Underwater Signature Management Solutions

Transcription

1 Underwater Signature Management Solutions Samantha Davidson Ultra Electronics PMES, United Kingdom INTRODUCTION The electromagnetic signature management process for a new vessel design begins at the concept stage. This can include, for example, evaluating the signature effects of the choice of drive type and hull coating. The electromagnetic design team are involved throughout the vessel design process determining the signature effect of changes proposed by the Naval Architect. Often the end customer specifies the target signatures to be achieved with the contractor taking complete signature responsibility. Once the vessel is in-service the signature management process continues with configuration of onboard electromagnetic signature reduction systems and periodic signature measurement and reconfiguration to ensure the signature is maintained below signature target through its lifetime. Signature measurement can be undertaken utilising multi-influence sensors (magnetic, electric, ELFE, acoustic, seismic). Over the last decade there has been much technological progress that has enabled the development of combined signature measurement ranges systems that gather electromagnetic, acoustic and other influence measurements on the same range system or upgrades which allow new influences such as electric field to be gathered at the same location as an existing magnetic and acoustic range. Acoustic bandwidths up to 100kHz may be gathered simultaneously with ELFE signatures up to 3kHz. A compact sensor package that is easily deployable has many applications for both commercial and commercial applications to range ships, submarines, ROVs and divers and also key in the field of hydrocarbon exploration. Rapid system deployment and straightforward user friendly data analysis are important features. VESSEL SIGNATURE MANAGEMENT Signature Management is a key element of operational capability. The primary goal of signature management technology is to reduce the likelihood of detection and thereby to increase survivability in the operational field. There are three prime requirements for vessel signature management: Vessel design for low signature vessel ( stealth ) Signature optimisation Threat prediction A vessel may be designed by its construction and countermeasures with regards to stealth. For the electromagnetic influence threats the design is generally achieved using finite element and boundary element methods. Finite element methods are used to model the ferromagnetic structure and the countermeasures (degaussing coils), whilst boundary element methods are

2 used to model electric currents flowing round the vessel which cause both electric and corrosion related magnetic (CRM) fields. Once built the electromagnetic signature for every vessel will be different. For the ferromagnetic components, the resultant magnetic field will be dependent on the handling of the magnetic materials used in its construction and the location or locations of its construction. This signature will then change gradually over time depending on the vessel s usage and transits made. The electric signature component will initially tend to be much more similar from vessel to vessel in a class but will over time diverge as the hull paint is damaged during its particular usage. In order to maintain a vessel s electromagnetic signature within target at the threat depth a knowledge of that vessel signature is required. This signature may be measured directly using an underwater range or alternatively inferred from knowledge of magnetic fields and electric hull potentials at or close to the vessel itself. The use of a modelling process enables signatures to be predicted at other locations other than the measurement points. REDUCTION OF MAGNETIC SIGNATURES Magnetic signatures can be reduced in several key ways that extends from initial design to through-life signature maintenance. The intrinsic magnetic signature of any vessel depends intrinsically and significantly on the vessel structure, material composition and design. These parameters are largely constrained by the operational requirements of the vessel. However as part of the design concept, system solutions can be put in place that will enable a high degree of signature reduction to be obtained using either internal of external solutions. In this paper we consider the following: Magnetic treatment or deperming Open loop onboard DG and ICCP system Multi-influence ranging Closed loop solutions MAGNETIC TREATMENT SYSTEMS A straightforward method of signature reduction that can be deployed even after the vessel design and build phases is a magnetic treatment process in which a sequence of large alternating magnetic fields are applied to a vessel via an external magnetic coil. This method can of course also be in used in conjunction with a permanently fitted onboard degaussing system and in this scenario can result in both power and cost savings in relation to the onboard signature reduction system. Magnetic treatment systems vary in scale, infrastructural complexity and cost. For many vessel types comparable performance can be achieved from any of the various common systems including the following coil system types: Over-run Drive in Close-wrap The choice of facility type is therefore largely dependent on the relative priority of cost, convenience and available real estate. Close wrap systems are the only systems that can be

3 configured in a transportable mode whereas over-run and drive in treatment systems are necessarily fixed systems. A facility type that is convenient for many locations and has the flexibility to treat many different ship types is the redeployable close wrap facility. The system can be made more flexible by having power supplied from transportable diesel generators removing reliance on dock yard power supplies and allowing greater freedom of deployment location. A typical system is shown in Figure 1 below. Figure 1. Actual System and Schematic Drawing of a Redeployable Magnetic Treatment System ONBOARD SIGNATURE CONTROL SYSTEMS Magnetic and Acoustic Influence Measurement Ranges Onboard open loop degaussing systems comprising current carrying coils within the vessel s structure can result in optimised magnetic signatures when the coil currents and/or turns are configured using a magnetic ranging facility. A modelling range comprises of typically 3 to 5 three axis magnetic sensors, which can be easily and speedily deployed on the seabed [Ref 1, 2]. Additional influences can alos be integrated or co-located. A typical Transmag periodic ranging uses vessel modelling techniques that require only two DG-off runs across the range to enable the signature to be minimised (once a first of class ranging has been completed for a vessel of the same class). This means that a typical ranging would be complete in approximately one hour. In contrast the manual signature optimisation technique often takes one day for a frigate and up to three days for a new MCMV. The main functions provided by ranging software are to enable modelling of the magnetic state of the vessel, to determine the magnetic effect of the vessel s degaussing coils and to optimise the ship s DG-on signature by calculating the optimum degaussing coil current settings. In addition the models can be used to calculate the threat to the vessel worldwide. A range may be used to determine the optimum DG-on state for a vessel for a particular local scenario.

4 This may be undertaken either at a range site prior to operational deployment or indeed by using a forward area range in the operational area to confirm the optimum vessel state has been achieved and maintained. Note that in contrast closed loop systema, which are considered later in this paper, will maintain the signature within a certain tolerance band but that its performance is governed by the quality of the input data gathered on the range and the data measured onboard the vessel. Thus it may often be possible to improve on the signature in the forward area for critical operations using a range. Redployable Measurement Ranges Transportable range systems are useful for forward area military measurement missions and also can be used at ship yard locations for verification of acoustic signatures of commercial platforms. Deployability is a key concept when considering sensor design. Compact sensors clearly can achieve improved man-handleability over traditional systems and can be deployed without the requirement for a large deployment vessel or crane. This provides the benefit of ease of deployment and recovery for transportable systems or as part of maintenance programme in a fixed mount system. Figure 2 Set up of a transportable range system office and tracking basestation

5 Figure 3 A Magnetic and Acoustic Influence Measurement Sensor in its Tripod Electric Field Ranges Historically magnetic and acoustic influences have been most commonly measured. In recent years the electric influence has come to the fore in both defence and hydrocarbon survey fields with experimental very low noise sensors being trialled [Ref. 3, 4] for detection of electric fields. Electric field ranges have been developed using 3-axis electric field sensors which can be easily deployed in parallel to or combined with an existing magnetic and acoustic range. Each sensor comprises 3 orthogonal pairs of silver-silver chloride electrodes and has been mounted on a flat concrete mattress to create a flat seabed area around each sensor to minimise distortion of the electric field by its mounting. The low noise electrodes can provide high accuracy electric field measurements in a spherical housing of 0.25m diameter; this is a considerable improvement over previous sensor technologies. Each sensor has a digital interface allowing Ethernet communications to an underwater network hub. The electric field range has been deployed in a line parallel to an existing magnetic and acoustic range which comprises eight multi-influence sensors. The magnetic and acoustic sensors have analogue outputs which are converted to digital signals at an underwater junction box. The two ranges are connected into a common fibre optic cable at a fibre optic interface box. The electric range has been developed such that it can be run as a standalone range or in parallel to the existing multi-influence range such that all three influences can be gathered at the same time.the development of compact redeployable high sensitivity, high bandwidth multi-influence digital sensors has only recently become a viable proposition. Sensors are can be supplied in magnetic only, electromagnetic or combined magnetic/electric/acoustic formats.

6 Figure 4 Compact Sensor with 3-axis electric field in Ultra s production test facility Electric Signature Field Control The interaction of a vessel hull with its environment, notably the sea water gives rise to corrosion where metallic areas of the hull are exposed. The corrosion of a vessel can be minimised by the suitable application and maintenance of coatings. Since coatings cannot be applied or maintained perfectly an impressed cathodic corrosion protection system, ICCP, are used in order to impress current on to the hull such that the relative potential of hull relative to the seawater is such that corrosion is rendered energetically impossible. Corrosion and the action of the anodes from the ICCP system lead to ionic current flow in the seawater and therefore there will be also be an associated electric and corrosion related magnetic, CRM field, due to the vessel in the seawater. The electric field and CRM of a vessel poses a potential threat to it from third party detection and thus it needs to be controlled and minimised. To enable the control or optimisation of the electric field or CRM, from a vessel it is necessary to have a detailed knowledge of the relative hull to seawater electrical potential or corrosion hull state. Knowledge of the vessel s corrosion hull state is necessary for the control of the signature because the systems placed on a vessel to control the signature will also impact on the hull state and thus affect its corrosion protection. Furthermore it is the hull state that ultimately determines the vessel s electric signature and also the majority of electric signature control systems employed are the same type of systems used to prevent corrosion of the vessel. The corrosion hull state of vessel continually changes with time due to both the physical state of the hull and its interaction with the environment, the seawater. Thus in order to control the electric signature of a vessel it is necessary to routinely characterise the corrosion hull state of a vessel. Numerical modelling techniques combined with onboard and off board ship measurement can be used to characterise the vessel hull corrosion state and have been shown to control the electric signature. Similar principles can also be applied to producing closed loops systems. CLOSED LOOP DEGAUSSING SYSTEMS Closed loop degaussing (CLDG) systems can be used to maintain the ferromagnetic signature of a vessel over extended periods of time. We shall not discuss possible CLDG methodologies

7 in any detail here other than to list several alternative methods that might be used to create such a system including: a) Vessel State Methods b) Compartment Modelling c) Enhanced Source Modelling d) Boundary Element Methods. In the following section we will look at an example of the development of a boundary integral method for CLDG. Boundary Integral Modelling Boundary Integral (BI) based algorithms are finding applications in vessel modelling especially in, for example, CLDG where the measurements are made close to the steel surface. At the heart of the BI method is an algorithm which calculates the keel signature from the field measurements at the hull. This type of algorithm is based on two main principles: Green s theorem; an application of which states that if the flux of a field is known all over a closed boundary, then the field elsewhere, external to the boundary can be calculated from this provided there are no other sources of flux in the domain external to the boundary; and the definition of a scalar magnetic potential which is possible because over a short period of time the field can be said to be magnetostatic. In practice, of course, for most practical implementations where sensors may be fitted, there will be sources external to the measurements. We may justify this by re-wording the Green s theorem statement to say that there are no sources which change significantly external to the measurement locations. In the case of CLDG, the field to be calculated in the domain is that of the threat signature and the boundary encloses the majority of the ferromagnetic material in the vessel. The flux is measured by magnetic sensors which measure the magnetic field. The number of sensors used is chosen to be both acceptable in an engineering context and also to be able to encompass the majority of the ferromagnetic material and hence represent all of the flux over the vessel. Again, in practice, this will be a compromise between coverage and practicability, and techniques must be employed to fill-in those areas where sensors cannot be located. The actual creation of the BI mesh is relatively straightforward as the geometry of the vessel is basically known and for submarines can be considered as combinations of cylindrical and spherical surfaces (Figure 5).

8 Figure 5 - Basic BI Formulation for Simple Hull Boundary Integral Results As stated above, there is a significant problem in the generation of data for testing algorithms designed to take on-board measurements as their input data. It is possible to carry out initial development using FE representations using models with a mesh high mesh complexity. Whilst the input data is not that of any specific vessel, sufficient variation can be applied to the material characteristics of the model to provide a suitable test for any algorithm under investigation (Figure 6). High Magnetization Area Figure 6 - Simple FE Model Magnetization Distribution Used in BE Evaluation Using such models it is possible to quickly investigate the optimum number of sensors required, the optimum distribution of these sensors and, perhaps more importantly, the consequence for performance of the effect of not locating the sensors in the ideal locations. Results from this type of modelling (Figure 7) can demonstrate the readiness of an algorithm to be tested against physical (real) data. In this case we illustrate the x and z signature components of the BI and FEM signatures at the depth of interest beneath the keel line of the vessel where the x-axis is aligned with the length of the vessel and the z-axis is vertically aligned.

9 Figure 7 - Comparison of BI and FEM Keel Signatures As has been previously stated, comparisons with real vessel signatures are more problematic. Obtaining contemporaneous close-in and range data measurements so that comparisons of the predicted and actual keel (threat) signatures can be made have been found to be difficult without suitable transportable ranges. Such data is invaluable however in comparing the magnetization distribution within real vessel steel and the FE approximation (Figure 8) and for confirming that the algorithms (in this case the BI algorithm) are effective in the real world (Figure 9). The three axes in Figure 9 represent the three signature components x, y and z for the measured signature and that predicted from the BI model at the depth of interest. This figure clearly demonstrates that the closed loop method is a valid choice as a means of magnetic signature control. Figure 8 - Magnetization Distribution on Real Hull cf. Figure

10 Figure 9 - Comparison of Actual (solid line) and BI Predicted (dotted line) Signatures CONCLUSIONS In this paper we have discussed Underwater Signature Management is an integrated process that extends from initial design to through-life signature maintenance with stages including:. Electromagnetic concept evaluation Magnetic treatment via deperming process DG and ICCP system design and through life signature control Multi-influence ranging Closed loop solutions We have looked at the use of Multi-influence ranges to configure both degaussing coils and ICCP systems to minimise the total electromagnetic signature. We have also shown that for closed loop systems with sensors fitted to the vessel hull, accuracies sufficient to determine the signature threat can be found using boundary integral (BI) methods. Using CLDG based algorithms ranging data can be supplemented using BI methods specifically to enable very near field measurements near the hull to be used in predicting threat depth signatures. REFERENCES 1. S.J.Davidson et al, 1994, Transmag a transportable degaussing range, UDT Europe S.J.Davidson et al, 2009, Electromagnetic Underwater Signature Management: Signature Design Techniques and Redeployable Measurement Systems, Marelec H.Jones, S.J.Davidson, P.Rawlins, 2004, Development of a Low Noise Electric Field Sensor For Measurement and Ranging Applications, Marelec Lucy MacGregor, 2004, Detecting and Characterising Hydrocarbon Reservoirs Using Marine Active Source Electromagnetic Sounding, Marelec 2004.