PILE ANCHORS 



Operation 



/foonila /^£ /irs& 



Definition . Pile anchors 

 achieve holding capacity by 

 mobilizing shear strength of 

 surrounding seafloor material. 

 Bearing pressure and/or skin 

 friction/adhesion are used to 

 achieve capacity. 



Cost . High installation costs 

 usually dictate pile anchor use 

 as last resort. 



Construction . Basic steel 

 shapes usually modified to act 

 as anchor piles. 



Installation . By driving, often in partially predrilled holes; in 

 hard strata, by grouting in fully predrilled holes. Screw-in pile 

 anchor (considered under plate anchors) [Refer to Chellis, 1961, 

 Havers and Stubbs, 1971, for detailed discussions of pile systems.] 



Pile Types/Methods to Improve Performance 



1. Mooring Line Connection 



Surface attachment - Inspection and maintenance possible 

 - Swivel/U- joint desirable to reduce 

 connection torsion (Ref Doris, 1977). 



Subsurface attachment 



Inspection not practical 



Applicable to unidirectional loading 



Enhances pile lateral load resistance; 



pile bending stress reduced 



Changes direction of pile load; higher 



vertical, less lateral load. 



Pile Head Burial 



- Places pile in deeper-stronger soil 



- Used for offshore moorings when drillship is available for 

 drilling and grouting 



- Load at pile can be reduced significantly, by mooring line 

 resistance (see drag anchor section) 



- In sand, pile anchors buried few ft to allow for scour. 



Near Surface Fins/Collars 



- Used to limit pilehead deflection/bending moment 



33 



