Seabed Topography

Example: Seabed topography, if extreme, can affect mooring and can raise concerns over long-term stability of the ocean floor.

From: Deepwater Drilling , 2019

Engineering Site Survey for Submarine Optical Cable

In Submarine Optical Cable Engineering, 2018

5.1.2.1 Water Depth and Seabed Topography

Measurements of water depth and seabed topography include plane control, elevation control, water level observation, water-depth measurement, water-depth data processing, and mapping, and ultimately require sufficient information and results to meet the needs of engineering site selection, design, and construction. Echo-sounding is usually used as a means of measurement; the echo sounder, especially a multibeam echo sounder, is the main measurement equipment.

The main measurement result of water depth and seabed topography is the seabed topographic map, which is able to reflect the seabed terrain, geomorphic features, such as submarine ups and downs, ditches, troughs, scarps, sand waves, sand ridges, and so on. The seabed slope is also of great concern in the cable burial installation. A slope of more than 6   degrees will pose a risk to the burial plow. Adequate digital terrain model (DTM) grid resolution should be maintained during the water-depth records and data processing, so that in these unique terrains, the geomorphology can be revealed, and to ensure that in these terrains, the geomorphology in different sources of data maintains consistency.

Fig. 5.1 shows the seabed pit group detected by a multibeam echo sounder in a cable-routing survey. The depth of the pit is more than 1   m, with a steep edge to the pit, and a flat bottom. It is easy to overturn the burial plow when it passes through the pits. The side-scan sonar record of this kind of microgeomorphology is not very obvious. Shallow stratigraphic records can reflect the profile of the pits, but cannot reflect the whole picture of the pit. A multibeam, three-dimensional water-depth map can reflect the distribution and terrain changes of the pits, and can provide more intuitive and valuable information for cable construction.

Figure 5.1. Seabed pits detected by multibeam sounding system (East China Sea continental shelf).

Fig. 5.2 shows the bare seabed bedrock found in a cable-routing survey. The side-scan sonar, the sub-bottom profiler, and the multibeam echo sounder have all detected this geological hazard phenomenon, but the location, height, and scope of the exposed bedrock given by the multibeam 3-D water-depth data are the most accurate and the most intuitive, and can be used as the main basis for the routing design.

Figure 5.2. Exposed bedrock detected by multibeam sounding system (Taiwan Strait).

Fig. 5.3 shows the seabed sand wave found in a cable-routing survey. The wave direction, wave height, and wave length of the seabed sand wave can be calculated by the multibeam 3-D water-depth data, and then the sand wave direction, erosion, or transformation by the current can be judged.

Figure 5.3. Seabed sand waves detected by multibeam sounding system (Taiwan Shoal).

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780128134757000059

Hazards and protection

K.J. Rawson MSc, DEng, FEng RCNC, FRINA, WhSch , E.C. Tupper BSc, CEng RCNC, FRINA, WhSch , in Basic Ship Theory (Fifth Edition), 2001

Abnormal Waves

Abnormal waves can be created by a combination of winds, currents and seabed topography. A ship may be heading into waves 8 m high when suddenly the bow falls into a long, sloping trough so that in effect it is steaming downhill. At the bottom it may meet a steep wall of water, perhaps 18 m high and about to break, bearing down on it at 30 knots.

Vessels can suffer severe damage. In 1973 the 12,000 grt Bencruachan had the whole bow section forward of the break of the 36.6 m long forecastle bent downwards at 7 degrees by the impact of a 15 m wave. A 15,000 dwt freighter was broken in two by a freak wave on its maiden voyage. The VLCC Athene met a 30 m wave which knocked out all the windows of the crows nest, nearly 18 m above the laden sea level. Even very large ships can be overwhelmed by exceptional sea conditions.

Mariners have always been aware that 'freak' waves could be experienced but for many years it was thought that they could not be predicted or quantified, and so could not be taken into account during the design process. By basing a new ship on a previously successful one it was hoped it could survive in other than very abnormal conditions. It did not follow, however, that the basis ship had met the most extreme waves that might be encountered in service.

As with so many other aspects of naval architecture this is a matter where statistics can be employed although our knowledge is still imperfect. The so-called freak waves are not unexplained quirks of nature and oceanographers can calculate their probability of occurrence, assisted by extensive wave data from buoys, ships and satellites. Buckley and Faulkner have done a lot of work in this area. They support retaining the present design methods for dealing with the normal operations of ships. They suggest these methods should be supplemented with procedures intended to give a ship at least some low safety capability to survive critical operational conditions under abnormal waves, unlikely as the ship may be to encounter these waves. To do this requires establishing the critical design conditions, associated seaway criteria and analytical methods and criteria.

Critical design conditions would include failure of main propulsion or the steering system, loss of rudder and so on. That is the designer should allow for the ship suffering a range of defects which degrade its capabilities. They should also, as far as may be possible, make allowance for human error. That is they should try to foresee the consequences of human actions which might not be the best in the circumstances. Whilst human error cannot be eliminated it may be possible to reduce the consequences by making the design more tolerant of such errors.

As to waves, opinions are hardening up as more data becomes available. Students should refer to the latest ideas, for instance, the work of Hogben and oceanographers. Longuet-Higgins has suggested that the extreme wave height during the chosen survival design storm is given by

H s [ 0 .5(In N -In(-In(1- p e ))) ] 0 .5

where p e is a small but acceptable probability of H s being exceeded, perhaps less than 5% during N wave encounters. N is given by dividing the length of time considered by the wave period T p. Separately it has been proposed that, in metric units, the range of waves considered should embrace

T p 2 from 13 to 30 times the significant wave height .

Amongst other things, the naval architect must design for the impact loads arising from the severe waves both on near vertical structures such as bridge fronts and on decks and hatch covers.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B978075065398550008X

Finite Element Analysis of In Situ Behavior

Qiang Bai , Yong Bai , in Subsea Pipeline Design, Analysis, and Installation, 2014

Seabed Model

The basis for constructing the 3D seabed model is data from measurements of the seabed topography from bathymetric surveys in the area where the pipeline is to be installed. From this information, a corridor of width up to 40 m and lengths up to several kilometers is generated in the FE model to ensure a realistic environment when performing analysis of the pipeline behavior.

The seabed topography is represented with four node rigid elements that make it possible to model flat or complex surfaces with arbitrary geometries. An advantage when modeling the three-dimensional seabed is the smoothing algorithm used by ABAQUS. The resulting smoothed surfaces, unlike the flat rigid element surfaces, have a continuous outward surface normal across element boundaries and model the seabed better. The smoothed surfaces do not match the faceted geometry of the rigid surface exactly, but the nodes of the rigid elements defining the seabed always lie on the surface.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780123868886000080

Finite Element Analysis of In-situ Behavior

Yong Bai , Qiang Bai , in Subsea Pipelines and Risers, 2005

7.5.4 Seabed Model

The basis for constructing the 3-D seabed model is data from measurements of the seabed topography (bathymetric surveys) in the area where the pipeline is to be installed. From this information a corridor of width up to 40 m and lengths up to several kilometers is generated in the FE model to ensure a realistic environment when performing analysis of the pipeline behavior.

The seabed topography is represented with four node rigid elements that make it possible to model flat or complex surfaces with arbitrary geometries. An advantage when modeling the three-dimensional seabed is the smoothing algorithm used by ABAQUS. The resulting smoothed surfaces; unlike the flat rigid element surfaces will have a continuous outward surface normal across element boundaries and model the seabed better. The smoothed surfaces will not match the faceted geometry of the rigid surface exactly, but the nodes of the rigid elements defining the seabed will always lie on the surface.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780080445663500099

Marine Natural Geography and Sea Boundary Division

In Submarine Optical Cable Engineering, 2018

2.2.2.1 Seabed Topography

From the Bohai Sea, the Yellow Sea to the East China Sea, the South China Sea and the eastern waters of Taiwan Island, the seabed topography of the China Sea is flat or undulating, a shelf connects with the slope, and the island arc lies adjacent to the basin, which formats the unique submarine topography features of China seas ( Fig. 2.4).

Figure 2.4. China sea area topography map (Ye et al., 2012).

The Bohai Sea is an inland shallow sea and is located in the northern part of China, which is surrounded by the Liaoning, Hebei, and Shandong provinces. The Bohai Sea connects with the Yellow Sea via the Bohai Strait and consists of Liaodong Bay, Bohai Bay, Laizhou Bay, and the Bohai Strait. Due to the major coastal rivers carrying a large amount of sediment into the sea for a long time, the seabed of the Bohai Sea is always maintained as a flat shallow basin. The average depth of the Bohai Sea is about 18   m, the maximum water depth is 86   m, the sea area being less than 30   m water in depth accounts for about 95%, the seafloor is flat, and the average slope is only 0.14‰.

The seafloor of the Yellow Sea tilts slowly from the north, east and west to the central part and southeastern part, and forms the Yellow Sea trough in the middle-eastern part of the South Yellow Sea. The average water depth of the Yellow Sea is 44   m, mostly being within 60   m. Inside the Yellow Sea trough it is 60–80   m, and in the direction of Jeju Island, the water depth increases to 90–100   m, with a maximum of 140   m. In the center are the vast shallow plains, while underwater ridge and groove development are near the Yellow Sea coast.

Seabed terrain of the East China Sea is shallow in the west and northwest but deep in the east and southeast, with an average water depth of 370   m and a maximum depth of 2719   m. The seabed is clearly divided into the western continental shelf shallow water area and the eastern continental slope-trough (Okinawa Trough) deepwater area, and the continental shelf break zone is their dividing line.

The average water depth of the South China Sea is 1212   m, with a maximum water depth of 5567   m, and the seabed is a deep basin that descends from the sides to the center and tilts to the center. This stepped slope is the continental slope of the South China Sea, with a water depth of between 150 and 3600   m. The Dongsha, Zhongsha, Xisha, and Nansha Islands are coral islands distributed in the continental slope. The central deep-sea basin of the South China Sea is tilted from north to south, and some of the submarine isolated peaks stand in the middle of the basin, rising above seafloor from about 1500 to 3500   m, with the highest up to 3904   m.

The Pacific sea area in the east of Taiwan Island with an extremely narrow continental shelf, a cramped and steep continental slope, and ocean basin is a great topographic relief. Most of the water depth is greater than 4000   m, and the maximum water depth is 7881   m (Wang, 2012).

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780128134757000023

Desktop Study of Site Selection for Submarine Optical Cable Engineering

In Submarine Optical Cable Engineering, 2018

4.1.1.2 Marine Route Selection

The cable landing point selection principles (1) and (5) mentioned herein are also applicable to the selection of cable route in the sea. In addition, the following principles should be followed in the selection of marine cable route:

1.

Selecting flat or less-undulating seafloor and avoiding unfavorable seabed topography such as reef, scarp, deep trough, and trench as much as possible.

2.

Generally, fine-grained seabed sediments are selected, with less variation of sediment type. Areas with exposed bedrock, manufactured and natural obstacles, severe seabed erosion, and siltation should be avoided.

3.

Avoiding geological hazard areas such as submarine landslide, turbidity, active fracture, strong earthquake, and high seismic activity as much as possible.

4.

Avoiding the area with frequent bottom fishing operations as much as possible, especially stow net and bottom trawl intensive areas.

5.

Avoiding port, wharf, and anchorage as much as possible. The cable route should pass through the navigation channel as vertically as possible.

6.

Avoiding offshore oil and gas exploitation areas, solid mineral mining areas, military sea areas, nature reserves, and other ecologically sensitive areas, etc., as much as possible.

7.

Avoiding crossing with existing submarine cables and pipelines or near a submerged repeater/branching unit as much as possible. If unavoidable, the crossing angle should not be less than 45   degrees. The distance between the route and the existing submarine repeater/branching unit cannot be less than three times the water depth.

8.

Avoiding unnecessarily close parallels to other existing cables and pipelines. When the route is parallel to existing submarine fiber-optic cables, power transmission cables, or submarine pipelines, the spacing cannot be less than three times the water depth in order to avoid damage to them during route survey installation and maintenance.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780128134757000047

Climate Change and Hydrovision

Derrick M.A. Sowa , ... Martin K. Domfeh , in Sustainable Hydropower in West Africa, 2018

4.2.1 Geomorphological and Met-Ocean Feasibility

Given the location of Ghana, the prevalent winds will aid in the successful implementation of this device. The prevalent south-westerly winds travel from the Atlantic onto the continent. These winds are of great magnitude with seasonal variations and transmit their energy into the ocean. Ly (1980) and Boateng (2009) have shown that the prevailing south-westerly winds cause an oblique wave approach to the shoreline. This wave approach generates an eastward littoral transport, which contributes to the erosion and accretion in the Eastern Coastal Zone. For this reason, the Eastern Coastal Zone is considered a high-energy beach (Ly, 1980). The nature of waves in this area demonstrates the possibility of installing the conversion system: The average wave height for the identified area is approximately 1.39   m with a mean period of about 10.91   s (Boatemaa et al., 2013). The issue of littoral transport will be addressed by the proper orientation and installation of the entire setup. Since calming of waves can be achieved by the installation of this system, coastal erosion in the area is likely to be significantly curtailed.

In addition, the sea bed topography of the estuary, from the continental shelf all the way into deep waters, consists of numerous canyons Manu et al. (2005). Waves that reach this point normally behave as though they were in deep waters, and this attribute aids in the propagation of relatively large waves that can be tapped by the WECD. Within this particular zone, the average depth of the coastal waters, the topography of the seabed, and the relatively low traffic density presuppose that implementation of the conversion system is practically achievable. In the event of SLR, the ensuing conditions are likely to favor the installation of the system.

When installed accurately, the device will reduce several environmental issues (Thorpe, 1999). The distribution of the generated energy will be done through an off-grid system. By and large, this is going to reduce the electric power deficit in the country. The advantages reaped from this project ranges from the availability of reliable electric power, which would inevitably be translated into the decreased dependence and pressure on the presently available sources of energy production. This will cause a parallel increase in productivity due to the availability of uninterrupted power supply. In terms of the coastal stability of the Eastern Coastal Zone, the engineering measures put in place will curtail the rate of coastal erosion due to the dampening of waves (Shaw, 1982) by the system, and enhance a healthy rate of accretion. The system will act as a protection between the vulnerable coastline and the ocean, and insulate the coastal communities from coastal erosion and subsequent flooding. The most remarkable aspect of this project is that the source of the energy (i.e., the ocean) is free, unlimited, and at Ghana's disposal.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780128130162000010

Route Optimization, Shore Approach, Tie-In, and Protection

Qiang Bai , Yong Bai , in Subsea Pipeline Design, Analysis, and Installation, 2014

Design of Shore Approach

The choice of a shore approach location is critical and demands a very careful engineering review of the coastal hydraulics, geotechnics, and environment, in parallel with a study of the proposed construction method. Normally, the following design issues are discussed in the shore approach design:

•

Coastal environment.

•

Pipe wall thickness.

•

Pipeline stability.

•

Cover depth.

•

Cathodic protection.

•

Environmental concerns.

•

Installation considerations.

Coastal Environment

As waves move toward a shore they become higher and steeper. A wave moving toward a shelving shore eventually becomes so steep that it breaks. Most waves break when the wave height is about 0.8 of the local water depth, although there is a variation between waves and an interaction with the flow created by the previous wave. The form of the breaker varies and depends primarily on the beach slope. Refraction alters the direction of propagation.

Waves that approach the shore obliquely induce a longshore current, which transports sediment stirred up by the wave-induced movement of the water close to the seabed. These currents modify the seabed topography through sediment transport processes, and these changes in turn modify the pattern of the breaking waves.

Pipe Wall Thickness

The selection of pipe wall thickness of subsea pipelines for the shore approach and landfall section can also be affected by pipeline on-bottom stability.

Pipeline Stability

The shore approach is usually characterized by relatively high environmental forces, due to waves and tidal currents. Pipelines in the shore approach are usually installed within a predredged trench and the sheltering effect of the trench should be taken into account in the stability analysis. In addition, significant embedment of the pipeline can be expected in sandy seabeds, which should also be considered in the analysis. Negative buoyancy of the pipe should take into account the increased soil density (liquefaction) during the artificial backfilling operations.

Cover Depth

Subsea pipelines should be buried in the shore approach; otherwise, alternative protection measures, such as rock dumping, are required. The depths of pipeline cover influence whether postinstallation trenching of the pipeline is feasible or preinstallation dredging and excavation should be performed. Unless a specialist trenching machine is to be used, which is confidently expected to achieve the specified cover requirements, preinstallation dredging is usually preferred.

Normally, it is suggested that the minimum depth of cover be 2.0–3.0 m to ensure that the pipeline stays buried well below any future erosion of the beach, and this has generally been adopted as a standard industry practice. The cover should extend to the location seaward at least 500 m from the beach or the final water depth of the cover is approximately 12 m below the lowest astronomical tide (LAT) to ensure the laying barge has an adequate draft. The cover seaward of this location should either be compatible with the offshore cover requirements or sufficient to provide long-term stability to the pipeline by decreasing exposure to environmental forces.

A typical backfill system comprises sand and gravel to a minimum of 800 mm on top of the pipe, a further layer (minimum 0.8 m) of rock on top of the sand, gravel of average size of 100 mm, and then a further layer of rock of average size of 350 mm.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780123868886000225

Autonomous environment and target perception of underwater offshore vehicles

Hongde Qin , in Fundamental Design and Automation Technologies in Offshore Robotics, 2020

4.1 Introduction

The earth's land resources will be exhausted, and the ocean is rich in resources, so the marine resources exploration using underwater offshore vehicles is a necessary development strategy all over the world. However, the complex underwater environment limits the application of autonomous underwater sensing technology [1]. Underwater autonomous environment and target perception technology has been applied for long-term underwater autonomous detection operation or avoiding obstacles [2–5]. Whilst underwater exploration is implemented with AUVs, environmental perception is the prerequisite for underwater operation [6,7] . AUVs have been applied in many underwater fields, such as seabed topography drawing [8], identification and tracking of subsea pipelines [9], visual navigation [10], and exploration of seafloor mineral resources [11].

A lot of research on land or air moving targets tracking is in progress. However, there are few researches on underwater target tracking. Ocean observation has experienced a rapid development in the past decade [12–14]. Underwater image processing is the key for understanding underwater environmental information. Poor underwater image quality will undoubtedly affect the perception and detection of underwater environment [15]. The absorption and scattering of underwater light can make the color of underwater image appear blue-green [16]. The scattering of underwater transmitted light decreases the underwater image quality [17]. In addition, the uneven auxiliary lighting on the AUV often results in organic matter and suspended particles in the underwater image [18]. In an underwater image captured by one underwater image, due to the limited view angle, there is insufficient information obtained [19]. Therefore, we are increasingly interested in underwater image filtering, enhancement, registration and mosaicking [20].

The research on underwater image enhancement and mosaicking has developed rapidly. However, the complex underwater detection conditions pose a huge challenge to the autonomous environmental perception. Besides, serious degradation of underwater image lacks effective feature information [21]. Convolutional neural network (CNN) has developed with a high speed, and it has made great progress in the field of computer vision [22]. An underwater image CNN based enhancement is proposed and implemented to filter the noise and generate a clear underwater image. VGG net is involved to extract more exact matching points. Underwater target features are difficult to be extracted, so it is hard to track multiple targets using acoustic image features [23].

Nowadays, great progress has been made in the field of image enhancement and panoramic image mosaicking technology, such as classical fuzzy image enhancement, which have achieved good results in terrestrial or air images. However, these algorithms cannot compensate for the decrease of sonar image quality. In the same way, the adaptive thresholding segmentation will result in false segmentation of noise into targets, which makes it not suitable for underwater complex environment. Accurate image segmentation is the key factor of multiple target identification and tracking. High threshold region growing algorithm will lead to "oversegmentation" of an image, which results in some target regions with low gray level segmented as background regions.

Traditional data association methods include NNDA [24], PDA [25], JPDA [26], multiple hypothesis tracking method [27], and clustering method [28]. In addition, the minimum cost flow method is used for determining the optimal data association algorithm. J. Neira of the University of Zaragoza [29] proposed a new measurement system with joint compatibility, which adopts restrictive standards to effectively find the best solution for data association. The traditional feature selection and generation methods include the analysis of Fisher linear discriminant, clustering method, artificial neural network, and so on. Carin has presented a relevance vector machine feature selection and classification method [30]. Pezeshki used the spatiotemporal correlation to demonstrate sonar data orientation characteristics for extracting underwater target features [31]. However, these methods are hard when generating the stability features of sonar images.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780128202715000092

Installation of the Submarine Optical Cable

In Submarine Optical Cable Engineering, 2018

6.1.2.2 Remote-Operating Vehicle

Similar to the new burial machine, the ROV is an important item of equipment on the modern submarine cable ship. It has the characteristics of a simple structure, including easy maintenance, alternative diving personnel to bear "saturated diving," and so on. It is widely used in the marine oil industry. With the rapid development of submarine optical cable technology, the new multifunctional, high-powered, more automated, and higher-burial ability ROV in the submarine optical cable installation plays a very important role. The main function of ROV is as follows.

1.

Basic performance of ROV

In the submarine optical cable installation, ROV SEALION III owned by S.B. Submarine Systems is a typical ROV with trenching capability (Fig. 6.9). It has 600   hp, using the world's most advanced ROV technology. Compared with the conventional ROV, a strong water flow produced by it can significantly improve the ROV burial capacity, especially the multifunctional manipulator with flexible operating performance.

Figure 6.9. SEALION III ROV

ROV SEALION III has the following characteristics:

a.

An intelligent power system, providing more powerful power support for the burial system;

b.

Advanced hydrodynamic design and high-power propulsion device that can carry out operations in conditions under 3   kt current;

c.

Large-flow-designed tools with 2–3   m burying capacity;

d.

With an adjustable jetting tool, it can be used for different-diameter submarine optical cables, repeaters, and pipeline burial operation;

e.

With an electronic sensor on the device, it is able to press down the cable and more accurately control the burying operation;

f.

Suitable equipment for submarine optical cable recovery.

The main performance indicators of SEALION III are shown in Table 6.2.

Table 6.2. Main Performance Indicators of SEALION III

Item Lightweight Type Crawler Type
Dimensions (L/W/H) 6.5   m/3.7   m/3.1   m 6.5   m/5.2   m/3.1   m
Weight in air 17,250   kg 18,400   kg
Operating water depth 5–2500   m 5–2500   m
Propulsion system Six horizontal propellers and four vertical propellers
Fore-Aft speed/ translational speed 3.0   kt/2.0   kt 1.1   kt
Power 440   V, 60   Hz, 3 DC, 1250   A
Power for jetting bury Pump power can reach 359   kW, pressure 4.2   Mpa, flow 1500   m3/h Pump power can reach 400   kW, pressure 5   Mpa, flow 2000   m3/h
Burial depth Reach to 3   m (cable), to 2   m (pipeline)
Burial for seabed strength Applicable to 0–150   kPa different seabed bed
Burying speed Maximum 600   m/h
Maximum diameter of cable and pipeline that can be buried 150   mm (cable), 180–380   mm (repeater/connector box), 300   mm (pipeline)
Camera equipment Four sets of color cameras, a low-sensitivity camera, a mini-color camera
Camera Platform Four sets of all-rotary heads, a rotating head
Sonar a set of Tritech Super SeaKing OA sonar
Cable tracking system TSS440 and 350 tracking systems
Altimeter A set of Tritech PA500 altimeter
Illumination Ten 250   W, adjustable halogen lamps 4500   m underwater
Sound positioning system 2 AAE Midi Beacon, 949
Gyro 1 Tritech Intelligent Fibre-Optic Gyro (IFG)
Manipulator Two Schilling Orion manipulators with seven kinds of functions
Cable Cutter NHS HCV-100 (cable and umbilical)
Cable-gripping clip PSSL TA17
Cable gripper Suitable for 2   m or 3   m burial tools with scissors
Digging tool 2   m/3   m two-legs-type burial gun, auxiliary burial device, hand-held burial device
Depressor 2 and 3   m hydraulic cable pressure plate with three cable sensors
Launch and recovery system Umbilical winch, A-frame, and two 110   kW hydraulic power packs

The data in Table 6.2 show that this ROV is a piece of underwater-operation, high-tech equipment that establishes precision-machining technology, hydraulic technology, pressure-resistance technology, antileakage technology, hydrodynamic technology, automatic-control technology, computer technology, and remote-sensing telemetry technology into one. Obviously, not only are new ROV technical staff of operations and maintenance required to have high professional theoretical literacy but also strong practical ability, and they also need to be trained by international professional institutions and be involved in induction operations after obtaining a certificate.

2.

Main function of ROV

ROV as the important operation equipment mainly has the following three functions:

a.

Burial/Reburial function

Due to the influence of water depth, seabed topography, and installation cost, it is unsustainable for the burial machine to perform installation of short-distance submarine optical cable. Currently, ROV is required to undertake submarine optical cable burial operations. ROVs have the characteristics of small size, flexible operation, and strong ability of jetting burial, and are suitable for burial operations under complex working conditions. When the buried submarine optical cable is repaired, it is necessary to conduct reburial of the exposed submarine optical cable, and it is most suitable to use the ROV for postburial. ROV, via a powerful function of the jetting system on the ship, can bury the exposed submarine cable until it meets the required burial depth.

b.

Recovery function

When submarine cable fails, but also when submarine conditions are more complex or for submarine cable crossing and so on, and when more commonly used salvage equipment cannot recover the submarine cable, the ROV can be used to salvage the submarine cable. As the ROV has a multifunction manipulator and cutting device and flexible control, it is very suitable for submarine cable salvage operations under the complex seabed conditions.

c.

Inspection function

When we need to understand the quality of submarine cable installation, the ROV can be launched to the seabed to locate the submarine cable for inspection. If you need to understand the complex seabed situation, the ROV can be placed in the sea to survey the seabed conditions through the equipped instruments and imaging devices.

It can be seen from this discussion that because the ROV has its own power-propulsion device, its movement mode is very different from the burial machine. ROV belongs to an "active" movement system, and the burial machine belongs to a "passive" movement system. In submarine cable installation, ROV is in a dominant position, that is, the submarine cable ship moves, followed by the ROV movement trajectory.

3.

Auxiliary equipment for ROV

a.

Cable tracking system

The tracking survey system (TSS) is special equipment for detecting and tracking submarine cable by utilizing the principle of electromagnetic induction injected on the submarine cable. The device has three types: TSS340, TSS440, and TSS350. Among them TSS340 can detect the submarine metal material, the advantage being, that it is more sensitive to the submarine cable and suitable for all current submarine cable detection. The disadvantage is that it is more sensitive to environmental information, vulnerable to external influences, and the depth of detection is shallow (accurate detection within 1.5   m). TSS440 is the updated product of TSS340. Its precise detection depth is up to 2   m. TSS350 is a fixed-frequency audio signal detection system, such as 25   Hz, 16   Hz audio signal. Its advantage is that it has strong antiinterference ability and deeper detection depth. The disadvantage is that it relies on the audio signal to the cable from the shore to apply. The general effective working distance is the range from the terminal to 600   km. In the case of seabed visibility being poor or even having no visibility, and to rely on the system, the ROV can continuously detect cable and measure the burial depth.

b.

Underwater acoustic positioning system

The USBL is the most commonly used hydroacoustic positioning reference, which uses acoustics to calculate the signal transmission/reception system response time, the relative position, the converting position of the submarine cable ship with the comparison, and the underwater positioning system. It is the system that actually determines the relative position of the underwater equipment and the submarine cable ship's positioning system. Submarine cable operations require precise underwater positioning to accurately record the operation process of the actual location of the submarine cable.

c.

Underwater camera and sonar system

The underwater camera and sonar system is the "eye" of the ROV. The pilot on the submarine cable ship can perform the remote control of the ROV through the underwater camera and the submarine's visual image obtained by the sonar system. Video recording and retention can also be performed through the underwater camera system when needed. When operating in a sea area with poor visibility of the seabed, the sonar system is required to visit or detect obstacles.

d.

Manipulator and cable cutter

ROV is usually equipped with a manipulator and a cable cutter. Table 6.2 shows that the new ROV has two manipulators with seven kinds of operating functions, the interaction between them being able to seize the submarine cable, connection rigging, and so on. If necessary, you can also use the cutter for submarine cable cutting.

e.

Hydraulic cable plate

In order to maximize the burial depth of the submarine cable, high-power ROV, especially with a 3   m burial capacity, in the upper part of the jetting gun, are designed to install a hydraulic pressure plate. The submarine cable can be pressed into the excavated trench bottom by a hydraulic pressure plate on the jetting gun, thus maximizing the burial depth. On the hydraulic pressure plate, three sensors are installed to control operating status of the hydraulic pressure plate.

In addition to the above auxiliary equipment, ROVs also have other auxiliary equipment and signal sensing systems. The central processing system of the cable ship performs power distribution, control balance, automatic tracking, and manual control.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780128134757000060