Structura l Per formance of F ibre Reinforced Polymer Mater ia ls (FRP)after Long Term

Immers ion in a Mar ine Subsea Environment

FRP in marine use

FRP widely used in marine environments:

• Corrosion resistance• Weight reduction• Strong and resilient• Leisure industry• Commercial applications• Naval vessels

FRP in Offshore Oil and Gas

FRP ideal in subsea marine environments:

• Significant weight reduction

• Corrosion resistance

• Cost saving solutions

What happens long term to FRP subsea?

Specified design life often 20 years +

How to justify this in design?- Some published reports available, but

many are anecdotal

Initiated research project:

• Lab accelerated ageing of FRP in subsea environment

• Measure changes in mechanical properties

• Compare to Real Time (7 years) aged data

Real Time FRP Subsea Aged Data

FRP Wellhead Protection – “Cocoon”

• Protects wellhead against fishing trawl gear

• 29 Installed in North Sea UK Sector

• FRP Cocoon retrieved as part of well maintenance

• Opportunity of 7 year real time aged data

(Installed subsea in 2006) (Retrieved in 2013)

Materials Tested

• Polyester and Vinyl Ester FRP Laminates• Glass reinforcements• Consolidated construction (Resin Infusion, Pultrusion)

• Virgin materials• Real-time aged materials from retrieved Wellhead protection structure,

7 years subsea, 94 metres depth at 5 deg C

Topic heading

Test Method

• Adopt Arrhenius approach, elevate temperature to accelerate ageing process

Arrhenius Calculated Time = Time of Laboratory Exposure * 2n[ Where n = (Temp of exposure in the lab - Temp in service) / 10 ]

Vinylester - measured Tg 115 deg C, so exposure temp selected = 55 deg CFor lifespan of 25 years at 5 deg C = exposure period in lab of circa 10 monthsTests complete and results summary presented in this paper

Polyester – measured Tg 59 deg C, so exposure temp selected = 30 deg CPolyester tests still ongoing

Test Procedure

• Prepared samples immersed in seawater (ASTM D1141-98 Sect 6)

• Vessels placed in air-circulating oven at set temp 55 Deg C

• Groups of samples retrieved and tested according to planned schedule

• Mechanical testing to recognised ASTM standards, 5 specimens per test

• Tensile, Flexural, Short Beam Shear, Hardness

• Longitudinal “fibre dominated” and Transverse “matrix dominated”

Results – Vinyl Ester materials

• Virgin and Wellhead samples immersed at non-elevated temperatures to achieve

saturation

• Weighed at regular intervals

• After 64 days, found to have achieved effective equilibrium state.

Summary of results follow…

• To aid understanding and comparisons, all data plotted together (Virgin and

Wellhead)

• A “best fit” line (solid line) of the Virgin data is shown, with upper and lower 2 x

Standard deviation (dashed lines)

Tensile Strength - Transverse

Virgin

Wellhead

Tensile Modulus - Transverse

Tensile Strength - Longitudinal

Virgin

Wellhead

Tensile Modulus - Longitudinal

Virgin

Wellhead

Tensile Results - comments

• Tensile strength in both Longitudinal and Transverse directions appeared to have reduced as a result of the ageing process, for both Virgin and Wellhead materials.

• The degradation rate of the Virgin and Wellhead protection structure materials appeared to be similar.

• There was significant scatter on the measured properties.

Flexural Strength - Transverse

VirginWellhead

Flexural Strength - Longitudinal

VirginWellhead

Flexural Results - Comments

• There was significant scatter on the measured properties.

• The specimen failure mode for almost all tests was classed as TBB (Tension, between loading noses, at the bottom surface) which is a valid failure mode.

• The flexural strength of the Virgin and Wellhead protection structure material appears to have degraded with different rates with time of ageing, but not significantly.

Short Beam Shear - Transverse

Virgin

Wellhead

Short Beam Shear - Longitudinal

Virgin

Wellhead

Short Beam Shear - Comments

• There was significant scatter on the measured properties.

• The failure mode in almost all specimens was classed as Interlaminar Shear (IS) which is a matrix dominated failure mode.

• Interlaminar shear strength of Virgin and Wellhead material seems to degrade at similar rates.

Hardness

VirginWellhead

Hardness - Comments

• Analysis of the Barcol hardness data showed that there was a change in hardness after saturation, but thereafter little significant change in hardness of either the Virgin or wellhead protection structure material over time.

Conclusions

• VE FRP material properties are affected by subsea ageing.

• Generally, a degrading (i.e. negative effect) process.

• Noticeable step change in properties when initially saturated.

• In most cases, degrade further over time, broadly in a linear fashion.

• Noted data scatter, however tension and flexure results for Virgin and Wellheadmaterials overlap within 2x SD boundaries.

Conclusions - continued

• Design guidance (eg DNV-OS-501C) requires the designer to measure thequantitative effect of an environment (in this case, sea water) on FRP laminateproperties so that reasonable material partial factors can be determined andapplied for the projected Design Life.

• These ageing tests provide quantitative data to allow reasoned partial materialfactors to be set for a given application to justify required Design Life.

Acknowledgements

The author acknowledges the contribution made to this project by:

Shell EXPRO (UK) - who helped to facilitate this project with NOV FGS Pipex andprovided materials from the retrieved wellhead protection structure to be included in this experiment.

Element (Hitchin) – The UK laboratory who devised the detailed experiment regime and undertook the sample exposure programme & mechanical testing, and compiled a summary report of the findings on which this paper is based.

Thank You!QUESTIONS

For all enquiries contact:Simon Eves – Technical Director | Pipex px® E: Simon.Eves@nov.com M: +44 (0) 7843 630 881W: pipexpx.com