Space frame structure with widened base for supporting offshore platform

Abstract: The present invention relates to a space frame structure with a widened base, for supporting an offshore platform, comprising four main legs (12) diverging in the lower region (C) and the transition region (B), parallel in pairs in the upper region (A) from where extend vertical sections (14), providing in a lower position a widened base (20) having dihedral external lateral faces, and fixed to the sea bottom through piles or groups of vertical piles (22) fixed to the lower ends of the said four main legs (12).

Description: The present invention refers to a space frame structure for supporting a platform for offshore operations such as well drilling, oil and gas exploration and production or for support of any other installations at sea. The most usually employed space frame structures for supporting offshore platforms are made up by spatial porches with four or eight main legs and shaped as a truncated pyramid, with square or rectangular base and top. The choice between four or eight legs for the structure depends basically on the required dimensions of the base and top as well as on the magnitude of the operational and environmental loads to be sustained, so that when these requisites can be met by a four-legged structure, this is normally the most simple and efficient solution. The eight-legged structures are normally installed by launching, using as launching rails two parallel internal faces. Consequently, they have few elements specifically dimensioned for the launching operation. As a drawback, they present larger structural complexity, with a larger number of strength elements to be manufactured and assembled, resulting further in an increase of the environmental loads. Alternatively, the four-legged type of structure is normally installed by lifting or launching, according to its dimensions and weight and the performance of the naval means available. When launched, the conventional four-legged structures require specific additional structures, such as a set of launching trusses, leading effectively to a six-legged structure wherein the four legs situated at the corners are basically used to absorb the operational and environmental loads while the two parallel internal legs are used in the installation phase. These additional structures are useless and counterproductive in the operational or production mode, since not only do they not contribute to the overall rigidity they also increase the dead weight itself as well as the acting environmental loads. US-A-3729940 discloses a square-section tower structure having a widened base as two base trusses extending outwardly from respective ones of two opposed sides of the square tower. Each of these base trusses is connected to the main tower by buttresses comprising inclined portions of the four main legs of the tower. GB-A-2214548 discloses a tower having a truncated triangular view in side elevation and a rectangular shape when viewed in front elevation, with the sides extended by box trusses to provide a widened base joined to the four main legs of the tower by divergent leg portions. With the objective of eliminating the above disadvantages, which are inherent to the conventional four or eight-legged offshore structures, the present invention provides a grid structure for supporting an offshore platform comprising four main legs which are divergent in a lower region and a transition region of the grid structure and are parallel in pairs in an upper region from where vertical sections extend upwardly, said structure having in the lower region a base which is wider than each parallel pair of main legs and which has four external lateral faces with a concave dihedral or approximately dihedral shape, said grid structure having a foundation adapted to be fixed to the sea bottom by vertical piles or groups of piles fixed to the lower ends of the four main legs, said main legs being extended by smaller diameter secondary legs which extend from the transition region at the point of divergence of the main legs towards the base of the structure and terminate at a lower horizontal bracing frame raised in relation to the sea bottom. Due to its peculiar geometrical configuration, the structure of the present invention does not present main elements specific to the installation mode or operation; instead all the main elements are dimensioned for the operating condition; also, the structure will present in the foundations loads that are relatively low, not only due to the presence of the widened base but also as a consequence of the low susceptibility to environmental loads, which in its turn originates from its slenderness and simplicity, notably in the regions closer to sea level. Thus, the space frame structure of the present invention for supporting offshore platforms with a high degree of optimisation has minimised weight and simplified geometrical configuration, causing appreciable reduction in manufacture and assembly time and costs besides allowing efficient transportation and installation by conventional methods. For example, the launching rails formed by the secondary legs and colinear main leg parts enable the structure to be launched-out onto a launching barge at the construction site and then launched from the barge at sea. The invention will now be described in more detail merely by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a perspective view of the frame structure according to the present invention; Figure 2 is an elevational view of the forward and aft faces of the structure; Figure 3 is an elevational view of the lateral faces of the structure; Figure 4 is a perspective view of the external lateral faces of the structure, including the spaces of the main legs situated in the transition zone; and Figure 5 is a plan view of the lower bracing frame of the structure. As can be seen from the drawings, in which identical numerical references identify corresponding parts, the structure for supporting an offshore platform is generally designated 10, and has four main legs 12, all divergent in the lower C and transition B regions, and parallel in pairs in the upper region A from where extend vertical lengths 14 to be adapted to engage the operation platform. The lower region C presents a widened base 20 which is fixed to the seabed by vertical piles or groups of vertical piles using pile guides 22 fixed to the lower end of the four main legs 12. To allow the loading-out and the launching of the structure by sliding, the main legs 12 are extended by smaller diameter branches, referred to as secondary legs 18, from (i) the point of divergence of the main legs 12 to (ii) the base of the structure; in this manner the secondary legs 18 define the launching rails. Thus, the structure of the present invention can be defined as a four-legged structure in its upper region A and as an eight-legged structure in its transition region B and lower region C, so it has the combined advantages of the two aforementioned conventional conceptions but without their inconveniences. A main feature of the structure 10 is that at least in the lower region C it presents external lateral faces with dihedral or approximately dihedral shape, which provide a widened base of optimum dimensions, as required, with a reduced number of strength elements and great structural simplicity. This simplified structure is also present in the transition region B, in which the only elements external to the main core defined by the secondary legs 18 are the divergent parts of the main legs. Such a structure presents little susceptibility to environmental loads, due to (a) its structural transparency and simplicity, notably in its upper region A but also in the other regions B and C, as demonstrated above, combined with (b) great stability and reduced stress in the foundations, due to its widened base 20 and to the aforementioned lower environmental loads, which make it particularly indicated for regions in which the soil load capacity is low and/or meteo-oceanographical conditions are severe. A main additional feature of the structure 10 is that, as shown in Figure 2, it has the lower horizontal bracing frame 24 raised in relation to the sea bottom, so as to avoid interfering with the well drilling template (when one exists), making the operation of docking the structure easier and still eliminating the need for expensive operations to clean up the sea bottom, normally obstructed by debris originating from drilling wells. The raised position of the lower frame 24 is possible without the need for additional structures, since the vertical reactions originating from the slanting lateral bracings are absorbed by the secondary legs 18. As a further favourable consequence of the raised positioning of the lower frame 24, there results a reduction in the overall weight of the structure due to its reduced length of its lateral faces defined by the main legs 12 in upper region A and secondary legs in transition region B and lower region C. Another main feature of the structure 10 due to its particular geometrical configuration, notably in the lower region C and transition region B, is that it enables operational and environmental loads to be transmitted directly to the foundations 22 through the main legs 12, minimising the part of the load transmitted by shear stresses throughout the concave lateral dihedral panels situated between the aforementioned main legs 12 and the secondary legs 18, and consequently optimising also the height necessary to the said lower region C and transition region B, each of these regions being in this fashion constituted by a single bay. As a further consequence of the particular structural shape of the aforementioned lateral panels, even the secondary legs 18 used for the installation phases, no longer need to have their maximum usage and their critical stress dimensioning associated with the operation phases, so that there are now main structural elements specifically dimensioned for the installation phases. Due to its particular shape and structural optimisation, the structure 10 of the present invention does not present any increase in weight in relation to the conventional, four-legged pyramid frustum body structure; it is free of a launching truss; also it presents a reduced transverse section in the vicinity of its centre of gravity. Consequently, the structure can be installed alternatively by lifting, with a particularly reduced radius of operation of the crane boom and consequently with maximised lifting capacity, making possible great flexibility as to the method of installation which can thus without losses be modified after the design/building of the structure, should the availability of naval means at the time indicate that such a decision is the more convenient. Furthermore, again due to the particular structural shape of the widened base and foundation by vertical piles, the operation of driving the aforementioned piles through guides 22 is made considerably easier. The piles are situated at a comfortable distance from the upper modules of the structure, minimising the risk of shocks and consequent damage to the structural strength elements, which then do not require the usual protection. The use of vertical piles eliminates also the need for secondary restraint structures (guides), reducing still further the weight and the overall cost of the structure, since the elimination of pile guides also allows its installation in a single body, without the costly operation of offshore welding. Also, due to the particular shape of the widened base of the structure with a low centre of gravity the reaction, in the rocker arms, of the launching barge is minimised, since the pivoting of the structure occurs late, when its upper part already finds itself totally submerged with the weight compensated for by the hydrostatic buoyancy. Furthermore, due to the low position of the centre of gravity, the structure can be easily self-aligned with the vertical (up-ended) following the launching, possibly with the use of buoyancy tanks in its upper part A.

Claims: A grid structure (10) for supporting an offshore platform comprising four main legs (12) which are divergent in a lower region (C) and a transition region (B) of the grid structure and are parallel in pairs in an upper region (A) from where vertical sections (14) extend upwardly, said structure having in the lower region (C) a base (20) which is wider than each parallel pair of main legs (12) and which has four external lateral faces with a concave dihedral or approximately dihedral shape, said grid structure having a foundation (22) adapted to be fixed to the sea bottom by vertical piles or groups of piles (22) fixed to the lower ends of the four main legs (12), said main legs (12) being extended by smaller diameter secondary legs (18) which extend from the transition region (B) at the point of divergence of the main legs (12) towards the base (20) of the structure (10) and terminate at a lower horizontal bracing frame (24) raised in relation to the sea bottom. A structure according to claim 1, wherein the main legs (12) which are parallel in pairs in the upper region (A) form, together with the secondary legs (18), launching rails for loading-out and launching the structure (10). A structure according to claim 1 or 2, wherein it has its lower region (C) and transition region (B) each comprising a single structural bay, having a particular geometrical shape to allow the transmission of operational and environmental loads directly to the foundation piles (22) through the main legs (12).

Inventors: MACIEL VITOR GIORDANI, MACIEL, VITOR GIORDANI
Applicants: PETROLEO BRASILEIRO SA, PETROLEO BRASILEIRO S.A. - PETROBRAS
Application year: 1993
Main classification: E02B17
IPC codes: E02B17