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Challenges of Good Facility Design

It is acknowledged that formidable challenges exist in the prediction of well performance and reduction of reservoir uncertainty. The challenges with the production facility aspect of the system are often viewed as inferior. This is also echoed from the viewpoint of research and development budgets for geological and downhole issues that normally far exceed the proportion spent on facilities.

Underestimating facilities leads to several risks. Since the most visible part of a production system is the facility, a failure in the facility captures attention. The catastrophic failure of the Petrobras P36 semi- submersible and consequent loss of life attracted worldwide negative publicity to both the company and the industry. With increasing awareness of environmental issues and global terrorism, safe facility design may be of paramount importance in comparison with factors that directly affect the profit margin, such as production flow rate. A good facility leads to less downtime during operations, thus indirectly enabling the production target to be achieved.

Flaring gas and oil spillage are common occurrences that need to be controlled. These are challenging issues that combine the expertise of reservoir, process and facility engineers. Poorly designed floating facilities may suffer from an unacceptable response in cyclonic conditions, or affect safe crude handling operations such as offloading on a day-to-day basis.

There has been publicity recently about possible terrorist attacks on offshore oil and gas facilities. Methods to combat a terrorist attack go back to the basics of fire and explosion. The industry has developed standards based on lessons learnt from well- known disasters such as the Piper Alpha. Most remedial strategies have revolved around common sense by separating key accommodation quarters from possible vulnerable areas of fire, for example. Considering that a terrorist attack would most probably occur at the sea level, hull subdivision and damage stability considerations of a floating system may need to be revisited, and better escape routes planned. These appear to be feasible within the existing operational and maintenance framework of most operators.

Extrapolation of Recommended Practice to Deep Water

Offshore regulations and recommended practice for deepwater floating facilities were derived largely from accepted criteria for design and operation of ocean- going vessels and those for shallow water fixed facilities. While this is logical, the limitations of extrapolation are often underestimated. For example, bilge keels do not work with FPSO vessels, but they are effective in containing the roll motion of small crafts. It is important to note the difference in forward speed of the two cases and the consequent effect on performance. Wider bilge keels have been suggested as a way of improving performance. Researchers may need to deviate from established practice, adopt a global perspective and get to the roots of the problem in order to find confidence in their answers.

Interesting problems have also arisen with spars. The motion of water in the moon pool results in an extra load on the risers, causing them to vibrate within the guides, thus causing fatigue cracks. Vortex-induced vibration (VIV) of risers was developed from an understanding of similar vibrations in aerodynamics and industrial applications. Helical strakes, commonly seen on chimneys, were adapted to risers and found to work, albeit at a considerable cost. However, strakes on spars do not appear to eliminate VIV entirely. The vibrations and consequent movement of the spar have resulted in several operational problems, not to mention personnel fatigue and riser tension. This situation has led to a rethink of the fundamentals of VIV on spars. Research on this subject is ongoing.

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