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Why Shrink-Fit Steel Flanges to Titanium Pipe?
Paul Brett, Karim Jan, Simon Luffrum, Subsea Riser Products
1 ABSTRACT
Producing hydrocarbons in deep and ultra-deep waters makes stringent demands on the riser system.
Consequently riser system types and configurations have been developed to serve the specific requirements of
the offshore operator, be that service life, cost, ease of installation etc. In many cases, steel catenary risers
(SCR) are the riser type choice. Depending on riser pipe wall thickness and water depth SCR’s offer an
overall material saving. The method of connecting the riser to the host production facility is ordinarily to use
an elastomeric flexible joint or a tapered stress joint (TSJ). Both have their advantages and disadvantages.
Recent finds of hot, sour, high pressure hydrocarbons have tilted the balance towards a slight preference for
tapered stress joints. With various considerations influencing floating production system hull selection, a
range of parameters can drive the eventual riser hang-off design and sometimes leads to a need for a longer
stress joint that is better able to cope with high bending angle or higher fatigue loads. The high flexibility of
titanium makes titanium TSJ’s an effective alternative to very long steel TSJ’s that can control the curvature
of a riser’s hang-off region to the same extent. This increased market demand for titanium tapered stress joints
has, at times, placed a considerable demand on a limited supply chain affecting cost. The cost of titanium is
much greater than that of steel. This serves as a commercial stimulus either to reduce the cost of the titanium
TSJ or to reduce the size of an equivalent steel TSJ.
Reducing the cost of titanium TSJ’s could be achieved by only using titanium for curvature control and using
another material for the (flanged) connection point. For many years shrink-fitting has been a standard
connection method in which a shaft is inserted into a pre-heated female component which is then left to cool.
Although common in many industries, the use of shrink-fitting in production riser component fabrication is
relatively new. Welding remains the primary pipe connection method. This paper describes qualification
testing conducted to understand the structural integrity of a shrink-fitted steel flange on titanium pipe and how
the assembly behaves in high load, fatigue driven applications.
2 INTRODUCTION
The body of work that is currently being carried out to qualify a shrink fit steel-titanium joint builds on the
work previously carried out by Subsea Riser Products Ltd to qualify a steel-steel shrink-fit, full bore (19in
internal diameter), 10ksi drilling riser for use in the North Sea [1]. The design work and testing for this riser is
complete and at the time of writing, the production phase is drawing to a close, with the first riser joints being
delivered.
It was noted early in the project that since the shrink-fit solution negates the need for welding, then it could be
used to join high strength, thick walled, or dissimilar materials. Welding steel to titanium is not possible, so
clearly this technique, properly developed and qualified, can provide a unique solution for joining titanium
pipe to steel flanges.
A well known example of titanium being used in an offshore drilling riser is the Heidrun Drilling Riser, which
had riser pipe, flanges, booster line and fasteners all fabricated from titanium. The mean water depth that the
riser was designed to drill at was 345m, and the material qualification and fabrication of this design lasted 4.5
years. This is an expensive and time consuming way of producing an entire riser, and whilst risers fabricated
entirely form titanium may find certain niches, it is much more common to find titanium in targeted areas of a
riser system. In curvature controlled high stress areas, a low Young’s modulus in conjunction with high yield
stress can be used to give technical advantage. An example of such an area would be production riser SCR
hang-off locations. In these situations, the stresses imparted to the joints due to bending through a pre-defined
angle are greatly reduced – improving the fatigue performance of the riser zone.

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Similar areas which could benefit from the mechanical properties of titanium are the rigid base jumper at the
foot of a hybrid riser, the touch-down zone (TDZ) of an SCR riser, and the stress joints of high pressure
drilling risers. An excellent summary of titanium riser components which have seen service as of 2007 can be
found in [2].
3 TITANIUM SHRINK-FIT CONFIGURATION
Titanium stress joints are currently manufactured by integrally forging entire joints, or by welding flanges
onto pipe. A combination of these methods has also been used (i.e. integrally forging one flange and welding
the other to create a flanged joint). This has some economic and schedule disadvantages when the specialist
nature of titanium welding and the need for large diameter integral forgings is taken into account. The flanges
of these joints are not highly stressed, and do not require the same flexibility. To use relatively low cost steel
at the flanges is technically acceptable, but not been possible due to the inability to join titanium flanges to
steel pipe. Overcoming this technical hurdle provides a number of advantages, primarily:
• Reduced lead time due to improved sourcing of titanium pipe or smaller forgings only,
• Reduced raw material cost due to thick, large diameter flange sections being constructed from steel,
• No titanium welding required,
• Fasteners and sealing gaskets can be standard and do not have to overcome galvanic coupling issues.
• Flange sealing surfaces can be CRA weld inlaid.
• Reduced complexity in interfacing with surrounding riser components that will generally be
constructed from steel.
The shrink-fit flange will be of similar proportions to the production steel-steel shrink-fit connections being
delivered by Subsea Riser Products for deployment in the North Sea [1], see Figure 3.1. The shrink-fit profile
is augmented with machined interlocking grooves and a back-up mechanical lock. Sealing between flange and
pipe is achieved by virtue of the high interference contact pressure generated near the flange shoulder, where
the pipe end bottoms out. Flange to flange sealing is through conventional BX or bore seal type gasket
designs, and a back-seat pressure port is included for seating tests. The shrink-fit design removes galvanic
coupling issues with bolting and gaskets, since the flanges are steel. A comparison between an integrally
forged and shrink-fit joint is shown in Figure 3.2.
In order to control the pull-out force, the length of the flange can be adjusted, as well as the interference of the
connection. Detailed FEA is carried out on production scale flanges to ensure that residual stresses due to the
shrink-fit are acceptable in conjunction with in-service, thermal, and hydrotest loads.

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Figure 3.1 – Shrink-Fit Connection, Typical Layout
Figure 3.2 – Integrally Forged Stress Joint (Above), and Shrink-Fit Stress Joint (Below)
4 DETAILED DESIGN
4.1 Pull-Out Prediction
Prediction of the load capacity of the shrink-fit connection needs to be easily calculable, to enable ‘what-if’
scenarios to be rapidly evaluated at the design stage and beyond. The theory used is based on elastic thick-
walled cylinder theory, adapted for compound cylinders with contact pressure generated by interference and
radial stiffness. Knowing the contact pressure, an estimate of the friction factor is required to predict the pull
out capacity – this can be provided by standard tables or from testing. In the case of the shrink-fit connection,
the pull-out prediction is critical, so lower bound values friction factors obtained from multiple tests are used.
The strength of this approach lies in its simplicity, and it has shown to accurately predict results with steel to
steel shrink-fit joints with a diameter of 11.7in. (23.4in diameter steel-steel joints are assembled for
production, but cannot be destructively tested owing to the extremely high pull-out loads expected)
Integrally Forged Ti Stress Joint
Shrink-Fit Stress Composite Joint
Steel Flanges
Ti Pipe
Standard Bore
Seal Gasket
Standard Fastener
Design
Flange-Pipe
Shoulder
Flange-Pipe
Transition
Area

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When compared to an identical steel-steel shrink-fit joint, a steel flange and titanium alloy pipe combination
should use a greater interference value to obtain the same contact pressure, since titanium alloys possess a far
lower Young’s Modulus (45% reduction on steel).
4.2 Titanium Grade Selection
Titanium ASTM Grade 23 (Ti-6Al-4VELI) titanium alloy is being considered for use in the shrink-fit
application. Grade 23 is commonly used in offshore petroleum applications, and is one of the primary alloys
considered in the DNV recommended practice document for design of titanium risers [3]. This material has
improved resistance to stress corrosion cracking (SCC) in seawater over standard ASTM Grade 5 (Ti-6Al-4V)
material.
Grade 29 material is identical to Grade 23, but has added 0.1 wt.% ruthenium – leading to automatic NACE
approval under sour service conditions, but also to higher cost due to the additional alloying element. In terms
of mechanical properties, Grades 23 and 29 are virtually identical.
Typical yield and ductility figures of ASTM Grade 23/29 material are given in Table 4.1.
Elevated Temperature Properties
ASTM Grade 23/29 Tensile Properties
Guaranteed
Minimum @
Room Temp. 200°F 300°F 400°F
Ultimate Strength (ksi) 126 120 111 104
Yield Strength, 0.2% Offset (ksi) 110 103 92 85
Elongation in 2inch Gauge Length (%) 10 13 15 16
Table 4.1 – Tensile Properties of Selected Titanium Grade
4.3 Titanium Corrosion
Although titanium is reactive, its good corrosion properties in most environments are related to the stable,
continuous, highly adherent oxide layer that forms on its surface. Since titanium has a high affinity for
oxygen, exposed metal develops an oxide layer (primarily 10nm thick TiO2) almost instantaneously if parts
per million of oxygen or water are present in the environment. Seawater alone can support the oxide film in
the absence of any dissolved oxygen (i.e. anaerobic conditions). Bending or machining has no impact on
corrosion resistance.
• General corrosion is characterised by a uniform attack of the exposed surface, provided that the
oxide film is intact, corrosion rates are typically much less than 40μm/year. In many environments in
which titanium is fully resistant, slight surface oxide growth can occur – resulting in slight specimen
weight gain. As a result of this very small rate of corrosion, titanium is often given a zero corrosion
allowance in seawater.
• Titanium crevice corrosion is a local form of corrosion that can occur at meal to metal seals. The
process of attack is similar to stainless steels, whereby anaerobic, reducing acid conditions develop
within tight crevices. Crevices exposed to hot (>70°C) chloride, bromide, iodide, fluoride, or
sulphate-containing solutions can be subject to localized attack. Crevice corrosion is mitigated in the
shrink-fit design through a combination of high contact pressures and the use of encapsulating
compounds and/or corrosion resistant weld inlay.
• Galvanic corrosion is a particular problem when titanium is electrically coupled with low alloy steels
and immersed in seawater; polymeric encapsulation is an effective method of minimising external
galvanic corrosion when applied across and beyond metal transitions. Another technique used is to
clad the low allow steel with nickel alloy 625 at all points in contact with the titanium, however, the
requirement for this in a shrink-fit application is still to be fully assessed.
4.4 Fatigue Performance
Some studies have been undertaken to determine the fatigue strength of titanium for offshore use and the
results of these studies have been used for designing titanium joints, and in some cases, entire riser strings. S-
N design curves for titanium do not currently offer improvements over steel fatigue curves, however, due to

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the improved flexibility of titanium, deflection controlled stress ranges are reduced – offering better fatigue
performance through this route.
Marintek have undertaken to produce a design curve for ASTM B348 Grade 23/29 riser systems, the one
shown in Figure 4.1 is the latest curve (published 2002) which is more conservative than their previous curve
(published 1998). DNV also have a design curve for ASTM B348 Grade 23/29 for both weld and base metal.
The base metal curve is shown in Figure 4.1, and is defined in the the DNV recommended practice for the
design of titanium risers [1]. A steel parent material design curve (in air) is included for comparison [6].
Surfaces under contact pressure can be subject to a different type of fatigue mechanism: fretting fatigue,
which is driven by cracks initiated at the mating surface by the rubbing of the shrink-fit surfaces near the
pipe/flange transition area. Fretting fatigue tests conducted by Subsea Riser Products are described in Section
5.2.
Figure 4.1 – Comparison of Design Curves
4.5 Assembly/Manufacturing
The machinability and formability of titanium is now well documented and many fabricators worldwide
possess the capability to machine titanium components. Machine shops liken the machinability of
commercially pure titanium as similar to 18-8 stainless steel, with the alloys being more challenging. Carbide
tools and super high speed steels are favored, and due to its low thermal conductivity (like stainless), generous
use of cutting fluid is recommended to dissipate heat. For a comprehensive description of how titanium
behaves under various machining and forming operations, consult [5].
Assembly of the steel shrink-fit flange to the titanium pipe is done by elevating the steel flange temperature
till its inside diameter is larger than the pipe outside diameter (which remains at room temperature). The
flange is brought to temperature slowly, monitored by multiple thermocouples attached to a digital controller
– this ensures that thermal hotspots do not locally temper the flange. An assembly jig is required that ensures
accurate axial alignment, this can be done vertically or horizontally (although vertical assembly has practical
disadvantages when assembling long joints). See Figure 4.2 for photographs of shrink-fit riser joint assembly.
Cooling the titanium pipe with liquid nitrogen (-320°F) is possible, but was discarded since the contraction is
insufficient to achieve clearance: titanium alloy has a lower thermal expansion coefficient than steel (42%
lower). In addition, there are practical issues with condensation and titanium ductility at cryogenic
temperatures that make this option unattractive.

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There are two other key aspects of titanium that require special consideration in titanium shrink-fit assembly:
a much lower thermal conductivity (13% of low alloy steel) and reduction in yield strength at high
temperature see Table 4.1, which could result in plastic deformation of the titanium as the steel cools and
tightens.
Figure 4.2 – Photographs of Shrink-Fit Joint Assembly
5 INITIAL TESTING
5.1 Load Capacity/ Cyclic Load Testing
Development testing was carried out by SRP to destructively test titanium/steel shrink-fit connections
subjected to axial load, see Figure 5.2. This was conducted in order to determine the strength of the shrink-fit
due to different surface finishes and different. As an additional test, two thirds of the predicted capacity was
repeatedly loaded and unloaded in order to assess the response of the connection to cyclic loading. The
materials used in the test were ASTM Grade 5 for the shaft, which was readily available and similar to Grade
23/29 - and nickel alloy 625 for the hub. The selection of alloy 625 was based on learnings from previous
offshore projects, which suggested cladding the interface may be appropriate. Further testing with low alloy
steel/titanium is planned.
Predictions were in very good agreement with testing, the force required before slip occurred was always
higher than theory would suggest. This was on average 4.7% and a maximum of 12% higher. A robust
secondary effect was that after initial slip of the shrink-fit connection, subsequent slip required higher loading
– in one case, after 6.5mm of slip, a load 75% greater than the initial slip load was required to induce
subsequent slip (see Figure 5.2). This is a desirable effect which, along with the locking features of the SRP
shrink-fit flange, introduces a ‘fail-safe’ aspect to the shrink-fit connection.
In the cycled load test it is shown that over twenty cycles of 66% slip load, no irreversible effects on the
connection are observed (see Figure 5.3).
The future development of the shrink fit connection will focus on large scale testing to simulate the effects of
internal pressure and secondary locking mechanisms combined with the shrink-fit connection. This would
likely be a half scale model of a final production connection.
Half scale model scaling works as follows:
• Interference should be halved, to produce identical contact pressures between pipe and flange,
• Applied pressure shall remain the same as for production hydrotest, this means that end-cap loads are
one quarter of the actual,
• Applied jacking force shall be one quarter of the actual load to produce similar axial stress,
22
io RRF −∝
• Any applied bending shall produce the same stress in the outer skin of the pipe (one eighth).
o
io
R
RR
M
44
−
∝

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Figure 5.1 – Diagram and Photograph of Small Scale Test Rig
Figure 5.2 – Increased Pull-Out Load Following Initial Slip
Locknut
Test Stand
Hollow
Hydraulic
Cylinder
Hub
Pipe
Displacement
Sensor

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Figure 5.3 – Cycled Load Test Showing no Accumulated Deflection
5.2 Fretting Fatigue Testing
One of the design’s areas of concern was the pipe-flange transition area, where it is believed that changing
tension and bending loads could cause the pipe to move relative to the flange under interference. This
movement generates wear that leads to fretting fatigue.
Since no data is readily available for titanium to nickel alloy 625 fretting fatigue behaviour under specified
contact pressures, a programme of testing was proposed. This testing emulates the pipe-flange transition area
using jaws and fatigue coupons, with the jaws gripping the coupon at a bearing pressure comparable to the
contact pressure between pipe and flange. The jaws and coupon are held in a fatigue testing rig (see Figure
5.4, and are cycled at high rates until failure or run-out.
Coupons are cycled at a variety of stress ranges with R=0 (i.e. the mean tensile stress is half the stress range).
A total of nine samples are tested, 4 of which are control samples to ASTM E466 geometry [7]. For each
fretting sample, the number of cyles endured is compared against the control samples to obtain a factor by
which the fretting life is de-rated. Testing is ongoing, to establish the factor for the combination of titanium
and nickel alloy 625..
Future tests are proposed to be carried out using full or half scale connections on a resonant rig in order to
give an even more faithful representation of the fatigue behaiour at the pipe-flange interface.

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Figure 5.4 – Titanium - Nickel Alloy 625 Fretting Fatigue Rig
6 CONCLUSIONS
The production of titanium riser joints, for application in offshore drilling and production risers can be
improved through the application of proven shrink-fit technology, initially developed for a steel high pressure
drilling riser.
Benefits are evident in terms of cost, lead time and availability, due to the reduced flange material cost, and
the avoidance of large diameter forgings and titanium welding. Secondary benefits also exist, in that gaskets
and fasteners can remain unchanged, and that previously disregarded unweldable grades of titanium can now
be considered as options for stress joint fabrication.
7 ACKNOWLEDGEMENTS
The authors would like to thank all members of Subsea Riser Products who contributed towards this paper.
8 REFERENCE
[1] K.Jan, J.Shield, P.Brett, “Ultra-High Pressure Risers for Deepwater Drilling” DOT International, 2009.
[2] Baxter, Shutz & Caldwell, RTI Systems, “Experience and Guidance in the Use of Titanium
Components in Steel Catenary Riser Systems” OTC 18624, 2007.
[3] DNV, “Design of Titanium Drilling Risers” DNV-RP-F201, October 2002.
[4] NACE MR0175, “Materials for use in H2S-Containing Environments in Oil and Gas Production”,2001
[5] Machinability Data Center Cincinnati Ohio, “Machining Data Handbook”, 3rd
Ed., 1997.
[6] DNV, “Fatigue Design of Offshore Steel Structures” DNV-RP-C203, April 2008.
[7] ASTM, “Standard Practice for Controlled Constant Amplitude Axial Fatigue Tests of Metallic
Materials” ASTM E466-07, November 2007.
Ti Fatigue
Coupon
Machine
Body
Nickel Alloy
Jaws
Steel
Clamp Plates
Hydraulic
Cylinder