Offshore concrete structures are vital for the oil and gas industry, serving as platforms for exploration and production in the open sea. These structures come in two main types: bottom-founded structures and floating structures, each designed for different purposes. The construction of such structures, however, is fraught with challenges, largely due to the unique and often hostile environment of the sea. This article explores the various methods and locations used to construct offshore concrete structures, highlighting their advantages, challenges, and real-world applications.
1. Introduction
Offshore concrete structures are essential for offshore oil and gas exploration and production. These structures provide a stable base for drilling, storage, and other operational functions in the ocean. There are two main types of offshore structures: bottom-founded structures, which are anchored to the sea floor, and floating structures, which remain buoyant at the water’s surface. Building these structures in the open sea presents numerous challenges, from dealing with harsh weather conditions to logistical difficulties in transporting materials and workers. This article examines the different construction methods used at various locations to build these vital offshore structures.
2. Locations for Construction of Offshore Concrete Structures
Offshore concrete structures can be built in several locations, depending on the nature of the structure and the resources available. The construction process might occur onshore or at sea, utilizing different platforms and techniques. The primary locations for constructing these structures include:
- Dry docks or graving docks
- Submersible barges
- Skid ways
- Precast facilities onshore
Each location offers unique advantages and constraints based on the project’s scale, budget, and site-specific conditions.
3. Offshore Structure Construction in Dry Docks or Graving Docks
The construction of offshore concrete structures in dry docks or graving docks is one of the most common methods, especially for bottom-founded structures. In this method, the concrete base of the offshore platform is constructed in a dry dock—a large, waterproof enclosure where water is removed to allow construction. Once the base has sufficiently stiffened and acquired enough buoyancy, the dock is flooded, allowing the base to float. The floating structure is then towed to a deeper water site, where construction continues until completion.
This method has been successfully used in many projects, including those in the North Sea, where harsh weather conditions make alternative methods more challenging. The construction sequence involves several key steps, including excavation of the construction area, pouring the base slab, and gradually increasing the height of the concrete platform. Once the platform is sufficiently buoyant, it is towed out to deeper water for further construction.
A notable example of this technique is the construction of the Isola Di Porto Levante LNG terminal in Italy. The terminal was constructed in a dry dock, and once the base was completed, the structure was floated and towed to its current offshore location. While this method is effective, it is also expensive, particularly when building or modifying a dry dock in areas where no existing infrastructure is available.
The cost of developing a dry dock can account for as much as 80% of the total project cost, due to the expenses involved in site excavation, dredging, and constructing additional infrastructure like roads, bridges, and sewage systems. Therefore, while dry docks offer precision and control, their high cost is a significant consideration for offshore projects.
4. Offshore Structure Construction on Submersible Barges
An alternative to dry docks is the use of submersible barges, which provide a more cost-effective and flexible method of constructing offshore structures. In this approach, sections of the offshore structure are built on large, submersible barges. The advantage of this method is that it eliminates the need for a dry dock, reducing costs and increasing the range of possible construction sites. The barges are designed to be submerged under water, allowing the structure to be constructed on their surface.
The construction process typically starts by mooring the barges near land, where construction equipment and materials are loaded from the shore. Once the materials are on the barges, the construction of the offshore platform begins. For smaller platforms, the entire structure may be built on the barge, while for larger platforms, construction may be completed in stages, with partially completed sections floated off the barges and moored temporarily until the remaining portions are finished.
A notable example of this method is the construction of the Snorre TLP foundation anchors, which were built on submersible barges. This approach allows for greater flexibility in terms of location, as it can be used in deeper or more protected areas where dry docks are not feasible.
5. Construction of Offshore Concrete Structures on Skid Ways
Skid ways are inclined slabs that stretch from the construction area into the water, providing a pathway for structures to be moved. This method is typically used for smaller offshore structures and involves constructing the base of the platform on the skid way, which is then floated or towed into deeper water for further construction.
For larger structures, however, the skid way method can be less effective due to the weight of the completed structure. In such cases, the skid way must be reinforced, which adds to the cost. Despite these limitations, skid ways are a practical solution for smaller projects and offer a more cost-effective alternative to other methods.
An example of this method is the construction of small platforms using skid ways, where the base is first constructed onshore and then moved into position. Once the platform is in deeper water, construction continues until the structure is fully completed.
6. Construction of Offshore Structures in Precast Facilities Onshore
Precast facilities onshore are often used to fabricate individual components of offshore concrete structures. This method involves producing structural elements such as decks and piles in a controlled environment before transporting them to the construction site for assembly. The prefabrication process ensures that components are built to high standards and reduces the time spent on-site.
One significant example of this technique is the construction of the Glomar Beaufort I, an Arctic drilling structure. Many of the structure’s interior components were fabricated in precast facilities before being transported to the offshore site for assembly. This approach is particularly useful for projects that require high precision and quality control.
Precast facilities are also beneficial for reducing on-site construction time, as much of the work is completed off-site. This can lead to significant cost savings, especially for projects located in regions with limited access to construction materials or where weather conditions make on-site construction difficult.
7. Conclusion
The construction of offshore concrete structures involves a variety of techniques, each suited to different project needs and conditions. Whether using dry docks, submersible barges, skid ways, or precast facilities, the choice of method depends on factors such as cost, location, and the size of the structure. While each method has its advantages, they all require careful planning and execution to overcome the challenges posed by the offshore environment.