Patent Publication Number: US-6668746-B1

Title: Lifting vessel and method for positioning, lifting and handling a platform deck and a jacket

Description:
The present invention is related to a lifting vessel having a U-shaped pontoon, with two longitudinal and one transversal pontoon, and columns connected to the pontoon extending upwards and through the water surface. 
     The present lifting vessel is designed with a hull where the buoyancy can be adjusted by ballasting/deballasting, for lifting and transportation operations at sea. 
     The invention includes also methods for positioning, lifting and handling a platform deck and a jacket, with the help of the lifting vessel of the present invention. 
     In connection with offshore activities such as gas and oil exploitation it is usual to install platforms on the field. These platforms often consist of large and heavy platform substructures fixed to the seabed. Such a platform substructure is normally a so-called “jacket”, which is a steel truss structure. On top of for example a jacket it is usual to place a platform deck, which is used in connection with drilling and production. The deck also often includes living quarters. 
     To transport and install the jacket and the platform deck described above, for example barges have been used to transport the jacket and platform deck out to the field, and large crane vessels have been used to install the platform on the field. 
     Heavy lift vessels using ballast to vary their draft have also been used to transport and install platforms offshore. 
     There are today a great number of offshore platforms installed to exploit oil and gas. When the oil and/or gas reservoirs are fully exploited the life span of the platform is usually over and it would in most cases be appropriate to remove the platform. 
     Some platforms are already removed, and removal of platforms will continue at an increasing pace the coming years. 
     The traditional way of removing platforms is to use large ocean going lifting cranes. The platform needs to be very thoroughly prepared prior to removal, and it must be cut into smaller parts since even the largest lifting crane vessels have limited lifting capacity. The same goes for the platform substructure (the jacket). 
     These operations are time consuming and costly, not only because the lifting cranes are large, expensive and need a large crew, but also because cutting a platform to smaller pieces in open sea is a very complicated task. It is also a risky operation. 
     The new technology, as described in this application, can be described as “single lift technology”, and will reduce the costs considerably. It will also make the operations less risky than present alternatives. Within the category “single lift technology” there are three other concepts that the applicant is aware of at the moment: 
     “Offshore Shuttle” is a vessel planned built as a frame work structure. The vessel has a significant length and the lifting of for example a platform deck is based on crossbeams spanning across the structure. 
     “Master Marine” is developing a U-shaped semi submersible with deck-structure connecting the top of columns. Lifting is based on load transfer to the deck-structure. 
     “Versatruss” is a concept involving two separate barges each supporting its own lifting frame. By pulling the barges together after positioning the lifting frames beneath the lifting points on the platform deck, the lifting of the deck can be performed. This method has already been used to remove small platform decks in calm waters. 
     One object of the present invention is to accomplish a removal operation of a platform in a fast and cost effective manner without cutting either the deck or the jacket into smaller parts. The removal operation shall be performed in a safe way where the safety of the operators is accomplished in the best possible way. 
     Another object of the present invention is that the lifting vessel is as flexible as possible and that it can be easily adjusted to fit different sized platform decks. Further the lifting vessel shall be able to lift and handle jackets of different sizes. In accordance with the invention the lifting vessel, a so-called Multi Purpose Unit, MPU, which also can transport e.g. the platform deck to shore, and then transfer the deck to a barge or a pier suitable to the vessel. 
     Another object is that the lifting vessel shall be able to be transported on a heavy lift vessel, to reduce the travel time to distant destinations like the Gulf of Mexico and the Persian Gulf. 
     Another object of the lifting vessel is that it shall also be able to install platforms, which basically is the reverse of removal. The lifting vessel should furthermore be applicable to a range of purposes where a large lifting capacity is required. 
     The objects described above is achieved according to the invention by lifting vessel with a U-shaped pontoon, a number of columns connected to the pontoon and extending upwards towards and through the water surface, characterised by the columns not being structurally connected above the pontoon. 
     Preferred embodiments of the lifting vessel is further described in the claims  2  to  12 . 
     The objects of the invention is further achieved by methods of positioning, lifting and handling of a platform deck and a jacket according to claims  13  and  14 . 
    
    
     The present invention is described below with the help of examples of use and with references to the figures, where: 
     FIG. 1 a  shows a lifting vessel with attached lifting gear according to the present invention, 
     FIG. 1 b  shows the lifting vessel according to the present invention, 
     FIG. 2 shows the lifting vessel positioned around a jacket with a platform deck, 
     FIG. 3 shows a steel tubular rotation beam for lifting and rotating a jacket structure, 
     FIG. 4 shows a device for lifting and rotating a jacket structure for installation or removal, 
     FIGS. 5 a-   5   c  show the vessel in connection with lifting and rotating a jacket structure where a special “cradle” is used, 
     FIG. 6 shows lifting frames for lifting of preferably a platform deck, 
     FIG. 7 shows hydraulic jacks for operating the lifting frame, situated between the lifting vessel and the inclined legs of the lifting frame and the figure also shows the steel tubular beam for lifting and rotation/removal of a jacket structure, 
     FIG. 8 shows a hydraulic lock bolt system for locking of the lifting frame in a certain position to a guide rail connected to the lifting vessel, 
     FIG. 9 shows one first alternative for a connection between the lifting frame and the jacket structure for removal of a platform deck, 
     FIGS. 10 a  and  10   b  show a second alternative for a connection between the lifting frame and the jacket structure for removal of a platform deck, 
     FIGS. 11 a  and  11   b  show a third alternative for a connection between the lifting frame and the jacket structure for removal of a platform deck, 
     FIGS. 12,  13 ,  14  and  15  show step by step the operation sequence for removal of a platform deck with the help of the lifting vessel of the present invention, and 
     FIGS. 16,  17 ,  18 ,  19  and  20  show step by step the operation sequence for removal of a jacket structure with the help of the lifting vessel of the present invention. 
    
    
     The lifting vessel according to the present invention will now be described with reference to the figures, especially FIGS. 1 a  and  2 . 
     The lifting vessel  1  (MPU) is, according to the present invention, developed as a floating concrete hull with a U-shaped pontoon foundation  2  containing two longitudinal pontoons  2   a,    2   b  and a transverse pontoon  2   c,  and with columns  5  through the water surface for hydrostatic stability and optimal behaviour in the sea. The columns  5  are not connected structurally at the top, which is made possible by a rigid and robust hull structure. A brim  3  along the lower edge of the pontoon foundation improves further the behaviour of the vessel in the sea. The vessel  1  is specially developed for operations offshore. The U-shape of the pontoon foundation  2   a,    2   b,    2   c  enables the vessel to position itself around a platform being installed or a platform being removed, be it the platform deck or a platform substructure. The lifting operation is performed according to Archimedes&#39; principle by ballasting/deballasting the vessel  1 . The lifting is mainly performed vertically, but the vessel  1  can be inclined in all directions to enable special lifting operations. 
     Positioning of the vessel  1  is considered done by tugs, but thrusters can be installed to make the vessel  1  self-propulsive. The vessel  1  is designed to operate in all oceans in all parts of the world. The vessel  1  is also designed to be transported on a heavy lift ship to ease transportation over large distances. 
     The vessel  1  is equipped with devices specially fitted for the operations the vessel  1  is intended for. Installation and removal of platforms (platform decks and platform substructures) for the oil and gas industry are examples of operations the vessel  1  is intended for. 
     Installation and removal of platform substructures are mentioned above as fields of operation for the lifting vessel  1  of the present invention. The vessel  1  will now be described in relation to these operations, especially in connection with the handling of jackets. Steel jackets are widely used all over the world in the oil and gas industry as substructure for offshore oil and gas production units. There are also many other situations where a jacket structure is suitable as a support structure. There will be a market for both installation and removal of jackets in the future. Below is described operations concerning removal of a jacket. For installation the operations will be performed in the reverse order. 
     Lifting brackets  25  are attached to the jacket legs on one side of the jacket at a certain, pre-established height. A circular tubular rotation beam  22  is fixed to the top of the transverse pontoon  2   c  of the lifting vessel  1 . The lifting vessel  1  is positioned around the jacket with the help of tugs and active use of a lifting frame  12 . This lifting frame will be described more thoroughly later in connection with lifting devices for positioning and lifting of a platform deck. The vessel  1  is hauled to a position where the transverse pontoon  2   c  of the vessel  1  is positioned close to the side of the jacket where the lifting brackets  25  are attached. The lifting vessel is ballasted to the desired draft and inclination of heel so that the tubular rotation beam  22  connects with the lifting brackets  25 , see FIG. 4, concurrent with the lower edge of the transverse pontoon  2   c  bear against the jacket legs with fenders between them. The lifting brackets  25  are locked to the tubular rotation beam  22  and by deballasting the lifting vessel  1  the jacket is lifted. When the jacket is lifted clear of the seabed or foundation the lower part is lifted to the surface using wires and winches (or buoyancy modules), thereby rotating the jacket about the tubular rotation beam  22 , before transportation to a new destination. 
     The lifting brackets  25  are made of steel of robust design and will absorb all forces introduced by the lifting and rotating operations. The lifting brackets  25  are designed to lock onto the tubular rotation beam. The lifting brackets  25  easily rotate on the tubular rotation beam  22 . 
     Pre-engineering is required with regards to the strength of the jacket structure before a lift can take place. The jacket legs must be reinforced if they cannot endure the loads introduced. The lifting brackets  25  can be shaped with two long tubular clamps with a plate between them, so that they can be mounted to the main leg and a diagonal bracing of the jacket. The brackets  25  will then absorb the forces from the tubular rotation beam  22  and distribute them to the tubular clamps, which in turn distribute the forces onward in axial direction of the legs and the braces of the jacket, and so avoiding the largest shear forces. This device must be dimensioned for each individual case. 
     For some jackets it may be difficult to dimension the support for the brackets  25 . If this is a problem a “lifting cradle” according to the invention can be used, see FIG.  5 . The lifting cradle is attached to the tubular rotation beam  22  and uses this as a rotation point as described above. The cradle  29  is a framework consisting of two triangular frames pointing outwards with a pointed end upwards, attached to the tubular rotation beam  22  on the pontoon. The triangular frames are connected with a tubular beam at the bottom of the perpendicular. The cradle  29  consists of tubes 2-3 meters in diameter that are filled with water when the cradle  29  is in its lowest position and will be emptied when the lift starts. The large dimensions secure structural strength and enough buoyancy to contribute to the lift. 
     The lifting vessel  1  is positioned as described above and the cradle  29  will embrace the jacket. Specially adjusted saddles are attached to the lower circular beam on the cradle  29 , resting against the jacket legs. To avoid the jacket from sliding off the cradle  29  during the lift the jacket is connected to the tubular rotation beam  22  through brackets attached to the jacket legs. On the back of the lifting vessel  1  winches are mounted on each side of the “docking area” i.e. the inner area of the U-shaped pontoon foundation surrounded by the two longitudinal pontoons  2   a,    2   b  and the transversal pontoon  2   c.  Winches onboard tugs can also be used. Through pulleys wires with a hook in one end is hooked to the lower corners of the cradle  29 . The cradle  29  is now lifted upwards rotating about the tubular rotation beam  22  and the jacket is lifted out of the water for safe transportation to shore. An alternative method is to ballast/deballast the vessel  1  combined with the use of buoyancy modules attached to the jacket. 
     Lifting devices for positioning and lifting of a platform deck will now be described with reference to the drawings. Platform decks exist in different sizes and to be able to handle them all, the lifting device must be large, strong and flexible/adjustable, with strict requirements to the shape for positioning around the substructure carrying the deck. 
     Lifting frame  12  is fitted with a horizontal robust lifting beam  13  at the top and is pin-connected  21  to the top of the longitudinal pontoons  2   a,    2   b  on each side of the docking area, see FIG.  1 . The lifting frame  12  consists of a near-horizontal structure  18 , preferably a truss structure, going from the horizontal lifting beam  13  to the upper anchorage point  10  on the lifting vessel  1 . Furthermore the lifting frame  12  consists of a vertical support structure  16 , preferably a truss-work, connected in its upper end to the lifting beam  13  and connected in its lower end to the lifting vessel through an anchorage point  11 , preferably a pin connection  21 . The lifting frames  12 ,  12  in the upright position stands taller than the top of the lifting vessel  1 , such that the lifting beams  13 ,  13  are always above the hull of the lifting vessel  1 . The lifting frames  12 ,  12  can, with the use of the hydraulic cylinders  20 ,  20  connected to the lifting vessel  1  and the lifting frames  12 ,  12 , see FIGS. 1 a  and  7 , be inclined towards the middle of the docking area to position the lifting beams  13 ,  13  under the lifting points on the platform deck. The two lifting frames  12 ,  12  can be run independently. The lifting frames  12 ,  12  are locked in the right position before the lift starts, with hydraulic bolts  9  through holes  8  in guide rails  7  connected to each of the four columns  5  on the hull of the lifting vessel  1 , see FIGS. 1 and 8. This ensures fixation in all directions included sea fastening during transport. Plane outer walls  6  tangentially fixed to the columns  5  are supporting the guide rails  7 . The plane walls  6  are furthermore perpendicular to the direction of the connection line between two columns  5 ,  5 . 
     The connection between the lifting beam  13  and the deck can be carried out in different ways. Below is described three ways that ensures adequate flexibility to absorb shocks during a lift off: 
     i) The lifting beam  13  can be equipped with a shock absorbing cover  14  while also placing shock absorbing cushions underneath the deck. If it is not possible to lift directly underneath the deck the upper part of the jacket can be fitted with brackets  26  with shock cushions so that the lifting beam  13  can get a proper hold, see FIG.  9 . Prior to lift off the jacket will be cut right below the brackets  26 . 
     ii) Hydraulic cylinders  30  are placed on top of the lifting beam  13  in well calculated positions to get direct contact with the lifting points on the deck structure (or brackets  26  on the upper part of the jacket). Shock absorbing cushions are placed between the deck structure and the hydraulic cylinders  30  to obtain maximum damping, see FIG.  10 . 
     iii) “Shock cells” consisting of cylinders  35  filled with sand or another shock absorbing material is placed on top of the lifting beams  13  in well calculated positions. Conical tube stubs  37  are placed in corresponding positions on the deck structure. The conical tube stubs  37  absorb shocks when they penetrate the sand-filled cylinders  35 , see FIG. 11 a.  An alternative is that both the tube stubs  37  and the shock cells  35  are mounted on the deck structure, see FIG. 11 b.    
     The MPU  1  is positioned around a jacket structure with deck and is made ready for lift off and removal of the deck. The lifting frames  12 ,  12  on each side of the docking area is actively used for positioning by inclining them against the jacket with the help of hydraulically controlled arms  20 , see FIG.  2 . Additionally the positioning is done by tugs. The lifting frames  12 ,  12  are pulled back into lifting position when the MPU  1  is in the right position, as described above. The MPU  1  is then deballasted slowly until the lifting beams  13  are touching the lifting points. Compensation for the vertical motions of the MPU  1  is partly done by flexible shock cushions mounted on the lifting beams and lifting points, and partly by the use of a flushing system that ensures a quick load transfer. When the deck has a safe clearance to the jacket the MPU is pulled away from the jacket before ballasted down to transport draft. 
     The flushing system consists of flushing (ballast) tanks  4  above the waterline with large area quick release trapdoors that enable the water to flush out. Trapdoors on different levels enable multiphase flushing, i.e. flushing in several steps. 
     This example describes the operations for removal of a platform deck. The different operations are illustrated in a sequence of figures; FIGS.  12 - 15 : 
     i) Positioning around a jacket with a deck. 
     With the help of tugs the MPU  1  is positioned around the jacket. The lifting frames  12 ,  12  are in upright position with good clearance to the jacket. The draft of the vessel  1  ensures good clearance to the deck, see FIG.  12 . 
     ii) Using the lifting frames  12 ,  12  to fine adjust the position around the jacket. 
     When the MPU  1  is approaching the correct position the lifting frames  12 ,  12  are inclined against the jacket to dampen the horizontal motions of the MPU  1  and also to fine-adjust the position. This is done by active use of hydraulics, see FIG.  13 . 
     iii) Deballasting the MPU  1 , ready for lift-off. 
     The MPU  1  is deballasted while the lifting frames  12 ,  12  glide along the jacket structure to dampen the horizontal motions. The deballasting proceeds until the lifting frames  12 ,  12  are right under the lifting points on the deck. The lifting frames  12 ,  12  are then locked into position and MPU  1  is ready for lifting off the platform deck, see FIG.  14 . 
     iv) Lift-off of the deck 
     When the MPU  1  is ready to lift off the deck, water in the flushing tanks  4  are let out quickly by opening the quick release trapdoors in the columns  5  thereby achieving a rapid lift. The deck is prepared in advance by cutting the connections between the deck and the jacket, see FIG.  15 . 
     v) Ready for transportation to shore 
     After lift-off the MPU  1  is pulled away from the remaining jacket. The MPU  1  is deballasted down to transportation draft when it is clear from the jacket. If necessary additional sea fastening to the locking of the lifting frames  12 ,  12  are added and the transportation to shore can start. It is also possible to transfer the deck to a barge for transportation to shore so that the MPU  1  is immediately available for new operations (e.g. removal of the jacket). 
     This example describes the operations for removal of a jacket structure. The different operations are illustrated in a sequence of figures; FIGS.  16 - 20 : 
     vi) Positioning around a jacket (without a deck). 
     With help from tugs the MPU  1  is positioned around the jacket. The lifting frames  12 ,  12  are in upright position with good clearance to the jacket, see FIG.  16 . 
     vii) Using the lifting frames  12 ,  12  to fine adjust the position around the jacket. 
     When the MPU  1  is approaching the correct position the lifting frames  12 ,  12  are inclined against the jacket to dampen the horizontal motions of the MPU  1  and also to fine-adjust the position. This is done by active use of hydraulics, see FIG.  17 . 
     viii) The MPU is inclined and deballasted, ready for lift-off 
     The MPU  1  is inclined and deballasted until the tubular rotation beam  22 , situated on top of the transversal pontoon  2   c,  gets a hold of the brackets  25  pre-installed on the jacket, see FIG.  18 . 
     ix) Lift-off 
     When the MPU  1  is ready to lift off the jacket, water in the flushing tanks  4  are let out quickly by opening the quick release trapdoors in the columns  5  thereby achieving a rapid lift. The jacket is prepared in advance by cutting the jacket legs, piles, risers etc., see FIG.  19 . 
     x) Tilting of the jacket, ready for transportation 
     After lift-off, the jacket is rotated to a near-horizontal position with the use of winches and wires mounted on the aft of the MPU  1  or winches and wires onboard tugs, see FIG.  20 . An alternative method is to attach buoyancy modules to the jacket. After sea fastening the transportation to shore can start. An alternative is to transfer the jacket to a barge for transportation to shore so that the MPU  1  is immediately available for new operations.