Offshore geotechnical engineering
[3] Today, there are more than 7,000 offshore platforms operating at a water depth up to and exceeding 2000 m.[2] A typical field development extends over tens of square kilometers, and may comprise several fixed structures, infield flowlines with an export pipeline either to the shoreline or connected to a regional trunkline.
These include the following:[1][4] Offshore structures are exposed to various environmental loads: wind, waves, currents and, in cold oceans, sea ice and icebergs.
[7] In shallow water, waves may generate pore pressure build-up in the soil, which may lead to flow slide, and repeated impact on a platform may cause liquefaction, and loss of support.
Because of the Bernoulli effect, they may also exert upward or downward forces on structural surfaces and can induce the vibration of wire lines and pipelines.
[10] Information on the potential risks associated with these phenomena is acquired through studies of the geomorphology, geological setting and tectonic framework in the area of interest, as well as with geophysical and geotechnical surveys of the seafloor.
[5] Examples of potential threats include tsunamis, landslides, active faults, mud diapirs and the nature of the soil layering (presence of karst, gas hydrates, carbonates).
Bathymetry, regional geology, potential geohazards, seabed obstacles and metocean data[17][18] are some of the information that are sought after during that phase.
One is to study the bathymetry in the location of interest and to produce an image of the seafloor (irregularities, objects on the seabed, lateral variability, ice gouges, ...).
They serve to ground truth the results of the geophysical investigations; they also provide a detailed account of the seabed stratigraphy and soil engineering properties.
Shallow penetration geotechnical surveys may include soil sampling of the seabed surface or in situ mechanical testing.
The purpose of deep penetration geotechnical surveys is to collect information on the seabed stratigraphy to depths extending up to a few 100 meters below the mudline.
[27] The latter provides undisturbed specimens, on which testing can be conducted, for instance, to determine the soil's relative density, water content and mechanical properties.
Sampling can also be achieved with a tube corer, either gravity-driven, or that can be pushed into the seabed by a piston or by means of a vibration system (a device called a vibrocorer).
The set-up used to sample an offshore structure's foundation is similar to that used by the oil industry to reach and delineate hydrocarbon reservoirs, with some differences in the types of testing.
[38] The shaft capacity for the piles making up each of the structure's legs was estimated on the basis of conventional design methods, notably when driven into siliceous sands.
For instance, the design and installation of suction piles has to take into account the soil properties, notably its undrained shear strength.
[41] These structures either rest on the seabed, or are placed inside a trench to protect them from fishing trawlers, dragging anchors or fatigue due current-induced oscillations.
[45] Buckling potential induced by the axial and transverse response of the buried pipeline during its operational lifespan need to be assessed at the planning phase, and this will depend on the resistance of the enclosing soil.
As offshore developments move into deeper waters, gravity based structures become less economical due to the large required size and cost of transportation.
