Subsea Survey

The subsea survey is described as a technique that uses science to accurately determine the terrestrial or 3D space position of points and the distances and angles between them in the seabed area for subsea field development.

Subsea Survey Requirements

Geophysical and geotechnical surveys are conducted to evaluate seabed and subsurface conditions in order to identify potential geological constraints for a particular project.

Survey Pattern for Selected Subsea Field and Each Pipeline Route

The base survey covers the whole subsea field development, which includes the infield pipelines, mobile offshore drilling unit (MOPU) footprints, pipeline end manifolds (PLEMs), manifolds, Christmas tree, umbilicals, etc. The nominal width of the pipeline route survey corridor is generally 1640ft (500 m) with maximum line spacing of 328ft (100 m). Different scenarios can be proposed provided full route coverage is achieved.

Geotechnical Study

A geotechnical study is necessary to establish data that allows an appropriate trenching design and equipment to be selected. It is also important to identify any possibility of hard grounds, reefs, shallows, and man-made debris. The electromagnetic properties of the soil are also of interest, and the potential effect that the ferrous content may have on the sacrificial anodes of certain subsea equipment such as manifolds and PLEMs needs to be assessed.

A grab sample/cone penetration test (CPT) is conducted at locations determined from review of the geophysical survey. Based on the test, the characteristics of the seabed soil around the subsea field development area can be determined. If there is a drastic change in one of the core samples, additional samples will be taken to determine the changes in condition. A piezocone penetration test (PCPT) should be obtained at the MODU and PLEM locations and FSO anchor locations.

Geotechnical gravity core, piston core, or vibracore samples are obtained 5 to 10 m from the seabed of the subsea equipment locations such as PLEMs, umbilicals, PLETs, etc. Samples should be suitable for a laboratory test program geared toward the determination of strength and index properties of the collected specimens. On board, segments (layers) of all samples at 1-m intervals will be classified by hand and described.

Samples for density measurements are also taken. At least one sample from each layer shall be adequately packed and sent to the laboratory for index testing and/or sieve analysis and unconsolidated, undrained (UU) triaxial testing. Cohesion will be measured on clayey parts of the core by torvane and a pocket penetrometer on board and by unconfined compression tests in the laboratory. The minimum internal diameter of the samples is generally 2.75 in. (70 mm).

Survey Vessel

The vessel proposed for survey should be compliant with all applicable codes and standards. The vessel must follow high safety standards and comply with all national and international regulations, and the marine support must be compatible for survey and coring operations. The survey vessels provided are collectively capable of the following:

Survey Vessel 1

• Minimum offshore endurance of 2 to 3 weeks.
• Operating in a maximum sea state of 2.5 to 3.5 m.
• Survey at speed of 3 to 10 knots.
• Supplying the necessary communication and navigation equipment.
• Supplying minimum required survey equipment: multibeam echo sounder, precision depth sounder, side-scan sonar, sub-bottom profiler, grab sampler/CPT, piston/vibracore coring equipment, and differential GPS (dual system with independent differential corrections).
• Supplying lifting equipment capable of safely deploying, recovering, and handling coring and geophysical equipment.

• Supplying adequate AC power to operate all geophysical systems

simultaneously without interference.

• Accommodating all personnel required to carry out the proposed survey

operations.

• Accommodating a minimum of two representative personnel
• Providing office space/work area. The area is fitted with a table/desk large enough to review drawings produced on board and to allow the installation of notebook computers and printers.
• The vessel should have radio, mobile telephone, and fax equipment. This equipment should be capable of accepting a modem hookup.
• The vessel should have a satellite or cellular phone link to report progress of the work on a daily basis. Communications with this system should not cause interference with the navigation or geophysical systems.
• At the time of mobilization, a safety audit is carried out of the nominated vessel to ensure compliance with standards typical for the area of operations, or as agreed. A current load certificate is supplied for all lifting equipment to be used for the survey operations (i.e., deep towfish, coring). Safety equipment, including hard hats, safety boots, and safety glasses, should be worn during survey operations.

Survey Aids

The survey vessel is normally equipped with an A-frame and heavecompensated offshore cranes that are capable of operating the required survey equipment. Winches are used for handling of sampling and testing equipment in required water depths. The winches have a free-fall option if required, such as for hammer sampling and chiseling. However, the winch speed should be fully controllable in order to achieve safe deployment. Geotechnical sampling and testing equipment are remotely operated. The tools are guided remotely and in a safe manner off and onto the deck. The vessels have laboratory facilities and equipment that can perform routine laboratory work. A vessel in the soil drilling mode should include the following:

• Sampling and testing equipment that is fully remotely operated;
• A pipe centralizer in the moon-pool is used;
• A pipe stab guide that is used when pipe stringing.

Gyrocompass

It is a compass that finds true north by using an electrically powered, fast-spinning wheel and friction forces in order to exploit the rotation of the Earth. Gyrocompasses are widely used on vessels. They have two main advantages over magnetic compasses:

Cutaway of Anschutz Gyrocompass

• They find true north, that is, the direction of Earth’s rotational axis, as opposed to magnetic north.
• They are far less susceptible to external magnetic fields, for example, those created by ferrous metal in a vessel’s hull.

The gyrocompass can be subject to certain errors. These include steaming errors, in which rapid changes in course speed and latitude cause deviations before the gyro can adjust itself.

On most modern ships the GPS or other navigational aid feeds into the gyrocompass allowing a small computer to apply a correction. Alternatively, a design based on an orthogonal triad of fiber-optic or ring laser gyroscopes will eliminate these errors as they depend on no mechanical parts, instead using the principles of optical path difference to determine rate of rotation. A dedicated survey gyro is installed on the vessel and interfaced to the navigation computer. During mobilization, calibration of the gyros is carried out while the vessel is at the dock.

Navigation Computer and Software

The navigation computer and software is capable of:

• Simultaneous acquisition of all navigation and sensor data as interfaced;
• Generation of closures to all geophysical acquisition equipment recorders simultaneously;
• Helmsman display showing vessel and fish position, proposed pipeline route, and intended survey line.
• Producing a header sheet and fix printout containing all relevant survey constants with bathymetry and position fix information.

Personnel

Besides the full complement of vessel operations personnel normally aboard a survey vessel, additional qualified personnel may be utilized to safely and efficiently carry out the survey and geotechnical operations. The number of personnel should be adequate to properly interpret and document all data for the time period required to complete the survey work, without operational shutdown due to operator fatigue. Qualified personnel interpret data during the survey and make route recommendations or changes based on the information gathered. This geophysical data interpretation should be performed by a qualified marine engineering geologist or geophysicist experienced in submarine pipeline route analysis.

Subsea Survey Equipment Requirements

For the main survey vessel, survey equipment is used to meet this specification. Vessels must follow high safety standards and comply with all national and international regulations, and marine support will be compatible for survey and coring operations. All survey systems are able to operate simultaneously with minimal interference.

Multibeam Echo Sounder (MBES)

The MBES, or swath echo sounder, is a high-precision method for conducting bathymet ric surveys obtained at water depths and seabed gradients over the corridor along the proposed pipeline routes. During data acquisition, the data density should be sufficient to ensure that 95% of the processed bins contain a minimum of four valid depth points.

MBES Working Model

The following issues are required for subsea survey equipment:

• Equipment specifications;
• Method of integrating the system with the vessel’s;
• Surface positioning system;
• Method of calibration;
• Method of postprocessing of data;
• On- and off-line quality control, with particular reference to overlap swaths.

The swath bathymetric system provides coherent data across the full width of the swath. Alternatively, the portion of the swath that does not provide coherent data should be clearly identified and the data from that portion are not be used.

A 50% overlap of adjacent swaths is arranged to provide overlap of acceptable data for verifying accuracy. In areas where swath bathymetry overlaps occur, the resulting differences between data after tidal reduction are less than 0.5% of water depth. Line spacing is adjusted according to the water depth to provide sufficient overlap (50%) between adjacent swaths to facilitate correlation of the data of the adjacent swaths.

Consideration should be given to installing a tidal gauge(s) or acquiring actual tidal data from an existing tide gauge in the area. If no nearby benchmarks of known height are available for reference, the tide gauge must be deployed for at least one lunar cycle.

Side-Scan Sonar

Side-scan sonar is a category of sonar system that is used to efficiently create an image of large areas of the seafloor. This tool is used for mapping the seabed for a wide variety of purposes, including creation of nautical charts and detection and identification of underwater objects and bathymetric features. Side-scan sonar imagery is also a commonly used tool to detect debris and other obstructions on the seafloor that may be hazardous to shipping or to seafloor installations for subsea field development. In addition, the status of pipelines and cables on the seafloor can be investigated using side-scan sonar. Side-scan data are frequently acquired along with bathymetric soundings and sub-bottom profiler data, thus providing a glimpse of the shallow structure of the seabed.

A high-precision, dual-frequency side-scan sonar system can obtain seabed information along the routes for example, anchor/trawl board scours, large boulders, debris, bottom sediment changes, and any item on the seabed having a horizontal dimension in excess of 1.64 ft (0.5 m). Sidescan sonar systems consist of a dual-channel tow-fish capable of operating in the water depths for the survey and contain a tracking system. The equipment is use to obtain complete coverage of the specified areas and operates at scales commensurate with line spacing, optimum resolution, and 100% data overlap.

The height of the tow-fish above the seabed and the speed of the vessel are adjusted to ensure full coverage of the survey area. The maximum towfish height is 15% of the range setting. Recorder settings are continuously monitored to ensure optimum data quality. Onboard interpretation of all contacts identified during the survey is undertaken by a geophysicist suitably experienced in side-scan sonar interpretation.

Sub-Bottom Profilers

The sub-bottom profilers are tested under tow at each transmitting frequency available using maximum power and repetition rates for a period of half an hour. During mobilization, the outgoing pulse from the transducer /seismic source is monitored to ensure a sharp, repeatable signature utilizing a suitably calibrated hydrophone. The monitored pulse shall be displayed on board on an oscilloscope with storage facility and a copy generated for a printout of the signature should be included in the final report.

A static or dynamic pulse test may be used to demonstrate a stable and repeatable seismic source signal producing a far-field signature at a tow depth of 3.28 ft (1 m):

• Pulse in excess of 1 bar meter peak-to-peak;
• Pulse length not exceeding 3 ms;
• Bandwidth of at least 60 to 750 Hz (–6 dB);
• Primary to secondary bubble ratio > 10:1.

High-Resolution Sub-Bottom Profiler

A high-precision sub-bottom profiler system is provided and operated to obtain high-resolution data in the first 10 m of sediment. The operating frequency and other parameters are adjusted to optimize data within the first 5 m from the seafloor. Vertical resolution of less than 1 m is required. A dual-channel “chirp” sub-bottom profiler may be capable of operating within the 3.5 to 10kHz range with a pulse width selectable between 0.15 and 0.5 m and transmitting power selectable between 2 and 10 kW. The system is capable of transmitting repetition rates up to 10 Hz. Transmitting frequency, pulse length, power output, receiving frequency, bandwidth, and TVG (Time Varying Gain) are adjustable.

A heave compensator is required when using a sub-bottom profiler. The system may either be collocated with the side-scan sonar system utilizing the same tracking system or hull mounted/over the side and reference the ship’s navigation antenna. Onboard interpretation of subbottom profiler records is carried out by a geophysicist suitably experienced in interpretation of such records.

Low-Resolution Sub-Bottom Profiler

The generic term mini air gun covers a range of available hardware that uses explosive release of high-pressure air to create discrete acoustic pulses within the water column, of sufficient bandwidth and high-frequency component to provide medium-resolution data for engineering and geohazard assessment. The system is capable of delivering a stable, short-duration acoustic pulse at a cycle repetition rate of 1 sec. The hydrophone comprises a minimum of 20 elements, linearly separated with an active length not exceeding 32.8 ft(10 m), with a flat frequency response across the 100 to 2000Hz bandwidth.

Magnetometer

A magnetometer is a scientific instrument used to measure the strength and direction of the magnetic field in the vicinity of the instrument. Magnetism varies from place to place because of differences in Earth’s magnetic field caused by the differing nature of rocks and the interaction between charged particles from the sun and the magnetosphere of a planet. Magnetometers are used in geophysical surveys to find deposits of iron because they can measure the magnetic field variations caused by the deposits. Magnetometers are also used to detect shipwrecks and other buried or submerged objects.

A towed magnetometer (cesium, overhauser, or technical equivalent) has a sensor head capable of being towed in a stable position above the seabed. The sensor head comprises a three-component marine gradiometer platform synchronized to within less than 0.1 m and able to measure 3D gradient vectors. The tow position should be far enough behind the vessel to minimize magnetic interference from the vessel.

In normal operation, the sensor is towed above the seabed at a height not exceeding 5 m. In the case of any significant contacts, further profiles across such contacts may be required. In these circumstances, the magnetometer should be drifted slowly across the contact position to permit maximum definition of the anomaly’s shape and amplitude. The magnetometer has field strength coverage on the order of 24,000 to 72,000 gammas with a sensitivity of 0.01 nanotesla and is capable of a sampling rate of 0.1 sec. The equipment incorporates depth and motion sensors and operates in conjunction with a tow-fish tracking system. Onboard interpretation of all contacts made during the course of the survey should be carried out by a geophysicist suitably experienced in magnetometer interpretation.

Core and Bottom Sampler

The gravity corer, piston corer, or vibracore can be deployed over the side or through the A-frame of a vessel or operated from a crane configured with a 70 mm-ID core barrel and clear plastic liners. The barrel length has 5 to 10 m barrel options. The grab sampler can be of the Ponar or Van Veen type, which can be handled manually. Both systems are capable of operating in a water depth 135% deeper than the maximum anticipated water depth. At all core locations, up to three attempts are made to acquire samples to the target depth of 5 to 10 m. After three unsuccessful attempts, the site is abandoned.

Positioning Systems

Offshore Surface Positioning

A differential global positioning system (DGPS) is utilized for surface positioning. A DGPS is capable of operating continually on a 24-hour basis. Differential corrections are supplied by communications satellites and terrestrial radio links. In either case, multiple reference stations are required. A dual-frequency DGPS is required to avoid problems associated with ionospheric activity.

A secondary system operates continuously with comparison between the two systems recorded along with the bathymetry data (geophysical vessel only). The two systems utilize separate correction stations, receivers, and processors. The system is capable of a positioning accuracy of less than 3.0 m with an update rate of better than 5 sec. During geophysical operations, the receiver can display quality control (QC) parameters to the operator via an integral display or remote monitor. The QC parameters to be displayed include the following items:

• Fix solution;
• Pseudo-range residuals;
• Error ellipse;
• Azimuth, altitude of satellite vehicles (SVs) tracked;
• Dilution of position (DOP) error figures for fix solution;
• Identity of SVs and constellation diagram;
• Differential correction stations, position comparisons.

A transit fix at a platform, dock, or trestle near the survey area is carried out to ensure the DGPS is set up and operating correctly. The transit fix is carried out in both clockwise and counterclockwise directions.

Underwater Positioning

An ultra-short baseline (USBL) tracking system is provided on board the offshore survey vessel for tracking the positions and deployments of the towed, remote/autonomous vehicles or the position determination of geotechnical sampling locations. The positioning systems interface with the online navigation computer. All position tracking systems provide 100% redundancy (a ship-fit USBL may be backed up by a suitably high-precision portable system), encompassing a fully backed up autonomous system. In addition, a complete set of the manufacturer’s spares are kept for each piece of positioning instrumentation such that continuous operations may be guaranteed.

The system, including the motion compensator, is installed close to the center of rotation of the vessel. It incorporates both fixed and tracking head transducers to allow the selection of the most optimum mode of performance for the known range of water depths and tow/offset positions. The hull-mounted transducer is located so as to minimize disturbances from thrusters, machinery noise, air bubbles in the transmission channel, and other acoustic transmissions. In addition, the transponder/responder mounted on the ROV or AUV will require suitable positioning and insulation to reduce the effects of ambient noise.

A sufficient number of transponders/responders, with different codes and frequencies, are used to allow the survey operation to be conducted without mutual interference. The system should perform to an accuracy of better than 1% of slant range.

References

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