NEW ADVANCES IN UNDERWATER INSPECTION TECHNOLOGIES FOR RAILWAY
BRIDGES OVER WATER
Daniel G. Stromberg, P.E., S.E.
Chief Structural Engineer/Diver
Collins Engineers, Inc.
123 North Wacker Drive, Suite 300
Chicago, Illinois 60606
(312) 704-9300
NEW ADVANCES IN UNDERWATER INSPECTION TECHNOLOGIES FOR RAILWAY BRIDGES OVER WATER
DANIEL G. STROMBERG, P.E., S.E.
Collins Engineers, Inc. 123 N. Wacker Drive, Suite 300
Chicago, IL 60606 USA
ABSTRACT:
The inspection of submerged elements and the channel bottom surrounding these elements is
essential for ensuring safety and promoting long-term serviceability of our Nation’s railway
bridges over water. Although underwater technologies have been used to supplement
underwater bridge inspection by divers for many years, recent advancements, including newly
developed equipment using acoustic principals most often employed in the offshore oil and gas
industry, have resulted in new and improved underwater inspection, support, and documentation
methods. This paper will serve to discuss the latest in underwater inspection technologies that
can be used to supplement and augment the efforts of inspection divers, as well as to discuss case
study examples where those technologies were used: in advance of diving during flood
conditions; to establish baseline channel bottom conditions; to document before and after
conditions related to repair/rehabilitation operations; and to promote improved safety and
accuracy for ensuing dive inspections.
INTRODUCTION The importance of and need for the underwater investigation of railroad bridges cannot be
overstated, especially in light of the significant age of so many railroad bridges and the trend of
late towards severe storms, greater-than-ever high water conditions, and the potential for scour to
levels that have never previously been experienced. In addition, railroad bridges more-often-
than-not have larger, more closely spaced foundations that make them particularly more
vulnerable to scour and other waterway effects, which are by far the leading cause of bridge
failures from an underwater standpoint. Given these realties, it’s easy to see why it is so
important that the properly executed underwater inspection of railroad bridges should be
implemented as part of an effective bridge maintenance program, as well as to identify any
critical conditions looming out there before it is too late. In this regard, there are various
underwater technologies that can be used to supplement and augment conventional underwater
inspection by properly trained divers, which includes at times when diving itself can not be
safely accomplished, to adequately assess existing conditions and identify any potential problems
before they manifest themselves.
There are some 100,000 railroad bridges in the United States and best estimates are that more
than half of them are over some sort of waterway with substructures units in the water. While
there is less available data for railroad bridge failures due to underwater effects, partly because
there tends to be less publicity unless there is loss of life associated, a good approximation for
the underwater situation that may exist for the Nation’s railroad bridges can be gleaned from
Federal Highway Administration (FHWA) data for U.S. highway bridges. The FHWA bridge
inventory includes about 480,000 structures located over water. Currently, there is no estimate
available for the number of bridges that have experienced a waterway related failure or for which
and underwater related failure was averted through emergency measures. The FHWA
unequivocally states, however, that the percentage of such bridges is not miniscule or
insignificant by any means, with an appreciable number of highway bridges over water in the
U.S. inventory succumbing to underwater problems. Furthermore, the FHWA indicates that an
even greater number of bridges have had a situation addressed before underwater problems can
cause havoc by virtue of the underwater inspections that are required by FHWA regulations.
In most instances, the underwater related problems of the Nation’s highway bridges relate to the
effects of scour, and in that regard, the FHWA does have good estimates pertaining to the
number of bridges in their inventory that have a significant risk for scour and the damage it can
cause. In particular, FHWA data indicates that approximately 18% of the highway bridges are
currently highly scour susceptible or scour critical, which means that the effects of scour, if not
detected soon enough, have a great potential to adversely affect the stability and safety of those
bridges. It is important to note though, that these projected scour problem numbers may be
understated because they are related to flood analysis values that are continually needing to be
revised upward as flood levels, which should never be actually experienced, are experienced.
That is, once a hypothetical 500-year design flood is actually experienced, the flood value must
be increased since it is intended to represent an upper limit for analysis that should not be
reached. This reality relates back to the aforementioned trend of late toward more extreme
weather and related high water and flow events that can cause more and more underwater
problems for bridges.
In addition to the known scour concern bridges, the FHWA has another 22% of bridges over
water with unknown foundations, which may or may not have the potential to succumb to
underwater problems depending on if the actual foundation design is suitable for the expected
scour effects. This aspect could have particular applicability to railroad bridges, because as
mentioned earlier, there is a generally older inventory of railroad bridges, which as a result can
mean that foundation design data is no longer available or that the original foundation design is
no longer suitable for the current demands of the environment and/or the waterway
characteristics.
If the FHWA highway bridge data was to be used as a “measuring stick” for potential for
underwater problems for the Nation’s railroad bridges over water, it would be very reasonable to
conclude that somewhere in the neighborhood of 20% of the total number of existing railroad
bridges over water may develop or may have already developed below water conditions that
could lead to a failure. Now even if that percentage is reduced by up to 50%, in light of the
generally more conservative nature of railroad designs produced by their age-old recommended
practices, that still yields some 5,000 railroad bridges with a reasonably good potential for
existing or future underwater problems of a serious nature. These potential problems that may
loom out there do not, however, have to come to fruition, resulting in a loss of life, in the most
severe case, or at the minimum, the closure of a bridge, disrupted rail service, lost revenue and/or
significant after-the-fact rehabilitation costs. This is because the various means of underwater
assessment presently available can be very effective in the detection of below water conditions of
concern, especially and most notably those that are related or are the result of scour, with these
means including not only diving inspection, but also some rather sophisticated underwater
inspection technologies.
Since the importance of underwater inspection first became apparent, the underwater assessment
of the submerged portions of railroad bridge substructures has been most commonly
accomplished with divers. There are, however, many cases including, but not limited to, water
depth, flow currents, and environmental conditions where it is not possible to adequately or
safely conduct an underwater bridge inspection by diving. In those cases though, there now are
technologies available to inspect or monitor the below-water bridge elements and the
surrounding channel bottom. Although these technologies, which include various sonar devices
and ROVs are not as capable as a “hands-on” inspection by a qualified underwater inspector,
they most often can render important and useful data, and in many instances, may be the only
way to get any data.
Furthermore, even when an underwater bridge inspection can be conducted by diving, there is
still the ever present issue of the inherent danger in performing such work. To this end, these
technologies can be used prior to the physical underwater inspection to collect up-front data on
possible structural deficiencies, scour depressions, exposed footings, and underwater
construction or debris accumulations. Collecting this data not only increases diving safety by
identifying potential hazards, but also identifies potential structural deficiencies and produces
data that can be used during the report writing and structural evaluation phases of the inspection.
Fathometers have been used for years to locate and quantify scour depressions, exposed footings,
and underwater debris accumulations; however, high definition sonar systems take this ability to
a new level. In addition to the enhanced safety aspects, these systems can also be used during
the underwater inspection to direct divers to specific areas of concern on the below water
structure.
Another limitation to underwater bridge inspection conducted solely by diving techniques is
presentation of inspection observations to the client or bridge owner. Typically, this is
accomplished through inspection drawings developed in accordance with design or as-built
structure drawings and detailed inspection notes relayed from the diver to topside personnel.
These drawings are often supplemented by underwater photography or videography. Although
underwater photography and videography are useful tools and have been used for many years,
they are not effective in waterways with limited visibility. A clear water box can be used in
these cases; however, these devices are cumbersome and can be difficult to position and
manipulate, especially if the waterway has an appreciable current. High definition sonar systems
are an invaluable tool in this regard, as they can provide near photo-quality images of the below-
water portions of a structure regardless of water clarity, thereby affecting or “big picture” of the
existing conditions below water.
AVAILABLE UNDERWATER INSPECTION TECHNOLOGIES
Many of the underwater inspection technologies available pertain to what is known as sonar
technologies. Sonar (Sonar Navigational and Ranging) uses transmitted and reflected
underwater sound waves to detect submerged objects and measure distances. This technology
has been used for a great many years for water depth determination, underwater object detection,
and underwater communications. The sonar devices primarily used for underwater
investigations are fathometers, multi-beam swath sonar, side-scan sonar, sector scanning sonar,
lens-based multi-beam sonar, and geographical sub-bottom sonar profilers.
FATHOMETERS
The most common sonar device, which has been used for water depth detection and limited
underwater investigation for many years, is the black and white recording fathometer. The
simplest fathometers consist of an acoustic sending/receiving device (transducer) suspended in
the water and a digital or paper recording device. A fathometer works by emitting acoustic
pulses through the water column toward the channel bottom by way of the transducer. The
recording device measures the time it takes the pulse to reflect off the channel bottom and return
to the transducer, and then converts that time into water depth. Fathometer frequencies typically
range between 24 kHz and 340 kHz, with higher frequencies yielding higher resolution, but little
or no channel bottom penetration. As channel bottom penetration is typically not desired when
performing a fathometer survey, a higher frequency is usually used (commonly 200 kHz). Many
transducers available on today’s market offer a variable beam angle. Using a larger beam angle
covers a larger area of the channel bottom; however, as it is typically desired to get the best
possible reading directly below the transducer, the smallest available beam angle is usually
preferred. When operating at high frequency with a small beam angle, many transducers can
attain depth accuracy of better than one inch.
More advanced fathometer systems include a Global Positioning System (GPS) receiver or
robotic total station. When a fathometer is coupled with one of these devices, water depths can
be referenced to a state plane or other horizontal coordinate system. This allows for very
accurate channel bottom surveys, which can be easily compared to future surveys. Although
more advanced fathometer systems are not necessary for typical bridge inspections, as the bridge
itself can be used for reference, they are invaluable for surveys in open water where no such
reference is available. Many GPS receivers and total station systems can attain horizontal
positioning accuracy of better than one inch.
The primary benefit of a fathometer is the ability to develop accurate channel bottom profiles
with a low cost, compact, and easy-to-use unit. The profiles can be used to locate and quantify
scour depressions, areas of infilling, and channel bottom objects such as exposed pier footings or
debris accumulations. Performing a fathometer survey prior to the diving inspection can direct
the underwater inspector to some potential problems with the bridge, as well as alert the
inspector to certain potential below-water hazards. Overlaying and comparing channel bottom
profiles from successive underwater bridge inspections can alert engineers to possible channel
related problems.
The primary limitation of a fathometer is its inability to collect data outside the path of the vessel
transporting the transducer. This limitation prevents detection of channel bottom irregularities or
objects unless the vessel passes directly over the top of the irregularity or object. For instance, a
fathometer survey conducted during a typical underwater inspection may include recording
channel bottom profiles along the bridge fascias and 100 feet and 200 feet upstream and
downstream of the bridge. If a previously removed bridge was located 50 feet downstream of
the bridge, and the substructure units were removed to some distance below the waterline, the
fathometer would not detect these waterway obstructions. If a larger transducer beam angle is
selected, an irregularity or object close to but not directly below the vessel may appear briefly;
however, most times an operator will disregard this anomaly as a fish or other object within the
water column.
MULTI-BEAM SWATH SONAR
As mentioned previously, single beam echosounders are one of the most common forms of sonar
used for underwater applications. A single beam transducer is used to transmit and receive a
series of sound waves to the benthic layer. The time lag between the transmission and reception
is used to calculate the water depth to point of first sound wave response. With this type of
system, a single depth location is received and recorded. Single beam sonar is limited in that it
does not have the ability to obtain 100 percent data coverage of the channel bottom as only one
single point is returned to the transducer.
Multi-beam sonar systems also referred to as Swath echosounders, function as the name implies.
This type of system uses a fanned array of sound beams that typically give 100 percent coverage
of the seafloor or channel bottom. Different sound velocities and beam angles can be used to
obtain the required data. For instance, a typical multi-beam survey may have a fanned array that
is capable of a “swath width” of seven times the water depth. This means that if the water depth
is 100 ft deep, bathymetric data can be obtained up to a swath of 700 ft wide, or 350 ft to the port
or starboard side of the survey vessel. The accuracy of the outer edges tapers off to the outside
of the fanned array, so it is good practice to have survey track lines overlap. The accuracy of
multi-beam data is quite good if the system has been calibrated and proper sensors are used.
Since the direction and angle of the beams can change with the heave, pitch, and roll of the
survey vessel, it is necessary to have motion and relay this information back to the on board
processor. Calibration checks known as “patch tests” are also performed to calibrate the sensors
and account for pitch offset, roll offset, and position time delay. These tests are performed prior
to the survey using the appropriate software. Calibration tests are absolutely necessary to obtain
quality data.
There are many advantages of using multi-beam sonar systems. Large areas of the seafloor or
channel bottom can be mapped in an efficient manner. By using multiple or overlapping passes,
the hydrographic surveyor is able to obtain 100 percent bottom coverage of the area. The shape
and size of underwater anomalies or obstructions can be ascertained from this data. It also has a
wide range of uses that include, but are not limited to: sea floor mapping, dredging support
surveys, and channel obstruction detection and identification.
SIDE-SCAN SONAR
Commercial side-scan sonar was first introduced in the early 1960s and has been successfully
used for submerged object detection and investigation for many years. Side-scan sonar typically
consists of a dual transducer and below-water electronics assembly fitted into a hydro-
dynamically shaped housing (towfish), which is towed through the water by a vessel. The
transducer and below-water electronics of many side-scan sonars can also be mounted directly to
the hull of a vessel below the waterline. A cable connects the below-water electronics to the
surface component of the system, which provides both power and either hardware-based or
software-based system control. The surface component of the system also generates the image
using either a digital or paper recording device.
Side-scan sonar works by emitting fan-shaped acoustic pulses through the water column
perpendicular to the path of the transducer. The beam is narrow in the horizontal plane (typically
less than 1°) and wide in the vertical plane (typically between 35° and 60°). Refer to Figure 1
for an illustration of side-scan sonar beam pattern and coverage (figure courtesy of Mark
Atherton). The resulting images from the channel bottom and objects located on the bottom or in
the water column are representative of the echoed (backscattered) target intensity within the
geometric coverage of the beam. When the images are stitched together along the direction of
travel, they form a contiguous image of the bottom and objects located on the bottom or in the
water column. Side-scan sonar operating frequencies usually range between 100 kHz and 800
kHz, with higher frequencies yielding better resolution, but less range. As an example, side-scan
sonar with an operating frequency of 100 kHz will typically have a range of up to 1,600 feet,
while side-scan sonar with an operating frequency of 800 kHz will typically have a range of less
than 250 feet.
Figure 1 The primary benefit of side-scan sonar is the ability to quickly and efficiently generate detailed
images of large areas of the channel bottom regardless of water clarity. Side-scan sonar can be
used for many purposes, including delineation of exposed sediment and geologic formations and
detection of underwater debris or objects that may be hazardous to marine operations. In
addition, the general location and configuration of submerged structures, pipelines, and cables
can be investigated using side-scan sonar.
SECTOR SCANNING SONAR
Scanning sonar was developed in the early 1980s as a piloting assistance device for ROVs.
Although scanning sonar was first used to investigate submerged structures in the early 1990s, it
was not until around 2000 that a high enough resolution was available to perform detailed
structural investigations. Scanning sonar consists of a sonar transducer that mechanically rotates
on a fixed base suspended below-water. A cable connects the below-water electronics housed in
the base to the surface component of the system, which provides both power and software-based
system control. The surface component of the system also generates the image using a digital
recording device.
Scanning sonar works similar to side-scan sonar in that the transducer emits fan-shaped acoustic
pulses through the water; however, unlike side-scan sonar, which requires vessel movement to
develop an image, scanning sonar works best when it remains stationary. The acoustic images
are recorded in a series of slices generated by the rotation of the transducer. Refer to Figure 2 for
an illustration of scanning sonar beam pattern and coverage (figure courtesy of Mark Atherton).
Computer software, available as part of these systems, stitch these slices together to form a
contiguous image. Scanning sonar operating frequencies usually range between 330 kHz and
2.25 MHz, with a common frequency used for channel bottom and structural imaging of 675
kHz. Although 675 kHz, which has a range of approximately 500 feet, is less than the side-scan
sonar upper limit of 800 kHz, frequency is only one component of resolution. The ability to
resolve a target is a combination of head stability, frequency, acoustic geometry, transducer
beam width in the vertical and horizontal planes, pulse length, receiver bandwidth, signal to
noise ratios, and target size, shape and acoustic impedance. As a result of the stable head, wide
band width, narrow transverse beam widths, and small pulse length, images generated using
scanning sonar are highly detailed even with an operating frequency of only 675 kHz.
The primary benefit of scanning sonar is the ability to produce highly detailed images of the
channel bottom and vertical components of submerged structures regardless of water clarity.
Scanning sonar can be used for many purposes, including detection and identification of scour
depressions, areas of infilling, exposed pier footings, debris accumulations, and some underwater
structural deficiencies. Scanning sonar can also be used prior to and during diving operations to
direct the underwater inspector to potential deficiencies, as well as direct the inspector around
potential below-water hazards. Near photo-quality images depicting entire or large portions of
structures can also be generated for inclusion into inspection reports.
Figure 2
The primary limitation of side-scan sonar is the inability to generate detailed images of the
vertical components of submerged structures. This is true even if the towfish transducers are
rotated so the beams scan vertically through the water column. As a result, scanning or lens-
based multi-beam sonar are better solutions for generating images of the vertical components of
submerged structures. Other limitations to side-scan sonar include inability to detect narrow
linear targets parallel to the beams, difficulty keeping the towfish at a constant location behind
the vessel and at a constant elevation in the water column, keeping the vessel along a consistent
line at a constant speed, and vessel pitch and roll, especially if using a hull-mounted application.
The primary limitation of scanning sonar is the inability to quickly and efficiently generate
detailed images of large areas of the channel bottom. This is due to limited range and the need
for the sonar to be located close to the bottom in a stable position by way of a tripod or other
deployment device. As a result, side-scan sonar is a better solution for overall channel bottom
mapping or searching for a submerged object. As developing highly detailed images using
scanning sonar is heavily dependent on sonar positioning and stability, additional limitations
include operator experience, structure geometry, and waterway current.
LENS-BASED MULTI-BEAM SONAR
Multi-beam sonar was first developed after WWII as a device for low resolution underwater
object detection. In the late 1990s, the Space and Naval Warfare Systems Center (SPAWAR)
funded the development of a prototype lens-based multi-beam sonar at the University of
Washington Applied Physics Laboratory to identify swimmer intruders with almost video-quality
resolution. Although the viability of using this sonar to investigate submerged structures was
immediately recognized, it was not until around 2004 that the offshore oil and gas industry began
using it for that purpose. It is estimated that currently only 30 lens-based multi-beam sonar units
are being used for structural investigations, with no units currently being used for underwater
bridge inspection on any consistent basis.
Lens-based multi-beam sonar is essentially scanning sonar that does not rotate. Scanning sonar
consists of one beam that mechanically moves each transmit/receive cycle to create an image line
by line. Lens-based multi-beam sonar consists of numerous beams placed side by side to create
an image in one transmit/receive cycle. Refer to Figure 3 for an illustration of lens-based multi-
beam sonar beam pattern and coverage (figure courtesy of Dr. Ed Belcher). Many lens-based
multi-beam sonar systems have manually selectable frequencies that allow for longer range for
locating objects and higher resolution for investigating objects. Operating frequencies usually
range between 0.7 MHz and 1.8 MHz, with higher frequencies yielding better resolution, but less
range. As an example, lens-based multi-beam sonar with an operating frequency of 0.7 MHz
will have lower resolution with a range of up to 240 feet, while lens-based multi-beam sonar
with an operating frequency of 1.8 MHz will have higher resolution with a range of less than 50
feet.
Figure 3 Similar to scanning sonar, the primary benefit of lens-based multi-beam sonar is the ability to
produce highly detailed images of the channel bottom and submerged structures regardless of
water clarity. However, by using numerous beams simultaneously, lens-based multi-beam sonar
provides real time images, which result in a sonar that is not as dependent on operator
experience. This is primarily due to the considerably shorter length of time required to hold the
sonar stable, which allows the operator to quickly move and view the structure until an optimal
position is determined. As lens-based multi-beam sonar provides real time images, it can
produce near photo-quality videos, as opposed to simply near photo-quality stills produced with
scanning sonar. In addition, battery operated units with a mask-mounted display can be carried
by an underwater inspector. Using a diver carried unit, an underwater inspector can navigate
himself to potential deficiencies as well as around potential below-water hazards.
The primary limitation of lens-based multi-beam sonar is range. A lens-based multi-beam sonar
unit set for lower resolution and longer range typically has a range of less than 240 feet, which is
less than half of the scanning sonar unit range of 500 feet. This is primarily due to higher
operating frequencies. As a result, scanning sonar is a better solution for producing highly
detailed still images of large submerged structures.
GEOPHYSICAL SUB-BOTTOM SONAR PROFILERS
High resolution sub-bottom profilers were first introduced in the mid 1960s and have been
successfully used for defining sediment stratification and detecting bedrock for many years. A
sub-bottom profiler typically consists of a transducer and below-water electronics assembly
fitted into a tow vehicle, which is towed through the water by a vessel. A cable connects the
below-water electronics to the surface component of the system, which provides both power and
either hardware-based or software-based system control. The surface component of the system
also generates images of the sediment stratifications, bedrock, and objects embedded in the
channel bottom using either a digital or paper recording device.
A sub-bottom profiler works by emitting low frequency modulated acoustic pulses through the
water column toward the channel bottom by way of the transducer. As the pulses are lower
frequency than a fathometer, only a portion of the pulses are reflected back to the transducer by
the channel bottom, with the remaining pulses penetrating into the channel bottom. The pulses
that penetrate the channel bottom are reflected back to the transducer by the acoustical
impedance between the various sediments (stratifications), bedrock, or objects embedded in the
channel bottom. When the acoustic returns are passed through a processor, they form an image
of the stratifications, bedrock, and embedded objects. Refer to Figure 4 for an illustration of sub-
bottom sediment stratifications detected with the use of a sub-bottom profiler. Sub-bottom
profiler operating frequencies usually range between 500 Hz and 24 kHz, with lower frequencies
yielding better penetration, but less resolution. As an example, a sub-bottom profiler with a 500
Hz to 5 kHz frequency modulation can typically achieve penetrations of 65 feet in course sand
and 650 feet in clay, while a sub-bottom profiler with a 4 kHz to 24 kHz frequency modulation
can achieve penetrations of only 6 feet in course sand and 130 feet in clay. Although color
fathometers can detect limited sub-bottom data by displaying materials of different densities with
different colors, they have limited range (0 feet in course sand and 20 feet in clay), and as a
result, they are not frequently used to collect sub-bottom data.
Figure 4
The primary benefit of sub-bottom profilers is the ability to accurately locate sediment
stratifications, bedrock, and objects embedded in the channel bottom. As a result, sub-bottom
profilers are frequently used prior to marine structure construction. With regard to underwater
bridge inspection, sub-bottom profilers can be used to measure the true depth of scour
depressions and locate embedded pier footings. Scour is most prevalent during a flood event;
however, hazardous site conditions including complex flow patterns and the presence of drift and
debris frequently prevent personnel from safely positioning instruments or diving during these
events. After a flood event, the waterway current decreases and sediment is typically deposited
into the scour depression. As the deposited sediment will typically consist of a different material
or have a different density than the true channel bottom sediment, the sub-bottom profiler will
depict the location of the true channel bottom. With regard to locating embedded pier footings,
sub-bottom profilers are only able to determine the top of footing location. Although
determining the footing location, thickness and supporting elements (piles, caissons, etc.) would
be the ideal situation, with time consuming and costly methods available to accomplish this,
knowledge of the top of the footing location alone is also useful. For example, if the top of
footing location is determined to be deeper than the theoretical maximum scour depth, a bridge
with an unknown foundation will likely not have the potential for future scour related issues.
The primary limitation of sub-bottom profilers is acoustic interference, which results in sub-
bottom images that are more difficult to interpret. Acoustic interferences include multipath when
operating in shallow water and side lobes when operating near in-water structures. Multipath
occurs when the transducer receives acoustic pulses that have reflected off the channel bottom,
water surface, and channel bottom again. Side lobes occur when acoustic pulses encounter
vertical objects, such as a bridge pier. As sub-bottom profilers use significantly lower operating
frequencies than fathometers, the beam angles are typically much wider. As a result of these
wider beam angles, collecting good quality sub-bottom images close to in-water structures is
challenging.
REMOTELY OPERATED VEHICLES
Beyond the sonar related techniques, which represent the majority of available underwater
inspection technologies, there are also devices known as remotely operated vehicles. Remotely
operated vehicles (ROVs) are tethered, self-propelled, underwater robots that are controlled by
an above-water operator. The U.S. Navy was responsible for most of the original ROV
technology development in the 1960s. The driving force behind this development was the need
for deep-sea rescue and object recovery capabilities. Building off the Navy’s development, the
offshore oil and gas industry created ROVs to assist in the development of offshore fields and rig
structures. Since their development, ROVs have been extensively used for deep-sea exploration,
and most notably, to locate and survey many historic shipwrecks, including the RMS Titanic, the
Bismarck, and the USS Yorktown. Presently, ROV technology allows for exploration at water
depths of up to 10,000 feet, although even at these depths, more than half of the earth’s oceans
are beyond current ROV working depth limitations.
A wide range of ROVs are available on today’s market, with the equipment generally
categorized or classified based on size, weight, and power. The smallest classification is the
‘micro’ ROV classification, which consists of small ROVs that typically weigh less than 5
pounds. These ROVs, which have propulsion power limitations, are frequently used for the
inspection of sewers, pipelines, small cavities, and structures where a diver is unable to
physically enter. The next larger classifications, which are the most frequently used
classifications for underwater inspections, are the ‘mini’ and ‘general’ classifications. The
‘mini’ classification consists of ROVs that weigh up to 40 pounds and the ‘general’ classification
consists of ROVs that weigh up to 100 pounds with typically less than 5 horsepower of overall
propulsion. Refer to Figure 5 for an illustration of a ‘general’ class ROV being deployed from a
small vessel. Beyond the ‘general’ classification are the ‘light work’, ‘heavy work’, and
‘trenching/burial’ classifications, which generally have up to 50 horsepower, 220 horsepower,
and 500 horsepower, respectfully. These ROVs are large pieces of machinery, at times as large
as a small truck, which are outfitted with manipulators used to conduct heavy work operations
and cable laying/trenching.
Figure 5 The primary benefit of ROVs is the ability to spend unlimited time at depth with none of the
decompression or other potential health risks that affect divers. As a result of the increased
safety and reduced liability, ROVs are the preferred means of accomplishing underwater work in
the offshore oil and gas industry. Heightened potential for diver entanglement, physical size
limitations, and contaminated water are additional reasons ROVs are used to perform underwater
inspections. To perform inspection operations as well as route the ROV around potential below-
water hazards, ROVs used for underwater inspections are commonly outfitted with a video
camera, scanning sonar, and/or lens-based multi-beam sonar. Video and acoustic images (if
using a sonar system) can be recorded for inclusion with inspection reports.
The primary limitation of ROVs is the inability to quickly and efficiently inspect large below-
water structures. This is primarily due to underwater visibility limitations, which restrict the
amount of useable data collected. Although scanning and lens-based multi-beam sonar systems,
which allow better coverage than video, are frequently installed on an ROV, maneuvering and
positioning the ROV properly with no visibility is difficult, even if there is no appreciable
current and the operator is highly experienced and knowledgeable in both ROV operation and
bridge construction and inspection techniques.
CASE STUDIES
The following case studies are intended to demonstrate the applicability and usefulness of
underwater inspection technologies, primarily with consideration for the use of scanning sonar
technology, for potential underwater inspection aspects that relate to railroad bridges. In
particular, the case studies present the important aspects of various applications either in lieu of
or in conjunction with diving inspection to provide an appropriate underwater assessment of
submerged conditions.
CASE STUDY 1: SCOUR MONITORING DURING MIDWEST FLOODING
During May of 2008, there was considerable rain and subsequent high flows and flooding for
numerous rivers throughout the midwest of the U.S. One of the states that was hardest hit by
these extreme events was Iowa, and as a result, there was great concern expressed by a number
of railroads for their bridges over water in that state. For quite a period of time, river levels were
up well above flood stage, with conditions such that inspection of submerged bridge
substructures by diving could not be safely accomplished. There was, however, a great need to
determine the submerged state of these bridge substructures, as well as to monitor various
locations while the extreme waterway conditions persisted. The solution to this predicament was
to implement scanning sonar technology at the bridges to generate images of the submerged
conditions, initially at the peak of the extreme high water event, and then at subsequent times
based on the initial results and the potential for further, more detrimental changes as the flooding
went on.
Ultimately, very good data and imagery was obtained at every bridge that was scanned by
utilizing Kongberg Mesotech scanning sonar apparatus, equipped with both imaging and profile
transducers and deployed from a 22-foot powerboat. The scanning process included a rather
stout bracket system for the sonar head assembly, as well as a specifically designed nosing on the
boat and a large horsepower outboard motor, all of which that allowed for a very stable
positioning of the scanning system, that in turn produced well defined “pictures” of the below
water conditions. Overall, from the scanning efforts, it was learned in some instances that either
newly developed foundation exposure was present or that the amount of preciously noted
exposure had increased notably. Reassurance was gained, however, that none of the conditions
initially identified required immediate action, although in some cases, the results did suggest that
further monitoring should be accomplished. Once the flooding subsided and diving inspection
could be conducted, the scanning images were of further vital use to specifically gear the drive
efforts in regard to the anticipated extents of foundation exposure. Furthermore, the data from
the scanning was also used to promote diver safety, since many of the scan images revealed the
presence of submerged timber drift, which could then be planned for during the dive operations.
Refer to Figures 6 and 7, which represent examples of the typical images obtained during the
2008 Midwest flooding, and that show the level of detail to which exposed substructure
foundations could be assessed, at times with submerged timber drift also readily identified.
Figure 6 Figure 7
CASE STUDY 2: BASELINE DATA AND UNDERMINING DETECTION
As part of the routine underwater inspection process at a railroad bridge, it is always key and
important to identify how the channel bottom is contoured around the substructure, and then
even more importantly, how that relates to the extent of foundation exposure, or in some
instances, the extent of foundation undermining. By using scanning sonar and the detailed
images that can be obtained, it is possible to gather a “big picture” of the existing channel bottom
and foundation exposure configuration at a bridge substructure unit, which in turn will allow the
bridge owner to see for themselves what the situation is, as well as can serve as a baseline from
which future inspections can track any changes. As part of the routine inspection of a railroad
bridge in Wisconsin, scanning sonar techniques were implemented to assess, prior to diving
inspection, the existing conditions at the bridge substructure units, which were all known to have
scour issues to varying degrees. In addition, the scanned images would prove to be very useful
with regard to foundation geometry and makeup, since like many railroad bridges, the bridge had
seen several owners, experienced various repairs over the years, and had no design plans
available.
Ultimately, the scanned images derived for the Wisconsin railroad bridge proved to be very
beneficial in defining the current extents of scour, providing details of the foundation designs,
and indicating the amounts of foundation exposure, by virtue of providing scalable “pictures” of
the existing submerged conditions. Furthermore, there were instances where the scanning sonar
results revealed some apparent undermining and foundation pile exposure at the base of the
substructure unit. With the quantified foreknowledge of the current conditions from the scanning
effort, the diving inspection was then more effectively orchestrated to confirm the scanning
results, because with no underwater visibility at the bridge, it would have been a much more
daunting task to quantify the conditions solely from the diving inspection. Refer to Figure 8,
which depicts the type of existing condition data, which can serve as a baseline for future
inspections, that was gathered at the Wisconsin railroad bridge.
Figure 8
CASE STUDY 3: VESSEL IMPACT AND SUBMERGED DAMAGE
In 2009, an emergency inspection of a State Route bridge over the Illinois River in Central
Illinois was made necessary to document the extent of damage to the bridge substructure and
surrounding channel bottom resulting from a barge striking and overturning a steel sheet pile
protection cell. In this case, the implementation of scanning sonar to obtain below water
acoustic images was an excellent choice as a means of information and documentation gathering
in advance of a diving inspection, especially since the prevailing river currents of 2 to 3 feet per
second at the bridge would make the diving operation appreciably challenging. Of much
importance, the scanning sonar images afforded a highly detailed “big picture” for the outcome
of the barge impact, depicting to-scale the protection cell position and damage, as well as how it
related to the bridge substructure, all of which that could prove to be useful in any litigation
resulting from the barge mishap. Furthermore, the scanning data was very useful for the
subsequent diving inspection, by allowing the underwater operations to be best coordinated to
the configuration of the damaged protection cell and timber drift accumulation and scour effects
caused by the existing conditions. Refer to Figures 9 and 10, which depict the protection cell
conditions resulting from the most recent (Figure 9) and a prior striking (Figure 10) due to an
errant barge.
Figure 9 Figure 10
CASE STUDY 4: SUBMERGED OBSTRUCTION DETECTION
As part of the superstructure replacement for a railroad bridge across a bayou in Louisiana, it
was proposed to drive piling for new steel bents between the existing concrete substructure units
that were to remain, and thereby carry a series of new bridge spans half the length of the existing
ones. Early on in the reconstruction effort, however, the contractor expressed concern and the
potential need for an “extra” because of the presence of abandoned timber piling that created
obstructions for the driving of the new bent piling. Although diving inspection could have been
used to identify the presence of any abandoned piles in the vicinity of the new construction,
acoustic imaging derived from scanning sonar was instead employed to render more precise
documentation of what may or may not be below water in the general vicinity of the proposed
bridge bents. Ultimately, the scanning efforts produced detailed, to scale, sectional images along
the bridge through the existing spans, on which the proposed locations of the new bent piling
could be located, and from which exact measurements could be made to any detected abandoned
piling or other possible obstructions. Overall, the scanning sonar images did reveal abandoned
timber piling throughout the channel at the bridge; however, in all but one instance, the locations
of the piling or obstructions could be shown to lie outside of the limits of what could pose a
problem for the driving of the new piling. Refer to Figures 11 and 12, which present typical
scanned images along the bridge spans showing the proposed new bent locations with respect to
various abandoned piles found to be sticking up from the channel bottom.
Figure 11 Figure 12
7151 Words
CASE STUDY 5: UNDERWATER CONSTRUCTION CONFIRMATION Along a portion of sheet pile bulkhead construction on the Indian River in Delaware, very high
riverine and tidal flow velocities were the source of an area of considerable scour and resulting
undermining at the base of the wall construction. Initially, because of the difficulty in
adequately accessing the scour conditions through diving inspections, scanning sonar apparatus
was used to develop highly accurate, to scale images of the extent of scour and the gap between
the bottom of the wall sheeting and the existing channel bottom. Based on the scanning
documentation, repair plans and specifications were then developed for the placement of stone
riprap to restore the bulkhead construction embedment, as well as to armor the channel bottom
against future scour activity. During the implementation of the riprap placement measures,
acoustic images derived from scanning sonar technology were again employed to confirm the
specified placement limits of the rock material. Based on the images obtaining, however, it was
determined that a portion of the bulkhead, and in fact the portion of bulkhead most affected by
the scour and undermining, did not receive the riprap placement that was specified by the repair
plans. Consequently, it was possible to precisely demonstrate this deficiency in the contractor’s
work and thereby direct the placement of additional riprap that met the requirements of the repair
plans, with the subsequent proper placement ultimately confirmed through additional scanning
sonar images. Refer to Figures 13 and 14, which depict scanning sonar images of the bulkhead
showing the lack of riprap placement along the critical section of wall, and the subsequent
placement of riprap that then addressed that portion of wall as specified in the repair plans.
Figure 13
Figure 14