Case Study • Installation of a 2,000m Marine Winch • Robertson Geo Logo

Case Study • Offshore Wind Farm Seabed Investigation • Robertson Geo

Vesterhav Offshore Wind Farm
Vesterhav Offshore Wind Farm
Seabed Investigation
Service Provider
Hjelmevej, Denmark
The project was to investigate the small strain moduli of the sea bed down to 45m using the Robertson Geo PS Logger. The acquired data would then be used to calculate the parameters for constructing the Vesterhav Offshore Wind Farm off the North West coast of Denmark. The geologic formations were typically sands, clays and mudstone.

Logging was conducted from the support vessel Freja, which has a compensated drilling platform.  The PS Logger provided high resolution, shear wave and compressional velocity data in rock and soils at depths up to 500m, from measurements within a single borehole. Often it proves technically superior and more cost effective than a downhole or cross-hole seismic survey.

Thirteen boreholes were logged over a period of 10 weeks to obtain the P and S wave velocities of the sea bed down to approximately 45m at 1m intervals. These velocities were then applied to the small strain moduli equations with the addition of density, to give Shear, Bulk and Youngs modulus.

The logging probe was successfully deployed with 100% reliability; boreholes were logged from approximately 45m up to the overburden casing. Following logging operations, the operator would quickly process the data acquired. Robertson Geo Engineers were on-board the Freja throughout the project and available for logging through 24 hours each day.
Sensors & Instruments
Robertson Geo's PS Logger Probe is a low-frequency acoustic sonde designed to measure compressional (P) and shear-wave (S) velocities in soils and soft rock formations. It operates using indirect excitation rather than mode conversion as in a conventional sonic. It is capable of acquiring high resolution P and S wave data in borehole depths of up to ... More600m in water.

The PS Logger probe contains a unique design of powerful hammer source and two receivers, separated by acoustic damping tubes. To acquire data, the probe is stopped at the required depth and the source is fired under surface command. Firing causes a solenoid-operated shuttle aligned across the borehole axis to strike plates on opposite sides of the probe in turn, setting up a pressure doublet in the surrounding fluid. The resultant fluid motion produces a tube wave at the borehole wall with velocity close to the shear velocity of the formation together with a compressional wave.

As the waves propagate parallel to the borehole axis, they set up corresponding fluid movements that are detected by the two neutral buoyancy 3D hydrophone receivers and geophones, allowing the wave velocity to be determined. The facility to stack multiple shots and filter the data as in normal seismic data acquisition is included in the operating software.
Related Case Studies
Robertson Geo service engineers are highly trained and fully certified for offshore work. Logging on offshore locations demands versatility of both crews and equipment to get the job done. Here, the 2,000m marine winch has been suspended in the gantry to deploy geophysical probes for offshore subsurface investigation near New Bedford, Massachusetts, ... MoreUSA.

If you look carefully you can just about see the Robertson Geo logging crew behind the marine winch in this image of the set up prior to logging.

Working experience by Robertson Geo offshore logging crews has led to the introduction of the 2,000m marine winch, designed to resist corrosive and saline conditions. Precision engineered for reliable use in the most challenging offshore applications, the marine winch is fully compatible with the Micrologger2 surface data acquisition system and the full range of probes to depths of 2,000m.
The customer required information to assist with constraining parameters for modelling the development and planning of a proposed sea wall and port. This required logging a framework to investigate boreholes in a shallow water tidal area where the proposed sea wall and marine offloading facility would be constructed off the north coast of Anglesey.... More

Borehole depth was 10-80m - a suite of probes was used to identify fractures, fault patterns and ground stability, including Optical Televiewer, High Resolution Acoustic Televiewer, Full Waveform Sonic and Borehole Geometry with inbuilt X and Y calliper. Surface equipment was a 2,000m winch with 3/16” core cable, connected to a Micrologger system.

Logging of 26 holes was successfully completed over a period of two months. The Robertson Geo team supported two jack-up rigs, with Engineers on 24 hours availability over the duration of the project. The jack-up rigs were accessed from a small rib boat that docked at nearby Cemaes Bay.
GeotechnicalInstrumentationOffshore RenewablesOffshore Wind FarmOWF
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