Offshore geotechnical engineering

Offshore geotechnical engineering is a sub-field of geotechnical engineering. It is concerned with foundation design, construction, maintenance and decommissioning for human-made structures in the sea. Oil platforms, artificial islands and submarine pipelines are examples of such structures. The seabed has to be able to withstand the weight of these structures and the applied loads. Geohazards must also be taken into account. The need for offshore developments stems from a gradual depletion of hydrocarbon reserves onshore or near the coastlines, as new fields are being developed at greater distances offshore and in deeper water, with a corresponding adaptation of the offshore site investigations. Today, there are more than 7,000 offshore platforms operating at a water depth up to and exceeding 2000 m. 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. An offshore environment has several implications for geotechnical engineering. These include the following: Offshore structures are exposed to various environmental loads: wind, waves, currents and, in cold oceans, sea ice and icebergs. Environmental loads act primarily in the horizontal direction, but also have a vertical component. Some of these loads get transmitted to the foundation (the seabed). Wind, wave and current regimes can be estimated from meteorological and oceanographic data, which are collectively referred to as metocean data. Earthquake-induced loading can also occur – they proceed in the opposite direction: from the foundation to the structure. Depending on location, other geohazards may also be an issue. All of these phenomena may affect the integrity or the serviceability of the structure and its foundation during its operational lifespan – they need to be taken into account in offshore design. Following are some to the features characterizing the soil in an offshore environment: Wave forces induce motion of floating structures in all six degrees of freedom – they are a major design criterion for offshore structures. When a wave's orbital motion reaches the seabed, it induces sediment transport. This only occurs to a water depth of about 200 metres (660 ft), which is the commonly adopted boundary between shallow water and deep water.The reason is that the orbital motion only extends to a water depth that is half the wavelength, and the maximum possible wavelength is generally considered to be 400 metres (1,300 ft). 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. Currents are a source of horizontal loading for offshore structures. 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. Currents are responsible for eddies around a structure, which cause scouring and erosion of the soil. There are various types of currents: oceanic circulation, geostrophic, tidal, wind-driven, and density currents. Geohazards are associated with geological activity, geotechnical features and environmental conditions. Shallow geohazards are those occurring at less than 400 metres (1,300 ft) below the seafloor. 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. Examples of potential threats include tsunamis, landslides, active faults, mud diapirs and the nature of the soil layering (presence of karst, gas hydrates, carbonates). In cold regions, gouging ice features are a threat to subsea installations, such as pipelines. The risks associated with a particular type of geohazard is a function of how exposed the structure is to the event, how severe this event is and how often it occurs (for episodic events). Any threat has to be monitored, and mitigated for or removed. Offshore site investigations are not unlike those conducted onshore (see Geotechnical investigation). They may be divided into three phases: