Work Packages
The aims of this research project will be delivered via six work packages (WPs), starting with initial scoping of the soil parameter variation (WP1), bounding the physical experiments (WP2) and numerical modelling (WP3). The outputs of WP2 and WP3 will be used to develop a CPT-based anchor penetration toolkit (WP4) which will be applied to existing geotechnical data to produce decision making mapping layers (WP5) and then disseminated to the wider community (WP6).
WP1: Contextualisation & case identification
Identification of case studies and data gaps based on existing BGS North Sea data. In addition to British Geological Survey (BGS) archive data, site-specific data will be provided by project partners (see Letters of Support), ensuring that soil parameters used for subsequent WPs reflect real-world seabed conditions. WP1 will scope out the bounds of the experimental/numerical programmes (see WP2/3) through: (i) classification of soil types to be considered (for example, range of in situ densities/soil strengths) based on actual quantitative datasets from the North Sea OffShore Wind (OSW) development sites; (ii) identify scope of the layered studies through analysis of borehole and Cone Penetration Test (CPT) datasets; and (iii) identify sites that exhibit potentially problematic ground conditions, through engagement with OSW developers and subsea cable installation con- sultants and contractors, that may require additional investigation.
WP1 outputs: quantification of the range of soil types and extents of in situ densities/soil strengths, definition of layered soil and special/problematic cases for WPs 2 and 3.
WP2: Physical testing
Scaled centrifuge model testing will be used to simulate the complete process of ploughing (and backfilling if required), followed by anchor penetration, using a large centrifuge box previously developed for cable/pipeline plough studies (EP/M000397/1, EP/N006054/1). This will minimise equipment development costs and allow the simulation of cable interaction in post-trenched conditions (realistic disturbed zone around the cable). Due to the potential for the anchor and cable to interact at any angle, a new actuation system will be developed that allows cable-anchor path angles of 0 to 90 degrees. It is important that the nature of the material above the cable is appropriately characterised to allow proper simulation of the initial conditions and quantification of how this affects anchor behaviour. To this end, existing actuation systems will need to be upgraded to include a mobile CPT device such that characterisation operations can be done in flight and at any stage during the installation processes. As well as centrifuge testing, parallel 1g testing will be undertaken to verify if the process can be simulated using simpler, appropriately scaled, 1g testing (allowing more rapid progress and wider parametric investigation in the same time frame). UoD has appropriate existing equipment that can be easily modified to undertake these works. The studies will start in sand and then move to clay before investigating layered soils. Model testing will focus on the extremes of soil type variation (identified in WP1), allowing validation of the numerical approaches, which are a more efficient platform for parametric analysis. The impact of different realistic anchor geometries will be investigated via 3D metal/plastic printing for centrifuge/1g testing.
WP2 outputs: High quality experimental data set quantifying the above points, validation cases for the WP3 numerical model. Articles submitted to geotechnical engineering journalsx and presented at appropriate conferences.
WP3: Numerical modelling
A key advantage of numerical modelling is that it allows straightforward investigation of detailed deformations around the anchor and stresses exerted on the cables that would be difficult and impossible during lab testing and field trials, respectively. Once validated, they also provide a cost and time-effective platform for parametric studies, beyond what can be achieved via physical experimentation within the project duration. The computational tool will be developed in an existing Material Point Method (MPM) framework developed and used by DU successfully in other recent EPSRC-funded projects also on offshore geotechnics (EP/N006054/1, EP/M000397/01). The existing frameworkxi will be extended to include: (i) solid-fluid coupling based on a single layer of MPs and a displacement-pressure formulation; (ii) inertia effects to account for the dynamic nature of anchor penetration; (iii) and frictional boundary conditions at the soil-anchor interface. The selected solid-fluid coupling approach is pragmatic in terms of computational cost (not introducing multiple material points) whilst conserving key physical quantities, such as mass, and being able to capture the salient features of anchor penetration. Unlike the vast majority of MPM codes available in the literature that spuriously dissipate energy, we will develop an energy conserving coupled dynamic formulation for the MPM that only dissipates energy via material failure, via fluid drag through the solid medium and at the frictional anchor-soil interface. Common with other MPM developments at Durham University, an implicit solution framework will be adopted as that is a more accepted approach in the geotechnical community and has been shown to be more computationally efficient compared to explicit methods. Finally the anchor-soil interface model will be based on previous DU experience in modelling frictional boundaries in other methods and application of boundary conditions in the MPM. These elements will be prototyped in Durham's open source MATLAB-based MPM code, AMPLE, before implementation in high-performance code. Code development will be followed by verification and validation based on the physical modelling from WP2. Once validated the code will be used to conduct parametric studies to explore anchor penetration variation in: (i) different seabed conditions, including layered strata (scoped in WP1), beyond the WP2 physical tests; and (ii) different anchor geometries across a range of different soil types to generalize the findings from WP2 (b). Numerical CPTs will be simulated alongside each anchor test, and validated from parallel tests in WP2. The stresses exerted on a cable by a close proximity anchor pass in different soil conditions will also be quantified in terms of cable damage potential.
WP3 outputs: Documented open-source MPM codes. Articles in computational mechanics journals, such as Comp. Meth. Appl. Mech. Engrg, Int. J. Num. Meth. Engrg. Conference publications (e.g. COMPLAS, NUMGE). Development/validation test cases for WP4.
WP4: CPT-anchor penetration tool development
Development of a CPT-based anchor penetration prediction methodology based on the results from WPs 1, 2 & 3. The UoD team has a track record of developing CPT-based methods for screw pile installation torque/vertical force requirements and cable plough tow force prediction, as well as determining input parameters for the rapid load testing of piles. UoD will distil the results from WP1, 2 and 3 to develop a CPT based toolkit that can first assess the variation of backfill material and material over the cable and how this varies from insitu conditions due to the installation process. This change will be correlated with CPT cone resistance. Based upon the results of the previous WPs they will identify how anchor installation depth varies under different soil conditions that can be correlated with well-developed existing CPT soil property determination approaches. Once these linkages have been identified for a wide parametric variation in WP2 & 3 it will be possible to relate cone resistance measurements directly to general anchor performance and the change that will occur when one encounters the disturbed zone above the cable.
WP4 outputs: CPT based anchor depth prediction toolkit for standard soil types and backfilled/trenched zones. Articles submitted to relevant journals, such as Geotechnique or Ocean Engineering. Conference publications (e.g. ISOPE, OMAE).
WP5: CPT-anchor penetration tool application
Application of the CPT-based anchor penetration prediction methodology to existing sediment profile maps within the North Sea to deliver two decision-making mapping layers for CBRA: (i) an anchor penetration layer based on the underlying sediment profile and (ii) a cable-anchor interaction risk layer, focused on areas of high marine traffic. Areas of the UKCS that are known to have specific stratigraphic sequences that correlate with those identified in WP1 and tested in WPs 2-4 can be effectively scored for potential anchor penetration depths through an ArcGIS-compatible algorithm. This will create a digital visual representation of anticipated anchor penetration depths in real-world seabed conditions, rather than hypothetical sedimentary sequences. This layer will greatly benefit decision- makers from an early stage in the project life cycle (e.g. The Crown Estate) when assessing risks associated with the positioning of future OSW projects, through to subsea cable and pipeline installation contractors in their assessment of burial depth requirements. The mapping layers will be accompanied by a data confidence layer, so users can understand where areas of low confidence (linked to areas of data paucity) are located, and conversely where areas of high data density have resulted in high levels of confidence in the anchor penetration values. In addition to the 2D GIS layers, where data density allows, 3D ground models for the shallow subsurface (sub 10m) at a site-specific to regional scale will be generated using BGS available software (GSI3D/Groundhog). These models will allow stakeholders, and general interested parties, to visualise the subsurface easily and rapidly, in relation to layered soils and the associated anchor penetration depths. These ground models will be made publicly accessible, alongside the 2D GIS decision-making mapping layers.
WP5 outputs: 2D GIS decision-making mapping layers. 3D ground models for high-density data regions. Technical report regarding the use of the 2D layers/3D models. Articles submitted to relevant journals, such as Ocean Eng./Eng. Geology & Hydro. Conference publications (e.g. ISFOG, Ocean. Int.).
WP6: Dissemination and CPT-anchor toolkit
Disseminate the outputs and findings to the industrial and research communities. Engagement activities beyond the wider supporting team, including a more-general end of project workshop focused on geotechnical risk in offshore renewable energy. The CPT-anchor penetration tool will be released through focused webpages, documenting its use and assumptions, be spreadsheet based and easily implementable by industry within existing CBRAs.
WP6 outputs: CPT-anchor penetration toolkit. Briefing event for project partners. Guide for industry report. Dissemination workshop.
