Structural Monitoring of the Keystone ''Twisted Jacket'' Foundation: A Case Study off the Hornsea Coast-Paper

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The paper reports on the first 15 months of structural monitoring of a Keystone “twisted jacket” foundation supporting a meteorological mast in 28 m of water off the Hornsea coast, UK. The foundation comprises a central vertical pile, a guide structure and three raking piles grouted into guide sleeves. Its lightweight design was intended as a cost-effective alternative to conventional jackets for offshore wind structures.<br /><br />Four accelerometers recorded north–south and east–west motion. Researchers used Frequency Domain Decomposition to identify the structure’s first natural frequency and compared acceleration patterns between the inner guide member and outer sleeve to assess possible twisting.<br /><br />The measured average natural frequency was 1.13 Hz, compared with the design value of 1.08 Hz—about 4.6% higher. This indicates that the foundation behaved more stiffly than anticipated, possibly because the in-situ soil was stiffer than assumed in design. The frequency remained stable throughout the monitoring period. On selected dates, the outer sleeve showed directional motion relative to the inner member, suggesting a twisting response; these instances were associated mainly with waves arriving along an approximate northwest–southeast axis. The maximum wave height recorded since installation was 8.9 m, and inspections found no visible structural deterioration.<br /><br />Interpretation was limited by the accelerometers’ 4 Hz sampling rate, which allowed only the first global sway frequency to be examined. The authors recommend sampling at approximately 100 Hz in future monitoring to identify higher modes, including torsion, and to assess damping more reliably.

Keywords

offshore wind

twisted jacket foundation

structural monitoring

natural frequency

Frequency Domain Decomposition

accelerometers

torsional response

wave loading

meteorological mast

soil stiffness