This paper describes the first application of Nonlinear Fourier Analysis to the quantification of internal soliton current speeds in offshore engineering design. Large amplitude solitary internal waves produce strong, rapidly varying currents that may cause hazards to offshore operations in several regions of the world. These phenomena are commonly referred to in Industry as “solitons.” Soliton quantification was undertaken using the latest methodologies and software available from the Nonlinear Fourier Analysis Spectral Tools (NFAST) Joint Industry Project.

Solitons require rapidly sampled in-situ data for reliable quantification. Such measurements are typically of very short duration compared to the time scales needed for engineering quantification. Similarly, numerical models capable of representing solitons are computationally expensive, and thus have limited capabilities for efficiently developing the long-term simulations required to supplement in-situ data. NFAST aims to address these issues by enabling new Hyperfast Nonlinear Fourier Analysis computational techniques.

Interface displacements, derived from temperature measurements, were the primary input to soliton quantification. Associated current speeds were estimated from relevant theory and validated with available measured current data. In this particular case, the temperature measurements are considered to be more reliable than using the measured current data directly.

Application of NFAST codes produced a synthetic dataset of soliton amplitudes and speeds with an effective duration of approximately 100 years, a period that is considerably greater than the duration of available measured data. This provided extreme values consistent with extrapolation of the measured soliton data, but with a considerable reduction in uncertainty.

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