Unmanned systems present new opportunities10 July 2018
Stephen Ferretti of Martac Commercial presents details of a new method to survey bathymetry and inspect water structures, and details how it was used on the Keokuk Dam in the US.
The use of unmanned systems is changing the way we inspect structures and survey environments. Whether in the air, on the land, or in the water, unmanned systems are saving lives and money.
Unmanned aerial vehicles (UAV’s) have become powerful tools finding many new commercial applications in inspection and surveying. While UAV’s have proliferated, adoption of unmanned maritime inspection and surveying systems have lagged. Many structure inspections have traditionally been completed by divers, manned surface vessels and/or remotely operated unmanned underwater vehicles (ROV’s). However, issues continue to exist with safety, high acquisition costs, accuracy and repeatability when using these methods.
That’s all changed with the successful inspection and bathymetric mapping of the Keokuk Energy Center and Dam on the Mississippi river. Improved communications and powerful surface and subsurface sensors combined with the MANTAS Unmanned Surface Vehicle (USV) presented a safer and more cost-effective opportunity to inspect structures and survey bathymetry, especially in the challenging water environments of an operational hydroelectric dam facility on the Mississippi River.
The MANTAS Unmanned Surface Vessel (USV)
Maritime Tactical Systems, the parent company to Martac Commercial, has been working with military partners over the last five years to develop the MANTAS USV for many of their operations. During this period, it became apparent that many of the military capabilities could be utilized in commercial markets. The MANTAS USV vessel is a catamaran hull design based on a powerboat racing vessel that set world speed records. This catamaran design is stable at both high and lows speeds and in challenging water environments. The stability comes from the hull which provides for continuous laminar flow forward to aft as the vessel moves through the water. The result is a high-performance surface vessel capable of burst speeds greater than 84 kilometers per hour and cruise speeds of 20 – 30km per hour while maintaining stable operations.
Combined with a Lithium Battery powered drive train the MANTAS vessel has cruise ranges from 43 to 278km, depending on the vessel size and on-board battery configuration. Vessel sizes range from 1.2m to 6m and have payload capacities ranging from 2.7kg to 318kg, respectively. These payload capacities allow the integration of many single, multibeam and side scan sonars as well as sub-bottom profilers. Another benefit of the stable hull design and resulting decrease in cavitation is that sonars can be placed at minimum imaging sonar projection depth and still provide high resolution imaging.
Hydrographic surveys and inspections may not require high burst or cruise speeds; however, many occur in water environments with currents and turbulence where other USV’s, ROV’s or divers cannot successfully navigate. This allows the MANTAS USV to work in areas, and on projects not possible by manned systems or slower imaging platforms.
The MANTAS USV is controlled by a simple, easy to use control system known as TASKER. The MANTAS USV can be controlled via a command center, laptop, tablet or smartphone in either fully autonomous, semi-autonomous or full operator modes. The interface is user friendly and requires minimal training to operate. Located within the control center is a pilot center that provides situational awareness including a mission map, onboard-camera view and user sensor interface.
TASKER is designed so that MANTAS can provide effective coverage area with multi-craft capability. Users can control multiple MANTAS vessels from a single point giving them the ability to survey and inspect over large distances and areas.
Depending on the working environment and communications requirement, the MANTAS USV can be configured for control and communication through S and C-band networked radios, 4GLTE, and/or satellite. By integrating powerful sensors and communications capabilities into the MANTAS USV, users now have a powerful tool to perform hydrographic surveys and inspect assets from a highly maneuverable, autonomous vehicle from the surface. It is safer than diver inspections and has better coverage than ROV’s. Because of its autonomy, it can be programmed to run the same survey or inspection one, two or three years apart with the same degree of accuracy. This provides repeatability and a temporal view of the bathymetry allowing users to better plan existing and/or future projects.
Dams, bridges, canals, harbors and ports are examples of assets that can be surveyed and inspected by the MANTAS USV. The draft of a 3.75 meters MANTAS USV is just 18 centimeters, and combined with its maneuverability, makes it proficient in performing in open ocean, riverine, and inland waters.
Ameren’s Keokuk Energy Center & Dam Problem
In the past, Ameren Missouri, a leading Midwestern, US public utility, has been challenged to acquire accurate subsurface water views of the Keokuk Energy Center and Dam. They either used manned survey vessels to do soundings or divers in performing visual inspections. What they found is that inspecting structures on the Mississippi River can be dangerous for both divers and manned vessels. Strict safety regulations prohibit manned vessels from working within 200 meters of the dam front and any inspection of powerhouse inlets or ice fender is not possible with divers or manned vessels without interrupting power generation due to high intake currents. This presented a problem as they needed accurate views of the structures for several planned projects.
Robert Kennedy, Project Lead of Ameren’s Innovations Team, had experience with UAV’s and realized that this could be a new opportunity to get a complete subsurface view of the Energy Center and Dam safely and cost-effectively.
MANTAS USV inspection system configuration, set-up and imaging
For the Energy Center & Dam survey, MARTAC combined a MANTAS USV measuring 3.75 meters in length and a payload capacity of 70 kilograms with the TASKER control system. Vessel control and communication was established through a 4GLTE connection along with line of sight radio as backup. Because of the high-resolution requirement in inspecting the structures, a Teledyne Reson T20P multibeam Echosounder was integrated for the image acquisition phase. To improve survey accuracy and resolution, an Applanix POSMV Wavemaster II Inertial Navigation System was incorporated with a connection to the Iowa real-time network (RTN). This gave the platform a survey run accuracy of 2 centimeters and feature resolution of 5 centimeters, depending on the depth.
A 4GLTE modem was linked to the imaging system to allow the onshore hydrographer to view the images in real time to assess scan quality and make survey run adjustments. Since the MANTAS is a catamaran hull, the Echosounder transducer was installed mid-way along the hull, between the two sponsons and positioned about 0.3 m into the water. This set-up was easily mobilized and demobilized utilizing a boat trailer shoreline launch.
Hydrographic survey and inspection
Surveys were performed at 3 – 7km per hour and MANTAS USV’s performance capabilities allowed it to access and perform in areas not possible with manned platforms.
The MANTAS USV Platform was required to survey and inspect four areas: (1) the upstream dam front consisting of 120 gates spanning East to West for 1.5 kilometers, (2) the ice fender protecting the powerhouse inlet bay spanning Northwest to Southeast at 0.5km, (3) the powerhouse inlets, and (4) the powerhouse inlet bay area on the west side of the powerhouse.
As part of a proposed automated gate opening project, Ameren required a complete scan of the front dam face to assess wall condition and debris buildup along the gates. For the upstream dam front, the MANTAS USV performed three survey runs parallel to the dam face running East to West at a distance of 8m from the dam face. The objective was to image the wall and riverbed out to 13m. The MANTAS USV was able to successfully navigate several open gates along this path with each survey pass taking approximately 30 minutes.
The ice fender spans Northwest from the powerhouse out to 0.5km. The water flow through the ice break results in a current of 11 – 17km per hour. This flow varies based on the number of gates open as water is diverted to the powerhouse inlets. The flow appeared to be laminar so scanning this area required positioning the MANTAS USV at the powerhouse from the Southeast to the Northwest along the ice break with sufficient acceleration to maintain the 3 – 7km per hour required to scan. This required a relative speed of 15 – 24km per hour to overcome the 11 – 17km per hour current. The MANTAS USV was able to run a straight course along both sides of the fender and achieve a 360-degree view of the submerged ice fender support system. It took approximately 1.5 hours to perform multiple survey runs on this structure.
The MANTAS USV then surveyed the condition of the powerhouse inlets to provide a bathymetric map and assess debris build up for a project to install a trash rake system. At normal turbine operation water flow is turbulent flowing into these inlets at 11 – 17 kilometers per hour clearly showing the surface debris built up against the powerhouse inlet wall. Unmanned, manned and diver operations are normally prohibited unless power generation is halted.
The MANTAS USV performed several runs along the powerhouse inlet wall at 7 meters from the wall. The MANTAS USV’s high performance profile enabled it to overcome the flow dynamics and acquire accurate imaging of these inlets while avoiding the surface debris. Not only did imaging provide a map of the subsurface debris, the system achieved views into the inlets grates up to 6 meters. These views were obtained without any disruption in turbine operations.
Additionally, post-processing software measurements of debris and sediment buildup allowed Ameren to achieve more accuracy for their project planning.
The final requirement imaged the bay area leading into the powerhouse inlet for debris and sediment buildup. This area is shared by both the Keokuk Energy Center and the US Army Corp of Engineers, which operates Lock #19. The objective of this imagery was to provide a bathymetric map for heavy barge navigation from March through December. The pool area is approximately 0.9 km long and 0.6 km at its widest span. It took approximately three hours to perform several survey paths including the powerhouse inlet imaging.
The bathymetric mapping of the inlet bay revealed features that had not been discovered in previous inspections. One notable feature was a cement house located about 13.2 meters off the powerhouse inlet front in the northeast corner. The approximate measurement of this structure was 37.1 meters long by 9.5m wide and 7.5m high off the riverbed. This was the original cement house utilized during construction and left standing when water was diverted back through the inlet pool. The structure was only 3.4m below the water’s surface, just below the required clearance for barge traffic at 2.8m. Locating and mapping this structure was important since barge traffic occasionally drifts towards the northeast corner of the powerhouse and Ameren and the US Army Corp of Engineers now know to act sooner when these drifts occur.
An additional feature noted in this pool was the significant sediment build up in the southeast corner of the bay near incomplete inlets for a powerhouse expansion project. This build-up was high enough that the MANTAS USV imaged in just 0.3m of water. This area is highlighted in red and represents an area that may require dredging. The MANTAS USV easily navigated the entire inlet pool to develop the bathymetric map.
The MANTAS USV was able to achieve the requirements for the Energy Center & Dam survey and inspection in a fraction of time that it would have taken a manned team, ROV’s or divers. All surveys were completed from either the dam, powerhouse or shoreline and represent a safer alternative than divers. The images and maps were of sufficient accuracy to allow Ameren to use them to plan their upcoming projects.
This dam imaging project confirmed that the MANTAS USV can be used by dam owners/operators, public utilities, oil & gas companies, harbor and port municipalities, Departments of Water Resources and Transportation and anyone else that desires to get safe and accurate structure images and bathymetric maps.
Finally, the MANTAS USV is multi-role. By integrating water quality sensors, it can be used as a mobile, in-situ water quality laboratory. It can also be utilized to protect both above and subsurface assets by integrating electro-optical, thermal and infrared sensors along with subsurface sonars and scanners to locate and track nefarious assets and personnel.
Stephen Ferretti is Executive Vice-President of Martac Commercial.