Quest.

Results

A static magnetic model of CFAV Quest has been produced for FLUX3D using the structural finite-element model of CFAV Quest. The model accounts for both the structural and non-structural components of the ship. Using this model, the Magnetic Anomaly Signature of CFAV Quest was predicted for several scenarios. An underwater electric potential model of CFAV Quest was created for prediction of the underwater electric potential using BEASY. The model accounts for the hull-shaft metals and their polarization behaviour. The model was used to predict the underwater electric potential for two different ICCP configurations at various ICCP current output levels. The predicted results indicate that the electric potential varies linearly with ICCP current for a given configuration and the potential decreases rapidly with distance.

Significance

The models will allow for prediction of the underwater electric potential and magnetic fields generated by CFAV Quest. The models provide a computational tool in which to access the influence of various system operation and design parameters and will allow the controlled study of the effects of these parameters on the non-acoustic signatures of the vessel. This is important in studying signature reduction measures that may be applied to DND vessels. Recommendations for future work involves using the models developed in the current project to further investigate signature reduction measures for CFAV Quest.

John Wallace and Ken Mackay. 2005. Electromagnetic Modelling of Canadian Forces Auxiliary Vessel (CFAV) Quest: Phase I Report. DRDC Atlantic CR 2005-083. Defence R&D Canada – Atlantic.

DRDC Atlantic CR 2005-083 iii

Sommaire

Introduction

Les signatures électromagnétiques sous-marines des navires à coque d'acier faisant l’objet de mesures de réduction de la signature active peuvent être modélisées exactement à l’aide de progiciels commerciaux. Les signatures magnétiques statiques peuvent être calculées avec FLUX3D, code à éléments finis perfectionné développé par le Groupe Cedrat, de Grenoble, en France. Le potentiel électrique sous-marin peut être adéquatement modélisé à l’aide de codes à éléments finis comme BEASY, développé par le Groupe BEASY, de Southampton, R.-U. RDDC doit faire la démonstration de modèles pour la prédiction de signatures magnétiques et électriques du navire de recherche NAFC Quest. Martec Limited possède une vaste expérience des deux progiciels et a été maître d’oeuvre pour le développement des modèles utilisés par RDDC dans l’étude des signatures électriques et magnétiques sous-marines d’un navire de guerre à coque d’acier. L’équipe de projet de Martec possède des compétences et une expérience étendues applicables au développement de modèles pour le NAFC Quest.

Résultats

Un modèle magnétique statique du NAFC Quest a été produit pour FLUX3D à l’aide du modèle structural à éléments finis du NAFC Quest. Le modèle s’applique aux composants tant structuraux que non structuraux du navire. Il a permis de prédire la signature d’anomalie magnétique du NAFC Quest pour divers scénarios. Un modèle de potentiel électrique sous-marin du NAFC Quest a été créé pour la prédiction du potentiel électrique sous-marin à l’aide de BEASY. Le modèle s’applique aux métaux de la coque et de l’arbre d’hélice et à leur comportement en polarisation. Le modèle a permis de prédire le potentiel électrique sous-marin pour deux différentes configurations ICCP (protection cathodique par courant imposé) à divers niveaux de courant de sortie ICCP. Les résultats prédits indiquent que le potentiel électrique varie linéairement avec le courant ICCP pour une configuration donnée et qu’il diminue rapidement avec la distance.

Portée

Les modèles permettront la prédiction du potentiel électrique et des champs magnétiques sous-marins produits par le NAFC Quest. Les modèles offrent un outil de calcul pour déterminer l’influence de divers paramètres de la conception et du fonctionnement du système et permettront l’étude contrôlée des effets de ces paramètres sur les signatures non acoustiques du navire. Cela est important pour l’étude de mesures de réduction de signature qui pourront être appliquées aux navires du MDN. Les recherches futures recommandées font appel aux modèles élaborés dans le cadre du projet en cours afin d’étudier plus en profondeur les mesures de réduction de la signature du NAFC Quest.

John Wallace and Ken Mackay. 2005. Electromagnetic Modelling of Canadian Forces Auxiliary Vessel (CFAV) Quest: Phase I Report. DRDC Atlantic CR 2005-083. Defence R&D Canada – Atlantic.

iv DRDC Atlantic CR 2005-083

Table of contents

1. Introduction ................................................................................................................... 1

2. Static Magnetic Model of CFAV Quest ........................................................................ 2

2.1 3D Model Preparation ...................................................................................... 2

2.2 Static Magnetic Model Creation....................................................................... 3

2.3 Magnetic Anomaly Signature Prediction ......................................................... 7

2.3.1 Ranging at Bedford ............................................................................. 7

2.3.2 Ranging at Ferguson’s Cove ............................................................... 9

2.3.3 Magnetic Anomaly Detection (MAD)............................................... 11

2.3.4 Near field studies I ............................................................................ 13

2.3.5 Near field studies II ........................................................................... 18

3. Underwater Electric Potential Model .......................................................................... 23

3.1 Model Generation........................................................................................... 23

3.2 Polarization Data And Boundary Conditions ................................................. 26

3.3 Underwater Electric Field Prediction ............................................................. 27

3.3.1 Current Densities ............................................................................... 27

3.3.2 Aft to Forward Current ratio 4:1 ....................................................... 30

3.3.3 Variation with depth.......................................................................... 33

4. Conclusions and Recommendations............................................................................ 38

DRDC Atlantic CR 2005-083 v

List of figures Figure 2.1 Original MAESTRO model of Quest........................................................................2

Figure 2.2 Final merged finite element model...........................................................................3

Figure 2.3 FLUX3D model with non-structural components....................................................4

Figure 2.4 FLUX3D Meshed Ship ............................................................................................5

Figure 2.5 Earth’s Magnetic Field .............................................................................................6

Figure 2.6 Total Magnetic Field for Bedford Range ..................................................................8

Figure 2.7 Total Magnetic Field Anomaly for Bedford Range ..................................................9

Figure 2.8 Total Magnetic Field for Ferguson’s Cove .............................................................10

Figure 2.9 Total Magnetic Field Anomaly for Ferguson Cove ................................................11

Figure 2.10 Magnetic Field Anomaly along Longitudinal Axis for MAD...............................12

Figure 2.11 Magnetic Field Anomaly along Athwart Axis for MAD ......................................13

Figure 2.12 Total Magnetic Field for Near Field Studies I (2 m depth)...................................14



Figure 2.15 Total Magnetic Field Anomaly for Near Field Studies I (2 m depth) ...................16

Figure 2.16 Total Magnetic Field Anomaly for Near Field Studies I (4 m depth) ...................17

Figure 2.17 Total Magnetic Field Anomaly for Near Field Studies I ( 8 m depth) ..................17

Figure 2.18 Total Magnetic Field for Near Field Studies II (2 m offset) .................................18



Figure 2.21 Total Magnetic Field Anomaly for Near Field Studies II (2m offset) ..................21

Figure 2.22 Total Magnetic Filed Anomaly for Near Field Studies II (4m offset) ..................21

Figure 2.23 Total Magnetic Field Anomaly for Near Field Studies II (8m offset) ..................22

Figure 3.24 Hull geometry........................................................................................................24

Figure 3.25 Propeller (disk) and shaft bossing geometry .........................................................25

Figure 3.26 Rudder geometry...................................................................................................25

Figure 3.27 Assembled boundary element model of CFAV Quest ..........................................26

Figure 3.28 Position of model above display surface...............................................................26

Figure 3.29 Potential (mV) contour plots at 14m depth with current density equal at the aft and forward anodes. ICCP current output (from top to bottom) 16A, 8A, 4A, and 2A. Note the 16A plot uses an expanded contour scale ..........................................................28

vi DRDC Atlantic CR 2005-083

Figure 3.30 Potential and electric field at 14m depth, longitudinal direction. Current density equal at aft and forward anodes. .......................................................................................29

Figure 3.31 Potential and electric field at 14m depth, midships , athwart direction. Current density equal at aft and forward anodes. ..........................................................................29

Figure 3.32 Potential (mV) contour plots at 14m depth with aft anode current 4 times forward anode current. ICCP current output (from top to bottom) 16A, 8A, 4A, and 2A. Note the 16A plot uses an expanded contour scale. ........................................................................31

Figure 3.33 Potential and electric field at 14m depth, longitudinal direction. Aft anode current 4 times the forward anode current. ...................................................................................32

Figure 3.34 Potential and electric field at 14m depth, midships, athwart direction. Aft anode current 4 times the forward anode current. .......................................................................32

Figure 3.35 Potential (mV) contour plots at depths of 14m (top), 28m (middle), and 56m (bottom) with 16A total ICCP current, current density equal at the aft and forward anodes. ..............................................................................................................................34

Figure 3.36 Potential (mV) contour plots at depths of 14m (top), 28m (middle), and 56 m (bottom) with 16A total ICCP current, aft anode current 4 times the forward anode current...............................................................................................................................35

Figure 3.37 Potential and electric field at depths of 14m, 28m, and 56m in the longitudinal direction with 16A total ICCP current, current density equal at the aft and forward anodes. ..............................................................................................................................36

Figure 3.38 Potential and electric field at depths of 14m, 28m, and 56m with 16A total ICCP current, aft anode current 4 times the forward anode current. .........................................36

Figure 3.39 Potential variation with depth with 16A total ICCP current, current density equal at the aft and forward anodes............................................................................................37

Figure 3.40 Potential variation with depth with 16A total ICCP current, aft anode current 4 times the forward anode current. ......................................................................................37

DRDC Atlantic CR 2005-083 vii

List of Tables Table 1 Magnetic Field for MAD (longitudinal line)...............................................................12 Table 2 Magnetic Field for MAD (athwartship line)................................................................12 Table 3 Total Magnetic Field for Near Field I (2 m depth)......................................................15 Table 4 Total Magnetic Field for Near Field I (4 m depth)......................................................15 Table 5 Total Magnetic Field for Near Field I (8 m depth)......................................................16 Table 6 Total Magnetic Field for Near Field II (2 m offset) ....................................................20 Table 7 Total Magnetic Field for Near Field II (4 m offset) ....................................................20 Table 8 Total Magnetic Field for Near Field II (8 m offset) ....................................................20 Table 9 Polarization data for Propeller material.......................................................................27 Table 10 Compositions of manganese nickel aluminum bronze (C95700) and nickel

aluminum bronze (C95800)..............................................................................................27

viii DRDC Atlantic CR 2005-083

1. Introduction The electrostatic and magnetic fields produced during the operation of naval surface ships and submarines represents the non-acoustic signatures which pose a mine threat to surface ships and submarines. Minimizing all signatures, including underwater electrical potential and magnetic signatures, is the essence of stealth technology for naval platforms. Stealth provides delayed detection, identification and target acquisition by hostile forces which enhances both the first strike capability and survivability. Therefore, it constitutes an important design consideration for naval platforms. Recent investigations demonstrated the possibilities of the finite elements (FE) and boundary elements (BE) methods to model the electromagnetic field of a steel-hulled vessel. Models were built using the FLUX3D FE package and the BEASY BE package respectively, now owned by DRDC Atlantic. The models are an important part of a larger effort to minimize the ship electromagnetic (EM) signature and to optimize its degaussing and active cathodic protection systems. These investigations have demonstrated that the underwater electromagnetic signatures of steel-hulled vessels with active signature reduction measures can be modeled by these commercially developed software packages. One of the difficulties of the signature reduction program at DRDC has been the availability of experimental data to validate model predications and design theories. Investigations up to this point have focused on a steel hulled naval vessel. The duties of these ships allow for very small windows-of-opportunity for conducting experimental trials and limit the ability to conduct the experiments in a controlled manner. To overcome these difficulties DRDC will now also focus some of its signature reduction investigations on the research vessel CFAV Quest. This report describes the work by Martec Limited to develop underwater electric potential and static magnetic field models of CFAV Quest.

DRDC Atlantic CR 2005-083 1

2. Static Magnetic Model of CFAV Quest

2.1 3D Model Preparation Before any finite element or boundary element analyses of CFAV Quest could be performed suitable numeric models had to be generated. In the case of a computational magnetics analysis of the ship, a model must include the correct three-dimensional geometry of the ferromagnetic structure, proper location of non-structural ferromagnetic masses, field coils, and material properties to accurately predict magnetic signatures. Specifying the structure geometry, be it for FLUX3D or any other finite element program, can be a daunting undertaking. In order to expedite this task an existing finite element model, supplied by DRDC Atlantic, was used. This model was to be modified to suite the requirements of both FLUX3D and BEASY. In the case of FLUX3D, the modified model was imported via a DXF file translator. This translator reads descriptions of geometric faces and converts them into FLUX3D surface regions that include geometric and material descriptions. These surface regions are then meshed thereby producing the elements that are passed to the finite element solver. The original finite element model, generated by MAESTRO, was developed for purposes of structural analyses. As such, it was a relatively refined model with 14,123 elements. It is shown in Figure 2.1. All structural components, including decks, frames, bulkheads, floors and stiffeners, were explicitly described in this model. This level of detail was, not only not required for the computational magnetic analyses, it was disadvantageous since it would result in an excessive number of degrees-of-freedom in the assembled finite element matrix and hence excessively long computation times.

Figure 2.1 Original MAESTRO model of Quest

After receiving the MAESTRO model a process of element merging began. Using software customized for this purpose, groups of plate elements were merged into a single element thereby producing a significantly coarser mesh. At the same time all beam elements were smeared into neighbouring plate elements. These merging and smearing algorithms were designed to produce plate elements of equivalent mass. This was accomplished by adjusting the thicknesses of each resulting plate element to reflect the total mass of all elements that were smeared or merged into the new element. The merging process had to proceed with

2 DRDC Atlantic CR 2005-083

caution. It was necessary to ensure that at each intersection of two or more adjoining merged plates there was a common edge. One plate could not simply line up with the interior of an adjoining plate. In such a case, the FLUX3D mesher would probably fail to create a mesh. Ease of meshing was also facilitated by closing off open areas with elements of near-zero thickness. When the merging process was complete the total number elements had been reduced by a factor of ten (down from 14,123 to 1,340) while the total mass, and overall distribution of mass, was unchanged. Figure 2.2 shows the final model. The final model was then converted to an “extended” DXF file using a stand-alone program developed at Martec Limited. This file could then be imported into Flux3D.

Figure 2.2 Final merged finite element model

The original plan called for the same merged finite element model to be imported into BEASY. Unfortunately, this was not possible and a BEASY model had to be generated within BEASY starting with lines-of-form. Fortunately, the required lines-of-form could be generated by Trident FEA, a finite element program suite created by Martec Limited.

2.2 Static Magnetic Model Creation A proper assessment of the magnetic field surrounding the Quest required the correct three-dimensional geometry of the ferromagnetic structure, proper location of non-structural ferromagnetic masses, field coils, magnetic material properties and a model of the surrounding domain. In FLUX3D the surrounding infinite domain is modelled by means of an “infinite box” which links the “real open domain to an image closed domain by means of a spatial transformation” [1].


From previous studies of the magnetic fields surrounding ships, it was found that good results were obtained for:

• fine meshes (1 meter element size) used in the plane of interest, • this plane could be aligned with an inner face of the infinite box, • the box had a thickness between inner and outer faces of 10 meters, • there were at least 2 second-order elements through the thickness of the

infinite box, • second-order elements were used throughout the model.

These guidelines were used in creating domains for the various analyses described in this report. The geometry of structure was generated and saved in a DXF file, as described in the previous section of this report. Since the DXF file importation feature in FLUX3D can read only lines and faces, it was not possible to include volumetric descriptions of any non-structural ship components that are made of ferromagnetic materials. Consequently, descriptions of these components had to be entered directly into FLUX3D. Volumetric descriptions of the generators, engines and the aft-most portions of the propeller shafts were created. Sizes and locations of these components were approximated by scaling Quest drawings supplied by the DRDC scientific authority. Figure 2.3 show these components as they were generated in FLUX3D. It was not possible to include the entire length of shafts since such a description would interfere with the faces that describe the ship structure.

Figure 2.3 FLUX3D model with non-structural components

In the absence of specific information on the magnetic properties of the materials used in the ship, or in the percentage of ferromagnetic material within the non-structural components, or experimental measurements with which to calibrate material properties, all materials were assumed to be linear isotropic with a relative permeability of 300. Each of the three types of


non-structural components was assigned to a different volume region. This approach permits quick and simple changes to material and geometric properties, should future studies need to be done. As there was no requirement to analyze the effect of degaussing coils in this first phase of the contract, no coils were included in the model. All ship geometry was assigned a mesh size of 1 meter. Second-order elements were used in all models. This resulted in a mesh for the ship that is shown in Figure 2.4.

Figure 2.4 FLUX3D Meshed Ship

The magnetic field of the earth was specified using values obtained from the website of the “NOAA Satellite and Information Service” of the “National Geophysical Data Center” (www.ngdc.noaa.gov/weg/geomag/jsp/struts/calcPointIGRF). Using the location of Halifax harbour, the earth’s magnetic field, as used in these analyses, was calculated. Figure 2.4 shows the computed field.


http://www.ngdc.noaa.gov/weg/geomag/jsp/struts/calcPointIGRF

Figure 2.5 Earth’s Magnetic Field

For each analysis the longitudinal axis of the ship was aligned north-south with the bow of the ship pointed north. Following recommended guidelines in the FLUX3D User’s Guide [2], the FLUX3D scalar model was used for all magneto-static analyses. Also, the reduced scalar potential, with respect to Hj (MS3RED), formulation was used throughout as the following conditions applied:

• sources consisted of a fixed field of constant value, and DC current in non-meshed coils (phase 2),

• magnetic properties were linear isotropic, with permeability equal to that of a vacuum in the region of interest,


2.3 Magnetic Anomaly Signature Prediction The scientific authority requested the simulation of five different sea trials:

1. Ranging at Bedford, 2. Ranging at Ferguson’s Cove, 3. Magnetic Anomaly Detection (MAD), 4. Near field studies I, 5. Near field studies II.

2.3.1 Ranging at Bedford For simulation of ranging at Bedford, the scientific authority requested that the magnetic field on a horizontal plane be computed. Details of the plane were as follows:

• length of plane = 2 times Quest length (152.4 m), • width of plane = 2 times Quest width (25.6 m), • depth of plane = 14 meters below sea level and centered with respect to the ship

center. Since the Bedford, Ferguson’s Cove and near-field studies I simulations all involved computation of magnetic fields on a horizontal plane beneath the ship, one model and one analysis was used for all of these cases. From this analysis, results were extracted on the various planes requested by the scientific authority. This model used an “infinite” box, as defined in FLUX3D, that had inner dimensions that were 4 times the ship length and 4 times the ship width. On each side the outer box extended out an additional 10 meters. The ship and the horizontal inner box face below the ship were meshed using a specified mesh point size of 1 meter, thus producing a fine mesh in the region below the ship. On the face of the box above the ship, a much coarser mesh was used in order to reduce the overall problem size. Figure 2.6 shows the total magnetic field (FLUX3D variable MODV(HMAG1), in A/m) on the specified plane.


Figure 2.6 Total Magnetic Field for Bedford Range

It should be noted that this plot shows the resultant field value. The resultant value for the earth’s field was 42.03 A/m. The predicted signature has considerable spatial detail and varied in magnitude as shown in the following table. The model does not include a residual permanent magnetization and so the measured signature is expected to be larger.

Magnetic Field Minimum value Maximum value X component 14.12 15.08 Y component 4.58 5.52 Z component -38.92 -40.12 Resultant 41.89 43.01

The next figure shows the total field less the fixed (earth) field (FLUX3D formula MODV(HMAG1)-42.03, in A/m). Magnitudes vary from a minimum of -0.15 to a maximum of 0.98.


Figure 2.7 Total Magnetic Field Anomaly for Bedford Range

2.3.2 Ranging at Ferguson’s Cove For simulation of ranging at Ferguson’s Cove, the scientific authority requested that the magnetic field on a horizontal plane be computed. Details of the plane were as follows:

• length of plane = 4 times Quest length (304.8 m), • width of plane = 4 times Quest width (51.2 m), • depth of plane = 21 meters below sea level and centered with respect to the ship

center. Since the Bedford, Ferguson’s Cove and near-field studies I simulations all involved computation of magnetic fields on a horizontal plane beneath the ship, one model and one analysis was used for all of these cases. From this analysis, results were extracted on the various planes requested by the scientific authority. This model used an infinite box that had inner dimensions that were 4 times the ship length and 4 times the ship width. On each side the outer box extended out an additional 10 meters. The ship and the horizontal inner box face below the ship were meshed using a specified mesh point size of 1 meter, thus producing a fine mesh in the region below the ship. On the face of the box above the ship, a much coarser mesh was used in order to reduce the overall problem size. Figure 2.8 shows the total magnetic field (FLUX3D variable MODV(HMAG1), in A/m) on the specified plane.


Figure 2.8 Total Magnetic Field for Ferguson’s Cove

The contour levels in the figure above are the same as those in the plot of the field for Bedford. The predicted signature has less spatial detail than that at the shallower Bedford range. The magnetic signature magnitude varied as shown in the following table. The model does not include a residual permanent magnetization and so the measured signature is expected to be larger.

Magnetic Field Minimum value Maximum value X component 14.55 14.79 Y component 4.94 5.18 Z component -39.32 -39.01 Resultant 41.98 42.24

The next figure shows the total field less the fixed (earth) field (FLUX3D formula MODV(HMAG1)-42.03, in A/m). Magnitudes range from a minimum of -0.05 to a maximum of 0.20.


Figure 2.9 Total Magnetic Field Anomaly for Ferguson Cove

2.3.3 Magnetic Anomaly Detection (MAD) For simulation of Magnetic Anomaly Detection (MAD) measurements, the scientific authority requested that the magnetic field along horizontal axes (longitudinal and athwart) be computed. Details were as follows:

• length of longitudinal axis = 10 times Quest length (762 m), • length of athwart axis = 10 times Quest width (256 m), • height of axes = 80 meters above sea level and centered with respect to the ship

center. This analysis required the use of a different domain. The infinite box had inner dimensions that were 10 times the ship length and 10 times the ship width. On each side the outer box extended out an additional 25 meters. The ship was meshed with a point size of 1 meter and the horizontal inner box face above the ship was meshed using a specified mesh point size of 3 meters, thus producing a fine mesh in the region above the ship. On the face of the box below the ship, a much coarser mesh was used in order to reduce the overall problem size. Figures 2.10 and 2.11 show the magnetic field (FLUX3D variable MODV(GRADVR1), in A/m) along the specified axes. The predicted magnetic field components vary in magnitude as shown in the following table. The model does not include a residual permanent magnetization and so the measured signature is expected to be larger.


Table 1 Magnetic Field for MAD (longitudinal line) Magnetic Field Minimum value Maximum value X component 14.6773 14.6815 Y component 5.0598 5.0600 Z component -39.0323 -39.0593 Resultant 42.0323 42.0373

Table 2 Magnetic Field for MAD (athwartship line) Magnetic Field Minimum value Maximum value X component 14.6777 14.6798 Y component 5.0584 5.0615 Z component -39.0634 -39.0599 Resultant 42.0329 42.0355

Figure 2.10 Magnetic Field Anomaly along Longitudinal Axis for MAD


-0.005

-0.004

-0.003

-0.002

-0.001

0.000

0.001

0.002

-150 -100 -50 0 50 100 150

athwart location (m)

Mag

netic

Fie

ld A

nom

aly

(A/m

)

north component (X)west component (Y)vertical component (Z)

Figure 2.11 Magnetic Field Anomaly along Athwart Axis for MAD

2.3.4 Near field studies I For simulation of the first near-field studies, the scientific authority requested that the magnetic field on three horizontal planes be computed. Details of the planes were as follows:

• length of plane = 1.5 times Quest length (114.3 m), • width of plane = 1.5 times Quest width (38.4 m), • depth of plane = 2 meters, 4 meters and 8 meters below the keel and centered with

respect to the ship center. Since the Bedford, Ferguson’s Cove and near-field studies (I) simulations all involved computation of magnetic fields on a horizontal plane beneath the ship, one model and one analysis was used for all of these cases. From this analysis, results were extracted on the various planes requested by the scientific authority. This model used an infinite box that had inner dimensions that were 4 times the ship length and 4 times the ship width. On each side the outer box extended out an additional 10 meters. The ship and the horizontal inner box face below the ship were meshed using a specified mesh point size of 1 meter, thus producing a fine mesh in the region below the ship. On the face of the box above the ship, a much coarser mesh was used in order to reduce the overall problem size. Figures 2.12 – 2.14 shows the total magnetic field (FLUX3D variable MODV(HMAG1), in A/m) on the specified planes.


Figure 2.12 Total Magnetic Field for Near Field Studies I (2 m depth)




Figures 2.12 through 2.14 all use the same contour levels. Hence, the plots can be directly compared. Ranges of computed field values, for the three depths, are summarized on the following tables.

Table 3 Total Magnetic Field for Near Field I (2 m depth) Magnetic Field Minimum value Maximum value X component 10.60 16.55 Y component 1.78 9.01 Z component -45.15 -38.15 Resultant 40.65 48.16




The next three figures show, the total field less the fixed (earth) field (FLUX3D formula MODV(HMAG1)-42.03, in A/m) for each depth.

Figure 2.15 Total Magnetic Field Anomaly for Near Field Studies I (2 m depth)


Figure 2.16 Total Magnetic Field Anomaly for Near Field Studies I (4 m depth)

Figure 2.17 Total Magnetic Field Anomaly for Near Field Studies I ( 8 m depth)


2.3.5 Near field studies II For simulation of the second near-field studies, the scientific authority requested that the magnetic field on three vertical planes be computed. Details of the planes were as follows:

• length of plane = 2 times Quest length (152.4 m), • height of plane = 4 times Quest width (102.4 m), • offset of plane = 2 meters, 4 meters and 8 meters off the side of the ship and centered

with respect to the ship center. This model used an infinite box that had inner dimensions that were 4 times the ship length and 4 times the ship width. On each side the outer box extended out an additional 10 meters. The ship and the vertical inner box face on the port side of the ship were meshed using a specified mesh point size of 1 meter, thus producing a fine mesh in the region of interest. On the face of the box on the starboard side of the ship, a much coarser mesh was used in order to reduce the overall problem size. Figures 2.18 – 2.20 shows the total magnetic field (FLUX3D variable MODV(HMAG1), in A/m) on the specified planes.

Figure 2.18 Total Magnetic Field for Near Field Studies II (2 m offset)





Figures 2.18 through 2.20 all use the same contour levels. Hence, the plots can be directly compared. The predicted signatures on the near field horizontal and vertical display surfaces have considerable spatial detail. The variation in magnitude is greater on the horizontal display surface, as shown for Near Field Studies I. The following tables summarize the range of computed field values for the

Table 6 Total Magnetic Field for Near Field II (2 m offset) Magnetic Field Minimum value Maximum value X component 13.44 15.19 Y component 3.45 7.18 Z component -39.68 -37.33 Resultant 40.23 48.78



The next three figures show the total field less the fixed (earth) field (FLUX3D formula MODV(HMAG1)-42.03) for each offset.


Figure 2.21 Total Magnetic Field Anomaly for Near Field Studies II (2m offset)

Figure 2.22 Total Magnetic Filed Anomaly for Near Field Studies II (4m offset)


Figure 2.23 Total Magnetic Field Anomaly for Near Field Studies II (8m offset)


3. Underwater Electric Potential Model

3.1 Model Generation An underwater electric potential model of CFAV Quest was generated for analysis with BEASY [3]. The model development required generation of a 3D geometry of the wetted surface of the vessel, proper location of impressed current and sacrificial anodes, and the appropriate material properties (polarization data). Due to symmetry, only one side of ship was required. Initially it was proposed to use a modified version of the mesh generator from Martec’s CPBEM boundary element software, CPBGEN, to automate the mesh generation process. CPBGEN input data consists of the ship lines-of-form (LOF) for the submerged portion of a surface ship, plus separate descriptions of the propeller, shaft rudder and stern, plus hull locations for the shaft penetration and anodes points. A description of the shaft/propeller, rudder and stern geometries are contained in a separate file and CPBEM automatically merges these geometries rudder and stern geometries to the BEASY file. While CPBGEN has been used successfully in previous projects to develop models of naval vessels for analysis within BEASY, the geometry of CFAV Quest could not be accurately reproduced. The main difficulty was that CPBGEN uses 3D surface splines to develop the hull shape and could not reproduce the sharp angles required for the aft portion of the keel. The second difficulty was that CPBGEN assumes the propeller shaft intersects the hull at a single location whereas with CFAV Quest, the bossing surrounding the propeller shaft intersects the hull for several meters. While it would have be possible to modify CPBGEN to overcome these limitations, in the interest of time it was decided to develop the model within BEASY. The hull geometry (Figure 3.1) was developed using ship LOF data extracted from the MAESTRO structural model of CFAV Quest using MGDSA. The LOF data was manually converted into a BEASY “OUT” file format, containing definitions of the points and lines. The OUT file was then read into BEASY resulting in LOF at intervals of 2.4384 m. Each line was then divided into segments and surface patches generated between adjacent lines. Surfaces representing the angular section of the aft portion of the keel were then added. The hull geometry contains an opening for the propeller shaft bossing. The opening was determined by importing the bossing geometry and generating lines at the intersections between bossing surfaces and hull surfaces. The intersection lines were then used to modify the surface patches on the hull to produce the opening, and to clip the planes of the bossing where they extended through the hull. The geometries representing the propeller/bossing (Figure 3.2) and rudder (Figure 3.3) were developed separately. As a starting point some features of each component were developed within AutoCAD, exported as a DXF file, and then imported into BEASY as lines. These lines were then used to generate surfaces within BEASY. The propeller is represented as a disk with the equivalent surface area of the propeller blades and hub. The blade surface area was calculated based on an expanded area ratio of 0.461. The surface area of the propeller hub was estimated from drawings supplied by the technical authority. The bossing and rudder geometries were also based on supplied drawings.


The individual components were assembled to create the ship model shown in Figure 3.4. The hull geometry contained mesh refinement in the anode areas and near the extended keel and bossing in the aft portion. The hull geometry was generally imported followed by the propeller/bossing and rudder. The numbering of the propeller/bossing and rudder components were offset during the import to allow for easy identification once the model was assembled. Typically numbering of points, lines, surfaces, and elements of the propeller and bossing started at 2000, and for the rudder 3000. Locations of anodes, reference electrodes, and other details were determined from structural drawings of CFAV Quest. Drawings of the cathodic protection system show that the ship contains 4 impressed current anodes, 2 located in the aft section, and 2 located forward, and 2 reference electrodes, 1 aft, and 1 near the bow thruster. The ship is also equipped with 16 zinc sacrificial anodes; 4 in the bow thruster compartment, 8 in the seabays, and 4 in the discharge seachest. As the sacrificial anodes are located in enclosed spaces within the hull, they were not included in the model. The model also included a display surface at 14m depth. The display surface was 225m long and 50m wide, and extended 75 in front of the bow. The location of the hull relative to the display surface is shown in Figure 3.5. In some cases display surfaces were also added at depths of 28m and 56m water depth. Internal points on the surface were used to determine results along the longitudinal and athwart directions.

Figure 3.24 Hull geometry.


Figure 3.25 Propeller (disk) and shaft bossing geometry.

Figure 3.26 Rudder geometry.


Figure 3.27. Assembled boundary element model of CFAV Quest.

Figure 3.28. Position of model above display surface.

3.2 Polarization Data And Boundary Conditions Once the model was assembled, boundary conditions and material behaviour were assigned. The hull, rudder and bossing surfaces were treated as insulated surfaces (perfect paint). Anode currents were set by assigning a current density to the anode elements. The polarization curve assigned to the propeller disk is given in Table 9 below. The polarization data is a calibrated curve used in previous investigations of naval vessels that is based on the potentiostatic polarization curve for nickel aluminum bronze (C95800) from Hack et al [2]. The polarization for the actual propeller material, manganese nickel aluminum bronze (C95700), was not available in the open literature. However, as shown in Table 10, the compositions of the two alloys are similar, and as the cathodic reaction is controlled primarily by oxygen transfer and the buildup of calcareous deposits, the polarization behaviour is expected to be similar.


A water conductivity of 2.9 S/m was used for the analyses

Table 9: Polarization data for Propeller material Potential

(mV vs Ag/AgCl Current Density

(mA/m^2) -375 0 -400 37.3 -500 80 -600 132 -800 298

-1000 328

Table 10: Compositions of manganese nickel aluminum bronze (C95700) and nickel aluminum bronze(C95800).

Min./Max Cu Al Fe(1) Pb Mn Ni Si C95700 71.0 min 7.0-8.5 2.0-4.0 0.03 max 11.0-14.0 1.5-3.0 0.10 max C95800 79.0 min 8.5-9.5 3.5-4.5 0.03 max 0.8-1.5 4.0-5.0 0.10 max

3.3 Underwater Electric Field Prediction The model was used to predict the electric current and potential field surrounding the ship for a number of load cases provided by the Technical Authority. The load cases were total ICCP current of 2A, 4A, 8A, and 16A with 2 different ICCP configurations:

• aft and forward current densities equal (aft anode current 15% greater than forward anode current)

• ratio of aft anode current to forward anode current equal to 4:1 The predicted underwater potential fields for the above load cases are presented in the following sections.

3.3.1 Current Densities Figures 3.6 through 3.8 show the predicted underwater potential field strength at the 4 different of ICCP current levels with the current densities at the aft and forward anodes equal. Due to the variation in the anode element size the resulting aft anode current is 15% greater than the forward anode current. For this ICCP configuration the potential varies linearly with ICCP current.


Figure 3.29 Potential (mV) contour plots at 14m depth with current density

equal at the aft and forward anodes. ICCP current output (from top to bottom) 16A, 8A, 4A, and 2A. Note the 16A plot uses an expanded contour scale.


3.3.2 Aft to Forward Current ratio 4:1 Figures 3.9 through 3.11 show the predicted underwater potential at the 4 different of ICCP current levels with the aft anode current 4 times greater than the forward anodes current. Compared to the results of the previous configuration (Section 3.3.1) the predicted potential is reduced by approximately a factor of two, while the spatial variation is similar. Once again the potential varies linearly with ICCP current. At typical range depths, the peak longitudinal electric field is typically 1 mV/m while the peak athwart field is a factor of 2-3 smaller.


Figure 3.32 Potential (mV) contour plots at 14m depth with aft anode current 4 times forward anode current. ICCP current output (from top to bottom) 16A,

8A, 4A, and 2A. Note the 16A plot uses an expanded contour scale.


3.3.3 Variation with depth The variation in field strength was investigated for both of the ICCP configurations described above, with an ICCP current of 16A. Figures 3.12 and 3.13 show contour plots of the potential at depths of 14m, 28m, and 56m. The potential distributions in the longitudinal direction at these depths are given in Figures 3.14 and 3.15. The maximum and minimum potentials at these depths are plotted in Figures 3.16 and 3.17. Figures 3.16 and 3.17 also include the potential distribution along a vertical line below midship. The results indicate that the field strength deceases rapidly with depth and the profile of the potential field change shape with depth. The division of the total current between the aft and fore anodes has significant impact on the overall signature level. This suggests that the electric field signature can be minimized by optimally positioning anodes and selecting an optimal division of the total current.


Figure 3.35 Potential (mV) contour plots at depths of 14m (top), 28m (middle), and 56 m (bottom) with 16A total ICCP current, current density equal at the aft

and forward anodes.


Figure 3.36 Potential (mV) contour plots at depths of 14m (top), 28m (middle), and 56 m (bottom) with 16A total ICCP current, aft anode current 4 times the

forward anode current.


-25

-20

-15

-10

-5

0

5

10

15

20

25

-75-50-250255075100125150

Position Relative to Bow (m, negative is ahead of ship)

Pote

ntia

l at 1

4m d

epth

alo

ng c

ente

rline

(m

V, A

g/A

gCl)

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5Po

tent

ial d

eriv

ativ

e (m

V/m

)

Potential, 16A, 14m depthPotential, 16A, 28m depthPotential, 16A, 56m depthdV/dx, 14m, 16AdV/dx, 28m, 16AdV/dx, 56m, 16A

Figure 3.37 Potential and electric field at depths of 14m, 28m, and 56m in the

longitudinal direction with 16A total ICCP current, current density equal at the aft and forward anodes.

-25

-20

-15

-10

-5

0

5

10

15

20

25

-75-50-250255075100125150

Position Relative to Bow (m, negative is ahead of ship)

Pote

ntia

l at 1

4m d

epth

alo

ng c

ente

rline

(m

V, A

g/A

gCl)

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

Pote

ntia

l der

ivat

ive

(mV/

m)

Potential, 16A, 14m depthPotential, 16A, 28m depthPotential, 16A, 56m depthdV/dx, 14m, 16AdV/dx, 28m, 16AdV/dx, 56m, 16A

Figure 3.38 Potential and electric field at depths of 14m, 28m, and 56 m with 16A

total ICCP current, aft anode current 4 times the forward anode current.


4. Conclusions and Recommendations A static magnetic model of CFAV Quest has been produced for FLUX3D using the structural finite-element model of CFAV Quest. The model accounts for both the structural and non-structural components of the ship. Using this model, the Magnetic Anomaly Signature of CFAV Quest was predicted for several scenarios. An underwater electric potential model of CFAV Quest was created for prediction of the underwater electric potential field using BEASY. The model accounts for the hull-shaft metals and their polarization behaviour. The model was used to predict the underwater electric potential for two different ICCP configurations at various ICCP current output levels. The predicted results indicate that the electric potential varies linearly with ICCP current for a given configuration and the potential decreases rapidly with distance Recommendations for future work involves using the models developed in the current project to further investigate signature reduction measures for CFAV Quest. The variation of degaussing coil parameters, optimal positioning of additional degaussing coils, and the role of a permanent magnetization can be investigated using the static magnetic model developed for FLUX3D. The electric potential model can be used to investigate the effect of littoral geometry paint damage, and the influence of ship velocity and shaft grounding affects.


References [1] “FLUX3D Version 3.10 User’s Guide”, CEDRAT, Ref # K301-A-310-EN-03/00, March

30, 2000. [2] H.P. Hack, “Atlas of Polarization Diagrams for Naval Materials in Seawater”, Naval

Surface Warfare Center, Carderoc Division, CARDIVNSWC-TR-61-94/44, April 1995. [3] “BEASY User Guide”, BEASY Version 9.0, Computational Mechanics Inc.,

Southhampton, UK, 2004.


Annexes


Distribution list

List A (Full Report)

Internal (DRDC Atlantic)

6-Library

1-GL/ISM

1-GL/UEMS

1-Z. Daya

1-M. Birsan

1-B. Nelson

1-Y. Wang

Internal (DRDC Pacific)

1-D. Lenard

External

2- Martec Limited

1888 Brunswick Street, Suite 400

Halifax, NS B3J 3J8

Attn: Ken MacKay



DRDC Atlantic mod. May 02

DOCUMENT CONTROL DATA(Security classification of title, body of abstract and indexing annotation must be entered when the overall document is classified)

1. ORIGINATOR (the name and address of the organization preparing the document.Organizations for whom the document was prepared, e.g. Centre sponsoring acontractor's report, or tasking agency, are entered in section 8.)

Martec Limited1888 Brunswick Street, Suite 400Halifax, NS B3J 3J8

2. SECURITY CLASSIFICATION !!(overall security classification of the document including special warning terms if applicable).

UNCLASSIFIED

3. TITLE (the complete document title as indicated on the title page. Its classification should be indicated by the appropriate abbreviation (S,C,R or U) in parentheses after the title).

Electromagnetic Modeling of Canadian Forces Auxiliary Vessel (CFAV) Quest: Phase I Report

4. AUTHORS (Last name, first name, middle initial. If military, show rank, e.g. Doe, Maj. John E.)

Wallace, John and MacKay, Ken

5. DATE OF PUBLICATION (month and year of publication ofdocument)

June 2005

6a. NO. OF PAGES (totalcontaining information IncludeAnnexes, Appendices, etc).54

6b. NO. OF REFS (total citedin document)3

7. DESCRIPTIVE NOTES (the category of the document, e.g. technical report, technical note or memorandum. If appropriate, enter thetype of report, e.g. interim, progress, summary, annual or final. Give the inclusive dates when a specific reporting period is covered).

CONTRACT REPORT 8. SPONSORING ACTIVITY (the name of the department project office or laboratory sponsoring the research and development. Include address).

Defence R&D Canada – AtlanticPO Box 1012Dartmouth, NS, Canada B2Y 3Z7

9a. PROJECT OR GRANT NO. (if appropriate, the applicable researchand development project or grant number under which the documentwas written. Please specify whether project or grant).

9b. CONTRACT NO. (if appropriate, the applicable number underwhich the document was written).

W7707-04-2694 10a ORIGINATOR'S DOCUMENT NUMBER (the official document

number by which the document is identified by the originatingactivity. This number must be unique to this document.)

Martec Technical Report: TR-05-26, Rev 1

10b OTHER DOCUMENT NOs. (Any other numbers which may beassigned this document either by the originator or by thesponsor.)

DRDC Atlantic CR 2005-083 11. DOCUMENT AVAILABILITY (any limitations on further dissemination of the document, other than those imposed

by security classification)( X ) Unlimited distribution( ) Defence departments and defence contractors; further distribution only as approved( ) Defence departments and Canadian defence contractors; further distribution only as approved( ) Government departments and agencies; further distribution only as approved( ) Defence departments; further distribution only as approved( ) Other (please specify):

12. DOCUMENT ANNOUNCEMENT (any limitation to the bibliographic announcement of this document. This will normally correspond to theDocument Availability (11). However, where further distribution (beyond the audience specified in (11) is possible, a wider announcementaudience may be selected).

DRDC Atlantic mod. May 02

13. ABSTRACT (a brief and factual summary of the document. It may also appear elsewhere in the body of the document itself. Itis highly desirable that the abstract of classified documents be unclassified. Each paragraph of the abstract shall begin with anindication of the security classification of the information in the paragraph (unless the document itself is unclassified) representedas (S), (C), (R), or (U). It is not necessary to include here abstracts in both official languages unless the text is bilingual).

The underwater electromagnetic signatures of steel-hulled vessels with active signaturereduction measures can be accurately modelled by commercially developed softwarepackages. Static magnetic signatures can be computed with FLUX3D which is a sophisticatedfinite element code developed by Cedrat Groupe, Grenoble, France. Underwater electricpotential can be suitably modelled using boundary element codes such as BEASY, developedby BEASY Group, Southampton, UK.

DRDC has a requirement to demonstrate magnetic and electric signature prediction for theresearch vessel CFAV Quest. This report describes the work by Martec Limited to developunderwater electric potential and static magnetic models of CFAV Quest.

A static magnetic model of CFAV Quest has been produced for FLUX3D using the structuralfinite-element model of CFAV Quest. The model accounts for both the structural and non-structural components of the ship. Using this model, the Magnetic Anomaly Signature of CFAVQuest was predicted for several scenarios.

An underwater electric potential model of CFAV Quest was created for prediction of theunderwater electric potential using BEASY. The model was used to predict the underwaterelectric potential for two different Impressed Current Cathodic Protection (ICCP) configurationsat various ICCP current output levels. The predicted results indicate that the electric potentialvaries linearly with ICCP current for a given configuration and the potential decreases rapidlywith distance.

14. KEYWORDS, DESCRIPTORS or IDENTIFIERS (technically meaningful terms or short phrases that characterize adocument and could be helpful in cataloguing the document. They should be selected so that no security classification isrequired. Identifiers, such as equipment model designation, trade name, military project code name, geographic location mayalso be included. If possible keywords should be selected from a published thesaurus. e.g. Thesaurus of Engineering andScientific Terms (TEST) and that thesaurus-identified. If it not possible to select indexing terms which are Unclassified, theclassification of each should be indicated as with the title).

Finite element, boundary element, magnetic models, underwater electric potential, FLUX3D,BEASY


Date post:	14-Sep-2018
Category:	Documents
Upload:	vanhuong
View:	214 times
Download:	0 times

Electromagnetic Modelling of Canadian Forces Auxiliary...

Documents