Stiftelsen For, Industriell Og Teknisk Forskning Ved Norges Tekniske Ho/yskole Karel
| 1. | Basement to be located on the sea bed, comprising a floatable and submersible structure, with buoancy/ballast compartments, c h a r a c t e r i z e d b y having the buoancy/ballast compartments constructed as tanks made to withstand the ambient water pressure, which in a floating position, preferably in a submerged towing position, can be pressurized to make the internal pressure correspond to the water pressure at the depth of the location. |
| 2. | Basement according to claim 1, c h a r a c t e r i z e d b y having the tanks made of steel. |
| 3. | Basement according to claim 1 or 2, c h a r a c t e r i z e d b y having the tanks placed side by side between an upper and a lower horisontal platestructure which is mutually supported. » . |
| 4. | Basement according to claim 3, c h a r a c t e r i z e d b y having the two plate structures made of reinforced concrete. |
| 5. | Basement according to claim 3 or 4, c h a r a c t e r i z e d b y having the tanks acting as supporters between the platestructures. |
| 6. | Basement according to one of the preceding claims c h a r a c t e r i z e d b y having provisions for one or more drilling templates. |
| 7. | 5 7. |
| 8. | Basement according to one of the preceding claims, c h a r a c t e r i z e d b y having penetration skirts located on its underside. |
| 9. | Method for establishing a basement provided with 0 buoancy/ballast compartments on the sea bed, whereby the basement after construction is towed to the site of installation and lowered unto the sea bed, c h a r a c t e r i z e d b y towing the basement in a completely submerged position with the buoancy/ 5 ballast compartments pressurized to a pressure corresponding to the hydrostatic pressure at the depth of installation. |
| 10. | Method according to claim 8, c h a r a c t e r i z e d 0 b y hanging the basement in surface floats by means of strainabsorbing elements, for instance wires, during towing of the basement. |
| 11. | Method according to one of the preceding claims 49, 5 c h a r a c t e r i z e d b y employing the tanks as formwork during casting of the upper plate structure. |
TECHNICAL FIELD
This invention relates to a basement to be located on the sea bed, comprising a floating and submersible construction with buoyanc /ballast compartments.
The invention also relates to a method for establishing of a basement which is provided with a buoyanc /ballast com¬ partment. The basement after construction is towed to the site of employment and lowered onto the sea bed.
BACKGROUND ART
The construction of an independant basement which can be placed on the sea bed, " onto which basement offshore - equipment or an offshore installation afterwards can be placed, is advantageous for many reasons. By constructing the foundation independently, one may shorten the construc¬ tion time for the whole project, and the time needed for making the installation ready for production, may also be substantially reduced, because the foundation may be brought onto the site at an early stage in the schedule and the settlement in the often loose masses at the bottom may be finnished before for instance the lowering and position¬ ing of a gravity platform onto the basement. By employing a basement on the sea bed one can also drill wells, either from a ship on the surface or by means of subsea equipment. A basement located on the sea bed, also makes it possible
to place by way of example platforms made as a steel trass- work contruction on the sea bed, without the elaborated * piling- and securing-operations needed today. The founda¬ tion may be given a design that makes it a stable basement for a steel tower, and may also in a simple way be provided with suitable securing means for such a steel tower. Both gravity platforms of steel reinforced concrete and other types of platforms, for instance a steel tower, can in this way be placed on such a foundation and relatively easily be removed after production of the field is finished. The foundation may naturally be used for other kind of equip¬ ment, by way of example for the construction of a subsea station for one purpose or another.
DISCLOSURE OF INVENTION
The present invention is particularily aiming at providing a basement which in a particulary quick way can be installed on the sea bed, thus making the operation less dependant on the weather conditions. Unfavourable weather conditions may often result in considerable delays and interruptions during towing and submerging of.offshore constructions.
Therefore, according to the invention there is proposed a foundation as mentioned, having buoyancy/ballast compart¬ ments consisting of tanks which are able to withstand the external water pressure, which in a floating position, preferably in a submerged towing position, can be placed under an internal pressure corresponding to the water pres¬ sure at the depth of the installation site.
By having said tanks already pressurized to a pressure corresponding to the pressure at the depth of the installa- tion site, such a designed basement can be towed out in a submerged state and when on the installation site, the basement can be lowered very quickly onto the seabed. Calculations shows that a basement designed in this way can
be lowered in one day, compared to the 2-3 days or more, usually required. Conventional offshore structures have to be filled with gas during submerging to compensate for the increasing hydrostatic pressure and this requires long time and large compressor capacity. By employing a basement according to the invention, one can pressurize the tanks of the basement in calm waters, that is before the towing operation takes place. One therefore has no need .of the demanding compressor equipment when on the field, and as mentioned, the submerging-operation can proceed very rapid¬ ly because the tanks are already pressurized.
Preferably the tanks should be made of steel.
The tanks are preferably placed side by side between an upper and a lower plate structure which are mutually sup¬ ported. These two plate-structures can by a preferred embodiment of the invention be made of reinforced concrete, in which case the basement will be a hybrid construction (concrete-steel) .
Preferably the tanks can act as supporters between the plate structures, in which case the tanks will absorb stress and strain.
The basement is preferably equipped with one or more drill¬ ing templates, making predrilling feasible as soon as the basement is located on the sea bed, and before any platform is placed on the basement.
Underneath the platform there is made arrangement for pene¬ tration skirts for penetration of the sea bed.
BRIEF DESCRIPTION OF DRAWINGS
The invention will be described in more detail with refer¬ ence to the drawings, where:
Fig. 1 shows a cut through perspective view of the basement according to the invention, placed on the sea bed,
Fig. 2 shows a section along the line II-II in Fig. 1.
Fig. 3 shows the basement in submerged state during towing Fig. 4 shows the basement during the lowering at the desired site of location on the sea bed, and
Fig. 5 shows a possible placing of a gravity platform of reinforced concrete on the ' basement.
MODES FOR CARRYING OUT THE INVENTION
The basement shown in fig. 1 is constructed as a platform structure and consists essentially of an upper plate 2 of concrete, a lower plate 3 of concrete, supporters or shear- walls 4 between the plates 2, 3, and penetration skirts in the form of downwardly open cylinder bodies 5 protruding beneath the lower plate. All of said elements of the base¬ ment are made of reinforced concrete and are cast together in such a way as to make a monolithic structure. Between the upper basement-plate 2 and the lower basement- plate 3 there are arranged a greater number of standing sylindric steel tanks 6. These are desinged as pressure- tanks able to withstand a substantial internal overpressure. The steel tanks 6 are connected to the con- crete structure in a suitable way which makes the tanks become an integral part of the structure of the basement, acting as elements absorbing stress and strain between the concrete plates 2 and 3.
The basement in fig. 1 has a triangular outline. This shape is not essential and in this context only arbitrarily cho¬ sen, in order to accomodate a known gravity platform, con¬ fer fig. 5.
The basement is provided with four depressions 7, 8, 9 and 10. The depressions 7, 8 and 9 have only a constructive purpose in connection with the accomodation of the platform shown in fig. 5, in that the depressions and the
accompanying shearwalls 4 are located under the legs of the platform. The centrally arranged depression 10 in the base¬ ment provides space and means for a drilling template. The depressions moreover give reduced price and weight.
In fig. 1 and 2 the basement is located on the sea bed II and the penetration skirts 5 have penetrated the sea bed.
The basement may be constructed in known and suitable fash- ion in dry dock for later towing of the structure to calm waters, where completion may take place. After construction of the basement, it is connected to surface floats and submerged to a towing position. A stable towing position is provided by ballasting the basement so that the basement hangs in the surface floats. Such a stable towing position is shown in fig. 3, where the basement is shown hanging in several wires 12, 13 and 14, which run up to surface floats 15, 16 and 17. The basement 1 hangs in the floats at a considerable depth, for instance about 200 m, and each of the ballast tanks 6 is pressurised to an internal pressure corresponding to the hydrostatic pressure of the intended depth. When the basement has arrived at the site of loca¬ tion in this way, the basement can be rapidly lowered down to the sed bed, by way of example as shown in fig. 4, where the basement, during the submerging operation is guided by guidelines 18, 19 running between surface assistance ves¬ sels and anchors 20, 21 on the sea bed 11. The basement is lowered towards the sea bed while hanging in the wires 12, 13, 14. The wires are connected to suitable winches (not shown) placed on each of floats 15, 16 and 17.
As soon as the basement has reached the sea bed the buoancy tanks are filled with water and the basement is made to sink into the sea bed, possibly by employing reduced pres- sure or suction underneath the basement. As soon as the basement has come to rest on the sea bed 11, a gravity
platform for instance, may be placed on the basement. If the basement has been properly installed, i.e. after the main settlement of the basement has taken place, placing a platform in this way will produce only small settling move- ments, and the whole installation can therefore be put more rapidly in production.
The platform 22 in fig. 5 is lowered onto the basement 1 guided by lines 23 connecting the basement 1 to the plat- form 22 through fair leads 25, 26 on the platform. This makes a simple procedure for lowering the platform 22 onto the basement.
The skirts may be made of steel and also be spray coated with a material giving a filter action.
In some instances, where the sea bed is soft, it may be favourable to keep the ballast tanks pressurised with air. The tanks can very favourably be employed during the cast- ing of the upper deck, in that the top part of the tanks thereby are used as formwork, possible in a somewhat extended shape, while standard formwork elements are employed between the tanks.