Corrosion monitoring within offshore wind foundation structures Anders Rosborg Black, FORCE Technology, Brøndby / Denmark

Outline

Activities overview Corrosion inside monopile foundations Monitoring for control

Corrosion of Monopile Foundations

Many parallel activities at FORCE Technology: - Coatings. Specification/inspection - Cathodic Protection. Modelling - Inspection. NDT - Corrosion assessments - Strain gauges - R&D projects - White book / literature survey - Committee work / NACE / Eurocorr - Seminars / DanCorr - Corrosion monitoring devices - Appropriate testing methods

Governmentally supported activities at FORCE Technology

R&D projects focused on innovative and cost reducing solutions: Fabrication and monitoring of green offshore structures (RKI).

•Optimizing welding processes, laser welding •Tests and monitoring of the in service corrosive condition

Monopile cost reduction and demonstration by joint applied research (EUDP)

•Improved design basis of welded joints in seawater •Cost optimization of welded joints •Automation of the production process •Validation of fatigue resistance •Corrosion properties and monitoring

Design criteria inside monopile foundation

• Airtight and closed • Water level below service

platform • Oxygen only during inspections • No exchange of media – low

microbial activity • Corrosion rates decreasing with

time

No corrosion protection

Corrosion inside a monopile

Experience so far: • Ingress of sea water • Localised corrosion ~ ”ALWC” type • Moderate corrosion rates • Pits up to 3 mm in 6 yrs

Major concerns • Service life reduction, J-tube • Safety – access • Design of CP internally • Cracking risk for welds, loaded details

Wall thickness by ultrasonic testing

• Good contact – cleaning necessary • Calibration, reproducible results • Internally or externally – wall thickness estimates

Manual UT Automated UT, subsea also possible

Monitoring for control and action

Monitoring corrosion rates • Accumulated (coupons) or real time (probes) • Corrosion rate changes – varying conditions • Checking effect of mitigation efforts

Risk of localised corrosion – tubercles, growth rate changes Technique: • Sensitive to mechanism • Zone/media • Probe design • Working life time without service • Transfer of data

Corrosion coupon installation

tAmm

V 21corr ∆⋅

−=

Determine weight loss after cleaning:

Basic technique, high reliability (low sensitivity)

Quality of results depends on accuracy of measurements and placement of probes

Option of determining pit depths etc.

ER - remote and real time monitoring

Measures thickness reduction as change in electrical resistance All marine zones On-line real time data Can be coupled to CP system

Automated logging and transmission Cabinet placed above airtight deck

reference coupon

Corrosion threats inside monopile

Corrosion monitoring options in mudzone

Inspection difficult – need for monitoring

Summary – be prepared for surprises

• Inspections as basis for risk assesment and mitigation efforts

• Risks related to structural integrity and safety must be identified

Corrosion control is continous, monitoring!

• Remote & numerous individual sites • Minimize repair, maintenance and

surprises • Certainty in design and predictable

service life

Knowledge sharing and training

Marine corrosion in offshore structures, 2 day course in Copenhagen area, 8-9 April 2014 DanCorr ATV SEMAPP Knowledge sharing October 2014: Seminar on corrosion in offshore wind farms European Federation of Corrosion, Working party Marine Corrosion – marine renewable energy at Eurocorr, September 2014, Italy NACE International, TG 476 - Corrosion Protection of Offshore Wind Power Units

State-of-the-art of standards and regulations for the corrosion protection of offshore steel constructions

Read more – recent papers

• L.R.Hilbert et al: Inspection and monitoring of corrosion inside

monopole foundations of offshore wind turbines, Eurocorr 2011.

• A.R.Black, P.K.Nielsen: Corrosion protection of offshore wind farm structures – present understanding and future challenges, Eurocorr 2011.

• L.R.Hilbert et al: Corrosion control inside offshore wind farm monopile foundations, Eurocorr 2012.

• L.R.Hilbert et al: Mud zone corrosion in offshore renewable energy structures, Eurocorr 2013

• Eurocorr 2014: 2 papers (on HISC and coating repair)

Thank you for your attention