Metocean criteria are required for the design and operation of offshore and coastal structures. In order to develop criteria the available data must be carefully assessed, the underlying physical processes must be thoroughly understood, and appropriate statistical techniques must be applied.
OCG can deliver metocean criteria for a wide range of applications and have experience of doing so for numerous locations around the world. An involvement in both research and standards enables OCG to avoid situations where traditional approaches would lead to non-conservatism and to take advantage of situations where over-conservatism can be removed in order to reduce client costs.
Examples of recent projects are as follows: response based design criteria for platforms in the North Sea, current criteria for pipelines in the Mediterranean and Caspian, and non-stationary tidal analysis for a port in the Persian Gulf.
Structural reliability analyses (SRA) are often required in the reassessment of existing structures; this is particularly the case for fixed platforms which would experience wave-in-deck loading at high return periods. A SRA allows the risk of platform failure to be quantified and the effect of any measures to mitigate the consequences to be understood. A key input into this analysis is the metocean loading in extreme conditions. Another is the probability of those conditions occurring.
OCG are able to conduct SRA using models that incorporate the latest understanding of hydrodynamic loading - this includes the full nonlinearity, irregularity and directionality of the wave field along with any wave-current interactions. Furthermore, by applying advanced statistical techniques OCG are able to provide a robust assessment of the reliability of an offshore structure.
Metocean analysis often relies on both long term simulations of the metocean environment and the properties of individual waves within different sea-states. This can be achieved through wave modelling. OCG have extensive experience in the development and application of a number of different wave models. Hence, OCG are able to run both deterministic and spectral wave models for both offshore and coastal applications. This includes: linear, second-order and fully nonlinear simulations of directionally-spread random sea-states and focused wave groups; WaveWatch III; SWAN; and SWASH.
Calculating wave loading is an essential step in the design process of offshore structures. Particularly as waves often dominate the forcing. In order to accurately determine wave loading a thorough understanding of wave-structure interaction is required. It is necessary to assess the flow regime, calculate accurate kinematics, determine hydrodynamic coefficients and apply appropriate analytical theories, empirical methods or numerical simulations.
OCG can calculate wave loading using Morison’s equation for slender structures, linear and second-order wave diffraction for large-volumes structures or CFD simulations for more complex flow fields when viscous effects cannot be neglected. OCG can also supervise (and advise on suitable facilities for) physical model tests when numerical methods are not appropriate, such as for determining wave-in-deck loading.
Examples of recent projects involve calculating wave loading on the Icebreaker Wind Turbine for Lake Erie Energy Development Company (LEEDCo) in the shallow waters offshore Cleveland, Ohio and OCG are currently running the LOADS JIP investigating wave-in-deck loading.
Field Data Analysis
Field measurements are important for calibrating or validating hindcast simulations, from which design criteria are derived. An essential step of using field measurements is the quality control of the datasets and employing suitable analysis techniques. This requires a good understanding of the underlying physical processes and the instrumentation used to record the measurements.
OCG have experience in the quality control of raw field measurements of waves, winds and currents. We apply novel techniques such as machine learning and we also manually review key events within the measurements to ensure their suitability. The analysis of the data is performed using Wavelet, Stockwell and Hilbert-Huang Transforms in order to understand the time and frequency variation of field measurements.
This expertise was formed from many year experience working for large oil & gas companies as well as continual research in this area, as demonstrated in our publications.