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