Geological and geophysical site characterization for marine renewable energy development and environmental assessment
This is the first edition of CSA EXP03, Geological and geophysical site characterization for marine renewable energy development and environmental assessment. CSA EXP03 has been developed as a guideline and does not contain any mandatory language. It provides guidance and best practices for geological and geophysical site characterization for marine renewable energy development and environmental assessment.
This Guide is being issued to assist the Canadian marine renewable energy community with geological and geophysical site characterization for marine renewable energy development and environmental assessment. This Guide aims to be as comprehensive in scope as possible but does not aim to be exhaustive in the level of details provided. References to more detailed work are provided for readers to pursue their own requirements. The most commonly used survey methodologies and equipment are presented but it should be noted that other legitimate tools exist and their exclusion should not be taken as a refutation of their potential value.
CSA Group acknowledges that the development of this Guide was made possible, in part, by the financial support of Marine Renewables Canada through the ecoENERGY Innovation Initiative (ecoEII).
The initial document was authored by the Geological Survey of Canada and has been reviewed by the Development Committee on CSA EXP03, formed with diverse stakeholders holding expertise in geological and geophysical site characterization for marine renewable energy development and environmental assessment.
1.1 Objectives of the Guide
Marine renewable energy (tide, wave and offshore wind) is available in large quantities for integration into the Canadian energy mix (Cornett, 2006), but the ocean is a harsh and unforgiving environment that presents major engineering challenges to industry. The development of standards and best practices for site and submarine transmission corridor characterization and environmental impact assessment are important requirements for moving marine renewable energy development forward.
Seabed site characterization is or should be a critical part of the engineering design process required to install turbines as well as to understand environmental impacts. The Geological Survey of Canada (GSC) has conducted a preliminary review of site characterization requirements and methodology at a number of sites with high wave and tidal energy potential. Based on this experience and previous experience related to the offshore oil and gas industry, this document is a first attempt to summarize the range of geological and geophysical information that is required for characterizing the seabed at a marine renewable energy site. The aim of the document is to provide information on methodology and geologic interpretation that will be useful for both proponents evaluating a potential site for development and regulators attempting to identify potential environmental concerns related to the development.
The present document will be subject to both peer review and review by industry experts and government regulators. However, given the relative youth of the marine renewable energy industry, it should be regarded as preliminary in nature. As the industry matures, it is anticipated that such a guide would become more finely tuned to the specific needs of both proponents and regulators.
1.2 Scope of the Guide
The document aims to be as comprehensive in scope as possible but does not aim to be exhaustive in the level of details provided. References to more detailed work are provided for readers to pursue their own requirements. The most commonly used survey methodologies and equipment are presented but it should be noted that other legitimate tools exist and their exclusion should not be taken as a refutation of their potential value. Many instruments have been developed for specific purposes and may be applicable to specific problems encountered in site characterization.
The character of the seabed depends on a large number of factors, but primarily (1) the geological history of the region, which determines the type of underlying bedrock and unconsolidated sedimentary deposits that occur at the seabed; and (2) the present day sedimentary processes that actively move, erode and deposit sediments. Correspondingly, two broad areas of seabed characterization are covered in this guide. First, geospatial mapping of the geological character of the seabed is presented. Modern geologic mapping in the marine environment uses a variety of geophysical remote sensing and sampling techniques to determine the surface morphology and nature of the upper part of the bedrock or sedimentary column immediately below the seabed. This information can be usefully summarized in map form as geological or seascape units with unique or distinguishing characteristics. By inference, these units will also be characterized by particular geotechnical properties. Second, the principles and techniques of determining the dynamics of sediment movement at the seafloor are discussed. Whereas these processes vary spatially and can to some extent be mapped, they also vary temporally and typically require time series data sets to understand the natural variability of the dynamics.
As much as possible, the guide uses examples from high energy environments in areas of wave or tidal energy potential. In association with this report, there is a companion GIS-based map product that provides overviews of several of these study areas.
1.3 Geotechnical requirements for offshore structures
Knowledge of the geological and geophysical characteristics of the seabed is critical to understanding the geotechnical conditions on which wave and tidal energy conversion systems are founded or anchored. CSA Group's Code for the design, construction, and installation of offshore structures developed with and for the offshore oil and gas industry comprises several engineering standards for both gravity-based and anchored structures that would be generally applicable to offshore renewable energy structures (CSA Group, 2007; 2008; 2009).
Design and placement of offshore structures require detailed regional and site specific information relevant to the seabed and sub-seabed properties to typical depths of tens of metres (CSA Group, 2008; 2009). Geological and geophysical interpretation is necessary for understanding the horizontal and vertical distribution and variability of soil properties. The CSA Group standard for Foundations (CSA Group, 2009) requires site investigations to be performed for all fixed offshore structures with the objective of characterizing the site conditions and to define relevant bathymetric parameters, morphological features, geological context, and geotechnical design parameters. These site investigations have to take into account both near field and far field conditions. CSA Group (2009) specifically includes determining the effects of bathymetry and geomorphology, surficial geology, bedrock geology, seismicity, slope instability and the sedimentary environment, including erosional processes on the design of structures.
Det Norske Veritas (2011) has developed standards for offshore wind turbine structures that include specifications related to soil investigations and geotechnical data. These investigations are divided into geological studies, geophysical surveys and geotechnical soil investigations. Further details are provided in a guidance note: A geological study, based on the geological history, can form a basis for selection of methods and extent of the geotechnical soil investigations. A geophysical survey, based on shallow seismic, can be combined with the results from a geotechnical soil investigation to establish information about soil stratification and seabed topography for an extended area such as the area covered by a wind farm. A geotechnical soil investigation consists of in-situ testing of soil and of soil sampling for laboratory testing.
While anchored structures depend less on the sub-surface conditions, geological maps provide information on seabed and shallow substrate properties (CSA Group 2007). The International Electrotechnical Commission (IEC) Technical Committee 114 (IEC/TC 114) has developed draft standards for the assessment of mooring systems (IEC/TC 114, 2011). Knowledge of sediment transport conditions is of particular importance to anchorages in the high energy sites required for marine energy conversion.
1.4 Geotechnical requirements for submarine cable routing
Submarine electricity transmission cables from energy conversion devices to shore are an inevitable component of renewable energy installations. Cable routing and issues related to the laying, burial or trenching of cables are therefore critical aspects of marine renewable energy projects. The International Cable Protection Committee (ICPC), an association of cable industry companies and governments, has developed formal recommendations in a number of technical areas related to these issues (ICPC, 2007; 2010). Geological characterization of the cable corridor, typically 500 m on either side of the engineered route (ICPC 2010), is a recommended practice either through desktop study using existing data or through subsequent marine surveys (ICPC 2007). Included in the list of geological characteristics that are recommended for evaluation are the following:
- tectonic setting and associated seafloor morphology and lithology;
- geological history;
- surface faulting;
- turbidity currents;
- sediment transport;
- sand waves;
- beach and near shore seabed stability;
- other geohazards.
Submarine pipeline routing involves many similar issues related to seabed conditions and therefore similar principles of information gathering can be applied. Det Norske Veritas (2010) standards require consideration of the following:
- seabed characteristics (uneven seabed, unstable seabed, soil properties, hard spots, soft sediment and sediment transport);
- seismic activity.
Route surveys are required along the total length of the planned pipeline route to provide sufficient data for design and installation related activities. Survey results should be presented in map form indicating location of the pipeline, related facilities together with seabed properties, anomalies and all relevant pipeline attributes. Features that should be noted include obstructions and topographic features such as rock outcrops, large boulders, pockmarks, potentially unstable slopes, sand waves, valley or channelling and erosion in the form of scour patterns or material deposits.