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Generally, offshore concrete structures are constructed in an open sea. There are different types of offshore structures such as bottom founded structures and floating structures. Both structures are built for different purposes such as oil and gas exploration and productions. Since, the offshore structure is located inside sea, so their construction might be subjected to several difficulties and hinders. Locations where offshore structures are constructed on are discussed in this article.Construction of Offshore Concrete Structures at Different Locations
It is possible to construct both bottom founded and floating offshore structures partially or completely at the following locations:- Offshore structure construction in dry docks or graving docks
- Offshore structure construction on submersible barges
- Construction of offshore concrete structures on skid ways
- Built offshore structures in precast facilities onshore
Offshore Structure Construction in Dry Docks or Graving Docks
In this method of construction, the base of the concrete platform is constructed in a dry dock or graving docks, after the base of the offshore concrete structure stiffen adequately and posses enough buoyancy, the dock will be flooded and the base will be permitted to float. Then, the base is moved to a deeper water area and will be moored until the construction of the structure is completed. It is reported that, this technique is applied for the construction of most of offshore concrete structures in the North Sea. Sequences for the construction of concrete offshore structures in dry docks or grave docks are shown from Figure-1 to Figure-7.
Fig.1: Excavation of Construction Area
Fig.2: Base Slab Construction of the Platform
Fig.3: Concrete Platform Construction to Adequate Height for Tow Out
Fig.4: Flooding of Dry Dock or Graving Dock
Fig.5: Tow Out Platform to Deeper Water Construction Site
Fig.6: Continuation of Floating Base Construction from Floating Work Barges
Fig.7: Continue Construction Until the Base is Completed
The location of plat form construction might near or far away from onshore based on the availability of deep water. Offices and concrete production facilities would be on land, and construction staff and necessary construction material could be transferred directly from land using convenient means such as fixed or floating bridge provided that the construction site is close to shore otherwise. Extra barges are required for supporting offices, concrete batching plant, material lay down areas, and other principal equipments. In addition to Construction personnel who need to travel back and forth from land to the site regularly. Figure-8, Figure-9 and Figure-10 show partial concrete construction in a dry dock in Spain, flooding dry dock, towing out reinforced concrete Isola Di Porto Levante LNG terminal which was inaugurated in 2008.
Fig.8: Dry Dock Construction of Reinforced Concrete Isola Di Porto Levante LNG Terminal
Fig.9: Flooded Dry Dock of Reinforced Concrete Isola Di Porto Levante LNG Terminal
Fig.10: Moving Concrete Isola Di Porto Levante LNG Terminal to its Current Position Offshore of Rovigo –Italy
Finally, the most uneconomical part of offshore construction is building a dry dock provided that there is no one existed or modification of an existing dry dock to make it suitable for the construction. The cost of dry dock development might be dry dock site excavation, cofferdam construction, dewatering systems; dredging of channels for tow out; supporting quays construction, docks, and wharves, in addition to complete infrastructure enhancement to support the project for example roads, bridges, water supply, power supply, sewage treatment. The overall cost of dry dock development probably reaches about eighty percent of total project cost in distant areas.Offshore Structure Construction on Submersible Barges
The construction of parts of the entire offshore structure on a submersible barge is considered to be realistic answer to cut most of the dry dock construction cost. This technique increases construction-site location options because it removes the dry dock requirement. Any location for construction-site is considered to be suitable provided that adequate depth of water is available for barges and ship-supplier activities and priority should be given to those locations which are protected and not subjected to relentless seas. Not only does the required number of barges based on the size of the project but also depend on the barge capacity. Moreover, it is possible to use either specifically built large barges or number of welded smaller size barges to achieve the same construction goal. The former is assumed to a solution but the latter could be cost effective and the construction on its working platform progress as it would on land. Furthermore, the construction might be started with mooring barges near the land until all required construction equipments are loaded directly from shore. After that, the offshore concrete structure is constructed at one place on the barges if the platform is not large. Figure-11 shows Snorre TLP foundation anchors built at one location on barges and required all supports directly came from land.
Fig.11: Snorre TLP Foundation Anchor
However, when the platform is large, it is partially constructed on the barges. Then, the completed part of the offshore structure is floated off the barges, moored temporarily and the remaining portion is built during which the portion is in floating condition. Figure 12 show typical arrangements of barges.
Fig.12: Typical Barge Arrangement
Construction of Offshore Concrete Structures On Skid ways
Skid way is structural slab which has slight slope and stretched from construction area into the water. Structures constructed on skid ways could be either self floating or skidding onto barges. Small structures are often constructed by applying this method. However, weight of completed large structures is huge so the skid needs to be upgraded in this case which costly. Therefore, the base of most of structure can be built on skid ways after that that portion is floated and moved to deeper water to complete the construction of the platform. Figure 13 shows small platform on skid way.
Fig.13: Small Structure on Skid Way
Construction of Offshore Structures in Precast Facilities Onshore
Precast facilities are not only used for precast concrete decks and piles but also to prefabricate great number of structural elements. After that, the prefabricated members are transferred to the site and then assembled.
Fig.14: Glomar Beaufort I
For example, substantial numbers of concrete arctic drilling structure (Glomar Beaufort I, Figure-14) interior elements were constructed from precast facilities. Finally, the actual method is determined is based on economic of the project and existing site facility where construction is planned. Read More: Types of Offshore Concrete Structures – Their Details and Uses
