42
Underwater Work Cdr H Wardle, Subsea Consultant, Sovereign Oil and Gas PLC
R W Barrett MSc(Eng)
Research Manager, UEG (the underwater engineering group of CIRIA) Contents 42.1
Introduction 42.1.1 Diver employment
42/3 42/3
42.2
Diving operations 42.2.1 Decompression 42.2.2 Surface-orientated diving 42.2.3 Bell diving
42/3 42/3 42/3 42/3
Diving equipment 42.3.1 Standard diving apparatus 42.3.2 The aqualung or SCUBA 42.3.3 Surface-demand diving equipment 42.3.4 The fibreglass helmet 42.3.5 Bell diving system 42.3.6 Diver heating
42/4 42/4 42/4 42/4 42/4 42/4 42/4
42.4
Air-supply systems 42.4.1 Rigid free-flow helmet diving system 42.4.2 Surface-demand diving apparatus 42.4.3 The aqualung or SCUBA 42.4.4 Compressors 42.4.5 Mixed-gas supply
42/5 42/5 42/6 42/6 42/6 42/6
42.5
Size of the diving team
42/6
42.6
Planning underwater works
42.7
42.3
42.9.3 42.9.4 42.9.5 42.9.6
Poor visibility The underwater season Fatigue Sickness
42/7 42/7 42/7 42/7
42.10 Preparation for underwater work 42.10.1 General 42.10.2 Offshore
42/7 42/7 42/8
42.11 Construction work under water – preplanning
42/8
42/6
42.12 Use of tools 42.12.1 Pneumatic tools 42.12.2 Airlifting 42.12.3 Underwater television 42.12.4 Underwater photography 42.12.5 Production of drawings 42.12.6 Hydraulic tools 42.12.7 Underwater thermal cutting 42.12.8 Cutting using explosives 42.12.9 Underwater welding 42.12.10 Concreting underwater 42.12.11 Grouting underwater 42.12.12 Underwater bolt-firing gun 42.12.13 Nondestructive testing
42/8 42/8 42/9 42/9 42/9 42/9 42/9 42/9 42/9 42/9 42/9 42/9 42/10 42/10
Diver qualification
42/6
42.13 Diver-alternative systems
42/10
42.8
The importance of an accurate underwater survey
42/7
42.14 Conclusion
42/12
42.9
Factors affecting the diver’s work 42.9.1 The effect of tidal flow 42.9.2 Visual inspection
42/7 42/7 42/7
Bibliography
42/12
This page has been reformatted by Knovel to provide easier navigation.
42.1 Introduction Underwater work involves divers, submersibles, remotely operated vehicles (ROVs) or tools from above the surface. The operation of submersibles and ROVs is a highly specialized field which is at present mainly confined to work on offshore structures, pipelines and cables laid in open water. Surfaceoperated 'tools' include dredging and drilling operations (see Chapter 41), pile driving, etc. which are essentially surfacecontrolled activities extending underwater. The aim in this chapter is to discuss some of the problems associated with the utilization of divers by the construction industry and to describe some of the diver alternative systems available. 42.1.1 Diver employment Work for divers has increased considerably in the last three decades in line with man's increasing exploitation of the oceans. Some supertankers have a laden draught of over 30 m, with a consequent increase in the depth of water for berths and moorings. Increasing world population and improving environmental standards demand the disposal of waste by the construction of long sea effluent outfalls; land reclamation schemes are being carried out in many parts of the world. By far the most important development has been the production of oil and gas from wells many miles offshore in water of increasing depth; oilfield divers may be required to work in excess of 200 m of water.
42.2 Diving operations 42.2.1 Decompression Common to all diving operations is the concept of decompression which is the controlled procedure by which a diver returns to surface pressure from the ambient pressure of his working depth. As the working depth increases so does the time taken to decompress; in addition, the longer the diver stays under pressure the more the decompression time is needed. Decompression is necessary because the diver breathing under pressure absorbs inert gas into his tissues and blood. The decompression procedure allows this gas to be released gradually and thereby avoid decompression sickness. Decompression sickness (the bends) is directly attributable to a too-rapid reduction of pressure which can cause gas bubbles to form in the body tissues from the dissolved gases. Decompression schedules or tables have therefore been developed which allow the diver to be brought back to the surface in predetermined steps that avoid decompression sickness. Typical air-decompression tables allow for SOmin work at 15m before 'stops' are incurred. Stops increase rapidly as the depth and working time increase. Divers and others who work under pressure may suffer from bone necrosis, i.e. damage to small areas of the bone. Some divers may be more susceptible than others. Strict compliance with decompression procedures will minimize this risk. 42.2.2 Surface-orientated diving In this type of diving, the diver enters the water from the surface, proceeds to the working depth, carries out his work and then returns to the surface. Decompression, if necessary, can be carried out either by stopping at intervals in the water or by returning directly to the surface to be decompressed in a compression chamber. The choice depends on circumstances at the work site and other factors such as the decompression time required. Commercial diving of this type generally uses compressed air
as the breathing gas, which is pumped down to the diver from the surface via an umbilical hose. Because the nitrogen in air has a narcotic effect when breathed under pressure, compressed air should not be used in any diving operations at depths greater than 50 m. As most underwater construction takes place in less than this depth - about nine in ten dives are carried out in depths of 10 m or less - this constraint very rarely affects diving associated with conventional civil engineering works. Surface-orientated diving can also be undertaken from simple diving stages. These act as lifts to carry the divers to and from the work site. Decompression can again be carried out by stopping the stage at intervals or in a surface compression chamber. A variation of this technique is the 'wet bell' which is an open-sided bell carrying the divers' air supply and having an upper canopy in which air is trapped thus providing a refuge for the divers' heads. 42.2.3 Bell diving A diving bell is normally cylindrical or spherical in shape, has a bottom hatch, and is designed to withstand internal and external pressure. It can accommodate two or more divers and is provided with breathing gas from the surface. Bell diving is a technique in which divers are transported to the work site in a diving bell from where they carry out their operations on completion of their work; they return to the bell which is then sealed (to maintain the pressure as it was at the work site) and winched to the surface. At the surface, it is 'locked on' to a deck compression chamber via a transfer-under-pressure (TUP) system, and the divers are decompressed. If the decompression period is short, decompression can be completed in the bell. Bell diving is most commonly used in support of offshore oil and gas fieldwork, and is obligatory in UK and Norwegian waters when diving to depths greater than 50 m. It is sometimes used for work in shallower depths especially for long duration tasks when the additional support it affords the divers may be necessary. Bell diving is now well developed and practical operations in excess of 300 m have been carried out. Exceptionally, operational dives have taken place to 45Om and experimental dives down to 600 m have been achieved. There are two basic diving techniques associated with bell diving - 'bounce' diving and 'saturation' diving. Bounce diving means that the diver is not exposed to pressure long enough for the dissolved gas in his body tissues to reach saturation. On completion of his work, he is returned to the deck compression chamber for immediate decompression. In practical terms, this type of bell diving is generally used for simple tasks requiring a short bottom time. Saturation diving, as its name implies, involves maintaining the diver at pressure for a long enough time for the dissolved gas to reach saturation. Saturation means that his body tissues and blood cannot absorb any more inert gas from his lungs and once this condition has been reached the time need for decompression is the same, no matter how long he remains saturated. Saturation diving avoids the need for decompression to atmospheric pressure at the end of each working period; the divers live in the deck compression chamber (maintained at the same pressure as the working depth) for periods of up to 30 days. For this type of diving the divers usually breathe a mixture of oxygen and another intert gas, usually helium. They are transferred to and from the work site by the diving bell. The choice between bounce or saturation diving is decided primarily by the depth of water and the expected duration of the dive, which in turn is governed by the nature of the work to be carried out. For continuous work or work that will take a relatively long time, saturation diving has clear advantages. As the depth of water, and therefore the decompression time,
increase, the technique becomes more attractive. In relatively deep water even quite short dives require such a long decompression that a saturation facility is likely to be the most costeffective way of doing the work.
CORSELET LANYARDS SECURING AIR PIPE AND BREASTROPE
42.3 Diving equipment As over 90% of all diving is carried out on work in less than 50 m using air as the breathing gas, this chapter deals mainly with surface-orientated diving. Some factors apply equally to the deep diver but, should specific information be required on deep work, reference should be made to one of the specialist companies providing deep-diving services. The following is a summary of diving apparatus in general commercial use for air diving.
WEIGHT LANYARDS WING NUT
FRONT WEIGHT JOCKSTRAP SHEATH KNIFE
42.3.1 Standard diving apparatus The traditional standard diving apparatus consists of a heavy tinned copper helmet and corselet (attachment between helmet and the heavy twill diving dress), lead-soled boots and weights. Air is supplied via an armoured air hose from the surface to the helmet and escapes from the helmet through a relief valve which maintains the pressure at slightly above the ambient (water) pressure. The apparatus is used for relatively static and heavy work. Although very comfortable for the diver when on the bottom, it is cumbersome and difficult to work in when operating in a swell and when mobility and vision are restricted. Its use is now limited to 'old hands' in the diving profession, but it still has a place for localized heavy work and in protecting the diver from cold or pollution. It is sometimes referred to as 'helmet' or 'hard hat' diving gear (Figure 42.1).
BOOT LANYARDS
Figure 42.1 Standard diver (front view) 42.3.2 The aqualung or SCUBA The aqualung or self-contained underwater breathing apparatus (SCUBA) normally consists of twin high-pressure cylinders supplying air via a manifold and demand valve (reducer) which automatically adjusts the air breathed by the diver to ambient pressure. This automatic adjustment is achieved by the water pressure acting on a diaphragm which, in turn, operates a tilt valve supplying air to the diver. Thus air is provided to the diver at the correct pressure and 'on demand', the action of breathing operating the valve. The diver wears a rubber suit, fins, mask and a releasable weight belt. This equipment gives a very high degree of mobility but low endurance. It has inherent limitations for sustained hard work at any depth and as depth increases it becomes increasingly difficult for the diver to make a reliable assessment of gas consumption under varying work conditions. It is not therefore generally recommended except for short-duration observation work. It is widely used for survey work. 42.3.3 Surface-demand diving equipment The principle of this apparatus is the same as the aqualung except that the air is piped from the surface to the diver (Figure 42.2). It is used where a combination of endurance and mobility is required. Some aqualungs have a dual capability, with the diver normally supplied from the surface but carrying 'emergency* air, usually in a cylinder strapped to his back. 42.3.4 The fibreglass helmet This equipment is an attempt to provide the diver with the
comfort and good communications of the helmet of the standard diving apparatus while retaining the mobility of the frogman. The principle of operation is the same as the standard helmet but with the fibreglass helmet attached directly to the frogman-type suit. The volume of water displaced by this apparatus is less than with the standard diving apparatus and therefore the weight required to 'sink' the diver is less. 42.3.5 BeU diving system A bell diving system can be designed for either bounce or saturation diving. A basic system consists of one or more deck compression chambers, a diving bell complete with its handling system and equipment necessary for life-support, environmental control and communications. A saturation system normally has more or larger compression chambers and a more developed life-support and environmental-control system. It may be capable of accommodating only one saturation diving team or a number of teams or relays of divers in order to carry out work around the clock (Figures 42.3 and 42.4). Life-support and environmental-control systems ensure that the correct breathing mixtures are provided to the deck compression chamber and the diving bell, and that the divers are maintained in a safe environment and in thermal balance. 42.3.6 Diver heating It is important that each diver's thermal balance is kept within safe limits, neither too hot nor too cold. When diving operations are carried out at depths greater than 50 m, equipment must be
Figure 42.2. Diver wearing typical surface-demand diving equipment
Living chamber
Transfer chamber
Gas cylinders
Diving bell
Figure 42.3 Basic saturation diving system provided for heating the diver's body. At depths greater than 150 m the diver's breathing mixture must also be heated. This is due to the fact that a diver breathing oxy-helium mixtures, loses body heat very much faster than a diver breathing air. Currently, the most popular diver heating systems use hot water fed from the surface to the diver via the main bell umbilical and then distributed throughout the diver's suit. Heating is not usually necessary for air diving but it should be considered in special circumstances, e.g. when diving in very low-temperature waters.
42.4 Air-supply systems 42.4.1 Rigid free-flow helmet diving system A helmet diver requires approximately 421 of free air per atmosphere, e.g. 841 at 10m, 126 at 20m, etc. The airflow is calculated on the basis of ensuring sufficient ventilation to prevent the diver suffering from carbon-dioxide poisoning. The output of the compressor should be sufficient to provide air for a standby diver to go to the assistance of the working
42.4.4 Compressors All compressors used in connection with diving must supply air suitable for breathing purposes. Tool or similar compressors should on no account be used. To avoid contamination of the compressed air, compressors and cylinders must be maintained properly. It is also essential that the air intake of the compressors should be positioned to prevent foul or contaminated air reaching the cylinders. 42.4.5 Mixed-gas supply Supply of mixed gas is normally from banks of cylinders containing the appropriate mixture dependent on the water depth in which work is being carried out. Normally, the gas mix is of helium and oxygen with the oxygen content reducing as the depth increases.
42.5 Size of the diving team
Figure 42.4 Diver in lightweight equipment working from a diving bell
diver. Because the output of compressors drops with wear, it is suggested that a safety factor of 50% be applied when calculating the required output. A receiver should be in the line and have sufficient capacity to allow the divers to surface safely in the event of compressor failure. The correct air supply is of vital importance for safe diving operations. 42.4.2 Surface-demand diving apparatus This apparatus requires gas at pressure at the demand valve of not less than 4.5 bar above ambient before it will supply at the correct rate to the diver. The volume of air consumed by a diver varies in a number of factors including his work rate and lung capacity; however, an allowance should be made for an average consumption of 551/min measured at the working depth. Supply pressure is critical and compressor output must be carefully considered for the planned diving depth. A reserve emergency high-pressure cylinder is carried by the diver. 42.4.3 The aqualung or SCUBA The air supply required for SCUBA divers is not less than 65 1/ min measured at the working depth. High-pressure compressors operating between 140 and 280 bar are used to charge the cylinders. For major surveys, or other works where aqualungs are used extensively, high-pressure storage banks are used and the aqualung cylinders are charged by decanting from the banks. If the diver's SCUBA bottles were charged for more than 6 h before a dive they should be checked for correct pressure immediately prior to diving.
The diving contractor should set up a diving team with at least the minimum number of divers and support personnel to operate the plant and equipment necessary to undertake the diving operation safely. A diving supervisor is responsible for the diving operation and for all the members of the diving team. A minimum number of personnel in the diving team is usually legally specified but, typically (and as would be applicable to most civil engineering work) the team for air diving in less than 30 m of water where no decompression is planned would be: the diving supervisor, the diver in the water and a standby diver. The standby diver should be dressed with his equipment immediately to hand and ready to enter the water. In the UK, an additional diver would be required on the surface if the airdiving operation was connected with offshore installations or pipelines, if decompression stops were required, or if there was a special hazard, e.g. where a diver might be endangered by a current, become trapped or his equipment become entangled. Also in the UK, if the diving operation takes place in less than 1.5m of water, and there are no special hazards, the diving supervisor may also act as the attendant thereby reducing the size of the team to two.
42.6 Planning underwater works Construction work under water is self-evidently more difficult than similar work on land. Clearly, it is unreasonable to expect anyone under water to carry out all construction tasks to the same standard as a tradesman on land. The employer of diving services for simple recovery work has little problem. For more complicated work he can either redesign underwater fabrication so it is basic assembly work for the divers or, if this is not possible, can employ a specialist diving contractor with engineering backup.
42.7 Diver qualification Standards of diver training in the UK are laid down by the Health and Safety Executive and minimum qualifications have been established in four grades: (1) Part I: air divers who are trained to perform a wide range of air-diving operations and diving techniques including surface decompression and who have received basic training in the performance of work tasks using tools underwater at depths of up to 50 m. (2) Part II: divers who are trained in deep diving using diving bells and mixed gas and saturation techniques, involving open-water experience of these techniques to 10Om.
(3) Part III: divers who need to perform only a limited range of air diving using both types of surface-orientated equipment. The training covers operations to 30 m but after appropriate work-up dives, operations to 50 m can be undertaken by this category of diver provided they do not exceed 20min decompression time. This standard is often adequate for those working in inland/inshore locations involving underwater inspection of visual survey work but not for heavy manual tasks. (4) Part IV: as for Part III but restricted to the use of selfcontained SCUBA apparatus. This category includes those employed as scientists, archaeologists, photographers, scallop fishermen, etc. (Only divers qualified in Parts I and II may be employed in the offshore oil and gas industry.)
42.8 The importance of an accurate underwater survey Underwater surveying may be a highly specialized, expensive and painstaking task, but it is a vital preliminary to underwater construction or remedial works. A practical example can be quoted of how a comprehensive but expensive survey resulted in a cheap and completely successful repair being carried out to the clapping face and sill of a large dock gate which was losing water. On the other hand, in the past, inspections of vital bridge foundations have been known to have been carried out by a two-man diving team after a tender - at a price which could only be described as ridiculous. All that can be achieved in a case like this is the classic 'yard arm' clearer: 'The foundations were inspected by ... on. ...' Probably the worst results of all come when a well-meaning but misguided employer tries to get, as he sees it, maximum value for money from the survey diving contractors with requests like: Whilst you are there you can fix this, and: I want all the diving team to be working divers, and so on. The end result is that nothing gets done properly. The diving contractor must, of course, carry out the wishes of his employer, but it cannot be emphasized too strongly that surveying is a job requiring a special approach. Accurate records are vital and, basically, the surveying supervisor must be on the surface, diving only to clarify specific points. Work and surveying are not mixed on the surface nor should they be under water.
42.9 Factors affecting the diver's work 42.9.1 Effect of tidal flow The most important limiting factor which adversely affects a diver's work is the velocity of water flow. A flow of 1 knot has roughly the same effect on a diver as that of an 80 km/h wind acting on a man on land. The maximum tidal flow in which a diver can work effectively is, not surprisingly, about "1 knot. The ability to work in a strong tidal flow is dependent on the work task, the work location and adjacent physical support available to the diver. 42.9.2 Visual inspection A diver looking, as he does, through layers of air, glass and water observes objects apparently larger and closer than they actually are. He is therefore liable to report incorrect dimensions if he relies solely on observation, and he should always use a sea-bed ruler. A spirit-level may be used to establish levels in shallow water.
42.9.3 Poor visibility In rivers which run through highly populated or industrialized areas, visibility is often very poor. The converse usually applies in sparsely populated areas. Similarly, in coastal areas near river estuaries, the outflow of polluted rivers may adversely affect visibility many miles offshore although the prospect of good visibility improves further offshore. Apart from pollution caused by man, sand from the sea-bed brought into suspension by storms can reduce visibility to a few centimetres even well offshore. After a few days of good weather this can change to give a visibility in excess of 30m. In poor visibility, highcandlepower lights illuminate only the particles in suspension so the diver sees myriads of bright reflections from the particles. Special low-candlepower lights are available for close inspection in poor visibility. In areas of permanent low visibility, tactile measuring devices are invaluable and the diver's fingers become his eyes. Underwater floodlights are likely to be useless in shallow water where daylight has not penetrated. The effective use of floodlights is primarily limited to nightwork or at intakes and other areas where natural light cannot penetrate. 42.9.4 The underwater season In the summer months, weeds and other marine growth are at their most prolific, particularly in shallow coastal waters. Inspection of outfalls and other structures is therefore best carried out in the early spring. 42.9.5 Fatigue Breathing underwater involves appreciable effort. The muscles of the rib cage which draw air into the lungs have to work harder to ventilate the lungs with the denser air. With the demand-valve systems, some effort is also required to activate the tilt valve. The effort required to swim underwater is heavy and trials have shown that a diver in standard diving apparatus uses approximately the same effort when walking as the underwater swimmer. Two hours is considered the maximum time a diver can work efficiently using a demand-valve breathing system. Provided that he is working in one area, 4 h duration is possible in helmet gear. 42.9.6 Sickness Of necessity, divers are required to have a high degree of physical fitness and, generally, they are very rarely ill. However, working in cold water and experiencing temperature changes exposes them to the common cold. This can be serious since the presence of mucus in the eustachian tubes can prevent 'clearing his ears'. He is then unable to balance the pressure across his ear drums and the drums are forced inwards. The forcing can cause damage or infection of the ear. Consequently, divers should not dive with a head cold.
42.10 Preparation for underwater work 42.10.1 General Recognizing the difficulties under which divers work, great care is necessary in briefing the diving team before underwater work begins. Basic information should also be established. For example, before diving commences on a bridge survey: (1) record a fixed datum point above water level (if required, it can be tied to Ordnance Datum at a later date); (2) on a tidal river, fix a temporary tide board from which direct readings can be taken
during the survey; (3) establish chainages along the face of piers and abutments; and (4) establish the river-bed profile in relation to the water level and therefore the datum. Dimensioned elevation drawings of the structure to be surveyed can be produced from this basic information. As the divers proceed with their inspection, findings can be related to the chainage position and the water level or bed level. When working in tidal rivers and estuaries, reference should be made to the Admiralty tide tables to establish the time of high and low water predicted for the nearest primary or secondary port. Strong onshore winds can cause higher water levels than those predicted in the tables. The converse applies with strong offshore winds. 42.10.2 Offshore When working offshore, divers can normally work from a vessel in conditions in which vessels can moor and work. The sea states generated by a force 5 wind normally prevent operations unless land protection is provided as with an offshore wind. In most coastal areas, diving can take place only during slack water between high and low water. Large-scale tidal charts are available with times, velocities and directions tabulated for fixed locations. Times are related to the time of high water at a primary port in the tide tables. It is unlikely that work will be at a tabulated location so interpplation is necessary between the nearest locations for which information is available. A good knowledge of chart work and tide tables is essential for the offshore diver. The following two important factors should be remembered when assessing tidal speed and direction. The tabulated information refers only to surface tides (which affect shipping) - on the sea-bed the tidal flow could be in the opposite direction to that of the surface. Tabulated information can only give an indication of what may be expected and some adjustment will need to be established during the first day's work. The second factor is that the direction of tidal flow moves around through 180° during the change of the tide. Thus a ship at single anchor may start off over the site of work and swing steadily away as the tide slackens, just when the diver is experiencing the best conditions for working on the bottom.
42.11 Construction work under water preplanning The work the diver carries out underwater usually involves a steel or concrete structure. As a general rule, underwater structures are designed in the same way as similar structures on land and little consideration is given to the problems and slow progress associated with underwater construction. It is common to find vital bolts missing simply because the tolerances were too tight or access so difficult that the diver could not insert or set up the bolt. When planning construction work to be carried out by divers the following factors should be taken into account: (1) Visibility may be poor and can be virtually nil, therefore units should be designed in such a way that the diver can work by touch. (2) The diver's vision is distorted, making fine assembly difficult. Wider tolerances should be adopted. (3) The diver and the structure he is working on are affected by tidal flow. Simple connections should be used between new and existing works. (4) Water flow accelerates around massive units placed underwater. The final velocity depends on the size of the unit and,
of course, on the basic flow. Consideration should be given to means of protecting the diver from strong currents. (5) It is difficult to place heavy units accurately into position from surface floating craft. A diver can normally handle a unit that weighs approximately 50kg in air. The use of controlled buoyancy is practical for units weighing up to approximately 250 kg in air but it must be remembered that tidal forces act on the buoyancy unit as well as the unit being placed. A decision on the method of placement is an essential part of design work. (6) A diver is virtually weightless underwater; he can apply little downward force. Providing he can establish a footing he can lift his full strength, although some of his energy may be diverted to overcoming tidal currents. The one way in which a diver can exert his full strength is with his arms and shoulders - using one hand as a reaction point, he can pull with the other. When preparing the detailed design of an underwater structure, the use of this force should be exploited. Modifications to the design of concrete blocks to facilitate diver assembly is likely to increase design and casting costs, but these should be more than offset by increased output from the divers. (7) A diver's 'feel' when setting up bolts underwater cannot be relied upon. By breathing compressed air he is, in effect, breathing enriched air due to the higher partial pressure of oxygen. As a result of this stimulant he may apply too much force to the bolts. The use of a torque wrench is therefore recommended. (8) In many cases the diver needs one hand to hold himself in position against current flow and so, effectively, he has only one hand free for working. Where a large number of bolts have to be set up, provision should be made to lock the bolt heads. (9) Whenever possible, divers should be allowed to examine units to be installed under water while at the surface. This familiarization will reduce the time needed to complete the underwater task.
42.12 Use of tools It is difficult for the diver, with the many restraints on his performance, to use tools effectively. He has a number of tools at his disposal but careful planning is necessary to ensure their proper use. For example, if concrete saddles are to be fixed to a rock bed by rock bolts it is sensible to cast the lower section of the saddle block so that it forms a template for the drilling operation, i.e. with its hole of sufficient diameter to accept the drill bit and its thickness related to drill steel lengths to give the correct penetration. As a general principle, the use of tools should be kept to a minimum. By careful design, it may be possible to construct underwater by a combination of interlocking bolted concrete blocks and/or units. The diver then has a repetitive 'Meccanolike' assembly to carry out. 42.12.1 Pneumatic tools Pneumatic tools are frequently used by divers. Generally, all surface air-diving tools can be used effectively to about 30m depth. It will be obvious that extra care is necessary to maintain the tools. Unsatisfactory operation is often caused by lack of consideration of the following basic facts: (1) Pneumatic tools usually require an effective pressure 'at the tool' of approximately 4.5 bar. At 30 m the back-pressure of the sea-water is approximately 3 bar and the minimum pressure required would therefore be 7.5 bar.
(2) The diver is normally a long way from the compressor and the use of normal 19 mm tool hose causes a large pressure drop in the line. This must be corrected by the use of a larger-bore hose, the introduction of a 'pig' (receiver) in the air line near the diver, or a combination of both (which is recommended). The use of exhaust hose to the surface to remove back-pressure appears attractive, but the extra resistance to airflow and the drag on the exhaust hose removes this apparent advantage in carrying the exhaust air back to surface-pressure. 42.12.2 Airlifting The simplest form of airlift consists of a 150 to 300 mm diameter length of PVC pipe with an air connection and control valve inserted approximately 500 mm from the lower end. The diver locates the lower pipe end close to the material to be excavated and opens up the air supply. The air travelling up the tube expands as it ascends, drawing water and sea-bed material to the surface. With an adequate air supply a 10-m long by 200-mm diameter airlift will easily lift 150-mm stones approximately 2 m above the surface. A diver can operate and move such an airlift manually, like a vacuum cleaner, to clear an area of sea-bed. A PVC airlift can be operated entirely underwater where the materials removed from a cofferdam on the sea-bed can be distributed on the surrounding sea-floor. In a water depth of 25 m a short airlift will work quite effectively if the discharge point is less than 15 m underwater. At this depth the air does not expand as much as it would if the lift extended to the surface. It does not therefore have its full lifting capacity and a greater flow of air is thus required to deal with a given quantity of spoil. Purpose-built steel airlifts deployed from surface barges may also be used. Generally, where the 'lifted' materials are to be removed from the site, normal dredger techniques should be used. 42.12.3 Underwater television Underwater television is frequently used for instruction and control in offshore work. Because most inshore diving work is done in shallow water and the greater proportion of this work is carried out where visibility is poor, television is suitable only for limited localized inspection. However, manufacturers are developing new and more sensitive systems with complex lighting which improves the performance. This development, together with the gradual cleaning up of our rivers, should increase the suitability of television for routine inspection in shallow water. The use of television is justifiable where major repairs are considered necessary to an important structure. A videotape recording made in the best conditions, which the engineer can study at any time, has obvious attractions. 42.12.4 Underwater photography Again, owing to poor visibility in, for example, rivers, docks and harbours, photography is seldom used. As with underwater television, improvements in photographic techniques under water increase the possibility of obtaining useful photographic records. 42.12.5 Production of drawings For structural surveys there are obvious advantages in, using engineers or draughtsmen who can dive to produce adequate drawings illustrating findings.
42.12.6 Hydraulic tools Hydraulic tools are available but the logistic problems of special pumps limit their use to specialized tasks. In addition to hydraulic rotary tools, a number of hand-pumped hydraulic tools are available for underwater use, especially those used for cutting cables, hawsers, etc. 42.12.7 Underwater thermal cutting There are three basic types: (1) The oxy-hydrogen torch which is little used commercially. Divers need regular practice. (2) Oxy-arc equipment, consisting of a 'gun' which holds a hollow carbon or steel rod through which oxygen is supplied. Power to the rod is provided by a welding generator. An arc is struck with the rod, as for welding, with the oxygen jet removing molten metal in addition to providing fuel. (3) Thermal arc, which is similar in principle to the thermic lance but with a flexible plastic hose which incorporates a metal 'fuel'. The hose is expensive and the oxygen consumption high. The thermal arc is a quick if crude cutting system which is attractive in that no generator is needed. 42.12.8 Cutting using explosives The cutting of steel plate or tubular section using explosives is limited in underwater construction work owing to the possibility of damage to adjacent structures. For offshore work in deep or exposed waters, specially designed and shaped charges have been developed. These use the minimum weight of explosive to cut through a plate or tube of a given thickness. They are expensive but worth considering where, for example, divers' working time on the bottom is limited by depth or tide. 42.12.9 Underwater welding Simple underwater welding has been carried out for many years but quality has been poor in the past and applications limited. Highly sophisticated and therefore expensive systems have been developed for use in the oil industry, such as welding within specially designed underwater environments and automatic underwater welding machines. 42.12.10 Concreting underwater Placing concrete underwater by tremie tube has been practised for many years. The difficulties in handling the tremie tube and pouring concrete to avoid passing the mix through water (with the consequent loss of cement) are well known. As an alternative, the present-day efficiency of the concrete pump allows direct placement. 42.12.11 Grouting underwater Unlike concrete with its high weight per unit volume and viscosity, grout, consisting of sand, cement, water and retarding additives, can be pumped over long distances. It is used particularly to 'tie together' precast concrete units. It is very effective where both the paths and connections for supplying and controlling the grout flow can be built into the structure. The problems of grouting the foundations of structures in water are much the same as on the surface, i.e. the possible excessive loss of grout through coarse gravel or other open material must be guarded against.
42.12.12 Underwater bolt-firing gun These guns are capable of fixing bolts into bricks, steel or concrete. It is not normally practical to fix bolts into rock owing to its variable consistency. 42.12.13 Nondestructive testing Underwater nondestructive testing techniques are similar to those used on the surface. Time must be allocated for preparing and cleaning surfaces where, for example, metal thickness readings are required. Where potential differences are to be measured, the diver may carry only the instrument probe connected by cable to the instrument on the surface. In this way the divers are controlled by telephone from the surface where readings are taken and recorded. Encapsulated complete instruments are also available so that a trained diver/inspector may take his own reading underwater but, where practical, surface control is recommended.
42.13. Diver-alternative systems The most widely used diver-alternative system prior to 1960 was the observation chamber. This consisted of a watertight cylindrical chamber with a removable upper access door which could accommodate one man. Observation portholes allowed the occupant to observe the underwater scene in relative comfort at atmospheric pressure. Life support was simple - a manually controlled oxygen cylinder, whilst carbon dioxide was removed by breathing out through an absorbent canister. This system was widely used on salvage operations in deep water with the observer controlling a grab deployed from the surface. Probably the most well-known operation was the recovery of gold from the SS Egypt which sank in about 13Om of water in 1932, a depth which then was well beyond divers' capacity. Attempts to protect man from water pressure resulted in many versions of 4iron men'. These were articulated armoured suits (Figure 42.5) which had little success until fairly recent times when development of materials and joint design gave the 'diver' greater mobility. The most significant early development in diver alternatives prior to the start of major offshore oil exploration was the introduction of underwater television by the Royal Navy in 1951 during the search for the submarine HMS Affray, lost in the English Channel. A major boost in demand for sophisticated underwater services took place in the UK in 1956. Progress since that date has been enormous; diving techniques have improved and depth capabilities extended. Although the safety record of diving companies has been good, the danger to human life has remained. This, plus the fact that exploration was taking place in water depths greater than divers had ever previously reached led to a surge in investment in systems which could replace v divers. The introduction of manned submersibles to carry out surveys and inspection work using television with video recording was a fairly straightforward development. Some were fitted with a diver lock-out capability. Manned sea-bed crawler-type vehicles for working on pipelines were introduced, with both submarines and crawlers being fitted with hydraulically powered arms and specialized tools of increasingly sophisticated design (Figures 42.6-42.8). The systems still posed an element of risk to personnel. Associated with the production of oil offshore has been the enormous technical development in every branch of technology relating to the remote control of instruments, tools, machines and installations. These, plus a similar advance in navigational systems for guidance of submersibles, led the development of many types of unmanned underwater vehicles controlled entirely from the surface (Figure 42.9).
Figure 42.5 An atmospheric diving suit. A. Magnesium alloy pressure hull. B. View ports. C. Ballast weight. D. Special-purpose manipulator. E. Breathing hose. F. Oral-nasal breathing mask. G. Carbon dioxide scrubber. H. Articulated fluid-supported joints. I. Manipulator hand-levers. J. Articulated fluid-supported joints
Figure 42.6 A two-man minisub. A. Surface radio antennae. B. Transponder. C. Hatch. D. Crew sphere. E. Speed log. F. Trim sphere. G. Oilbag. H. Oxygen bottle. I. Badge bar. J. Light. K. Sonar. L Telechiric arm. M. Torpedo recovery arm. N. Control console. O. Viewing port. P. Receiver. Q Air-purification unit. R. Videotape recorder. S. Propulsion motor. T. Batteries. U. Machinery sphere. V. Main fuse panel. W. Sail. X. Emergency release buoy
Figure 42.7 A manned mobile observation chamber fitted with manipulators. 1. Flotation material. 2. Television camera and light. 3. Force feedback manipulator. 5. Sonar dome. 6. Battery pods. 7. Oxygen supply. 8. Thruster. 9. Hydraulic power supply
Figure 42.9 A typical advanced remotely operated vehicle. A. Umbilical cable. B. Syntactic foam buoyancy. C. Foam attachment straps. D. Trim foam. E. Lamp. F. Manipulator. G. Pan and tilt unit. H. Strobe lamp. I. Cine camera. J. Vertical thruster. K. Still camera. L. Colour television. M. Lateral thruster. N. Longitudinal thruster. O. Pan unit. P. Side-scan sonars. Q. Sector-scan sonar. R. Sub-bottom profiler
Figure 42.8 A one-man minisub. 1. Vertical thruster. 2. Acoustic communications transducers. 3. Forward acrylic viewport. 4. Manipulator. 5 Directional transducer for locating acoustic 'pinger'. 6. Lamp. 7. Variable buoyancy compressed-air tank. 8. Horizontal thruster. 9. Electronics container for thruster control
Today, the tendency in offshore operations in depths beyond the air diving range, i.e. 50 m, is to consider diver-alternative systems whenever possible. In response, the diving companies have not been slow to improve their performance capabilities: this coupled with the increased size of the offshore industry, has ensured a continued wide utilization of divers' services where these continue to be cost-effective. The requirement for relatively expensive diver-alternative systems in industries other than the offshore oil industry is likely to be small; for works on deep-water reservoirs or in contaminated waters, for example, it is considered that systems developed offshore could be worth evaluating. The most versatile tool in daily use offshore is the ROV (Figure 42.10). This varies from a simple swimming 'television eye' to powerful units with articulated 'arms' designed for a multitude of complex tasks formerly carried out only by divers. Control is achieved by an operator at the surface via an umbilical cable with visual feedback of the ROVs position using, for example, television, sonar, depth, heading, etc. Work by ROVs has been successfully carried out in the northern sectors of the North Sea in water depths greater than 600 m with a 2-knot effective current in addition. Navigational systems are available by which the vehicle can return repeatedly to a given position.
Having established a scheme, stick to it and aim to standardize assembly so that the diver can establish his working techniques. It is said that, given time, the sea will 'eat' anything in it. It is certainly true that the power of water should never be underestimated. The Victorian engineer's principle of good, strong and durable construction is not a bad example to follow for underwater work.
Bibliography UK Government Acts and Regulations The The The The
Health and Safety at Work, etc. Act 1974 (clause 37) Merchant Shipping Act 1974 (clause 34) Mineral Workings (Offshore Installations) Act 1971 (clause 61) Petroleum and Submarine Pipelines Act 1975 (clause 74)
Statutory Instruments The Construction (General Provisions) Regulations 1961 (number 1580) The Diving Operations at Work Regulations 1981 (number 399) The Merchant Shipping (Diving Operations) Regulations 1975 (number 116) The Submarine Pipelines (Diving Operations) Regulations 1976 (number 923) The Petroleum (Production) Regulations 1976 (number 1129) The Health and Safety at Work, etc. Act 1974 (Applications outside Great Britain) Order 1977 (number 1232) The Merchant Shipping (Submersible Craft Construction and Survey) Regulations 1981 (number 1098)
British Standards BS 1319:1976, Specifications for medical gas cylinders, valves and yoke connections BS 1319C:1976 Chart of colours for the identification of the contents of medical gas cylinders BS 4001 Recommendations for the care and maintenance of underwater breathing apparatus: Part I 1961 'Compressed air open-circuit type' Part II 1967 'Standard diving equipment' BS 5430 Specification for periodic inspection, testing and maintenance of transportable gas containers (excluding dissolved-acetylene containers): Part 1: 1977 'Seamless steel containers' Part 2: 1977 'Welded steel containers' Part 3: 1980 'Seamless aluminium alloy containers' BS 5500:1982 Unfired fusion-welded pressure vessels
Figure 42.10 The 'Pioneer' ROV with features as shown in Figure 42.9 42.14 Conclusion This chapter has outlined some of the services which can be provided by specialist underwater contractors. For a specific task it is recommended that the services of these contractors be employed. A number of companies provide both diver and diver-alternative systems and therefore should be able to offer an objective project analysis. Divers can, and do, carry out some remarkable tasks underwater. Such works are, however, invariably the result of sound practical engineering and a proper appreciation of the maritime problems associated with underwater working. Divers need to be used intelligently as part of the construction team. It is not sensible to ask the diver to carry out heavy manual work where this can be avoided. Fatigue reduces the mental faculties of the diver in the same way as it would any other person. Diving is hard work which, to some extent, accounts for the loss of memory associated with the job in hand.
Other publications including guidance codes and manuals: NOAA Diving manual (2nd edn) (1979) Health and Safety Executive (1980) Offshore construction (guidance booklet), HSE, London BR 2806: The Royal Navy diving manual US Navy diving manual (1974) Training standards published by or on behalf of the Health and Safety Executive Underwater Association Code of Practice for Scientific Diving
AODC publications AODC 010:1983 Testing, examination and certification of gas cylinders AODC 014:1983 Guidance note on minimum quantities of gas required offshore AODC 015:1983 Guidance note on surface orientation (air) diving from DP vessels AODC 016:1983 Guidance on colour coding and marking of diving gas cylinders and banks AODC 027:1984 Oil-lubricated compressors AODC 035:1985 Code of practice for the safe use of electricity underwater
UEG publications UR34: Control and monitoring of carbon dioxide in diving bells UR31: Tables for saturation and excursion diving on nitrogen—oxygen mixtures UR23: The principles of safe diving practice UR28: Thermal stress on divers in oxy-helium environments
UR18: Handbook of underwater tools UTN26: Procedures and language for underwater communication UTN25: peptic bone necrosis in commercial divers UR14: Underwater electrical safety - some guidance on protection against shock URIl: Oxy-helium saturation diving tables prepared by RNPL UR7: RNPL metric air diving tables
