Tow Ship Research Articles

Methods of suppressing tow ship noise with a horizontal linear array

Tow ship noise seriously influence the detection and direction of arrival (DOA) estimation performance of towed array. Null steering beamforming(NSBF), inverse beamforming(IBF) and non-data based beam-space transformation(NDBBT) are applied to suppress interference of tow ship in this paper. These methods are evaluated according to the improvement of signal interference noise ratio and DOA estimation precision. Simulation and experimental results show that all three methods can suppress interference of tow ship effectively so as to improve DOA estimation precision. In addition, IBF and NDBBT do better in suppressing interference than NSBF.

On the structure and dynamics of stratified wakes generated by submerged propagating objects

ABSTRACTThe structure and intensity of the intermediate wake generated by a submerged propagating body in a stratified fluid was studied using a combination of (i) numerical simulations, (ii) field measurements, and (iii) laboratory experiments. The numerical component offered guidance for the field work performed in Monterey Bay (CA, USA) in the summer of 2015. The field work focused on subsurface thermal signatures of a submerged propagating object. Vertical temperature profiles suggested that long-term changes in thermal stratification can occur after the passage of a towed body. Horizontal temperature variability, measured by an autonomous underwater vehicle facilitated the identification of the wake using perturbation temperature variance as the key diagnostic variable. Analogous thermal signatures of stratified wakes were found in ocean observations and in modelling results. The influence of the tow ship on the wake was shown to be minimal. Laboratory experiments focused on the surface expression of stratified wakes were used to complement numerical simulations and field measurements. All three components of this project indicate that detection of the wake of a submerged object based on its thermal signatures is a viable and effective approach.

Depth Enhancement of an Underwater Towed System using Hydrodynamic Depressor

<p>The underwater towed system described here consists of tow cables, a towed body, an acoustic module and tail rope towed behind a surface ship. The required depth at a particular speed of the towing ship is obtained by paying out specified length of cable from the winch. However the excessive drag forces on the various components of the towed system results in impractically large values of cable length, especially at higher speeds. A hydrodynamic depressor is designed to improve the depth performance. The design is evolved based on numerical analysis and towing tank tests. Estimation of depth attained is carried out based on steady state theory of tow cables. Validation of the numerical analysis results is carried out through field evaluation of depressor performance during sea trial of the towed system.</p>

Couple Model for Predicting Behaviors of Underwater Towed Systems under Effect of Heave and Pitch Motion of Ship

Underwater towed systems have the many applications in exploitation of underwater environments. While in the research exists which is about the numerical models of towed system, the relatively simple model of the towed body is used and the dynamic of the towing ship is neglected. The towing ship as part of towed system, its heave and pitch motion are coupled and highly non-linear. Considering the dynamic of three parts of towed system is important. In the paper, a newly couple model is developed to predict the behaviors of underwater towed systems under effect of the heave and pitch motion of towing ship. We use a finite difference method as the towed cable and is coupled with the non-linear numerical models of a towing ship and a towed body. Illustrative application of the coupling model is presented. The numerical results show the heave and pitch motion of towing ship affect the motion of towed body.

Modeling three-dimensional propagation in a continental shelf environment

An acoustic propagation model is applied to predict measurements of three-dimensional (3-D) effects recorded off the southeast coast of Florida. The measured signal is produced by a low frequency source that is towed north parallel to the shelf from a fixed receiving array. The acoustic data show the direct path arrival at the bearing of the tow ship and a second refracted path arrival as much as 30° inshore of the direct arrival. Notably, the refracted arrival has a received level more than 25 dB greater than that of the direct arrival. A geoacoustic model of the environment is created to explain the data. It is shown that the topography of the seafloor plays the largest role in controlling horizontal refraction effects, whereas the range-dependent sediment properties have the most influence on the received level. The modeling approach is based on a 3-D adiabatic mode technique in which the horizontal refraction equation is solved using a parabolic equation in Cartesian coordinates. A modal decomposition of the field provides insight into the variability in the arrival angle and received level of the measured signal.

The Simulation of Tow Ship Noise on Towed Linear Array

Based on the acoustical properties of the tow ship noise, this paper studies the effect of tow ship noise on the towed linear array sonar system by computer simulation. The tow ship noise which is fairly approximate to the truth tow ship noise is obtained by the typical mathematical mode simulation firstly. Then the near-filed characteristics of the tow ship noise are simulated by using a simplified channel model. Finally, this thesis studies the interference of tow ship noise through beam-forming. The simulation describes acoustic characteristics of the tow ship noise well, and also provides a good theoretical support for the other simulation systems designs.

正面向波中における曳船・被曳船の船体運動シミュレーション

A time domain simulation method for wave-induced motions of tow and towed ships in head seas is presented. The present method uses a 2D lumped mass method for expressing the dynamics of towline, and so can take into consideration the effects of self-weight and deformation of towline and the fluid resistance acting on it. The present method can calculate not only wave-induced motions of tow and towed ships but also tension acting on a towing point in time domain. In order to validate the present method, using a ship model with twin podded propellers as tow ship and SR108 container ship model as towed ship, the simulations of wave-induced motions of tow and towed ships and tension acting on a towing point were made in various wave lengths and the calculated results were compared with the model test results. As a result, although there is some room for improvement, the present method can roughly capture the wave-induced motions of tow and towed ships and tension behavior during towing in head seas.

Course Stability of a Ship Towing System

This paper proposes a numerical model for course stability of a towed ship coupled to tow ship through towline using both nonlinear and linear approaches. Captive model tests were conducted at the towing tank to obtain hydrodynamic derivatives of both tow and towed ships. Effects of unstable towed ship (2B) and stable towed ship (2Bs), towline, tow points and rudder autopilot parameters at tow ship were treated in the numerical analysis. The simulation results demonstrated better course stability of 2Bs compared to 2B. A longer towline and shifting the tow point towards the bow both for 2B and 2Bs made the towing system more course stable, which is indicated through attenuation in slewing motion. Lateral motion of the tow ship increased gradually with shifting the tow point forward of the ship’s centre of gravity. This degrades the course stability of the towing system. The rudder action enforced by the autopilot improved the course stability. In general, the increase of the tow ship’s dimension was still insufficient to improve the course stability of the ship towing system.

Dynamic modeling of cable towed body using nodal position finite element method

This paper analyses nonlinear dynamics of cable towed body system. The cable has been modeled and analyzed using a new nodal position finite element method, which calculates the position of the cable directly instead of the displacement by the existing finite element method. The newly derived nodal position finite element method eliminates the need of decoupling the rigid body motion from the total motion, where numerical errors arise in the existing nonlinear finite element method, and the limitation of small rotation in each time step in the existing nonlinear finite element method. The towed body is modeled as a rigid body with six degrees of freedom while the tow ship motion is treated as a moving boundary to the system. A special procedure has been developed to couple the cable element with the towed body. The current approach can be used as design tool for achieving improved directional stability, maneuverability, safety and control characteristics with the cable towed body. The analysis results show the elegance and robustness of the proposed approach by comparing with the sea trial data.

Transient behavior of towed cable systems during ship turning maneuvers

The dynamic behavior of a towed cable system that results from the tow ship changing course from a straight-tow trajectory to one involving steady circular turning at a constant radius is examined. For large-radius ship turns, the vehicle trajectory and vehicle depth assumed, monotonically and exponentially, the large-radius steady-state turning solution of Chapman [Chapman, D.A., 1984. The towed cable behavior during ship turning manoeuvers. Ocean Engineering 11, 327–361]. For small-radius ship turns, the vehicle trajectory initially followed a corkscrew pattern with the vehicle depth oscillating about and eventually decaying to the steady-state turning solution of Chapman (1984). The change between monotonic and oscillatory behavior in the time history of the vehicle depth was well defined and offered an alternate measure to Chapman's (1984) critical radius for the transition point between large-radius and small-radius behavior. For steady circular turning in the presence of current, there was no longer a steady-state turning solution. Instead, the vehicle depth oscillated with amplitude that was a function of the ship-turning radius and the ship speed. The dynamics of a single 360° turn and a 180° U-turn are discussed in terms of the transients of the steady turning maneuver. For a single 360° large-radius ship turn, the behavior was marked by the vehicle dropping to the steady-state turning depth predicted by Chapman (1984) and then rising back to the initial, straight-tow equilibrium depth once the turn was completed. For small ship-turning radius, the vehicle dropped to a depth corresponding to the first trough of the oscillatory time series of the steady turning maneuver before returning to the straight-tow equilibrium depth once the turn was completed. For some ship-turning radii, this resulted in a maximum vehicle depth that was greater than the steady-state turning depth. For a 180° turn and ship-turning radius less than the length of the tow cable, the vehicle never reached the steady-state turning depth.

Noise suppression using the coherent onion peeler

An innovative noise suppression algorithm, called the coherent onion peeler (COP) (patent detained) is derived by minimizing the error in the coherent subtraction of the data from a single plane wave model. The COP solution "collapses" for all beamforming algorithms to the same solution at the complex hydrophone FFT level. Thus, any beamformer can be applied to the residual FFTs after the COP has been applied. The COP is applied to the strongest interferer first and the residual FFTs represent the acoustic field with this interferer coherently "peeled back" like the outer skin of an onion. This procedure can be repeated any number of times to coherently suppress the noise from all interferers present. A new nonadaptive beamformer is developed that "blends" together the best beam pattern properties of conventional beamforming (CBF) near array design frequency and the generalized fourier integral method (GFIM) at very low frequencies (VLF). In between these frequency extremes, the GFIM-CBF Blend (patent pending) (G-C Blend) is a frequency-dependent weighted average of both individual beamformers. COP and G-C Blend are applied to the broadband noise emitted by the tow ship, which severely degrades the performance of the towed array; COP reduced this broadband noise by over 19 dB in a single pass.

最適曳航支援システムの性能評価

When disabled ship accidents occur, optimum towing operation for safety is important to prevent secondary accident such as collision and grounding, which induce oil spill leakage after drifting. Optimum Towing Support System (OTSS) is a simulation program, which has been developing by the National Maritime Research Institute to calculate towing condition of tow and towed ships under waves, wind and current. OTSS can predict drifting motion of disabled ship, trajectory of tow and towed ships, towline tension and so on. In order to investigate applicability of OTSS to the actual ship on the sea, towing test by using patrol vessels of the Japan Coast Guard was carried out. The results of straight and turn towing in calm and rough seas are compared with the calculated results by OTSS, and performance for OTSS is evaluated. These results demonstrate that predicted results of towing motion in wave by OTSS are in a good agreement with the measured results rather than those in calm sea. Based on experimental results of towing test in turning, turning characteristics of tow and towed ships during towing are clarified. Moreover, influence of towing on turning performance of tow ship is indicated in comparison with turning test results of a single ship. These results obtained by towing tests show that OTSS is available for the prediction of towing condition in rough sea.

Towed acoustic source array system for marine applications

A marine towed array has a plurality of tow bodies of positive buoyancy each containing a marine acoustic source, such as a vibrator source. Each tow body uses a reel containing an electro-mechanical tow cable connecting the sources, whereby reeling and unreeling of the cable, controlled from a towing ship, changes the spacing of the tow bodies. One of the tow bodies at an end of the array is attachable to the towing ship.

New towed-array shape-estimation scheme for real-time sonar systems

In real-time towed-array systems, performance degradation of array gain occurs when a line array that is not straight is assumed straight in the beamforming process. In this paper, a new method is proposed for array shape estimation. The novelty of this method is that it accounts for the variations in the tow ship's speed, which are typical during course alterations. The procedure consists of two steps. First, we solve for the tow-point induced motion in the time domain based on the constraints from the tow-point compass-sensor readings and from a discretized Paidoussis equation. At each time instance, the shape estimate is solved from a linear system of equations. We also show that this solution is equivalent to a previous frequency-domain solution while the new approach is much simpler. In the second step, we use the tail compass-sensor data to adjust the overall array shape. By noting that variations in the ship speed lead to a distortion in the normalized time axis, we first register the predicted tail displacement with the tail sensor readings along the time axis. Then, distortions in the estimated array shape over its length can be compensated accordingly. We also model a slow-changing bias between sensor zeros and remove systematic sensor errors. The effectiveness of the new algorithm is demonstrated with simulations and real sea-trial data.

Experimental investigation on a two-part underwater towed system

An experimental set-up is developed and proved to be effective for laboratory study of an underwater towed system. The experimental technique gives a practical method for monitoring the kinematic and dynamic performance of an underwater towed system in a ship towing tank. Both the theoretical and experimental results in the investigation indicate that the hydrodynamic response of a towed vehicle to the wave induced motion of a towing ship can be significantly reduced by applying a two-part tow method. A comparison of the numerical and experimental results in the investigation demonstrates that the numerical simulation results are close to the experimental data, overall agreement between experimental and theoretical results is satisfactory. The results qualitatively verify the mathematical model of a two-part underwater towed system proposed by Wu and Chwang [Wu, J., Chwang, A.T., 2000. A hydrodynamic model of a two-part underwater towed system. Ocean Engineering 27 (5), 455–472].

Volume acoustic backscattering measurements with a 68-kHz cylindrical array

Most sensors designed to measure volume acoustic backscatter over the entire water column have limited horizontal coverage. The 68-kHz cylindrical arrays of the U.S. Navy’s Toroidal Volume Search Sonar (TVSS), towed 80 m below the sea surface in 200-m water depth, have been used to measure volume backscatter over the entire water column with about 4.5 deg of angular resolution over a swath roughly 300 m wide. The data were analyzed to determine the volume acoustic backscattering strengths and cross sections, and the target strengths of (1) near-surface bubble clouds in the tow ship wake; (2) midcolumn zooplankton layers; and (3) near-bottom fish schools. Unlike previous acoustic studies of ship wakes, these results are obtained from data which sample the bubble field at a constant distance from the ship. Echo integration over a midcolumn scattering layer is used to assess the patchiness of zooplankton in the region. Target strengths of near-bottom fish schools are easily distinguished from the surrounding seafloor backscattering strengths. [Work sponsored by ONR-NRL (Contract No. N00014-96-1-G913).]

A hydrodynamic model of a two-part underwater towed system

A three-dimensional model of a two-part underwater towed system is studied. In the model, the governing equations of cables are established based on the Ablow and Schechter method. The boundary conditions for the two-part underwater towed system are derived. The six-degrees-of-freedom equations of motion for submarine simulations are adopted to predict the hydrodynamic performance of a towed vehicle. The established governing equations for the system are then solved using a central finite difference method. In this paper several algorithms are used to solve this special form of finite difference equations. The results in this paper indicate that the two-part underwater towed system improves the dynamic behavior of the towed vehicle and is an easy way to decouple the towing ship motion from the towed vehicle. Because the model uses an implicit time integration, it is stable for large time steps and is an effective algorithm for simulation of a large-scale underwater towed system.

The U-Tow: a system for sampling mesozooplankton over extended spatial scales

A new system is described for sampling mesozooplankton over extended spatial scales. This system, called the U-Tow, is small (-1 m long), can be towed at high speed (>10 knots) and collects mesozooplankton in up to SO discrete samples on a roll of filtering mesh. The spatial resolution of samples is dictated by the advance interval of the filtering mesh, which, in turn, is software controlled. So, for example, if each sample provides an integrated catch over 20 km, a single deployment may cover up to 1000 km. The U-Tow can carry a range of sensor packages with data being stored either in an internal data logger or transferred in real time up a conducting cable to be logged on the towing ship. Additionally, the U-Tow has the capability to undulate and hence sample at a range of depths. Initial results from deployments in the NE Atlantic and Irish Sea are described.

Reverberation rejection with a dual-line towed array

Low-frequency bottom reverberation from explosive sources was measured with a dual-line horizontal towed array during a 1991 system engineering trial. The receiver's parallel line arrays were processed coherently in order to unambiguously resolve the reverberation field in azimuth. Initial beamforming that assumed a straight-and-parallel array configuration led to poor or erroneous ambiguity resolution, suggesting the system was deformed significantly from the assumed shape. In the absence of reliable shape measurement data, the array shape was deduced acoustically using tow ship signals and the direct blast of an explosive source. The estimated shape was then used in a shape-compensated beamformer to produce azimuthally unambiguous estimates of the reverberation field. Backlobe rejection of more than 20 dB was attained for both direct blast and distant reverberation energy.

Shape calibration for a nominally linear equispaced array

We consider a thin flexible line array of equispaced hydrophones that is towed through the sea and develop a statistical test that allows us to decide whether the array is straight or not. The motion of the towing ship, the ocean currents and other forces induce deformations on the array and affect the performance of spatial processing of the data developed under the assumption that the array is straight. When the ship is maneuvering, the processing is generally turned off for long periods of time, an extremely penalizing situation that can be overcome by applying our scheme. Indeed, if the test-applied to the whole array-decides that it is out of shape, the same test is applied to parts of the array to determine the maximum size of admissible subarrays on which the standard processing can be pursued. By combining the bearings estimated by the different subarrays, we reconstruct a piecewise linear estimate of the shape of the array. The approach allows the handling of quite important deformations with no need for a cooperating source.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>