Opening the window

Weather is a major factor when planning offshore drilling campaigns. Asbjørn Stålesen of FMC Technologies says the company's Real Time Operating Limits system has allowed it to increase the time available for carrying out drilling work.

The biggest obstacle for operators drilling offshore in certain parts of the world is weather. Currents and waves can severely damage drilling equipment, and in order for an operator to fulfill a contract for an oil company, the operator must be able to anticipate inclement weather conditions and know when, and for how long, operations will be efficient and safe during any given season.

When designing or mobilizing equipment for use on a rig, an operating contract always specifies the worst possible combination of weather scenarios: for example, 3mHs (a measurement of wave height) waves and a one-year current represent the worst combination of wave height and direction. At their most conservative, operating limits in the North Sea require a two-week straight weather window within which wave heights do not exceed 3m. When this is not the case, the rig is down, but still has to be paid for.

Taking these conditions into account leaves operators with small drilling windows. In the North Sea, where 200 wells are susceptible to these conditions, drilling is limited to the summer months. Predicting actual weather rather than relying solely on worst-case scenarios could greatly extend operating windows.

One client, frustrated with the operational limits for the riser system in a project in the North Sea, posed this very problem to FMC Technologies, where to maximize efficiency, the contract only allowed operations from May to September. FMC determined the best solution was to replace assumed weather and current data used in formulating the contract, with real-time measured data to address this conservative approach.

Developing RTOL
FMC delivered its RTOL (real-time operating limits) system as the solution.

The effort began by performing extensive data collection from the North Sea project. Observations revealed waves rarely reached the height specified in the contract, and typically only occurred during the end of a storm or at the very outer edges of a weather system. In fact, the observations seemed to reveal that the North Sea is more ‘direct wind-driven' in its wave patterns and produces waves that are shorter than originally accounted in the project's operating limits. The ship's response to the shorter waves was much more favorable, given that shorter waves carry less energy and have less of an effect on equipment as they pass.

The actual occurrence of the one-year current was also observed. By definition, the one-year current should only occur once per year, so FMC analyzed the data for the average current at the North Sea project for the remainder of the year and discovered the normal current is substantially lower than recorded in the contract. With multi-year measured data, FMC could state that at this particular North Sea project, the current would normally not pose a major issue.

The direction of the wind compared to the current was also analyzed to see if any assumptions could be removed, but the conclusion was reached that significant data did not exist.

With the new data, it was obvious that using a measured value when formulating operating contracts instead of an assumed, conservative value, would provide longer and more efficient operating windows.

A great amount of data was collected. The analysis was run nearly 5000 times to ensure every single possibility was taken into account. The results of the analysis were stored in a database, which was used to formulate the first version of RTOL.

The evolution of RTOL
FMC felt that taking control of the current was the first priority in widening the North Sea project's operating window, so the first version of RTOL in October 2008 focused exclusively on current conditions. It included a database with the simulations as well as search capabilities for retrieval of data. The software was run on a laptop hardwired into a computer running the current measurements via an RS232 cable. This meant the physical location of the laptop was limited to the same room as the current-acquiring computer on the rig, and onshore personnel had no access to the system.

The first version of RTOL had a few drawbacks. The location of the computer was crowded and the rig personnel could not have constant access, as the laptop was not mobile. In response to the mobility issue, FMC introduced software allowing rig personnel to access the computer remotely and control or view the information from onshore when the laptop was deployed on the North Sea project offshore. A system with a second computer was also set up. During this iteration of the technology, the decision was made to move the system to a server onshore.

In the second version of RTOL, used in October 2009, all of the data was moved to a server, allowing the system to rely on network connectivity instead of hardwiring. This enhancement presented new possibilities to include weather forecasting in the RTOL system, and a real-time five-day forecast was programmed to be directly downloaded into the RTOL system. The second version also allowed FMC to look into backup for all the sensors and a rig with good weather radar in the area to be used as a secondary back-up. A rental wave rider buoy was also procured to measure wave and current data in real-time.

The wave rider turned out to be invaluable in the evolution of RTOL, as it is second-to-none at measuring smaller waves, though it underreports the size of larger waves. The buoy must be accelerated through the water up and down by the waves; when the waves grow larger, it simply falls behind and rides a smaller trough, causing it to underreport wave height. The wave radar has the opposite problem as it struggles to distinguish smaller waves, but measures larger waves very well. These findings illustrate the fact that the two technologies are complimentary and both are necessary for accurate wave measurements. To expand the functionality of the system, other modes were added to the second version of RTOL. A connected mode was added, which predicts the operating limits for a time period if the operator needs to disconnect. A subsea installation mode for running on wire was also included. In addition to a wave and current limit, this operation also has a wind strength limit.

RTOL today
With RTOL in place, it was determined drilling on the North Sea project could efficiently and safely proceed through November, allowing two additional months of operation. With this system onboard, the project can continue operating under broader conditions. It also has continuous access to the system's weather forecasting system, allowing the operator to plan operations up to five days in advance. The current RTOL system constantly provides the operator with ‘recommended operating limits' and ‘ultimate operating limits' so the operator will know how far the equipment may be stretched in case of an emergency.

This project's development is an ideal example of how the RTOL technology is being used. While there are many other opportunities for it to extend drilling windows in the North Sea, the technology is also applicable in other inclement weather and high current-prone areas, such as offshore eastern Canada.

The next phase of RTOL will include fatigue monitoring, which is especially valuable in a high dynamic load area like the North Sea, where there is a real chance of wearing out the equipment. The fatigue monitoring aspect of the RTOL system will allow the operator to set limits on equipment usage based on fatigue on temporary equipment such as the riser system, and on the permanent installed equipment such as the wellhead and the subsea tree. A fatigue database is under development. OE

About the Author
Asbjørn Stålesen
is group leader of well access systems analysis for FMC Technologies. He has a master's of science degree from University of Stavanger in offshore engineering and has worked in the offshore.

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