OFFSHORE MULTI-STAY PLATFORM STRUCTURE

Abstract: An offshore, bottom supported platform structure comprising a tower structure (1) of low bending stiffness which is laterally supported at several elevations by means of inclined, pre-tensioned stay cables (2). The tower structure (1) is composed of a number of vertical columns (5) which at each stay elevation below the elevation of the uppermost one, are interconnected by means of only horizontal bracing members (6). Method for constructing such offshore platform structure, the tower structure (1) being erected on and stayed to a floating box-like foundation structure (10), whereafter the completed platform structure is towed to its location and installed. The stay cables (2) are anchored to arms (11) cantilevering out from the foundation structure (10), said cantilevering arms being braced to the base of the foundation structure (10) by means of inclined bracing members or stays (14).

Description: 2-if! S ! 20l_!!!y IIι§I^-S ^IE25!!_ I3y£iyθε 1. This invention relates to an offshore bottom supported platform structure comprising a vertical touer structure of low bending stiffness uhich is laterally supported at several elevations by inclined, pre-teπsioned stay cables. The touer structure being of lou bending stiffness entails the major part of any horizontal loading on the touer is transferred douπ to the sea bottom - or douπ to the plat¬ form foundations - as changes in the stay cables tension forces. The invention describes the configuration of a touer structure feasible for such platform. Further, the invention includes a platform construction alternative uhere the touer is erected on a floating box-like foundation structure uhereafter the completed platform Is toued out to location and installed. The invention also may be used as subsea uell-head platform for large uaterdeDths. 2. In a structural sense, conventional, fixed platforms like piled steel jackets and gravity. latforms of concrete or steel are stiff touer structures spanning from the sea bottom up above the sea surface. For such structures the environmental loads are transferred douπ to the platform foundations as shear and bending forces in the structure. With increasing uaterdepth the size and ueigth of such conventional platform structures increase dramatically. The structural ueigth of a conventional steel jacket platform increases approximately in proportion to the square of the increase of the uaterdepth. The reasons are the environmental loads acting on the platform increase in proportion to the size of the structure uhile the bending moments at the platform foundations produced by said loads further increase uith increasing heigth of the structure. Exploitation of hydrocarbons and other resources at in¬ creasing uaterdepths implies need to identify more efficient and appropriate concepts for offshore platform structures than those in use today. The here disclosed invention represents such concept. 3. Guyed touers used as offshore platform structures have been proposed earlier. This previously proposed structure - Exxon's f Guyed Touer' (Fig. 1) - comprises a steel jacket structure uhich is laterally stayed at one single elevation some distance belou the sea surface by means of inclined, pre-teπsioπed guy lines. All the guy lines have the same length and inclination and are attached to anchorages at the sea bottom. The jacket structure may be founded on oiles or on a gravity 'spud can' foundation penetrated into the seaflααr. Horizontal loads acting on the 'Guyed Touer' platform uill in oart be balanced by changes in the guy line tension forces. Hαuever, the touer still needs have significant bending stiffness as it spans from the sea bottom up to the elevation uhere the guy lines are attached. The request for bending stiffness limits the acceptable horizontal deflections of the touer such reducing the efficiency of the guy line stay arrangement. In a structural sense the 'Guyed Touer' platform is a stiff touer structure. Concepts for offshore platform structures uhich do not make use of inclined stay arrangements, do not affect the invention presented here. 4. The main structural configuration of the platform in¬ vention disclosed here is sketched on Fig. 2a. Fig. 2b shous a horizontal projection of the structure. The platform structure comprises a vertical touer / \ / uhich is laterally supported at a number of elevations by means of inclined stay cables /2/. The stay cables are pre- tensioned tα a level uhich excludes slack in any cable for extreme environmental loading on the platform. The pre-tensioniπg of the stay cables introduces a comp- ressive force /T/ into the touer. The stay cables /2/ are attached to the stay anchorages /3/; the anchorage forces due to cable pre-tensioniπg are denoted /W/ and /S/. The platform touer /1/ may be founded on piles or on a box- like foundation structure (gravity type Dlatform). The stay anchorages /3/ may be integrated into the touer foundation /4/. Fig. 3 illustrates the load carrying principles of the proposed structure. Uhen the structure is exposed to a horizontal environmental load /ΔH/ uith the resultant /H/, the touer deflects uhich introduces the changes /Δ s/ of the stay cable forces. The related changes of the stay anchorage farces are denoted /ΔS/ and /ΔV/. The deflection of the touer also introduces bending moments /Mt/ and shear forces / l t/ in the touer itself; the magnitudes of these tuo contributions being functions of the touer bending stiffness. Force equilibrium of the structure is expressed by; - a. Overturning moment equilibrium: H-h = 2a-ΔS + Wt - b. Horizontal force equilibrium: H = 2-ΔV + Ut The horizontal force /H/ does not cause any change of the touer cαmpressive force /T/. Uith respect to offshore platform structures the here disclosed multi-stay arrangement of inclined cables represents a neu system for carrying load. The touer structure πou mainly is the cαmpressive chord member of a structural system uhere the horizontal forces are carried by the inclined stay cables. Similar to the chord of a truss, the cσmoressive chord member does not need much bending stiffness. Any significant bending stiffness of the touer structure is neither uaπted, since this uill reduce the efficiency of the stays and increase the stresses in the touer. Pre-teπsioπing of the stay cables intro¬ duces considerable compressive forces into the touer; hence, safety against buckling uill be governing the touer structural design. The touer deflection curvature for horizontal loading is controlled through adjusting the longitudinal stiffness (i.e the cross sections) αf the individual stay cables. The disclosed σlatfαrm uill have superior qualities uith respect tc dynamic behaviour due to the large amount αf system damoing in a multi-stay arrangement of cables of different lengths and inclinations. 5. The tαuer bending stiffness is of paramount importance for the here proposed multi-stay platform. The ratio betueen the touer bending stiffness and the longitudinal stiffness of the stays can be expressed as: Eo-Io !*■ Ec- Ac where : Eo Io = touer bending stiffness Ec Ac = longitudinal stiffness αf the stay cables 1 = heigth αf the touer. Large value αf K means the structure primarily uill behave like a stiff touer structure, the effect αf the stays being corresαondiπgly lou. Lou value of K represents a αlatfαrm structure for uhich horizontal loads primarily are carried by the stay cables implying correspondingly lou bending stresses in the touer structure. Fig. 4 shαus the structural configuration αf a touer uhich allous near optimum flexibility uith respect to tαuer bending stiffness uhile at the same time sufficient safety against buckling of the tαuer structural members is ensured. The touer structure comprises a number of vertical columns /5/ uhich at each stay elevation are interconnected by means of only horizontal bracing members /6/. The touer bending stiffness is adjusted by adjusting the bending stiffness of the horizontal bracing members. Above the elevation of the uppermost stay attachment the touer bending stiffness is increased by means αf cross bracings /!/ so as to reduce the horizontal deflections of the platform topside structure /8/. It might be bene¬ ficial to strengthen also the upper part of the tαuer just belαu the uopermost stay elevation by cross bracings so as to obtain a more even distribution of stay cable forces. Hσuever, for the structural system disclosed here it is imperative the touer horizontal deflections are governed by the stay cables longitudinal stiffness, not by the touer bending stiffness. The elevation of the upαermαst stay attachment should be as close to the too of the tαuer as possible as this uill reduce the tαuer bending stresses. Practical considerations e.g the traffic of boats close to the platform as well as the risk of damage to the stay cables, imply the elevation αf the uppermost stay is same distance belαu the sea surface. The touer configuration may easily be adapted to accommodate uell conductors, riser pipes and any other installation /9/ related tσ the platform function. The vertical distance betueen the stay elevations - and hence betueen the hori¬ zontal bracing members /6/ - may practically be chosen from the need for lateral support to the conductors and riser pipes. (This implies from 20m to 40m vertical distance between the stay elevations). The environmental loads acting on the conductors and risers then are transferred to the tower at the stay elevations. Fig. 4, Section A-A shous a tower structure comprising four vertical columns /5/, each column being stayed in tuα horizontal directions. The stays in same horizontal direction need not converge at the same stay anchorage /3/ as shαun on Fig. 2b. Fig * 4b shows a tαuer structure uhich is stayed diagonally by one horizontal stay direction to each column. At the stay elevations the columns /5/ are interconnected also by means of diagonal bracing members. Alternatively, each single column may be stayed in three - or preferably four - horizontal directions. For such arrangement, horizontal loads on the touer do not at all introduce any cαmpressive forces into the touer columns. The above examples just illustrate some of the possible stay arrangements. Practical considerations and costs uill determine uhich stay arrangement is the most feasible for each specific case. 6. Fig. 5 shows a gravity platform version of the invention. The platform structure can be completed in inshore uaters before it is toued out and installed. The tower /1/ is erected on top of a floating box-like foundation structure /1Q/. The stays are installed and the stay cables tensioned consecutively follouing the erection of the touer structure. To increase the incli¬ nation o f the stays these are anchored to arms /11/ canti- levering out from the foundation structure /IQ/.Thε canti- leveriπg arms /11/ are braced to the base of the foundation structure by means of inclined bracing members or stays /14/. Upon completion of the the touer erection the platform is toued tα its final location and installed. The platform may be equipσed uith temporary buoyancy units /13/ to ensure hydrostatic stability during the construction afloat and tou-out stages. The topside structure /8/ may be lifted on after the platform structure has been firmly installed on the sea bottom. The platform foundation structure may be equipped uith skirts /12/ penetrating into the seafloor so as tα improve the platform geotechnical safety. . 7. Fig. 6 shous the invention utilized for a subsea well¬ head platform for large waterdepths. The well-heads /15/ are placed on top αf the tower /1/ which'is discontinued some distance belou the sea surface /16/. By this apprpach the zone of maximum environmental load intensity is avoided, uhile the uell conductors /9/ are laterally supported by the tower for the larger waterdeoths. Use of the invention as disclosed on Fig. 6 will simplify the riser and con¬ ductor problems related to floating production installations, The well-head platform may be supported on piles or απ a gravity foundation. 8. The above examples dα not exclude other potential applications αf the disclosed invention.

Claims: . I _claim_ 1. An offshore,bottom supported platform structure (Fig. 2 ) comprising a vertical touer structure /1/ uhich is laterally supported at several - minimum three - elevations, and at each elevation in minimum three horizontal directions, by inclined, pre-tensiαned stay cables /2/, said touer struc¬ ture /1/ having lou bending stiffness as compared uith the longitudinal stiffness of the stay cables /2/ so that the major oart of any horizontal loading on the touer structure uill be transferred douπ to the sea bottom, or dαun to the platform fαuπdatioπ(s) , as changes of the stay cables tension forces. 2. The offshore olatfor structure of claim 1 (Fig. 2) uhereiπ only the uooer ends αf said stay cables /2/ are attached to the tower structure /1/, the stay cables louer " anchorages /3/ being outside the horizontal projection of the touer structure. 3. The offshore platform structure of claim 1, the tαuer structure /1/ of uhich is serving as suooort structure for uell tubes and/or riser oioes for hydrocarbons, uherein said tubes and/or pioes are laterally supported at each stay elevation of the tαuer structure. 4. Tαuer structure (Fig. 4) far the offshore olatform structure of claim 1, cα orisiπg a number αf columns /5/ uhich at each stay elevation belαu the elevation of the the uppermost one, are interconnected by means of only horizontal bracing members /6/. 5. The touer structure (Fig. 4) αf claim 4, uherein the tαuer structure /1/ comprises four columns /5/ each being laterally supported by stay cables in minimum tuo hori¬ zontal directions, the horizontal orojection of said touer structure being αf rectaπgulare shaoe. 6. The tαuer structure (Fig. 4a) αf claim 4,uherein said tαuer structure cαmorises four columns /5/ each stayed in one horizontal direction diagonally to the touer struc¬ ture horizontal projection, and each single column being interconnected to each one of the three other columns by means of horizontal bracing members /6/ at the tower stay elevations. 7. The offshore olatform structure (Fig.5) αf claim 1, wherein the tower structure /1/ is erected απ and stayed to a floating foundation structure /10/, uhereafter the completed platform structure is toued to its location and installed. 3. The offshore platform structure (Fig. 5) of claims 1 and 7, wherein the stay cables /?. / are anchored tα arms /11/ caπtilevering out from the foundation structure /10/. 9. The offshore olatform structure (Fig. 5) of claims 1, 7 and 3, wherein said cantilevering arms /11/ are braced to the base of the foundation structure /10/ by means αf inclined bracing members or stays /14/. 10. The offshore platform structure (Fig. 6) of claims 1 and 3, wherein the touer structure /1/ is discontinued some distance below the sea surface elevation /16/.

Inventors: ELLINGVAOG NILS A, ELLINGVAOG, NILS, A.
Applicants: ELLINGVAOG NILS A, ELLINGVAOG, NILS, A.
Application year: 1985
Main classification: E02B17
IPC codes: E02B17