Offshore concrete structures play a crucial role in various marine industries, particularly in hydrocarbon exploration, production, and other specialized applications. These structures are exposed to the open sea environment, where they must endure extreme weather conditions, constant wave motion, and corrosive saltwater. Constructing durable, long-lasting offshore structures is not only essential for operational efficiency but also for safety, as maintenance in such a hostile environment is both costly and extremely challenging.
Concrete used in offshore structures is typically made with locally available materials, ensuring that it can withstand the harsh marine conditions. The compressive strength of the concrete used ranges from 25 to 65 MPa, offering the strength required for these structures to stay intact for decades. Given the difficult environment and high costs of maintenance, ensuring the longevity and robustness of these structures is paramount.
Types of Offshore Concrete Structures
Offshore concrete structures can be broadly categorized into three main types: bottom-founded structures, floating structures, and other offshore structures. These structures differ in design, application, and their ability to adapt to varying sea depths and environmental conditions.
A. Bottom-founded Structures
- Gravity Base Structures (CBS) Gravity base structures are primarily used for hydrocarbon exploration and production. These structures rely on their massive weight to stay fixed to the seabed, providing stability even in rough waters. Typically, gravity base structures are installed in water depths ranging from 40 to 350 meters. They are constructed onshore or inshore and are then floated and moved to their final location. Once installed, they can be refloated for maintenance and repositioned as needed. The combined weight of the structure, along with additional ballasting weight, helps resist sliding and overturning forces caused by environmental loads.
- Concrete Cylinder Pile-Supported Structures These structures are made up of a group of prestressed concrete piles driven deep into the seabed. The piles are arranged to support a prefabricated deck that acts as the working surface of the platform. To protect the piles from damage, especially in splash zones or areas vulnerable to boat impacts, concrete jackets are placed around them. If the piles become too long, steel bracing may be used to reinforce the structure. These structures are commonly used in shallow waters, typically in depths ranging from 5 to 20 meters. They are ideal for supporting docks, bridges, and other infrastructure over water.
- Floatable/Bottom-founded Concrete-Hull Structures These structures consist of a barge-like concrete hull designed to float on water. Concrete or steel posts extend from the hull to support the platform, but the hull alone doesn’t provide enough weight for stability. Instead, the weight of the posts or piles keeps the structure grounded. These structures are versatile, as they can be refloated and repositioned for maintenance or repurposed for use in different locations during their lifespan. They are commonly used in water depths ranging from 4 to 30 meters.
B. Floating Structures
- Concrete Tension-Leg Platforms (TLPs) Tension-leg platforms are floating structures that consist of a base pontoon with columns extending upwards to support the platform deck. The structure is anchored to the seabed using long tethers, which are fastened to the structure at one end and to anchors on the seabed at the other. The tension in the tethers is carefully calibrated to hold the platform in place even during harsh sea conditions. TLPs are typically employed in water depths ranging from 300 to 1,500 meters, with their size governed by the operational weight they need to carry.
- Deep-Draft Concrete Floaters (DDCFs) Similar to TLPs, deep-draft concrete floaters feature a pontoon and columns. However, DDCFs rely on a normal mooring system and their large draft (more than 130 meters) to stabilize the structure. The weight and low center of gravity of the platform make it largely unaffected by sea motion, making it suitable for operations in water depths between 300 and 900 meters. These platforms offer greater stability in deeper waters compared to other floating structures.
- Industrial Plant Ships Industrial plant ships are custom-built prestressed concrete barges designed to serve as mobile support platforms for various industrial activities. These ships typically provide workspaces for processing machinery, storage, and living quarters for workers involved in gas or oil production, as well as other industrial applications. Some of the common applications include fertilizer production, refineries, desalination plants, and power stations. The concrete structure ensures durability and resistance to harsh marine conditions.
- Floating Bridges Floating concrete bridges offer a cost-effective solution for crossing rivers or other challenging locations where traditional bridge construction would be impractical. These bridges, made from concrete members, float on water and can accommodate large loads while adjusting to water level changes. Notable examples include the Hood Canal Bridge and Ford Island Bridge in the United States, both of which demonstrate the feasibility of using floating concrete bridges in real-world applications.
- Floating Piers and Docks Floating piers and docks are increasingly used in locations where seasonal water level changes or tidal fluctuations could otherwise pose challenges. These structures are moored securely, allowing them to rise and fall with changing water levels. They are ideal for a range of applications, from small fishing boat docks to large piers used for container ships and cruise liners. The Valdez floating container pier in Alaska is a prime example of a successful floating dock that has withstood these dynamic marine conditions.
C. Other Offshore Concrete Structures
Beyond the main categories, there are other specialized offshore concrete structures that do not fall into the above classifications. These include:
- Concrete Subsea-Oil Storage Tanks: Used for storing oil beneath the sea floor.
- Concrete Wall Caissons: Massive concrete structures used for marine construction, often in deep-water environments.
- Immersed Tunnels: Reinforced concrete segments that are submerged and connected to form tunnels under the sea.
Detailed Overview of Key Structures
A. Gravity Base Structures (CBS)
Gravity base structures are most commonly used in hydrocarbon production. They rely on their own weight to remain stable on the seabed, ensuring that they can withstand the forces generated by the ocean. They are often constructed inshore and then floated out to their designated location. Once in position, they are ballasted with water or other materials to ensure they remain fixed.
B. Concrete Cylinder Pile-Supported Structures
These structures are ideal for shallower waters, providing a reliable foundation for offshore platforms. The combination of prestressed concrete piles and protective jackets ensures their durability. Steel bracing is used to reinforce the pile structure when needed, offering stability in rough marine conditions.
C. Floating Structures (TLPs and DDCFs)
TLPs and DDCFs are designed for deeper waters, with each structure having its own stabilization mechanism. TLPs are anchored by tensioned tethers, while DDCFs rely on their deep drafts and low center of gravity. Both designs offer unique solutions for offshore projects where traditional fixed structures would not be feasible.
D. Industrial Plant Ships
Industrial plant ships offer versatility, providing both industrial space and living quarters. Their design allows for mobility, ensuring they can be relocated for different projects or purposes over their lifespan.
E. Floating Bridges and Piers
Floating bridges provide an innovative solution for difficult locations where traditional bridge construction is not viable. These bridges have been successfully used in places like Hood Canal and Ford Island, demonstrating their practicality and longevity. Floating piers, on the other hand, are ideal for handling the challenges posed by fluctuating water levels and tides.
Conclusion
Offshore concrete structures represent the pinnacle of engineering in marine environments. From gravity base structures and concrete cylinder piles to floating platforms and industrial plant ships, these designs ensure that industries can operate efficiently and safely even in the harshest sea conditions. As technology advances, the future of offshore construction will likely bring even more innovative solutions, ensuring the continued stability and longevity of these essential structures.