Marine construction
Some examples of marine structures include ships, offshore platforms, moorings, pipelines, cables, wharves, bridges, tunnels, breakwaters and docks.
Remotely operated underwater vehicles (ROVs) and other types of submersible equipment are a lower risk alternative, but they are also expensive and limited in applications, so when reasonably practicable, most underwater construction involves either removing the water from the building site by dewatering behind a cofferdam or inside a caisson, or prefabrication of structural units off-site with mainly assembly and installation done on-site.
Cost and time constraints may make it difficult to gather sufficient samples from borings to fully describe the substrate.
[1]: Ch 2.1 Bottom material is often sedimentary, and in deeper water, may range from extremely fine, low density silts to loose gravels, to dense, highly compacted sands.
Granular sediments may be subject to liquefaction if strongly disturbed, as by earthquakes, cyclic impact of storm waves, or crushing by sea ice.
[1]: Ch 2.3 The presence of large boulders in glacial till deposits can give a misleading impression of bedrock, hiding the further extent of softer sediment below.
[1]: Ch 2.7 Methane can occur at shallow depths in delta sediments with significant quantities of organic matter, and in arctic silts.
A sudden, large release may temporarily reduce water density sufficiently to cause vessels or drill rigs to sink, and can cause an explosion or fire at the surface.
[1]: Ch 2.14 Topics of concern: Marine construction materials subjected to coarse conditions including corrosion and temperature change.
[2][1]: Ch 4 Durability in the marine environment is affected by corrosion, both inside and outside of hollow structures, and can be particularly severe in crevices and cracks.
[1]: Ch 4.2 When reasonably practicable, workshop conditions of suitable humidity and protection from hazardous weather should be used when painting and coating steel structures, and keeping joint surfaces clean for welding.
[1]: Ch 4.2 Prestressed and reinforced concrete have been used for several decades for the construction of large offshore platforms, mostly in the North Sea.
[1]: Ch 4.3.1 Two forms of combined steel and concrete construction have been used to advantage: The hybrid approach: For these structures, the main problem is the working of the joint under cyclic-dynamic loads, which can frequently be managed by pre-stressing the connection.
[1]: Ch 4.4.1 The composite approach:[1]: Ch 4.4.2 Typical examples include: This type of composite construction appears to have advantages where offshore structures must withstand high local impact forces, such as ice or ship collision, and for the walls of concrete structures where cracks and leakage are unacceptable and difficult or impossible to repair.
Commonly marine and offshore operations include towing, mooring, ballasting, lifting and lowering heavy loads, personnel transfer, surveying, inspection, and diving.
[1]: Ch 7.1 Seafloor modifications to improve performance of foundations are intended to provide a stable base of sufficient strength to support the structure and to resist failure and degradation for both a single extreme event and repetitive dynamic loads.
Injection of cementitious material, following land-based grouting procedures, can be used to displace pore water and give a stronger substrate.
There are many reasons why a deep foundation may be preferred, such as large design loads, a poor soil at shallow depth, There are different terms used to describe different types of deep foundations including the pile (which is analogous to a pole), the pier (which is analogous to a column), drilled shafts, and caissons.
Finger piers are commonly used for transfer of petroleum products, and trestle supports are provided for access to loading platforms and wharves.
[1]: Ch 9.3 An offshore platform is usually a large structure with facilities to extract and process petroleum and natural gas that lie in rock formations beneath the seabed.
[5] Sizeable pipeline construction projects need to take into account many factors, such as the offshore ecology, geohazards and environmental loading.
