Underwater vision

Louise Smyth

Robin Stephens provides insight into the MACH project, which is developing a comprehensive ocean current hindcast database

Effective offshore oil and gas exploration, engineering design, field development and operational planning in deep water environments require a comprehensive understanding of the ambient and extreme ocean current conditions through the use of in-situ measurements and ocean modelling technology. Although in recent years, there have been significant advances in ocean modelling technology, ensuring fully ‘fit for purpose’ model results has continued to challenge the research community.

Some 7% of the world’s oil and gas resources are believed to be within the deep offshore regions such as West Africa and North and South America. Since the 1980s, the exploration and production of oil within deepwater reservoirs has steadily increased and by 2015, it reached 10 million barrels per day, equivalent to 10% of global oil output.

The oil and gas majors have the utmost respect for the challenging ocean conditions that characterise these regions and in turn, carry out the necessary due diligence to help minimise the uncertainties in engineering design and operational planning. However, producing a high quality, robust, fit for purpose ocean simulation is still proving to be a difficult challenge to overcome.

To better understand the ocean conditions within a potential development site, an oil and gas company would typically deploy an oceanographic mooring that 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 relatively short and can therefore limit the insight provided. 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.

Designing FPSOs

The design of deepwater offshore structures such as a floating production facility will comprise subsea components including risers and moorings, which will span right through the water column from the floating surface structure down to the seabed. This means that the loadings on subsea components will be strongly influenced by the current flow through depth. When designing such components that require a long life span in service, there is a need to assess ‘worst case’ design loading conditions, as well as through-life fatigue loading. The design of FPSOs 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 through-depth ocean currents and water density is also necessary for oil spill contingency planning. The availability of high quality, long-term oceanographic datasets can greatly minimise the uncertainties within the concept feasibility phase and allow for an optimum design to be delivered.

If a design engineer is faced with high levels of uncertainty in the ocean conditions, then a more conservative design with high factors of safety built in is likely. Equally, an engineer may unwittingly ‘under-design’ a component because of a lack of awareness of conditions. Either scenario is not ideal. The more detailed information you have for informing the design process, the less uncertainty there is in the conditions that you are 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 majors will use expensive installation vessels and subsea 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 that can quickly escalate from one day to the next. Therefore, detailed statistical analysis of favourable conditions to help complete the different phases of installation is key.

To help support oil and gas majors with offshore engineering design and operational planning in the mid-Atlantic region, BMT Argoss has joined forces with the Met Office, the UK’s national weather service, and Oceanweather Inc.  Funded by the partners, the main deliverable of the project, entitled the Mid-Atlantic Current Hindcast (MACH), is a 20-year high quality 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 organisations in data assimilation and large-scale, complex ocean circulation modelling. The pioneering work of Oceanweather’s hindcast approach and derivation of optimal wind fields for ocean model boundary forcing has been recognised internationally in both the scientific community and in applications for the offshore industry. This knowledge and expertise is 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 is taking measured data and using it 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 is available.  Available data can come from a number of sources including satellite, remote sensing of sea surface height and temperature, as well as 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.

Following completion of a pilot two-year integration and a detailed validation study against available comprehensive in-situ oceanographic measurements, a full 20-year integration is now underway to produce an ocean current hindcast database. There will then be a detailed phase of validation where BMT Argoss will play an integral role in testing the quality of the final product, ensuring it is fit for purpose.

A key focus for the project moving forward will be further engagement with the oil and gas majors through the International Association of Oil & Gas Producers’ (OGP) Metocean Committee. This group aims to encourage a better understanding of the value of applied oceanography and meteorology both within and outside the industry. The Committee promotes an awareness and use of the best practical metocean techniques in offshore and onshore structure design and operations.

Utilising state-of-the-art assimilative ocean modelling technology and high quality wind forcing, this hindcast will provide a strong framework for conducting fine resolution modelling in other parts of the mid-Atlantic basin, including Brazil. A robust, long term ocean simulation that has been validated and optimised 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 territories are explored.

Robin Stephens is Metocean group manager at BMT Argoss, a subsidiary of BMT Group.

Underwater glider agreement 

Ashtead Technology has secured a global asset management agreement with Blue Ocean Monitoring to store, maintain and supply underwater gliders for ocean data monitoring.

The deal will see Blue Ocean Monitoring expand its service offering globally with Ashtead providing asset management services and project support from its offices in Aberdeen, Houston and Singapore.

Unlike autonomous vehicles (AUVs), which are driven by conventional propellers, the gliders operate using either buoyancy or wave motion propulsion mechanisms, which allows for longer deployment periods and the collection of large datasets continuously over extended time scales.

The gliders are capable of transmitting data in real-time and can be deployed and recovered easily, at a fraction of the cost of traditional vessel-based or fixed-mooring monitoring approaches, lowering both project costs and health, safety and environmental risks.

Simon Illingworth, managing director of Blue Ocean Monitoring says: “Initially used extensively for academic and military applications, these gliders are now increasingly being embraced by the oil and gas community for a wide range of purposes.

“Oil and gas applications include pipeline leak detection, oil spill response, decommissioning studies, dredge/construction plume monitoring, environmental monitoring and metocean studies.

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