Abstract:
A method of transporting and installing an offshore structure including transporting the offshore structure to the installation site by barge with at least a portion of the structure being supported under the barge, and lowering the offshore structure to the water bed. In accordance with a preferred embodiment, the structure includes a tower and a base, with the tower being mounted offset from a center of the base through the use of bolts.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method of transporting and installing an offshore structure, such as an offshore production platform, in particular for oil or gas. 
     2. Discussion of the Prior Art 
     The offshore oil industry is highly competitive and therefore the cost of constructing, transporting and installing offshore structures is extremely important. Moreover, the cost of transporting and installing a platform may be even higher than its construction costs. 
     One known method of installing offshore platforms involves lifting the platform sections by a crane vessel and assembling them in situ. However this is expensive since it involves the long term use of installation vessels, which are expensive to hire. Another method involves building buoyancy tanks into a structure to allow it to be floated to its desired position and then ballasting the structure onto the water bed However this increases the cost of constructing the platform. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to provide a method of transportation and installation which is both simple and which is less expensive than known methods. 
     From a first aspect, therefore, the invention provides a method of transporting and installing an offshore structure comprising transporting the offshore structure to the installation site by barge with at least a portion of the structure being supported under the barge, and lowering the offshore structure to the water bed. 
     Thus the invention contemplates supporting at least a part of a structure below a barge, conveying the structure to its installation site and then lowering the structure into position. 
     This method has several advantages over the known methods described above. Firstly, it allows a standard barge to be used for transportation purposes, which is extremely cost effective. A fully assembled structure may be transported to the installation site and no assembly of the structure need take place at the installation site. Furthermore, the structure may be simply lowered into position from the barge, avoiding the need for expensive machinery for lifting the is structure from the barge. Thus the method of the invention offers significant advantages over the known methods. 
     The structure may be supported to the barge in a number of ways. In the preferred method of attachment, the barge is floated into position over the structure and the structure then attached to the barge. Thus preferably, the structure is constructed in a dry dock which is then flooded and the barge is floated over the appropriate part of the structure. 
     Although it would be possible to suspend the structure below the barge, for stability reasons, the structure is preferably made fast with the barge. To this end, the barge is preferably lowered over the structure into contact with an upper surface thereof and the structure then made fast with the barge. It may be possible, for example, to lower the barge over the offshore structure by at least partially emptying the dry dock so that the barge sins with the sinking water level. However most simply, the barge is ballasted down to the structure. The barge and structure are then attached to each other. 
     The structure and barge may be attached to each other by any means, for example by welding. Preferably however the offshore structure is attached and tensioned to the barge by standard cables using jacking means. The tensioning of the barge to the structure allows relative movement between them during transportation to be minimised. In addition, as will be seen, the jacking means can also be used to lower the structure to the water bed at the installation site. 
     In a preferred embodiment the barge and offshore structure are towed to the installation site by tugs. It will however be appreciated that any means of conveying them to the installation site might be employed. For example, they could be self propelled. 
     It will be understood from the above that the method of the invention could be used to install many forms of offshore structure. However, it is expected to be most useful for the installation of offshore production platforms. 
     A preferred structure for installation by the invention comprises a base, preferably of concrete, which when installed rests on the water bed and a tower, preferably a steel tower extending upwardly from the base and mounting suitable topsides. In such a structure the base or a portion thereof is supported beneath the barge, and the tower extends to one side or end of the barge. Thus the base may extend either sideways or lengthwise underneath the barge. 
     To facilitate such an arrangement, the tower of the platform is preferably offset from the centre of the platform base Since standard platforms known in the art are conventionally constructed with their tower at the centre of the platform base, this arrangement is believed to be novel and inventive in its own right, so from a further aspect the invention also provides an offshore platform comprising a base and a tower wherein the tower is offset from the centre of the base. 
     In order to counter the pitching or rolling moment generated by the tower offset during transportation, and thus improve the stability of the barge and platform while being conveyed to the installation site, the base is preferably provided with a suitably positioned counterweight. The counterweight may conveniently be formed as an upstand formed over part of the platform base. This may be sized and positioned such that when the platform is supported to the barge, it extends up the side of the barge opposite the side over which the tower extends. 
     In an alternative arrangement, when the base extends lengthwise under the barge however, the length of the base itself will act, to some extent as a counterweight, obviating the need for an upstanding counterweight. 
     To assist in installation, a positioning means may be provided on the water bed prior to the installation. The positioning means could be installed by any method, for example, lifting it into place or piling offshore. However it is preferred that it is also installed by the method of the invention. 
     The positioning means preferably comprises a large mass, for example of concrete, dimensioned to resist the forces generated during installation of the platform and having guide means for engagement with cooperating guide means on the platform. 
     As in the present invention, the base is held underneath the barge during transportation, the buoyancy of the structure is provided by the barge. Therefore, the thickness of the concrete base may be minimised to that required in the installed condition. However, traditionally the steel tower of an offshore structure is attached to the concrete base by being cast into the concrete during manufacture of the base. This method of attachment, however, requires the concrete base to have a thickness of at least 2 to 3 times the diameter of the steel tower Such a method is not, therefore, suitable for towers with relatively thin bases. 
     Thus in a preferred structure of the invention, the tower is attached to the base by bolting, most preferably by bolts extending upwardly from the base. 
     This method is believed to be novel and inventive in its own right. Thus, from a still further aspect, the present invention provides an offshore structure comprising a base and a tower extending upwardly therefrom, wherein the tower is attached to the base by bolts, most preferably by bolts extending upwardly from the base. 
     In this method, the bolts may be securely fastened in the base, for example by being pre-cast into the base or by being grouted into bores provided in the base, and the tower then assembled over the upstanding bolts and fastened into position. To maximise the strength of the mounting, the mounting bolts preferably extend substantially through the base. 
     To prevent corrosion, the portions of the bolts extending above the base may be coated with a suitable resistant coating. 
     The base of the tower is preferably formed with a fastening flange extending radially outwardly therefrom, for receiving the bolts. Most preferably circumferential stiffening ribs are provided between the flange and the tower to provide rigidity in that region, and bolt receiving holes are formed in the flange in the pockets defined between adjacent stiffening ribs. 
     In assembling the tower, the tower base flange may be assembled over the upstanding bolts, with a layer of grout positioned between the tower base flange and the upper surface of the concrete base to provide a satisfactory seating for the flange on the concrete base. An anchor block may then be positioned over the ends of the bolts to rest on the flange. An epoxy seating or the like may be provided for the block so as to prevent ingress of water to the bolt, thereby preventing corrosion. 
     After nuts have been tightened onto the bolts to securely attach the tower to the base, an enclosure may be defined around the nut and exposed bolt, which is filled with epoxy or some other sealant, again to prevent corrosion. 
     It will be appreciated that the tower need not be completely prefabricated before it is attached to the base. Only a tower base part need be constructed and attached and further tower sections then attached to the tower base part, for example by welding. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which: 
     FIG. 1 is a schematic representation of a first stage of an installation sequence for an offshore platform; 
     FIG. 2 is a schematic representation of a second stage of the installation sequence for an offshore platform; 
     FIG. 3 is a schematic representation of a third stage of the installation sequence for an offshore platform; 
     FIG. 4 is a schematic representation of a fourth stage of the installation sequence for an offshore platform; 
     FIG. 5 is a schematic representation of a fifth stage of the installation sequence for an offshore platform; 
     FIG. 6 is a schematic representation of a sixth stage of the installation sequence for an offshore platform. 
     FIG. 7 is a plan view of an offshore platform of the invention; 
     FIG. 8 is a sectional view showing the attachment method of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the Figures, an offshore platform  2  comprises a generally rectangular concrete base  4  with a steel skirt  6  protruding downwardly therefrom for penetrating the sea bed. A steel tower  8  is mounted to the concrete base at one end  10  of the base  4 , on the longitudinal axis of the base  4 . The tower  8  is attached to the base  4  in any suitable manner but preferably by bolting as will be described further below. At the top of the tower  8  is mounted a support frame  12  with topsides  14 . In addition, the concrete base  4  is provided with an upstanding integral concrete counterweight  16  extending from the other end  16  of the base  4 . As can be seen from FIG. 5, the counterweight  16  extends across the complete width of the base  4 . 
     The platform  2  is constructed essentially completely in a dry dock  20  using standard equipment. As shown in 
     FIG. 2, once the construction phase is completed, the dry dock  20  is flooded, and a standard industry steel barge  24  is then floated and manoeuvred over the base  4  of the platform  2  so as to lie between the tower  8  and the counterweight  16 . The barge  24  is of standard dimensions, and the relative positions of the tower a and the counterweight  16  are designed with such dimensions in mind such that, as shown in FIG. 3, the barge may be accommodated between the counterweight  16  and the tower  8 . 
     When the barge is properly positioned over the base  4 , the barge is ballasted down to rest on the upper surface  26  of the base  4 . The base  4  is then connected to the barge  24  using four or more prestressing strands  28  which are positioned around the periphery of the barge as shown in FIGS. 3 and 5. As shown, the strands  26  extend over the side of the barge  24 . Once the strands  28  are attached to the base  4 , they are tensioned by strand jacks so as to hold the base  4  firmly to the underside of the barge  24 . 
     The dock  20  is then reflooded, the barge level tried and the platform  2  then floated out of the dock  20  firmly attached to the barge  24 . The platform is then towed to the installation site. As will be appreciated, the counterweight  16  counters the mass of the tower  8  during transportation to the installation site, thereby increasing the stability of the barge  24 . It will also be appreciated that when installed, the counterweight will contribute to the anchoring of the structure to the water bed. 
     As shown in FIG. 6, once the barge  24  is in the desired installation position, for example over a pre-drilled well head  38 , the base  4  is lowered towards the seabed  30  using the jacks  28 . Prior to installation of the platform, a positioning means  32  having guides  34  is positioned on the water bed  30 . As shown in FIG. 6, the positioning means may simply be a large mass of concrete with suitable guide means  34  and which may itself have been installed by the method of the invention. During lowering, a guide  40  on the base  4  will engage in the guides  34  provided on the positioning means  32  so as to accurately position the base  4  on the water bed  30 . The exact position of the guide  40  on the base  4  is pre-determined by measurement of the position of the guides  34  of the positioning means  32  relative to the well head  38 . Once the skirts  6  penetrate the seabed  30  the weight of the base  4  is transferred to the seabed  30  and the base  4  is firmly secured allowing the strand jacks  28  to be disconnected. 
     As shown in FIGS. 7 and 8, the tower  8  of the offshore structure is attached to the base  4  by bolting. The tower comprises a radially outwardly extending flange  42  at its base, and  36  radially extending stiffening ribs  44  disposed about its circumference. In the particular embodiment shown, the tower diameter is about 3.8 m, the flange about 0.55 m wide and the ribs about 1 m high. 
     As shown more particularly in FIG. 8, bolts  46  are fixed into the concrete base  4 , preferably extending substantially through the entire depth of the base  4  (about 2 m) so as to be held sufficiently firmly within the base. The bolts  46  may be grouted into pre-drilled holes in the base  4  but preferably, they are cast directly into the concrete. The bolts  46  are positioned so that they will pass through apertures formed in the flange  42  which are regularly spaced around the circumference of the tower  8  between each pair of stiffening ribs  44  when the tower is positioned on the base. Thus in this embodiment there are  36  bolts, each of which is about 2 m long and 75 mm in diameter. 
     The flange  42  of the tower  8  is secured to the upper surface  48  of the base  4  and the bolts  46  extend above the flange  42 . A grout layer  60  is formed on the upper surface of the base  4  so as to provide an even seating for the flange  42  on the base. An anchor block  50  and nut  52  are then placed over each bolt  46  and the nuts tightened so as to hold the tower  8  to the base  4 . An epoxy resin layer  54  is provided under the anchor block  50  so as to protect against corrosion. In addition, a cover  56  is attached over the projecting end of the bolt  46  and attached to the anchor block  50  so as to form an enclosure  58  which is also filled with epoxy resin. Thus, all exposed areas of the bolt  46  are protected against corrosion. 
     Preferably only a bottom segment of the tower a is initially bolted to the base  4 . The other shaft segments which make up the tower may then be welded to this bottom segment. 
     It will be seen from the above description that the invention provides a method of installing an offshore structure which is simple to carry out and less expensive than existing methods. More specifically, the method of the invention requires neither the long term use of expensive installation equipment on site nor the construction of costly structures for the transportation and lowering of the offshore structure. This results in a very significant saving in the total cost of the construction and installation of an offshore production platform. 
     It will further be seen that the invention provides an alternative method of attaching a tower of an offshore structure to its base which allows the thickness of the base to be minimised so as to save on material costs. 
     The skilled person will appreciate that the invention is not limited to the specific embodiment described above. For example, it would be possible to carry out the installation method using more than one barge to support the offshore structure if so required, with respective parts of the structure arranged under respective barges. Also, for example, the counterweight need not take the particular form shown. For example, the base  4  may be of a generally constant thickness and suitably shaped to produce a counterweight effect.