Innovations in Subsea Technology

Introduction

The author has developed his career within the underwater engineering industry which has included some exciting roles, from Submarine Observer, 1983; R&D of Diverless Debris Recovery Techniques (Patent), 1987; Expert Witness, Piper Alpha Public Inquiry, 1989; contributor to the operational development of the Clear Well Subsea (world-first) remote hot tapping R&D project at -1000msw, 2009 and now employed by one of the principal contractors on a new-build nuclear power station, currently under construction on the banks of the Bristol Channel, Somerset, UK. The specifics of the construction may not be directly referred to as many components are still within the design phase and all hardware specifications are protected under a UK Government and EDF Energy security of supply agreement. Hence no direct references may be made.

1.1 The Bristol Channel Environment

The Bristol Channel geographically has a characteristic phenomenon of an exceptionally high tidal range (2nd largest worldwide); specifically, 10-13 metres between low and high tide. This tidal range affects each waterway within its catchment, resulting in deep gorges having formed over the ages as each river fell during low tide to meet the resulting channel water level.

The high velocity of tidal flow combined with filling and emptying of all the river gorges introduces large volumes of newly eroded soils every day. This water-borne solid added to the existing turbidity of the disturbed channel floor has the effect of making subsea visibility impossible. Almost as soon as a diver leaves the surface, visibility is reduced incrementally, within the length of his arm. It is therefore important to understand turbidity and any related key parameters.

1.2 Hinkley C, Subsea Asset Specifics (Agency, 2013)

Three cooling water tunnels are planned; two 6m diameter intake tunnels, 3.3km long and one 7m diameter outfall tunnel, 1.8km long. There will be two concrete diffuser heads per tunnel, placed on the seabed. A cumulative maximum flowrate of 134m3 per second will be discharged through the outfall tunnel, with a projected temperature increase of 12.5oC.

The planned offshore construction activities comprise:

Large tubular tunnel connections,

Drilling operations,

Multiple rigging and lifting operations,

Alignment checks,

Grouting operations

Installation of specialist systems.

Apart from the difficulty of conducting subsea operations within this environment, the risks include asset integrity and significant safety risks to human health, i.e. divers’. The ideal scenario would be to design-out divers at this early stage with the introduction of many ROV-friendly adaptations to the proposed hardware. However, with the state of the art in ROV-mounted sonar and laser systems, this is not possible, as ROVs are perceived to be inoperable with the vision available. Professional divers therefore remain essential contributors, using the sense of feel to further the project. Very specific procedures and clear understandings of the limitations will contribute to the mitigation of the hazards identified.

1.3 Post Installation and In-Service Inspections

Post-construction, it is imperative to provide evidence that the subsea (diffuser) structures are free of gross defects. Reliability of the cooling water systems will enable the power station to continue long term operation, as planned. To this end, this report also details options that the project team might consider, in order to conclusively assure the integrity of the assets.

2.0 Aims and Objectives

The aim of this project was to investigate the impediment of visibility in highly turbid waters, for pending critical and complex subsea operations, within the offshore construction phase of a new-build UK nuclear power station. The power station has an operating life of approximately 80 years. It is therefore prudent to try and project how safe thinking may develop, to predict, for example, that divers may not be considered risk-worthy within the design life of the power station. From Section 1.2, the perception that, ROVs cannot be employed for these operations, illustrates an immediate safety gain, should the visibility issue be resolved in the future.

2.1 Optical Issues Underwater and Light-based Tooling

This project has centred on an overview of light-based research, including specific tooling types currently available. A recently completed, European funded, collaborative research project was also covered (Case Study, Section 6.0).

2.2 Sound-based Technology and Underwater Tooling

A brief overview of sonar systems was researched, including a number of commercially available sound-based tools. A case study from the Rail Industry, published in 2013, with parallel objectives to the Hinkley project was also cited and summarised (Section 7.0).

2.3 Supporting Objectives

The supporting objectives for the study were:

Understanding Turbidity; review existing methods of quantifying turbidity and establish composition of the water column at the worksite. Two operating environments were contrasted.

Understanding how differing light wavelengths affect the scatter from suspended particles – and identify the passport to see through them.

Review subsea operations in comparable environments,

The long-way-round; techniques employed to circumvent the vision problem.

Autonomous vehicle options were considered, on cost saving and safety grounds.

Projected immediate and long-term benefit in the Bristol Channel,

It was thought that the project would encompass feedback via questionnaires from discussions with ROV Operators, Diving companies and Installation Contractors, however all but a few were reluctant to record their methods. It was thought essential to involve the suppliers or manufacturers of vision tools, varying from low light cameras to commercially available sonar and laser imaging systems. It was seen that individual companies researching in this field were likely to be working in isolation, unable to learn from others – for strong commercial reasons. Consequently, restriction in technical disclosure resulted with just one (out of 12) questionnaires being returned. Physical trials of the most promising solutions would have been an ideal way to illustrate any gain from the research, but this was not feasible, on grounds of the infrastructure required for such a trial and consequently the cost. Alternative technologies included illustrating options of passive integrity monitoring (Acoustic Emission Systems) and a concept of a simplistic mechanical centrifugal device was explored. Other passive aids such as non-sound-reflective coatings to aid sonar function was not mentioned! This study was not intended to become a development program, nor an in-depth analysis of electronics and componentry. Electronic competence (on the part of the Author) and Patent restriction dictated the level of individual tooling evaluation.

3.0 Background - use of light subsea

Light waves are known to be transverse, electromagnetic waves, but more accurately these waves are produced in random multiple planes. Polarised light is essentially filtered such that the light from being active in multiple planes is then directed to a single plane. Polarised light is more controllable as an output and more readable as a camera input.

3.1 Scatter and Backscatter

When light waves impact particulates, some are absorbed, some depending on particle geometry and density are deflected or scattered. These scattered light waves release photons, enabling the particles to be detected (or illuminated). As the incident light is generally random in direction scattering too is random and depending on the particle geometry, some photons will scatter back towards the emitter – known as back-scatter. Back-scatter can also impede or attenuate the emitted light. Drawing from operational experience when using white light illumination for professional low light subsea cameras; in most subsea video footage the use of white light raises few problems. However, during high Phytoplankton bloom periods or during seabed disturbance operations, this situation can change with back-scatter from organic particles or solids in suspension. This can obscure the subject due to the camera’s inability to process this additional light.

3.2 Wavelength Relating to Detection of Particles

If it is accepted that the wavelength of the light used subsea may affect the amount of reflected or back-scattered light, then it is important to consider particle sizes. If it were predictable that a particular wavelength of light could detect a range of particle sizes, then equally one could select an alternative wavelength, which would not detect those particles, in turbidity for example. Consequently, without scatter, the light wave would have passed this particle (and not illuminated it). The scatter is therefore dependent on either the particle size or the wavelength of light “illuminating” it. This association was further investigated. From Figure 3 below, the high frequency 10nm wavelength UV light is contrasted with a lower frequency >0.01cm wavelength. In visible light, 440nm is the wavelength at lower visible threshold (violet/blue) to 700nm upper visible (red) threshold. Near Infra-Red (NIR) wavelength ranges from 700nm to 1100nm.

According to Jochen Winkler (Winkler, 2003), “The light scattering ability of particles is incredibly dependent on their size, relative to the wavelength of light”. “The light scattering coefficient is a measure of the ability of a particle to scatter light backwards”, but is also related to the angle of incidence. The smallest particles detectable in visible light must be at the blue end of the spectrum – with the smallest wavelength (Larger wavelengths have a coarser passage through the medium and hence are more likely to miss small particles). This is referred to as the diffraction limit. In photography, usually during close-up – or low light exposures, the iris or lens can reach this limit, beyond which no further camera adjustment with benefit the resolution of the exposure. The light waves become self-cancelling or interference patterns result. For the purposes of viewing a subject through turbid waters, the objective is to avoid scatter from small suspended particles. The longer wavelengths red to near-infra red (NIR) should therefore be selected.

3.3 Attenuation

In order to predict light attenuation in water, within specified particle sizes – or conversely determine the particle size through which a specified wavelength of light is attenuated or completely absorbed - reference to absorption coefficients must be made. For complete absorption, this can be referred to as the attenuation or extinction coefficient (Van-de-Hulst, 1957). Attenuation coefficients are significant in the design of eye protection, anti-glare coatings, for example.

Absorption Coefficients

Absolute Spectra of Water – diagram, publication reference required?

The resulting graph indicates the least attenuation, for the largest range of particle sizes can be viewed using NIR wavelengths.

4.0 The turbidity impediment

Turbidity is the term given to the scattering of light due to suspended solids and other matter in the water column, which restricts visibility. The measurement of turbidity is a key test of water quality.

Silt Effects on Subsea Visibility

The size and density of the silt and sand particles found at the mouth of the Severn estuary – and along the length of the estuary floor, provide the opacity which prevents optical visibility. The average suspended concentration was thought to be 400mg/l (0.4kg/m3), according to HR Wallingford (HR Wallingford, 2015).

Turbidity can be quantified and enables this variable, by reference to a simple array of samples (Atkin, 2017), illustrated in Figure 1. Inside the base of jars is a Secchi disk, which becomes invisible, as the turbidity increases.

As defined; (U.S. Department of the Interior, 2016) “Turbidity is the measure of relative clarity of a liquid” and “when a light is shined through the water sample, the higher the intensity of scattered light, the higher the turbidity”

4.1 Measurement of Turbidity

Turbidity is an important metric in many industries, from food production, water processing and the brewing of beer. In most cases industrial measurement is carried out with the use of opto-electronics; essentially a light source and an integral light sensor. The cloudiness or opacity of the fluid is commonly compared to two recognised standards with units either expressed as Nephlometric Turbidity Units (NTU) or Formazin Turbidity Units (FTU). Other standards exist internationally.

There are a wide range of commercially available measurement instruments such as the RS Hydro (RS Hydro (UK), 2018) NEP160 hand-held portable turbidity meter, reading in Nephlometric Turbidity Units (NTU) (Refer to Appendix 2). Autonomous turbidity instruments, example illustrated below, have been installed directly in rivers facilitating consistent water course suspended particulate matter (SPM) monitoring. These measurements are all taken to monitor consumer product quality or habitats where known contaminants endanger marine life. However, in the ocean there is no such broad monitoring necessity, therefore for offshore operations only local monitoring of SPM is possible, but in practice is generally carried out with divers’ estimates and rarely using instruments.

4.2 Turbidity Study, offshore

A recently published study (P Larcombe, 2018) presented at OTC in Kuala Lumpur in March 2018 revealed significant background research in an oilfield of North Western Australia, the Browse Basin (depth -240m LAT). The objective was to quantify periods when it would be predictable that ROV operations would be more efficient and hence utilisation would be greater/more economic. At elevations of 2m to 6.5m above seabed (ASB) they were able to cite visibility banding in excess of 2 and 3m, whereby if selected randomly in time, visibility might have been between 1 & 2m. For subsea operations this is significant and pilot to worksite orientation would increase proportionately. They were also able to show daily, monthly and seasonal turbidity patterns. Similar studies in the Severn Estuary/Bristol Channel (Oceanographic Centre, Southampton???) have not provided significant “windows of opportunity” and generally conclude that ……… Unfortunately, this study does not aid problem solving in the Bristol Channel. Whilst seasonal changes in current speed (spring and neep tides), certainly are predictable, the minimum tidal rise and fall is 9.?? Metres, rising to a maximum of 13m.

4.3 Slack Water Solids Drop-out

At a central location within the Hinkley offshore construction site, water samples were obtained from the water column. The samples were recovered by divers at three depths, during a slack water period in September 2017 (Spring Tidal Cycle).

The first sample was taken within one metre of the surface, the 2nd (middle) approximately mid-water (-8m) and the 3rd was taken on the seabed, to illustrate the liquid mud (approximately 1m - waist deep – for divers), which was present at the time, but is known to move around the Channel floor and could be described as a dense liquid within less dense liquid (a little like a large non-breaking Mercury bubble). Refer also to Table 1.

The isolation of the solution in glass jars permits the solids to drop out of solution much quicker than would be possible in the offshore environment.

4.3.1 Organic Matter in Suspension

The organic matter observed in the samples appeared to be largely translucent, but more likely is that the silt in solution becomes flocculated by organic molecules, hence reducing their ability to sink quickly. What is known regarding river-borne colloidal materials (Hunter, 1982) is that they initially become “flocculated by sea salt (molecules) early in the salinity range”. i.e. almost as soon as freshwater organic matter comes into contact with salt water it becomes contaminated by the salt, although this contamination is almost certainly exacerbated by hydrodynamic factors. Those saline particles subsequently attain a negative charge. Hunter et al also discovered that the adsorption of ions can make large concentrations of colloids unstable – inducing the tendency to sink when otherwise they may have floated (on a positively charged surface). They Hunter (Hunter, 1982) concluded that “surface electrical properties of suspended material(within) the presence of organic films are matters of central importance in the estuarine transport of particles”. By way of crude confirmation, the sample jars number 1 & 2 had mould growing on the inside of the glass, within days of their recovery, but months later it could be perceived that the organic matter, stripped of its excessive charge (due to regular jar shaking), also excluded from fresh oxygen had become inert, within the mineral deposits of the samples. The times recorded for all matter to settle, leaving the seawater completely translucent varied, as Table 1.

As the density of suspended solids were approximately proportional to depth, i.e. the highest density solids were found at deepest sampling depths, the time taken for the lightest solids to settle out was (logically) greater than the larger volumes of heavier solids. Note; deciding the exact moment at which the water became translucent was very subjective and later results were discarded. The samples had been taken as a point of interest, unrelated to this report, although later adopted, but had taken too long to be brought ashore. Further, the settlement measurements were deemed too crude and too infrequent to be meaningful, save informing the author that significant time was required if basing subsea operations on vision resulting from any settlement potential.

4.4 Two Differing Environments at the Hinkley Offshore Site

Two distinct environments exist within the overall envelope of offshore operations; the external structure, includes rigging deployment and alignment activities, whilst there will also be key activities within the (sheltered) vertical shaft. Because of this, there are consequently different parameters for vision systems.

5.0 light-based Tools, Laser Imaging, Ir and NIR

Following on from Section 3.0, use of Light, this section looks at practical applications of Laser Imaging, Infra-red and Near Infra-red illumination of worksites. Laser Imaging is also referred to as laser radar, although the radar connotations are generally linked to longer wavelengths. Daniel Lidar, Physicist, developed pulsed laser light in the 1950s, which measures time of flight. The technique used today uses LIDAR as an acronym for the process (Light Detection and Ranging). Laser imaging is probably the current hub of subsea digital image research. As a result, there are many companies looking at differing idioms of this baseline technology.

5.1 Literature Search – Laser Radar, Pulsed Light

A patent (Stettner & Bailey, 1995) was filed in 1995, seeking to log a theory step in underwater laser radar. The patent is aimed at medium to long range subsea transmission, suited to survey, bathymetry and military purposes. The patent brief outlines the nub of the 3D measuring capability or limitations at that time, namely the issue with resolving the range-gated pulse arrival times, such that time of flight could be calculated. The electronics at the time prevented flexibility in receiving both strong and weak signals, near-simultaneously (return signals from depth are of the order 4 or 5 times weaker) whilst converting this catalogue of analogue signals to digital, in order to calculate what the pulses had found. There were problems from laser emission back-scatter (to the receiver), which was not able to be filtered out quickly enough. This back-scatter was also found to cause return signal attenuation within the water column. The resolution from this research was the ability to receive the reflected and scattered light with multiple capacitors, which automatically provided range-gated inputs. The switching between capacitors in 10ns effectively performed the function of signal storage and range gating was performed at the same time.

5.2 Literature Search – Underwater 3D LIDAR

In 2013, a joint venture with Lockheed Martin, Houston USA and 3D at Depth, Denver USA (McLeod, 2013) was formed to investigate the use of a free-swimming vehicle scanning objects on the seabed. This venture was funded by the USA Department of Energy and administered by Research Partnership to Secure Energy for America (RPSEA). 3D LiDAR is a Time of Flight – based, white light laser image acquisition system, notable for reproducing millimetre accurate dimensional surveys. This can provide great overall detail of structures, not previously possible. Lockheed Martin’s contribution is the “Marlin” AUV platform, fitted with the commercially available Coda Octopus Echoscope 3D sonar system.

Prior to this development, 3D LiDAR required fixed transponders on the seabed, for reference and the resulting data provided many hours of Client viewing. This trial revealed that 3D CAD models of the structures could be integrated, such that autonomous ROV could carry the LiDAR system without the need for fixed references. The principal benefit for the client is the (lengthy) post-survey processing time now culminates in a 3D computer model of the subject which is simplistic to view – in as much detail as required. Limitations of this hybrid system are that the Coda Octopus sonar provides an excellent stand-off view (5-10 metres),but lacks resolution for close observation. The 3D at Depth LiDAR system whilst now providing a more useable output, still requires significant post-survey processing time and the images are degraded by turbidity.

5.3 Literature Search - Infra-red Research in Limnoglogy

In Limnology, research carried out by oceanographers and biologists illustrate the same need to view subjects (invertebrates and fish), as Subsea Engineers do in near opaque underwater conditions. The conditions within sometimes humic or fulvic acid – indicating a contaminated environment - may result in heavily coloured waters. White light is generally considered invasive in deep water or within heavily coloured areas, as the visible spectrum is likely to change the behaviours of the subjects, potentially harming the eyes of bottom dwellers, according to Herring et al, 1999 (Chidami, 2007). White light is simply ineffective in heavily coloured waters. It was thought that this was the first video footage of fresh water fish behaviour in highly coloured waters, using infra-red illumination. The paper concluded that to obtain good video recordings in the absence of visual light – or at night, the prototype system required two hundred LEDs. The LEDs were mounted within four projectors, on a small frame (800mm x 800mm x 700mm), due to the absorption of IR wavelengths in water. The monochrome IR-emitting LEDs (850nm) were used in conjunction with a commercially available CCD underwater video camera, set up to record IR wavelengths up to 900nm. In 2016, a similar project to view shrimp aquaculture (Hung, 2016). Multiple cameras were used (up to 9) in an array, together with two x 3-watt IR (780nm) LED lamps. In daylight the shrimp ponds absorb or scatter approximately 99% of ambient light, due to turbidity.

5.4 Literature Search - Image Acquisition in an Underwater Vision System with NIR and Visual Illumination.

A Limnology project team (Bieganski, 2014), (Part of the Isfar Project) in Poland built a prototype lighting system with the ability to simultaneously record images from three sources; Visible Light, Ultra-violet and NIR radiation. However, in this paper, only visual light and the NIR results were discussed. The operation was referred to as Single Sensor Image Fusion, also called a Trinocular Vision System. The imaging system was intended to augment the capabilities of an autonomous underwater vehicle (AUV). The CCD camera was shielded with a rotating disc, comprising two sets of optical filters:

Excluded all wavelengths below 712nm permitting NIR to be viewed only.

A pair of filters permitting wavelengths between 380nm and 780nm

The light sources were power LEDs in both cases, but with the NIR lamps operating at 850nm. Various images were recorded of a steel cube in a test tank. The cube had different textures and coatings. The baseline assumption was that the images recorded from each light wavelength will contain different features. It was seen that the NIR images were sharper or had greater contrast than the visual images, although not all the visual information could be seen on the NIR channel. The paper concluded that the NIR image resolution was greater due to the NIR resistance to scattering – in comparison to white light absorption in the water column in addition to its greater scattering potential.

5.5 Building Blocks for Survey Systems, particularly suited to AUV Technology

There are sub-systems that are commonly used for survey, orientation and best-guess positioning, in the absence of an exact set of input data criteria. Autonomous systems, whether driverless cars or submersible craft will all utilise the building blocks, described below.

5.5.1 Dead Reckoning Technology (DR)

This technology hitherto used in ship navigation is also applicable to automotive “Satnav”, where GPS (satellite) signals are continuously used to update the tracking system. When identifying the vehicle location, relative to a map - until the vehicle enters a tunnel or built-up area where there is signal interference or signal loss. The receiver then utilises other inputs such as velocity and accelerometers, identifying change of direction, to extrapolate the last confirmed GPS position on the planned route.

5.5.2 Kalman Filter

The Kalman Filter (originally referred to as: Optimal estimation algorithm) uses a series of measurements with vector algebra, observed over time. This algorithm is commonly adapted and contains statistical noise and other errors, but produces estimates of unknown variables, therefore assisting accuracy. Also, was used in the Apollo space programme to predict trajectories and utilised for guidance and navigation systems including computer vision systems.

5.5.3 Laser Odometry

This is the continuous logging of location of a moving body relative to its surroundings. The cumulative odometry can be made up of several other odometric inputs, such as wheel rotation, video image input (for recognition) and sonar proximity inputs. All autonomous vehicles and AUVs utilise this ideology. In order to scan a body, for survey or subsequent intervention purposes, either visual-based or sonar-based scanning must be continuous and accurate. If an asset or fixed subsea object to be inspected, has known dimensions, then geometric referencing can be employed, to speed-up this odometry. The geometric referencing can originate from CAD drawings or other construction data.

5.6 Case Study; Sintef research - eagle-eyed subsea camera

SINTEF AS (Research) of Norway (Sintef Research (Norway), 2016) have been co-ordinating a EUR 5.7m funded project, since February 2015 with the objective of an Underwater Time of Flight Image Acquisition (UTOFIA) camera. Participants for the project were from Italy, UK, France, Germany, Spain, Denmark and Norway. The resulting system was a hybrid solution, incorporating Time of Flight laser pulses with variable range-gated technology (see Section 5.0). The project was closed-out in 2018 and the dissemination of findings was distributed through CORDIS. The lasers employed have a wavelength of 905nM – within the Near IR spectrum. Refer to datasheet in Appendix 5. The intended market for the technology although projected to be multi-disciplined, was primarily aimed at the Aquaculture Industry. Here, fish production efficiency is greatly hampered by the inability to inspect habitats and provide much-needed welfare information in order to increase management value and ultimately the quality of fish production. The in-water trials reported in offshore Denmark, offshore Cartagena, Spain and inshore Norway (Institute for Marine Research) show the camera system’s ability to model and hence size individual fish in real time and at different distances (possible via Time of Flight). This was contrasted with a stereoscopic camera that can only size objects at a fixed stand-off. A turbidity sensor (Turner C7F) was used to provide a water clarity reference. The NIR wavelength selected for the laser (950nm) was specifically chosen to minimise disruption and stress to the subject fish. Distributed results from this development programme include stills from video footage and comparisons with a commercially available underwater sports camera (Go Pro). The aquaculture industry could have been better served if the trials participants had access to the best subsea industry low light cameras. Ideally, this research would have been tested in parallel with the state of the art in sonar imaging systems.

The author has written to the UK participants; Odos Imaging Ltd (Odos Imaging Ltd, 2018), Edinburgh for more trials information. However, no response has yet been received.

5.7 Light-based Tools, Summary

The latest results from 3D Lidar systems are very positive, especially if they can be linked to CAD drawings and NIR lasers. The IR and NIR in-water trials have all revealed that photography within suspended organic matter can be achieved using light with longer wavelengths, together with range-gated cameras. However, when suspended minerals are also present, light-based systems lack penetration.

7.0 Sound-based Tools, SONAR

According to Klaucke (Klaucke, 2018) Sonar was originally developed in order to make sea captains aware of icebergs, post Titanic. In practice, this proved an impractical goal as surface waves caused reflections preventing return signals. Later, between WW1 (1914) and the mid-1950s, Willem Hackmann (Hackmann, 1984) described the chronological development of the now-essential acoustic technology, both for submarine utilisation and enemy submarine destruction. Early technology sharing agreements with the American navy is also recorded. The first patent for side scan sonar was filed in 1954 by Dr J Hagemann (Klaucke, 2018), a German who was brought to the USA. The Shadow Graph was eventually revealed to the public in 1980.

7.1 Piezo Electric Principal

The building block for Sonar (and Ultrasound) systems is the piezo electric crystal. The piezo electric crystal vibrates at a pre-determined frequency when initiated by an electric current or pulse. Not only can these crystals convert electrical to mechanical energy, but on reflection the sound wave can re-enter the crystal and produce an electrical signal. According to Noliac (Noliac, 2018), they are one of the few single crystal manufacturers that can tailor piezo electric crystals for prototype and low production applications. Their current promotions indicate that single crystal transducers are now able to achieve broader bandwidth and higher sensitivity, due to a higher energy density within their crystal. Essentially, the crystal is reported to be easier to energise than previously, giving out sound waves of a specific frequency with less energy input. Given that, piezo electric crystals have been in manufacture since the 1950s, it is revealing that such an apparently simple device can still provide scope for related hardware development, nearly seventy years later.

7.2 Sound Transmission Range-Gate Principal

When sound is transmitted via any medium, air/water etc., then every incidence or interruption of those waves causes either refraction - or a partial reflection. The measurement of this reflection is what sonar and indeed ultrasonics is all about. As illustrated on an oscilloscope when connected to an ultrasonic probe, all of the interruptions are fed back to the unit and appear as “grass” or noise on the baseline. This noise, if close to the probe interface is generally related to the surface finish of the inspected material or the lack of a coupling medium. Equally, other noise or reflections can appear between the emitting probe and the target to be investigated, causing confusion or masking the target. A method of removing this unwanted noise to set range gates, such that only signal reflections within the desirable range is shown (A similar effect can be engineered with light-based systems, Section). Careful selection of the range gates within sonar systems will enhance the image required and essentially program-out unwanted noise. Within a turbid environment, if close range is desired, the sonar unit will be unable to simultaneously transmit navigational information. A second unit is hence required, with different range gate settings, to enable the ROV to find the worksite, for example.

7.3 Implementation of Sonar Subsea

The factors that affect SONAR performance include:

Beam angle/width

Frequency

Water depth/foot print

Swath sector

Number of beams

Range gate set-up

Across track & along track considerations

Sonar utilisation is generally categorised by operating range, numbers of beams and emitted angle of incidence, e.g.

Long Range navigation, wide angle multi-beam. Can be fitted to ships hulls or Larger ROVs

Medium Range (10-20m), wide angle multi-beam, usually work in the range 400-1600kHz.

Close Range high resolution Sonar, range 3m – 10m, usually work in the frequency range 2mHz – 3mHz.

7.4 Market Leading Sonar Vision systems, ROV Deployed

The Coda Octopus was recommended by Ashtead Technology (Ashtead Technology, 2018) as being most suited to turbid waters, due to “sounding densities far in excess of other sonars”, the 16000 beams together with a statistical rendering technique, makes for easier image interpretation.

7.5 Beam Geometry, Ping Shapes

Most imaging sonars (single or multi-beam) use a fan-shaped Ping. Single beam, profiling sonars, use a conical shaped Ping. The narrower the angle of the cone, gives a higher resolution. The range of conical side-scan sonars vary between 600m and approximately 10,000m. The original side scan sonar systems were either mounted on a Tow Fish or had the transducers mounted directly on the vessel hull. They featured three distinct beams with two Ping shapes. Two conical beams were emitted horizontally, either side of from the vessel, perpendicular to the direction of travel. The third was directed vertically downwards (fan-shaped), providing a bathymetric check on the seabed, but also filling the tranducer dead zone, i.e. areas closest to the transducer which is least effective at detecting reflections. In this case the ping shapes or swath was perhaps 30m high, above the seabed, but only a few metres thick. This 180 degress scan or sweep would then be repeated, until the area of the seabed was covered as required.

7.6 Narrow Beam Imaging (NBI)

MOOG Inc. developed the Gemini NBI Sonar Imaging unit in 2016, which uses a 1-degree vertical beam, with a 130o swath in order to obtain high resolution images. The manufacturers claim a 10mm range resolution, which coupled with a refresh of 30 scans per second, makes this unit an ideal contender for turbid environments. In fact, MOOG inc. specifically target this machine at trenching, cable lay and subsea mining operations – all of which are renowned for disturbing the sea floor and hence high turbidity. NBI is designed to be used in conjunction with wide angle multi-beam sonar systems, to enable the ROV pilots to locate the worksite in the first instance. The Teledyne Marine Blue view P900 has similar attributes.

7.7 Case Study (US Railroad Bridge Inspections)

According to Daniel Stromberg (Stromberg, 2013), the gap in the Arema Inspection Manual for underwater railroad bridge inspections can now be filled with his guidance for the use of imaging systems. The principal advantage seen for imaging systems over diver visual and touch inspections is reporting consistency; each diver having slightly different yardsticks for declaring anomalies – or not. Stromberg goes onto discuss the lack of acceptable equipment specification in the manual and the absence of operator qualifications. However, in 2013 sonar imaging was accepted only as additional information (to Diver inspection). However, where diver access was restricted, in the case of rivers in spate for example, then any bridge integrity information received was better than none. Laser scanning (or LIDAR) was rejected due to multiple reflectors in turbid waters, but radar, specifically synthetic aperture radar had been used for imaging river bed topography. Ground Penetrating Radar (GPR) was acknowledged as being capable of recording sub-surface concrete features, whilst recording sub-bottom geotechnical data.

7.9 Sound Waves Doppler Effect

Named from an Austrian physicist, using the Doppler Effect may have a niche function at the Hinkley construction site. During the construction phase of the Hinkley Cooling water works, several very large (6m+ diameter) casings are to be lowered through guides, where visibility prevents the use of ROVs or divers. It may be possible to predict a sonar transducer approaching transitions in planar surfaces, such as the Hinkley cooling water concrete structures. This may permit deployment of components with some accuracy, when it would be too dangerous to have divers positioned beneath a load. The leading edge of a principal tubular may build momentum during deployment, as a pendulum set-up by the current and sea state. The Doppler Effect should flag the pendulum state which may become predictable, in order to align the casing, without damage.

7.10 Sound-based Tools, Summary

Sound-based tools have been in use/under development for a long time and could be considered relatively simplistic (as opposed to light). There are almost limitless applications for sonar, but in the subsea market the gains are all related to resolution and imaging and the application for AUVs. Sonar systems can be effective within turbid environments and careful range gate adjustment can enhance that. There is a wide range of off-the-shelf sonar systems, but tool selection, with parameters to suit a particular task is not clear cut. For close inspection of a construction event, for example, one may be left with a choice of 4 units. Due to the breadth of the market and without the ability to test all four tools side by side, there is no industry standard benchmark to guide the user. Advice from experienced operators or hire agents generally sways the decision. The parallel development of data processing aids the handling of subsea imaging systems, but in the 0.5m to 3m close-up range, even with narrow beam widths, sonar images are still open to interpretation. However, for gross defect inspection, even in fast flowing rivers, useful information can now be acquired that was not possible a few years ago.

8.0 SUBSEA OPERATIONS, IN COMPARABLE ENVIRONMENTS

It was considered important to review existing tools employed to perform subsea operations – in similar environments. On the same (west) coast of the UK, as the Bristol Channel, specifically in Morcambe Bay, large natural gas deposits have been recovered since the early 1980’s. Twelve fixed production platforms have been installed, maintained and inspected since then. The upper tidal range of 9m is not as dramatic during springs (as the Bristol Channel), but with a shallow basin, significant turbidity still results. Similar visibility problems are hence encountered, when conducting routine subsea operations. Direct experience and professional opinion were sought from (both Diving and ROV) companies known to have past experience or to be currently operating in both diving and ROV operations, in Morcambe Bay. Questionnaires were sent out to 18 ROV and Diving Operators, including equipment hire companies. A further 8 questionnaires were sent to independent ROV professionals. Only one was returned (Refer to Appendix 3).

9.0 Hinkley Subsea Construction and Integrity Monitoring

This section is designed to illustrate the passive changes that can be made within the project to mitigate risk incurred by conducting operations in the turbid, fast tidal waters of the Bristol Channel. Integrity monitoring options were also reviewed.

9.1 Dry Thinking – Hinkley Example

Dry thinking was the term used on this (Hinkley C) project to give a focus for lateral thinking, to remove as many complex or critical subsea operations from the water. Effectively, Dry Thinking will de-risk the project by giving visibility back to those operations.

9.1.1 Flooding, Venting and Latching Systems

A good example of Dry Thinking for these systems was deleting ROV control panels and taking all of this switching technology to the surface, via simple plug-in umbilical hot stabs. This change removed the potential for an ROV pilot to operate the wrong valve, with just a hot stab to plug-in. This will also remove a time constraint based on tidal flow, such that a diver or ROV may be unable to dive when the project is ready. Physical guides are to be incorporated, to aid the location of the Hot Stab receptacle (s), but these are to be duplicated with sonar stencil targets, such that these can be confirmed by either diver hand-held or ROV-mounted sonar systems. Wireless hydrophones may be considered, such that tunnel flooding can be heard on surface.

9.2 Large Shaft and Piles Installation

The guidance of 6m+ diameter tubulars requires the employment of remote/removable temporary guides and hydraulic release-able shackles. These subtract from the cumulative total of subsea operational hours, but also reduce diver risk. Examples here may be similar to Britania Engineering’s “Spring-lok” remote mechanical latch system (Britania Engineering Consultancy Ltd, 2018).

9.3 Use of Radar (P Larcombe, 2018) for Concrete Inspection

Post construction in-service inspections, looking for gross concrete defects could employ an application of Radar, using long wave radiation to detect delamination, cracks and material changes. Larcombe described such a system where the antenna was contained within subsea housing. The objective was to investigate the structural integrity of a concrete plug, submerged at -46 m in a water supply tunnel. He estimated that, the Radar system could penetrate 800mm into concrete.

9.4 Acoustic Emission, Condition Monitoring Techniques

The ultimate goal, post construction is to “present” the structures to the Client with a guarantee that they are as good as the day they were deployed into the Bristol Channel. All of the visual, acoustic and light-based systems potentially employed can only give an indication of the structural integrity and surface condition with large caveats. The size and lack of features of the structures, in conjunction with the many on- site impediments to inspection are likely to result in a measurable margin of error. Read John Henderson’s email There is an opportunity to “finely-tune” the completed structures, by locating receptacles, at critical nodes, for the temporary insertion of acoustic transponders. These transponders will enable an as-new acoustic signature to be recorded, as a baseline for future comparison. As part of future in-service inspections, the transponders would be re-inserted into the structures, for a re-listening period, such that signal comparisons can be made, at intervals over the lifetime of the structure. From an article (Mohammad Reza Hedayati, 2011) written for Information Technology Mechatronic Offshore (ITOM) and the Port and Maritime Organization (PMO) in Iran, the following summary was made: “It is a common observation that, when there were voids, mix separation or cracks the reflected waves detected by the receiving sensor were different than those from the perfect areas. The results showed that the analysis of surface wave testing has the ability to detect changes in the constructed structures. The vibration signals which appear on the perfect part of structure, give a characteristic vibration signature. This signature provides a base line against which future measurements can be compared”.

10.0 Autonomous Subsea systems

Autonomous systems, have been in development for many years, promoted by obvious cost savings of offshore vessels and on-site personnel. This aspect of subsea activity – suited mainly to subsea survey and inspection has many reasons to look for cost savings; primarily relating to new Oil & Gas reserves, with the need for pipeline routes, wellhead and manifold sites. All of these activities could be categorised under exploratory expenditure, where up-front costs must be borne by development budgets, without promise of returning revenue. Once preliminary survey results have been evaluated an enhanced level of funding can be allocated for the Execute phase; Production. At this stage investors have a tangible interest. The military too have interests in autonomous seagoing systems, a parallel to Drone technology, now completely accepted in modern warfare.

10.1 Autonomous Systems Collaboration Trials, 2018; Case Study

A UK Government initiative via Innovate UK, had funding available under the category Maritime Autonomous Systems (MAS), to the value of £3M. Following three years preparation, a trial was set up in July 2018 involving manufacturers of Vessels, ROVs and several sonar positioning and through water data transfer systems. The two-week trial was held at Loch Ness in Scotland, which gave access to deep water, whilst having the manageability of an inshore site.

We received funding from Innovate UK to develop the concept for a long-endurance marine unmanned surface vehicles (LEMUSV) which could use both existing and new sensor technology to gather data from the oceans for several months at a time. Phase 1 saw the development of the concept for the C-Enduro, a rugged self-righting vehicle that uses solar panels, a wind generator and a lightweight diesel generator as energy sources to keep the vessel at sea for up to three months. The success of this concept phase led us onto to phase 2 where we were awarded £390,000 to build the prototype. In November 2013 we launched the prototype C-Enduro for the first time in the Solent for initial trials and testing. Within three months of officially launching the C-Enduro, we received two orders, one from the National Oceanography Centre (NOC) and another from Heriot-Watt University.

“We are now able to send down new missions via acoustic communications to avoid the ALR having to surface from 6 kilometres deep. We are not only tracking, we are getting quality data back from the system via acoustics, so we can make informed decisions.”

Matthew Kingsland, Senior Robotics Systems Engineer, NOC

11.0 Novel Concept - Dynamic suspended solids displacement

In the absence of sophisticated electronic vision aids the observer, via a medium sized ROV, may be able to use a physical method to reduce the suspended solids from a fixed aperture, albeit with a fixed stand-off distance. There are obvious limitations with this technique, as observations can only be made within a very short distance of the jig’s aperture. However, for specific observation of dynamic states, ie valve movements, leak observations and enabling specific physical damage to be quantified, it may have commercial application.

11.1 Concept Design - Jig

A jig is proposed comprising a driven impeller, mounted within a cylindrical housing and a camera mounted off-centre, such that the contents of the housing are agitated in a centrifugal manner. If successful, the majority of the suspended heavy solids should be forced to the outer periphery of the housing, leaving a less restricted volume of water through which the camera may observe. The cylindrical housing becomes a partially sealed seawater reservoir which is allowed to refresh slowly, from an off-centre filtered inlet. An alternative might be to have an open aperture in-line with the camera. With careful positioning, the objective would be for the incoming water to immediately become part of the centrifugal action. The open aperture would ensure that the camera would never be obscured with a dirty housing. The ROV pilot/observer would eventually become accustomed to the distraction of the rotating impeller, much like racing karting drivers in heavy rain. The impeller there is driven by the passing air speed, but the principal is similar. The appearance of the jig design has resulted not unlike the proportions of a washing machine drum, but with the vision aperture looking the opposite way, to that in domestic mode. The “drum” in this case remains static, mounted to the ROV. This tool is not proposed to aid navigation through turbid waters, as the resulting (clearer) view is not infinite, merely the depth of the jig plus a short distance. This tool would be complimentary to the suite of commercially available ROV vision aids. Hence, only when located close to the intended inspection site, would it come into service.

11.2 Jig Manufacture

The jig would be manufactured in a combination of 316 stainless steel and translucent perspex. The perspex is specified for the front face of the jig, which will permit a greater spread of ambient light from the ROV lamps. The camera mounting aligned with an eccentric hole in the stainless steel back plate, permitting vision through the jig. The depth of the jig and hence camera stand-off is not thought to be practical beyond approximately 400mm, but the induced centrifugal effect is expected to continue for a short distance beyond the jig apperture, see footnote 40. The impeller should be hydraulically driven, for reliability and can utilise existing workclass ROV feed and return connections. The impeller originally was designed as a two spoke item, with inverted aerofoil section “peripheral agitators”, causing increased flow over the underside and hence drawing water and suspended particles from the centre towards the outside. (The aerofoil section was mounted such that the water in the centre was drawn outwards, being replaced only from the perimeter (and potentially the open vision aperture). It is anticipated that there would be a build-up of silt at the outer circumference, but with the inclusion of a small number of open slotted exits, this build-up would essentially over-load the exits and hence the housing would clear naturally.

11.2.1 Alternative Impeller

In the event of the original aerofoil section impeller being ineffective, a simpler paddle-type could be considered. The mode of operation is purely to generate water rotation, but with a risk of aeration. Experimentation would determine the optimum speed of rotation.

11.3 Water Jet Option

A modified design was considered, where potable water was introduced to the housing, replacing the impeller. The complementary, diametrically opposed water jets then generated the rotational and cetrifugal action. However, the water supply was required to be drawn from the surface and the supply hose would be time-consuming in launch and recovery modes.

11.4 Diver Aid, Using Principal of (Turbid) Water Displacement

Anecdotal evidence suggests that a diver-held clear water reservior has been trialed in a similar turbid water environment. In this case the diver held a translucent “beach ball” shaped water bag against items to be observed, then in-turn pressed his mask against the opposite surface, in order to view an object. When monitoring a pressure gauge for example, this tool fulfilled the need.

There were a number of drawbacks with this technique:

The object being observed must be static.

The depth range had to be constant, otherwise the beach ball became distorted with external pressure and hence recovery for pressure adustments became time consuming.

The inclusion of ambient light was problematic, to avoid reflections.

The face of the reservoir nearest the work site became contaminated easily and had to be returned to the surface for cleaning.

12.0 Conclusion

The objective of this study was to identify specific vision systems in order to carry out complex subsea construction and maintenance operations, relating to the integrity of the subsea cooling water assets in the Bristol Channel. From the research to date, it is apparent that the ability of users to choose vision systems to fulfil a particular need is clouded by Manufacturers’ advertised claims, backed by subjective examples. The scope of this project was widened to include other methods of integrity monitoring, primarily due to the author’s inability to glean technical detail from specific products.

Hybrid Light vs Sound

It is hoped that by considering longitudinal light waves, in conjunction with transverse sound waves, a multiplexed receiver could read the reflected images from each of the two diverse sources, within a short period of time. It may be possible to overlay the resulting reflections in order to produce consolidated images. What the study can conclude is what is not known than rather than the preferred what is (known), with regard to subsea vision systems. The sales masking of clear cut technical attributes makes definitive choice impossible. Bench-marking, using bona fide turbidity figures would seem to be a good place to start, but even that depends on the composition of the turbidity. Aerated water, silt entrainment and organic suspended mass – all have a best-methods for viewing through them. Passive systems like acoustic emission have a part to play in long term structural monitoring, but for the practicalities of real-time construction operations, the safest way is perhaps to design-out as many subsea decisions as possible, see “Dry Thinking”, Sect. ?? The subsea construction and maintenance industry is as disparate as the steel fabrication industry was prior to the Oil Industry requirements in the late 1970’s. Critical quality benchmarking evolved, as major steel fabricated structures were designed with twenty five year design lives for hostile ocean environments. The only way forward was to benchmark the quality of welding, personnel and inspection. The certificate of weldment inspection personnel (CSWIP) emerged. A case in point is the success of the European joint venture concluding in 2018 with a state-of-the-art vision product (UTOFIA (Sintef Research (Norway), 2016)) which was positively compared to a sports accessory camera, as a means of acceptance. From Sect. ?, with the UK’s largest hirer of subsea equipment (BP) specifying equipment to sensor level, it is apparent that a new joint industry body is required to establish benchmarks, which would enable comparison, calibration and subsequently acceptance for purpose.

Works Cited

Agency, U. E. (2013). Decision Documnt No.EPR/HP3228XT, relating to Hinkley C Water Discharge. Bristol: Environment Agency.

Bai, B. a. (2012). Subsea Surveying, Positioning and Foundation. In B. a. Bai, Subsea Engineering Handbook (pp. 104 - 110). Oxford UK: Elsevier Inc.

Bieganski, W. a. (2014). Image Acquisition in an Underwater Vision System with NIR and Vis Illumination. Poznan': Poznan University of Technology.

Boss, E. a. (2008). Real-Time Coastal Observing Systems for Ecosystem Dynamics and Harmful Algal Bloom. UNESCO.

Chidami, S. G. (2007). Underwater Infrared Video System for Behavioural studies in lakes. American Society of Limnology and Oceanography, 371 - 378.

David Havelock, S. K. (2008). Handbook of Signal Processing in Acoustics. New York: Springer-Verlag.

DOKI.okimarket.com (Printing). (2018, August 25). Light Waves Chart.

Doxaran. (2001). Spectral Signature of Highly Turbid Waters. In e. Doxaran, Application with Spot Data to Quantify Suspended Particulate Matter Concentrations (pp. 149 - 161). Plymouth, UK: Elsevier.

Environment Agency. (2013). Review of Nuclear Site Authorisations. Bristol: Assets Publishing Service.

Hackmann, W. (1984). Seek and Strike: Sonar, Anti-Submarine Warfare and the Royal Navy. London: Her Majesty's Stationary Office.

Hovenier, J. (2012). The Polarisation of Light Scattered by Small Particles: A personal view. Journal of Quantitative Spectroscopy and Radiative Transfer, Vol. 113, 2280-2291.

HR Wallingford. (2015). Numerical and Physical Modelling of the Hinkley Point C Intake Structures. Oxford: HR Wallingford.

Hunter, K. (1982). Organic Matterand the surface charge of suspended particles in estuarine waters. American Society of Limnology and Oceanography Inc., 322-335.

Kraus, L. a. (2009). 3D-Sonar Image Formation and Shape Recognition Techniques. Institute for Water-Acoustics, Sonar-Engineering and Signal-Theory. Bremen: University of Applied Sciences, Bremen.

Mohammad Reza Hedayati, A. A. (2011). Expert System Development for Acoustic Analysis in Concrete Harbour NDT. Information Technology Mechatronic Offshore (ITOM) and the Port and Maritime Organization (PMO), Iran, 222.

P Larcombe, M. G. (2018). Reduced Downtime in Subsea Operations Through Use of Predictions of Underwater Visibility for ROVs. OTC, Asia. OTC-28351-MS, p. 11. Kuala Lumpur: Offshore Technology Conference.

Stromberg. (2013). Guidelines for Underwater Inspection and Imaging of Railroad Bridges. Arema (p. 31). Arema.

Van-de-Hulst. (1957). Light Scattering By Small Particles. New York: Dover Publications Inc.

Weiderman, A. e. (2005). In Harbour Underwater Threat Detection/Identification, using Active Imaging. SPIE 5780. Orlando, Florida: SPIE.

Winkler, J. (2003). Eurpoean Coating Literature: Titanium Dioxide. Hannover: Vincentz.

Yentsch, C. (1962). Measurement of Visible Light Absorption By Particulate Matter In The Ocean. Falmouth, USA: Woods Hole Oceanographic Institution.

Agency, U. E. (2013). Decision Documnt No.EPR/HP3228XT, relating to Hinkley C Water Discharge. Bristol: Environment Agency.

Boss, E. a. (2008). Real-Time Coastal Observing Systems for Ecosystem Dynamics and Harmful Algal Bloom. UNESCO.

Chidami, S. G. (2007). Underwater Infrared Video System for Behavioural studies in lakes. American Society of Limnology and Oceanography, 371 - 378.

Doxaran. (2001). Spectral Signature of Highly Turbid Waters. In e. Doxaran, Application with Spot Data to Quantify Suspended Particulate Matter Concentrations (pp. 149 - 161). Plymouth, UK: Elsevier.

Hovenier, J. (2012). The Polarisation of Light Scattered by Small Particles: A personal view. Journal of Quantitative Spectroscopy and Radiative Transfer, Vol. 113, 2280-2291.

HR Wallingford. (2015). Numerical and Physical Modelling of the Hinkley Point C Intake Structures. Oxford: HR Wallingford.

Hunter, K. (1982). Organic Matterand the surface charge of suspended particles in estuarine waters. American Society of Limnology and Oceanography Inc., 322-335.

McLeod, D. J. (2013). Autonomous Inspection using an Underwater 3D LiDAR. IEEE. San Diego, USA: Intitute of Electrical and Electronic Engineers.

P Larcombe, M. G. (2018). Reduced Downtime in Subsea Operations Through Use of Predictions of Underwater Visibility for ROVs. OTC, Asia. OTC-28351-MS, p. 11. Kuala Lumpur: Offshore Technology Conference.

Stromberg. (2013). Guidelines for Underwater Inspection and Imaging of Railroad Bridges. Arema (p. 31). Arema.

Weiderman, A. e. (2005). In Harbour Underwater Threat Detection/Identification, using Active Imaging. SPIE 5780. Orlando, Florida: SPIE.

Yentsch, C. (1962). Measurement of Visible Light Absorption By Particulate Matter In The Ocean. Falmouth, USA: Woods Hole Oceanographic Institution.