Abstract:
A new jacket installation method is disclosed, especially for shallow water heavy jacket installation applications. The method utilizes a special type of air bags, called launching air bags, to provide low cost net buoyancy in order to make a shallow water jacket self afloat with a sufficient bottom clearance with seabed. At a floating state a minimum size installation crane vessel could be used to perform all needed jacket installation operations such as lowering, setting-down to seabed, driving foundation piles, grouting and the welding between foundation piles and jacket leg tops. For a shallow water heavy jacket installation, a crane vessel cost usually represents a majority cost of a jacket installation. Therefore, a significant savings could be achieved if a minimum size crane vessel could be used, instead of a large size crane vessel.

Description:
FIELD OF THE INVENTION 
       [0001]    The disclosure relates generally to an improved method for installing offshore fixed platforms, more particularly for shallow water jacket installation applications. 
       BACKGROUND OF THE INVENTION 
       [0002]    An offshore platform is generally composed of two sections: 1) a substructure such as a jacket for a fixed platform, and 2) a superstructure such as a deck to be installed on the top of a substructure. 
         [0003]    A deepwater substructure, deeper than 60 meters (about 200 ft) in water depth, of a fixed platform is normally fabricated as a single unit with battered leg onshore in a horizontal orientation and then skidded onto a transport vessel or a launch vessel, towed to the installation site in a horizontal orientation, launched or lifted off from the vessel, and placed at the seabed before upending/ballasting of the jacket to a vertical position. Finally, foundation piles are driven to fix the jacket with the seabed by grouting or welding. 
         [0004]    A shallow water substructure, less than 60 meters (about 200 ft) in water depth, of a fixed platform is normally fabricated as a single unit with vertical legs onshore in a vertical orientation and then skidded onto a transport vessel or a semi-submersible vessel, towed to the installation site in a vertical orientation, lifted off the transport vessel deck, or lifted off the semi-submersible vessel deck when it is submerged to a design draft, and placed at the seabed in a vertical orientation throughout the installation operations. Finally, foundation piles are driven to fix the jacket with the seabed by welding between foundation piles and jacket leg tops. 
         [0005]    For a typical shallow water jacket configuration, especially a large sized one, it is very difficult to gain sufficient net buoyancy. Therefore, a large crane installation with a lifting capacity larger than the weight of the jacket has to be utilized to lift the jacket as a whole off the transport vessel deck, or the semi-submersible vessel deck, and to place the jacket at the seabed. 
         [0006]    In recent years, shallow water jackets get heavier and heavier because the associated deck weights also get heavier and heavier. In many cases, the jacket weights exceed the lifting capacity of available crane vessel(s) and alternative jacket installation methods have to be considered. One common alternative method is to launch the jacket. If the launching method is adopted, the jacket orientation on the transport vessel is usually changed to a horizontal orientation. In addition, it has to face two common challenges: 
         [0007]    1. The jacket has to be a self afloat structure with necessary reserve buoyancy (usually &gt;12%, defined as (submerged buoyancy−total weight)/submerged buoyancy %). In order to satisfy this requirement, a large number of steel-made buoyancy tanks have to be installed and connected to the jacket and to make this jacket even heavier. Ballast tanks and flooding/venting systems have to be designed in order to lower the jacket to seabed through ballasting operations. These buoyancy tanks have to be removed after the installation and transported back onshore at a considerable cost. Other costs include fabrication and installation of the buoyancy tanks and the design and fabrication of the ballast tanks. Another issue is that the weight of steel makes the steel-made buoyancy inefficient to produce net buoyancy and very costly for each ton of net buoyancy. For example, one ton steel used for making buoyancy tanks could typically produce 3-ton buoyancy. If deducting the steel weight, each ton of steel could produce only 2-ton net buoyancy. Adding other costs such as design, fabrication, flooding/venting system, welding to a jacket, offshore cutting to remove from the jacket, lifting and the use of a transport vessel for returning the tanks back, the total cost of using buoyancy tanks could be very high. 
         [0008]    2. Due to the shallow water at the installation site, a launched jacket could easily hit the seabed during the launch operation. In such cases, the jacket is usually towed to a deeper water location, launched and wet towed from the launching site to the installation site. If the launching site is far away from the installation site, the cost associated with the wet tow could be high. 
         [0009]    A heavy shallow water jacket could be launched in a vertical orientation. However, it would require larger reserve buoyancy (&gt;20%) and the attached buoyancy tanks have to be placed at very low position, to pick up buoyancy immediately after the launch, which would impose extra difficulty for removing these buoyancy tanks because they would be all submerged after the launch. 
         [0010]    Therefore there is a need for a shallow water jacket installation method that is more efficient in producing net buoyancy and cost effective. 
       SUMMARY OF THE INVENTION 
       [0011]    An offshore jacket installation method using non-steel buoyancy tanks is disclosed. A special type of air bags, called launching air bags (SLAB), is utilized as buoyancy tanks to replace the steel buoyancy tanks. SLAB buoyancy tanks provide low cost net buoyancy in order to make a shallow water jacket self afloat with a sufficient bottom clearance with seabed. 
         [0012]    The jacket installation method includes preparing a plurality of non-steel buoyancy tanks for the installation, installing the prepared non-steel buoyancy tanks on the jacket, injecting air into non-steel, buoyancy tanks to achieve a predetermined internal air pressure level, transporting the jacket to the installation site with a transportation apparatus, removing the jacket from the transportation apparatus to let the jacket becomes self afloat with positive reserve buoyancy, lowering the jacket to the seabed, releasing air from each non-steel buoyancy tank to reach another predetermined internal air pressure level, and removing non-steel buoyancy tanks from the jacket. All attached non-steel buoyancy tanks together should contribute a reserve buoyancy greater than 20% of the jacket total reserve buoyancy (combining the one contributed by non-steel buoyancy tanks with the others contributed by the jacket members) when it is in a self-floating condition. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The drawings described herein are for illustrating purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. For a further understanding of the nature and objects of this disclosure reference should be made to the following description, taken in cornunetion with the accompanying drawings in which like parts are given like reference materials, and wherein: 
           [0014]      FIG. 1A  is a side view of a conventional Ship Launching Air Bag; 
           [0015]      FIG. 1B  is a side view a front end cone structure of the conventional SLAB in  FIG. 1A ; 
           [0016]      FIG. 1C  is a side view of a conventional SLAB with “ears”; 
           [0017]      FIG. 2A  is a side view of a conventional shallow water jacket with a large opening at jacket upper portion for a topsides floatover installation; 
           [0018]      FIG. 2B  is a front view of a conventional shallow water jacket with a large opening at jacket upper portion for a topsides floatover installation; 
           [0019]      FIG. 2C  is a plan section view of the bottom horizontal frame of the jacket; 
           [0020]      FIG. 3A  is a side view of the jacket with Type I buoyancy tanks (four SLABs as a group); 
           [0021]      FIG. 3B  is a front view of the jacket with the Type I buoyancy tanks; 
           [0022]      FIG. 4A  is a side view of the jacket with an alternative arrangement of Type I buoyancy tanks (three SLABs as a group); 
           [0023]      FIG. 4B  is a front view of the jacket with an alternative arrangement of Type I buoyancy tanks; 
           [0024]      FIG. 5A  is a cross section view of a single SLAB with one pair of side steel rings bonded to the SLAB middle section; 
           [0025]      FIG. 5B  is a front view of 6 single SLABs connected with side rings ready to wrap up with a jacket leg member; 
           [0026]      FIG. 5C  is a front view of 6 single laterally connected SLABs wrapped up with a jacket leg member; 
           [0027]      FIG. 5D  is a cross section view of a Type II buoyancy tank wrapped with a jacket leg member with stoppers; 
           [0028]      FIG. 5E  is a cross section view of a Type II buoyancy tank wrapped with a jacket leg member without stoppers; 
           [0029]      FIG. 6A  is a plan view of a semi-submersible vessel loaded with a shallow water jacket installed with Type I buoyancy tanks near the stern; 
           [0030]      FIG. 6B  is a side view of the semi-submersible vessel loaded with a shallow water jacket near the stern; 
           [0031]      FIG. 6C  is a front view of the semi-submersible vessel loaded with a shallow water jacket installed with several Type I and Type II buoyancy tanks; 
           [0032]      FIG. 6D  is a side view of the semi-submersible vessel with vessel deck submerged below water surface and the jacket afloat and off the vessel deck; 
           [0033]      FIG. 7A  is a side view of a crane vessel with an initial lift to the floating jacket at a designed hookload; 
           [0034]      FIG. 7B  is a side view of a crane vessel with lowering of the jacket at the seabed with several Type I and Type II buoyancy tanks; 
           [0035]      FIG. 7C  is a side view of a crane vessel separated from an installed jacket after the installation is completed; 
           [0036]      FIG. 8A  is a plan view of a launch barge loaded with a jacket and several Type I and Type II buoyancy tanks near the stern of the barge; 
           [0037]      FIG. 8B  is a side view of the launch barge loaded with a jacket and several Type I and Type II buoyancy tanks near the stern of the barge; 
           [0038]      FIG. 8C  is a front view of the launch barge loaded with a jacket and several Type I and Type II buoyancy tanks near the stern of the barge; 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0039]    Ship building in sand beaches started in 1980&#39;s in Southern China. Builders place wood blocks on a sloped sand beach and start ship construction on the tops of these blocks with land cranes. When the construction is complete, a special type of air bags, Ship Launching Air Bags (SLAB), would be placed under the ship keel longitudinally between two rows of wood blocks. Injecting air to these SLABs, the ship should be lifted off the wood blocks. After the lifting operation, the wood blocks would be then removed off the ship keel. Once cutting holding lines, the ship will be launched toward the sea along with the rolling of these SLABs. 
         [0040]    The ship launch using SLAB&#39;s method has been successfully deployed in China for quite some time already. Recently, the application of SLAB has expanded to other areas, such as ship salvage, a floatation tool for the transportation of a large concrete structure for a bridge. In these applications, “ears” used for tying-up with other structures are added on the middle section surface of the SLAB. These “ears” usually use the same material such as nature rubber and polyester nets and experience a vulcanization process together with the middle section in order to be bonded together. Nowadays, the SLABs have become a mature and off shelf product in China shipbuilding industry with excellent characteristics, such as light in weight, durable, scratch resistant, and tolerant of high internal pressure, etc. 
         [0041]    A standard SLAB  100  is made of a tubular middle section and two cone sections at the ends.  FIG. 1A  illustrates one embodiment of a standard SLAB  100 . As shown in  FIG. 1A , a standard Ship Launching Air Bag  100  is divided into three sections: a front cone section  101 , a middle section and a back cone section. The length of the middle section varies for each application. 
         [0042]    As shown  FIG. 1B , the front cone section  101  comprises a steel cone structure  103  covered with rubber layers with several attachments such as an air valve  105  for air inlet and exit, and an air pressure meter  104 . The back end steel cone is similar to the front cone section with a steel ring  106  attached at the end for handling the SLAB  100 . The back end cone section does not have the pressure meter or the air valve. 
         [0043]    The middle section and the surfaces of the two end sections are made of nature rubber mixed with several layers of polyester nets. The cone structure with nature rubber/polyester net layer is totally bonded with the steel cone structure  103  through a vulcanized process. During the SLAB  100  assembling process, the air bag is put into a sealed container with high temperature for a predetermined duration with a vulcanization process to make the rubber layers tightly bonded with the cone steel surfaces at both ends and the rubber bonded with layers of polyester nets at the middle section and the two end sections. 
         [0044]      FIG. 1C  is a side view of a SLAB  100  used as a buoyancy tank in offshore jacket installation. Several rows of “ears”  107  are circularly arranged at the surface of SLAB  100  middle section. These ears are utilized for tying up the SLAB  100  with other SLABs  100  or with other jacket structural members. 
         [0045]    This disclosure describes a method that applies SLABs  100  in offshore jacket installation. Referring now to  FIGS. 2A through 2C , a shallow water jacket  200  has horizontal structural members  201  and corner main jacket legs  202 . The top portion of the jacket  200  has a large opening used for deck offshore floatover installation. At the bottom layer of the jacket  200 , four mud mats  209  are located. 
         [0046]    In jacket designs, the lack of sufficient reserve buoyancy is always a big concern for all jackets. For shallow water large jacket, this concern becomes even greater. Most shallow water large jackets could not be designed to be self afloat (reserve buoyancy negative). Therefore, a large lifting capacity crane vessel is usually needed to perform the lifting operation as a part of the requirement for the offshore installation for these jackets. However, the lifting operation usually takes a majority of the jacket installation cost and the lifting operation is the only part of the jacket installation which needs a large capacity crane vessel. For all other tasks such as foundation pile installation and grouting operation, a small capacity crane vessel should suffice. Sometimes, a large capacity crane vessel may not be available locally for a large jacket offshore installation. 
         [0047]    In order to utilize a small capacity crane vessel for the complete installation of a large shallow water jacket, the jacket has to be self afloat with positive reserve buoyancy. In this disclosure, a new type of buoyancy tanks, SLAB buoyancy tanks, is introduced to replace conventional steel buoyancy tanks because SLAB buoyancy tanks have many advantages over conventional steel buoyancy tanks: 
         [0048]    1. More efficient in producing net buoyancy—with conventional steel buoyancy tanks each ton of steel-made buoyancy tank could produce about 2-ton of net buoyancy, whereas each ton of SLAB buoyancy tanks could produce more than 60-ton net buoyancy; 
         [0049]    2. Easy installation and offshore removal—without welding and offshore cutting, SLAB buoyancy tanks only requires to be tied up with jacket members which make the installation and offshore removal of SLAB buoyancy tanks easy. For underwater applications, ROV (Remote Operational Vehicle) could be used to cut off the tie-up connections and recover SLAB buoyancy tanks without the assistance of divers; 
         [0050]    3. Reusable at low cost—SLAB is designed for multiple uses. Therefore, the total cost of SLAB buoyancy tanks could be a small fraction comparing with conventional steel buoyancy tanks for jacket installation applications. 
         [0051]    Equipped with sufficient SLAB buoyancy tanks, a large shallow water jacket could be launched in a shallow water condition and the jacket could also be transported and floated-off from the deck of a semi-submersible vessel in a shallow water location. 
         [0052]    The key issue in applying SLABs in offshore jacket installation, especially in large shallow water jacket installation, is to develop a tie-up method between SLAB buoyancy tanks and jacket members, in which the tie-up connections should be strong enough to take potential loads such as jacket launching and these SLAB buoyancy tanks should also be easily released and recovered after an offshore jacket installation is complete. 
         [0053]    There are two common functions for buoyancy tanks: 1) the increase of reserve buoyancy to the jacket during the jacket installation operation: 2) the increase of jacket floating stability through an enlarged water plane area of the jacket during floatation at water surface. Accordingly, two different tie-up methods are introduced in this disclosure: Type I method for Type I SLAB buoyancy tanks and Type II method for Type II SLAB buoyancy tanks. The main objective of the Type I tie-up method is to increase the reserve buoyancy of a jacket in order to make it afloat. The main objective of the Type II tie-up method is to increase the floating stability of a jacket. However, easy tie-up and offshore recovery are the basic requirements for both methods. 
         [0054]    For Type I SLAB buoyancy tanks which aim to increase the jacket reserve buoyancy, a number of large diameter SLABs placed in a horizontal orientation, are tied-up together as a buoyancy tank group. In one embodiment, the SLABS are tied up through the “ears” at SLAB surfaces. The SLABS maybe tied up through other means. The locations of these grouped buoyancy tanks should be placed as low as possible inside a jacket bottom structure. 
         [0055]    For Type II SLAB buoyancy tanks which aim to increase the jacket floating stability when the jacket is afloat, they are usually placed near the water suffice area along jacket corner main legs. In one embodiment, the Type II SLAB buoyancy tanks, usually placed in a vertical orientation, are tied-Lip with jacket main corner legs near the upper portion of these jacket main legs. 
         [0056]    With ample and lower positioned reserve buoyancy for a jacket, the jacket could be launched in a shallow water condition with a vertical orientation and with a sufficient bottom clearance to seabed. This self-vertical floatation configuration in post launch condition simplifies the offshore operation and saves offshore installation time. 
         [0057]    In additional to the launch method for a shallow water jacket described above, as semi-submersible vessel could also be used for a jacket transportation and installation. The semi-submersible vessel loaded with a shallow water jacket transports the jacket from the fabrication yard to an installation location, then submerges her deck below the water surface and the jacket then floats off the vessel deck. 
         [0058]    Referring now to  FIG. 3A  and  FIG. 3B , six groups of Type I SLAB  100  are located near the bottom portion of the jacket  200 , in  FIG. 4A  and  FIG. 4B , an alternative Type I SLAB application is illustrated. Each group of Type I SLAB  100  is tied-up together through ears  107  or other means. Based on the size of the each group, restrain structural members  208  are installed. Each group of SLABs  100  is in a flat condition during the installation. Once properly placed inside the restrain structural members  208 , air is injected and the group of SLABs  100  is expanded and totally restrained by the restrain structural members  208 . No physical tie-up between the group of SLABs  100  and the jacket  200  members is needed to form the Type I SLAB buoyancy tanks. During transportation, a pre-determined air pressure will be maintained for all SLAB  100  tanks. Once arrived at the installation site, re-injection of air to some SLAB  100  tanks should be available with a pre-installed air compressor and an associated piping system. 
         [0059]    Once the installation of jacket  200  is completed offshore, air in each group of SLABs  100  will be released through a control center located at the top of the jacket  200 . Once a group of Type I SLABS in flat condition, this group of Type I SLABs should be easily towed out from the restrain structure  208  by a tug from the side of the jacket  200  by a wire connected to rings  106  at one end of these SLABs. Air release should be controlled so that some residual air makes the SLAB group afloat and floating at water surface for easy recovery. 
         [0060]    Referring now to  FIG. 5A , a single SLAB  100  with a pair of side rings  203  are prepared for a Type II SLAB  100  application. The side rings  203  are totally bonded with the SLAB  100  middle section. A Type II SLAB may have multiple pairs of side rings  205  along the surface of middle section with a designed distance apart to each other. 
         [0061]    Referring to  FIG. 5B , six SLABs  100 , each with three pair of side rings  203 , are connected to form a sheet to wrap around a jacket main leg  202 . The side rings  203  of the SLABs  100  are connected through a connection wire  204 . The length of the connection wire  104  between adjacent SLABs  100  is specially designed so that the tightness of the SLAB  100  sheet wrapping around the jacket leg is as designed. 
         [0062]      FIG. 5C  illustrates one embodiment of the final installed configuration of Type II SLAB  100  buoyancy tanks. Prior to the installation, all SLABS  100  are flat and six stoppers  205  are installed at the jacket leg  202  with the same elevation as the top row of these side rings  203 . Additional six stoppers  205  are also installed at the jacket leg  202  with the same elevation as the bottom row of these side rings  203  The stoppers  205  are made of steel tubular members with designed cut-off at the top to let the connection wire  204  through. The purpose of the stoppers  205  is to restrain the vertical movement of Type II SLABs  100  along the jacket leg  202  by the connection wires  204 . During the installation of Type II SLAB  100  buoyancy tanks, all SLABs  100  are kept flat. After all connection wires  204  are installed and connected with associated stoppers  205 , air will be injected to all SLABs  100  and these SLAB  100  buoyancy tanks are tightly wrapped around jacket legs  202 . During transportation, a predetermined air pressure will be maintained for all SLAB  100  tanks. Once arrived at the installation site, re-injection of air to some tanks should be available with a pre-installed air compressor and an associated piping system. 
         [0063]      FIG. 5D  shows the cross section view of the final installed Type II SLAB  100  buoyancy tanks with associated stoppers  205 . As described above, the function of these stoppers  205  is to restrain the vertical movement of the SLABs  100 . At the tops of the stoppers, special cutting is made to restrain vertical movement of these connection wires  204 , but leave a room for Remote Operational Vehicle (ROV) under water cutting if needed.  FIG. 5E  shows the cross section view of the final installed Type II SLAB  100  buoyancy tanks without the associated stoppers  205 . 
         [0064]      FIG. 6A  through  FIG. 6D  illustrate the transportation and offshore installation of a shallow water jacket, equipped with SLAB  100  buoyancy tanks, using a semi-submersible vessel. 
         [0065]    Referring to  FIG. 6A  through  FIG. 6C , a semi-submersible vessel  300  is loaded with a shallow water jacket  200  near the stern. Two stability columns  301 , usually located at the stern, are relocated in front of the jacket  200 . The jacket  100  sits on eight support blocks  305  plus some seafasternings during the transportation. Type I buoyancy tanks and Type II buoyancy tanks are pre-installed with a predetermined air pressure before the sail. After arrival at the installation site, air pressure inside each SLAB  100  should be readable and air maybe re-injected, if necessary, to the SLABs  100  to make their internal pressure at the predetermined pressure level. Associated equipment such as an air compressor and a piping system is needed for this air re-injection operation. 
         [0066]    Looking at  FIG. 6D , as the deck of the semi-submersible vessel  300  is submerged under water surface  310  to a predetermined water depth, the jacket  200  becomes afloat and could be moved off the vessel  300  by pre-connected tugs  309  or a crane vessel  312  with a lifting hook  302 . 
         [0067]    Referring to  FIG. 7A  through  FIG. 7C , a sequence of site jacket  200  installation is illustrated. Once the crane hook  302  is connected with jacket  200  top padeyes by lifting slings  303 , a hook load is applied to the jacket top in order to control the floating jacket  200  during the lowering operation. 
         [0068]    Looking at  FIG. 7B , a constant hook load is maintained during the lowering process until the jacket  200  sits on seabed  311  for remaining jacket  200  installation activities such as pile driving, grouting operation and weld-off between jacket  200  leg tops and foundation pile tops. During the time as the lowering operation, air will be released from proper SLAB  100  tanks through a control center at the jacket top to balance the required total buoyancy. During the driving piles and grouting operation period, the air pressure inside each SLAB buoyancy tanks is maintained at a designed level. 
         [0069]    Referring to  FIG. 7C , after the installation completion, air will be released from all SLAB  100  buoyancy tanks. In one embodiment, all tanks are flat. In another embodiment, all tanks are at a predetermined low air pressure in order to be afloat. For Type I buoyancy tanks, a tug  309  will be utilized to pull out each Type I tank group using a pre-installed wire set connected to SLAB rings  106 . For all Type II tanks, one cutting of the connection wire  204  at middle elevation and two cuttings of the connection wires  204  at top and bottom elevations for each whole Type II SLAB  100  buoyancy tank should be sufficient to release and recover it. These recovered buoyancy tanks could be re-used for next application. When the installation is completed, the crane vessel  312  will be de-mobilized from the installation site. 
         [0070]    Referring to  FIG. 8A  through  FIG. 8C , a launch barge  306 , loaded with a shallow water jacket  200 , is towed by a tug  309  from a jacket fabrication yard to a jacket installation site. The launch barge  306  is equipped with two rocker arms  307  and two sets of launchways  308  at barge deck surface. The jacket  200  has two matching launch cradles with the two launchways  308 . Once arrived at the installation site, launch barge  306  will be ballasted down with a designed trim by the stern. Jacket  200  will be launched at a shallow water condition and the jacket  200  will be afloat in a post launch condition with a vertical orientation. The rest of the jacket  200  installation activities will be the same as described for ones using a semi-submersible  300  vessel. 
         [0071]    The present invention has been described in terms of specific embodiments incorporating details to facilitate the understanding of principles of construction and operation of the invention. Such reference herein to specific embodiments and details thereof is not intended to limit the scope of the claims appended hereto. It will be readily apparent to one skilled in the art that other various modifications may be made in the embodiment chosen for illustration without departing from the spirit and scope of the invention as defined by the claims.