Inspect holes in steel sheeting for loss of backfill material through the opening and subsidence of adjacemtn ground surface.

Table 3-4

Underwater Voltmeter Values for Steel Structures

Table 3-4

Underwater Voltmeter Values for Steel Structures

Voltage Measured (V)


0.0 to -0.7

Steel is cathodically unprotected. The rate of corrosion depends on the effectiveness of paint or tar coatings, marine growth, and local water chemistry and water currents. On some structures, the hard layer of marine growth may provide some protection. The closer to 0 volts the more active is the corrosion potential. Note: -0.6 volt is the potential of bare, unprotected steel in seawater.

-0.7 to -0.82

The steel is partially protected.

-0.83 to -1.1

The steel is adequately protected. Cathodic protection systems are working effectively.

-1.1 or higher negative values

The steel is "overprotected." Note: Under some circumstances, the metal surface can be made more britde when overprotected. Surface coatings may be damaged or "lifted off" by the excess formation of hydrogen bubbles.

• UWMT Applications - Underwater magnetic particle testing is used primarily as a quality assurance tool to support underwater welding on ship structures. It can also be used to inspect hulls or other magnetic components for surface discontinuities such as cracks and lack of fusion in welds. UWMT can be used to define the true length (and locate the true ends) of discontinuities detected visually and to help determine where corrective measures (e.g., stop drilling) should be applied.

• UWMT Limitations - As with any inspection method, UWMT has some limitations. These include:

1. Underwater magnetic particle testing has limited subsurface capability. For use on U.S. Navy applications, it is considered to be stricdy a method for detecting and measuring surface discontinuities. It is not an approved method for detection of subsurface discontinuities.

2. The adequacy of inspection with UWMT (as with most nondestructive test methods) is largely a function of the operator's knowledge and skill. Inspections with UWMT are to be performed only by personnel trained and specifically certified in UWMT.

3. UWMT is limited to ferromagnetic materials, which include most steels. For most applications, a simple check with a magnet is sufficient to determine suitability for UWMT.

• Certification Requirements for UWMT - Personnel performing UWMT require certification in UWMT. Certification can only be obtained by training and examination.

Divers may be trained either by a certified UWMT examiner or by NAVSEA's agent. Training can be done at either the diver's activity or at the agent's facility, as determined by NAVSEA.

* Specific Preparation Requirements for UWMT - Preparation for conducting UWMT entails assembling all necessary material and personnel required to safely satisfy the plan requirements. Divers must determine that the water current will not affect the application of magnetic particles where UWMT is to be conducted. Water currents greater than 1 knot make it difficult to perform UWMT. Divers must also determine that the underwater visibility is adequate for the interpretation of the test results.

The following paragraphs describe inspection equipment required to conduct UWMT. Inspection equipment specific to UWMT, along with general surface preparation and recording equipment are:

- Inspection Equipment

Hydraulic or pneumatic hand-held grinder or high-pressure water jet Anti-corrosion coating (epoxy)

- Recording Equipment

Stereo and/or still camera (optional) Video and monitor system (optional) Measuring devices (tape or rule) Plexiglas writing slate, grease pencil, arrow punch

For fleet users, the equipment for UWMT is specified in NAVSEA drawing number 6653063. This equipment meets the requirements of MIL-STD-271F. Only this equipment may be used. UWMT equipment includes:

• Electromagnetic Yoke - An electromagnetic yoke (Figure 3-25), which meets the specifications of MIL-STD-271 F and is listed in NAVSEAINST 10560.2C, is used to induce a magnetic field in the material. The articulation of the yoke's legs allows any pole spacing between 2 inches and 8 inches, and the yoke can accommodate plate offsets of up to 6 inches and joint angles from about 45 degrees to 270 degrees. To minimize the risk of electric shock, there are no controls on the yoke. A topside operator energizes the yoke.

Electronic yoke with power cable Ground fault interrupter (GFI) White light source UAV-1 magnetic particles with applicator Magnetic field indicator

Surface Preparation and Restoring Equipment

Diver staging (optional)

• Power Cable - A two-conductor power cable with a braided external ground and a protective jacket (Figure 3-26), listed in NAVSEAINST 10560.2C, delivers power to the electromagnetic yoke. Typical cables are 250 feet long to permit operation in the majority of locations accessible by a surface-supported diver. The external ground braid generates a ground fault whenever the cable is cut; the power conductors cannot be reached except by first penetrating the ground braid.

Figure 3-25. Electromagnetic yoke.

External ground braid

/ V- conductors

L. Neoprene jacket

Figure 3-26. Two-conductor power cable with braided external ground and neoprene jacket.

• Ground Fault Interrupter (GFI) - The diver's primary protection from electric shock is an approved isolation transformer/ GFI device (Figure 3-27), listed in NAVSEAINST 10560.2C. The GFI interrupts power when it senses a drop in the resistance between the isolated system power leg and the power supply ground.

• White Light Source - The primary illumination source is a Remote Ocean Sys tems model TUBE-LIGHT, listed in NAVSEAINST 10560.2C.

• Magnetic Particles - Magnetic particles are finely divided ferromagnetic particles having a low magnetic retentivity and a high permeability. The magnetic particles used for UWMT must meet the specifications of MIL-STD-271F. They are dyed pink to be visible under normal lighting. The particles are mixed with wetting agents and corrosion inhibitors to enhance their underwater performance.

• Magnetic Particle Applicator - The magnetic particle applicator is a reservoir of magnetic particles and water that the diver uses to deliver magnetic particles to the surface of the inspection area. A simple and effective magnetic particle applicator is a plastic squeeze bottle that contains a marble-sized object to aid in mixing.

• Magnetic Field Indicator - The magnetic field indicator is a small device with crack-like discontinuities on its face. The indicator is used to determine if the inspection site has adequate magnetic flux. The diver places the indicator at the inspection site, topside energizes the yoke, and the diver delivers the particles. A clearly visible accumulation of particles (indications) should then form along the crack-like discontinuities on the pie-shaped magnetic field indicator (Figure 3-28). In the pie-shaped gauge, the crack-like discontinuities are furnace-brazed joints between adjacent steel wedges. Though a simple test to measure the magnetic field strength inside the material being inspected is unknown, it is assumed that an adequate field just outside the material signifies an adequate field inside as well.

Underwater The Ground Anchor
Figure 3-27. Ground fault interrupter.
Underwater Pile Hammer Weight
Figure 3-28. Magnetic field indicator.

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