66- www. world- petroleum. org 5.3- Technology: pushing boundaries Usually, subsea systems consist of an in-tricate network of gadgets installed by that vessel and connecting it to the hydrocarbons miles below. The most important pieces of kit include the wellhead, the component at the surface of an oil well that acts as the interface for drilling and production equipment. It pro-vides a pressure barrier connecting the casing strings that run from the source of the well to the pressure- control equipment. Traditionally, the wellhead was located on oil platforms, but in deeper waters it sits on the seabed. Once the well has been drilled, a comple-tion is placed in the well to provide the conduit for the well fluids. The surface pressure con-trol is provided by a Christmas tree, which is installed on top of the wellhead. Named for its crude physical resemblance to a tree, this is an assembly of valves, spools and fittings, that helps control and regulate the flow of oil out of the well; provides numerous chemical injection points, allowing the oil to be treated; and also sensors, enabling temperature, flow-rates, and flow composition to be measured. Subsea wells and trees connect through flowlines ( or risers) to a fixed or floating pro-duction platform or to a storage vessel. The riser is the conduit for the oil and must be Located miles beneath the surface of the oceans, and sometimes hundred of miles from the shoreline, subsea wells are by their nature remote. That makes drilling and planting these wells a problem. And it makes going back to repair them a nightmare. Maintenance is comparatively easy to per-form on equipment based onshore, or in shallower seas, where the wellhead is usu-ally sited on the platform. Access to dry trees is easier and can be accomplished more quickly, boosting the total recovery factor. The average onshore well needs between four and five days of intervention a year, but because it takes longer to repair and main-tain wet- tree wells, their productivity is low-ered. The recovery factor of a subsea well averages about 20%, compared with around 50% for dry- tree wells, says Andy Hendricks, head of Schlumberger's subsea division. Then there's the wasteful cost of charter-ing the kit necessary to make the repairs. This typically means hiring a drilling rig, which has all the hardware required to lower equipment to the floor, and on- board personnel - maybe 100 people. But that vessel is designed to perform other functions, besides repairs, such as drilling. " You don't need all that equipment just to do an intervention," says Hendricks. And it's undesirable economically. Deep- water rigs can cost $ 600,000- 800,000 a day. In depths of 400 metres or more - in the Gulf of Mexico, for example, or offshore Brazil, or the North Sea west of Shetland - this expense has been almost unavoidable. Until now. But Schlumberger has developed a technology that it says could enable oil producers to halve maintenance costs. Says Hendricks: " We've found a commer-cially viable solution - a system that uses a light vessel." This vessel, he says, will be much smaller and cheaper to run than a drill-ing rig, and is purpose- built, deploying a sys-tem that allows producers to drop special tools through the top of a wellhead, using a wire that can run 3,000 metres down to the seabed. These tools are remotely- operated and can be controlled from the vessel. The system imitates technologies that al-ready exist at wells at the level of continental shelves; but Schlumberger's device will be a first for deep- water wells - assuming there is sufficient customer interest to justify fur-ther investment. If so, Hendricks estimates the vessel will go into production next year and will be at work by 2012. With Brazil's pre- salt offshore fields open-ing up for production in the coming years, Hendricks expects demand to grow. " As the industry moves into deeper waters, we expect an increase in demand for services like this," he says. " Between 2003 and 2008, a total of 1,900 wet trees were installed in the ocean. But between 2009 and 2014, we're expecting that figure to grow to 3,600. So there's a big ramp- up of wet trees to be serviced." ?? Schlumberger says it can halve subsea maintenance costs