Section 2 Guidelines for site investigation

2.1.1 The methods of investigation are to be adequate to give reliable information for anchor design and should take account of, but not limited to, the following:

• Nature and stability of the seabed.
• Geomorphology and engineering properties of the strata underlying the seabed.
• Seabed topography in sufficient detail for the type of anchor point being installed.
• Presence of sand waves, ripples & other mobile seabed features.
• Surface deposits, rock outcrops and debris.
• Variations in soil conditions at the anchor point locations.
• Stability of sloping seabeds and other geohazards.
• Natural eruptions and erosion of the seabed due to emissions of gas, mud, fresh water springs etc.
• Presence of shallow gas.
• Other seabed infrastructure.
• Obstructions (manmade or otherwise).

2.1.3 The extent of investigations is to be sufficient in area, depth and detail to adequately cover the anchor design. The site and complexity of the proposed anchor point arrangements and the anticipated seabed soil conditions to be encountered at the anchor point locations are to be considered in determining the extent.

2.1.4 Site investigation is to consist of the following phases:

• Desk study.
• Geophysical site investigation.
• Geotechnical site investigation.
• Integration of geophysical and geotechnical data including update of desk study.
• Determination of design parameters.

2.1.5 Where necessary, site investigation phases are to be updated or repeated to ensure sufficient data is available for anchor design.

2.2.1 The desk study is to be performed prior to commencing other forms of site investigation. In offshore areas where detailed geological data already exist, this information is to be obtained and used to aid determination of the scope and method of site survey.

2.2.2 The desk study should also consider other anchors or structures that have been installed nearby and take account of information such as installation records or scour inspection reports.

2.3.1 In absence of a geophysical site investigation standard published by ISO, other good industry practice such as that published by the ISSMGE is to be taken into account when planning and implementing geophysical surveys.

2.3.2 The geophysical survey is generally to be performed over an area centred on the proposed floating installation location. The number and spacing of survey lines are to be appropriate for the site characteristics and type and number of anchor points.

2.4.1 The geotechnical site investigation should be performed in accordance with the requirements of ISO 19901-8. Geotechnical site investigation data is considered to comprise of sample data & associated laboratory testing and in situ test data that has been appropriately interpreted.

2.4.2 Geotechnical site investigation data is required at each anchor point location and the geotechnical data shall be sufficient to characterise each soil strata found across the site. Where site conditions are geotechnically uniform a reduced amount of geotechnical investigation locations may be justified at an anchor point cluster. Such a reduction in geotechnical site investigation data is to be supported by proper integration of the geophysical and geotechnical data.

2.4.3 Geotechnical site investigation should extend to a depth greater than the maximum anticipated depth of influence of the anchor.

2.4.4 More than one geotechnical site investigation location for a gravity base anchor may be required where the soil conditions are variable. For gravity base anchors at least one geotechnical site investigation location should extend to a depth greater than the maximum anticipated lateral dimension of the proposed gravity base. This deeper data may be supplemented by shallower data to provide an understanding of soil variability across the gravity base footprint.

2.4.5 When planning a site investigation and selecting equipment, particular consideration should be given to issues that may affect the likely anchor type(s). For example, particular attention should be given to identifying any thin weak strata which may be critical to sliding capacity of gravity anchors, but of relatively little significance to the design of friction piles.

2.4.6 The depth accuracy class, defined according to ISO 19901-8, should be selected to be appropriate for the anchor type. Some anchor types, such as a skirted gravity base, may be particularly sensitive to differences in depth of soil strata. In general this should mean a depth accuracy class of Z3, or better (i.e. Z1 or Z2), shall apply.

2.4.7 The sample class, as described in ISO 19901-8, should be appropriate for the anchor type and design requirements. In general this should mean Class 3 samples, or better (i.e. Class 1 or Class 2), are obtained.

2.5.1 In situ testing is to be performed in accordance with ISO 19901-8.

2.5.2 For in situ tests (such as cone penetration tests or field vane tests) the Application Class is to be determined based on the anchor type and site conditions anticipated from the desk study.

2.6.1 The results of the geotechnical investigation are to be interpreted and presented to allow understanding of soil and seabed conditions across the site. If less than one geotechnical investigation location per anchor is supplied then the interpretive report shall include justification for this on the basis of the desk study, geophysical data and geotechnical data.

2.7.1 Site investigation in unconventional soils will require special consideration.