Seabed anchor

Abstract

A suction anchor ( 10 ) comprising a caisson ( 12 ) with side wall ( 14 ), open bottom ( 22 ) and closed top ( 26 ) that define the interior volume if the caisson. When the anchor is placed on a seabed ( 100 ) of fluent mud, a top connector ( 28 ) allows temporary connection of a pump ( 200 ) to suck seabed mud ( 102 ) through the open bottom into the caisson's interior, the bottom edge ( 20 ) of the caisson wall ( 14 ) allowing the anchor to cut into, and become embedded within the seabed ( 100 ). To increase resistance of the embedded anchor ( 10 ) from being withdrawn, the caisson is internally fitted with a seabed soil retaining means ( 30 ) that receives and retains mud during anchor embedment. The seabed soil retaining means comprises an open-topped container secured within the caisson, the open top ( 38 ) being located just below the caisson's closed top ( 26 ). By forming the container wall ( 36 ) as a downwardly convergent cone, the passage ( 102 ) of mud upwardly past the outside of the container ( 30 ) during embodiment of the anchor is minimally impeded. As well as improving the stability and performance of suction anchors, the invention can also be applied to similarly improving the stability and performance of embeddable gravity bases or to seabed anchors embedded by downwardly directed dynamic mechanical forces, rather than by suction. The anchors can be embedded vertically or non-vertically.

1 . A seabed anchor in the form of a caisson having a longitudinal axis and comprising a caisson side wall, an open caisson bottom and a closed caisson top that together define an interior volume of the caisson, characterized by seabed soil retaining means for retaining seabed soil displaced during embedment of the anchor in seabed soil in a direction generally downwardly along said longitudinal axis such that the weight of seabed soil retained by the seabed soil retaining means adds to the force required to pull the embedded anchor out of the seabed.  
   
   
       2 . An anchor as claimed in  claim 1 , characterized in that said anchor is provided with a fluid connection to the interior volume, whereby suction can be applied to cause embedment of the anchor in seabed soil.  
   
   
       3 . An anchor as claimed in  claim 1 , characterized in that said seabed soil retaining means comprises at least one container having an opening arranged to admit seabed soil during embedment of the anchor in the seabed.  
   
   
       4 . An anchor as claimed in  claim 1 , wherein said soil retaining means has a downwardly reducing external cross-section to minimize resistance to upward movement of seabed soil past the soil retaining means during embedment of the anchor.  
   
   
       5 . An anchor as claimed in  claim 4 , wherein said soil retaining means comprises at least one conical hopper, having an apex oriented to penetrate the soil during embedment.  
   
   
       6 . A suction anchor as claimed in  claim 1 , characterized in that the seabed soil retaining means is located entirely within the interior volume of the caisson.  
   
   
       7 . A suction anchor as claimed in  claim 1 , characterized in that the seabed soil retaining means is located adjacent the caisson top to receive and retain seabed soil displaced during latter stages of anchor embedment.  
   
   
       8 . A method of embedding a seabed anchor as claimed in  claim 1  in a seabed composed of soil, characterized in that the method comprises the steps of deploying the anchor onto the seabed with the longitudinal axis of the anchor aligned substantially in a predetermined direction such that an open lower end of the anchor, or an opening in the lower end of the anchor, contacts the seabed soil, and applying forces to the anchor directed generally downwardly along the longitudinal axis of the anchor such as to force the anchor into the seabed soil and cause seabed soil to enter the interior of the anchor eventually to displace seabed soil into the seabed soil retaining means of the anchor whereby the anchor is embedded in the seabed substantially in said predetermined direction and the weight of seabed soil retained in the seabed soil retaining means adds to the force required to pull the embedded anchor out of the seabed soil.  
   
   
       9 . A method of embedding a suction anchor as claimed in  claim 8 , characterized in that said applied force is derived by applying suction to the interior volume of the anchor.  
   
   
       10 . A method as claimed in  claim 8 , characterized in that said predetermined direction is substantially vertical.  
   
   
       11 . A method as claimed in  claim 8 , characterized in that said predetermined direction is partly vertical and partly horizontally directed in a selected bearing such as to embed the anchor into the seabed substantially in a predetermined non-vertical direction that optimises resistance of the so-embedded anchor to withdrawal by non-vertical loads.  
   
   
       12 . A gravity base comprising a single open-bottom, closed-top cell, characterized in that said cell is provided with a seabed soil retaining means in like manner to the seabed anchor as claimed in  claim 1 , such that the gravity base can be embedded in fluent seabed soil.  
   
   
       13 . A gravity base comprising a plurality of open-bottom, closed-top cells, characterized in that each said cell is individually provided with a respective seabed soil retaining means in like manner to the seabed anchor as claimed in  claim 1 , such that the gravity base can be embedded in fluent seabed soil.  
   
   
       14 . A method of embedding a gravity base as claimed in  claim 12  in fluent seabed soil, characterized in that said method corresponds to the method as claimed in  claim 8.