Underwater Inspection and Evaluation
“A Critical Component” for Preventative Maintenance, Operation and Relicensing
William J. Castle, P.E.
W.J. Castle, P.E. & Associates,
P.C. 693 Main Street, Building B, Suite 1
Lumberton, New Jersey 08048 USA
E. Levels of Inspection
There are three basic types or levels of inspection used while inspecting dam facilities:
Level I – General Visual
This type of inspection involves no cleaning of any structural elements and therefore is the most rapid of the three types of inspections. The purpose of the Level 1 inspection is to confirm as-built structural plans, provide initial input for an inspection strategy, and detect obvious major damage or deterioration due to overstress, impacts, severe corrosion, or extensive biological growth and attack.
Level II – Close-Up Visual
This level is directed toward detecting and identifying damaged/deteriorated area which may be hidden by surface biofouling or deterioration and towards obtaining a limited amount of deterioration measurements. The data obtained should be sufficient to enable the gross estimates of the facilities load capability. Level II examinations will often require cleaning of structural elements. Since cleaning is time-consuming, it is generally restricted to areas that are critical or which may be representative of the entire structure. The amount and thoroughness of cleaning to be performed is governed by the necessity to determine the general condition of the overall facility.
Level III – Highly Detailed
This level will often require the use of Non-destructive Testing (NDT) techniques. It may also require the use of partially destructive techniques such as core sampling into concrete and wood structures, physical material sampling, or surface hardness testing. The purpose of this type of evaluation is to detect hidden or interior damage, loss in cross-sectional area, and material. A Level III examination will usually require prior cleaning. The use of homogeneity NDT techniques are usually limited to key structural areas, areas that may be suspect, or to structural members which may be a representative of the underwater structure. Level III inspections will require considerably more experience and training than Level I or Level II inspections and should be accomplished by qualified engineering or nondestructive testing personnel. This type of inspection is classified as a specialized inspection within the United States. (U.S. Navy Standards)
Special Testing, Level III Examination
The types of testing described in this section are among those which may also be used for Level III examinations. Level III examinations are employed when Level I and Level II examinations cannot conclusively determine the structural condition of the underwater items. Findings of previous inspections or the age of the structure may also dictate the level of examination needed. A Level III examination is not required for all inspections.
In steel structures, the inspector is often concerned with measuring the remaining thickness of corroded members. This can be done with a graduated scale, caliper and ultrasonic thickness measuring devices.
Graduated Scales – For measuring the exposed edges of flanges, a rule, or graduated scale is the most basic tool.
Calipers – Another simple method of thickness measuring is to use a set of calipers. Calipers are compact and easy to use under most conditions.
Ultrasonic Measuring Devices – Ultrasonic devices are also available for measuring remaining steel thickness. The device sends a sound wave through the member. It then measures the travel time of the sound wave and calculates the thickness of the steel. An advantage to this device is that it only needs a transducer to be placed on one side of the member.
Several nondestructive tests can be performed on concrete; however, the nondestructive testing instruments must be modified for underwater use.
V-Meter – The V-meter is an ultrasonic testing device. Using ultrasonic to check the condition of materials such as concrete requires two transducers. When taking measurements, the transducers can be arranged in three different positions. The direct transmission method provides the most accurate data. The semi-direct and indirect methods require correction factors to interpret the data. The V-meter measures the time it takes a sound wave to pass through a material. Location of discontinuities in the material, such as cracks and voids are determined by abnormal velocities. Data has to be interpreted by a trained technician or information may not be accurate. The Schmidt Hammer is a mechanical device which measures the compressive strength of in-place concrete. For underwater use, the hammer is placed within a housing and the equipment modified somewhat, including a special scale.
Coring is a partially destructive test method. It can be used alone or to verify and correlate data from non-destructive test methods. Cores obtained underwater can be tested in a laboratory in accordance with standard procedures. The actual levels of inspection to be used for a particular task must be decided early in the planning phase. This cannot be overemphasized, because the time and effort required to carry out the three different levels of inspection are quite different. The time required will also depend on environmental factors such as visibility, currents, wave action, water depth, severity of marine growth, and the skill and experience of the inspector.
Inspector-divers usually move from dive site to dive site. Often divers must enter waters of unknown quality. Local water quality monitoring agencies should be contacted to determine the degree of hazard the water presents, and appropriate precautions must be taken. These may include additional immunizations and the employment of special diving equipment to provide complete diver encapsulation. In some cases, it may be necessary to obtain water samples and have the samples tested prior to diving. Diving managers must ensure that proper precautions are observed. The long term effects on the diver of these and other occupational hazards may result in future liabilities.
F. Modes of Diving & Inspection Equipment
Within air diving, two principle modes are used: SCUBA, in which the diver carries his air supply with him in a tank, and surface-supplied diving in which the diver’s air source is in a boat or on shore. While some organizations may be predisposed to one mode over the other, both modes are permitted by OSHA standards and both have a place in dam inspections. In some situations, one mode may have significant economic benefits over the other, while providing all the inspection information required without in any way compromising safety. Their appropriateness for any specific diving situation depends on a number of factors including depth, bottom time, and the experience and capability of the diver. Each mode has unique operational advantages and disadvantages.
Scuba is an acronym for Self-Contained Underwater Breathing Apparatus. Scuba is generally recognized today in the open-circuit form: air is inhaled from supply tanks and the exhaust is vented directly to the surrounding water.
SCUBA is well suited for inspection work because if its portability and ease of maneuverability in the water where there are many dives of short duration at different depths than one sustained dive. SCUBA equipment weighs about 75 pounds and requires no elaborate support operation. It has the advantage that the diver does not have to drag an air hose behind him. The use of SCUBA is limited by OSHA to depths of 130 feet and the bottom time is limited by the amount of air the diver can carry with him.
Surface to diver communication is possible using SCUBA with either hardwire or wireless systems. Communication may be desirable in deep water or for complicated structures. Wireless systems are also available which allows for greater mobility.
Surface Supplied Air
There are two types of surface-supplied equipment: deep-sea (hard-hat) and lightweight. Deep-sea equipment consists of a helmet and breastplate, diving suit and weighted shoes. The equipment worn by the diver alone can weigh more than 200 pounds. Add to this the air compressor, hoses, lines and possibly a diving launch to work from, and the problems of mobility and transportation become significant. This equipment is cumbersome, and generally not considered economical for most modem diving operations since the development of lightweight equipment. The deep-sea hard-hat has changed little in the last 150 years.
Lightweight equipment usually consists of a full face mask or helmet, safety harness, weight belt, boots or fins, back-up air supply, and an exposure suit (a wet or dry suit). Early helmets were free-flow air hats in which a constant stream of air was supplied to the diver. Today, demand regulators similar to those used in the second stage of SCUBA equipment have been incorporated in helmets and full face masks.
In a surface-supplied system, the air is supplied by a high volume, low-pressure compressor or from a bank of high-pressure cylinders equipped with a regulator to reduce the high pressure.
The diver’s umbilical (Life-Line) is a combination air hose, safety line, communication cable, and pneumofathometer hose.
The primary advantage of surface-supplied air diving is the “unlimited” air supply. Longer bottom times can be obtained on surface-supplied dives if decompression schedules are used. Dives in accordance with OSHA standards can be conducted to a depth of 190 feet or, if bottom times are less than thirty minutes, to depths of 220 feet. The major disadvantage of Surface Supplied diving is the lack of mobility. Inspection work generally requires the diver to constantly change depth or travel around structures or obstacles. In doing so, the diver using Surface Supplied equipment may become entangled in his umbilical. As a minimum, he has the added effort of dragging it after him.
This is part 2 of a 4-part series.
You can find part 1 here.
Read part 3 here.
Read the 4th of this 4-part series here.