High-tech underwater maintenance increases safety, reduces costs
Using new inspection technologies and highly-trained divers, diving contractors provide much-needed services to power plants
By American Inland Divers Inc.
It is a virtual certainty that any power generating facility using cooling water for its processes will eventually require underwater maintenance. Since the first days of large-scale electric generation, underwater maintenance projects have involved some form of diving activity. In the days of the hard-hat divers, the technology was primitive and the hazards even more significant. Consequently, project engineers would plan diving operations very carefully, and diving was viewed as a method of last resort.
Fortunately, improvements in diving technology have removed much of the worry over worker safety and cost issues. Power plants routinely employ independent diving companies to provide maintenance, repair and inspection services. Nuclear fuel pool diving, cooling tower cleaning and maintenance, dam surveillance, and intake screen maintenance and inspection are just a few of the jobs carried out by specialized diving contractors.
Water: nature`s radiation defense
Nuclear power plant managers use commercial divers for a variety of reasons, but one of the best reasons is based on the fact that water is a natural shield against radiation exposure. In the nuclear industry there is a concept known as “As Low As Reasonably Achievable” (ALARA). This term is meant to guide personnel working in nuclear environments to limit radiation exposure to dose rates as low as possible by reasonable means.
With ALARA comes three critical elements: time, distance and shielding. An individual`s radiation exposure is decreased by minimizing his exposure time, maximizing his distance from the radiation source, and shielding him from the source whenever possible.
The benefits of using commercial divers trained to work in the nuclear environment are directly related to limiting radiation exposure. Alpha, beta, gamma and neutron are the four types of ionizing radiation commonly present at nuclear power plants. Each type of radiation has certain characteristics that make it more or less dangerous in certain environments. The common denominator for these ionizing radiation particles is that water is an effective shield to all of them.
So, work in the spent-fuel pool, transfer canal, cask-loading pit, reactor core or torus, most of which are usually full of demineralized water, is best completed by a commercial diver because work is completed under water.
A recent diving project conducted by American Inland Divers Inc. (AID), of Houston, Texas, is a good example of how the nuclear industry works to lessen exposures and dose rates to maintenance personnel. The project involved the maintenance and repair of the plant`s fuel transfer cart during a critical-path operation. The transfer cart is used to transfer new and used fuel rods between the reactor core and the spent fuel pool. Because it directly handles the fuel rods, the cart has much higher surface contamination and radiation emission rates. Plant personnel could have completed the project without the assistance of commercial divers, but the workers` exposure risk and dose rate would have been greatly increased had they isolated and drained the transfer canal for repair.
Within the transfer canal sump pit, certain materials can emit radiation as high as 10 rems, a very high individual radiation exposure rate. But with the water still in place, the divers receive only approximately 150 millirems. This was a much more acceptable exposure rate and made using a diver the logical choice because the job was completed faster and at a lower cost.
On-line cooling tower cleaning
Maintenance workers clean cooling towers to remove sludge and sediment from the tower basin, to provide clean circulating water for the system, and to maximize tower efficiency. Clean water also minimizes the risk of exchanger fouling and reduces the amount of required treatment chemicals. Furthermore, cleaning the basin restores the tower`s capacity to its original design parameters.
To clean a cooling tower, workers begin by taking the unit out of service, removing the water from the tower basin and then removing the sludge. Using this procedure, all water and sludge is waste, and an operator is faced with disposing of a substantial amount of material. AID developed a cleaning procedure using divers that is more cost effective and reduces the amount of waste material.
There are three main benefits to using divers for cooling tower cleaning. First, using divers maximizes sludge removal, minimizes water handling and further reduces waste disposal. Cooling tower water is considered a waste item because it is contaminated with various treatment chemicals. The diving procedure decants the water system instead of sending waste water to a landfill.
The second benefit is that the tower is cleaned on line. This gives the operator the latitude to schedule the procedure to meet personal work schedules or to take advantage of low demand periods. Planning the procedure in advance also reduces overtime costs by allowing diving crews to work regular hours instead of during hectic outage periods.
The third benefit is a diver`s ability to thoroughly inspect the basin, underwater columns and supports for possible cracks and corrosion. These are the tower`s main support areas and are often overlooked during normal inspections because they are not visible. Divers can also inspect the expansion joints for deterioration which can contribute to leaking.
The diver-based method of on-line cooling tower cleaning proved effective at Koch Refinery in Corpus Christi, Texas, where the company previously used a manual service to clean its towers on-line from the surface. After inspecting the towers, divers found and removed a 3.5 foot-high mound of sludge that the surface cleaners missed in the center of one tower. Robotic and manual surface cleaning procedures do not insure thorough cleaning because they cannot negotiate the columns and supports like a diver. In addition, these types of devices cannot perform inspections or retrieve large pieces of debris.
Safe dam inspection
Dam operators generally perform subsurface dam inspections in response to a problem or suspected problem that cannot be confirmed from the surface. There are no federal or state mandates which require dam operators to carry out scheduled underwater inspections, so they perform inspections on an as-needed basis. Once operators visually confirm a problem, they employ an underwater contractor to repair the dam. Contractors perform inspections using three methods: remote operated vehicles (ROV`s), sonar systems and manual diving operations.
ROV`s can provide good visual data in clear water and dive to great depths, but they can be limited by high currents and poor visibility. General sonar can provide some detailed information regarding debris build-up on trash racks, but does not have the resolution to delineate small surface conditions such as stress fractures or concrete spawling. Trained divers provide the best tactile inspection. However, diving operations are labor intensive and can be restricted by water depth and altitude. Often, ROV`s, sonar and divers are used on the same dam inspection project to provide the most thorough and detailed analysis possible.
Underwater inspections can confirm a variety of problems including dam cracking, debris build-up on the trash racks, and scour or erosion at the dam`s base and downstream structures. Scour or erosion severely impact the dam`s structural integrity and undermine it to the point where it could shift and then fail. Often, dam operators use divers to inspect for scour conditions, later providing the remediation services through the diving contractor. Although divers can provide a great deal of needed information to operators, such data may be limited in scope.
In some cases, dam undermining is so extreme that it becomes too dangerous for divers to make inspections. For this reason, AID developed a high-tech sonar scanning device that provides complete data on every inch of the structure, including the vertical surfaces. Dam operators can use this information to make objective decisions based on computer-generated technical sonar illustrations.
Early in 1995, the City of Houston suspected that the Lake Houston dam had a scour condition, exacerbated by Houston`s late 1994 flood. The city retained an engineering firm which in turn hired an underwater contractor to perform the inspections. The inspectors immediately determined that severe scour had undermined the dam enough to make it too dangerous for divers to enter the water. The contractor decided to use the sonar system to inspect the area. Inspectors deployed the system from a small floating platform with the unit`s position tied into existing XY controls. The sonar inspection confirmed the scour conditions and the city remediated the structure as a result of this underwater technology.
Debris 15 feet thick?
As the power industry has pursued economies of scale, the average plant`s circulating water intake has grown in size and complexity, and higher flow rates have contributed to increased levels of trash on the intake screens. Removing the screens for inspection and cleaning creates a very real possibility that accumulated debris will find its way into the pump`s intake cell, possibly causing damage to the pump or condenser tubes. Screen rakes used on a continuous basis remove most of the debris. However, extended rake downtime can result in thick and entangled debris that requires some other method for removal.
In 1992, Houston Lighting & Power (HL&P) faced this situation at its Cedar Bayou Station when standard surface survey methods revealed 15-foot thick debris piles on the intake screens. To remove such a large pile would require dredges with cutting heads or a crane with grapples. To avoid damaging the screens in the process, HL&P planned to use divers to guide the operation.
AID suggested using a color sonar device to quantify the debris pile before beginning the clearing process. To HL&P`s surprise, the sonar indicated that the debris pile thickness ranged from a few inches to three feet. After verifying this measurement, HL&P used divers assisted by intake screen service cranes to remove the trees and other large objects which were preventing the trash rakes from removing small items. With dredging estimates near $100,000, HL&P saved tens of thousands of dollars by quantifying the debris load.
As HL&P and others discovered, utilities can save money by using trained divers to perform a variety of functions. Using them in conjunction with sonar technology can be the most efficient and effective way to complete underwater inspection and maintenance. END
The following American Inland Divers staff were contributing authors to this article: Mitch Adams, Kansas City, Mo.; Gordon Barksdale and Mark Coyle, Houston, Texas; and John Deats, Lafayette, La. Gerald Young, a registered professional engineer with Houston Lighting and Power Co., was also a contributing author.
(Left) Cooling towers can remain on line when divers are used to remove sludge and sediment in the basin.
(Below) Underwater scour or erosion can undermine a dam and cause it to shift. Periodic inspections reduce this risk.