Method and apparatus for reducing set-down of a tension leg platform

Abstract

An Auxiliary Mooring System (AMS) supplements a Tension Leg Platform (TLP) tendon-based mooring system, to provide lateral resistance to offset movement of the TLP in extreme survival events with minimum impact to the TLP or permanent tendon system design. Limiting platform offset also limits set-down of the platform. The mooring system of the invention provides a method and apparatus for supplementary mooring of a Tension Leg Platform. The apparatus includes a plurality of catenary mooring lines attached to the platform's lower columns or pontoons and secured to the seabed by means of piles or other established anchoring means. The method of the AMS includes installing compensating buoyancy devices on the platform.

1 . An offshore platform comprising: 
 a buoyant hull;    a plurality of tendons each having a first end anchored to the seafloor and a second end attached to the hull such that the tendons are loaded in tension and the hull is held by the tendons below its free-floating draft;    a plurality of fixed-length mooring lines each having a first end anchored to the seafloor and a second end attached to the hull such that lateral movement of the hull is limited to a pre-selected distance and a restoring force is applied by the mooring lines if the hull moves to a position wherein the tendons are not substantially vertical.    
   
   
       2 . An offshore platform as recited in  claim 1  wherein the hull is a monocolumn hull.  
   
   
       3 . An offshore platform as recited in  claim 2  wherein the monocolumn hull has a plurality of radially-extending subsurface pontoons.  
   
   
       4 . An offshore platform as recited in  claim 3  wherein the attachment points of the tendons are on each pontoon and the attachment points of the mooring lines are on each pontoon outboard of the attachment points of the tendons.  
   
   
       5 . An offshore platform as recited in  claim 4  further comprising a buoyancy compartment on each pontoon between the attachment points of the tendons and the attachment points of the mooring lines.  
   
   
       6 . An offshore platform as recited in  claim 5  wherein the buoyancy of the compartment is substantially equal to the vertical component of the force exerted by the mooring lines attached to each pontoon when the tendons of the platform are substantially vertical.  
   
   
       7 . A method of reducing set-down in a tension leg platform comprising: 
 providing a plurality of fixed-length mooring lines each having a first end and a second end;    anchoring the first end of each mooring line to the seafloor; and,    attaching the second end of each mooring line to the hull such that lateral movement of the hull is limited to a pre-selected distance and a restoring force is applied by the mooring lines if the hull moves to a position wherein the tendons are not substantially vertical.    
   
   
       8 . A method as recited in  claim 7  wherein the tension leg platform comprises a monocolumn hull.  
   
   
       9 . A method as recited in  claim 8  wherein monocolumn hull has a plurality of radially-extending subsurface pontoons.  
   
   
       10 . A method as recited in  claim 9  wherein the attachment points of the tendons are on each pontoon and the mooring lines are attached to sites on each pontoon that are outboard of the attachment points of the tendons.  
   
   
       11 . A method as recited in  claim 10  further comprising a providing a buoyancy compartment on each pontoon between the attachment points of the tendons and the attachment points of the mooring lines.  
   
   
       12 . A method as recited in  claim 11  wherein the buoyancy of the compartment is substantially equal to the vertical component of the force exerted by the mooring lines attached to each pontoon when the tendons of the tension leg platform are substantially vertical.  
   
   
       13 . A method as recited in  claim 7  further comprising providing a buoy on each mooring line at a point intermediate the first end and the second end.  
   
   
       14 . A method as recited in  claim 13  wherein the buoy is a subsurface buoy or buoys.  
   
   
       15 . A method as recited in  claim 7  wherein the mooring lines comprise a section of chain configured to rest on the seafloor when the tendons of the tension leg platform are substantially vertical.  
   
   
       16 . A method as recited in  claim 7  wherein the mooring lines are attached to the hull using fairleads which comprise a chain stopper.  
   
   
       17 . A method as recited in  claim 7  further comprising pre-tensioning the mooring lines by passing the mooring line through a fairlead attached to the hull and pulling the second end of the mooring line using a winch on an attending vessel.  
   
   
       18 . A method as recited in  claim 7  further comprising pre-tensioning the mooring lines by passing each mooring line through a fairlead attached to the hull and hoisting the second end of the mooring line.  
   
   
       19 . A method as recited in  claim 18  wherein the hoisting is performed by a derrick barge.  
   
   
       20 . A method as recited in  claim 18  additionally comprising applying tension to the mooring lines by also using a winch mounted on an attending vessel.  
   
   
       21 . An offshore platform comprising: 
 a buoyant hull including a generally cylindrical central section and a plurality of subsurface pontoons radially extending from the central section;    a plurality of tendons each having a first end anchored to the seafloor and a second end attached to a pontoon such that the tendons are loaded in tension and the hull is held by the tendons below its free-floating draft;    a buoyancy compartment attached to the outboard end of a pontoon such that the buoyancy compartment is substantially outboard of the tendon attachment point on the pontoon;    a plurality of fixed-length mooring lines each having a first end anchored to the seafloor and a second end attached to a buoyancy compartment such that lateral movement of the hull is limited to a pre-selected distance and a restoring force is applied by the mooring lines if the hull moves to a position wherein the tendons are not substantially vertical.    
   
   
       22 . An offshore platform as recited in  claim 21  wherein the buoyancy compartment has a buoyancy that is approximately equal in magnitude to the downward force exerted by the mooring line attached to the buoyancy compartment.  
   
   
       23 . An offshore platform as recited in  claim 21  further comprising at least one buoyancy compartment attached to a side surface of a pontoon.  
   
   
       24 . An offshore platform as recited in  claim 23  wherein the combined buoyancy of the buoyancy compartment at the end of the pontoon and the buoyancy of the buoyancy compartment attached to a side surface of the pontoon is approximately equal in magnitude to the downward force exerted by the mooring line.  
   
   
       25 . An offshore platform comprising: 
 a buoyant hull including a generally cylindrical central section and a plurality of subsurface pontoons radially extending from the central section;    a plurality of tendons each having a first end anchored to the seafloor and a second end attached to a pontoon such that the tendons are loaded in tension and the hull is held by the tendons below its free-floating draft;    a buoyancy compartment attached to the outboard end of a pontoon such that the buoyancy compartment is substantially outboard of the tendon attachment point on the pontoon, the buoyancy compartment having an interior passageway extending from an upper surface to a lower surface thereof;    a plurality of fixed-length mooring lines each having a first end anchored to the seafloor and a second end extending through the passageway attached to an upper surface of the buoyancy compartment such that lateral movement of the hull is limited to a pre-selected distance and a restoring force is applied by the mooring lines if the hull moves to a position wherein the tendons are not substantially vertical.    
   
   
       26 . An offshore platform as recited in  claim 25  wherein the opening of the passageway on an upper surface of the pontoon is generally circular and has a first diameter and the opening of the passageway on a lower surface of the pontoon is generally circular and has a second diameter which is greater than the first diameter.  
   
   
       27 . An offshore platform as recited in  claim 25  further comprising a chain stopper on an upper surface of a buoyancy compartment for securing a mooring line comprised of chain at its second end.