Abstract:
A semi-submersible vessel has a pair of vertically spaced pontoons with varied buoyancy. The lower pontoon is retained in a close vertical proximity to the upper pontoon when the vessel is in transit The lower pontoon is ballasted at the deployment site, dropping the pontoon to a depth of about 32 meters below the first pontoon baseline. As a result, stability and motion characteristics of the vessel are significantly improved.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates to offshore structures, and more particularly to offshore structures adapted for supporting oil and gas exploration/production operations at sea.  
         [0002]     Semi-submersible vessels represent one type of an offshore structure that is used for conducting operations at sea. Semi-submersible units are often used in deep waters, where conventional bottom-supported structures are difficult to install. A typical semi-submersible unit has a superstructure deck or decks supported by columns. The columns are supported by buoyant pontoons with ballast chambers that allow the vessel to be transported to a deployment site and be flooded with water to submerge the pontoons below the water surface. Once the pontoons are lowered to the desired depth, they provide stability and reduce vessel motion response to wind and water waves.  
         [0003]     Some semi-submersible vessels are designed with a pair of parallel horizontal pontoons, which support vertically extending columns. Other designs provide for the use of the so-called “ring pontoon,” which is not necessarily circular. In most cases, the term “ring pontoon” is applied to square or rectangular pontoons that are defined by four interconnected pontoon portions. The object of the pontoons is to facilitate stability of a vessel in deep waters.  
         [0004]     Various solutions have been offered to solve this problem. For instance U.S. Pat. No. 4,040,265 discloses a jack-up platform supported by legs that extend down and engage a mat structure resting on an ocean floor. The mat is a rectangularity-shaped structure having a plurality of chambers, where hydrocarbons may be stored. The end portions of the mat extend beyond the position of the support legs.  
         [0005]     U.S. Pat. No. 4,907,912 illustrates another example of a jack-up rig using a submersible storage barge. During transit, the submersible storage barge-provides the flotation means for the rig hull. Once the unit reaches the drill site, the submersible barge is lowered together with the support legs until they rest upon a sea bottom. The submersible barge is at least flooded to stabilize its position on the sea bottom.  
         [0006]     U.S. Pat. No. 6,374,764 discloses a self-floating offshore structure with one or more pontoons that are provided with lifting supports. The lifting supports, which may be rack-and-pinion gear driven jacks, telescoping hydraulic rams, system of cables and pulleys, are attached to a deck and move the deck relative to the pontoons. The pontoons may be lifted out of the water to provide an additional deck area.  
         [0007]     While these designs work satisfactory under certain conditions, there exists a need for an offshore structure that is particularly adapted for development of small marginal fields that are located in deep waters and harsh environment.  
       SUMMARY OF THE INVENTION  
       [0008]     It is an object of the present invention to provide a semi-submersible unit that has enhanced resistance to wave actions.  
         [0009]     It is another object of the invention to increase the added mass of the unit in a simple and cost effective way by using the second tier pontoon (STP) in the ESEMI. ESEMI allows the added mass of the vessel to be increased without any increase of displacement, as would be the case with conventional semi-submersibles designs. Instead of increasing the displacement to achieve higher added mass, this invention seeks to separate the total displacement into two different locations in the operation mode. One part of this displacement is located in the conventional semisubmersible hull and the other part, called STP, is situated at a certain distance below the hull. This separation of the total displacement results in higher overall added mass for the ESEMI compared to the conventional semi-submersible of similar displacement.  
         [0010]     It is another object of the present invention to provide an offshore structure that has significantly reduced motion responses to wave actions.  
         [0011]     Solution to the above-mentioned problems is given according to one embodiment of the invention as defined in the patent claims below.  
         [0012]     The major wave- and current-influenced forces act on the columns and the pontoons at the upper levels below the sea surface. These forces normally decrease with increasing depth below the surface. The vessel of the present invention provides for the use of a two-tier pontoon system. An upper ring pontoon supports stabilizing columns, which in turn support the upper hull and the deck structures. The lower pontoon moves vertically in relation to the upper pontoon through the use of an independent ballasting system housed in the lower, or second tier pontoon. By dropping the second tier pontoon (STP) to a depth well below the sea surface, preferably below the most strongly wave-influenced zone, the semi-submersible unit of the present invention achieves motion response characteristics normally only achieved by large purpose built fixed units.  
         [0013]     According to a preferred embodiment of the invention, the second tier pontoon supports a plurality of vertically movable retractable legs. The legs move through leg guides carried by exterior of the stabilizing columns and the upper pontoon. The legs ensure that the second tier pontoon is retained in a parallel relationship to the upper pontoon.  
         [0014]     When in transit, the lower buoyant pontoon is secured in close proximity to the upper pontoon, and the legs are fully retracted. Once the vessel reaches the operational site, the second pontoon is ballasted and lowered such that the upper pontoon baseline is about 32 meters above the second tier pontoon baseline. The second tier pontoon remains locked in relation to the upper pontoon and the upper hull through a series of winches tensioning handling lines, which connect the second tier pontoon to the main deck.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The invention is illustrated in the following drawing figures stating non-limiting examples of an embodiment of the invention.  
         [0016]      FIG. 1  is a perspective view of the semi-submersible vesel in accordance with the present invention in transit confition.  
         [0017]      FIG. 2  is a perspective view of the semi-submersible vesel in accordance with the present invention in operating condition.  
         [0018]      FIG. 3  is profile of the semi-submersible vessel of the present invention in transit condition.  
         [0019]      FIG. 4  is a bow view of the vessel of the present invention in operating condition.  
         [0020]      FIG. 5  is a plan view fo the main deck of the vessel of the present invention.  
         [0021]      FIG. 6  is a schematic view of the main pontoon bottom; and  
         [0022]      FIG. 7  is a schematic view of the second tier pontoon bottom. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0023]     With reference to the drawings in more detail, the semi-submersible vessel of the present invention is designated by numeral  10 . The vessel or unit  10  comprises an upper hull  11  having an operating deck  12 . The upper hull  11  is designed as a big box or grillage structure. The hull  11  and the deck  12  support various machinery and equipment, as well as crew housing. For handling goods delivered by supply ships there are cranes  14 .  
         [0024]     The upper hull  11  and the operating deck structure  12  are carried by four vertical columns  16 , which in turn are supported by a main pontoon structure  18  formed as a square ring Each column  16  has a rectangular cross-section and extends from a corner of the square ring pontoon  18 . The main pontoon  18  is comprised of four box-like structures, each having a four-sided cross-section. One of the main advantages of the ring pontoon is a significant torsional strength in the vessel structure, which is particularly important in harsh environments. The main pontoon  18  has a quadrate opening  20  allowing production risers  22  to extend therethrough.  
         [0025]     The main pontoon  18  houses a plurality of ballast tanks  24 , storage tanks  26 , pump rooms  28  located in each corner of the pontoon  18  and associated machinery for delivering, storing and exporting water and hydrocarbons necessary in the operation of the production vessel  10 . The main pontoon  18  is rigidly secured to the columns  16 .  
         [0026]     A second tier pontoon (STP)  30  is secured in parallel vertically spaced relationship to the main pontoon  18 . The STP  30 , similarly to the main pontoon  18  is made of four box-like structures  32 ,  34 ,  36 , and  38 , each having a four-sided cross-section. The pontoon  30  has a quadrate opening  40  vertically aligned with the quadrate opening  20  of the main pontoon  18  to allow production risers  22  to extend therethrough. The second tier pontoon  30  has a shallower depth than the main pontoon  18 .  
         [0027]     The portion  32  of the pontoon  30  has a pair of horizontal extensions  42 ,  44 . The portion  36  of the pontoon  30  has a pair of horizontal extensions  46 ,  48 . The extensions  42 ,  44 ,  46  and  48  each support a vertically extending retractable extension leg  50 , such the bottom of each retractable leg  50  rests on a respective extension  42 ,  44 ,  46 , or  48 .  
         [0028]     The legs  50  connect the upper hull  11  and the main pontoon  18  with the second tier pontoon  30 . A guide member  52  is attached to each of the columns  16 ; a guide member  54  is attached to the main pontoon  18  in vertical alignment with the guide member  52 . Each leg  50  is received through an opening formed by the guide member  52  and the guide member  54 , The legs  50  move in relation to the hull  11  and the main pontoon  18 , thereby moving the second tier pontoon  30  to the operational draft.  
         [0029]     When in transit ( FIGS. 1 and 3 ), the STP  30  is located a short distance below the main pontoon  18 . In transit draft, the deck  56  of the main pontoon  18  is lightly above the water line  58 . The main pontoon baseline  60  is submerged below the water surface, while the baseline  62  of the second tier pontoon is located about 3 m below the main pontoon baseline  60 .  
         [0030]     The second tier pontoon  30 , similarly to the main pontoon  18 , is divided by vertical and transverse bulkheads into a plurality of compartments that house ballast tanks  70 . Suitable pumps in pump rooms  72  are provided facilitate variable ballasting of the tanks  70 .  
         [0031]     When the vessel  10  is deployed at the operating site, the second tier pontoon  30  is lowered well below the water line  58 , such that the baseline  62  of the second tier pontoon  30  is about 32 m below the baseline  60  of the main pontoon  18 . To facilitate mooring and handling of the second tier pontoon  30 , the vessel  10  is equipped with eight mooring lines  74 , two at each corner of the vessel  10 . A handling line  76  is provided for each extension  42 ,  44 ,  46 , and  48 . One end of the STP handling line is secured in an attachment member  78  mounted on the top surface of each extension member  42 ,  44 ,  46 , and  48 .  
         [0032]     The vessel  10  is designed to operate in water depths up to 2500 m using a pre-laid mooring system. To facilitate station keeping of the vessel  10 , there are provided four double drum traction winch assemblies  80 . Wire storage reels are located inside the columns  16 . The winch assemblies  80  serve dual purpose: they carry mooring lines  74  and the pontoon handling lines  76 . Each winch assembly  80  uses the double traction winch for mooring and a single traction winch for handling of the second tier pontoon  30 . Preferably, there is one common shaft for driving the traction winches of the STP handling winches connected individually and in turn through external clutches.  
         [0033]     The winch assemblies  80  are driven by variable speed motors of conventional design. Control panels for each winch assembly  80  are operationally connected to a centralized control panel to synchronize tension in the mooring lines  74  and pay out of the STP handling lines  76 .  
         [0034]     The process of lowering the second tier pontoon provides for the use of a static holding brake (not shown), which is part of each winch assembly  80 . The second tier pontoon is free-flooded from se chests provided in the second tier pontoon.  
         [0035]     Once the operator ensures that the ballast water amount does not exceed a pre-determined amount, the locking pins of the winch assemblies are released. With the help of a dynamic brake, the handling lines  76  are unwound from the winches  80  and the second tier pontoon is allowed to submerge to the desired depth under water. Once the pre-determined depth has been achieved, the operator applies a static holding brake and engages a locking mechanism (not shown) associated with each handling line  76 .  
         [0036]     While the pontoon  30  is being lowered, the legs  50  slidably move within the guide members  52 ,  54 , retaining parallel position of the second tier pontoon  30  in relation to the hull  11  and the main pontoon  18 .  
         [0037]     To raise the second tier pontoon  30 , the operator applies a static holding brake and unlocks the pins of the retractable legs  50 . The second tier pontoon is de-ballasted, while the winch assemblies  80  maintain tension of wires  76 , raising the second tier pontoon to a draft of about  26  meters. The static holding brake is then applied, and the second tier pontoon is again de-ballasted. With the winch assemblies  80  pulling on the handling lines  76 , the second tier pontoon is raised again to a draft of about 10.5 meters.  
         [0038]     This process continues until the second tier pontoon  30  reaches the desired elevation. Then, the static holding brakes are applied and the second tier pontoon  30  is locked to the columns with the help of special pins, which are capable of taking the STP static weight and the dynamic forces. It is envisioned that the pumps of the second tier pontoon  30  will be capable of complete de-ballasting of the second tier pontoon to facilitate retraction of the pontoon  30  in eight hours.  
         [0039]     The ballast system of the main pontoon and the second tier pontoon is capable of restoring the vessel  10  to a normal operating condition or transit draft and a level trim condition, when subject to damage and flooding conditions. In case of emergency, de-ballasting and retraction of the second tier pontoon may be accomplished in about 4 hours.  
         [0040]     The vessel  10  provides for two functionally different ballast systems: first is located in the second tier pontoon and its purpose is to facilitate deployment and retraction of the second tier pontoon. The second ballast system is provided in the main pontoon; its purpose is trim adjustments, ballast and de-ballast between transit, operational and survival drafts and to enable restoration of the unit from damaged conditions.  
         [0041]     Once the second tier pontoon is extended to the desired water depth and locked to the main pontoon  18  and the hull  11 , the dual-purpose winch assemblies  80  are changed over to use in mooring the vessel  10  at the deployment site.  
         [0042]     The vessel  10  of the present invention allows for the topside, the main pontoon and the second tier pontoon to be manufactured at a shipyard as an integrated unit, which eliminates the need for integration on location. As a result, significant time period can be saved for a vessel owner. The second tier pontoon not only provides a larger added mass but also significantly reduces the vessel&#39;s motions. The vessel motion response is substantially equal to responses that may be achieved by large purpose built fixed bottom units. The lower motion characteristics translate into less “down time” and riser fatigue.  
         [0043]     The components forming part of the vessel in accordance with the present invention may be varied within the scope of the appended claims.