Chapter 16

16-1.1 Purpose. This chapter provides maintenance activities with the minimum design, fabrication, and installation requirements for the use of cofferdams on U.S. Navy ships and submarines during waterborne maintenance and repair operations.

16-1.2 Scope. Underwater Ship Husbandry (UWSH) requires standardization of practices to ensure consistently safe and cost-effective operations. This chapter provides approved NAVSEA policy and requirements for the design, fabrication, and use of cofferdams on U.S. Navy ships and submarines. Specifically, this chapter provides: maintenance planners with a broad view of the function and use of cofferdams in underwater ship maintenance and repair; diving personnel with the requirements for the safe installation of cofferdams; designers with fabrication requirements for common size flat and box patch cofferdams; and ship’s force or repair personnel with the interior space requirements during cofferdam operations.

16-1.3 Cofferdams are defined as: any plug, patch, or dry chamber installed externally to the hull of a ship or submarine at or below the waterline in order to secure or dewater an area or system to enable shipboard or diver personnel to conduct maintenance or repairs to the hull or system. They can be as simple as a wooden plug inserted into a round opening or as complex as a dry chamber for a shaft coating repair.

16-1.4 This chapter is divided into ten major topic sections: introduction, general information, operations planning, plugs, patches, dry chambers, stern tube seals, templating, cofferdam design, and compliance and performance standards for contractor cofferdam work.

16-1.5 Applicability. All personnel involved with the design, selection, fabrication, installation, and removal of cofferdams on U.S. Navy ships and submarines must be familiar with the appropriate sections of this chapter. This includes not only the engineers responsible for the design of the cofferdam and the divers responsible for the installation, but the ship fitters responsible for fabricating the devices, the ship’s force responsible for tag-outs, the repair personnel responsible for removing systems that have been blanked off, and the riggers who will assist in properly placing and removing the cofferdam.

16-1.6 Training. All divers (Navy and Contractor) shall be trained in the fundamental cofferdam knowledge identified in Appendix E prior to cofferdam installation tasking. Regional Maintenance Centers shall be responsible for approval of contract diver organization training plans and execution.

16-1.7 Background. There have been recent (circa 1994) casualties during UWSH operations, including flooding of aircraft carriers and submarines. These casualties have prompted a new emphasis on cofferdam engineering and installation practices, leading to the development of this chapter to the Underwater Ship Husbandry Manual.

16-1.8 General Safety Precautions. Section 6 “Safety Precautions” of Chapter 2 “General Information and Safety Precautions” of the Underwater Ship Husbandry Manual describes the safety precautions to be observed during UWSH operations.

SECTION 2 - GENERAL INFORMATION

16-2.1.1 Plugs. The simplest blanks used in UWSH operations are plugs. There are two types of plugs used in UWSH applications, the wooden damage control (DC) plug and the expandable mechanical plug. Plugs can be used to seal round hull penetrations that have no screens or bars to use as attachment points. Examples of these kinds of penetrations include pump discharge piping, blow-down piping, and dead lights. See Figure 16-1.

16-2.1.2.1 Flat Patch. The simplest form of a patch is one made out of a single plate of metal or wood. As the name implies, this form of patch lies flat on the hull when it is secured in place.

16-2.1.2.2 The unit is sized and shaped to rest flat and uniformly against the surrounding hull surface, thereby blanking off the hull opening. If the area to be covered is contoured, accurate templating is necessary to properly shape the unit during fabrication, depending on the stiffness of the material and curvature of the hull area. Because of its configuration, a flat patch is effective only if the hull surface is reasonably fair and free of any projections, distortions, or fittings. As the size and load carrying requirements increase, flat patches are often reinforced with stiffeners. Figure 16-2 illustrates a typical installed flat patch.

16-2.1.2.3 Box Patch. The box patch is essentially a flat patch with sides, with the open side of the box facing the hull. It mates to and seals against the hull surface through a flange and gasket along its open side. Sealing a main circulating pump or fire pump sea chest to allow maintenance to be performed from inside the ship is a typical box patch application. It can be constructed to cover any projections or fittings near the opening to be enclosed. If the area to be covered is contoured, then accurate templating is necessary to properly shape the unit during fabrication. Figure 16-3 illustrates a typical installed box patch.

16-2.1.3 Dry Chambers. There are two common types of dry chambers used in UWSH work: open bottom and open top. Open bottom cofferdams allow divers direct access to a particular hull area, system, or opening. The flange sides of the chamber secure and seal against the hull, acting as an airtight boundary. Dry chamber use considerations typically include items such as diver work space for rigging or welding and ventilation for welding or epoxy cure. Open top dry chambers are similar in design but allow surface access to the work area.

16-2.1.3.1 Several Navy shipyards have designed and built various large unique application surface access dry chambers. These dry chambers allow work to be performed by topside workers below the waterline. Design of the larger units normally requires sophisticated design techniques not addressed in Section 9 of this chapter. However, NAVSEA 00C5 has collected many such designs and cofferdams from closing shipyards and will act as the focal point for identifying available units. Figures 16-4 and 16-5 illustrate typical dry chambers.

16-2.1.4 Stern Tube Seals. Stern tube seals are used to dewater the stern tube to allow maintenance, repair or replacement of the packing gland or Syntron seal. Often stern tube repairs only require the resurfacing of internal face seals which are inboard of the boot seal. Special blanking plates (dummy seat, see Section 7, Stern Tube Seals) are available which are installed after the face seals are removed. The blanking plates provide double-valve protection when combined with the inflated boot seal. In this case, diver-installed external stern tube seals are not required. Shaft seals may be a single device or a composite system. Surface ship shafts are sealed either by wrapping, a specialized box patch, or a mechanical shaft seal. The cooling ports are normally closed off with small flat patches or plugs. Submarine shafts are normally sealed using a mechanical shaft seal.

SECTION 3 - OPERATIONS PLANNING

16-3.1 Operations planning involves the assessment and assignment of resources necessary to accomplish a task. While each cofferdam task possesses its own unique aspects, the requirements of the planning process are identical. The following sections present a listing, in the typical order of occurrence, of the requirements for UWSH cofferdam work. For cofferdam tasks this assessment should be viewed as impacting three resource areas: technical, personnel, and organizational.

16-3.2 Task Assignment. The identification of the work required on the affected system, i.e., blanking cofferdam or dry chamber, is accomplished by a thorough review of the tasking documentation which may include 2-Kilo, Automated Work Request (AWR), and message information. These documents may assist the planner by providing reference to specific drawing numbers and technical manuals that are required to plan cofferdam work.

16-3.3 Retrieve Drawings. A thorough review of applicable drawings is required to properly plan and select the type and size of device to accomplish the repair. The use of these drawings will provide the planner with the required information such as location, details, interference, and curvature of the work area:

16-3.4 Ship Check. A ship check is required to establish liaison with the ship’s Engineering Officer, Main Propulsion Assistant, and respective work centers. This allows the planner to retrieve information on conditions that may exist with the system being worked on or associated systems that could affect the installation and success of the cofferdam. The planner shall conduct a thorough internal inspection of the repair area for material condition and interference. An external inspection is required to determine accessibility to the repair area, positioning of the ship, and the availability of work area. The following are the minimum checks required during a ship check.

16-3.4.2.1 Double-valve or single-valve protection. A primary concern of UWSH cofferdam operations is double-valve protection. Double-valve protection is defined as maintaining any two of the following boundaries simultaneously:

16-3.4.2.4 Special conditions (other shipboard interference, fuel or other tanks).

16-3.4.2.5 Liaison with the Chief Engineer (CHENG) to discuss associated systems:

16-3.5 Identify Support. To prevent scheduling conflicts, support required to accomplish the repair must be identified early in the planning stage. The services of several activities may be required simultaneously, such as a crane from public work center and a YD from port operations. Proper planning and scheduling will prevent unnecessary delays. The following are some typical support services that may be required for the installation of a cofferdam.

16-3.6 Inspection Dive. Except for repetitive routine tasks, an inspection dive is required whenever a cofferdam is to be installed. There are certain steps that must be considered to perform an inspection dive for the use of a cofferdam such as confirm correct system, cleaning, templating, and planning for rigging. The type of cofferdam used will determine the details of the inspection.

16-3.7 Select and Inspect Cofferdam. The requirement of the cofferdam selection process is to provide an engineering approved cofferdam that will ensure and maintain watertight integrity. Engineering documentation for every cofferdam must be on file with the installing activity and accessible for review by installation personnel. It must also provide a means of being held firmly and securely against the hull in the event of a loss of differential pressure. All patches must be secured such that they can not rotate or move out of alignment with the hull opening. The following are the minimum considerations for the selection and inspection process:

16-3.8 Engineering Evaluation. All cofferdams used for UWSH must meet the engineering design criteria of Section 9. Section 9 of this chapter also provides flat and box patch designs that meet this requirement and cover over 80 percent of U.S. Navy hull openings. Included are: material type and thickness criteria for unstiffened flat patches and the NAVSEA Standard Cofferdam drawing.

Caution
The flat and box patch designs provided have adequate strength and gasket loading area for application over the opening sizes identified in each design. Prior to application on a specific hull opening attachment and rigging plans must be developed for the specific ship and hull opening.

16-3.10 Templating. Except for flat hull surfaces, templating is the process to pattern hull shape data (hull contour) for use in forming the sealing surface of cofferdams. The requirement is to achieve acceptable leakage of the cofferdam without the continuous use of dewatering equipment. Successful templating requires trained personnel using properly designed equipment.

16-3.11 Fabrication or Modification of the Blank or Seal. All fabrication, modification, or repair of a cofferdam must be done in accordance with an approved engineering design specification. Engineering design criteria is specified in Section 9.

16-3.12 Rigging Plan and Assembly. A rigging plan is required for installation of a cofferdam. When planned properly, it ensures the safety of personnel and equipment and the successful installation of the cofferdam. The rigging plan includes the installation or attachment method and the selection of safe working loads for rigging components. The rigging plan must identify the following:

16-3.13 Installation Confirmation. Dive Supervisors shall confirm all installations are proper and complete by signing the Appendix D Patch and Plug Installation Checksheet. All installation checksheets shall remain on file at the installation activity while the cofferdam is installed on a vessel.

SECTION 4 - PLUGS

16-4.1 Though plugs are the most basic, simplest form of sealing devices, the proper use and selection is critical to safe, successful UWSH operations. Two types of plugs, wooden damage control (DC) and mechanical expandable, are commonly used for UWSH. Of the two types, mechanical expandable plugs are preferred since they provide a reliable positive seal, are reusable, and are easy to install and remove.

16-4.2 DC plugs have been used for salvage and UWSH for years; however, their safe use for UWSH requires special consideration. The diver must always ensure that the DC plug is completely seated and sealed (i.e., Bintsuke sealing DC plug to hull) before checking for leakage. (See Appendix A for Bintsuke recipe).

16-4.3 Mechanical expandable rubber plugs provide a simple and reliable means of sealing any small round hull opening. They are easy to install and remove and will remain securely in place when correctly installed. The expansion of the plug, rather than a differential pressure, secures the plug in place and forms the seal.

16-4.4 Typical uses for plugs are sealing uncovered discharges and dead lights. Suctions are normally covered with a grating or screen that prevents the use of plugs for routine blanking. There may be instances, however, where an individual suction within a sea chest requires blanking while leaving the other systems operational. In such cases, it is necessary to remove the screen or grate and use a plug to blank only the desired system.

16-4.5 The installation of a tag line from the plug to the surface will greatly assist in relocating the plug for removal. Further, a highly visible tag line alerts other diving activities (hull cleaning) and outside traffic, such as tugboats, pusher boats, or other craft of the plug’s existence when coming alongside. The tag line also alerts shipboard personnel to avoid mooring camels or ship waste offload barges in the vicinity of the plug. This is especially important when the plug is installed on the vertical sides of the hull (above the bilge keel or turn of the bilge). Some activities prefer use of hull bellybands to tag lines since bellybands can help to secure the plug in addition to locating the plug.

16-4.6 For plugs installed on the flat bottom of a hull, a tag line from the plug to a C-clamp secured on the bilge keel will assist the diver in relocating the plug, but will not interfere or obstruct traffic alongside.

16-4.7.1.4 Prior to installation of a plug, ship’s Commanding Officer or his designated representative shall be notified of the location of the plug and level of protection provided (single or double-valve protection).

16-4.7.1.5 Emergency plan (flooding) - Whenever single valve protection is in place, the repair activity shall include in their written notification to the ship a specific plan for immediate installation of a replacement piping component or internal sealing blank as discussed in 16-4.7.1.1 or 16-4.7.1.2 to restore double-valve protection. The ship shall calculate the worst case possible scenario for flooding and provide a plan of action and additional emergency dewatering equipment as required. Except for the brief period of single valve protection provided for in 16-4.7.1.1 or 16-4.7.1.2, the equipment shall be rigged in place and operationally ready prior to commencing work and for the entire time single valve protection is in place.

16-4.7.1.6 Prior to installation, all plugs shall be visually inspected in accordance with the Patch and Plug Inspection checksheet (Appendix C). The dive supervisor shall sign off the Patch and Plug Inspection Checksheet approving the inspection. The Patch and Plug Inspection checksheet shall remain on file at the installing activity while the device is installed on a vessel. Any deficiencies found during inspection must be corrected, repaired, or replaced, prior to each use.

16-4.7.1.7 All components and subsystems of the affected system must be identified and tagged-out using an approved tag-out procedure.

16-4.7.1.8 Zero leak rate seal shall be confirmed at the internal repair area prior to complete removal of piping connection fasteners. If a repair activity is unable to achieve a zero leak rate and all efforts are exhausted, a leak rate acceptable to personnel performing internal repair work and ship’s damage control personnel must be established. The established leak rate must be confirmed at the internal repair area prior to complete removal of piping connection fasteners. Regular assessment (measurement) of the plug’s leakage to ensure leakage is maintained at or below the acceptable leak rate is required.

16-4.7.1.9 Dive supervisors shall complete and sign the Patch and Plug Installation Checksheet (Appendix D). The Patch and Plug Installation Checksheet shall remain on file at the installing activity while the plug is installed on a vessel.

16-4.7.1.10 Divers shall not leave the dive station until double-valve protection is established, repairs are completed, or ship’s Commanding Officer or his designated representative permission is granted.

16-4.7.2.1 The top plate or washer must have an outside diameter larger than the opening being sealed.

16-4.7.2.3 The size range for each plug must be stamped on the end plate or on a permanent waterproof tag attached to the plug.

16-4.7.2.4 The plug must have a seated pressure rating at least one and one half times the static pressure head it will seal.

16-4.7.2.5 Due to the potential for galvanic corrosion, mechanical plugs with aluminum components are not authorized for use.

CAUTION
The old Underwater Work Techniques Manual suggested the use of tapered mechanical plugs.
These plugs are unreliable and tend to dislodge
during the tightening process. They are not
authorized for UWSH applications.

16-4.8.1 Mechanical plugs for UWSH applications may be locally manufactured or bought commercially. Figure 16-6 illustrates the typical features of these plugs. Figure 16-7 and Table 16-1 provide size and construction details for locally manufactured mechanical plugs. Commercial plugs must meet the requirements of Section 16-4.7.2. Tables 16-2 and 16-3 list two manufacturers and recommended models.

16-4.8.2 Preparation. Using ship's drawings (Docking Plan), determine the diameter of the hull opening. All expandable rubber plugs have a limited range of the sizes of openings they will effectively seal. Select a plug that matches the diameter of the opening to be sealed. Commercially available mechanical expandable plugs are well suited to UWSH applications.

16-4.8.3 Thoroughly inspect the condition of the plug. Disassemble the plug and examine all components for deterioration, corrosion, stripped threads, or any other obvious damage. If the material condition of any single component is questionable, either replace that component or the entire plug assembly, whichever is most appropriate.

16-4.8.4 Establish a communications protocol with the on-site repair personnel. Direct communication is required to verify the integrity of the plug upon installation and prior to removal.

16-4.8.5 All components and subsystems of the affected system must be identified and tagged-out using an approved tag-out procedure. Any increase in internal system pressure or movement of fluid in the piping could dislodge the plug. Consult all applicable drawings such as piping diagrams (tab books) and backup dewatering systems when planning the tag-out. Surface tag lines to the plug also should be clearly marked.

16-4.8.6 Plug Installation. Thoroughly clean the area of all fouling, rust, and debris. The plug must have a smooth clean surface to seal against. Use a wire brush, scraper or “greenie pad” to thoroughly clean the hull surface and opening of any fouling, cleaning at least as far into the opening as the plug will reach.

16-4.8.7 Insert the plug into the hull penetration and verify that the backing plate is firmly seated against the hull prior to tightening. Do not over tighten and strip the threads of the compression assembly.

CAUTION
Do not use cheater bars when tightening the plug.

16-4.8.9 Have personnel inside the ship bleed off the pressure inside the piping to ensure a positive seal has been made. One method of accomplishing this is by opening a bleed valve installed in the system. Another method is to slowly loosen an internal flange. Only loosen the flange; do not completely remove any fasteners until a positive seal is established.

CAUTION
Use caution when performing this procedure. Depending on the type and size of the affected system, a large amount of residual water may be trapped within the piping. It may take a long time to drain all the trapped water and confirm seal integrity.

16-4.8.10 Once the seal is confirmed, repair personnel may remove the item requiring repair. Repair personnel must be ready to internally blank the piping off or immediately return it to its original configuration, i.e., swap out a damaged item with a new or repaired one. The plug acts as one part and the installed blank acts as the other part of the required double-valve protection.

WARNING
If an internal blank has been installed, the integrity
of the plug must be re-established prior to removal
of the blank. Repair personnel must not assume
a positive seal.

16-4.8.13 After removal, the diver should remain at the work site, ready to reinstall the plug, until he receives verification that no leaks exist within the ship.

16-4.8.14 After each use of a mechanical expandable plug, thoroughly inspect the condition of the plug. Disassemble and freshwater wash the plug and examine all of its components for deterioration, corrosion, stripped threads, or any other obvious damage. If the material condition of any single component is questionable, either replace that component or the entire plug assembly, whichever is most appropriate.

16-4.9.1 Preparation. Table 16-4 gives National Stock Numbers (NSN) and sizes for standard DC plugs. Table 16-5 gives the minimum and maximum size opening restrictions for standard plugs. Using Table 16-5, select the appropriate size DC plug for the opening. Plugs longer than 8 inches may need to be trimmed so that a maximum of 3 to 4 inches protrudes from the opening after installation. Excessively long plugs are harder to install and pose a greater risk of becoming fouled and dislodged.

16-4.9.2 Thoroughly inspect the opening for fouling and corrosion; clean as required to obtain a smooth sealable surface.

16-4.9.3 Installation. Using a hammer, firmly seat the plug in the opening. Seal the plug to the hull with a sealing compound (e.g., Bintsuke) if required.

16-4-9.5. Have personnel inside the ship bleed off the pressure inside the piping to ensure a positive seal has been made. One method of accomplishing this is by opening a bleed valve installed in the system. Another method is to slowly loosen an internal flange. Only loosen the flange; do not completely remove any fasteners until a positive seal is established.

16-4.9.6 Once the seal is confirmed, repair personnel may remove the item requiring repair. Repair personnel must be ready to internally blank the piping off or immediately return it to its original configuration, i.e., swap out a damaged item with a new or repaired one. The plug acts as one part and the installed blank acts as the other part of the required double-valve protection.

WARNING
If an internal blank has been installed, the integrity
of the plug must be re-established prior to removal
of the blank. Repair personnel must not assume
a positive seal.

16-4.9.8 When the system is restored to its original configuration by repair personnel, the diver loosens and removes the plug. Use a hammer to dislodge and loosen the plug by striking the side of the plug. If the plug is too firmly seated to dislodge by hammering alone, use a chisel to crush the seal area between the plug and the hull to assist in removal.

16-4.9.9 After removal, the diver should remain at the work site ready to install a new plug until he receives verification that no leaks exist within the ship.

SECTION 5 - PATCHES

16-5.1 General Information. This section discusses the types and application of patches for use in Underwater Ship Husbandry. When properly planned and executed, they provide a safe and cost effective method to conduct repairs to a ship waterborne versus dry docking.

16-5.1.1 Flat Patch. The flat patch is constructed of metal or marine plywood. Figure 16-8 shows typical flat patch details. Depending on the size, it may be provided with stiffeners for strengthening and padeyes for lifting. The sealing surface is made by gluing a layer of sealing material to the mating side of the patch.

16-5.1.2 Box Patch. The box patch is basically a flat patch with four sides to allow the patch to stand-off from the hull, thus clearing obstructions and providing sufficient height to contour the sealing surface to match the hull curvature. Figure 16-9 shows typical box patch details. Box patches can have interchangeable sealing flanges. The design and fabrication of only a new sealing flange for different hull openings can save time and expense and reduce or eliminate additional engineering analysis.

16-5.2 Requirements. Patches, flat or box, used for UWSH maintenance and repair must meet the following minimum specific requirements to safely accomplish the task.

16-5.2.1 Repair personnel shall install a replacement component or an internal sealing blank with a gasket immediately after the damaged component is removed. Immediate installation of an internal sealing blank in combination with a closed sea chest valve meets the double-valve protection requirement without a diver installed patch, provided that the ship's Commanding Officer or his designated representative has been notified per Appendix G and understands that there is only single valve protection during the brief period occurring between the time the repair component is removed and the internal sealing blank is installed and between the time the internal sealing blank is removed and the repaired component is replaced. Prior to removal of the internal sealing blank for replacement of the repaired component the blank fasteners shall be loosened and the seal cracked to check for leakage.

16-5.2.2 If a sea chest closure valve is being worked or can not be secured, a diver installed patch in combination with an internal sealing blank with a gasket installed immediately after the damaged component is removed meets the double-valve protection requirement provided that the ship's Commanding Officer or his designated representative has been notified per Appendix G and understands that there is only single valve protection during the brief period occurring between the time the repair component is removed and the internal sealing blank is installed and between the time the internal sealing blank is removed and the repaired component is replaced. If no other internal vent exists, the internal sealing blank shall have a less then ½ inch diameter vent valve installed. The vent valve facilitates internal vent cofferdam monitoring per requirement 16-5.2.18. Prior to removal of the internal sealing blank for replacement of the repaired component no leakage shall be confirmed at the internal vent or blank vent valve.

16-5.2.3 The design and fabrication must meet the requirements of Section 9 of this chapter. Design for sealing gasket and rigging loads discussed in Section 9 must be considered in addition to the patch structural design. Section 9 also provides standard flat and box patch designs that meet this requirement.

CAUTION
The flat and box patch designs provided have adequate strength and gasket loading area for application over the opening sizes identified in each design. Prior to application on a specific hull opening attachment and rigging plans must be developed for the specific ship and hull opening.

16-5.2.4 Sea chest gratings and splitter bars can be considered in the sizing of flat patch material. To consider support from recessed grating or bars, they must be shimmed to flush with plywood or other suitable material.

16-5.2.5 An uncompressed gasket sealing material thickness of 2 inches. Gasket material is specified in Section 9.

16-5.2.6 The patch must be positively secured to the hull to prevent loss of the patch and rotation or movement of the patch out of proper alignment with the hull opening. The outer perimeter dimensions of the patch shall be marked on the hull prior to patch installation to ensure proper patch alignment with the hull opening. Marking on the hull shall not be by scribing into the hull paint system. Divers shall inspect the sea chest strainer bars for excessive deterioration, weld cracks and permanent deformation. Strainer bars shall be free of defects prior to cofferdam installation. If the strainer bars are bolted, verify that the fasteners are in good condition.

16-5.2.7 All patches must have an installed data plate or engraved serial number which corresponds to supporting documentation. The repair activity having responsibility for patch installation shall maintain supporting documentation and maintenance records for verification while the device is in service. Supporting documentation shall include patch rated depth and maximum hull opening size, fabrication drawing, gasket requirements, eductor and vent line requirements, and patch specific hull opening attachment and alignment requirements.

16-5.2.8 Prior to installation, all patches and attachment hardware (J-bolts, turnbuckles, etc.) shall be visually inspected in accordance with the Patch and Plug Inspection checksheet (Appendix C). The dive supervisor shall sign off the Patch and Plug Inspection checksheet approving the inspection. The Patch and Plug Inspection checksheet shall remain on file at the installing activity while the device is installed on a vessel. Any deficiencies found during inspection must be corrected, repaired, or replaced, prior to each use.

16-5.2.9 Prior to installation of a patch, ship’s Commanding Officer or his designated representative shall be notified in writing of the location of the patch and level of protection provided (single or double-valve).

16-5.2.10 Emergency plan (flooding)- Whenever single valve protection is in place the repair activity shall include in their written notification to the ship a specific plan for immediate installation of a replacement piping component or internal sealing blank as discussed in 16-5.2.1 or 16-5.2.2 to restore double-valve protection. The ship shall calculate the worst case possible scenario for flooding and provide a plan of action and additional emergency dewatering equipment as required. Except for the brief period of single valve protection provided for in 16-5.2.1 or 16-5.2.2 the equipment shall be rigged in place and operationally ready prior to commencing work and for the entire time single valve protection is in place.

16-5.2.12 All components and subsystems of the affected system must be identified and tagged-out using an approved tag-out procedure.

16-5.2.13 All patches shall be sealed to the hull opening by a gasket compressed under differential pressure. Differential pressure is applied when the seachest covered by the patch is dewatered and brought to atmospheric pressure.

16-5.2.14 Venting: All patches shall be vented to atmospheric pressure by an internal seachest vent (steam blow down valve) or an external non-collapsible vent line installed directly on the patch and run to the surface. A vent line (internal or external) must be installed and open prior to operation of a dewatering eductor to ensure eductor suction will not draw the enclosed space pressure below atmospheric pressure and overload the patch. If the internal vent line is greater than or equal to ½ inch IPS, a temporary reducer shall be installed to make the opening less than ½ inch IPS. When using an internal vent, communications must be established between topside and internal space workers to ensure that the internal vent valve is open prior to eductor operation. All internal vents shall be caution tagged "External cofferdam vent valve. If water present or pressurized air released when opened, take action to confirm cofferdam adequacy". When an external vent line is used, it must be a non-collapsible hose. All patch pipe nipples used to attach external vent lines must have valves installed to secure the space when dewatering is complete.

16-5.2.15 Dewatering: All patches shall be dewatered by draining the seachest at an internal drain valve or piping connection or by use of an eductor installed directly on the patch. Eductors shall be secured after dewatering is complete. Eductors shall not be left running to maintain a differential pressure seal. All patch pipe nipples used to attach the suction side of dewatering eductors must have valves installed to secure the space when dewatering is complete.

16-5.2.16 Retightening: After sealing, a final check of all support rigging is necessary to verify integrity. Sealing the patch will compress the seal, causing slack in the rigging that was not present before dewatering. Readjust the rigging or tighten the J-bolt(s) as necessary to ensure a tight fit.

16-5.2.17 Leakage: Zero leak rate seal shall be confirmed at the internal repair area with dewatering equipment secured prior to complete removal of piping connection fasteners. If a repair activity is unable to achieve a zero leak rate after exhausting all efforts and the patch application has an internal vent, a leak rate with dewatering equipment secured acceptable to personnel performing internal repair work and ship’s damage control personnel must be established. The established leak rate with dewatering equipment secured must be confirmed at the internal repair area prior to complete removal of piping connection fasteners. Regular assessment (measurement) of the patch leakage with dewatering equipment secured to ensure leakage is maintained at or below the acceptable leak rate is required. If a repair activity is unable to achieve a zero leak rate after exhausting all efforts and the patch application has an external vent, a departure must be processed.

16-5.2.18 Monitoring: The differential pressure seal on all patches (with dewatering equipment secured) shall be monitored during the time that the patches are actually providing single or double-valve protection for the seachest.

16-5.2.18.1 For seachests and piping systems with internal means of venting, the location of the affected seachests and internal vents shall be reported to the ship's Chief Engineer per Appendix G. Internal vents shall be inspected for signs of leakage by ship's sounding and security detail on an interval not to exceed every seven days. The isolation valve for the internal vent shall be carefully opened during the inspection and the vent observed for the presence of water, or the release of pressurized air (indicating entry of water past the cofferdam seal causing the remaining air bubble to compress). The isolation valve for the internal vent may then be closed until the next inspection to avoid leakage between inspections from entering the bilge or, upon approval by the Ship, the isolation valve for the internal vent may be left continuously open to maintain cofferdam differential pressure.

16-5.2.18.2 For seachests and piping systems with no means of internal venting, the patch shall be visually inspected by divers for reduced gasket compression indicating loss of seal on an interval not to exceed every seven days, or install a pressure gauge with one psi gauge resolution onto the surface end of an external vent line which has been extended to a location where ship's sounding and security detail can monitor the pressure gauge for a one psi(g) pressure increase, or install flood detection sensors on or inside the patch that will alarm in a continuously manned space if flooding occurs. NAVSEA Drawing 6698919, Wireless Alarm Flood Monitoring System, details a cofferdam patch alarm system.

16-5.2.19 Dive supervisors shall complete and sign the Patch and Plug Installation Checksheet (Appendix D). The Patch and Plug Installation Checksheet shall remain on file at the installing activity while the patch is installed on a vessel.

16-5.2.20 All divers (Navy and Contractor) shall be trained in the fundamental cofferdam knowledge identified in Appendix E prior to cofferdam installation tasking. Supervisor of Ship Building (SUPSHIP) activities shall be responsible for approval of contract diver organization training plans and execution.

16-5.2.21 Divers shall not leave the dive station until double-valve protection is established, repairs are completed, or ship’s Commanding Officer or his designated representative permission is granted.

16-5.3.1 All patches (single or double-valve protection) should be scheduled for installation just prior to the requirement for internal work and seals confirmed immediately prior to commencing internal work.

16-5.3.2 Selection. The first reference drawing used when selecting a patch is the Docking Plan. The Docking Plan provides the underwater profile of the ship, the plan view of its hull, the exact location and size of the opening, the location of underwater appendages, and reference points and measurements to other openings and appendages. The “List of Hull Openings” on the Docking Plan provides drawing numbers applicable to each hull opening. These references are used to obtain details of openings, fittings, and appendages attached to the plating in the area of repair.

16-5.3.3 Templating. The requirements for templating can be found in Section 8. The additional time to template pays off in ease of installation and a watertight seal.

16-5.3.4 Eductor Use. Figure 16-10 shows a typical patch and eductor configuration. Figure 16-11 shows typical eductor and vent details. Venting during eductor use prevents rapid dynamic loading on the patch and drawing a vacuum, both of which will over stress the patch materials. Once the eductor is secured, the vent line provides an easy method for checking the cofferdam seal prior to breaking any internal connections. This can be accomplished by closing an installed stop valve on the surface end of the vent line after the patch has been dewatered, waiting a period of 30 minutes and opening the valve to check for pressure. If a discharge of air does not occur, an effective seal is indicated. This does not eliminate the requirement for verifying cofferdam seal from within the ship’s workspace as internal connections are broken. The eductor is used only for dewatering the void space of water retained in the system while establishing a differential pressure and not as a securing device or to compensate for an ineffective seal. Once a system is dewatered, eductors are secured to assess the performance of the patch. The sizing of the eductor should take into consideration the volume of water to be removed and the GPM rating of the eductor. The use of strain reliefs for the eductor water supply and patch vent lines should be incorporated into the rigging plan to prevent the weight of the hose from being carried by the patch. The eductor supply valve should be tagged, as appropriate, to clearly identify its condition.

16-5.3.5 Rigging Plan. The development of a rigging plan is important no matter how simple the task may seem. The type and size of the patch to be installed determines the extent of the rigging plan. The plan should identify the proper selection of equipment (safe working loads) to lift the device in air and water, secure it in place, and remove it after completion of the repair. The primary method of securing a patch to the hull is the attachment of J- bolts or hogging lines to permanent structures such as sea chest grates, splitter bars, or the main deck. When hull configuration limits the force applied by the hogging lines, the use of a stand-off should be considered to provide the sealing force required. (See Figure 16-12). Temporary devices, such as spiders, may be used as an aid to initial seating. However, an additional means of securing the patch to the hull, such as hogging lines, must also be used. If a NAVSEA-approved temporary securing device is used as a positive attachment, a hogging line is not required. Positive securing ensures the patch will stay on the hull in position if the differential pressure seal is lost. This will ensure the seal can quickly be re-established by the divers.

16-5.4.1 System Localization. The first step to installing a patch is locating and confirming against the Docking Plan that the valve or system to be removed corresponds to the system being blanked. Localization may be accomplished by the use of locator lines suspended from known frames on deck. If the opening is below the bilge keel, the diver can follow the vertical butt welds to the corresponding strake. Another method of localization is to hammer on the valve or adjacent structure inside the ship. The diver then locates the opening by noise and vibration. Some systems are configured to allow connection of an air source (sea chest steam valve connection) which will allow a diver to follow the bubbles to the source. This method is helpful on large ships when there is poor visibility.

16-5.4.2 Surface Preparation. Removal of all fouling and corrosion from the immediate hull area is essential to achieve a positive seal. When using J-bolts to attach a patch to a sea chest grating or spreader bars, the patch should be centered over the opening. A paint stick or marker may be used to extend contour lines out from the opening for reference in positioning the patch.

16-5.5.1 Flood/Equalize. Prior to flooding the patch for removal, establish communications with repair personnel inside the ship. Flooding can be accomplished by slowly opening a valve internal to the ship to equalize the pressure or by removing the vent line if installed. When it has been verified that of no leaks exist within the ship, the patch can be de-rigged and removed.

16-5.5.2 Inspect and Refurbish. After the patch has been removed and prior to placing back in stowage, an inspection should be performed as during the preparation phase to determine its condition and refurbish as necessary.

SECTION 6 - DRY CHAMBERS

16-6.1 General Information. This section discusses the application of dry chambers and specific types available for use in Underwater Ship Husbandry. The installation of a dry chamber is a means of providing a dry envelope in which divers can work to accomplish specific requirements of a repair, or to maintain a minimum freeboard in cases of hull or system repairs at or near the waterline. Typical repairs requiring the use of dry chambers require that personnel are trained to accomplish the repair method and in some instances be certified in the procedure.

16-6.2 Dry chambers typically are a four-sided open bottom or top cofferdam with a sealing surface to match the curvature of the hull. They may be constructed of metal or marine plywood with metal stiffeners. Dry chambers can have interchangeable sealing flanges. The design and fabrication of only a new sealing flange for different applications can save time and expense and reduce or eliminate additional engineering analysis. Figure 16-13 shows a typical dry chamber configuration.

16-6.3 The attachment of a dry chamber to the hull or appendage is accomplished by the use of rigging designed to provide the required force to maintain a seal.

16-6.6 Requirements. All dry chambers used for UWSH maintenance and repair must meet the following minimum requirements to safely accomplish the task.

16-6.6.1 The design and fabrication must meet the engineering requirements of Section 9. The design shall also include the attachment method to ensure sufficient force is applied to make and maintain a seal during the repair.

16-6.6.3 The dry chamber must be positively secured to a permanent hull structure or temporary welded padeyes.

16-6.6.4 All dry chambers must have an installed data plate or engraved serial number that corresponds to the supporting documentation. The repair activity having cognizance or ownership of the dry chamber shall maintain original design and maintenance records for verification while the device is in service.

16-6.6.5 Prior to installation of the dry chamber, the chamber and attachment hardware (J-bolts, turnbuckles, etc.) shall be visually inspected for cracks, corrosion, and deformation. Inspect all gasket material for damage, cleanliness, adhesive bond, and pliability. Any deficiencies must be corrected, repaired, or replaced, prior to each use.

16-6.6.6 Dry chambers shall be provided with sufficient lighting to illuminate the chamber and work area.

16-6.6.7 Dry chambers must have an air supply/vent system capable of meeting the requirements of NSTM S9086-CH-STM-030 Chapter 074, Volume 3 Gas Free Engineering, and the U.S. Navy Diving Manual, Volume 1, 0094-LP-001-9010, “Enclosed Space Diving,” if work involves the requirement for ventilation (welding, shaft laminate installation, etc.) or if divers plan to remove their UBA to perform work.

16-6.6.8 Prior to commencing any work or repairs in the chamber the seal must be confirmed. With the dewatering system secured, the maximum leakage rate must not exceed 4 inches per hour rise in water level when the chamber is fully dewatered.

16-6.6.9 The water level at the bottom of an open bottom dry chamber shall be continually monitored.

16-6.7.1 References. Since dry chambers are used for various types of repairs on appendages and hull plating, the use of the ship drawing index (SDI), which lists all applicable drawings to that particular ship, must be used to find the drawing applicable to the particular area to be worked. Drawings numbers are arranged in the SDI by functional group and they are numerically listed within these groups.

16-6.7.2 Monitoring. Monitoring for dry chamber work falls into two areas: the dry chamber requirements and the work re-quirements. For the dry chamber, when work is being performed by divers in the chamber, continuous monitoring of the water level and periodic checks outside the chamber shall be performed to assess the performance of the seal. Continuous monitoring is required to prevent flooding of the chamber. When divers are not at work, this may be accomplished by placing a DUCTS (Diver Underwater Color Television System) camera inside the chamber to view the waterline. The chamber may then be dewatered by topside personnel as required. Considerations for the work include: cure time, humidity, and temperature for epoxy and pre- and post-heat treatment for welding. Whenever welding is performed, and as appropriate, gas freeing and fire watches of the internal space opposite of the repair area are required.

16-6.7.3 Templating. The requirements for templating can be found in Section 8. The additional time to template pays off in ease of installation and an airtight seal.

16-6.7.4 Inspection and Acceptance. Prior to the installation of a dry chamber, a thorough inspection of its condition shall be performed. The user must perform a visual inspection of the lifting points, padeyes, welds, staging, air supply and vent line connections (if installed) for any signs of deformation, cracks, or bending which will require repair and testing before use, and possibly an engineering re-evaluation. Gasket material shall be inspected for complete adhesion to the flange and replaced when permanent indentations exist that may prevent the achievement of acceptable leakage. When a new dry chamber is fabricated, the design and fabrication drawings will provide initial testing and inspection requirements. The verification of testing to an approved dry chamber design shall be by means of an attached data plate or engraved serial number which corresponds to supporting documentation. The repair activity having cognizance or ownership of the dry chamber shall maintain records for verification while the device is in service.

16-6.7.5 Dewatering and Ventilation Equipment. The dewatering and ventilation equipment selected shall be capable of delivering the required amount of air for initial dewatering and ventilation of the chamber throughout the repair. The air supply requirements are determined by calculating the volume of the chamber and converting to standard cubic feet (scf) required for the operating depth. The air source (compressor or air banks) must be capable of meeting these requirements. The sizing of the air supply and exhaust hose shall be capable of meeting the ventilation flow requirements of NSTM 074, Volume 3. Air supply and exhaust hoses should be equipped with stop valves at the chamber for diver operation and at the surface for topside control during monitoring. The discharge side of the air supply in the chamber should be equipped with an air diffuser or deflector to prevent injury to personnel. Strain reliefs are recommended to support the weight of the hoses at the chamber and topside.

16-6.7.6 Rigging Plan. The development of a rigging plan is required during the engineering design phase to ensure sufficient lifting force is provided. The location and size of the dry chamber to be installed determines the extent of the rigging plan. The plan shall identify the proper selection of equipment (safe working loads) to lift the device in air and water to the job site, secure it in place, and remove it after completion of the repair.

16-6.8.1 Surface Preparation. Removal of all fouling and corrosion from the sealing surface is required to achieve a positive seal. When using an existing dry chamber for a specific repair, marking the outline of the seal area on the hull with a paint stick or marker for reference in positioning the chamber will ease the installation.

16-6.8.2 Open Bottom Dry Chamber Dewatering. When the dry chamber has been installed, place a mark on the wall inside the chamber (grease pencil or paint stick) approximately 4 inches from the bottom. This will be the reference water level line. With the exhaust valve shut, open the air supply valve and dewater the chamber down to the mark and secure the air. Divers then inspect for excessive leakage and correct as necessary. In most cases, minor leaks will be present. Minor leaks are defined in terms of the rise in water level with the inlet and exhaust secured. A rise of not greater than 4 inches per hour is considered to be minor. Prior to commencing any work, a stable water line shall be established and confirmed. As divers and equipment enter and leave the chamber, the volume of air displaced will change. The divers will need to either add or exhaust air to maintain the water line.

CAUTION
Never dewater the chamber completely. This will cause air to exhaust out the bottom of the chamber
and create a rocking effect. The rocking can cause
the rigging to slack and break the seal, thereby
flooding the chamber.

SECTION 7 - STERN TUBE SEALS

16-7.1 General Information. This section provides the requirements for sealing shaft stern tubes externally to allow for removal and repair of internal shaft seals.

16-7.2 Often stern tube repairs only require the resurfacing of internal face seals which are inboard of the boot seal. Special blanking plates (dummy seat) are available which are installed after the face seals are removed. The blanking plates provide double-valve protection when combined with the inflated boot seal. In this case, diver-installed external stern tube seals are not required.

16-7.3 Figure 16-16 shows the internal stern tube seal configuration for a CG 47 Class ship. Figure 16-17 shows the “face insert,” “seat,” and “splash guard” removed and the dummy seat installed. Similar arrangements exist for other ship classes. Detailed installation procedures and fabrication drawings for dummy seats are available from NAVSEA 00C5.

16-7.4 There are two types of stern tube sealing methods commonly used on surface ships. The first method is the use of a special box patch which is templated to fit around and seal the propeller shaft, stern tube, and hull. The second method is the use of packing material to wrap and seal the opening between the propeller shaft and stern tube fairwater. The packing material wrap method is only practical for ships with an exposed, accessible stern tube fairing which can be wrapped with material. Inaccessible or recessed stern tube openings require the fabrication of a box patch. The design, templating, and fabrication of a box patch stern tube seal can be initially time consuming. However, ease of reapplication to all ships of the same class and a high degree of watertight integrity more than pay back the initial effort.

16-7.5 Submarines typically use mechanical shaft seal devices installed around the shaft at the forward end of the mudtank. NAVSEA Drawing 203-6618843, SSN 688 Class Temp Seal Stern Tube Assy & Details, details a SSN 688 Class stern tube seal device. Modifying the design with a new inside and outside diameter at the seal ring (item 4) and the clamp ring (item 5) as applicable for other shaft sizes, would allow for application to other submarines or surface ships with exposed accessible stern tube fairings. A mechanical seal of this type is preferable to the wrapping method.

16-7.6.1 Regardless of the sealing method used, the seal must be installed and secured to the propeller shaft, stern tube, and/or hull as applicable prior to any dewatering effort.

16-7.6.2 A stern tube box patch or a mechanical sealing device must meet all the requirements of Section 5, Patches, except as stated below.

16-7.6.3 When eductors are used to dewater a stern tube cavity sealed by a mechanical sealing device or wrap type seal the eductor shall be positively secured to the hull to prevent loss.

16-7.6.4 Emergency plan (flooding) - When the stern tube seal is providing the only barrier to the sea (single-valve protection), the repair activity shall notify the ship's Commanding Officer or his designated representative in writing. The ship shall calculate the worst case possible scenario for flooding and provide a plan of action and additional emergency dewatering equipment as required. The equipment shall be rigged in place and operationally ready prior to commencing work.

16-7.6.7 Divers shall not leave the dive station until double- valve protection is established, repairs are completed, or ship’s Commanding Officer or his designated representative permission is granted.

16-7.7.1 Selection. The first reference drawing used when selecting a stern tube sealing method (box patch, mechanical shaft seal, or wrap) is the shafting arrangement drawing. It gives the general measurement of the shaft and a general idea of the stern tube accessibility. The reference block of this drawing lists the detail drawings of the stern tube and stern tube fairwater, usually included in the “rope guard and fairwater” drawing. Careful review of the stern tube and stern tube fairwater drawings may show hull access plate locations that would not be obvious during an inspection dive. These areas would need to be sealed also.

16-7.7.2 Monitoring. Whenever a stern tube seal is installed, ship's force should have a thorough understanding of the location and protection it is providing. This allows shipboard watch standers to be fully aware of and monitor its condition during normal watch standing rounds.

16-7.7.3 Templating. When a repair activity makes a decision to develop a ship class box patch stern tube seal, accurate templating is the key to making an effective seal. The requirements for templating can be found in Section 8. NAVSEA Drawing 6697929, Shaft Repair Top Section Assembly, gives detailed procedures for templating the hull contour and shafting in the stern tube area using the NAVSEA contour tool (NAVSEA Drawing 6698079).

16-7.7.4 Inspection. Conduct an inspection dive of the entire stern tube area, including fairwater, shaft, and hull surface, prior to installation of a stern tube seal to assess cleaning required, configuration, or access issues that were not clear from the drawing review. The inspection dive will also locate additional cracks or openings, weld seams, or hull access plates not shown in the drawings that will need to be sealed. Box patch stern tube seals shall have a material inspection and verification of approved design prior to use in the same manner as for all patches. (See Section 5, Patches.)

16-7.7.5 Rigging Plan. Similar to all patches, a box patch stern tube seal requires a rigging and installation plan. An important aspect of stern tube box patches is fixed, closed, positive buoyancy tanks. Positive buoyancy ensures that the installing divers do not have to fight with excessive rigging loads at the same time they are trying to optimize the exact position of the patch for the best possible seal. Each time a stern tube box patch is used on a different ship of the same class the optimum position for best seal will be slightly different. A series of position adjustments and inspection swims of the entire sealing surface will eventually locate the optimum position for the patch. Trial and error adjustment of the patch buoyancy by adding or removing weight will optimize buoyancy at depth. If an eductor and vent line are used, strain reliefs should be provided to support the weight of hoses.

16-7.8.1 Surface Preparation. Remove all fouling and corrosion from the immediate hull area, shafting, and stern tube to achieve a positive seal. If possible, avoid sealing on damaged surfaces.

16-7.8.2 Dewatering. When the stern tube seal has been secured in place, rapidly drain the trapped water to establish a differential pressure and assess the performance of the seal. One common reason for failing to establish an initial differential pressure is draining the stern tube too slowly from inside the ship. To properly dewater, flow into the stern tube seal from the outside (minor leaks in the external seal caused by lack of differential pressure acting on the seal) must be overcome by flow out of the stern tube seal at the internal drain. Quickly opening the internal stern tube drain all the way should provide sufficient flow to obtain differential pressure on the external seal. A second reason for failing to get adequate drainage to achieve a differential pressure seal is a failure to secure the ship’s stern tube flushing or bearing cooling water supply. Sometimes an unsecured port to starboard cooling water cross-connect or a secured valve which is leaking-by frustrates all attempts to drain the system and achieve and confirm a seal.

16-7.8.3 In some ship classes, a large amount of water is trapped in the stern tube by the external seal, so it may take 20 minutes or more to see reduction in the drainage flow as an indication the seal is working. In some cases, partial external dewatering can be used to reduce the volume of water drained to bilge. Inspection of the seal during the dewatering process will reveal problem areas with the seal.

16-7.8.4 After dewatering, a final check of all support rigging must be made. Dewatering the stern tube will compress the seal, causing slack in the rigging that was not present before dewatering. Adjust as necessary to ensure a tight fit. If flat patches with T-bolts are used to seal dead lights, the wing nuts must be retightened because the differential pressure will compress and seal the patches more.

16-7.9.1 Flood/Equalize. Flood the stern tube with internal stern tube flushing or bearing cooling water supply to equalize the external pressure. When no leaks are verified at the internal seal, the external seal may be removed.

16-7.9.2 Inspect and Refurbish. All stern tube sealing equipment should be inspected after use and refurbished as necessary to ensure it is ready for the next operation.

SECTION 8 - TEMPLATING

16-8.1 Introduction. Templating is the process required to pattern hull shape data (hull contour) for use in forming the sealing surface of cofferdams. Except for flat hull surfaces, templating is required to achieve the goal of zero leakage. If done properly, templating is precise and time-consuming work; however, the time and effort expended up front in templating gives a pay back of quicker installations with good seals for blanks and habitats. Poor fit is the major result of poor templating procedures and the primary cause of rework. Rework may result in the removal, disassembly, re-templating, and modification of the cofferdam. This results in lost time, rescheduling of logistic support, and possible damage to and replacement of equipment. A poorly fitting cofferdam is a clear safety hazard to the diver, the ship or submarine, and interior workers. Templating and pattern fit checks must be repeated until a good fit and seal is obtained.

16-8.2 Requirements. Templating is required in underwater ship husbandry cofferdam applications when the repair activity can not define the hull contour to +/- 1 inch at the location of cofferdam installation using ship’s drawings only. If templating is required, the specific requirements of the templating fixture are:

16-8.2.1 Two-dimensional box structure allowing divers to perform one continuous template layout which matches the footprint of the cofferdam to be installed.

16-8.2.5 Attached levels allowing divers to confirm proper horizontal and vertical attitude.

16-8.2.6 All divers involved in the templating process must have prior training in the use of the templating fixture.

16-8.2.7 A templating fixture similar to the simple, proven accurate NAVSEA Contour Template Fixture (NAVSEA Drawing 6698079), Figure 16-18, meets these requirements and can be locally manufactured.

16-8.3.1 Review Drawings. The ship’s drawings are useful in helping to plan the templating process. They are used to determine location, position, and size of fixture; identify possible interference and location references; and estimate length of battens and set the initial fixture stand-off required. Interpretation of these drawings may require the assistance of or training by an engineer.

16-8.3.2 Site Survey/Marking Location. Clean, inspect, and mark the area to be templated. The hull surface must be completely clean of all fouling for the template fixture to produce an accurate measure of the hull contour. Inspect the area to be templated for any surface irregularities such as dents, bumps, or doubler plate installations. If possible, shift the eventual cofferdam location to avoid having to seal on hull surface irregularities. If the cofferdam location can not be shifted, mark the irregularities so special attention can be focused on detailed templating to avoid seal problems. Confirm that no interference exists for placement of the template fixture or the cofferdam. Consider the rigging plan for the cofferdam when selecting the final location for the cofferdam. Changing the footprint (sealing surface) of the cofferdam may simplify the rigging requirements. Use of the Diver Underwater Color Television System (DUCTS) to document the survey can provide all hands with a good understanding of the task. When the area has been surveyed, mark the hull to help with initial positioning of the template fixture. This ensures that the fixture is aligned with the intended position of the cofferdam sealing surface on the hull. Mark the hull with underwater paint sticks or grease pencils. Do not scribe into the paint on the hull, since this will cause hull plate corrosion.

16-8.4.1 Assemble Template Fixture. To minimize handling, the templating device should be assembled as close as possible to the repair staging site. Prior to placing the templating device in the water, ensure that the templating rods and all moving parts move freely and are easy for the diver to set. Number all battens to help provide a means to record location of additional detailed measurements taken in problem areas. Set the position of the corner battens (stand-off) using the measurements taken from the drawing review. The minimum fixture to hull standoff distance (length of shortest batten extension) for the leveling process should be 4 inches.

16-8.4.2 Position Fixture. Before setting battens or taking any measurements, the templating fixture must be secured firmly in place. Any templating performed with a lively fixture will produce an inaccurate contour. Secure the fixture by using hogging lines. Compare the structural strength and weight of the fixture to the size of the equipment used in rigging. Most templating fixtures are lightweight and are designed to be neutral. The rigging must not distort the fixture. Therefore, use lightweight securing equipment. Lower the fixture to the work site and attach rigging to secure it in place. Make sure to align the fixture inside the markings from the survey. Level the fixture by observing the spirit levels attached to the fixture while adjusting the four corner battens of the fixture. Normally, the cofferdam should be installed level in the fore-aft and the port-starboard orientations. However, blanks mounted to the side of the ship’s hull may only require leveling in the fore-aft orientation. For such blanks, the port and starboard ends of the template fixture should be offset by an equal amount to ensure a uniform hull contour is measured. Once the fixture is in position, mark all four corners to form reference points for the template. These reference marks are used to confirm that the template stays in position while adjusting the battens. Figure 16-19 illustrates the installation of the NAVSEA templating fixture for templating a shaft stern tube area of the hull.

16-8.4.3 Adjust Battens. Adjust, secure, and mark the remaining battens to match the hull contour. If there are irregularities in the hull which the battens do not accurately describe (i.e., irregularities are between the batten spacing), more detailed measurement of that area is required. Commercially available contour gauges or a straight edge and ruler provide detailed templating or measurements of small problem areas. (See Figure 16-20). Make a final inspection of the fixture, checking reference points to verify that the fixture did not move while adjusting the battens. Scribe the final position of each batten before sending the fixture to the surface.

16-8.4.4 Mark Exact Location of Fixture on Hull. Prior to removing the templating fixture from the hull, mark the outline on the hull along each of the battens. This outline approximates the centerline of the sealing surface. Extend the marks outward at the corners to compensate for half the width of the proposed sealing flange. These marks are critical for accurate positioning of the cofferdam. Incorrect positioning by as little as a few inches can negate all templating efforts. The hull should be marked without scribing or otherwise damaging the hull paint system.

16-8.4.5 Retrieve Fixture. Ensure that the rigging does not become fouled between the battens. To prevent distortion of the fixture, do not use excessive strain when retrieving it. Position the fixture on the pier, using standing blocks to provide sufficient height to prevent damage or movement of the battens. Keep the fixture level and stable to prevent distortion. Avoid unnecessary handling and movement of the fixture. Use the scribe mark on each batten to confirm that no movement has occurred.

16-8.4.6 Make/Check Patterns. Using ¼-inch plywood as pattern material, trace the hull curvature outline from the tops of the fixture battens. Cut the pattern along the hull contour, being sure to include any additional measurements or contours taken around irregularities or doubler plates. Make a pattern for each of the template fixture’s four sides. Take each pattern down to its respective hull positions as marked on the hull before the template fixture was removed. Depending on the size of the pattern, more than one diver may be required. Test fit each pattern to the hull contour. Mark gaps, return to the surface for adjustment, and repeat contour verification until each pattern accurately represents the hull contour. There should not be any gaps larger than ½ inch. This will ensure that gasket material will provide an effective seal. Figure 16-21 illustrates making patterns for a shaft stern tube hull contour.

16-8.4.7 Training/Practice. Templating is a demanding skill. Like all skills, it requires training and practice to be perfected. While there are manuals explaining the techniques for templating, these manuals only give the basics. Merely reading about them will not give the diver the skills required to template properly. Practice and training are required. Since most UWSH work takes place in low visibility, on-the-job training is a less-than-ideal training environment. A ship in dry-dock provides an excellent opportunity as a training platform on which to practice. Other dry-land sources of training can be found on any structure with an irregular or regular contour, such as storage tanks, large drums, or work boats that have been hauled and blocked for maintenance.

SECTION 9 - FLAT PATCH, BOX PATCH, AND DRY CHAMBER DESIGN

16-9.1.1 NAVSEA Flat and Box Patch Designs. The following designs allow divers to blank most existing Fleet hull openings without additional engineering support.

For blanking applications where the patches below do not work, the design criteria in Section 16-9.2 support unique patch design.

16-9.1.1.1 Unstiffened Flat Patches. Unstiffened flat patches applied over flush mounted or shimmed to flush gratings or splitter bars with maximum 5-inch spacing shall be minimum ¼-inch 5000 series aluminum, ¼-inch mild steel, or 1-inch marine grade plywood. Uncompressed gasket sealing material thickness shall be 2 inches. Submarine main ballast tank patches shall be minimum 1/8-inch 5000 series aluminum or 16 gauge stainless steel sheet.

16-9.1.1.2 Stiffened Flat Patches for Large Unsupported Openings (Scoop Injections). NAVSEA-approved standard stiffened flat patch designs are provided in NAVSEA Drawing 805-7370421. These patches will cover large unsupported (no grates or grates recessed too far to practically shim) openings such as some scoop injections.

16-9.1.1.3 Box Patch for Discharge Openings with Protrusions. NAVSEA approved standard box patch designs are provided in NAVSEA Drawing 805-7370421.

16-9.2.1 Introduction. This specification provides experienced design engineers with criteria necessary for the design of cofferdams for use in Underwater Ship Husbandry.

16-9.2.2 Applicability. This design specification establishes a minimum standard for the design of any flat patch, box patch and dry chamber used for sealing, blanking or establishing a dry work environment on U.S. Navy ships and submarines for waterborne maintenance and repair actions. The criteria set forth in this specification are applicable to all new and existing flat and box patches and dry chambers.

16-9.2.3.1 Steel Construction. Flat patches, box patches, and dry chambers of steel construction shall meet the requirements of the American Institute of Steel Construction, Inc. as defined by the latest issue of the Manual of Steel Construction (Allowable Stress Design), reference 16-9.6.1.

16-9.2.3.2 Aluminum Construction. Flat patches, box patches, and dry chambers of aluminum construction shall meet the requirements of the American Aluminum Association, Inc. as described in the latest issue of the Specifications for Aluminum Structures, reference 16-9.6.2, using the basic allowable tensile stresses and factors of safety specified for Building and Similar Type Structures.

16-9.2.3.3 Marine Grade Plywood Used as Sheathing. Marine grade plywood can be used for sheathing in metal framed dry chamber or box patch structures or for unstiffened flat patches. Frame work of steel or aluminum must meet the criteria of Paragraph 16-9.2.3.1 or 16-9.2.3.2 above, as applicable. Marine grade plywood construction shall meet the requirements of the American Plywood Association, as described by the latest issue of the Plywood Design Specification, reference 16-9.6.3.

16-9.2.3.4 Minimum Factors of Safety for Rigging, Lifting, Handling, and Securing Components. The factors of safety listed in Table 16-6 shall apply to the design of all bolted or welded attachments and their supporting structure to the flat patch, box patch, or dry chamber. All cofferdam applications must have a basic rigging plan which defines the direction and magnitude of expected rigging loads for safe installation, use, and retrieval of the cofferdam. An example of an open bottom dry chamber rigging plan design is provided in Appendix B. J-bolt attachment devices shall meet the minimum requirements specified in Appendix F.

16-9.2.3.5 Total Equivalent Design Head Loading. Other than the weight of the patch or dry chamber and any live loads generated by the weight of personnel and equipment in a dry chamber, the primary loading of patches and dry chambers comes from hydrostatic (water pressure) and hydrodynamic (current) forces. The total equivalent design head (hd), expressed in feet of salt water (64 lb/ft3), is the summation of hydrostatic and hydrodynamic forces as follows:

16-9.2.3.6.1 Steel and Aluminum Plating Panels. The required plating thickness for aluminum and steel panels is based on simply supported panel edges, with the maximum bending stress occurring at the center of the plate, using the following expressions:

16-9.2.3.6.2 Marine Grade Plywood Sheathing Panels. The minimum required thickness of marine grade plywood for use as sheathing panels or patch material is determined by Section 4.1 of reference 16-9.6.3, Plywood Design Specification. The following expressions is derived from the Plywood Design Specification and can be used for selection of the minimum thickness of marine grade plywood required:

16-9.2.3.6.3 Used as Sheathing in Stiffened Patches or Dry Chambers. Where used as sheathing resisting hydrostatic loading in stiffened patches or dry chambers, the marine grade plywood shall not be used in a manner where tear-out forces are produced on fastener connections. The material shall be used only in a manner where hydrostatic loading results in bearing reactions along the perimeter of the sheathing panel. Fastener connections shall be limited to holding plywood panels to the framing and for maintaining watertight integrity.

16-9.2.3.7 Design of Stiffeners. All stiffener design shall meet the requirements of references 16-9.6.1 or 16-9.6.2 as applicable, except for wooden dry chamber hull interface seals which may be stiffened with wood planking or beams.

16-9.3.2 Gaskets for Sealing Between Bulkheads or Interconnecting Flanges of a Bolt-Together Cofferdam.

16-9.4 Use of Fasteners. Fasteners used in flat patches, box patches, and dry chambers shall be either steel or stainless steel and must meet the requirements of the American Institute of Steel Construction, Inc. as defined in reference 16-9.6.1.

16-9.5.1 Deflection of Dry Chamber Walls. For cases where the dry chamber seals to the hull and air is captured under the vessel, the deflection of the walls away from the hull sealing surface must be considered. Dry chamber walls can be designed so that they are under the minimum stresses allowed by the governing specification, but deflect under load so that they lose contact with the sealing surface. Even though the members of the wall are below the allowable stress, the deflection of the wall violates the chamber seal and the chamber suddenly floods. This failure mode must be considered during the design process to ensure that the chamber walls have sufficient stiffness to inhibit excessive deflection. The maximum deflection of the dry chamber sealing surface should be no larger than 1/4 inches.

16-9.5.2 Rigging Loads on Box Patches, Flat Patches, and Open Top Dry Chambers. A cofferdam design package must include a rigging plan noting the direction and magnitude of expected loads through installation, use, and retrieval of the cofferdam. Rigging points and supporting structure should be designed with the factors of safety from Table 16-6. A value of 2.0 psi gasket compression is recommended to estimate rigging loads for sizing of wire rope and rigging hardware. Installation loads are determined in practice by observation of gasket compression and actual achieving of a seal. The 2.0 psi compression allows for initial gasket seating during installation so that patches or open top dry chambers can be dewatered and allow hydrostatic pressure to take effect. Side loading of padeyes should be avoided wherever possible. If side loading cannot be completely avoided, the predicted side loads must be considered in the padeye design.

16-9.5.3 Buoyant Forces on Flat Patches, Box Patches, and Dry Chambers. Buoyant forces can be significant forces in patches and dry chambers. Buoyant forces need to be resolved for the installation, use, and retrieval phases to ensure that rigging and structure is adequate for all loading circumstances. Buoyant forces are of particular concern in open top (topside access) dry chamber designs.

16-9.5.4 Load Paths and Stiffener Location. Framing members (stiffeners) should be properly supported along the boundaries of the patch or dry chamber such that all reaction load paths transmit loads directly to the hull.

Available from:

American Institute of Steel Construction
One East Wacker Drive, Suite 3100
Chicago, Illinois 60601-2000
(312) 670-2400

Available from:

The Aluminum Association
900 19th Street, NW
Washington, DC 20006
(301) 645-0756

Available from:

American Plywood Association
P.O. Box 11700
Tacoma, Washington 98411-0700
(206) 565-6600

CHAPTER 16 COFFERDAMS

SECTION 1 - INTRODUCTION

SECTION 2 - GENERAL INFORMATION

SECTION 3 - OPERATIONS PLANNING

SECTION 4 - PLUGS

SECTION 5 - PATCHES

SECTION 6 - DRY CHAMBERS

SECTION 7 - STERN TUBE SEALS

SECTION 8 - TEMPLATING

SECTION 9 - FLAT PATCH, BOX PATCH, AND DRY CHAMBER DESIGN

SECTION 10 - COMPLIANCE AND PERFORMANCE
STANDARDS FOR CONTRACTOR COFFERDAM WORK

16-10.1. SCOPE

CHAPTER 16 COFFERDAMS

SECTION 1 - INTRODUCTION

SECTION 2 - GENERAL INFORMATION

SECTION 3 - OPERATIONS PLANNING

SECTION 4 - PLUGS

SECTION 5 - PATCHES

SECTION 6 - DRY CHAMBERS

SECTION 7 - STERN TUBE SEALS

SECTION 8 - TEMPLATING

SECTION 9 - FLAT PATCH, BOX PATCH, AND DRY CHAMBER DESIGN

SECTION 10 - COMPLIANCE AND PERFORMANCE STANDARDS FOR CONTRACTOR COFFERDAM WORK

16-10.1. SCOPE

SECTION 10 - COMPLIANCE AND PERFORMANCE
STANDARDS FOR CONTRACTOR COFFERDAM WORK