Underwater engineering1 January 2002
Underwater engineering can be a cost-effective alternative to dewatering but it is not without risk. Safety must be paramount. To ensure underwater projects are carried out successfully, dam owners must select an experienced and qualified diving contractor capable of executing a specific work plan
When it comes to the rehabilitation, repair and inspection of dams, carrying out the work underwater can prove a cost-effective alternative to dewatering. Underwater maintenance offers many advantages for the dam owner. This method can not only help minimise impacts on power generation and recreation but can also help eliminate adverse structural and environmental impacts associated with dewatering.
Conducting an effective underwater operation however requires more than simply hiring a diving company. A sensible work specification must first be developed, coupled with a qualified and experienced contractor capable of executing the work.
Specifications and procedures
Properly specifying diving services requires more than a simple understanding of the work to be performed. An understanding of the capabilities required of the diving contractor as well as the limitations of various underwater working methods is also essential. Ultimately the engineer must match the job to the skills of the contractor, realising that by nature, divers have a can-do attitude that leads them to overestimate their capabilities. Sufficiently detailed specifications that fully define the work scope and contract performance requirements permit the diving contractor to carefully consider his suitability to the task.
While engineers will usually stop short of specifying means and methods, there are things that can be included that help the contractor better assess job requirements. Information about the expected conditions may be important. Environmental conditions such as the depth range of the work area, underwater visibility, water temperature and water flow will be essential in helping the contractor plan his crew size, diving mode and equipment requirements. Certain operational information may be relevant even though it is not part of the scope. Leaky valves, gates that do not close properly, and structural defects in concrete are examples of things that may pose a hazard to the contractor. For complex jobs requiring special work procedures or diving techniques it may be necessary to employ a consultant or involve the diving contractor in developing a work package.
Successful underwater projects are often characterised by a collaborative relationship between the dam owner, the engineer and the diving contractor. This generally includes involving the diving contractor in careful consideration and scope of the work, and how underwater tasks will be approached. Likewise the contractor must ensure that project personnel understand the effect that diving procedures have on the safety and success of the work.
Commercial versus scuba divers
Before developing a work plan, owners and engineers must first understand the difference between scuba divers and commercial divers. There are professional scuba divers. They often teach recreational diving, perform research, work for law enforcement agencies or rescue organisations and occasionally perform light underwater work. The key differences between professional scuba divers and professional commercial divers lie in the training they receive and the equipment they use.
Although scuba divers are very well trained for certain types of work, their training is specific to activities like instruction, rescue or research. They are generally not trained or experienced in tasks such as welding, cutting, rigging or concrete work. The conditions that scuba divers are expected to work under are usually relatively good - shallow, clear, warm water. And the equipment they use is designed for these conditions.
Scuba divers generally operate individually or in two-person teams independent of any surface support. They have no physical link or method of direct communication with surface support personnel. This is one of the main reasons that commercial divers rarely use scuba. It is simply unsafe to perform hazardous work completely isolated from any form of assistance.
Recently, a tragedy occurred in which divers using scuba attempted to perform work in a flooded tunnel. Without a lifeline and communications, they became disoriented and were unable to radio the surface for assistance. Their air supply, limited to what they carried in their scuba tanks, was exhausted before they could find the exit. When the divers failed to surface a second dive team was dispatched to attempt a rescue. The rescue divers, who were also using scuba, became disoriented and ran out of air. Many died.
The standard diving mode for commercial diving is surface.
The diver receives a continuous supply of air from the surface via an umbilical that includes a safety line and hard wire communications.
A surface-supplied dive crew comprises at least three people but deep diving or complicated work tasks can increase this number to six or eight. Surface support personnel are in constant physical and voice contact with the diver via an umbilical. Surface supplied diving is not only safer, it is more productive. Divers can spend more time on the bottom and can sustain heavy workloads that would be impossible for scuba divers to achieve. Good communication and tools like underwater video allow engineers and project managers to take an active role in directing underwater work. Most types of commercial diving work like welding, pumping concrete or heavy rigging cannot be performed safely in scuba.
Diving is, of course, just a means to get to the work site. What the diver is able to safely accomplish after arriving there determines the success of the job. Not all commercial diving contractors are qualified to perform work on dams. In addition to all of the standard skills, a contractor's personnel very likely need to be proficient in deep diving, mixed gas diving, concrete and mechanical inspection, welding, concrete repair, and have specific dam-related experience.
Managing an underwater project is difficult at best. Each step of the project must be carefully planned and monitored. Daily safety and work co-ordination briefings are essential to this process. On a complicated job, each day's work should be choreographed and discussed in sufficient detail so that each person understands how his tasks affects and is affected by all other tasks. The procedures are often alien even to experienced engineers and a large portion of the work is hidden from direct observation.
The primary responsibility for diver safety rests with the diving contractor. They should arrive with a trained crew properly staffed to support the job. Their equipment should be functional and appropriate to the work at hand. However, these basics are often insufficient. Even the most detailed bid specifications rarely describe all of the things a contractor should know about the work site. This is particularly true of dams. One of the most dangerous situations in which a diver can be placed is one in which differential pressures exist. On a dam, this is the rule rather than the exception.
Identifying, closing and locking-out gates and valves solve most problems. A responsible dive supervisor will always verify lock-outs. But the dive crew may not be knowledgeable enough to ensure all potentially dangerous openings have been addressed; it is here that the help of operators and engineers familiar with the structure become invaluable. The divers and owner's personnel must work together to confirm that all operating equipment with the potential to pose a safety hazard if operated has been positively deactivated. Most sites will have a standard tag-out procedure in place. Still, it should be reviewed at the start of each job to ensure that changing conditions do not need procedural modifications. On dams, there are often openings that cannot be controlled. Stuck gates, leaking valves, or large structural cracks may produce a dangerous suction or flow. If these areas cannot be avoided special precautions must be taken. One of the most effective methods for ensuring diver safety when uncertain conditions exist is the use of a remotely operated vehicle (ROV). A small inexpensive ROV can be deployed to investigate areas of suspected leakage before a diver is committed.
An ROV equipped with a video camera can gather information about structural conditions as well as safety hazards. It may even be possible to undertake some of the preliminary inspection before sending in the dive team. This can be especially advantageous in deep water since ROVs are not limited by depth, water temperature or other environmental hazards.
Support equipment such as boats, cranes, or pumps can be dangerous on any construction site. Co-ordinating underwater work with topside support equipment operators is difficult even under the best conditions. Operators may not understand the underwater component and divers may not be well versed in the subtleties of operating the equipment that supports them. To make matters more difficult, communication will be difficult due to high ambient noise levels.
Planning and implementation
As with safety, the planning and execution of the work is the contractor's responsibility. But if a good working relationship has developed, the diving contractor will feel comfortable sharing the details of his approach with the engineer and in turn, the engineer will be able to provide helpful information and suggestions. This makes it easier to react to the problems that always develop when things don't go exactly as planned. Three resources will prove infinitely valuable to the project engineer:
• The diver radio.
• The underwater video.
• The divers themselves.
If the right contractor has been selected, their divers will be knowledgeable in dam construction and operation as well as in the construction methods to be used. This will allow the divers to communicate effectively with the engineer in real-time via the diver radio. Add underwater video and the engineer can now 'see' what the diver sees.
This ability to put the engineer at the work site via radio and video often makes the difference between a successful project and a failure. On many occasions, it has meant the difference between a safe job and a disaster.
A word of caution: the first duty of topside personnel is to ensure the safety of the divers. The radio is a vital link in this regard and all other transmissions are secondary. The diving supervisor must have the immediate and final say on when and how communication with the diver is used.
To work effectively with divers, the engineer must become familiar enough with the diving routine to know when it is acceptable to step in and when the divers just need a minute to sort things out. It also helps if the engineer has at least a passing knowledge of the capabilities and limitation of the divers, diving equipment and underwater tools.
For underwater work, inspection tools include underwater video, sonar, laser imaging, digital crack monitoring, dye tracing, acoustic leak detection and acoustic positioning. Visual imagery is one of the most effective means of documenting inspection findings. Numerous tools are available depending on the image quality and level of detail desired.
Underwater cameras are the workhorse of inspection documentation. They are usually helmet-mounted to allow hands-free operation and permitting surface observers to view the inspection in progress. Cameras are available that produce high image resolution, even in low light, but underwater video has limitations.
Sonar imaging does not approach the resolution of a photograph, but it will produce an image regardless of water clarity, and the image quality will be sufficient to detect gross defects or changes in the geometry of a structure. Sonar can be used for positioning.
In the case of imaging sonar, rough locations are determined based on recognisable structures. Certain types of sonar allow the diver or ROV position to be determined by triangulation. Using this method, accuracy in the centimetre range is possible. Sonar can 'see' for quite long distances even under conditions of absolute zero visibility.
With such tools available, conducting a safe and effective underwater operation is an attainable goal. Add to this a diving contractor working under a sensible work specification in cooperation with the owner and engineer, then the risk to all parties will be minimised to the greatest extent possible.
Inspection and condition evaluation
The clay liner of a 340ha reservoir supporting a 1872MW pumped storage facility developed large trench-like features. The reservoir depth ranged from 4m to 34m.
Before a repair could be designed and implemented, it was necessary to determine the extent of the cracking. The size of the reservoir and the depth would make this a difficult task. Engineers working with a diving contractor and marine navigation firm developed a mapping plan that included the use of side scan sonar, short baseline acoustic diver positioning, and differential global positioning (DGPS) of the support vessel. Sonar was used to scan large areas and roughly characterise the cracking. Divers equipped with helmet-mounted video cameras then inspected each feature.
The divers' position relative to the support vessel was mapped by computer using an acoustic tracking system. The position of the support was plotted by DGPS. By correlating this data, a computer map could be generated to show the precise position of features in the liner. Divers also entered the features, some of which were 3m wide and 6.25m deep to provide additional data in support of the sonar findings. Video images and depth readings from the diver's pneumo-fathometer helped to ground truth the sonar data.
Engineers were able to use the data to develop a grout mix design and accurately estimate quantities. Survey maps were then used to quickly relocate features during the repair phase.
A concrete gravity dam with an earth core and rock/soil foundation was completed in 1944. The dam is 2600m long, 65m high with a 40m head. Maximum storage is 7.6Bm3 and its purpose is navigation, flood control, power production and recreation. Downstream of the dam is a radial gate controlled spillway with a maximum discharge of 29,715m3/sec.
Eddies created by a change in the gate operation sequence deposited gravel on the downstream concrete apron. A resulting eddy between the downstream flow blocks and the apron toe caused a ball mill type action that eroded the concrete apron. The repair design called for the placement of number six rebar in each erosion area to eliminate a drowning hazard.
Thirty centimetre deep holes were to be drilled on 1m centres and the reinforcing steel was epoxy grouted in place. Since underwater visibility was limited to a few centimetres, the engineer directed divers to construct a wire template to aid in aligning each hole. Measurements were then taken so that each piece of reinforcing could be sized to leave 10cm of cover at the repair surface. Concrete would be placed by bucket and designed so that it could be dropped through up to 2m of water (see diagram).
The success of the repair depended largely on the concrete mix design. Because the diving contractor was experienced in placing concrete underwater, he was able to make helpful recommendations to the engineers designing the concrete. The mix would need excellent cohesive strength in the submerged application and had to be resistant to washout. Its ability to self-compact and self-level and its workability at low water to cement ratios would also be important. Because of the poor visibility, the divers would need as much working time as possible to place the concrete.
Divers used oxy-arc cutting torches to remove damaged steel reinforcing. Loose debris was removed from the repair areas using a combination of low pressure water jetting and hand dredging. Damaged concrete was then removed by 5,000 psi high pressure water.
Before placing concrete, divers conducted a video inspection so that the engineer could verify the preparation of the repair areas. A cable was anchored to the apron adjacent to the repair area. The concrete bucket was tethered to the cable at the surface and then lowered to the bottom. Once the bucket reached the apron, the diver could safely move in and adjust its position. Buckets were positioned within 30-45cm of the steel reinforcing and then opened to release the concrete. Only minimal hand towelling was required to provide the desired surface. A final video inspection was performed to approve each repair.