Ice-resistant offshore platform for seismic regions

 

(57) Abstract:

The invention relates to the construction of offshore marine structures designed for drilling and/or production of oil or gas in seismic areas with ice period. Ice-resistant platform has a floating hull with ballast tanks, equipped at the bottom of a vertically oriented farm, and the mooring system with inclined anchor links, located in the star order hawse and bottom of pile anchors. In addition, it is equipped with extra suspension system with inclined links that through the hawse holes in the lower part of the farm is based floating body, with angled connection of the suspension is stretched vertically in the direction opposite to the direction of tension of the mooring system with inclined anchor links and rely on the Central beams piles piles with elevation above the bottom of the sea more than the distance from the bottom of the sea the hawse suspension with inclined links on the bottom of the farm. The buoyancy of the hull of the platform is made less than the sum of its weight and the vertical projection downward force from the mooring system with inclined anchor links. The proposed design allows you to provide simultaneously against the practical progress of the seabed and Umilenie movements from the effects of rough seas. 1 Il.

The invention relates to the field of construction of offshore marine structures designed for drilling and/or production of oil or gas from offshore fields in seismic areas of the ice age.

Known for numerous marine structures (see, for example, D. A. Mirzoev. Oil and gas resistant construction of the shallow shelf. - M.: VNIIEM, 1992, page 15, 26 or So Dawson. Design of offshore structures. Leningrad: Sudostroenie, 1986), installed on the bottom of the sea: the gravitational - presses to the ground due to the ballast, pile on pile Foundation and pile-gravity. Solid construction (steel or concrete), made of slabs, forming a tight loop, reliably withstand the ice fields, giving them the power to the ground, but at a depth of more than 60-80 m platform of this type become cumbersome and bulky, and their cost increases in cubic degree from the depths of the sea. In addition, for gravity platforms in seismic areas need a lot of ballast to ensure stability, along with the growing pressure from the ice loads can lead to invalid razrushauschiesa.

Much less sensitive to the seismic impact of the platform on a floating base for tension ties (foreign terminology TLP), anchored gravitational or pile anchors (see, for example, Recommended Practice for Planning, Designing and Constructing Tension Leg Platforms, API RP 2T, American Petroleum Institute, 1997, S. 3, 12-28, 55). These platforms are used mainly for medium and large depths of the sea, and classifying their characteristic is that anchor ties are strained by excess buoyancy of the hull. The specified type platforms feel the seismic shift of the ground only through the bottom, usually in the form of piles or beams piles, anchors, support which is easy to prevent.

Closest to the proposed decision of a kind specified type platforms platform type Spar (see J. E. Halkyard, J. Murray "Spar As a Production Platform in the Arctic Environment", Proceedings of the Third International conference "Development of the Arctic seas of Russia" RAO'97, S. Petersburg, 1997, S. 17-22, 25, 31, 34), "truss" option which is taken as a prototype. Floating anchored platform - prototype (see Fig. 2), has to support the upper structure 1 floating body 2 in the form of a vertically extended column with ballast tanks and attached to the chassis bottom vertical (if necessary). Farm 3 ballast ensures the stability of the platform as floating structures in operational working position of the platform. The mooring system of this platform with inclined anchor links 5, located in the star order, hawse holes 6 and the bottom of pile anchors 7 holds the platform above the wells so that movement of the floating body 2 from the effects of external disturbances did not exceed the allowable use of drilling and production risers 8. Such platforms are installed at relatively large depths (200 m and more), along with the seismic disturbance and can easily withstand the effects of rough seas, having a small amplitude of pitching due to the substantial differences in the period of natural oscillations (as inverse pendulum on strained ties) and periods of the waves.

However, this type of structures is poorly resists the global ice load, because the drifting ice its pressure displaces the floating body structure in a horizontal direction to an invalid value, nakrekal and protoplanet it due to the strong tension ties located from the application of the ice load, and the small area of the waterline. To prevent excessive source of Finance for load i.e. the "vicious circle" in the design.

The present invention is to solve the problem of providing ledostojkoj structures, i.e., preservation of body movements of the platform in a valid (riser) limits when exposed to ice, preserving, at the same time, sufficient resistance to seismic and wave impact.

To achieve the desired results support the base platform is made, as in the prototype, of the vertically stacked: a buoyant hull (for example, a vertical waterproof columns) and a vertically oriented farm abuts the bottom end farm in additional mooring system - a suspension system, while the lower end of the farm does not reach the bottom of the sea to a distance defined by the mounting technology, but not less than the maximum in the area of sea-level change (caused, for example, the tide-the tide and/or storm surge). Thus, the platform has two mooring system. The upper mooring system (prototype) has the hawse holes, placed on the floating body or the upper part of the farm, and inclined (to oppose the ice load) tension connection, fixed bottom anchors, and additional the e farm and inclined (oriented vertically opposite relations top systems that the total force is directed upwards) short communications, located in the horizontal projection of the stellar image is fixed on the pile anchors. Pile anchors represent clusters (fascicles) of piles fastened together, the Central of which rises above the bottom to the height, the greater the distance the hawse on the farm from the bottom of the sea. The total buoyancy of the column and farms with less weight structures (together with superstructure) in the amount of vertical projection forces of the tension ties the top of the mooring system so that part of the weight of the structure was perceived lower suspension system. The resulting so negative buoyancy of the platform (the value of which must be greater than the amplitude of the vertical lifting force from the maximum wave), is provided by a corresponding limitation of the floating volume of the body and is additionally regulated by the admission of water into the ballast tanks.

Due to the presence of such a design is achieved, firstly, sufficient ledostojkoj platform due to the fact that the floating bearing housing through the farm to the additional lower suspension prevents peoplenew and the inclination of the support base and, thus, the conditions DOPOMOGA base and overturning moment. Secondly, remains characteristic for floating structures low sensitivity to seismic shock through the free bearing of the lower end of the farm. Third, increases resistance to wave loads as by maintaining a small area of the waterline of the hull, and by the possible regulation of the period of oscillations of the structure by choosing the optimal tension of the upper links with the transfer of part of the load on the lower link.

The proposed structure is illustrated in the drawing.

The drawing shows a General view of the platform in operational working position. The platform consists of the upper structure 1 (modules for various purposes, rig etc), the support base including a floating body 2 with ballast tanks 3 and supporting farm 4, and two mooring systems. Top (traditional) system includes hawse 5, the floating body 2, the anchor connection 6 and bottom anchors 7 (preferably pile type). The lower the mooring system - a suspension system includes hawse holes 8 on the lower end of the farm 4, the beams piles 9 and the tension connection 10. Both the upper and lower links can be composed of steel beams or synthetic ropes, or chains, or cords t the sediment structures may be regulated due to ballast tanks 3 floating body 2 when you install the platform and in the process of operation depending on the load, environmental conditions, season, sea level to optimize counter external loads.

The resistance of the platform with respect to external loads and its stability is ensured in the following way. When earthquakes shake the ground, manifested in the form of vertical and horizontal accelerations and motions. When this occurs predominantly horizontal displacement of pile anchors 7 and 9, leading thanks to the free suspension farm 4 and the damping effect relationships 6 and 10 only to a small safe (for the upper structure 1 and equipment ties with wells) the concussion of the housing 2. All vertical efforts from the earthquake and from the sea or periodic fluctuations in the level of amortized vertical compliance of the links 6 and 10 and the freedom of the suspension 10. In the case of a violent storm arising wave forces and moments do not lead to large amplitude oscillations of the floating body 2 due to the selection of such tensions links 6 and 10, which provide the periods of oscillations of the structure, far from the resonant periods of rough seas. And, finally, in the event of exposure drifting ice fields the existence of two systems mooring allows komiya and heavy loads on the bottom of the sea. A characteristic property of the proposed design (absent in the prototype) is that when exposed to ice (approximately at the level of the waterline) the top of the mooring system to counter panning and tilting is provided by the maintenance side of the lower suspension system that prevents peoplenew the substructure.

The reaction of plants to the load may be regulated due to the optimal (for the season) the ratio between the buoyancy body 2 and the tension of the top 6 and the bottom 10 ties. So, for example, to better address the ice load tension ties should be strengthened to increase the rigidity of the system, and during the summer ice-free period, the tension must be, on the contrary, weakened, which will increase the period of your own pitching structures and, consequently, to avoid resonant vibrations in the band main periods of rough seas. Regulation of buoyancy of the hull is made by appointment or removal of ballast water from the ballast tanks 3.

Thus, the proposed design allows you to achieve a new result, providing (previously unattainable) to execute simultaneously four qualities necessary for obespechenie fixing structure as in the top, and in the lower part (the two systems, mooring and suspension), 2) prevent unacceptable impacts on the ground - due to the partial compensation of the weight of the structure and vertical loads buoyancy of the case support base 3) repayment of accelerations from seismic movements of the sea floor due to the rolling of elastic knot fastening anchor farm, 4) Umilenie movements from the effects of rough seas due to the adjustable buoyancy of the hull and pull the anchor links of the upper and lower systems.

Ice-resistant offshore platform for seismic regions, containing the supporting base in the form of a buoyant hull with ballast tanks, equipped at the bottom of a vertically oriented farm, and the mooring system with inclined anchor links, located in the star order, hawse and bottom of pile anchors, characterized in that the ice-resistant platform equipped with an additional mooring system in the form of a suspension with inclined links that through the hawse holes in the lower part of the farm is based floating body, with angled connection of the suspension is stretched vertically in the direction opposite to the direction of tension of the mooring system with inclined anchor links and rely on the Veski with inclined links on the bottom of the farm, thus the buoyancy of the hull is less than the sum of its weight and the vertical projection downward force from the mooring system with inclined anchor links.

 

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