A Subsea Housing Design Checklist Loading Considerations > Stress Analysis > Materials Selection and Corrosion Design > Seal Design

Loading Considerations

Has the housing been designed for the highest practical external pressure? For instance, if an ROV application, will the housing be rated for the full tether cable length?

Have fastener loads due to internal pressure build-up from temperature variations or off-gassing of internal components been considered? If rechargeable batteries are being installed you should consider installing a reliable pressure relief valve.

Have deployment and recovery loads been analyzed and considered? Will the housing support the weight of other components during these operations and if so are the fasteners and lift points sized for this additional load?

Have the loads imparted by the mounting structure been considered? Will the mounting structure accommodate the deformation of the housing under pressure?

Have shock and vibration loads been considered? Is it necessary to isolate internal equipment from handling shock or the shipboard/ROV vibration spectrum?

How would a catastrophic failure of this housing impact adjacent equipment? Could a failure of this magnitude result in a cascading failure of critical systems? Back to Top

Stress Analysis

Has a general factor of safety for this application been established and consistently applied? Does it reflect the following considerations:

• Cost of failure at the program level • Level and accuracy of analysis performed • Material toughness in a sea water environment (sensitivity to stress risers)

If the housing material is plastic: has a conservative working strength been established for this material which includes consideration of time and temperature effects?

Have stresses at structural discontinuities and seal areas been looked at in detail? (FEA?)

Has the increase in both stress and deflection due to all end plate penetrations been considered in detail? (FEA?)

Will local deformation (rotation) in seal areas adversely affect the seal pressure rating?

Have all failure modes been considered and analyzed (shell failure, bucking, tangential moment, radial moment, flange shear, seat bearing)?

Have all stress critical calculations been three dimensional and then compared against an established three dimensional failure criteria?

Will deformation of the tube and end caps under pressure result in loads on internal components or structure? Back to Top

Materials Selection and Corrosion Design

Can the subsea housing material effectively meet the depth requirement and stay within the system weight budget?

Does the selected material have a life expectancy in sea water which exceeds your system operational plans?

Is the base material, in its planned temper or heat treated state, subject to stress corrosion cracking?

Is the base material galvanically compatible with adjacent materials?

Do any all galvanically dissimilar metals in electrical contact have a favorable area ratio? If not can they be electrically isolated?

Does the design avoid or minimize crevices and other potential corrosion initiation sites?

Are through bolts used in lieu of blind tapped holes wherever possible?

Are stainless steel inserts used for all threaded holes in aluminum?

Is there a well defined electrode attachment feature for all anodized aluminum parts?

Back to Top

Seal Design

Are redundant seals used wherever possible to maximize reliability?

When redundant seals are employed, are they different types (one piston, one face, one crush,….etc) to minimize susceptibility to common failure modes?

Is the seal material compatible with the all fluids across the entire operational temperature range?

Has the seal durometer been selected to provide a reliable seal at maximum pressure while minimizing installation forces?

Do all piston seals have generous lead-in chamfers to protect them for pinching during assembly?

Have all sharp corners which the seal may come contact with during installation been removed?

Are critical seal surfaces protected from damage during handling and assembly?

Are there inspection plans and assembly procedures in place to insure critical surfaces are not damaged and that seals are lubricated and installed correctly? Remember 8 out of 13 leaks past o-ring seals result from improper installation!

About Subsea Housing Materials Factors which have to be considered in selecting a subsea housing material include: STRENGTH DENSITY CORROSION RESISTANCE COMPATABILITY WITH OTHER MATERIALS GALVANIC COMPATABILITY COMPATABILITY WITH WORKING FLUIDS THERMAL COEFFICIENT OF EXPANSION COST OF RAW MATERIAL MACHINING COST COATING COST TOUGHNESS OR IMPACT STRENGTH THERMAL CONDUCTIVITY ELECTRICAL CONDUCTIVITY MAGNETIC PERMEABILITY WATER ABSORBSION/PERMEATION

Relevant Properties of Materials Commonly Used in Subsea Applications: Standard PREVCO materials are in RED. Values are for comparison purposes only and should not be used in design.

Strength Corrosion Resistance

Ultimate Yield Impact Youngs Modulus Density

Approx. Half-Cell

Galv. Voltage

Thermal Cond.

Material

(Kpsi) (Kpsi) Charpy (ft*lb) (Mpsi)

(lbs/Cu In)

General

(volts)

Susept. to Pitting

Special Considerations

Btu/hr*ft^2*FNoble Metals/Alloys

Titanium (GRADE 5) 150 120 --- 16 0.161 Excel. -0.1 No Compatability(1) 4 Hasteloy C 83 48 10 to 23 29.8 0.323 Excel. -0.08 No Compatability(1) 6.5

INCONEL 686 110 50 --- 30 0.315 Excel. -0.2 No Compatability(1) --- Copper Nickel

90-10 52 36 --- 20.3 0.323 Good -0.28 No --- 23 Aluminum

6061-T6 45 40 --- 10 0.098 Good -0.76 Yes Anodize Reqd 104 5086-H34 47 37 --- 10.3 0.096 Good Yes 73

5083-H112 42 21 --- 10.3 0.096 Good Yes 68 7075-T6 83 73 --- 10.4 0.101 Poor Yes Anodize Reqd 70

Stainless Steels

304 82 35 --- 28 0.29 Good -0.08 to -.53 Yes Crevices 9.4 316 80 30 --- 28 0.29 Good -0.05 to -.45 Yes Crevices 0.94

17-4PH H1150 150 125 50 29 0.281 Good -0.05 to -.45 Yes(2) Crevices(2) 10.4 Mild Steel

A36 58 36 5 to 20 30 0.283 Poor -0.61 No Predictable Rate 27 Non Metals

PVC N/A 8 0.5 (3) 0.000375 0.052 Excel. N/A No Non-Corrosive 1 Acrylic N/A 9 0.4 (3) 0.00045 0.043 Excel. N/A No Non-Corrosive 1.4 Acetal N/A 8.8 1.5 (3) 0.000375 0.05 Excel. N/A No Non-Corrosive 1.9

Notes:

(1) To avoid accelerated corrosion care should be take to avoid electrical coupling of these more noble materials to other less noble metals. (2) While 17-4PH is susceptible to pitting and crevice corrosion, unlike most other stainless steels, it can be protected galvanically. (3) Plastic impact strengths shown are Izod, notched and in ft*lb/in.

The Relationship Between Weight, Internal Volume and Maximum Depth Rating For 6061-T6 ALUMINUM and GRADE 5 TITANIUM Subsea Housings.

Preliminary Subsea Metal Housing Sizing: Wt. vs. Vol.

Estimating the Cost of a Subsea Housing This chart may be used as budgetary pricing guide for subsea housings. Caution should be exercised as actual prices will vary significantly depending on the number of connectors, quantity of order, housing aspect ratio, lead time, and material availability. PREVCO will be happy to provide a ROM or FFP quote for your specific application when you require a more accurate estimate. Use the convenient quote form on this site, or contact us directly. ROM quotes usually are available the same day while FFP quotes require 5 to 7 working days.

Example: A 2000 meter Titanium housing with a 5” ID and 10” internal length is required. Internal Volume = 52 p/4 x 10 = 196.35 Cu In. Max Depth = 2000 meters

Budgetary cost estimate from chart = 2000 x 196.35 x .02 = $7,853

Care and Installation of Seals > Intro > Storage > Seal Surface Inspection > Clean Seal Surfaces > Prepare Seal Surfaces > Seal Inspection > Seal Preparation > Installation >

The predominant failure mode of subsea housings is seal failure.

Subsea housings employ O-ring seals for vent ports, relief valves, connectors and end cap closures. Analysis of O-ring seals in certain underwater connectors that have been in use for decades show that roughly 8 out of 13 leaks past the O-rings result for improper installation and assembly or from improper quality control and inspection procedures at the time of assembly. 1

Therefore the care and maintenance of O-ring seals may be the most important component of the assembly process to insure a long and successful operating life. The following abbreviated steps should be followed as a general guide for the handling and installation of O-ring seals.

Note that these steps should be repeated at every assembly; that is remove, inspect and reinstall all O-ring seals prior to each assembly.

Dirt and air borne debris (particularly human hair) can often lead to seal failure. Always clean and lubricate seals and components immediately prior to assembly.

Storage NITRILE (BUNA-N) is the PREVCO standard subsea housing seal material. Alternate materials which may be supplied for special applications may have different storage and handling requirements – see manufacturers recommendations.

NITRILE is subject to aging damage when exposed to ultraviolet radiation, ozone or elevated temperature. Always store spare seals in a clean environment protected for direct sunlight, ozone and elevated temperatures. Discard any seal with damaged packaging or a cure date that is over 5 years old.

Note: O-ring seals discarded for any reason should be cut completely through with a pair of scissors to prevent accidental re-use.

1 Sandwith, C. J., O-ring Installation for Underwater Components and Applications, NRL Memorandum Report 4809, April 15, 1982

Back to Top

Seal Surface Inspection Prior to installing O-ring seal inspect all seal surfaces for cleanliness, proper finish and absence of defects. Surfaces and edges must be free of all contaminants, dirt, nicks, scratches, gouges, marks and burrs. Minor burs can be removed by “touching” them with 400 grit emery paper, provided that the surface coating is not compromised.

Do not install O-rings on components that are not free of burrs or other imperfections.

Back to Top

Clean Seal Surfaces Clean sealing surfaces and all surfaces that the O-ring may come in contact with during installation. Use Isopropyl Alcohol for all surfaces other than polyacrylate and polyurethane.

Back to Top

Prepare Seal Surfaces Mask any sharp edges over which the O-ring must pass during installation (threads, holes,..etc). Do not mask the seal groove edge. Apply seal lubricant as a uniform thin film over entire seal groove and mating seal surface. For long life applications or for added anti-corrosion protection put sufficient lubricant in the groove that the groove will be full after the O-ring is installed.

Back to Top

Seal Inspection Verify that packaged seal is the correct part number listed bill of materials and remove seal from package. During handling, carefully protect the seal from damage by finger nails, tools, dirt, contamination or chips. Thoroughly inspect seal for cracks, nicks, dents or flat spots which might inhibit sealing. MIL-STD-171 and MIL-STD-413 can be used as guidance.

No defects are allowable. Again, any O-ring seals discarded for any reason should be cut completely through with a pair of scissors to prevent accidental re-use.

Back to Top

Seal Preparation Clean seal using Isopropyl Alcohol for all materials other than polyacrylate and polyurethane. Apply a thin continuous film of seal lubricant over the entire O-ring seal surface. While applying the lubricant, pass the entire seal through your fingers several times to insure complete coverage and simultaneously inspect for defects and debris which might have become trapped in the lubricant.

Back to Top

Installation Do not use metal tools to install or remove O-rings from their grooves. Install the O-ring seal in it’s groove without excessive twisting or stretching. Preferably O-rings should not be stretched more than 50% of their initial ID. Push the seal down to the bottom of it’s groove and all the way to the back, if it is a piston seal. Back is defined as the side that the O-ring will be pushed against during assembly. Inspect that the seal is evenly distributed and the same height around the groove. Remove excess lubricant or add lubricant, if desired, to fill the groove.

After the above installation steps are completed and prior to the next assembly it is recommended that an independent inspection operation be performed. The goal of this inspection is to provide a redundant check that all O-rings are installed and fitted up in their grooves correctly prior to closing the seal.

Galvanic Series of Metals and Alloys Marine galvanic corrosion is driven by the voltage potential in sea water between two electrically connected conductors (ie, metal connectors and housings or fasteners and marine hardware, etc..). To minimize this form of attack, materials in electrical contact, if required, should be selected so as to minimize their relative potential.

The galvanic series of metals lists common materials in order of their electrical potential relative to a recognized standard. Materials widely separated on this list will rapidly corrode in sea water when in electrical contact, the anodic material suffering rapid material loss. Materials close together on this list are more compatible and will suffer less damage due to this effect.

CORRODED END (ANODIC OR LEAST NOBLE) MAGNESIUM MAGNESIUM ALLOYS ZINC ALUMINUM 5052, 3004, 3003, 1100, 6053 CADMIUM

MANGANESE BRONZE (CA 675), TIN BRONZE (CA903, 905)SILICONE BRONZE NICKEL SILVER COPPER - NICKEL ALLOY 90-10 COPPER - NICKEL ALLOY 80-20

ALUMINUM 2117, 2017, 2024 MILD STEEL (1018), WROUGHT IRON CAST IRON, LOW ALLOY HIGH STRENGTH STEEL CHROME IRON (ACTIVE) STAINLESS STEEL, 430 SERIES (ACTIVE) 302, 303, 321, 347, 410,416, STAINLESS STEEL (ACTIVE)NI - RESIST 316, 317, STAINLESS STEEL (ACTIVE) CARPENTER 20CB-3 STAINLESS (ACTIVE) ALUMINUM BRONZE (CA 687) HASTELLOY C (ACTIVE) INCONEL 625 (ACTIVE), TITANIUM (ACTIVE) LEAD - TIN SOLDERS LEAD TIN INCONEL 600 (ACTIVE) NICKEL (ACTIVE) 60 NI-15 CR (ACTIVE) 80 NI-20 CR (ACTIVE) HASTELLOY B (ACTIVE) BRASSES COPPER (CA102)

430 STAINLESS STEEL NICKEL, ALUMINUM, BRONZE (CA 630, 632) MONEL 400, K500 SILVER SOLDER NICKEL (PASSIVE) 60 NI- 15 CR (PASSIVE) INCONEL 600 (PASSIVE) 80 NI- 20 CR (PASSIVE) CHROME IRON (PASSIVE) 302, 303, 304, 321, 347, STAINLESS STEEL (PASSIVE) 316, 317, STAINLESS STEEL (PASSIVE) CARPENTER 20 CB-3 STAINLESS (PASSIVE), INCOLOY 825NICKEL - MOLYBDEUM - CHROMIUM - IRON ALLOY (PASSIVE) SILVER TITANIUM (PASS.) HASTELLOY C & C276 (PASSIVE), INCONEL 625(PASS.) GRAPHITE ZIRCONIUM GOLD PLATINUM

PROTECTED END (CATHODIC OR MOST NOBLE)