Offshore structures

The fossil fuel and renewable energy industries both rely heavily on offshore platforms and infrastructure constructed on the seabed of the shallow seas around the UK. While the platform designs vary, for example to suit water depth, injected concrete is commonly used to transfer the loads between steel substructures within their foundations. Wind turbines, for example, use a range of foundation designs including the gravity (or monopile) foundation for shallow water depths (less than 30 m) or the tripod foundation for deeper water depths (down to 60 m). The foundations in all cases incorporate load transfer through steel-concrete-steel substructures (Figure 1).

Inspection of concrete grouts

The foundation substructure can be represented by a three-layer system bound by water inside the cylindrical pile and the open sea surrounding the substructure, shown in the inset in Figure 1.

This three-layer system can be inspected using a low-frequency beam of ultrasound. Incident ultrasound is partially reflected and transmitted from the substructure’s outer surface. The transmitted wave enters the structure to be partially transmitted and reflected at each boundary surface between the steel and concrete layers. Thus, the ultrasound waves backscattered from and transmitted through this layered sequence are the result of a superposition of a series of the original incident wave that has been modulated by partial reflection and transmission, and delayed via multiple transmissions across the thickness of the bound structural layers (Figure 1 inset).

The echo-reflected signals are modulated and exhibit characteristic notches in their magnitude spectra that are signature characteristics not only of the material layers, but also of any small water gaps between individual solid layers.

Wave propagation through foundation models

The concrete is subjected to large, complex stresses leading to progressive deterioration in the foundation condition, manifested in a range of defects such as debonding and the growth of gaps at its interfaces with the steel substructures, or even a complete loss of concrete in parts of the annulus (Brett et al. 2018). 

We modelled the wave propagation through this three-layered system to evaluate the potential sensitivity of an echo ultrasound inspection method to these effects. The model outcomes include predicted spectra for a three-layered foundation system in good condition, i.e. with no gaps between the concrete and either of the bounding steel substructures (shown as the red plots in Figure 2), and the spectral characteristics related to front gaps (left-hand plots in Figure 2) and rear gaps (right-hand plots). This modelling yielded notable results.

Results

Increasing the aperture of a front gap from 0.1 mm to 1 mm causes the notch at 53.3 kHz to diminish in depth and shift to lower frequencies. The notch at 60.75 kHz also shifts to slightly lower frequencies but deepens and the notch at 73.8 kHz shifts to lower frequencies (Figure 2, left-hand plot).

Increasing the aperture of a rear gap from 0.1 mm to 1 mm causes the minimum at 53.3 kHz to shift to lower frequencies while maintaining a constant depth. The minimum at 60.75 kHz diminishes while also shifting to slightly lower frequencies and the minimum at 73.8 kHz grows and shifts to lower frequencies (Figure 2, right-hand plot).

Diagnosing concrete condition in foundations

Our numerical modelling indicated that it was feasible to inspect the condition of offshore foundations using an echo-ultrasound method. The model outcomes provided the baseline echo spectra used to distinguish defected foundations from those in good condition. Experimental replication of these baseline spectra would then provide very compelling evidence and the impetus for development of an inspection platform for offshore deployment.

Figure 3 shows a large ultrasound transducer (150 mm²) used to transmit ultrasound pulses towards the layered steel-concrete-steel target and to detect the echoes from this target. The black plots in the left- and right-hand graphs in Figure 3 show the spectrum for a target in ‘good condition’, i.e. where the concrete is in contact with both steel plates.

In Figure 3, note how the notch at 54 kHz diminishes and shifts to lower frequencies with the introduction of a front gap of increasing aperture; note also how the notch at 61 kHz deepens and shifts to lower frequencies.

Note how the notch at 54 kHz deepens and shifts to lower frequencies with the introduction of a rear gap of increasing aperture; note also how the notch at 61 kHz diminishes and shifts to lower frequencies. These are the same responses as the modelling outcomes and these very different responses verified the great potential for applying echo ultrasound to inspect offshore foundation condition.

Field ultrasound inspection system

The laboratory bench top modules were condensed onto a ROV-deployed field system shown in Figure 4, which comprised:

• a PC laptop
• a combined arbitrary wave generator (AWG) digital storage oscilloscope (DSO)
• a sonar pulser-receiver
• a sonar probe (or ultrasonic transducer)

This system was deployed from a remotely operated vehicle (ROV), which had complete and independent yaw, pitch and roll (Figure 5). The sonar probe was centrally mounted on the front of the ROV along with lights and cameras to aid visibility.  For example, Figure 5b shows a view of an underwater target about to be inspected. Note that the spacer bars are visible on either side of each view. These were used to maintain a constant echo offset for all inspections.

This platform was tested in a diving pool by recording echo reflections from a 0.5 m × 0.5 m square target comprising a 50 mm front steel plate, a 70 mm concrete centre layer and a 50 mm rear steel plate (Figure 6). The field trials consisted of a similar matrix of experiments to the laboratory trials whereby echoes were recorded from targets in ‘good condition’ and with defects including front and rear gaps.

Our early field trials were very encouraging because we gathered data that confirmed our findings from the numerical models and also from the laboratory experimentation. For example, Figure 7 compares the spectra for a steel-concrete-steel target in good condition (black plot in both right- and left-hand graphs) with the spectra from targets with gaps between the concrete and the front steel plate (left-hand graphs) or the rear steel plate (right-hand graphs). Note how the higher frequency notch at 59 kHz develops as the front gap increases, whereas the lower frequency notch at 57 kHz develops as the rear gap in aperture. 

Our aim is to follow up these field trial with further trials on actual offshore infrastructure.

The BGS Drilling Facility operates out of BGS Keyworth. The drilling team is comprised of staff from different science areas across BGS, all trained to an in-house level as either lead driller or assistant driller status. The facility has been in operation since the end of 2005 and uses a Dando Terrier 2002 rig to perform shallow drilling to depths of 10–15 m.

Core recovery methods

The drill rig has three methods of recovering core depending on the geology being drilled.

Research

The BGS Drilling Facility has been involved with a number of research themes. These have ranged from drilling estuarine sequences in the Thames basin looking at the depositional evolution of the River Thames to coring various glacial till deposits in Wales and trying to understand the deposition of basement till, glacial lake sediments and moraines. We have been involved with providing sample and installing piezometers at a test site on the River Trent floodplain to understand the groundwater response to rainfall. We have also played a large part in the re-mapping process of certain areas of the UK such as the Cretaceous of the Isle of Wight, the Jurassic in Oxfordshire and Quaternary deposits in Somerset.

Further improvement of our in situ downhole drilling tests means that the team now has the capability to perform standard penetration tests, cone penetration tests and dynamic probe tests. This has been used to determine the strength characteristics of landslides.

Drilling services

The team aims to provide a complete service and if necessary can provide additional services to a project. In particular the team have developed skills and contacts in:

• searching for utilities and cables
• organising the field site
• recording all drilled core as a standard driller’s log (to BS5930)
• remediation of the site after drilling has been completed
• transfer and storage of all cores back to BGS Keyworth
• registering of boreholes and logs in the BGS system
• performing downhole tests and completing piezometer installation if necessary

We collect the undisturbed field samples required for measurement of in situ geotechnical and geophysical properties. Undisturbed sampling offers the potential for geological characterisation to include the property attribution critical for understanding engineering performance and ground behaviour. Our capability covers undisturbed sample collection from trial pits and a shallow DANDO drilling facility.

Undisturbed sample collection

Our bespoke sampling ensures suitable capture of formation heterogeneity including interval moisture – density sampling down exposed elevations, 100 mm and 300 mm kubiena tins and pit floor sampling for geotechnical strength testing.

The geotechnical properties from tests on samples collected from the pit shown in Figure 1 contributed to a field experiment that used shear wave velocity imaging (Figure 2) to assess the changes in the ground stiffness due to infiltration of water leaking from a buried utility pipe. This was part of a project funded by the UK Engineering and Physical Sciences Programme called ‘’  to develop non-invasive techniques for ground and asset condition imaging. For more information, download this case study by Dashwood et al. (2019) from our Publications page.

Our laboratories go beyond standard index tests for soils and weak rocks. Using new technologies we have developed more efficient apparatus to replace old or unsafe test standards. We also undertake experiments to quantify the controls of moisture content and porosity on soil consistency, strength, electrical and seismic geophysical properties.

The Geo-lab facility includes wet and dry sample storage and preparation of samples for testing in our geotechnical and geophysical laboratories. 

Geotechnical laboratories

The Geotechnical Soil Laboratories are a comprehensive testing facility that includes both standard (e.g. BS1377) and specialised test equipment and forms part of the BGS’s laboratory research facilities. In-house designed equipment simplifies and improves standard, commercially available apparatus.

Soil testing facilities

• GDS stress-path triaxial PC: computer-controlled, temperature-controlled testing (50–100��mm); local strain; mid-plain pore pressure measurement; CU; CD; CIU; SHANSEP; permeability; saturation; consolidation,;cyclic loading; creep testing on undisturbed, compacted, or remoulded specimens
• triaxial: standard PC computer-logged testing (35–100��mm) UU, CU, CD, CIU; compressed-air system with low-friction cell
• consolidation: 1D, PC-controlled oedometer testing (50–100��mm) with digital strain and load gauges; swelling tests, collapse tests; undisturbed, compacted, remoulded specimens (stresses to 3.0��MPa)
• shear box: PC-logged testing (60–100��mm) on undisturbed, shear plane, or remoulded specimens; cut-plane and multi-reversal tests
• ring shear: PC-logged testing (Bromhead); residual strength; remoulded specimens (200��g)
• shrinkage limit (SHRiNKIT) (in-house method); remoulded, undisturbed or compacted specimens
• 1D swelling pressure to BS1377 (1990)
• 1D and 3D swelling strain to ISRM (1981)
• soil suction tests: extractor plate and dew-point potentiometer methods; soil characteristic curve derivation; remoulded specimens
• index testing to BS 1377 (1990): Atterberg limits; shrinkage limit; linear shrinkage; particle size; specific gravity; density; water content

Bespoke apparatus

Our facilities include bespoke apparatus, for example to automate some test procedures that formerly have either been labour intensive or have required for some highly specialised handling procedures.

Contact

Please contact Matthew Kirkham for further information.

Geophysical laboratories

We have developed specialist apparatus and methods to study the inversion of interrelated parameters of porosity, saturation, density, suction and stiffness from shear wave velocity and electrical resistivity ground surveys. Innovative geotechnical and geophysical property testing has driven the emergence of geotechnical property imaging via geophysical proxy.

This an area that we have led for over a decade (case studies). Laboratory calibration of our field geophysical images has led to new advances in monitoring natural landslides and the condition and deterioration of railway embankments.

Core resistivity imaging system

This system is used for surface scanning of reservoir rocks and electrical imaging of fine sedimentological structures at centimetric scale (Figure 1).  Fine-scale sedimentary features and open fractures in saturated rocks are interpreted from the measurements with reference to established relationships between electrical resistivity and porosity.

Our results successfully characterise grainfall lamination and sandflow cross-stratification in a brine-saturated, dune-bedded core sample representative of a southern North Sea reservoir sandstone (shown in Figure 1), studied using the system in constant current, variable voltage mode. In contrast, in a low-porosity marble, identification of open fracture porosity against a background very low matrix porosity is achieved using the constant voltage, variable current mode.

See for more information or view more case studies.

Geophysical/geotechnical proxy

This apparatus is for conditioning samples and testing moisture content, matric suction, density and resistivity (Figure 2). Cells and methods have been devised to control sample wetting and drying phases for the development of geotechnical/geophysical property relationships (Figure 2). We have applied these relationships to field monitoring data to visualise the effect of cyclic water movement on the stability of natural and engineered slopes (Figure 3).

For further information, see .

Research undertaken within BGS may require unique equipment that is not available commercially, often because the application is entirely novel and provides an innovative solution. The Research and Design Engineering Facility fills this gap with its conceptualising, designing, testing and manufacturing services based on the BGS campus at Keyworth.

Our services

The R&D Engineering Facility incorporates the following capabilities:

• 3D modelling package (Autodesk Inventor): this software is used as a design tool at several other NERC technical facilities. It has a powerful FEA capability and graphics that are suitable for posters and technical reports. Drawings can be emailed in all industry standard CAD files.
• Tool room type workshop: turning, milling, surface and cylindrical grinding, and hand fitting.
• Fabrication tools: horizontal and vertical band saws suitable for all materials, sheet metal guillotine, folders and rollers.
• Welding bay: equipped with MIG, TIG, spot, and MMA welding. Oxyacetylene, brazing and silver soldering. Ability to weld most common materials (including some plastics).
• Joinery and wood machine shop: equipped with band saw, circular saw, planer thicknesser, disc and bobbin sander.
• Material and consumable parts storage and procurement: in-house storage of common metals and plastics in sheet, rod and bar form; rapid procurement of special materials and forms.

Case studies

The National Geotechnical Properties Database (NGPD) primarily holds data and information from commercial site investigations carried out for civil engineering purposes, including:

• borehole details
• lithological description
• field and laboratory geotechnical tests
• engineering
• environmental chemical testing

A full description of the database structure and fields can be found in .

Where does the data come from?

The distribution of our data holdings (Figure 1) often follows major UK infrastructure developments and the coverage of our research, including UK rocks and soils engineering geology studies or the areas covered by ground information for sustainable development and geohazards projects.

Much of the data is from site investigations for major trunk road construction schemes, often provided by Highways England (formerly the Highways Agency) and other major engineering projects. A small proportion of the data is from BGS laboratory test results.

The final investigation data and information are supplied to BGS by clients, consultants and contractors either as final paper records or as (AGS) digital data transfer format files. AGS digital data transfer format files (.ags) are the preferred method of receiving data because they can be added to the database much more quickly and accurately than adding data by hand from paper records.

51���� has an and API available to validate your AGS files against the AGS v4.x standard and BGS data deposit requirements.

• Please contact 51���� Enquiries for advice on depositing your data.

What do we do with the data?

Our engineering geology studies include analysis and the collation of the geotechnical properties in to technical reports covering formations important to strategic UK development and construction. These can be downloaded and include:

• Gault Formation
• Mercia Mudstone Group
• Lias Group
• Lambeth Group

History and evolution of the database

Contact

Contact Marcus Dobbs for further information about the National Geotechnical Properties Database.

The Rock Mechanics and Physics Laboratory (RMPL) undertakes research on the properties and behaviour of rocks and geomaterials at near-surface to shallow crustal depth for georesources (energy; storage), geoengineering (tunnelling) and geohazards (faults; earthquakes).

Beneath our feet, rocks continuously deform, slide and move differently across a large spectrum of environmental conditions (pressure; temperature; fluid chemistry, etc.) and time — very slowly in creep mode for instance, or very rapidly during an earthquake. These coupled processes take place at all scales, from atomic level mineral/fluid interfaces to continental faults, mostly at inaccessible, deep locations precluding direct observations.

Laboratory capabilities

RMPL is the home of BGS large-scale rock deformation apparatus and specialises in standard (ISRM and ASTM) and bespoke geomechanical and rock physics testing.

Testing

• Strength (triaxial and uniaxial)
• Deformability
• Thermal properties
• Geophysical properties
• Permeability
• Porosity
• Density

Equipment

• MTS 815 Rock Testing System (4.6��MN capacity servo-controlled, hydraulic load frame)
• Servo-controlled confining pressure up to 140��MPa
• Servo-controlled pore fluid pressure up to 100��MPa (water and brines)
• Heating system up to 180°C
• Four internal load cells (from 250 to 2600��kN)
• Direct contact axial and circumferential strain gauges (extensometer type)
• Ultrasonic measurements (up to 18 electrical feedthroughs)
• Point Load Strength Index