Underwater Service Inspections

Read more about service inspections on acrylic viewports.

This article focuses on windows used in human occupied submarines - but some information can be applied to aquaria and semi-submarines.

One of the most important operations that must be continuously performed on all windows during their service life from the instant that they are installed are visual inspections. They are grouped into operational and maintenance inspections. Regularly performed visual inspections are the best insurance that catastrophic failure will never take place since crazing which precedes cracking, and ultimately fracture would be detected at its initiation.

Inspections should take place regardless of the window's service age. Some vessel operators think that because the window is less than 10 years old, inspections are superfluous since the-v are still within their stated design life. They erroneously consider design life as warrants against failure. Nothing could be further from the truth, as the window may begin to craze and crack soon after installation. This could be due to high residual stress which remains after incomplete annealing during the fabrication process, improper mounting procedure, or some unknown factor in the service environment not taken into account during the design of the window.

1. Operational visual inspections of windows
Operational visual inspections of windows are tied to the operational schedule of the vessel in which the windows are mounted. As a rule, they should be performed prior to each pressurization. Those that are pressurized several times daily need to be inspected only before the first pressurization on that particular day. This inspection, which does not require removal of the window from its mounting, is limited to the viewing faces; in the case of submerged windows, the inspection of both window surfaces may be performed solely from the dry side of the window.

2. Maintenance inspections
Maintenance inspections are also of visual nature but much more thorough and tied to regular inspection schedule. Not only the viewing surface, but also the bearing surfaces edges are inspected for most window configurations provided this can be accomplished without removal of the window from its mounting. Where those surfaces are inaccessible inspection from outside the window, the window may have to be removed from its seat in viewport. Plane disc windows need to be removed only for inspection of cylindrical edge surfaces if they are in contact with 0-ring seals. The time interval between maintenance inspections will vary. It is much longer when the window is in its design life phase, since occurrence of crazing generally does not appear until the window is several years old. When the windows are past the design age, the maintenanance inspection should be scheduled more frequently The severity of the service environment has also a large effect on the frequency of maintenance inspection. The more severe the environment, the more frequent the scheduled inspections .

If during the window inspections, whether of operational or maintenance type, deterioration of, or damage to the surfaces inspected, the inspector must form a judgment whether the detected irregularity (i.e., departure from brand new window appearance) poses a safety hazard or can be ignored. If, in his judgment, the irregularity poses a safety hazard, he must present to the vessel operator a recommended course of action to eliminate the safety hazard (i.e., replace window, remove and repair, or repair in situ).

DESCRIPTION OF SAFETY HAZARDS
Whether a surface defect is designated by the inspector to be a safety hazard depends on three factors: the type and size of defect, and its location on the windows.

The types of surface defects can be grouped into six categories: (1) scratches, (2) dings, (3) gouges, (4) crazing, (5) depressions, and (6) cracks. When ranked in terms of severity, the most severe are the cracks, while the most benign are depressions. In terms of size, the critical factor is the sharpness of the penetration edge and its depth below the surface. Cracks have the sharpest edge, and thus are more likely to initiate fracture, even when they are shallower then gouges or dings.

The length of a crack, scratch, or gouge has only a minor influence on fracture initiation. Similarly, the number of defects can be ignored in most cases in the evaluation of the structural condition of the window since its condition is determined by the single most severe defect and its location that will initiate the fracture during pressure loading.

The location of the defect on the window/panel is of profound importance on the effect that the discontinuity will have on the structural performance of the window under pressure. The magnitude of the effect on the structural performance of the window depends primarily on the type of stresses found on the given window surface during pressurization of the window.

If the stresses on the surface at that location are positive (i.e., tensile) the defect will have a big effect on the structural performance; i.e., it will reduce its short and long-term critical pressures significantly. Thus, for example, the low pressure face of flat window/panels, conical frustum windows with t/D; < 0.5 in thickness, spherical sectors with square edges, and both low and high pressure faces of cylindrical windows under internal pressure are under tensile stresses when pressurized and, for this reason, the presence of surface defect deeper than 0.01 in (0.25mm), is not acceptable. The depressions resulting from local repair of defects by in situ hand sanding and polishing are acceptable up to 0.02 in (0.05 mm) depth because of their gentle slope. Because of the above restriction on depth of surface damage, crazing cannot be tolerated in surfaces of windows which are under positive stress.

Minimum Intervals Between Maintenance Window Inspections

During Design Life	Past Design Life
(Protected Environment) Indoor Location 3 yrs	(Protected Environment) Indoor Location 2 yrs
(Weathering Environment) Outdoor Location 1.5 yrs	(Weathering Environment) Outdoor Location 1 yr

1. This schedule of maintenance frequencies is based on the design life defined by ASME/PVHO-1 Safety Standard 2001. The design life may vary from 10-20 years depending on the window configuration.
2. If the vessel equipped with viewports containing several windows with different design lives, the maintenance inspection shall be performed based on the window with the shortest design life.
3. During some of the maintenance inspections for windows, a seat/seal inspection shall be performed also.
   A minimum of one viewport assembly from each vessel shall be disassembled every 10 years and the condition of all viewport components inspected for indications of deterioration (i.e., rusty metal seat, brittle/cracked or permanently deformed gaskets). At that time the bearing surfaces and edges of that window previously not accessible during window maintenance inspections shall also be examined for presence of crazing and cracks.
4. If window surfaces are crazed or cracked, metal seat rusted, or elastomeric gaskets and seals cracked or permanently deformed, the remainder of the viewports in that vessel shall be disassembled assembled and inspected.
   This by itself does not reduce the critical pressure of the window. However, during depressurization of the window, the yielded material will be stretched by the surrounding relaxing acrylic material that did not yield during pressurization and local cracking may originate there. The major difference between crack origination in a tensile stress field by a discontinuity, and the crack origination in a compressive stress field is that the former occurs during rising pressure increase while the latter occurs during pressure decrease.

The continuing pressure increase will invariably propagate the crack in a window surface under positive stress until catastrophic fracture takes place with instantaneous pressure release resulting in loss of equipment and life. The crack initiated in the originally compressive stress field, however, will not propagate during pressurization, as the surrounding material is still in compression and the pressure is decreasing. Ultimately, when the pressure and the compressive stress decrease to zero, the crack will propagate because the yield material at the tip of the discontinuity is now in tension tending to open the crack while the surrounding material is relaxing. During succeeding pressurizations, the crack will close preventing catastrophic fracture. If repeated, a sufficient number of pressure cycles leakage will take place during depressurization.

The courses of action available to the vessel operator for removing the surface defects whose depth exceeds the critical magnitude defined in the ASME PVHO-2 Standard are:

1. Disassemble the viewport and remove the window from the viewport assembly and replace it with a spare on a permanent basis.
2. Disassemble the viewport, remove the window from the viewport assembly and replace it temporarily with a spare. The damaged window is either repaired by the vessel operator if the damage is slight, or is sent to a window fabricator for repair. Upon return from the fabricator, the spare window is removed from the viewport assembly and the repaired window is re-installed.
3. Do not disassemble the viewport if the damage to the window surface is slight (i.e., < 0.02 in or 0.5 mm) enough to allow the vessel operator to repair the window himself. Upon completion of repair, the window may be immediately put back into service.

Detailed discussion of the defects and their location on window surfaces is found in ASME PVHO-2 Standard.

Generally speaking, surface defects in the form of faint crazing, scratches and dings with < 0.02 in depth are considered slight damage and may be removed by hand sanding and polishing without removal of the window from the viewport. Such a repair of the window surface performed by vessel operator in situ by manual sanding does not require post annealing operations. Surface defects with > 0.02 in depth, however, are considered to be severe damage and the window should be removed, sent to a window fabricator who refinishes the damaged surface using power tools, followed by annealing, dimensional inspection and pressure testing prior to reinstallation. If the dimensional inspection determines the refinished window to be undersize in thickness, the pressure rating of the window should be reduced accordingly. Window with severe damage may be refinished also in situ, provided that the work is performed by a technically qualified representative of the window fabricator utilizing only manual sanding. Upon completion of repair, the window must be pressure tested in situ to design pressure prior to resuming regular service.