The Monopod Bucket Foundation Hamburg Offshore Wind 2009 12. May 2009 www.dongenergy.com Recent Experience and Challenges Ahead Christian LeBlanc Bakmar Offshore Technology, DONG Energy Power
Agenda 1. Introduction Background and history of the monopod bucket foundation concept. 2. Motivation Why is there an interest in commercialising the monopod bucket foundation? 3. Recent experience Successful installation of The Mobile Met Mast at Horns Rev 2 Offshore Wind Farm. 4. Challenges ahead What are the main challenges for commercialisation of the monopod suction caisson. 5. Concluding remarks 2
Introduction The Monopod Bucket Foundation: A hybrid of a monopile and a gravity based foundation. Cost efficiency is improved if the ratio diameter/skirt length is approximately 1. 3
The installation technology Downward pressure on lid, due to suction Reduction of tip resistance, due to flow induced in soil Skirt tip injection for vertical alignment and reduced tip resistance 4
Previous research & development, since 2001 Development has been undertaken MBD Offshore Power A/S. Test field in Frederikshavn: Medium-scale testing in sand (2x2 m and 4x4 m) Aalborg University: Laboratory testing in sand Numerical modelling Tests performed: Installation tests Monotonic moment loading tests Cyclic moment loading tests Methods and techniques derived for: Installation using suction Vertical installation using skirt tip injection Static and cyclic moment resistance 5
Previous prototypes Frederikshavn, Denmark 2003 Designed for 3 MW Vestas turbine Successfull installation Post-installation monitoring program was conducted Skirt dimensions: D = 12 m, L = 6 m Wilhelmshaven, Germany 2005 Designed for a 5-MW Enercon turbine The installation failed. The installation barge floated sideways and colided with the bucket during the installation process. The impact with the bucket skirt initiated buckling after only 7 m penetration. Skirt dimensions: D = 16 m, L = 15 m 6
MBD Offshore Power A/S October 2006 MBD Offshore Power A/S became a subsidy of DONG Energy. The group holds patents encompassing the installation technology. DONG Energy is promoting the further development of the monopod bucket foundation through MBD Offshore Power A/S. 7
Agenda 1. Introduction Background and history of the monopod bucket foundation concept. 2. Motivation Why is there an interest in commercialising the monopod bucket foundation? 3. Recent experience Successful installation of The Mobile Met Mast at Horns Rev 2 Offshore Wind Farm. 4. Challenges ahead What are the main challenges for commercialisation of the monopod suction caisson. 5. Concluding remarks 8
Motivation Commercialization of the monopod bucket foundation: We estimate that the bucket foundation can be more costeffective than the monopile. The monopod bucket foundation is a feasible foundation concept for several sites in DONG Energy's current offshore portfolio. The concept is environmentally friendly (silent and reversible installation). Patented technology - possible commercialization of IPR and use in other projects. 9
Some pros and cons Pros Less steel is required compared to the monopile Installable without the use of heavy cranes No transition piece is required No hammer is required Stiffer structure than the monopile, resulting in less dynamic amplification of loads. Important for deeper waters The need for scour protection can be eliminated The installation process can be reversed Silent installation process - no pile driving is required Floatable foundation - limited storage area needed during fabrication Cons More complicated structure to fabricate Require grouting beneath the bucket lid Can only be used in certain types of soil Float-out to site requires sufficient water depth in the harbor More high-tech than the monopile more can go wrong More vulnerable concept 10
DONG Energy's Offshore Portfolio In operation Under construction Use of monopod bucket foundations: Non-applicable Partly or maybe applicable Frederikshavn Offshore Applicable Wigtown Bay Walney II Barrow Walney I West of Duddon Sands Shell Flat Burbo Westermost Rough Horns Rev II Horns Rev I Borkum Riffgrund II Borkum Riffgrund I Tunø Knob Vindeby Nysted Middelgrunden Gunfleet Sands Scarweather Sands London Array II London Array I 11
Agenda 1. Introduction Background and history of the monopod bucket foundation concept. 2. Motivation Why is there an interest in commercialising the monopod bucket foundation? 3. Recent experience Successful installation of The Mobile Met Mast at Horns Rev 2 Offshore Wind Farm. 4. Challenges ahead What are the main challenges for commercialisation of the monopod suction caisson. 5. Concluding remarks 12
The Mobile Met Mast "The Mobile Met Mast" is a prototype of a monopod bucket foundation designed as a support structure for a met-mast. Purpose: To gain confidence that a monopod bucket foundation can be successfully installed offshore. To obtain a movable met-mast, which can be used in several offshore wind farms. Specs Total height: 38 m Weight: 165 tons Skirt length: 6 m Skirt diameter: 12 m Fabricated in Aalborg in August 2008. Installed at Horns Rev 2 Offshore wind farm in March 2009. 13
Fabrication Fabricated by Bladt Industries A/S, Denmark Steel structure Plate girder lid Cold rolled and welded skirt 14
Fabrication The geometric skirt imperfertions was measured by a 3D point cloud laser scanner The maximum out-of-roundness was ± 50mm The largest imperfections were along the vertical weldings 15
Pumping Equipment Pumping equipment was installed for: Adding suction in the bucket Skirt tip injection Adding or emptying water from parts of the foundation body Adding air into the bucket skirt 16
Launching 17
Site for installation Horns Rev 2 Denmark Wind turbines: 91 Siemens 2.3MW 200 MW Scheduled installation: - 2008: Foundations - 2009: Turbines The Mobile Met Mast 3 installation tests were planned at different locations (depending on weather) Was only installed at the final location No data from CPT or borings are available (yet) Horns Rev 1 Esbjerg 18
Float out to site Floated to site using 2 tug boats 40 m 3 water was pumped into the head of the Mobile Met-Mast in order to ensure a horizontal orientation when floating 19
Up-ending procedure Up-ended with M/S SeaPower by A2SEA The lift was performed with a heave-compensator Water depth (15 m) was close to the lower limit only 1 meter of water below skirt during up-ending. Inflation/deflation of air in the bucket skirt was necessary during up-ending 20
GENERATION - HCASA 21
Up-ending procedure 22
Up-ending procedure 23
Adding connections 24
Installation A penetration velocity of 2 m/ hour was obtained After 2.5 m penetration, the foundation started tilting. At 4 m penetration, the process was reversed until 3 m penetration. Then the penetration process was repeated without tilting problems. We believe the tilt problem was caused by a stone along the skirt circumference. The flow induced in the soil was blocked, indicating that a clay layer was present. Installation continued. The foundation was successfully installed with a 0.1 degree inclination out of vertical. 25
Scour development Scour is critical during and after installation. During up-ending and installation there were currents of ± 1 knot. Local scour holes, approximately 1m deep, had developed on two sides of the bucket. No scour protection has been installed. Monitoring of the scour development continues. 26
Installation time Time from jacking-up to complete installation was 32 hours. In comparison, the first monopile was installed at Horns Rev II in 31 hours and at Horns Rev I in over 2.5 days. Mounting and unmounting of connections was very time consuming. Room for much optimization - it is estimated that the operation could be performed in less than 10 hours. Time distribution Mounting connections 11.5 h Other 5.5 h Jack-up and preload 1.5 h Installation 10 h Preparation of lifting equip. 2 h Up-ending foundation; 1.5 h 27
Agenda 1. Introduction Background and history of the monopod bucket foundation concept. 2. Motivation Why is there an interest in commercialising the monopod bucket foundation? 3. Recent experience Successful installation of The Mobile Met Mast at Horns Rev 2 Offshore Wind Farm. 4. Challenges ahead What are the main challenges for commercialisation of the monopod suction caisson. 5. Concluding remarks 28
Further research & development Motivation for further research & development: Foundations are a vital part of an offshore wind farm foundation failure is crucial! Statistically, 1 of 3 prototypes have failed so far. This clearly underlines the vulnerability of the concept. The monopod bucket foundation will not be commercialized before we are completely confident that we are able to handle all risks successfully! We are not there yet! Further research, development and prototype testing are necessary in order to: a) eliminate all major risks associated with the foundation concept b) increase the cost-efficiency of the foundation concept 29
Managing risks Installation failure; complete penetration cannot be obtained Accurate prediction of the installation process is important in order to ensure a successful installation. Our experience is mainly in installation of monopod bucket foundations in sand. Further work is undertaken to gain knowledge with: Installation in layered soils Installation in clays and silts The effects of skirt imperfections on penetration resistance Structural failure; collapse of the bucket during installation Monopod bucket foundations fall into the category of thin shell structures and are therefore particularly exposed to structural buckling. Further work is undertaken in order to: obtain accurate prediction of buckling limits investigate ways in increasing the robustness of future buckets in a costeffective manner, e.g. by addition of stiffeners 30
Managing risks Unforeseen ground conditions; risk of installation failure due to e.g. boulders in the soil. The geotechnical site investigations may be extended in order to minimize the risk of unforeseen ground conditions. For example, acoustic intensity imaging may be used to determine sub-seabed stratigraphy and identify buried geohazards, such as boulders, e.g. the Acustic Corer by PanGeo Subsea, which is a seabed deployed unit with subsurface scanning sonar heads attached to a boom rotating to cover 360 degrees. Acustic Corer by PanGeo Subsea 31
Concept development Design and fabrication The bucket lid: Plate girder lid Conical transition Concrete lid + dry dock construction The bucket skirt: methods to reduce geometrical imperfections use of stiffeners? other? Installation methodology There are several possibilities for installing a monopod bucket foundation: Horizontal floating transport + up-ending using a crane or an external pile gripper on a jack-up/vessel or by buoyancy alone. Vertical transport on barge or jack-up + lift with crane on jack-up Vertical floating transport in arrays + lift by vessel Other? 32 Monopile and transition pieces on a barge, Horns Rev 2 Offshore Wind Farm
Technical development Coupling device Adding connections, hoses etc., is very time-consuming. A module for rapid coupling needs to be developed. The module will attach to the bucket lid and contain pumping equipment, sensors, hoses etc. Lid excavation The installed capacity of the bucket foundation may be significantly increased, if it is possible to excavate soil beneath the lid and penetrate the bucket further into the ground. This may also eliminate the need for scour protection. 33
Future prototypes Onshore test bucket, diameter 8m The primary purpose is to develop and test a lid excavation system and coupling device. Installation site: Onshore test facility in Frederikshavn Commencement date: fall 2009 Full-scale prototype Offshore installation of a full-scale prototype, probably for a 3.6 MW wind turbine. Installation site: To be decided A location at Borkum Riffgrund is a possibility. Expected installation date: Late 2010 / Early 2011 34
Agenda 1. Introduction Background and history of the monopod bucket foundation concept. 2. Motivation Why is there an interest in commercialising the monopod bucket foundation? 3. Recent experience Successful installation of The Mobile Met Mast at Horns Rev 2 Offshore Wind Farm. 4. Challenges ahead What are the main challenges for commercialisation of the monopod suction caisson. 5. Concluding remarks 35
Technology jump? 36
Thank you for listening 37