Jack up
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installation and removal of jack-ups offshore structures, civil engineering, M. Tech
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Jack up
- 1. M.GEORGE BUSH M170281CE INSTALLATION AND REMOVAL OF JACK-UPS
- 2. Contents Introduction Types of supports for jack-ups Pile supported Jack-ups Mat supported Jack-ups Geotechnical Problems Related to Jack-ups During the installation of the platform During the removal of the platform References
- 3. Introduction Jack-up platforms are mobile units which rest on the sea bottom during the operation and they may be used for exploratory drilling or production. At present there are about 150 jack-ups in service throughout the world operating in water depths ranging from 30 to 100 m.
- 4. Types of supports for jack-ups The jack-up platforms basically be divided into two types 1. Platforms resting on piles ending in bases or caissons (known as spud cans). 2. Mat-supported jack-up The stability of the jack-ups mainly depends on the type of support to the nature and mechanical characteristics of the terrain.
- 5. Pile supported Jack-ups The jack-ups were simply resting on the sea-bed without any base for the piles. Spud cans are typically conical structures with sloping top and bottom. The spud cans are usually cylindrical with diameters ranging from 5 - 15 m and with tip angles of 60 - 120.
- 7. Mat supported Jack-ups When the jack-ups are to be set up in very soft deposits, the bases or spud cans would penetrate to deeper depths. In such situations, the mat-supported jack-ups are the ideal . They have been widely used in the Gulf of Mexico, Pacific coast, West African Coast etc. The platform is generally supported on a single rectangular mat with a central opening or may be on one or two mats linked together by struts with hydrodynamic shape for easier movement.
- 9. Geotechnical Problems Related to Jack-ups During the installation of the platform Stability during drilling / production During the removal of the platform. The method of installation and removal, stability during the operation, clearly varies with the type of the platform and the nature of the soils.
- 10. During the installation of the platform The installation of a jack-up platform is the penetration of the piles which depends on the nature and consolidation of the surface material, the geometry and type of support. The maximum penetration of the supports during the installation of the jack-ups is Dmax = H - ( W + Amin ) D max = maximum penetration of the supports H = total length of piles W = depth of water + maximum wave amplitude Amin = total minimum height above the water line
- 11. The maximum penetration D is the sum of the penetration under the effect of static loads and that under the action of horizontal forces. The jack-ups supported on bases or spud cans commonly penetrate to about 15 - 20 m in soft soils and may be up to 30 - 40 m in very soft soils. The mat supported jack-ups usually penetrate to about 3 - 4 m. Whereas, in the case of consolidated materials the penetration is quite less such as 1 - 2 m on dense sands or about 5 – 10m in medium consolidation clay.
- 12. Penetration of Supports On installing the jack-up platform, the supports will continue to penetrate until the bearing capacity of the soil balances the unit loading caused by the weight of the structure. The supports will further penetrate during the operation of the jack-up or due to the wave forces and the consequent to dynamic loads. The further penetration can be eliminated / restricted by preloading the structure
- 13. Supports on Clays or Muds The general equation of bearing capacity can be written as qu = Cu Nc + γ'.D D = penetration of the supports For bases or cylindrical (or square) spud cans with diameter B, buried to a depth D Nc = 6 ( 1 + 0.2 D/B) the value of Nc ranges from about 6 - 9. For the mat supported structures with a mat dimension of B and L, buried only a little, Nc can be taken as Nc = 5 (1 + 0.2 B/L)
- 14. Supports in sand The general bearing capacity equation can be written as qu = 1/2 γ' B Nr + γ' D Nq B = diameter or width of the supports
- 15. During the removal of the platform The jack-ups are removed by simultaneously jetting around the mats or spud cans and pulling out the piles by suitable pulling devices. Jetting is carried out in turn in each support and it is essential to prevent any leg from settling into the soil due to jetting for a long period. The duration of jetting needed to release the supports obviously varies with the nature of the soil, number of supports, depth of penetration etc.
- 16. In the case of spud can jack-ups, the total load required to be raised is approximately equal to the weight of the sediment filling the hole, created by lowering the supports and the friction of the sediment on the racks and braces. In the case of mat supported jack-up platforms resting on a soft soil, the weight to be raised in proportional to the force of suction between the mat and the soil.
- 17. References Lovie, P.M. - "Jack-ups, a Future in the North Sea", Northern Offshore, No.4, 1974. Tubb, M. - "Jack-ups", Ocean Industry, Sept. 1976. stability of jackups,Design guides for offshore structures-”Clarom”