Remote Tank Monitoring and Inspection Methods

E. Lemieux, A. Webb, K.E. Lucas, P.F. Slebodnick, M. Krupa, and F. Martin
Chemistry Division
E.A. Hogan
Materials Science and Technology Division

Summary: Preservation of tanks and voids on U.S. Navy ships expends more than 25% of maintenance funds annually. The MIL-P-24441 system that has been installed in most tanks and voids has a 5 to 7-year service life. Two major thrusts have been made to reduce maintenance costs of tank and void preservation: (1) replacement with high solid epoxy coating systems that are approximately 98% solids, are edge-retentive, and have a service life of 20 years; and (2) implementation of condition-based maintenance technology via electrochemical in situ sensors and remote optical inspection technologies for routine assessment of the "state of preservation" of shipboard tanks and voids. This article reviews improvements in technology currently being developed with regard to the second thrust area. Various inspection techniques, including the Insertable Stalk Imaging System (ISIS), the Remotely Operated Paint Inspector (ROPI), and the Corrosion Detection Algorithm (CDA) are discussed.

Introduction: Ballast tank spaces include seawater tanks for ballast and damage control, compensated fuel tanks, fuel/oil service, potable water storage, and combined holding tanks (CHT). These spaces have coatings as the primary corrosion control element and a cathodic protection system as the secondary element to minimize coating degradation and effects of galvanic corrosion. Currently, U.S. Navy maintenance practices for ballast tank spaces include Fleet-wide inspection of the 20,000 tanks. Approximately 4,000 of these occur annually, at a conservative cost of $24M. Inspection typically requires the opening of all tank hatches, cleaning, maintenance of a gas-free environment, and entry of trained personnel to evaluate tank integrity. Operationally, each tank may see different degrees of service depending on mission requirements, thus creating widely variable maintenance concerns, in addition to those problems routinely anticipated for each tank type. As a result, up to 50% of current tank maintenance is due to hidden damage or unplanned work. Costs for tanks identified for refurbishment soar to $250M/year for a fraction of the total tanks Fleet-wide.

NRL has developed a strategy by which the "state of preservation" can be determined by the implementation of Tank Monitoring Systems (TMS), which is essentially an unmanned tank entry method for inspection and qualification of tank integrity. The TMS systems include (1) an in-situ corrosion sensor (Fig. 4), which is installed in the tank to monitor coating integrity, the corrosion status, and cathodic protection functionality; (2) insertable optical systems (Fig. 5), for periodic remote visual and analytical assessment of coatings damage; and (3) software to integrate the results of the corrosion sensor and optical measurements, which allow maintenance needs and dollars to be predicted and assessed on a "condition basis" rather than the traditional "time interval" method.

FIGURE 4
Schematic of the corrosion sensor for remote monitoring of tanks and voids. Fig5 Image

FIGURE 5
Remote Tank Inspection Hardware.

Optical Inspection Techniques: The optical systems of the TMS strategy include three significant elements, an Insertable Stalk Inspection System (ISIS); a Remotely Operated Vehicle (ROV) for remote inspection termed the Remotely Operated Paint Inspector (ROPI); and an image analysis software package utilized as the Corrosion Detection Algorithm (CDA). ISIS currently incorporates a 72:1 zoom-capable charge-coupled device (CCD) camera, 70-W lighting, a hatch-mountable pole (stalk) for camera insertion, and a video recording device. ISIS is inserted up to 3 m into the tank through a personnel entry hatch and is mounted to the hatch. The operator then records high-resolution images (stills) and video of all tank surfaces for later analysis.

The ROPI system, is essentially a mini-ROV, whose dimensions allow entry through the ~ 13 X 21-in. tank hatches. The ROPI is ideal for use in tanks that are ballasted and those having numerous obstructions that would prevent useful implementation of the ISIS system. The ROPI is outfitted with an autohover system that will allow for smooth vertical evaluation of tank surfaces. A total 340 W of lighting with intensity control are included onboard for adequate lighting in a variety of conditions. As in the ISIS system, a zoom-operable CCD camera is also onboard. One of the unique features afforded the ROPI is the inclusion of dual reference electrodes that measure the tank condition from a corrosion standpoint. These corrosion sensors provide a number of advantages over both the TMS sensor and ISIS in that both a global view of the tank condition and highly localized measurements can be obtained in mission-critical spaces. In addition, these sensors ascertain whether the cathodic protection system is adequately controlling corrosion. The data obtained from the reference electrode are integrated with the video so that the condition is immediately viewable with the image.

FIGURE 6
Original image (left) and analyzed image (right) revealing a 7.1% damage.

Finally, the CDA provides an analytical tank assessment from acquired video imagery to provide the coatings inspector with a "percent damaged" coating, an example of which is shown in Fig. 6. With the use of ROPI and ISIS in conjunction with the CDA, it is expected that a repeatable and objective evaluation of the coatings damage can be achieved. This represents a significant development in tank coatings inspection since, to date, human inspectors have been expected to accurately distinguish tank damage from 0 to 10% within 1% of the actual. This ability is not only difficult but is subjective, especially where different inspectors are used.

Conclusions: Ship husbandry using TMS, either as individual elements or as an integrated system will provide the U.S. Naval Fleet a useful tool by which a variety of tanks and their coatings systems can be evaluated on a regular basis without manned entry. In addition, these components will significantly reduce the manpower and costs associated with tank inspection and preservation and minimize the high costs of unplanned tank represervation.

Acknowledgments: The authors gratefully acknowledge the support, coordination, and guidance provided by NAVSEA 05N and 05M in performing much of the work discussed in this paper. Specifically, the authors acknowledge the many contributions provided by Captain William Needham, Mr. E. Dail Thomas, and Mr. Andy Seelinger.

[Sponsored by NAVSEA]