An estimated 7% of the world’s oil and gas resources are believed to lie within deepwater regions such as offshore West Africa, North America and South America. Production from such reservoirs has steadily increased since the 1980s to the point where it is projected to reach approximately 10 MMbbl/d by year-end, equivalent to 10% of global oil output.

Oil and gas companies have the utmost respect for the challenging ocean conditions that characterize these regions and, in turn, carry out the necessary due diligence to help minimize the uncertainties in engineering design and operational planning.

Effective offshore exploration, engineering design, field development and operational planning in deepwater environments require a comprehensive understanding of the ambient and extreme ocean current conditions through the use of in situ measurements and ocean modeling technology.

Robust simulation
Although in recent years there have been significant advances in ocean modeling technology, ensuring fully fit-for-purpose model results has continued to challenge the research community. Producing high-quality and robust ocean simulation is still a difficult problem to overcome.

To better understand the ocean conditions within a potential development site, an operator will typically deploy an oceanographic mooring, which would be equipped with current meters as well as temperature and conductivity sensors to acquire detailed site-specific measurements.

Although such an approach can provide a detailed insight, measurements are seldom more than a year in duration, which is a relatively short time period and can therefore provide only limited insight. When designing an offshore structure, it’s important to consider the longer term variability in conditions at a given location—a year’s worth of measurements may not necessarily capture all of the characteristics of the ocean’s current conditions.

Floating design requirements
The design of deepwater structures such as floating production facilities comprises subsea components including risers and moorings, which span right through the water column from the surface structure down to the seabed.

This means that the loadings on subsea components will be strongly influenced by the current flowthrough depth. When designing such components that require a long life span, there is a need to assess worst-case design-loading conditions as well as through-life fatigue loading.

The design of FPSO vessels also requires an assessment of dynamic loadings associated with combined response to winds, waves and near-surface currents. An evaluation of spatial and temporal variation in throughdepth ocean currents and water density also is necessary for oil spill contingency planning.

The availability of high-quality, long-term oceanographic datasets can greatly minimize the uncertainties within the concept feasibility phase and can allow an optimum design to be delivered.

More conservative designs
If a design engineer is faced with high levels of uncertainty in ocean conditions, then a more conservative design with high factors of safety built in is likely.

Equally, an engineer may unwittingly “underdesign” a component because of a lack of awareness of conditions. Either scenario is not ideal. The more detailed information an engineer has for informing the design process, the less uncertainty there is in the conditions that the engineer is designing the subsea components to withstand.

Metocean support is equally crucial within the installation process. There are various stages of this process that can be extremely sensitive to strong currents, for example, and will therefore create operational limits.

Oil and gas operators use expensive vessels and ROVs during the installation process. If detailed information regarding the environmental conditions has not been made available, this can cause extended and very costly delays, which can quickly escalate from one day to the next. Detailed statistical analysis of favorable conditions to help complete the different phases of installation is therefore key.

Mid-Atlantic region
To help support oil and gas companies with offshore engineering design and operational planning in the Mid-Atlantic region, BMT ARGOSS joined forces with the Met Office (the U.K.’s national weather agency) and Oceanweather Inc. Funded by the partners, the main deliverable of the project, titled the Mid-Atlantic Current Hindcast (MACH), is a 20-year ocean current reanalysis for the Mid-Atlantic region, with nested high-resolution grids covering principal oil and gas concession areas.

The Met Office is one of the leading organizations in data assimilation and large-scale, complex ocean circulation modeling. The pioneering work of Oceanweather’s hindcast approach and derivation of optimal wind fields for ocean model boundary forcing has been recognized internationally in both the scientific community and in applications for the offshore industry. This knowledge has been coupled with BMT ARGOSS’ extensive experience in providing fit-for-purpose metocean services.

Key to MACH is the data assimilation scheme that is used within the model. Data assimilation uses measured data to accurately condition the starting fields in the model. In effect, a computer simulation is conducted and objectively adjusted to match the realities of the observed data that are available.

Available data can come from a number of sources, including satellite, remote sensing of sea surface height and temperature, and autonomous Argo profiling floats that are commonly used in oceanography.

With approximately 3,600 deployed worldwide, these drifting profiling floats continuously measure ocean temperature and conductivity to derive density, which is important in determining large-scale motions in the ocean.

MACH One
During the initial pilot phase of the MACH project, the system was run over two one-year periods with 12 different permutations of model configuration.

Extensive validation of all results was undertaken against in situ measurements and drogue trajectories in order to determine the best setup. An optimized 20-year “production run” was then conducted. Detailed validation of the full hindcast dataset was undertaken to demonstrate the performance of the model and the suitability of the outputs for use in offshore applications.

Utilizing state-of-the-art assimilative ocean modeling technology and high-quality wind forcing, the hindcast can provide a strong framework for conducting fine-resolution modeling in other parts of the Mid-Atlantic Basin, including Brazil. A robust
long-term ocean simulation that has been validated and optimized against data measured in the given region will provide vital support in helping to meet the demanding needs of the oil and gas industry as new deepwater territories are explored and developed.

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