Multiline anchors for floating offshore wind towers

Roughly 60% of potentially exploitable offshore wind power is located beyond the range of water depths suitable for fixed foundations, where floating offshore wind towers (FOWTs) moored to the seabed are required. Anchors comprise a critical component of the mooring system. A variety of anchor types are potentially suitable for this purpose, but all have limitations in regards to the types of seabed soils in which they may be deployed and the type of mooring systems (catenary, taut) for which they are suitable. Additionally, foundations for offshore wind make up a large portion of project cost; therefore, minimizing the costs of fabrication, transport and installation of anchors is a key aspect of overall project feasibility. In contrast to offshore oil-gas installations, offshore wind towers are deployed in arrays, which offer the possibility of reducing project costs by attaching more than one mooring line to a single anchor. In addition to direct cost savings, the multiline anchor concept permits a reduced scale of costly offshore geotechnical site investigations. This paper first examines different anchor types that are potentially suitable as anchors for FOWTs, largely within the context of their traditional usage in securing a single mooring line to the seabed. Then, the potential for adapting these anchors to multiline systems is assessed. Anchor types examined include: driven piles, dynamic piles, suction caissons, drag embedded anchors, vertically loaded anchors, pile driven plate anchors (PDPA), dynamically embedded plate anchors (DEPLA), and suction embedded plate anchors (SEPLA). Performance considerations for each anchor include: Soil profile constraints, vertical load capacity, horizontal load capacity, precision of positioning, installation cost, efficiency, performance under sustained loading, potential loss of embedment, as well as other anchor specific considerations.

[1]  Mark Randolph,et al.  The static equilibrium of drag anchors in sand , 1996 .

[2]  C Madshus,et al.  Idealized load composition for determination of cyclic undrained degradation of soils , 2015 .

[3]  J. D. Murff,et al.  Vertically loaded plate anchors for deepwater applications , 2005 .

[4]  C. Aubeny,et al.  Lateral Undrained Resistance of Suction Caisson Anchors , 2001 .

[5]  Clarence J. Ehlers,et al.  Technology Assessment of Deepwater Anchors , 2004 .

[6]  C. Aubeny,et al.  Recent Advances in Soil Response Modeling for Well Conductor Fatigue Analysis and Development of New Approaches , 2015 .

[7]  M. F. Bransby,et al.  Drag anchor fluke-soil interaction in clays , 2003, Numerical Models in Geomechanics.

[8]  E. Zimmerman,et al.  Efficient Gravity Installed Anchor for Deepwater Mooring , 2009 .

[9]  V. N. Vijayvergiya,et al.  Design And Installation Of Piles In Chalk , 1977 .

[10]  David White,et al.  Novel Anchoring Solutions for FLNG - Opportunities Driven by Scale , 2015 .

[11]  Haydar Arslan,et al.  Remaining Capacity of the Suction Pile due to Seabed Trenching , 2015 .

[12]  T. Tjelta The suction foundation technology , 2015 .

[13]  Syed Rajab Ali,et al.  Uplift behaviour of horizontal anchor plates in sands , 2001 .

[14]  L. Reese,et al.  Analysis of Laterally Loaded Piles in Sand , 1974 .

[15]  Katja Reimann,et al.  Validation of Sound Source Simulation due to Offshore Pile Driving , 2015 .

[16]  Charles Aubeny,et al.  Mechanics of Drag Embedment Anchors in a Soft Seabed , 2010 .

[17]  Conleth O'Loughlin,et al.  Loss in Anchor Embedment during Plate Anchor Keying in Clay , 2009 .

[18]  Mark Randolph,et al.  Suction anchors for deepwater applications , 2005 .

[19]  Aaas News,et al.  Book Reviews , 1893, Buffalo Medical and Surgical Journal.

[20]  M. Randolph,et al.  Influence of the installation process on the performance of suction embedded plate anchors , 2006 .

[21]  Wim Turkenburg,et al.  Cost Reduction Prospects for Offshore Wind Farms , 2004 .

[22]  G. Houlsby,et al.  Foundations for offshore wind turbines , 2003, Philosophical Transactions of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences.

[23]  Michael W. O'Neill,et al.  Evaluation of p-y relationships in cohesionless soils , 1984 .

[24]  James Forrest,et al.  Design Guide for Pile-Driven Plate Anchors. , 1995 .

[25]  E.A.L. Smith,et al.  Pile-Driving Analysis by the Wave Equation , 1960 .

[26]  P. Jeanjean,et al.  Long-term axial capacity of deepwater jetted piles , 2014 .

[27]  Mark Randolph,et al.  SCIENCE AND EMPIRICISM IN PILE FOUNDATION DESIGN , 2003 .

[28]  Christophe Gaudin,et al.  Installation and capacity of dynamically embedded plate anchors as assessed through centrifuge tests , 2014 .

[29]  Itai Einav,et al.  Cyclic shakedown of piles subjected to two‐dimensional lateral loading , 2009 .

[30]  K Andersen,et al.  Cyclic soil parameters for offshore foundation design , 2015 .

[31]  M. Randolph,et al.  Penetration of dynamically installed anchors in clay , 2013 .

[32]  Conleth O'Loughlin,et al.  Installation of dynamically embedded plate anchors as assessed through field tests , 2015 .

[33]  Bob Wilde,et al.  Field Testing of Suction Embedded Plate Anchors , 2001 .

[34]  Mark Randolph,et al.  Numerical investigation of dynamic installation of torpedo anchors in clay , 2015 .

[35]  Gilberto Bruno Ellwanger,et al.  Undrained Load Capacity of Torpedo Anchors in Cohesive Soils , 2009 .

[36]  J. D. Murff,et al.  Inclined load capacity of suction caissons , 2003 .

[37]  E. A. Dickin Closure of "Uplift Behavior of Horizontal Anchor Plates in Sand" , 1988 .

[38]  H. Matlock Correlation for Design of Laterally Loaded Piles in Soft Clay , 1970 .

[39]  M. Cassidy,et al.  Improving Plate Anchor Design with a Keying Flap , 2014 .

[40]  Christophe Gaudin,et al.  Incorporating shank resistance into prediction of the keying behavior of suction embedded plate anchors , 2015 .

[41]  Donna Heimiller,et al.  Assessment of Offshore Wind Energy Resources for the United States , 2010 .

[42]  E. Davis,et al.  The behaviour of anchor plates in sand , 1982 .

[43]  J. D. Murff,et al.  Behavior of Suction Embedded Plate Anchors during Keying Process , 2012 .

[44]  Robert B. Gilbert,et al.  The Performance of Drag Embedment Anchors (DEA) , 2011 .

[45]  Sanjay R. Arwade,et al.  Efficient Multiline Anchor Systems for Floating Offshore Wind Turbines , 2016 .

[47]  Hans Petter Jostad,et al.  Penetration Resistance of Offshore Skirted Foundations and Anchors in Dense Sand , 2008 .