Generally, offshore concrete structures are constructed exposed to an open sea environment. Offshore structures are designed to either stay semi permanently or permanently fixed to seabed by applying proper means such as piles, anchors, or gravity or stay floated and fastened securely.
There are many applications of offshore concrete structures includes hydrocarbon exploration and production and other specific utilizations.
Concrete of offshore structure are prepared using locally available materials and compressive strength is between 25 to 65 MPa.
It is extremely significant to construct durable structures because not only do offshore structural maintenance is uneconomical but also seriously difficult in hostile environment.
- Types of Offshore Concrete Structures
- Bottom-Founded Offshore Concrete Structures
- Floating Offshore Concrete Structures
- Other Offshore Concrete Structures
Types of Offshore Concrete StructuresDifferent types of offshore concrete structures are: 1. Bottom-founded structures
- Gravity base structures (CBS)
- Concrete cylinder pile-supported structures
- Floatable/bottom-founded concrete-hull structures
- Concrete tension-leg platforms (TLPs)
- Deep-draft concrete floaters (DDCFs)
- Industrial plant ships
- Floating bridges
- Floating piers and docks
Bottom-Founded Offshore Concrete Structures
Gravity Base Structures (CBS)are usually used for hydrocarbon exploration and production. Gravity base structure is remained and fixed on the seabed due to its massive weight and typical water depth range is 40 to 350 m. they are constructed at either onshore or inshore locations after that moved to the final position. For maintenance purposes, it can be refloated and then installed at its location. The combination of structural weight, ballasting weight within the structure, and the operating weight on the structure withstand sliding force and overturning moments that induced by peak environmental loads. Figure-1 shows typical gravity-based offshore concrete structure.
Fig.1: Typical Gravity –Based Offshore Concrete Structures
Concrete Cylinder Pile-Supported Offshore Concrete StructuresThese are composed of groups of prestressed piles which are forced into the seabed and organized in such a way that prefabricated deck is positioned on the arranged group of piles to create structure working surface. In splash exposure condition and platform areas that prone to boat-impact, concrete jackets are placed around piles. In the case where piles are become too long, steel bracing might be applied to stiffen the overall arrangement of piles. Typical depth for this platform is between 5 to 20 m and could be used for support of docks, bridges, and roads over water. Figure 2 illustrates a typical concrete-cylinder pile-supported structure.
Fig.2: Typical Concrete-Cylinder-Piled Structure
Floatable / Bottom-Founded Concrete-Hull Offshore StructuresThese consist of barge-like concrete hull that is designed to float. Column or posts, which could be constructed from concrete or steel, are extended from the hull and support platform. Floatable / bottom-founded concrete-hull structure does not have enough weight on the bottom by itself, that is why the weight of posts or piles resist sliding and overturning actions. The practical depth for this type of structure ranges from 4 to 30m. They can be refloated and reinstalled for maintenance purposes or sometimes employed again in different location many times in the life span of the platform. Figure-3 show floatable/bottom-founded concrete structure.
Fig.3: Floatable / Bottom-Founded Structure
Floating Offshore Concrete Structures
Concrete Tension-Leg Platforms (TLPs)This consist of base pontoon, columns shafts which are extended from the base to the deck that rests on the shafts or columns and all components can be constructed from concrete. The structure is kept at its position by long tethers that joined to the large anchors on seabed at one end and fastened to the corners of the structure at the other end. The tension in the tethers is determined before its application. The practical water depth to employ this type of structure is between 300-1500m. Concrete tension-leg platform size is governed by the amount of operational weight that must be carried. Figure-4 shows concrete tension leg platform.
Fig.4: Concrete Tension Leg Structure
Deep-Draft Concrete Floaters (DDCFs)This type of structure which is composed of pontoon, column, and any bracing, is similar to concrete tension leg platform apart from using normal mooring system in deep-draft concrete floaters. In operational conditions, the DDCFs are stayed in its position by a substantially long drafts that mooring from lower part of the hull. The length of the draft which is more than 130m, its large weight, and low centre of gravity stabilize the structure to an extent that is nearly not sensitive to the sea motion. This platform is workable at water depth of 300-900m. Figure 5 shows DDCF platform.
Fig.5: Deep Draft Concrete Floater
Industrial Plant ShipsThese are custom-constructed prestressed concrete barges which grant a support surface for processing machinery, works and storage spaces, and living quarters that required for gas or oil production or any other industrial applications. Process applications for reinforced concrete plant ships are fertilizer production, manufacturing plants, refineries, desalinization plants, electrical power stations, chemical treatment facilities, tidal power generation facilities, and liquefied natural gas (LNG) and liquefied petroleum gas (LPG) terminals. Figure-6 demonstrated the concrete production barge.
Fig.6: Concrete Production Barge
Floating BridgesThese types of concrete members, which are used for crossing rivers, produce an economical and extremely effective solution for special applications or for problematic or difficult sites. There are number of outstanding floating concrete bridges for example Hood Canal Bridge shown in Figure-7 and Ford Island Bridge shown in Figure 8 both are constructed in the United States.
Fig.7: Hood Canal Floating Concrete Bridge
Fig.8: Ford Island (Admiral Clarey) Bridge, with the precast prestressed floating draw span being retracted under fixed bridge spans.
Floating Piers and DocksThere are large number of docks, piers, and quays which are built all over the world with different sizes such as small fishing boat docks, huge docks for container ships, ferry boats, and cruise ships. Floating piers and docks can withstand seasonal water level changes or tidal fluctuations due to mooring that provide up and down movements with rising and lowing water level. Precast and prestressed concrete container that is built in Valdez/ Alaska and shown in is an example of floating piers.
Fig.9: Valdez, Alaska, Floating Container Pier
Other Offshore Concrete StructuresThere are other types of offshore concrete structures that are not fallen into the above categories for example concrete subsea-oil storage tanks shown in Figure-10, concrete wall caissons shown in Figure-11, immersed tunnels which construct from reinforced concrete segments.
Fig.10: Concrete Subsea Storage Tank
Fig.11: Concrete Retaining-Wall Caissons