We present a selective review of geohazards that have influenced recent offshore development projects. The examples have been chosen to show how recent developments in geophysical interpretation have addressed the problems posed by various types of geohazards. Three types of geohazards are considered: environmental issues, geohazards affecting sub-sea foundation design and deepwater geohazards. Environmental issues may not be conventionally regarded as "geo" hazards; however, many environmental issues present significant constraints on offshore developments and must be treated in the same way as traditional geohazards. They are features of, or on, the seabed, which must be located, mapped and characterised so that they can be avoided or mitigated. Examples include coral reefs, herring spawning grounds, algal mats and many other types of benthic biology. Geohazards affecting sub-sea foundation design are relatively well known (shallow channels, lateral variations in soil properties, hardgrounds etc.) and there has been a progressive improvement in the geophysical interpretative methods available to characterise these hazards. Deep-water geohazards have been widely discussed and the range of hazards that may be encountered is widely appreciated, but there is still scope for new surprises. Our examples show how geophysical interpretation can characterise some of the major hazards such as slope instability, complex topography and mud volcanoes. Developments in geophysical interpretation include increasing use of digital processing and interpretation of "analogue" datasets, use of seismic velocities to characterise soil types, tight integration of geophysical and geotechnical datasets, interpretative use of visualisation software.
The types of seabed and shallow sub-seabed constraints on offshore developments have expanded over the last ten years. This has been caused by several factors including the move into deepwater with its many associated geohazards, the rise in environmental constraints and the increase in sub-sea developments with smaller, lighter structures being more sensitive to very shallow soil conditions. There has been a concurrent improvement in site survey data acquisition and introduction of new methods such as swathe bathymetry. There has also been a continuing development in the way we look at the data, how we interpret it and how the resultsare presented.
In addition to the standard geohazard identification scope of work for site surveys, many oil companies are including an environmental scope of work as part of the survey work. The information required is usually obtained from the standard survey programme, but in some cases, site specific surveys are carried, often in conjunction with seabed sampling and ROV inspection. The following describes some of the issues that FSL have encountered and attempted to resolve through remote sensing.
Deep-water coral (e.g. Lophelia Pertusa) accumulations are extremely sensitive to changes in their local environment and therefore oil companies are now taking great care in avoiding and preserving these unique communities. The accumulations of coral identified in recent surveys within the Norwegian North Sea can be grouped into two main categories based on shape, size and location.
