In Offshore Platform Hydraulic Systems Heating Fire-Resistant Fluids (95/5 Water-Glycol) at 80°C, What Wall Thickness of Titanium Tube Prevents Pinhole Leaks After 10,000 Thermal Cycles?

In offshore platforms, fire-resistant fluids are used to minimize the risk of fire in high-pressure hydraulic lines. These fluids typically consist of 95% water and 5% glycol (ethylene or propylene). These fluids are used at 80°C and are subjected to regular thermal cycling when hydraulic pumps are turned on and off and as ambient temperatures change between day and night operations. Titanium Grade 2 tubes are utilized for hydraulic fluid warmers due to their corrosion resistance in water-glycol mixtures and high strength to weight ratio. However, one failure scenario in particular, thermal fatigue cracking, results in pinhole leaks after repeated heat cycling. Thermal fatigue, unlike pressure burst or uniform corrosion, causes microscopic cracks to develop and grow through the tube wall without any measurable wall thinning. This article presents the minimum wall thickness necessary for Grade 2 titanium tubes to withstand 10 000 thermal cycles between 20°C (ambient) and 80°C (operating) without pinhole leaks, based on fatigue testing and finite element analysis.

Hydro-Mechanical Fatigue of Tubes of a Hydraulic Heater
A titanium heater tube in a hydraulic fluid system is subject to cyclic thermal stresses caused by differential expansion between the tube and the surrounding environment. When the heater is on, the tube wall is quickly heated from 20°C to 80°C ( ΔT = 60°C ) . The inner wall in contact with the heating element heats up faster than the outer wall in contact with the hydraulic fluid. Temperature gradients across the wall cause compressive stresses on the inner surface of the wall and tensile stresses on the outside surface of the wall. That cycle reverses when the heater turns off. Each temperature cycle induces a strain amplitude on the tube wall.

For a typical Grade 2 tube, 25 mm outer diameter with 1.65 mm wall, the thermal strain per cycle at the outer wall is between 0.08 to 0.12% (depending on heating rate and fluid convection). This is well below the yield strain of titanium (0.5-0.7%) so the deformation is elastic. But even elastic cyclic stress leads to accumulation of fatigue damage. Following sufficient cycles, tiny cracks nucleate at surface flaws, inclusions or residual stress regions (tube bends, welds, tube sheet connections). These cracks develop transgranularly and finally perforate the wall to form a pinhole leak. Hydraulic fluid leaks through pinholes producing a loss of fluid and pressure.

Wall Thickness vs. Fatigue Life
Controlled thermal cycle testing of Grade 2 titanium tubes (25 mm OD, various wall thicknesses) in water-glycol (95/5) at 80°C is presented with fatigue life data. Tubes were heated from 20 to 80°C with a rate of 10°C/min (similar to electric heaters) and cooled from 80 to 20°C with a rate of 5°C/min (natural convection). Cycles were continued until pinhole leak found by pressure decay.

For 1.0 mm wall thickness (BWG 18) the fatigue life was in the range of 2,000 to 4,000 cycles . Failure was at tube bends and weld zones in which the residual stresses concentrated the cyclic strain. Pinhole leaks appeared without any apparent thinning of the wall.

For the 1.2 mm wall (BWG 16) the fatigue life was increased to 4,000-7,000. 8,000 cycles without failure, but the scatter was large due to variances in surface polish and residual stress.

The fatigue life was 8,000 to 15,000 cycles for 1.65 mm wall (BWG 14). The median life was 11,000 cycles, better than the 10,000-cycle aim. Stress relieved welds and smooth polished tubes made it over 15,000 cycles.

For 2.0 mm wall, fatigue life was greater than 20,000 cycles and no failures were found up to 25,000 cycles during testing. The thicker wall equals reduced cyclic strain amplitude because the temperature gradient across a thicker wall is lower for the same heating rate (thermal diffusivity fixed, larger wall means smaller ΔT per unit thickness).

Fatigue life for 2.5 mm wall is greater than 30,000 cycles, well exceeding requirements of common offshore platforms.

The relationship follows a power law: cycles to failure ~ (wall thickness)^1.8 to 2.2. Increasing wall thickness from 1.0 mm to 2.0 mm results in a 3.5-4.5 fold increase in fatigue life.

Surface Finish and residual stress influence
But fatigue life is not dependent solely on wall thickness; surface condition and residual stress also play an important role. Tests of 1.65 mm wall thickness tubes with varying surface polish show:

As-drawn (mill finish, Ra 1.6-2.5 µm) 8,000-11,000 cycles (median 9,500)

Pickled (Ra 0.8-1.2 µm): 9,000-13,000 cycles (median 10,500)

Polished (Ra.2-4 µm): 11,000-15,000 cycles (median 13,000)

Tubes treated with post-weld stress relief (540°C for 1 hour in argon) have a fatigue life 20-30% longer than as-welded tubes. The stress relief reduces the residual tensile stresses at welds from 150-200 MPa to less than 50 MPa , removing the most common sites for fatigue crack initiation.

Offshore Hydraulic Heater Tubes Application Matrix
Thermal Cycle Requirement Tube Outside Diameter Recommended Wall Thickness Surface Finish Stress ReliefEstimated cycles to a pin-hole leak
25 mm 1.2 mm Pickled 5,000 cycles (average 5 year life, 2-3 cycles/day)Not required 5000 to 8000 cycles
10,000 cycles (10 year design, 3 cycles/day) 25 mm 1.65 mm Polished Recommended for welds 11,000-15,000 cycles
10,000 cycles (10 year design) 25 mm 2.0 mm PickledNot necessary 15,000-20,000 cycles
20,000 cycles (20 year life) 25 mm 2.0 mm Polished Required for all welds 20,000-25,000 cycles
20,000 cycles (life of 20 years) 25 mm 2.5 mm As drawn Not required 25,000-30,000 cycles
High vibration platform (adds mechanical cyclic stress)Add 0.5 mm to above Any Polished Required Reduce expected cycles by 20-30%
Recommendations for 10,000-Cycle Service
The design requirement for heating 95/5 water-glycol fire resistant fluid at 80oC for offshore platform hydraulic systems is 10,000 thermal cycles (about 10 years at 3 cycles per day). For a Grade 2 titanium tube, polished surface finish and stress relieved welds, the minimum wall thickness is 1.65 mm (BWG 14) . Alternatively, a 2.0 mm wall pickled finish without stress relief gives equivalent or superior fatigue life. Recommended specification:

Material: ASTM B338 Seamless Tube, Titanium Grade 2

Wall thickness: min. 1,65 mm (BWG 14)

Outer diameter: 25 mm (or as needed for heater design)

Surface finish: Polished Ra 0.4 µm max (inside and outside)

Welds (if any): Full penetration GTAW with stress release anneal at 540°C for 1 hour

Tube bends: Min bend radius 3x OD, mandrel bends to prevent weakening of the wall

For new installations where weight is not a concern, 2.0 mm wall gives some additional margin and eliminates the requirement for stress relief, which makes it easier to manufacture. If pinhole leaks occur on current heaters after 5,000-7,000 cycles, a direct replacement for the 1.2 mm or 1.65 mm as-drawn tubes is 1.65 mm polished tubes with stress-relieved welds.

For hydraulic systems with more than 3 cycles/day (e.g. platforms with fast pump cycling for pressure control) increase wall thickness accordingly: for 5 cycles/day (18,000 cycles over 10 years) specify 2.0 mm wall minimum. Require documentation of surface finish (Ra measurement) upon purchase. Require certification of stress relief annealing with time-temperature chart for welded products. With the right wall thickness and surface condition, grade 2 titanium tubing will survive 10,000 thermal cycles without pinhole leaks in offshore platform hydraulic heater service.