Section 1 Site conditions assessment

1.1 General notes on site investigation

1.1.1 As presented by Thomas (2017), in the context of investigation for wind farms, site characterisation should be designed to provide the right information, at the right time to optimise the location, design and installation of the required structures. This should enable mitigation of geohazards by avoidance or through design. The SUT Guidance notes for the planning and execution of geophysical and geotechnical ground investigations for offshore renewable energy developments present a basis for planning and conducting ground investigations such as defining data requirements, specifying appropriate equipment and typical work scopes.

1.1.2 Generally speaking, the design and installation process for a structure needs to take into account the seabed conditions which are considered to include the geotechnical properties of the soil and any other seabed features or hazards.

1.1.3 For a site with many structures it is general practice to divide a site into one or more clusters and perform foundation design for each cluster. The clusters are not necessarily allocated in a spatial sense, but may be dominated more by other factors such as water depth, seabed features or soil types and thus may also be considered as a categorisation of turbine conditions.

1.2 Identification, assessment and mitigation of seabed hazards

1.2.1 The SUT Guidance notes for the planning and execution of geophysical and geotechnical ground investigations for offshore renewable energy developments provide a list of typical features that may need to be assessed by site investigation. Where any seabed features or hazards are identified then the impact of them on the proposed construction concept should be assessed. This assessment should generally consider, but not be limited to:

changes to the hazard that may occur from the initial survey, through to construction, operation and decommissioning of the structure;
location and distribution of the hazards or features;
impact on the selected foundation concept(s); and
avoidance, mitigation or engineering required of foundation concept to ensure that the impact of the hazard or feature is reduced below an acceptable level.

1.2.2 A few examples of seabed hazards, their impact and avoidance, mitigation or engineering are provided in Table 3.1.1 Examples of seabed hazards and mitigation:

Table 3.1.1 Examples of seabed hazards and mitigation

Hazard	Impact	Avoidance, mitigation or engineering
Global seabed gradient of 0,75 degrees across the footprint of a mudmat jacket structure and the surrounding area. No indications the gradient will vary over the life of the structure.	May cause jacket out of level in excess of requirements that can be handled during installation	Cannot be avoided. Potential mitigation measures include jacket levelling (in addition to that normally allowed) Seabed dredging or preparation
Presence of sandwaves or megaripples at structure location may be subject to future variation.	Local gradients may impact structure installation. Indicative of scour/deposition environment.	Where appropriate consider use of seabed preparation to install foundation below trough of sandwaves or ripples; or inclusion of scour allowance. It should be noted that deposition may affect loading on structural members or other appurtenances. Consider use of scour protection provided it can be designed to remain effective.
Geophysical survey shows indications of shallow gas anomaly below structure location. Unsure how the indicated gas will behave in future.	May cause hazard to the structure during installation and operation.	Consider relocating the platform. Assess and/or survey the shallow gas anomaly in further detail to establish how active and extensive it is and to assess future behaviour.
Initial geotechnical site investigations reveal thick peat layer at shallow depth below gravity base structure location.	May affect capacity, settlement, stiffness of gravity base and ease of installation of skirts (if used).	Consider further site investigation to ensure that the properties and extent of the peat are fully understood so that a detailed assessment can be made.
Mobile sand veneer overlying clay layer around gravity base location.	Sand may or may not be present when the structure is installed or for in-service capacity.	Consider sliding capacity according to sand or clay methodology and use the most onerous result; or include skirts sufficient to mobilise capacity in clay layer.

1.3 Planning for future jack-up rig operations

1.3.1 A key aspect when planning wind farm foundations and site investigation requirements is to plan what vessels may be used for the site investigation, through to construction and maintenance in-service. Where jack-ups may be used their operation should be carefully considered to minimise the potential for seabed damage around the turbine locations and for interaction between the jack-up spudcans and foundations. For example, the following issues may be encountered:

creation of seabed footprint depressions that may impact future deployment of jack-ups or installation of foundations;
jack-up spudcan penetration and retrieval may cause soil displacements that can induce loads into the foundation and structure leading to overstress of the foundations or structure; or cause movement of the foundation;
in addition to any seabed hazards already present, jack-up operations may be affected by scour pits that form around foundations after installation; and
repeated reuse of the same spudcan depressions may cause soil damage and impairment of the foundations.

1.3.2 The degree and impact of jack-up interaction is dependent upon the spudcan to foundation edge to edge spacing, spudcan penetration and soil conditions with the issue shown schematically in Figure 3.1.1 Potential effects of jack-up spudcans on a foundation. Generally speaking, for stiff seabed conditions where spudcan penetration is very limited the potential for interaction is small. However, in softer conditions with significant spudcan penetrations and smaller spacing, the interaction and impact of spudcans may become significant.

1.3.3 Generally speaking, in excess of one spudcan diameter spacing the effects are limited. However, it should be noted that this rule of thumb does not apply to deeper spudcan penetrations where a more detailed assessment may be required.