Geotechnical Divers Tool Impact Corer

Figure 4-15. Fathometer.

future reference. Fix marks are inscribed on the chart by a FIX MARK switch (a remote switch may be connected).

The transducer is designed for precision survey work and other applications where a very narrow beam pattern is required. Such requirements result when extreme accuracy is needed in underwater laying of pipe or cable. The narrow beam pattern is also useful for more accurately defining steep sloping bottom contours and outlining relatively small bottom features.

4.4.4 Side Scan Sonar

Bathymetric surveys may be supplemented by side scan sonar. Side scan sonar consists of a towed receiver-transmitter, a cable to the tow ship, and a shipborne recorder. An acoustic pulse, similar to a fathometer's pulse, is sent out across the bottom. The pulse is returned from topographic variations and obstructions. Side scan sonar is particularly useful for finding pinnacles and other obstructions that may be missed by the fathometer. Planning, operating, and interpreting readouts of side scan sonar require a trained and experienced operator. Side scan sonar operations in less than 60 feet of water require special planning and operational techniques.

4.4.5 Visual Surveys

Once a cable or pipeline route has been established or an underwater construction site has been selected, a visual survey of the route should be made. Either divers or an ROV may be used. If divers are used then the Diver Navigation System is the preferred method of navigation. If an ROV is used then an Ultra Short Baseline Navigation System is the preferred method of navigation. Visibility permitting, photographs and video coverage of the features offer a valuable means of interpretation. With limited visibility, diver sketches may be substituted for photographs. Targets of interest may include: large rocks and boulders, large debris, cables or pipelines, and rock or coral outcroppings. Observations are recorded via diver-to-surface communications, underwater television, still photographs, or sketches prepared on either waterproof paper or a Plexiglas slate.

4.4.6 Geotechnical and Geophysical Surveys

Geotechnical data along a cable or pipeline route or for a shallow water array are often required. On soil seabeds it is necessary to classify and determine the area! and vertical extent of soil layers. It is extremely important to accurately establish and record the location of all samples and in-place tests. The specific types, numbers, and locations of samples and tests required depend on the nature of the project and should be established in consultation with the "user" of the geotechnical survey.

A set of six geotechnical diver tools is available to allow divers to collect accurate geotechnical site survey data and samples. The set includes the six tools plus items needed for operation and maintenance. The geotechnical tool kit (available through OCEI) is highly portable and designed to be transported and operated without the necessity of a large ship deployment or other platform-based systems. The tools, illustrated in Figure 4-16, are divided into two groups: six hand-powered and auxiliary-powered tools.

The hand-powered tools are:

• Impact Sediment Corer. This tool is capable of obtaining core samples to subbottom depths of 2.5 feet. It consists of a plastic core tube, a stainless steel head with attached handles, and a steel impact hammer.

• Miniature Standard Penetration Test. This tool is used to determine in-situ density profiles of noncohesive sediments. It consists of four basic components: cone, drive shaft, drive head and guide tube, and a drive weight.

• Vane Shear Tool. This tool consists of a torque wrench, drive shaft, and a four-bladed vane. Different sizes of vanes are provided for use in various types of soil to determine shear strength properties.

• Rock Strength Tool. This tool consists of a Schmidt test hammer enclosed in a watertight Plexiglas cylinder. The tool measures the in-situ compressive strength and modulus of exposed rock.

The auxiliary-powered tools are:

• Vacuum-Assisted Sediment Corer.

This tool consists of three major components: a jet eductor, a core tube apparatus, and a gasoline-engine-powered water pump. Core samples in sediments to subbottom depths of 10 feet may be obtained with this tool.

• Water Jet Probe. This device consists of a probe and a gasoline-powered water pump. The same pump is used with the vacuum-assisted sediment corer.

In addition to these diver-operated tools, geotechnical surveys frequently involve deeper coring and sampling using devices such as the vibracore, cone penetrometer, expendable doppler penetrometer, and resistivity probes. Personnel from NFESC Code ESC50 can provide assistance with this specialized equipment.

The geotechnical report may provide a summary of soil conditions along the pipeline route and soil test results obtained from the drop cores. The results of soil tests conducted by soils laboratories usually include:

• Core number, location, and water depth

• Soil classification

• Grain size analysis

• Atterburg limits

• Water content

• Specific gravity

• Undrained shear strength (Su)

• Consolidated undrained shear strength (Cu)

Geophysical surveys are used to remotely sense die geology of an area. The most common types of surveys are: seismic reflection, seismic refraction, and magnetics. The first two are used to profile the geologic strata, while magnetic surveys are usually concerned with location of bedrock. Reasonable estimates of the depth of soil strata can be obtained using near shore acoustical subbottom profiling techniques (reflection

Geothecnical Tools

Vane Shear Tool (3 feet)

Impact Sediment Corer Sampler (3 feet)

Miniature Standard Penetration Test Tood (6 feet extended; 3 feet stowed)

Rock Strength Schmidt Hammer (1.5 feet)

Vane Shear Tool (3 feet)

Cotton Template

Watet Jet Probe (10-foot long probe)

Vacuum-Assisted Sediment Corer (10-foot long probe)

Watet Jet Probe (10-foot long probe)

Vacuum-Assisted Sediment Corer (10-foot long probe)

Figure 4-16. Geotechnical diver tool.

and refraction). An experienced operator, however, is required to obtain reliable readings from this equipment, and trained engineers should interpret the data.

4.4.7 Specialized Underwater Construction Surveys

4.4.7.1 Construction Surveys. To accurately lay -:ut an underwater construction site may require very specialized techniques. The accuracy of the Diver Navigation System is 3 feet. If more accurate underwater surveys are required then other methods will be necessary. With high frequency acoustic systems it is possible to get accuracies of a couple of inches under the right conditions.

The most accurate surveys are conducted using conventional surveying hardware, theodolites, and tapes. Consider the problem of establishing a Third Order survey of two hydrophones located on the seaf-loor. If they were 1,000 feet apart, a Third Order survey requires 1/10 foot accuracy. An approach would use triangulation from two onshore theodolites, located over Third Order benchmarks to a target at the top of a controlled-buoyancy survey spar, which is used in place of the standard rod. The length of the spar is known and the survey spar is instrumented to measure its angle in two orthogonal directions. The "rod" is located over a point on the seabed by the surface support vessel and lowered into position. Two theodolites each track the spar, and upon reception of a "go ahead" through the communications link, readings are recorded. These readings should be taken when an inclinometer inside the spar reads close to vertical. Five to ten successive readings should be taken to obtain reproducible results. For nighttime surveying, a flashing strobe light can be fixed to the top of the spar. The spar is lowered and raised by ballasting and deballasting. This method is especially effective for construction site surveys where high accuracy is desired. The bottom location can be fixed by installing a stake driven into the seabed.

4.4.7.2 Quality Control Surveys. The

UCT may be tasked with assisting in the quality control inspection of underwater facilities. This type of survey requires that the UCT become familiar with the intended arrangement and details of the facility prior to making the inspection. Divers then inspect the facility to determine if it was constructed in accordance with design drawings. A visual record of the inspection should be made using still photography or underwater video.

4.4.8 Cable Surveys

When conducting an inspection of an oceanographic cable, determining the location of the cable is second in importance only to visually inspecting the cable. In this case, the Diver Navigation System (Section 4.4.2.4) would be used. The range of the DNS is 3,000 feet with a mirror image. Figure 4-17 shows a typical setup. The master transmitter is located to the left, when looking from shore. The slave transmitter is approximately 3,000 feet away and both master and slave are positioned 3,000 feet offshore. Using this configuration a nautical mile of cable can be surveyed with one reference station setup. As the diver tracks the cable and logs his observations, the diver also records fixes. At this point the data is all relative to the master and slave baseline. But after processing the X-Y data, using the real world position of the master and slave spar buoy, the data is in real world coordinates.

o//c ounvcr rtiOi*cuuricct

Beach

Beach

Underwater Construction

Slave Transmitter

Master

Transmitter

Figure 4-17. Cable route survey using diver navigation system

Slave Transmitter

Master

Transmitter

Figure 4-17. Cable route survey using diver navigation system

4.4.8.1 Onshore Cable Surveys. Sufficient benchmarks should be established so that all of the cable to a depth of 100 feet is visible from two benchmarks. If the site has been previously inspected, permanent benchmarks should have already been established. In this case, they should be checked to see that they are still in good condition and still provide the required visibility. If so, then all that is required is to confirm their positions. If new benchmarks are required, they should be established as described in Section 4.3.5 and with the following points in mind:

• Visibility of cable track from the position.

• Distance from other benchmarks (needs to be great enough for triangulation, about 0.75 mile).

• Accessibility of position during different weather and tide conditions.

4.4.8.2 Underwater Cable Surveys. The goal of the underwater survey is to determine the position and depth of the cable at 100-foot intervals and be able to relate these charted positions back to the cable. The cable will be surveyed using the diver navigation system. The master and slave reference stations are set up, perpendicular to the cable track (see Figure 4-17), with the master on the left (facing offshore). The location of the master and slave are recorded using Hydro-I or II to locate the DNS spar buoys.

After recording a baseline, the diver tracks the cable, using either the cable tracking system (Section 5.2.3), the BC&PL (Section 2.2.8), or visual observation. Assuming the diver starts at the shore end, he would record his first fix, track the cable until the Y value had decreased by 100 feet, then record the next fix. This method (measuring a 100-foot change in Y value) does not estimate the length of the cable. If needed, the length of the cable can be computed assuming a straight cable between fixes using:

This would continue until the baseline was crossed (i.e., 3,000 feet of the cable had been surveyed). Once the baseline had been crossed the fixes would be recorded based on increasing Y values. After 6,000 feet, the reference stations would have to be moved seaward 6,000 feet and resurveyed.

4.4.8.3 Marking the Cable When the Cable is Visible. In all portions of the track where the cable is visible, a permanent brass tag will be placed at each of the 100-foot stations. These tags, shown in Figure 4-18, will be provided by

NFESC Code ESC55 in packages of 1 to 60. The tags will assist a returning diver in relating his physical position back to the chart. Should there be more than one cable at a particular site, a 1/4-inch hole will be drilled in the comer of one set of tags to differentiate one cable from the other. Should three cables be on-site, two holes will be drilled in the third set and so on. The tags will begin with the zero tag placed as close to the intersection of the cable with mean sea level as the surf will allow, and then increase numerically as the cable progresses seaward. Some tags are inscribed with the cable number (i.e., MBS S-Y" or similar).

Along with attaching the tags at each station and taking a DNS fix, the bottom type and water visibility will be recorded on the diver's slate. The DNS depth sensor is in the receiver, thus the diver should ensure that he is either prone at the tag, kneeling at the tag, or standing at the tag when the fix is taken, and that he follows the same procedure at each tag. The fix number and tag number should be recorded on the slate.

Should the cable be more than 3 feet off the bottom, the height of the cable should be noted.

If the cable appears to be buried in the bottom for its entire length, the assumed path of the cable will be searched out to a depth of at least 60 feet. This will be done to see if the cable appears at some short lengths at certain spots. If die cable appears, its position will be surveyed and the standard depth/visibility information taken. The search can be conducted by starting at the shore end, just beyond the surf zone, and moving up the beach (increasing X) 50 feet, then moving offshore (decreasing Y) 100 feet, then searching down the beach (decreasing X) by 100 feet. As the survey proceeds offshore, the distance up and down the beach can be increased to produce a fan-shaped survey.

It should be noted that in cases where the cable is buried less than 6 inches, its path may be followed by probing the bottom with a knife or rod or by using the cable tracker as discussed in Section 5.2.3.

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