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&#39;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.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     none  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates to offshore platforms. More particularly, it relates to a supplemental anchoring system for deepwater tension leg platforms.  
         [0004]     2. Description of the Related Art  
         [0005]     A tension-leg platform (TLP) is a vertically-moored, floating structure normally used for the offshore production of oil and/or gas, and is particularly suited for water depths greater than about 1000 feet. A TLP typically comprises a buoyant hull that is at least partially submerged and one or more decks supported above the surface of the water by the hull.  
         [0006]     The TLP is connected to a fixed foundation (or piles) by pre-tensioned tendons. The tendons are normally parallel, near vertical elements, acting in tension, which usually restrain the vertical plane motions of the TLP in heave, roll and pitch. The platform is usually compliant in surge, sway and yaw.  FIG. 1  shows an example of a tension leg platform.  
         [0007]     The platform is permanently moored by means of tethers or tendons grouped at each of the structure&#39;s corners or at the ends of pontoons extending from a column-shaped hull. A feature of the design of the tendons is that they have relatively high axial stiffness (low elasticity), such that virtually all vertical motion of the platform is eliminated. This allows the platform to have the production wellheads on deck (connected directly to the undersea wells by rigid risers), instead of on the seafloor. This makes for less expensive well completions and workovers and allows better control over the production from the oil or gas reservoir. The platform is also an excellent facility for subsea wellhead applications and Steel Catenary Risers due to the reduced platform motions.  
         [0008]     Although a TLP permits almost no vertical motion, it can move sideways under the influence of wind, waves and ocean currents. As is illustrated in  FIGS. 1 and 2 , lateral movement of TLP  10  in response to Force F to displaced TLP  10   a  results in set-down—a decrease in the distance between the surface of the sea W and the lowest deck of superstructure  20 —owing to the geometry of the tendons  16 , the tendon anchors in seafloor S and the tendon attachment points on the pontoons  14  of the TLP. The hull of a TLP is commonly designed such that its buoyant force increases with lateral displacement (and set-down) of the platform. This effect generates a restoring force which tends to move the platform back into a position directly above the tendon anchor points with the tendons oriented vertically. However, the force of the wind, waves and/or current may be such that the equilibrium point of the platform is displaced from the vertical and hence there is some degree of set-down of the platform.  
         [0009]     TLP&#39;s, like all other offshore platforms, must be designed to withstand certain extreme environmental conditions that may be encountered during a storm or hurricane. Commonly, these conditions are defined in terms of the strongest storm likely to be experienced by the platform in a certain time period—e.g., “a 100-year storm” or “a 1000-year storm.” Perhaps the most important environmental criterion that a TLP must be able to withstand is wave height. The height of the lowest deck must be sufficient to clear the highest wave likely to be encountered. As explained above, lateral displacement of a TLP results in set-down of the platform which reduces the height of the deck(s) above the water. Limiting the lateral displacement of a TLP therefore reduces the deck height necessary to meet the worst-case environmental scenario. This both enhances the safety of the platform and reduces its size and cost.  
         [0010]     The TLP encounters increasing design challenges if extended into very deep water and under harsh environmental conditions. Firstly, tendon weight must increase as water depth increases in order to maintain adequate tendon stiffness. This is typically achieved by increasing tendon cross section, which means increased tendon weight, requiring more buoyancy and hence a larger hull, which in turn boosts tendon stiffness requirements. Secondly, the requirements regarding air gap increase, owing to the set down effect. Air gap is the distance between the surface of the water and the underside of the lowest deck of the TLP. Both of the aforementioned characteristics are a negative effect both technically and commercially. Furthermore, the number of suppliers of large diameter thick-walled high-strength tendons is extremely limited and can easily result in a single worldwide source, which may result in prohibitive costs.  
         [0011]     In recent years there has been an increasing number of severe weather events, and the resulting substantial platform damage has called into question the validity of the survival event design data that has been employed by the industry. A TLP is more sensitive to changes in survival weather criteria than traditional catenary-moored vessels. This can be compensated for in the design phase of a TLP through adequate weight management and tendon detail design optimization, but at a significant cost. One impact of an increased survival event is to cause significant increases in TLP offset (surge and sway) and set-down, and results in substantially higher maximum tendon tensions and angles, and the potential for a slack tendon condition.  
         [0012]     An auxiliary mooring system is a cost-effective method of supplementing either existing, operational TLP&#39;S, those in mid-fabrication cycle, or new build designs, in order to meet new, enhanced-survival events.  
       BRIEF SUMMARY OF THE INVENTION  
       [0013]     An auxiliary mooring system is used to constrain the offset (surge and sway) of a tension leg platform. The auxiliary mooring system additionally provides a redundant restraint system for the platform in the event of tendon failure. In one embodiment, the auxiliary mooring system comprises a plurality of catenary lines secured by anchors on the seafloor. Combined fairleads/chain stoppers mounted on submerged pontoons of the TLP may serve as the termination point of the mooring lines on the TLP. Additional buoyancy devices may be attached to the TLP to compensate for the weight of the mooring lines. Subsurface buoys attached at selected locations on the mooring lines may be used to further direct and modify the forces exerted on the TLP by the mooring lines. The auxiliary mooring system of the invention may also be used to retrofit existing tension leg platforms.  
         [0014]     The Auxiliary Mooring System (AMS) of the present invention has been developed so that the hurricane survival event does not dictate the tendon design and instead the tendon system works in conjunction with a catenary mooring system to provide a composite and cost-effective means of platform vertical and lateral restraint. The AMS also provides additional restraint in deep water under the enhanced-survival event. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)  
       [0015]      FIG. 1  depicts the set-down phenomenon in a tension leg platform of the prior art.  
         [0016]      FIG. 2  is an enlarged view of the upper sections of the TLP depicted in  FIG. 1 .  
         [0017]      FIG. 3  illustrates a TLP having an auxiliary mooring system according to a first embodiment the present invention.  
         [0018]      FIG. 4  illustrates a TLP having an auxiliary mooring system according to a second embodiment of the invention which comprises subsurface buoys on the mooring lines.  
         [0019]      FIG. 5  shows one method for installing an auxiliary mooring system on a TLP using a surface vessel.  
         [0020]      FIG. 6  depicts an alternative method for installing an auxiliary mooring system of the invention.  
         [0021]      FIG. 7  shows several views and embodiments of chain stopper/fairlead devices for terminating a mooring line on the pontoon of a TLP.  
         [0022]      FIG. 8  shows an embodiment wherein the auxiliary mooring system includes added buoyancy devices on the TLP.  
         [0023]      FIG. 9   a  is a partially-sectioned side view of a TLP equipped with an auxiliary mooring system according to an embodiment of the invention which comprises a chain chute through each pontoon of the TLP.  
         [0024]      FIG. 9   b  is a side view of the pontoon end of the TLP shown in  FIG. 9   a.    
         [0025]      FIG. 9   c  is a plan view thereof. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]     A Tension Leg Platform Auxiliary Mooring System according to the present invention may comprise a floating platform secured by a plurality of vertical tendons comprising steel pipe with an effective diameter-to-wall thickness ratio that provides a nearly neutrally buoyant condition. The system may also be used in conjunction with tendons having neutral, positive or even negative buoyancy. The lower ends of the tendons may be anchored to the sea bed and extend to near the sea surface while being held in tension by the buoyant floating platform such that the platform stays in a near vertical position. The mooring system additionally comprises a plurality of platform auxiliary moorings arranged in a catenary pattern extending a distance from the platform to seabed anchors.  
         [0027]     Referring now to  FIG. 3 , a TLP  10  comprising hull  12  equipped with a plurality of wings or pontoons  14  is secured by tendons  16  secured to piles  24  in seafloor S. Wings of pontoons  14  may be buoyant structures. Superstructure  20 , which may comprise a plurality of decks, is attached to hull  12  by means of jacket structure  22 . Risers  18  (shown in  FIG. 1 ) connect equipment on the deck(s) to well heads, pipelines and related apparatus on seafloor S.  
         [0028]     For the single-column TLP design shown in  FIG. 3 , a single catenary mooring line ( 26 ) may be terminated at the end of each pontoon ( 14 ), providing an auxiliary, multi-leg, symmetrical or non-symmetrical, catenary mooring system. The seabed termination of the mooring lines may be the same system as that used by the tendons—i.e., driven piles. Alternatively, the anchoring means  28  for the mooring lines may comprise suction anchors, vertically loaded anchors (VLAs) or similar devices. The connection of mooring line  26  to TLP  10  at pontoon end  32  may be significantly simplified compared to systems of the prior art because it requires no operational adjustment, tension monitoring equipment or chain lockers.  
         [0029]     As illustrated in  FIG. 4 , mooring line  26  may be equipped with subsurface buoy  30  to at least partially support line  26  in multiple catenary segments and modify and direct the forces applied to TLP  10  by mooring line  26 . Mooring line  26  may be equipped with multiple subsurface buoys. The attachment means of subsurface buoy  30  to mooring line  26  may comprise a triplate or other means well-known in the art.  
         [0030]     Mooring line  26  may comprise chain sections, wire rope sections, and/or synthetic fiber sections. In some embodiments, lines  26  may be formed of a single material. In one preferred embodiment, at least the lower end of line  26  comprises heavy chain which rests on seafloor S when TLP is in a position such that tendons  16  are vertically oriented. Lateral displacement of TLP  10  from this position tightens the upwind or upstream mooring line(s)  26  which tends to raise the chain section at the seafloor end. The force of gravity acting on the raised chain section provides a restoring force to TLP  10 , urging it back towards its original position wherein the tendons  16  are vertical. The water weight of line  26  may be compensated for by means of additional buoyancy in hull  12 . Alternatively, additional buoyancy devices may be added to hull  12  to at least partially compensate for the downward component of force exerted on TLP  10  by lines  26 . One such embodiment comprising additional buoyancy devices is shown in  FIG. 8 .  
         [0031]     The embodiment illustrated in  FIG. 8  comprises pontoon extensions  52  on the distal ends of pontoons  14 . Pontoon extensions  52  may be located outboard of tendon porches  38 . Alternatively, pontoon extension  52  may be attached to the side(s), bottom or top of pontoon  14 . In certain embodiments, it may be advantageous to attach a plurality of pontoon extensions  52  to a plurality of locations on pontoon  14 . Fairlead  40  or a combination chain stopper/fairlead may be attached to the outboard end of pontoon extension  52  to provide a termination of mooring line  26 .  
         [0032]     The embodiment illustrated in  FIG. 8  is particularly well-suited for adding an auxiliary mooring system to an existing TLP design inasmuch as some or all of any additional hull buoyancy necessitated by the addition of the AMS may be provided by the pontoon extensions  52 . Moreover, the additional buoyancy is provided close to the attachment points of the mooring lines  26 . This feature also helps to avoid having to change any of the existing design parameters of the TLP.  
         [0033]     Yet another embodiment of a pontoon extension  52  is shown in  FIGS. 9   a ,  9   b  and  9   c . This embodiment employs a flared chain chute  54  through the interior of each pontoon extension. Chain chutes  54  may obviate the need for fairleads  40 . Chain stopper  48  may be located at the upper end of chain chute  54  to provide a termination for mooring line  26 . Space within pontoon extension  52  not occupied by chain chute  54  may be employed to provide additional buoyancy. This space may comprise one or more watertight compartments.  
         [0034]     The AMS mooring lines  26  may be pre-installed and laid out on the seafloor S. The tendons  16  also may be pre-installed and buoyed off. A derrick vessel may be used to deploy the components. The hull  12  may be towed to site and the tendons  16  firstly connected to the hull  12  (or pontoons  14 ). During hook-up, a derrick vessel may support the hull through its main crane and may effect the connection of the tendons to the hull. After the tendons are connected to the hull and tensioned, the AMS may be attached to the hull, using the derrick vessel to support the main weight of each mooring line ( 26 ) and apply tension one line at a time. The AMS of the present invention may be installed and attached to the hull of the TLP either before or after the deck superstructure is installed.  
         [0035]     Alternative AMS installation methods are illustrated in  FIGS. 5 and 6 . As shown in  FIG. 5 , a winch line  34  from attending vessel V may be attached to the upper end of mooring line  26 . Attending vessel V may be dynamically positioned and/or be secured by anchoring means of its own (not shown). A winch on attending vessel V may be used to apply a selected amount of pretension to mooring line  26 . Once the desired pretension is applied, a device such as those illustrated in  FIG. 7  may be used to secure line  26  with a fixed length.  
         [0036]     Another installation method (illustrated in  FIG. 6 ) uses a winch  36  mounted on hull  12  to pretension mooring line  26 . This method may employ a single, moveable winch used to successively pretension each mooring line  26  or may employ a plurality of winches which may be used simultaneously or seriatim to apply the desired amount of pretension to each mooring line  26 . The methods illustrated in  FIG. 5  and  FIG. 6  may be used in conjunction. In such a situation, a plurality of winch lines  36  may be attached to mooring line  26  in order to provide the selected amount of pretension. This method may have the added advantage of reducing frictional forces in fairlead  40  on pontoon end  32  as the mooring line  26  passes through the fairlead because the direction of the net force being applied to line  26  may be selectively altered.  
         [0037]     Attachment means for mooring line(s)  26  are shown in  FIG. 7 , a top plan view of the distal end of pontoon  14 .  FIG. 7   a  is a side view thereof, taken along line A in  FIG. 7 .  FIG. 7   b  is a partially-sectioned side view of an alternative embodiment taken along line B in  FIG. 7 .  FIG. 7   c  is and end view of pontoon  14  taken along line C in  FIG. 7 .  FIG. 7   d  is a plan view of hull  12  showing three pontoons or wing  14 , each having a pair of mooring line fairleads  40  at its distal end.\ 
         [0038]     As seen in  FIG. 7 , pontoon  14  may be equipped with tendon porches  38  for tendon attachment. Additionally, pontoon tip  32  is equipped with pivoting fairleads  40  whose rotation may be restricted as shown in the drawing to prevent entanglement of mooring lines  26 . In the embodiment illustrated in  FIG. 7   a , chain laydown rack  42  is provided on chain storage area  44  of the upper surface of pontoon  14  to accommodate excess chain comprising at least the end portion of mooring lines  26 . An alternative embodiment is shown in  FIG. 7   b  wherein hawse pipe or chain locker  46  is provided within pontoon  14  for securing excess chain.  
         [0039]     As may be seen in  FIGS. 7   a ,  7   b  and  7   c , chain stoppers  48  may be integrated with fairleads  40  to provide a termination of mooring lines  26 . Alternatively, chain stopper  48  may be separate from fairlead  40 . In certain embodiments, fairlead(s)  40  may be equipped with integral load cell  50  for monitoring the forces on fairlead  40  and/or mooring line  26 .  
         [0040]     Although the invention has been described in detail with reference to certain preferred embodiments, variations and modifications exist within the scope and spirit of the invention as described and defined in the following claims.