Studies update the industry's knowledge about deepwater's impact on production operations.

As oil and gas activity progressively moves into deeper water, there is an increasing need to understand and quantify ocean currents. Currents influence rig selection, riser design, many aspects of offshore operational planning, and the design and installation of production systems, moorings, subsea components and pipelines. Of particular importance is fatigue associated with dynamic response to current loading.

The characteristics of current behavior differ significantly between key oil and gas regions. In the Gulf of Mexico, the dominant feature for the offshore oil and gas industry is the "Loop Current." The Loop Current is an intrusion of warm water from the Caribbean Sea that flows northward through the Yucatan Straits into the eastern Gulf of Mexico, and then bends to the east to enter the Atlantic Ocean through the Florida Straits.

Loop current

The Loop Current is a persistent feature in the Gulf, characterized by strong surface current velocities, typically 2 to 4 knots with its position and intensity varying over time. Further problematic features of the Loop Current include anti-cyclonic warm-core eddies that break away from its northern extremity. Such eddies are characterized by intense current velocities around their outer edge, and can also cause serious impact to offshore operations as they propagate towards the western Gulf, decreasing in intensity as they go.

Typically one or two eddies form each year, although timing, frequency and behavior are highly variable. Recent studies have also identified the rarely observed and poorly understood "submerged" currents as having a significant impact on the design of offshore installations. A major challenge in the Gulf of Mexico is the characterization and forecasting of these energetic phenomena. This challenge also extends to other oil and gas regions around the world that are impacted by strong currents, such as offshore Brazil, Trinidad, Indonesia, Vietnam, the United Kingdom and India.

Design planning

For offshore engineering, there is a need to understand currents to support design and also planning of marine operations. For operational support of offshore exploration and production, real-time current measurement systems are generally installed. The technology associated with current velocity measurement is now well advanced since the introduction of Acoustic Doppler Current Profiling (ADCP) techniques some 15 years ago. It is now possible to acquire good quality current profile measurements, in real-time, through the water column, even in very deep water. However, for effective planning and decision-making, reliable forecasting of future current conditions is ideally required.

For the estimation of extreme current criteria to support engineering design, it is important to assess seasonal and inter-annual variability in dynamic conditions, for example to account for the El Niño phenomenon. It is very seldom possible or cost-effective to undertake multiple-year site-specific measurement programs in support of oilfield developments. Therefore, competent long-term computer model simulations (hindcasting) are required to complement available short-term in-situ current measurements to form the basis for detailed statistical analysis.

Models

The use of numerical ocean circulation models in hindcast and forecast modes represents the only viable technology available to provide long term data for design purposes and to predict future current conditions needed for operational planning. This is an exciting emergent technology that will greatly improve our ability to understand and quantify ocean currents. This technology is still in the development phase, with many research efforts now being undertaken to validate the use of these models. An important aspect of the models is the implementation of "data assimilation" techniques, whereby recent observations are incorporated into the computer model representation, such that the model is "nudged" towards a best representation of reality.

Data that are assimilated into ocean circulation models include sea surface height (SSH) and sea surface temperature from satellites, as well as temperature and salinity data from the innovative global ARGO profiling float program. Data assimilation significantly improves the ability of numerical models to reproduce ocean circulation characteristics with accuracy and reliability, both for hindcasting and forecasting applications. Figure 1 provides an example of a satellite altimeter derived sea surface height chart for the Gulf of Mexico using data available on October 8, 2003.

Both eddies and the Loop Current are indicated by large values of SSH (highs) and the surrounding Gulf waters are have relatively low values of SSH. Strong currents exist where there are strong gradients in SSH. In this example, the Loop Current flows through the Yucatan Straits and follows a path almost directly into the Florida Straits. The region of high SSH just north of the Loop Current at about 25° north latitude indicates the recent formation of a large anti-cyclonic eddy, which may reconnect or completely separate from the Loop Current. A smaller high, centered at about 27° north and south of the Mississippi Delta formed in the summer of 2003 and has been named Eddy Sargassum. Figure 2 shows the SSH and surface velocity vector field from a HYCOM model for 7 October 2003.

3-D models

There are a number of initiatives now being undertaken by the academic, governmental and commercial scientific community to develop and verify 3-D current models of key oil and gas basins around the world. The CASE (Climatology and Simulation of Eddies) joint industry project used the CUPOM (Colorado University Princeton Oceanographic Model) to derive a first current hindcast for the Gulf of Mexico.
Oceanweather's WANE (West Africa Normals and Extremes) hindcast of wind, wave and 3-D current data is already in use for the derivation of extreme and operating criteria for the deepwater region. The current modeling component of WANE was conducted by Ocean Numerics, a joint venture between the oceanographic consultancy Fugro GEOS and the Nansen Environmental and Remote Sensing Center, an independent non-profit research institute affiliated with the University of Bergen. This grouping is looking to develop and provide practical applications of advanced ocean current modeling. Development work is based around use of the HYCOM model that is also being implemented by the US Navy as the basis for ocean current forecasting. An Atlantic Ocean wide version of the HYCOM model forms the platform on which more detailed regional current models are developed.

Figure 2 shows a test forecast using a HYCOM system for the Gulf of Mexico. The forecast used all SSH data available on October 1, 2003 to predict the SSH field and surface velocity vector field shown in the figure for October 7, 2003. The similarity in the features shown in the model SSH field and the altimeter SSH field is obvious. The joint venture has recently been awarded funding from the National Oceanographic Partnership Program (NOPP) to evaluate and demonstrate ocean model performance in the Gulf of Mexico.

With models and initiatives such as this now in place and under development, and prototype-forecast services now available, the advent of ocean forecasting for the oil and gas industry is imminent. However, it should be remembered that some of the first weather forecasts for coastal regions were issued in 1861 by Captain Robert Fitzroy of the former UK Meteorological Department. Over the years there have been major advances in weather forecast capability, which provide us with the extensive, global services available today. While today's ocean forecasting programs start out with a significant advantage over Fitzroy in terms of data, computing technology and understanding of the physical environment it will still take time, investment and the custom of offshore industries to develop this capability to a level that will match that of today's weather forecasting.