Offshore Platforms
Intensive activities for exploration and exploitation of the oceans, especially the growing demand for hydrocarbons, led to the development of a wide range of offshore structures during the last decades. These activities are expanded worldwide in permanently increasing sea-areas and in greater water depths.
Because of their variability with regard to employment and function, geometrical configuration, size and method of construction, a strict subdivision of the offshore structures is not possible.
For the wide class of offshore platforms the mobility defined as the ability to change the location of operation can be considered as a criterion for the global subdivision in the groups of fixed and mobile (movable) units.
Two types of fixed platforms are dominant, the gravity platforms which rest directly on the sea bed by virtue of their own weight and the piled structures.
The mobile offshore structures can be categorized as follows:
• Jack-up platforms;
• Semi-submersible platforms;
• Submersible platforms;
• Tension Leg Platforms (TLP);
• Drilling ships;
• Lay barges;
• Semi-submersible lay barges;
• Offshore terminals and storage units;
• Buoys
Furthermore, in many cases the separate investigation of the group of the compliant structures in which the effects of their motions significantly affect the structural requirements is efficient. This group includes the moored floating structures, tethered platforms, articulated towers, guyed towers, and single point moorings.
The main factors influencing the selection of the platform types and especially the structural
configuration are the utility requirements, the water depth, the lateral and vertica1 loads resulting from the environmental factors and the lateral and vertical resistance of the fluid-structure-soil system (restoring forces, soil resistance).
Furthermore, the fabrication techniques, the towing and installation methods and last but not least the aspects of maintenance and reliability can affect essentially the selection of a structural system.
The utility requirement is reflected in the size of the superstructure needed to support the necessary equipment for drilling and production. Therefore, the most important factor affecting the size of the deck structure is the number of and the productivity of the wells.
Steel piled structures of tubular space-frame construction, the so-called jacket platforms, have been installed and operated in a great number in several sea-areas.
A jacket platform consists of a tall vertical section made of tubular steel members supported by piles driven into the seabed with a deck placed on top, providing space for crew quarters, a drilling rig, and production facilities. It will be fixed to seabed by means of tubular piles either driven through legs of the jacket (main piles) or through skirt sleeves attached to the bottom of the jacket (skirt piles).
The structural weight and thereby the capital cost of a fixed steel platform increase exponentially with the water depth. In figure 3, the weight of jackets installed in different offshore regions is plotted as a function of the water depth.
NoDoC cost model for the offshore structures includes:
• onshore construction
• loadout and seafastening
• transportation
• luanching, installation and pilling
in this connection the model has features to estimate all steel structure, vessels, boats, services, and all required resources costs.
Because of their variability with regard to employment and function, geometrical configuration, size and method of construction, a strict subdivision of the offshore structures is not possible.
For the wide class of offshore platforms the mobility defined as the ability to change the location of operation can be considered as a criterion for the global subdivision in the groups of fixed and mobile (movable) units.
Two types of fixed platforms are dominant, the gravity platforms which rest directly on the sea bed by virtue of their own weight and the piled structures.
The mobile offshore structures can be categorized as follows:
• Jack-up platforms;
• Semi-submersible platforms;
• Submersible platforms;
• Tension Leg Platforms (TLP);
• Drilling ships;
• Lay barges;
• Semi-submersible lay barges;
• Offshore terminals and storage units;
• Buoys
Furthermore, in many cases the separate investigation of the group of the compliant structures in which the effects of their motions significantly affect the structural requirements is efficient. This group includes the moored floating structures, tethered platforms, articulated towers, guyed towers, and single point moorings.
The main factors influencing the selection of the platform types and especially the structural
configuration are the utility requirements, the water depth, the lateral and vertica1 loads resulting from the environmental factors and the lateral and vertical resistance of the fluid-structure-soil system (restoring forces, soil resistance).
Furthermore, the fabrication techniques, the towing and installation methods and last but not least the aspects of maintenance and reliability can affect essentially the selection of a structural system.
The utility requirement is reflected in the size of the superstructure needed to support the necessary equipment for drilling and production. Therefore, the most important factor affecting the size of the deck structure is the number of and the productivity of the wells.
Steel piled structures of tubular space-frame construction, the so-called jacket platforms, have been installed and operated in a great number in several sea-areas.
A jacket platform consists of a tall vertical section made of tubular steel members supported by piles driven into the seabed with a deck placed on top, providing space for crew quarters, a drilling rig, and production facilities. It will be fixed to seabed by means of tubular piles either driven through legs of the jacket (main piles) or through skirt sleeves attached to the bottom of the jacket (skirt piles).
The structural weight and thereby the capital cost of a fixed steel platform increase exponentially with the water depth. In figure 3, the weight of jackets installed in different offshore regions is plotted as a function of the water depth.
NoDoC cost model for the offshore structures includes:
• onshore construction
• loadout and seafastening
• transportation
• luanching, installation and pilling
in this connection the model has features to estimate all steel structure, vessels, boats, services, and all required resources costs.
