Systems and Methods for Tethering Subsea Wellheads to Enhance the Fatigue Resistance of Subsea Wellheads and Primary Conductors

Abstract

A system for tethering a subsea wellhead including a plurality of anchors disposed about the subsea BOP and secured to the sea floor. In addition, the system includes a plurality of tensioning systems. One tensioning system is coupled to an upper end of each anchor. Further, a plurality of flexible tension members. Each tension member extends from a first end coupled to the subsea wellhead to a second end coupled to one of the tensioning systems. Each tensioning system is configured to apply a tensile preload to one of the tension members.

What is claimed is: 
     
         1 . A system for tethering a subsea wellhead, the system comprising:
 a plurality of anchors disposed about the subsea BOP and secured to the sea floor;   a plurality of tensioning systems, wherein one tensioning system is coupled to an upper end of each anchor;   a plurality of flexible tension members, wherein each tension member extends from a first end coupled to the subsea wellhead to a second end coupled to one of the tensioning systems;   wherein each tensioning system is configured to apply a tensile preload to one of the tension members.   
     
     
         2 . The system of  claim 1 , further comprising a plurality of pile top assemblies, wherein one pile top assembly is mounted to an upper end of each anchor, and wherein each tensioning system is coupled to one of the pile top assemblies. 
     
     
         3 . The system of  claim 2 , wherein each pile top assembly is removably mounted to the upper end of one of the anchors. 
     
     
         4 . The system of  claim 3 , wherein each pile top assembly includes an adapter and a plurality of circumferentially-spaced locking rams coupled to the adapter;
 wherein each adapter receives the upper end of the corresponding anchor;   wherein each locking ram includes a linear actuator and a gripping member coupled to the linear actuator, wherein the linear actuator is configured to move the gripping member between a first position engaging the corresponding anchor and a second position spaced apart from the corresponding anchor.   
     
     
         5 . The system of  claim 1 , wherein the plurality of anchors comprises at least three anchors, and wherein each anchor is a driven pile or a suction pile; and
 wherein each tension member extends horizontally from the corresponding tensioning system to the first end.   
     
     
         6 . The system of  claim 1 , wherein each tensioning system is a winch configured to pay in and pay out the corresponding tension member;
 wherein each winch includes a spool rotatably coupled to the corresponding anchor and a locking mechanism configured to prevent pay out of the corresponding tension member from the spool, wherein the spool has an axis of rotation.   
     
     
         7 . The system of  claim 6 , wherein each locking mechanism includes a spool ring coupled to the spool, a hub fixably coupled to the anchor, and a lock ring slidably mounted to the hub;
 wherein the spool ring includes a plurality of internal splines, the hub includes a plurality of external splines, and the lock ring includes a plurality of external splines and a plurality of internal splines;   wherein the external splines of the hub mate and intermesh with the internal splines of the lock ring;   wherein the internal splines of the spool ring are configured to mate and intermesh with the plurality of external splines of the lock ring   wherein the lock ring is configured to move axially along the hub between an unlocked position with the external splines of the lock ring axially spaced apart from the internal splines of the spool ring and a locked position with the external splines of the lock ring intermeshing with the internal splines of the spool ring.   
     
     
         8 . The system of  claim 6 , wherein the spool is rotatably mounted within a housing, and wherein the locking mechanism is a is a ratchet including a ratchet wheel fixably coupled to the spool and a pawl pivotally coupled to the housing, wherein the pawl is configured to pivot between an unlocked position spaced apart from the ratchet wheel and a locked position engaging the ratchet wheel. 
     
     
         9 . The system of  claim 1 , wherein the first end of each tension member is coupled to a wellhead adapter mounted to the subsea wellhead. 
     
     
         10 . The system of  claim 9 , wherein the wellhead adapter includes an annular hub disposed about the subsea wellhead and a plurality of circumferentially-spaced arms extending radially outward from the hub. 
     
     
         11 . The system of  claim 10 , wherein the wellhead adapter includes a plurality of circumferentially-spaced locking devices coupled to the hub, wherein the locking devices are configured to lock the adapter to the subsea wellhead. 
     
     
         12 . The system of  claim 1 , wherein each tension member comprises a chain, a wire rope, or Dyneema® rope. 
     
     
         13 . The system of  claim 1 , further comprising a load cell coupled to each tension member and configured to measure the tensile load in the corresponding tension member. 
     
     
         14 . A system for drilling, completing, or producing a subsea well, the system comprising:
 a subsea wellhead extending from the well proximal the sea floor;   a plurality of circumferentially-spaced anchors disposed about the wellhead and secured to the sea floor, wherein each anchor has an upper end disposed proximal the sea floor;   a plurality of tensioning systems, wherein each tensioning system is coupled to one of the anchors;   a wellhead adapter mounted to the wellhead;   a plurality of flexible tension members, wherein each tension member is coupled to one of the tensioning systems and has a first end coupled to the wellhead adapter, wherein each tension member is in tension between the corresponding tensioning system and the first end.   
     
     
         15 . The system of  claim 14 , wherein the adapter includes an annular hub disposed about the wellhead and a plurality of circumferentially-spaced arms extending radially outward from the hub. 
     
     
         16 . The system of  claim 15 , wherein the first end of each tension member is coupled to one of the arms of the adapter. 
     
     
         17 . The system of  claim 15 , wherein the adapter includes a plurality of circumferentially-spaced locking devices coupled to the hub, wherein the locking devices are configured to lock the adapter to the subsea wellhead. 
     
     
         18 . The system of  claim 14 , wherein the plurality of anchors comprises at least three anchors, and wherein each anchor is a driven pile or a suction pile; and
 wherein each tension member extends horizontally from the corresponding tensioning system to the first end.   
     
     
         19 . The system of  claim 14 , wherein each anchor is disposed at a radial distance R1 measured horizontally from the wellhead, and wherein each radial distance R1 is the same. 
     
     
         20 . The system of  claim 14 , wherein each tensioning system is a winch configured to pay in and pay out the corresponding tension member;
 wherein each winch includes a spool rotatably coupled to the corresponding anchor and a locking mechanism configured to prevent pay out of the corresponding tension member from the spool, wherein the spool has an axis of rotation;   wherein each locking mechanism includes a spool ring coupled to the spool, a hub fixably coupled to the anchor, and a lock ring slidably mounted to the hub;   wherein the spool ring includes a plurality of internal splines, the hub includes a plurality of external splines, and the lock ring includes a plurality of external splines and a plurality of internal splines;   wherein the external splines of the hub mate and intermesh with the internal splines of the lock ring;   wherein the internal splines of the spool ring are configured to mate and intermesh with the plurality of external splines of the lock ring   wherein the lock ring is configured to move axially along the hub between an unlocked position with the external splines of the lock ring axially spaced apart from the internal splines of the spool ring and a locked position with the external splines of the lock ring intermeshing with the internal splines of the spool ring.   
     
     
         21 . The system of  claim 14 , further comprising a load cell coupled to each tension member and configured to measure the tensile load in the corresponding tension member. 
     
     
         22 . A method for tethering a subsea wellhead, the method comprising
 (a) securing the plurality of anchors to the sea floor about the wellhead;   (b) coupling a flexible tension member to each anchor;   (c) coupling each tension member to the wellhead; and   (d) applying a tensile preload to each tension member after (a)-(c).   
     
     
         23 . The method of  claim 22 , wherein the plurality of anchors are uniformly circumferentially-spaced about the wellhead. 
     
     
         24 . The method of  claim 22 , further comprising:
 mounting a wellhead adapter to the wellhead;   coupling each tension member to the wellhead adapter.   
     
     
         25 . The method of  claim 22 , wherein (d) comprises applying a minimum tensile load to each tension member necessary for the tension member to extends linearly from the wellhead to a tensioning system coupled to the corresponding anchor. 
     
     
         26 . The method of  claim 22 , further comprising measuring and monitoring the tension in each tension member during (d). 
     
     
         27 . The method of  claim 22 , wherein a winch is coupled to each anchor and the corresponding tension member;
 wherein (d) further comprises:   (d1) paying in each tension member with the corresponding winch;   (d2) locking the winch to prevent the winch from paying out the corresponding tension member after (c1).