The present invention relates to an equipment and a method of positioning and mounting large installations on the εeafloor or on mainland.

Examples of such positioning/mounting operations are the mating operation for a large offshore construction onto a previously mounted template on the seabed _* or building-like constructions onto a construction already standing on the ground, and where separately installed directing or docking piles are used. For simplicity reasons mainly seabed operations will be dealt with in the following.

A typical such mating operation may involve that a supporting structure or "jacket" is to be lowered down in order to be secured in a predetermined position on/above a template or an equivalent construction underneath. The jacket shall be maneuvered downwards so that docking sleeves suitable thereto can be threaded down onto docking piles which have been driven or drilled down into the seafloor around the template, in order that the jacket may be placed exactly in position over the template. Two conflicting requirements can always be set up in connection with mating operations of this kind:

On one hand there exists a requirement that the hori¬ zontal deviation between jacket and template be as small as possible in relation to the completely ideal position. This in order to secure the subsequent tieback operation, i.e. coupling together conductor strings to the already completed wells (well heads) in the template.

On the other hand one would like to have large hori¬ zontal clearances or tolerances in the guiding system, i.e. between the docking piles and the guiding or docking cylinders which are to be threaded down onto the piles in order to simplify and secure this operation itself, which operation is critical regarding weather, wave and current conditions.

Traditionally a compromise is made, and one selects tolerances in the guiding system to make the mating operation feasible within a specified "weather window" based upon statistic on such conditions.

The previously known technique regarding such positioning and mating is principally based upon three systems, all using

two or more docking piles to effect guidance both "sideways and directionally" during the mating operation. As previously mentioned, these piles have been installed in advance. They have been driven or drilled down into the seabed under guidance from guide members mounted for instance on the ends of the previously mounted template. Then the jacket is floated or hoisted in above these docking piles.

A) "Fixed" system: When hoisting the jacket down, an open cylinder, the docking sleeve, which is fixed to the jacket, is firstly guided down onto the corresponding one of the docking piles. Thereafter the jacket is rotated until the other docking sleeves are in position right above their corresponding docking piles, and the jacket is lowered down the last part of the way, until the jacket is standing in its position on the seabed.

B) "Active" system: This system is similar to the "fixed" system above, however, the real docking sleeves are mounted loosely each in a respective fixed cylinder, i.e. the docking sleeves may be moved vertically. The loose docking sleeves are suspended in respective wires, and thus may be lowered indivi¬ dually down onto the corresponding docking piles. In this system there is a possibility of raising the docking sleeves again if necessary.

C) "Passive" system: This system is also similar to the previous one, however, the loose docking sleeves are not suspended in wires and therefore can not be re-raised.

The previously used systems are burdened with several drawbacks:

The ultimate and essential point is being able to achieve the joining together of the oil conductor strings belonging to the platform and the pipes protruding up through the template (i.e. the tieback operation) . Because of the above mentioned compromises, which must be made regarding tolerances in the dimensional measures of piles and sleeves as well as positions and angles thereof, angular deviations as well as deviations in position may become larger than advisable during the tieback operation. In other words: Good, i.e. small tolerances for the tieback operation entails a greater risk when effecting the very critical mating operation, when using these prior art systems.

Moreover, with the known systems there exists a possi¬ bility that the parts may get stuck during the mating operation if the tolerances are exceeded. This may prove fatal.

As a secondary point, it should also be mentioned that the previously known sleeve system is always large and heavy, and i connection with these large steel constructions it is also ^■ necessary to use corrosion preventing electrical systems of considerable size.

On the other hand, when the present invention is put into use, the following advantages are gained:

The tolerances in connection with the tieback operation are radically improved.

The mating operation itself may be effected with larger clearance, which simplifies the operation and makes it less critical. The mating operation is very critical per se, becaus the jacket is afloat during the operation and is exposed to changing weather conditions.

The construction allows the use of a centering gimbal which is fixed by welding to each one of the docking piles. Suc gimbals provide reduced local positional and angular deviations in relation to the guide members on the template when the piles are drilled down into and cemented to the seabed, compared to a pile without such a gimbal.

The preliminary analysis of the mating operation is simplified because the number of variables entering into the calculation, is reduced. Thereby also the reliability of the analysis results may be increased. The traditional method of using long sleeves down onto the piles, results in a large number of possible combinations of stiffnesses, since the stiffness is changed the further down the sleeve is threaded onto the pile. There are also uncertainties in determining a single stiffness. When using the system in accordance with the present invention, only the stiffness of the pile itself is a variable parameter during lowering down, while the previous variations relating to the sleeve are eliminated.

There is achieved a weight reduction of the mating system of about 90 % in relation to the sleeve systems used to-day.

As a consequence of this reduction in weight and surface area, a corresponding reduction of the number of corrosion preventing anodes is also achieved.

The production of the present system is clearly simpler than the conventional systems.

A further and apparent advantage of the system in accordanc with the present invention is the cost savings which can be appreciated in connection with several of ^" the items men-tioned previously.

The advantages mentioned above are gained by putting into use a positioning equipment and a method of mating for a large construction on the seafloor or on mainland as indicated in the enclosed patent claims.

A closer explanation of the invention is following, referring to the enclosed drawing figures, in which: FIG. 1 shows previously known mating systems, FIG. 2 shows a traditional system with fixed sleeves, FIG. 3 shows a section along the line A-A in FIG. 2, FIG. 4 shows the fundamental embodiment of the positioning equipment in accordance with the invention, shown in a corresponding manner as in FIG. 3 , FIG. 5 shows the positioning equipment in accordance with the present invention, with a view in the arrow direction at B-B in FIG. 4, FIG. 6 shows a cross section view of ring and slot in accordance with the invention, FIG. 7a shows docking piles equipped with centering gimbals, made possible as a result of the invention, as well as the corresponding reference positions, FIG. 7b shows reference positions when using an alternative system without gimbals, FIG. 8 shows a sequence of different stages during lowering down in accordance with the invention, FIG. 9 indicates a few of the sources of error which a traditional system must take into consideration, FIG. 10a shows the static system when using traditional equipment,

FIG. 10b shows in a corresponding manner the static system when using the equipment in accordance with the invention, and FIG. 11 shows a variant of the present invention.

In FIG. la-c there are shown previously known techniques of positioning large constructions on the seafloor, as mentioned above. All the three solutions which have been sketched are based upon the use of large sleeves which are threaded down onto docking piles in order to direct the construction into its position. The present invention does not depend upon such sleeves, as will appear below.

FIG. 2 also shows a traditional system using fixed sleeves 3, as in FIG. la. The angle between the directions of the two pipes 6 and 7, as well as the sideways deviation S , are paramete to be minimized in connection with the tieback operation. The docking sleeves 3 are long cylinders extending beteen the two lower levels of horizontal bracings of the con-struction 1, and they are threaded down onto docking piles 4. A frame or template 2 mounted on the seafloor forms a starting point for the installation of the piles 4 in the seafloor 5. Thereafter, not shown guide members are removed by burning and are hoisted away from the remaining stumps 8 , so as to avoid transmitting the shock loads from the mating operation through the bottom template itself and thereby inflicting damage to the well heads.

In FIG. 3 the same thing is viewed from above, or as a section along A-A in FIG. 2. The docking piles 4 are shown in black inside the sleeves 3.

In FIG. 4 the central parts of the positioning equip-ment in accordance with the present invention appear in a view of the same type as FIG. 3, however in which the template has been left out for the sake of clarity. The docking piles 4 are here also shown in black colour.

FIG. 5 shows the same situation in a side view. The central features of the invention are as follows:

The docking sleeves (3, FIG. 2 and 3) have been exchanged for a guide ring 10 and a guide frame with a rectangular slot 11. The ring 10 and the slot 11 are in this case located in the same plane, for instance as shown in FIG. 4 and 5, in the

lower bracing level. However, it is equally possible to place ring and slot in their respective horizontal planes, vertically spaced from each other. Furthermore the guide slot 11 is arranged in such a manner that its longitudinal center line is directed to the center of the ring 10. Both ring and slot are suitably equipped with downwardly opening directing parts for reception of the docking piles 4. Thus the guide ring 10 is equipped with a conical directing collar 10a with a lower and . larger diameter D _Q , which directing collar in its upper part is gradually curving into the smaller ring diameter dø. which diameter is fitted to the diameter of the docking pile , see also FIG. 6. In a corresponding manner the guide slot 11 is constructed with a directing skirt 11a, which preferably has th same cross sectional shape as the guide ring 10, 10a, when viewing that section which is perpendicular to the above mentioned center line, in other words a view from the position of the guide ring 10. Thus, in this section one sees the same lower maximum opening D _Q and the same minimum opening dg. When mentioning "ring" and "slot" hereafter without further explanat statement, there shall be understood the ring and the slot where the openings are the smallest. When constructing these minimum openings it is only necessary to take into consideratio construction tolerances of piles and guide parts.

By making the lower opening (D ₀ ) of the directing parts 10a, 11a (see FIG. 6) sufficiently large, the requirement for maximizing horizontal clearances/tolerances in order to secure the installation operation within a given "weather window" , is complied with. A value of this largest opening may be calculat from the "mating analysis", i.e. from the movement characterist of the construction during the mating operation.

The system itself with ring 10 and slot 11 eliminates the consideration for horizontal deviation on top of the docking piles (because of vertical deviation on the pile itself) , see FIG. 8 in connection with FIG. 9. From FIG. 9 the increased tolerances which must be taken into consideration by the traditional sleeve system appear clearly, the highest points of the docking piles possibly being erroneously placed within margins as sketched, having dimensions _] _, respectively δ.2 .

When using the system including ring and slot, such erroneously positioned points do not pose a problem, since the ring will slide down along one of the piles, the slot at the same time allowing a sliding movement along the other pile, independent o angular errors. At the same time it should be noted that the angles in the drawings have been partly grossly exaggerated in order to clarify these considerations.

Thus the present invention utilizes the fact that the ring arrests the construction from movement in the xy-plane, while the slot stops or determines the rotation in the same plane. A previously mentioned, ring and slot may thereby be constructed as narrow as desired, i.e. it is only necessary to take into consideration the construction tolerances of ring, slot and pil

In a preferred embodiment of the invention the system is constructed in such a manner that that point of a docking pile 4 which is to have the smallest horizontal deviation relative to the template 2, will be situated in the same level as the narrowest horizontal plane in respectively ring 10 and slot 11 on the construction 1 when the last mentioned stands in its final position, i.e. on the sea floor. This is tentatively illustrated in FIG. 7a and 7b, in which in both cases the hori¬ zontal deviation shall be minimized in relation to the distance x' , which represents the theoretically correct horizontal dis¬ tance between a reference point 15 on the template 2 and an idea position 14 of the docking pile 4.

Thus, when looking at the arrangement according to FIG. 7b, ring and slot shall in the end, i.e. when the construc¬ tion 1 is standing on the seafloor 5, be brought into the same level as the point 14. This requires of course in the shown case that the guide members 12 have been removed prior to the mating operation, which removal is normally made by burning off and hoisting up the guide member 12 itself, see also FIG. 2, reference numeral 8.

However, in the most preferred embodiment of the invention a system as shown in FIG. 7a is used. Here the docking piles 4 have been equipped in advance with centering gimbals 13 which have been fixed by welding and are adapted to cooperate with the guide members 12. Such a centering gimbal provides a pile point

14 which deviates to an even lesser degree from its ideal position than the pile point 14 of FIG. 7b. Thus, in this manner it is possible to achieve a further reduction of the horizontal deviation in relation to the reference point 15. However, the welded gimbals 13 entail that the guide members 12 cannot be hoisted up after burning. For this reason the guide members 12 are placed in a sufficiently high position on the template 2 that when being burned off, they may fall down to the seafloor 5, thereby being brought in such a distance from the parts of the construction 1 when the latter is lowered down, that said guide members 12 do not constitute an obstacle to the mating operation. Simultaneously it should be remarked that loosely mounted gimbals are not always desirable, since these gimbals must be removed by means of some mechanical system. Furthermore it must be remarked that also FIG. 7b shows guide members 12 in a high position, so that also in the situation indicated here, it is possible to utilize the advantage of merely dropping burnt-off guide members 12 down to the seafloor.

Thus, in the situation shown in FIG. 7a the guide ring 10 and the guide slot 11 will be lowered down to the level indicate by the point 14, the construction 1 simultaneously reaching the seafloor 5. With ring and slot in this level a minimum value of horizontal deviation is ensured. Accordingly it will be appreciated that the combination of the features guide slot and guide ring, securing the final level of the latter parts in the same level as the pile point 14 in question, as well as guide members located in a sufficiently high position and the use of centering gimbals, imply an essential decrease in the horizonta deviation possibly appearing between the construction 1 and the template 2. This because that point 14 of a docking pile 4 which has the smallest horizontal deviation in relation to the template 2 , determines the relative horizontal deviation for the construction 1 in relation to the template 2.

In FIG. 8 is shown a sequence of the different stages of the mating operation. Docking piles 4 with gimbals 13 have been drilled down in advance. Thereafter the guide members have been burned off, and the latter are now lying on the seafloor, see reference numeral 16. The construction 1 to be hoisted down is

equipped in its lower bracing level with ring and slot, in line with what is shown in the preceding drawings. The ideal horizontal distance (x' , see FIG. 7a) from the reference point 17 in the template 2 to the ideal position of point 14 above the centering gimbal 13 on pile 4, is found to be the distance from point 18 in the construction to the center of the ring. It is now a goal to bring point 18 into horizontal co¬ incidence with point 17, i.e. point 18 is supposed finally to b located exactly vertically above point 17. From position I the construction 1 is lowered with the ring 10 down towards the top of left docking pile, the size of the lower opening of the directing collar 10a securing and simplifying this operation. Lowering down of the construction continues through phase II, the ring following the pile downwards with a small clearance. At the same time the construction 1 is now adjusted rotationall about the left pile, so that there is achieved accomodation of the slot (to the right in the figure) onto the right hand pile. The directing skirt of the slot simplifies admission of small movements in the critical moment of the mating, i.e. just befor phase III. During the further lowering the construction is guided by both ring and slot which are situated around their respective docking piles. When the lower parts of the construc tion touch the seafloor and stop, ring and slot are located in the same level as point 14. The possible .horizontal deviation now existing regarding point 18 in relation to point 17, is due to errors in the position of for instance the center of the rin relative to the ideal position of point 14. As mentioned pre¬ viously, these errors have now been minimized.

In FIG. 10a and 10b a coarse and simplified comparison is shown between the traditional configuration with a sleeve and the configuration in accordance with the present invention, regarding the "static system". In the tradtional configuration, FIG. 10a, two engagement spots appear for a docking pile inside a docking sleeve, shown by arrows F^ and F ₂ • For a comparison only one engagement spot appears in the configuration in accordance with the present invention, shown by the arrow F ₃ . The deformed condition of the docking pile in the two cases is shown symbolically beside. Analytically, the configuration in

accordance with the present invention (FIG. 10b) constitutes an essentially simplified system. As previously mentioned in this specification, the mating analysis comprises a number of variable parameters, for instance stiffnesses. The number of possible combinations of stiffnesses constitutes a problem since stiffnesses are changed as the sleeve slides further down onto the pile. In addition there exist uncertainties when determining a single stiffness. Since the system in accordance with the invention only has one engagement spot, the stiffness of slot or ring does not change, but only from the pile itself during the lowering operation. Since the number of variables in the analysis is reduced, the reliability of the analysis results is also increased.

In the shown figure examples the construction 1 to be lowered appears in the form of a typical steel jacket. How¬ ever, the invention finds general application in any installing operation of a construction above an already installed structure on the seabed or on mainland, when sideways tolerances are of vital importance. Typical examples are, in addition to the shown jacket over a template, a concrete platform, a protecting structure or an underwater installation on top of a frame or template.

When comparing two actual cases, namely the Oseberg-B Jacket and the Oseberg-II Wellhead Platform, as existing in May 1987, calculations have been made to show a reduction in the tieback moment which can be transformed directly into tolerances, of about 70 %. However, it must be noted that lesser parts of this reduction are due to other matters than those related to the invention, namely the use of three piles instead of two. However, on the other hand "as installed" tolerances, i.e. known and certain tolerances were applied regarding Oseberg-B, while in the Oseberg-II calculation example all theoretical uncer¬ tainties were taken into consideration, i.e. larger uncertainties Accordingly, this resulted in a conservative calculation of the reduction in tieback moment.

The way the invention has been referred to previously in this specification, it is related primarily to, and has also been developed from a "fixed" system, compare the prior art

techniques mentioned in the introductory part of the specifica¬ tion. However, variants of the principle according to the invention, i.e. including ring and slot, may also be constructe starting from the previously mentioned "active" and "passive" systems.

For example both slot and ring, or possibly only one of these, may be constructed to be movable and thereby the effect of these movable systems is achieved.- The characteristic feature of the movable systems (active/passive) is that they ar able to enclose the docking pile rapidly when the construction has been brought into position, in addition to the fact that th construction is made less sensitive to vertical blows from the docking pile against the guide part of the construction, which docking pile may thrust into the edge under unfortunate circum¬ stances.

The slot and the ring in accordance with the present invention may for instance be constructed as movable elements simply by placing each of them inside a larger cylinder which corresponds to the traditional docking sleeve, whereafter the docking sleeves can be hoisted and lowered as desired. In this connection it is referred to FIG. 11, where the docking sleeves 19 can be lifted up and down by means of wires 20, and where th docking sleeves comprise guide parts 10, 11 in ac-cordance with the present invention, which guide parts are located inside the docking sleeves.

As previously mentioned the invention may also be realized by locating the ring and the slot in different levels in the construction to be lowered down, on the condition that the heights of the two piles are adapted to such a situation. As long as the ring can be threaded onto "pile no. 1" first, and thereafter a possibility exists for rotation in order to fit th slot down onto "pile no. 2", the invention will be able to function as intended in this regard. The point is that the gui member and the guide part (i.e. ring or slot) in the final stag are supposed to be located in the same level for each single pi

Nor are a directing collar/directing skirt for the ring and the slot necessary features per se, but practical addi¬ tional features of the invention. A corresponding directing

effect during the mating operation is also achieved for instance by sharpening or rounding the tops of the docking piles.