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Robin Rigg | WTG Decommissioning Project
Decommissioning Project Review and Lessons Learnt
25th January 2017
Robin Rigg
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Situated 11km from the Dumfries and
Galloway coast & 13km North West
of the Port of Workington.
58 x Vestas, V90 3MW Wind Turbine
Generators (WTG)
2 x offshore substations (East &
West)
2 x132kV cables connect near
Seaton, Cumbria where they travel
2km inland to the onshore
substation.
Commissioned 2009
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Foundation Integrity Management – What is Required?
Conduct bathymetric surveys to identify;
- Any movement of the seabed or,
- Local changes to the seabed as a result of installation of foundations i.e. scour
development around the foundations
Monitor Natural Frequency of the WTG;
- The frequency of vibration or oscillation in which a system will inherently adopt
according to its structure given a suitable excitation e.g. wind
- The foundation structures are designed to operate between 1P and 2P/3P
frequencies of the WTG i.e. prevent resonance of the system.
- A change to the water depth, scour or seabed movements, will reduce the
natural frequency of the structure, potential to meet the natural frequency of the
WTG which would result in resonance and performance issues.4
Post- Commission Foundation Management Strategy
Bathymetric surveys to be completed annually.
3 Annual Bathymetric Surveys showing no change
Survey results up to and including 2012 evidenced that the seabed was
relatively stable although deeper than anticipated at design scour pits were
recorded in the south eastern corner of the site (deep water locations).
To abate further scour develop a retrospective scour installation campaign was
completed in October 2016.
No significant seabed movements or scour pit development (greater than
anticipated by design) were identified in any other areas of the site.
EON continued with a ‘monitor’ strategy with respect to foundation integrity and
seabed stability based on the available data.
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Identification of Issue and Action Taken
In Autumn 2013 natural frequency of WTG A1 was at ‘high’ end of operational
frequency range
Jan 2014 a significant frequency drop was observed
May 2014 the natural frequency was recorded at lower operational limit of the
WTG.
Decision Made to Resurvey
A bathymetric survey was completed in May 2014, a reduction in seabed
of approximately 15m since the previous bathymetric survey was
recorded at WTG A01
Post-May 2014 Survey
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Immediate steps taken;
Discussion with MHI Vestas to assess operational limits of WTG –
agreement to reduce safety margin allowing continued operation.
Review of storm and environmental events that may have resulted in
significant mobility changes to seabed i.e. identification of immediate
root cause of geomorphological changes to seabed.
Tidal surge events had occurred in Dec 2013 & Jan 2014
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2012 Bathymetric Survey
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2015 Bathymetric Survey
Seabed Profile at Location A1
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Assessment of Structural Integrity
2 independent elements of work were commissioned in order to:
(1) identify any potential geotechnical and / or structural solutions to avoid or
defer decommissioning the turbines and
(2) complete a review of foundation stability under revised environmental load
conditions subject to significant seabed lowering.
Assessment of Structural Integrity
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Findings
Initial assessment carried out identified 2 WTG which would need to be
decommissioned due to risk of collapse
Further work was conducted to assess the suitability of scour protection systems
to abate further seabed lowering at other ‘at risk’ locations.
Further Options
EON engaged EngD through ‘REMS’ University research programme to assess
the structural contribution of rock fill / soil infill solutions and determine if an
alternative option to decommissioning could be developed.
Options
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Rock fill and ‘reinstatement’ of the eroded material were identified as possible
solutions.
However they were not considered feasible options due to:
No engineering basis to justify that rock fill / replenishment would re-
establish foundation stability & may exacerbate structural integrity /
stability issues.
Unproven offshore (installation methodology and long term performance),
Strong tidal currents likely to restrict / inhibit placement and performance.
Structural solutions identified carried significant risk from both a long term
performance and engineering / installation complexity
All options carried significant H&S risks during install
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Structural Integrity Assessment & Access Safety
Assessment
Structural assessment under revised environmental conditions to
understand the effect of seabed lowering on foundation stability.
Analysis confirmed stability
severely compromised in
extreme storm events.
Further work undertaken by
EON to demonstrate
conditions under which
foundation stable and safe
for personnel to access
(required to complete
essential enabling works
prior to decommissioning).
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Independent Assessment of Dynamic Changes to Seabed
Aim was to:
Review dynamic changes to seabed and overall impact on the Robin Rigg
sandbank area,
Identification of ‘at-risk’ turbines,
Predication of future changes to seabed levels and impact of ongoing subsea
channel migration on the wind farm.
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Seabed Erosion
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Seabed Erosion Review
Key points of the review:
Overall extent of scour hole local to A01 was 120m x 60m x 20m in
March 2015 – further analysis in June 2015 indicated this had
increased further.
Changes in seabed level driven by global effects, further lowering of the
seabed local to A01 was predicted with a further reduction in
embedment depth.
WTG B01 had also seen a significant lowering of the seabed and
corresponding reduction in natural frequency.
Extent of scour and effect of global lowering identified that installation of
scour protection in the existing scour hole would not abate ongoing
global lowering and associated scour i.e. no intervention options at
WTG A01.
Seabed Erosion Review - March 2015
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In March 2015 monopile embedment depth was circa 10m, design
embedment depth was 20m.
Seabed erosion modelling work indicated that ongoing erosion local
to WTG A01 had the potential to reduce the seabed further with a
final embedment depth of 5m anticipated.
Key Issues & Considerations
Robin Rigg site is an operational site, so how to decommission 2 WTG
during operations?
What method could you use to do this which does not put the WTG,
personnel completing the works and vessels at risk whilst doing so?
Lease arrangements consider decommission as the ‘whole’ site at end of
life?
Has WTG decommission been done previously? What methods were
available?
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Works Planning – Site and Operational ConsiderationsEngagement of all key stakeholders essential to deliver the project within the
required timescales (requirement to decommission identified April 2015, works
to be completed Summer / Autumn 2015)
Key Stakeholders had to be consulted:
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Works Planning – Site and Operational Considerations
Marine License Operational issues
Requirements as identified in the S36
consent,
Current licensing requirements
Interim or ‘full’ decommissioning
Impact on array cable string layout,
Isolation of WTG B2
Impact on back-link for A and B strings.
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Impact on Site Activities Occupational Safety Issues
CTV requirements,
Marine coordination,
24/7 operations,
Prioritisation of decommissioning works
affecting statutory maintenance
Safety case approach to assess risks to
personnel for continued access to
WTG’s
Wind Farm Site Layout and Cable Strings
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Planning - Decommissioning Specific Considerations
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Outcome of Assessment:
Decommissioning was only envisaged at end of operational life, as such no
established methodology or consents / licenses in place to allow works to be
completed
No known partial decommission completed to date
Many methodologies unproven
Need to look at other industries and to look at potentially unproven techniques
Planning - Decommissioning Specific Considerations
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Project team conducted a review of existing oil and gas and conventional inshore
decommissioning techniques. Issues identified included;
Robin Rigg Monopile diameter greater than any decommissioning undertaken in
O&G,
Grouted connection rather than flange connection introduced added complexity,
Length of Monopile to be removed required decommissioning of foundation
structure to be completed in two sections.
Limited planning time meant that full fracture mechanics failure assessment not
completed, therefore all cutting required the foundation to remain on the hook of the
main vessel crane to ensure stability of the foundation structure throughout the
decommissioning works.
Review of Decommissioning Methodology
Robin Rigg is an environmentally challenging site, vessel stability issues during
construction and scouring around jack-up legs was an identified issue so
decision made to utilise the existing Charter with MPI who had previous
experience operating on Robin Rigg to minimise the risk
Marine Warranty Surveyor utilised throughout planning stages and present on
the vessel for the majority of the campaign.
Avoidance of spring tides and monitoring or scouring around legs during
operations essential.
WTG and WTG Tower
Adopted a ‘reverse’ engineering approach as no established methodology.
On review ‘bunny ear’ configuration not suitable with respect to back deck
laydown and transportation post decommissioning. 25
Decommission Operations as follows:
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Blades removed individually and stacked on deck in a 3 x 2 configuration,
Nacelle and hub lifted as one unit,
Tower removed as a single or two section lift (dependent on available lifting
beam)
6 x Major lifts were required per location to dismantle the WTG and tower.
Review of Decommissioning Methodology
Marine License
The proposed decommissioning the first of its kind, no precedence set.
EON presented as an interim decommissioning with opportunity to revisit or
complete further works at the end of the operational life of the wind farm.
MS LOT agreed to a cut off height of the MP 2m above the current seabed
level on this basis.
The 2m above seabed level allowed use of external cutting tool without
requirement to dredge or excavate around the base of the MP.
.
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Review of Decommissioning Methodology
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Cabling and Impact on Continued Operation
WTG’s A01 and B01 are located at the end of array cable strings, therefore
impact on individual strings negligible.
The backlink between A and B array cable strings is situated between A01
and B01, therefore decommissioning of these WTG’s would cut the backlink
increasing production risk.
WTG B02 would be isolated by decommissioning of A01 and B01 so there
was a requirement to complete offshore joints between existing cables or lay
new cables between WTG’s to maintain array cable connections
Review of Decommissioning Methodology
An initial review confirmed that the foundation would need to be removed in
two lifts to be accommodated on the back deck. For ease and simplicity for
transportation, the cuts would be located as follows;
1st cut 1m below the bottom of the grout skirt / transition piece,
2nd cut 2m above the current seabed level.
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Review of Decommissioning Methodology
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Transition Piece
Exposure of the TP / WTG tower interface flange provided solid lifting
points of known integrity.
TP would be sea fastened vertically utilising grillage already on the MPI
Adventure.
Monopile
2No. Holes would be cut through the top can of the remaining MP stub,
shackles and chains would be attached to provide stable lift.
Internal sand column would be dredged prior to cutting to reduce lift weight
minimum (steelwork only)
CDM Original decommissioning strategy in the H&S File assumed all
decommissioning at the end of life of the site – very limited information in
O&M manual, did not have an ‘in service’ decommissioning requirement
No indication on ‘how’ to decommission a WTG on a live site
UK Regulator were kept in the loop on the proposed works
All procedures written from scratch, very detailed review process completed
to ensure all interfaces identified and understood
Construction Phase H&S Plan developed to manage full work scope.
HSE involvement - Risks openly addressed in early discussions – kept in
loop and invited to observe works on the vessel.
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WTG / Tower Removal
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WTG / Tower Removal
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TP Removal
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TP Removal
Project Lessons Learnt
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The Decommissioning Strategy should be should be reviewed by the O&M Team
post construction
A Decommissioning Strategy Should:
identify restrictions and constraints from a decommissioning aspect
The clear proposed decommission method options and configurations
rather than ‘let’s figure it out later’
Equipment assumptions that should be used should be listed
Designs & Layouts should consider any potential need for disconnection
points for EOS and WTG/Strings
Vessel capability requirements should be listed
Decommission method options should aim to minimise diving
Project Lessons Learnt
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Bathymetric Surveys should not be treated as ‘Routine Maintenance’
Surveys should be planned Annually as a minimum
After major storm events and Tidal Surge Events, additional surveys
should be considered, particularly in at risk locations
Surveys should extend outside of the WTG site a reasonable distance in
order to consider wider sea bed movements in local vicinity, which may
indicate potential threats to operational Site areas
Sites which are in close proximity each other should consider sharing
survey data
