Gravity Platforms

Fixed Platforms Gravity platforms

Gravity Structure GBSGBS- Grav Gravit ity y Ba Base sedS dStr truc uctu ture re Condeep - Concrete GBSGBS- Conc Concre rete te,, Ste Steel el

Gravity Platform • Design: production; • large reinforced concrete bottom mounted structure • uses its weight to resist environmental loads, • not attached to the bottom with piles • Depth: medium… up to 350 m

Advantages • (1) support large deck loads, • (2) possible reuse, • (3) construction and testing may be completed before floating and towing to site, • (4) large field, long term production (supports a large number of wells), • (5) may have large storage capacity, • (6) more tolerant to overloading & sea water exposure than steel jacketed platforms

Disadvantages • Disadvantages: • (1) cost increases exponentially with depth, • (2) foundation settlement, • (3) subject to seafloor scour, • (4) may require more steel than steel  jacket structures

Field examples • Three concrete gravity platforms for the North Sea oil industry at Ardyne Point on the West Coast of Scotland between 1974 and 1978. • Weighing over 300,000 tonnes, at the time of construction Cormorant ‘A’ was the largest oil production platform ever built. • The 100m square, 56m high caisson had a storage capacity of one million barrels and carried four 116m high towers.

Gravity Platform

Description • Concrete Gravity Base Structure have been constructed using a base manufactured from reinforced concrete. • The design of base includes void spaces or caissons to provided the structure with a natural buoyancy which will enable it to be floated to field development location. • Once on location the void spaces are flooded on the seabed whilst the topside modules are lifted into place.

Description • The void spaces then used as storage compartments for crude oil, or filled with permanent iron ore ballast. • The colossal weight of concrete structures obviates the need to install foundation piles, hence the name gravity base structure.

Where to built it? • Template structures are suited to soft soil regions like Gulf of Mexico • Deeply driven piles are needed to fix the structure in place and carry required loadings • This is not suitable in hard soil conditions since pile driving is difficult • Alternative structures with large foundation elements are developed • Relies on its own weight to resist lateral environmental loads • Usually the foundation elements are ballasted and contribute significantly to the weight • Such structures are called Gravity Structures

GRAVITY PLATFORMS – CONCRETE (STEEL also possible) Mostly Concrete Gravity Platforms - Very massive.. Typical weight 300,000 tons - Stability is through the weight of the platform - Just rests on the sea bottom and Environmental forces (wind, wave) are resisted through the friction between the platform bottom and sea-bed Gravity structures are possible only with strong sea-bed strata. In locations where seabed strata is poor (soft clay or loose sand) which cannot offer required bearing capacity, these structures cannot be constructed. In these situations pile foundations are adopted. Water Depth: 70 – 200m CONDEEP – Concrete deep water structure

 Variety of designs – Commonly adopted one is discussed The tower design consists of  Three basic components (1) Caisson or a cellular base  – cellular concrete foundation (serves as oil storage tanks) (2) Two or three or four hollow-concrete shaft or tower extending above to support deck structure (steel) (3) Deck (steel) Foundation (Caisson) bases are circular or square or hexagonal or octagonal

CONDEEP • •

CONDEEP – Concrete deep water structure CONDEEP platform has a caisson with 19 vertical interconnected cylindrical cells, with spherical domes at each end

•

Cells provide buoyancy during construction/towing and later serve as oil storage space.

•

A few (three) of the cells are extended upwards as shafts or towers

•

Below the spherical dome of the bottoms of the cells, the cylinder walls extend as 0.95m thick concrete skirts to a level of 0.5m below the bottom of domes

•

Beneath the concrete skirts, steel skirts extend (3.5m) from the twelve outer cells and the central cell

WHY STEEL SKIRTS? •

IMPROVE FOUNDATION STABILITY

• • •

ACT AS EROSION RESITANT FEATURES ASSIST IN PROPER GROUTING OF THE CAISSON BASE PROVIDES TRANSVERSE RESISTANCE TO PLATFORM SLIDING

•

BESIDES STEEL SKIRTS, THREE STEEL DOWELS ARE PROVIDED ON THE PERIPHERY OF THE CAISSON. DOWELS EXTEND 4m BELOW THE LEVEL OF STEEL SKIRTS

•

DOWELS HELP TO PREVENT POSSIBLE DAMAGE TO STEEL SKIRTS DURING TOUCH DOWN BY PROPERLY ANCHORING THE CAISSON

 ADVANTAGES OF GRAVITY PLATFORM OVER JACKET PLATFORM GREATER SAFETY FOR PERSONNEL AND FACILITIES TOWING TO SITE WITH DECK – MINIMIZING INSTALLATION TIME AND COST LOW MAINTENANCE COST (CONCRETE IN SUBMERGED WATER…LESS OF A  PROBLEM RATHER THAN STEEL)  ADJUSTABLE CRUDE OIL CAPACITY  CAPABILITY OF SUPPORTING LARGER DECK AREAS STEEL RISERS ARE PROTECTED AGAINST CORROSION BY PLACING IN THE CENTRAL SHAFT POSSIBLE ACCESS TO THE SEA FLOOR FROM THE CELL COMPARTMENTS IN THE FOUNDATION

ADVANTAGES OF GRAVITY PLATFORM OVER JACKET PLATFORM

•

GREATER SAFETY FOR PERSONNEL AND FACILITIES

•

TOWING TO SITE WITH DECK – MINIMIZING INSTALLATION TIME AND COST

•

LOW MAINTENANCE COST (CONCRETE IN SUBMERGED WATER…LESS OF A PROBLEM RATHER THAN STEEL)

•

ADJUSTABLE CRUDE OIL CAPACITY

•

CAPABILITY OF SUPPORTING LARGER DECK AREAS

•

STEEL RISERS ARE PROTECTED AGAINST CORROSION BY PLACING IN THE CENTRAL SHAFT

•

POSSIBLE ACCESS TO THE SEA FLOOR FROM THE CELL COMPARTMENTS IN THE FOUNDATION

End