Abstract:
A system of lashing a tender assist drilling unit (TADU) to a floating production platform includes a plurality of lashing lines reeved over a hydraulic-pneumatic tensioning system sheave assembly and a plurality of fixed anchor points to the above water line section of the production platform hull structure. The hydraulic-pneumatic tensioning device will, when in tensioning mode, compensate and control the relative movement between the two floating bodies. The hydraulic-pneumatic tensioning system will also enable lashing pre-tension adjustments during operations and fine-tuning of system performance can be done site-specific based on lessons learned. The hydraulic-pneumatic tensioning device will, when in hydraulic lock mode, enable a fixed lashing line system. This will by the means of the first units mooring winch system enable; a winch-in for close proximity during heavy lifting; or an efficient lashing pay put during extreme weather preparation for increased separation between the two units.

Description:
RELATED AND CO-PENDING APPLICATIONS 
       [0001]    This application claims priority to co-pending U.S. Provisional application entitled “ Compensated Lashing of Tender Assist Drilling Unit to A Floating Production Facility” Ser. No. 61/811,057, filed on 11 Apr. 2013, the entirety of which is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    The present subject matter relates to floating offshore structures and particularly to a system for connecting two floating structures by wire rope lashing to obtain a controlled relative separation and to achieve the lashing loads to be within predefined maximum lashing tension load values. 
         [0003]    The need for this type of lashing technology is especially applicable when implemented with a tender assist drilling unit (TADU) for an offshore production facility based drilling and or completion operation. 
         [0004]    In many deep water drilling and production installations, a floating tender assist drilling unit (TADU) is lashed to an adjacent offshore production facility or platform to assist in the drilling and production operations. This TADU can be any type of semi submersible or barge hull form. Both the TADU and the platform are typically moored to the seabed, and they are lashed to each other so as to restrict relative movement between the two structures, thereby to facilitate the transfer of drilling consumables, supplies and personnel from one structure to the other and hook-up of control and fluid lines between the two structures. The lashing mechanism must be capable of maintaining the relative movement within predefined limits that allow normal operation throughout environmental conditions that can be expected during the course of a normal year (a “one-year environment”), and that allow limited operations, including the maintenance of drilling circulation and control, throughout worst-case conditions to be expected during a typical ten-year period (a “ten-year environment”). The one year and ten year environments are established and described by Metocean. 
         [0005]    In a 100-year extreme weather condition the system must be capable of increasing the separation distance to a storm safe distance and at the same time function as a 100-year storm safe coupled mooring system. This is obtained by having the four off lashing lines connected to the TADU forward mooring winches for separation control during storm separation and pull back for normal operation. 
         [0006]    The present system for coupling two bodies is either a fixed wire/rope/chain coupling lines by doing a fixed length connection between the two bodies. 
         [0007]    The downside of this fixed length type of lashing is the lack of control over the lashing loads acting between the two floating structures. As a consequence this can add to the maximum overall horizontal acceleration on the drilling equipment set installed onto the offshore production facility, with the consequence of potentially de-rating of the drilling facility operational specifications and subsequent reduction of drilling efficiency (drilling up-time). 
         [0008]    Additionally, this type of lashing will require a full disconnect between the two floating structures during an extreme weather event to be storm safe. This disconnect will require assistance of additional support vessels and add time for the overall extreme weather preparation window required. 
         [0009]    An alternative solution has been to utilize a wire/nylon/wire lashing hawser for a spar based production facility. This system will require an extensive length of nylon section that can only be achieved by routing the nylon-based hawser down the spar hull. This system is described in patent publication US2007/0119359 the entirety of which is incorporated by reference. 
         [0010]    The down side of this system is that it is specific to spar hulls. It is partly submerged by having the majority of the nylon-hawser routed down the spar hull. Additionally, this is not a system that lends itself to be retrofitted. It is not possible to adjust any stiffness characteristic for this system after installation. Also, this system will require separate fixed lashing lines to be installed during close proximity of the two floating structures during heavy lift operation. 
         [0011]    The present disclosed subject matter addresses these shortcomings, for example, not being floating structure independent. It may be used on any type of floating structure like; semi-submersible, tension leg platform, spar, barge or mono hull. The disclosed subject matter also addresses the need for relative separation control between the two floating bodies and the lashing tension load control. It also eliminates the necessity for disconnect during extreme weather to make the structures storm safe. It also addresses the need to have an additional fixed close proximity lashing during heavy load transfer between the two floating structures. 
         [0012]    Broadly, the present subject matter is a system for lashing a first floating structure, such as a TADU, to a second floating structure, such as a production platform, which include a set of lashing lines, each extending from an anchor point of the first floating structure through a hydraulic-pneumatic tensioner system to a fixed anchor of the second floating structure. 
         [0013]    In a preferred embodiment each of the lashing line first or free ends are connected to a winch cable that is wound on a winch on the first structure or TADU. The lashing is preferably a wire rope connected by suitable connection means connected to an anchor wire winch of the first floating structure. 
         [0014]    The lashing wire is reeved over upper and lower sheaves, or sheave cluster of the hydraulic-pneumatic tensioner system. The lashing wire is routed over to the second floating structure and secured to an anchor mechanism at the structure. This anchor can either be a fixed anchor or a rotating anchor for the purpose of easing slipping and cutting of the lashing wire as part of the scheduled maintenance of the lashing system. 
         [0015]    These and many other advantages of the present subject matter will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of preferred embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is an elevation view of the TADU and a second floating Tension Leg Platform based structure linked together by a lashing system in accordance with the present invention; 
           [0017]      FIG. 2  is a plan view of a TADU and a platform of  FIG. 1  linked together by lashing system of the present invention. 
           [0018]      FIG. 3  is a schematic drawing of the hydraulic-pneumatic tensioning system including showing generic valve assembly, generic sheave cluster arrangement, generic lashing wire routing and the cylinder safety device to prevent “sling shot”. 
           [0019]      FIG. 4  is a flow diagram of a method of lashing two floating structures together according to an embodiment of the disclosed subject matter. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Referring now to the drawings, a first floating structure  10  is shown, in  FIGS. 1 and 2 , linked to a second floating structure  20  by a lashing system in accordance with a preferred embodiment of the disclosed subject matter. In this exemplary embodiment, the first floating structure  10  is a tender assist drilling unit (TADU), and will be referred to as such in the following description. The second floating structure  20  is an offshore drilling and production platform of the Tension Leg Platform (TLP) type, and will be referred to as a “platform” in the following description. It should be understood, however, that the present disclosed subject matter is not limited to use with the specific types of floating structures described herein. For example, the second floating structure  20  can be any other platform or structure supporting a deck structure. Similarly, the first floating structure  10  can be any type of floating structure that needs to be linked or lashed to another floating structure, such as a platform. It is further noted that the disclosed subject matter may also be implemented where the second structure is not a floating structure, however, its benefits are greater when both structures are floating. 
         [0021]    As shown, the TADU  10  includes a hull that has a submerged portion  12 , from which extend a plurality of columns  14  that support a deck  16  on which various equipment and structures used in drilling and production operations are located. Also secured to the deck  16  is a gangway  18  having a distal end adapted to be secured to the platform when the structures  10 ,  20  are lashed together by the present subject matter, as described below. 
         [0022]    In the described embodiment, the platform  20  is a TLP platform, the platform  20  has a hull comprising a pontoon  22 , columns  24 , tendons  26  for mooring and a deck structure  28 . A deck structure  28  is secured to the top of hull  20 , supported above the surface of the water, and it is adapted to receive and secure the distal end of the gangway  18  when the structures  10  and  20  are lashed together by the present subject matter. The TLP structure  20  is well-known in the art. 
         [0023]    As shown in  FIGS. 1 and 2 , the platform  20  is secured to the seabed by a plurality of platform tendons  26 . Similarly, the TADU  10  is secured to the seabed, at a location closely adjacent to the platform  20 , by a plurality of TADU mooring lines  50 , after having been moved to that location, either under its own power (if self-propelled), or by tugboats (not shown). 
         [0024]    A plurality of winches  40  are provided near the forward end of the TADU  10 , preferably on the two forward-most columns  14  of the TADU hull. In a preferred embodiment, each of the two forward-most columns  14  carries two winches  40 , for a total of four winches  40 . Mounted on the forward-most columns  14  below each of the winches  40  is the hydraulic-pneumatic tensioner system  41 . A plurality of fixed anchor points  44  mounted on the upper part of the platform hull  20 , such as on the column  24 , on the side that would be lashed to the TADU  10 . 
         [0025]    Thus, as shown in  FIG. 2 , in the exemplary embodiment disclosed herein, there are four fixed anchor points  44 , two at approximately the 10 o&#39;clock position and two at approximately the 5 o&#39;clock position on the TLP column  24  (looking from the TADU  10 ). It may be advantageous to provide idler sheaves at the 2 o&#39;clock and 10 o&#39;clock positions as well, to allow the TADU  10  to be lashed to either side of the platform  20 . Additional idlers or fairleads may also be utilized. For each winch  40  and hydraulic-pneumatic tensioner unit  41  there is a fixed anchor point  44 , on the platform hull  20 . In the exemplary embodiment disclosed herein, each of the fixed anchor points are fixed to the platform column  24 . In some situations, two or more of the hydraulic-pneumatic tensioner units  41  may utilize a common anchor point  44 . 
         [0026]    A plurality of lashing lines  48 , preferably around 3 in. diameter, are employed to lash the TADU  10  to the platform  20  by means of the winches  40 , hydraulic-pneumatic tensioner unit  41 , and fixed anchor connections  44 . 
         [0027]    As best shown in  FIG. 3  each lashing line  48  is preferably made of wire rope. The lashing line is at one free end connected by a suitable connector  60  to the mooring winch  40  mooring line  62 . The lashing line is reeved over the hydraulic-pneumatic tensioner unit lower sheave assembly  64  and upper sheave assembly  66  back down to sheave assembly  64  and over to the platform  20  fixed anchor point  44 , preferably located at the column  24 . By having both sheave assembly  64  and sheave assembly  66  made up by dual sheaves system a 4:1 motion ratio between the cylinder stroke and connection line travel, a desired separation envelope of +/−30 feet can be obtained. Any number of sheaves (N s ) may be employed with the understanding while the motion ratio will increase as a function of N s , the force required to maintain the desired tension in the lashings will also increase as a function of N s . 
         [0028]    The hydraulic-pneumatic tensioner system as shown in detail in  FIG. 3  comprises of several components well known to the industry. The system includes a lower sheave assembly  64  fixed to a cylinder body  70 . The cylinder rod  72  is outfitted with an upper sheave assembly  66 . The cylinder is connected to a gas/fluid accumulator  76  (gas and fluid are shown as  76   a  and  76   b  respectively) by a fixed pipe spool  77 , with an in line “sling shot” valve  78 . The purpose of the “sling shot” valve  78  is a rapid closure of the hydraulic line between the accumulator and the cylinder to prevent an uncontrolled out stroke of the cylinder rod  72  in the event of a sudden lashing line  48  failure during operation, the “sling shot” valve can also be described as a fail safe valve. The references to upper and lower are for ease of description only, as several orientations of the sheaves are envisioned. 
         [0029]    The accumulator  76  is sized to accommodate the required gas volume needed to maintain a predefined tension system stiffness characteristic and also to accommodate the required nominal and maximum pressure anticipated during operation. Generally the larger the accumulator&#39;s volume, the less the differential in pressure between the maximum stroke state, in which the rod is fully extended and the minimum stroke state in which the rod is fully retracted. 
         [0030]    The accumulator is protected by pressure safety valve  80 . A valve control panel  82  controls the accumulator. Increases in pressure are made by opening valve  84  and valve  86 . Operating valve  86 , which is operably connected to the accumulator  76 , and valve  88  reduces pressure. Valve control panel is connected to high-pressure air system  90 . The valves may be automatically controlled. Additionally, a pressure gauge is preferably included on the control panel. Manual control of the sling shot or fail safe value may also be enabled on the valve control panel. 
         [0031]    In operation, as shown in method  400  of  FIG. 4 , the TADU  10  is brought to the vicinity of the moored platform  20 , as mentioned above. At least one of the winch cables  48  is assembled by connecting the mooring line section  62 , as shown in block  401 , and reeved through the hydraulic pneumatic tensioner system sheave assembly, as shown in block  403 , and the second end connected to the platform  20  and fixed anchor point  44 , as shown in block  405 . This can be done on an anchor handling tug or “AHT” (not shown). 
         [0032]    The TADU  10  is brought to the desired separation distance from the platform  20 , and each remaining lashing line  48  is connected  60  with the mooring winch cable  62  and run through its associated sheave assemblies  64  and  66  and then has its free end connected to the anchor point  44  at the platform  20  column  24 . 
         [0033]    With the lashing lines  48  assembled and secured between the TADU  10  and the platform  20 , the winches  40  may be employed to adjust the separation distance between the two floating structures to the optimum separation distance, which would typically be the optimum distance for securing distal end of the gangway  18  to its appropriate attachment fixture or location on the platform  20 . The hydraulic-pneumatic tensioning system then tensions the lashing line with the hydraulic ram as shown in block  407 . The hydraulic-pneumatic tensioning system will provide the required compensation of the relative distance between the TADU  10  and the platform  20  and lashing pretension to maintain a controlled separation while keeping the two structures at an optimum desired operating distance, typically a minimum of about 30 ft. in ordinary environmental conditions. The tension in the lashing line is a function of the set gas pressure and the stroke position of the rod (the stroke position can also be expressed as a percentage of maximum stroke). It is important to note that the range may be narrow as a function of the accumulator size, however, the hydraulic-pneumatic tensioning system increases the tension in the lashing line in response to the first structure  10  moving apart from the second structure  20 . 
         [0034]    The hydraulic pneumatic tensioner based lashing system of the disclosed subject matter meets a number of significant design criteria. Among others, one differentiator between this and current designs is that it is not dependent of a specific hull design, for example a deep draft spar. The hydraulic-pneumatic tensioner system maintains the lashing lines within a desired tension band while maintaining a nominal separation range without introducing the deleterious shock loads associated with fixed lines. The tension band may advantageously be informed by the maximum lashing tension load valves. 
         [0035]    The disclosed system may be easily retrofitted into an existing structure, as all modifications are typically above water line of any of the floating structures. 
         [0036]    The system maintains the ultimate objective of: full drilling and production operations can be conducted through a one-year environment, and limited operations can be conducted through a ten-year environment. Moreover, the connection of the gangway  18  and associated electrical, hydraulic and pneumatic connections, can be maintained through a ten-year environment. In more extreme environmental conditions, such as a 100-year storm, the gangway  18  can be disconnected from the platform  20 , and the lashing lines  46  paid out to increase the separation distance substantially. 
         [0037]    The hydraulic-pneumatic tensioning system via the associated sling shot valve  78  the system can be set into hydraulic lock; this will enable the system to becoming a fixed lashing system. This is beneficial during an initial rig up or final rig down of the TADU here the TADU  10  is brought into a close proximity of platform  20  due to potential rig-up crane hook reach limitations and a strict separation control is needed. 
         [0038]    The particular connection mechanisms for connecting the lashing  48  to the anchor  44 , and suitable alternatives will suggest themselves as equivalents to those skilled in the pertinent arts and thus are not described herein. Also, the number of lashing lines  48 , as well as their particular structure, in terms of the materials and dimensions of the hawsers  48 , may be varied considerably for different applications, such as the types of floating structures to be lashed together and the environmental conditions to be endured during their operation. 
         [0039]    The type of fixed anchor connection  44  can be in the form of a rotating anchor on the platform  20  side. This may ease any operations in connection with slipping and cutting of the lashing line due to the lashing wire ton/mile maintenance requirements. The use of additional fairleads, pulleys and hawsers in directing the lashings and mooring lines is also envisioned. 
         [0040]    While embodiments of the disclosed subject matter are discussed with the adjectives hydraulic and pneumatic, those terms are used in their fullest scope, hydraulic relating to fluids and pneumatic relating to gases. Petroleum fluids, water, oils and other non-compressible liquids and gels etc. are all considered encompassed by the use of hydraulic, while air, nitrogen, as well as other gases are encompassed by the use of pneumatic. 
         [0041]    While preferred embodiments of the present invention have been described, it is to be understood that the embodiments described are illustrative only and that the scope of the invention is to be defined solely by the appended claims when accorded a full range of equivalence, many variations and modifications naturally occurring to those of skill in the art from a perusal hereof.