4.4. Site Preparation Activites

Prior to the construction phase of the Array, a number of site preparation activities will be required to be undertaken. It is assumed that site preparation works will continue throughout the construction phase as required, therefore, these works may be undertaken at any point within the construction programme. A summary of site preparation activities is provided in this section.

Pre-construction surveys, including geophysical and geotechnical surveys, may be carried out to provide further information of:

Geophysical surveys will be undertaken within the Array to provide further information of Unexploded Ordnance (UXO), bedforms and mapping of boulders, bathymetry, topography and sub-surface layers. Geophysical survey techniques to be employed include Multibeam Echosounder (MBES), magnetometer, Side-Scan Sonar (SSS), Sub-Bottom Profiler (SBP) and Ultra-High Resolution Seismic (UHRS).
Geotechnical surveys will be carried out at specific locations within the Array and will employ techniques such as Cone Penetration Tests (CPTs), vibrocores, box cores, piston cores and boreholes.

The possibility exists for UXO originating from World War I or World War II to be present within the Array. Due to the health and safety risks posed by UXO and potential interactions with planned locations of installed infrastructure and vessel activities, it is necessary for UXO to be surveyed and managed carefully before the construction phase and installation of offshore infrastructure commences.
A desk-based study of the Array (Ordtek, 2022) reviewed the relevant military history in the vicinity of the Array and the likelihood of encountering UXO. Based on known military activity the desk-based study concluded that there was a low background risk of UXO within the Array, and the likelihood of encountering different types of UXO within the Array was considered to be unlikely, meaning that it would be unusual for UXOs to be encountered within the Array. However, due to existing evidence of use in the wider area, potential for unrecorded activities such as munitions dumping, and potential for burial and migration of UXO due to natural seabed processes, the potential presence of UXOs cannot be discounted (Ordtek, 2022).
Methodologies considered within the PDE to avoid/clear UXOs are as follows:

Due to the health and safety risks that UXOs pose, the Applicant would seek to either avoid UXOs entirely, avoid UXOs via micrositing, or relocate UXO where practicable. If methods cannot be employed to avoid UXOs, a specialist contractor will clear UXOs in advance of further site preparation and construction works taking place. The preferred clearance method for UXO is use of a low order technique with a single donor charge of 0.25 kg Net Explosive Quantity (NEQ) for each clearance event. Up to 0.5 kg NEQ clearance shot will be required for neutralisation of residual explosive material at each location. Detailed design work would be required to confirm planned locations of infrastructure, prior to conducting any UXO surveys. The Applicant have assumed that up to 15 UXOs may require clearance based upon the desk-based study (Ordtek, 2022) and experience from other offshore wind farms in the region such as the Seagreen 1 Offshore Wind Farm. As a risk remains that unintended high order detonation may occur, 10% of clearance events have been assumed to have the potential to result in high order detonation.
Table 4.23 Open ▸ presents the maximum design envelope for UXO clearance.

Existing sand waves may need to be cleared in some areas of the Array prior to the installation and burial of inter-array and interconnector cables. There are two main reasons for undertaking sand wave clearance:

To provide a relatively flat seabed surface for cable installation and so that cable burial tools can work effectively: if cables are installed up or down a slope over a certain angle, or where the cable burial tool is working on a camber, the ability to meet target burial depths may be impacted.
In order for cables to be buried at the target burial depth and remain buried for the operational lifetime of the Array (35 years): as sand waves are generally mobile in nature, the cable must be buried beneath the level where natural sand wave movement could uncover it. Therefore, for this to be achieved, mobile sediments may need to be removed before cables are installed and buried.

No large bed forms were observed as being prevalent across the site. It is expected based on geophysical data that if sand wave clearance is required it will be undertaken in specific discrete areas of the Array (e.g. along inter-array and interconnector cables) and could occur throughout the construction phase.
Sand wave clearance techniques could include pre-installation ploughing which flattens sand waves and pushes sediment from wave crests into adjacent troughs to level the seabed may be employed. It is not anticipated that large scale dredging would be required within the site boundary.
Table 4.24 Open ▸ presents the maximum design envelope for sand wave clearance. A geophysical survey campaign will be completed prior to construction which will allow the final parameters for sand wave clearance to be defined.

Boulder clearance may be required in some areas of the Array prior to installation of offshore infrastructure, in particular, along inter-array cables and interconnector cables. A boulder is defined as being over 256 mm (Wentworth Scale) in diameter and/or length. A DP1 vessel is likely to be used to undertake the boulder clearance campaign.
Boulder clearance is required to aid cable installation and increase the success rate for achieving minimum target burial depth during cable burial, therefore, reducing the risk of further cables burial works and/or the need for cable protection. Boulder clearance also reduces the risk of cable damage during installation and subsequent burial. It may also be required in the vicinity of the OSP jacket foundation locations (including within the jack-up vessel zone around the OSP foundation locations), to avoid disruption to installation activities and to ensure stability for the jack-up vessel. The maximum design envelope for boulder clearance in the Array is presented in Table 4.25 Open ▸ .
Boulders may be cleared using a plough or boulder grab, however, the geophysical and pre-construction surveys, and the parameters of any boulders present (e.g. size, density and location of boulders), will inform the methodology to be used. It is possible that more than one method of boulder clearance may be deployed across the Array. Cleared boulders will be relocated to an appropriate location within the site boundary.

The maximum design envelope for vessels to be used during site preparation activities is presented in Table 4.26 Open ▸ .