Anne M. Fullerton

Shipboard Measurement of Ocean Waves

Over the past several years a number of techniques have been utilized for the measurement of ocean waves from shipboard platforms. These systems have ranged from commercial off the shelf (COTS) navigation radar and Light Detection and Ranging (LIDAR) systems to specially developed in-house instrumentation systems. Most of these systems have been utilized to measure the directional wave spectra around the ship. More recently, the Naval Surface Warfare Center, Carderock Division (NSWCCD) and others have begun to utilize these techniques for shipboard measurement of individual ship generated waves as well as open ocean waves. NSWCCD has used a number of these methods on various Office of Naval Research (ONR) and Naval Sea Systems (NAVSEA) sponsored field tests. These field tests were performed on a variety of naval platforms over a range of sizes, including some fixed platforms, for various sea states. While each of these tests has had individual measurement goals and objectives, the series of tests has also provided an environment for testing and developing new instrumentation and exploring their capabilities. As a result of these efforts, instrumentation has grown in sophistication from qualitative video-based observations of the wave field around an underway vessel to laser and radar based imaging and ranging measurements of free surface dynamics. This work has led to higher fidelity data, as well as data that were previously unobtainable. In this paper we provide an overview of these systems and techniques and summarize the basic capabilities of each method by providing measurement examples/applications. These systems include a shipboard array of ultrasonic distance sensors for measuring directional wave spectra, a COTS wave radar system, and a COTS scanning LIDAR system. While not intending to be exhaustive, this paper seeks to highlight the insights gained from the recent applications of these techniques, as well as the difficulties and issues associated with shipboard measurements such as ship motion and logistical constraints.

Measurement and Modeling of the Motions of a High-Speed Catamaran in Waves

Wetdeck slamming can be defined as a large vertical acceleration event that occurs when ship motions cause an impact between the cross deck and the ocean’s surface. The use of Computational Fluid Dynamics (CFD) and other simulation tools to accurately predict wetdeck slamming loads and ship motions has become the objective of a number of efforts (Hess, et al, 2007; Lin, et al, 2007; Faller et al, 2008; for example). The Sea Fighter, FSF-1, is a high-speed research vessel developed by the U.S. Office of Naval Research (ONR). Christened in 2005, she is an aluminum catamaran propelled by four steerable water jets capable of speeds up to 50 knots. In 2006, Sea Fighter underwent a series of rough water trials to assess its operational profile in high sea states (Fu, et. al., 2007). Along with this assessment, ONR sponsored an effort to obtain full-scale qualitative and quantitative wave slamming and ship motion data. One of these rough water trials took place 18–20 April 2006 as the ship transited from Esquimalt, British Columbia, Canada to San Diego, California, USA. During this trial, the significant wave height ranged from 1.5 to 2.7 m and the ship speed ranged from 20 to 40 knots. This paper describes the results of the effort to characterize the Sea Fighter’s motion in waves. To provide suitable full-scale validation data, the incoming ambient waves had to be characterized. A Light Detecting and Ranging, (LiDAR) system, an array of ultrasonic distance sensors, and several video cameras were used to characterize the incoming wave field. In addition, three fiber optic gyro motion units were deployed to record ship motions. Additionally, a GPS unit was utilized to measure ship speed, pitch, roll, and heading. Several slam and near slam events are discussed over the range of ship’s speed, heading, and sea states tested. Similarities and differences between these events are also noted. Additionally, this data was used to develop a simulation of the Sea Fighter’s motion in waves similar to previous work done utilizing model test data (Hess, et al, 2007; Faller et al, 2008).Copyright

Comparison of Incoherent and Coherent Wave Field Measurements Using Dual-Polarized Pulse-Doppler X-Band Radar

Radar-based remote sensing for measurement of ocean surface waves presents advantages over conventional point sensors such as wave buoys. As its use becomes more widespread, it is important to understand the sensitivity of the extracted wave parameters to the characteristics of the radar and the scatterers. To examine such issues, experiments were performed offshore of the Scripps Institution of Oceanography pier in July 2010. Radar measurements in low wind speeds were performed with a dual-polarized high-resolution X-band pulse-Doppler radar at low grazing angles along with two independent measurements of the surface waves using conventional sensors, a GPS-based buoy, and an ultrasonic array. Comparison between radar cross section (RCS) and Doppler modulations shows peak values occurring nearly in-phase, in contrast with tilt modulation theory. Spectral comparisons between Doppler-based and RCS-based spectra show that Doppler-based spectra demonstrate greater sensitivity to swell-induced modulations, whereas RCS-based spectra show greater sensitivity to small-scale modulations (or generally have more noise at high frequency), and they equally capture energy at the wind wave peak. Doppler estimates of peak period were consistent with the conventional sensors, whereas the RCS differed in assignment of peak period to wind seas rather than swell in a couple of cases. Higher order period statistics of both RCS and Doppler were consistent with the conventional sensors. Radar-based significant wave heights are lower than buoy-based values and contain nontrivial variability of ~33%. Comparisons between HH and VV polarization data show that VV data more accurately represent the wave field, particularly as the wind speeds decrease.

Distribution of Wave Impact Forces From Breaking and Non-Breaking Waves

Impact loads from waves on vessels and coastal structures are highly complex and may involve wave breaking, making these changes difficult to estimate numerically or empirically. Results from previous experiments have shown a wide range of forces and pressures measured from breaking and non-breaking waves, with no clear trend between wave characteristics and the localized forces and pressures that they generate. In 2008, a canonical breaking wave impact data set was obtained at the Naval Surface Warfare Center, Carderock Division, by measuring the distribution of impact pressures of incident non-breaking and breaking waves on one face of a cube. The effects of wave height, wavelength, face orientation, face angle, and submergence depth were investigated. A limited number of runs were made at low forward speeds, ranging from about 0.5 to 2 knots (0.26 to 1.03 m/s). The measurement cube was outfitted with a removable instrumented plate measuring 1 ft2 (0.09 m2 ), and the wave heights tested ranged from 8–14 inches (20.3 to 35.6 cm). The instrumented plate had 9 slam panels of varying sizes made from polyvinyl chloride (PVC) and 11 pressure gages; this data was collected at 5 kHz to capture the dynamic response of the gages and panels and fully resolve the shapes of the impacts. A Kistler gage was used to measure the total force averaged over the cube face. A bottom mounted acoustic Doppler current profiler (ADCP) was used to obtain measurements of velocity through the water column to provide incoming velocity boundary conditions. A Light Detecting and Ranging (LiDAR) system was also used above the basin to obtain a surface mapping of the free surface over a distance of approximately 15 feet (4.6 m). Additional point measurements of the free surface were made using acoustic distance sensors. Standard and high-speed video cameras were used to capture a qualitative assessment of the impacts. Impact loads on the plate tend to increase with wave height, as well as with plate inclination toward incoming waves. Further trends of the pressures and forces with wave characteristics, cube orientation, draft and face angle are investigated and presented in this paper, and are also compared with previous test results.

Free-Surface Measurements in a Tow Tank Using LiDAR

Light Detection and Ranging, or LiDAR, is a remote sensing technique that can be utilized to collect topographic data. These systems have been used extensively to measure open ocean and ship generated waves. Recently LiDAR systems have been used to measure the transom wave of the R/V Athena I and ambient ocean waves. This work has primarily focused on providing the time averaged, and spectral content of the wave field, by scanning the laser to measure wave profiles evolving in time. This paper describes recent efforts to utilize LIDAR systems to measure free-surface elevations in laboratory tow tanks. LiDAR measurements are limited to the white-water breaking regions of the flow, due to the limited strength of the signal return from non-breaking regions. In extending LiDAR measurements to a laboratory tow tank environment the lack of surface roughness and hence the lack of surface light scatterers needed to be addressed. A number of laboratory measurement applications will be described including a tow tank measurement similar to the R/V Athena I effort, and also measurement of regular and irregular breaking waves.

Wavemaker Limitations and Difficulties in Model Testing of Small High Speed Craft

Model testing of marine platforms has always focused on preserving geometric similitude and Froude number. While Froude scaling does provide a good approximation to true dynamic similitude, there are viscous effects which are not scaled. Additionally, the dynamic testing of small high speed marine craft presents difficulties due to their small size, so while one would prefer to minimize these effects by testing large models, model basin characteristics limit the physical size of the models that can be tested. These issues as they relate to model testing, including sea state scaling and scale effects, are discussed.

Measurements of Surface Waves Using Low-Grazing Angle High-Resolution Pulse-Doppler Radar

Techniques for extracting surface wave characteristics from radar backscatter have been investigated and improved over the last several decades. Much of this research has focused on the use of backscatter intensity from navigational radars for characterization of wave period and direction, and has clearly demonstrated accurate measurement of these wave characteristics. However, the precise determination of significant wave height has been more problematic due to the required application of a modulation transfer function. Furthermore, low sea states generally do not provide enough backscatter intensity for evaluation of wave characteristics, and thus, navigational radar measurements are restricted to relatively high sea states. More recently, techniques using Doppler velocities as a measurement of surface waves have been an area of increasing focus and development. An advantage of this approach is that no modulation transfer function is required, and only phase information is used from the backscattered radar signal. Recent research suggests that the relationship between Doppler velocities and wave height may be more consistent than that between radar backscatter intensity and wave height. In July 2010, surface waves were measured during an experiment at the Scripps Institution of Oceanography pier. Radar measurements were performed using a high-resolution pulse-Doppler instrumentation radar at low grazing angle (∼1 deg) with a pulse repetition frequency of 800 Hz and spatial resolutions of 10–30 cm. Radar data for X- and Ku-bands using both VV and HH polarizations were collected. Concurrent buoy measurements were also performed, along with the collection of wind speed and direction data. Measured seaways consisted of small significant wave heights (glassy conditions to <1 m), and contained combinations of wind sea and swell. Doppler processing of the radar data provided estimates of surface wave orbital velocity spectra in wavenumber and frequency domains. The velocity spectra were converted to sea surface elevation spectra. Using these spectra, peak periods were computed as well as RMS wave heights, thus providing approximate significant wave heights. The methods for extracting wave spectra, peak periods, and significant wave heights are discussed, and results are compared with buoy measurements. When sufficient capillary waves existed on the sea surface, the radar and buoy measured wave spectra were in agreement, and analysis indicates that the instrumentation radar was able to detect and spectrally distinguish between wind seas and swell.

Statistics of Greater Sea States

Accurate representations of seaway statistics are important for physical and computational predictions of ship motions. The spectra that are most typically used in these applications are the Pierson-Moskowitz or Bretschneider. While these spectra are useful for fully developed seas, the larger sea states (Sea State (SS) 7 and higher) are typically not fully developed. In these cases, other spectral models may be more appropriate. It is critical to ship motion prediction, for both physical and numerical models, to accurately capture the frequency range for the sea state of interest. Sea state statistics, including wave heights, periods, and spectral bandwidths from various buoys and a platform in the North Sea are collected and compared with statistics from lower sea states. The spectral data are then averaged to generate a typical spectrum under the measured conditions. These developed spectra are compared with the ideal spectra mentioned previously.

Analysis Methods for Vessel Generated Spray

The droplet sizes and velocities contained in vessel generated spray are difficult to quantify. This paper describes three different methods to quantify velocity and size distributions from high speed video of spray from a planing boat. These methods include feature tracking, displacement tracking and video inversion. For the feature tracking method, the images were preprocessed using contrast limited adaptive histogram equalization, and then converted to binary images with a specific intensity cutoff level. Image statistics were then generated from this image, including droplet area and effective diameter. These images were processed using commercial PIV software to obtain velocities. For the displacement tracking method, the images were also converted to binary images with a specific intensity cutoff level. Image statistics were again compiled from this binary image. A droplet filter was then applied using a binary erosion image processing technique, where large droplets were removed because the entire droplet may not be in frame, and small droplets were removed because they might not overlap between frames. Droplets were then tracked by comparing the bounding boxes of two droplets between time frames. The video inversion method consisted of the manipulating the original high speed videos from spatial x-y frames in time space to time-y frames in x-space, where the x-axis is longitudinally along the ship and the y axis is vertical to the ship. From this orientation, the speed of the general spray mass could be determined by summing the pixels in time columns for each × frame. Comparisons of droplet size distribution between the feature and displacement tracking method yield qualitatively similar results, with some disagreement likely due to the different threshold levels. The trend of the distribution curve suggests that both methods are unable to resolve the smallest droplet sizes, due to the processing filters applied as well as the field of view of the camera. The three analysis methods compare well in their spray velocity computation, and are also similar to spray speed predictions found in the literature for a given geometry and vessel speed.

A Comparison of the Model and Full Scale Transom Wave of the R/V Athena

Over the past few years the U.S. Office of Naval Research has sponsored a series of measurements of the transom wave of the R/V Athena and of a 1/8.25-scale model (NSWCCD Model 5365) of the ship. The objectives of the testing were to characterize the free surface wave behind the ship’s transom at both model and full scale for use in identifying hydrodynamic features and for developing and validating numerical simulation tools. The focus of this paper is the comparison of these full scale and model scale measurements, specifically a comparison of the time-averaged free-surface stern wave profiles and the dominant hydrodynamic features, the rooster tail for example. Both the field measurements and the model scale tow tank measurements were made in as calm as possible ambient conditions. Full scale data was collected in the relatively protected waters of St. Andrews Bay, Florida. The winds, which typically build as the day progresses, were minimal, and it was a new moon during the test period, so tidal excursions were also minimized. While measurements were obtained for ship speeds ranging from 3.1 to 6.2 m/s (6 to 12 knots), equivalent to Froude number range based on length (47 m) of 0.14 to 0.29, respectively, the focus of the comparison is for the 0.24 Froude number (10.5 knots full scale) case. Measurements of the full scale stern wave were made by a scanning laser altimeter, while measurements at model scale were made using a traversing set of conductivity finger probes.Copyright