Riserless drilling pushes forward

Overcoming some of the most prevalent top hole drilling challenges and offering the industry fresh hope after a long search for such technology.

The unique challenges of offshore drilling, unable to be overcome by traditional drilling approaches, have forced the industry to aggressively pursue alternative methods to ‘pump and dump’ top hole drilling. After nearly a decade of significant expenditure in the areas of research and development of dual gradient drilling technology, the offshore drilling community has seen fruition in the form of AGR Subsea’s Riserless Mud Recovery (RMR) system.

AGR Subsea first introduced its patented RMR system (also known as riserless dual gradient drilling) to the industry three years ago and it has since penetrated the offshore drilling landscape, helping to relieve the growing challenges presented by offshore drilling.

The fundamental challenges of offshore drilling are commonly augmented by distinct geology unique to particular regions. Seabed geology that possesses shallow geohazards introduces conditions such as soft soils, swelling clay, shallow gas and water flows, and even mud volcanoes, making it increasingly difficult to drill in those specific areas and increasing the likelihood of damaging wells by soil movement.

The traditional top hole approaches involve either drilling with full mud circulation to the surface through a riser or utilizing the ‘pump and dump’ methodology, which simply spills the mud and cuttings onto the seabed. Drilling top holes using riser is considered very risky in relation to shallow gas and is banned in many places such as the Norwegian continental shelf. The most prevalent top hole drilling problem associated with the use of these approaches is the dispelling of mud and cuttings, which has obvious environmental implications. Heightened focus on the discharge to the marine environment has propelled the adoption of RMR technology, as authorities have begun to place restrictions on the amount of discharge emitted to sea.

RMR differs from traditional drilling approaches in that instead of a riser pipe through the water column to extend the annulus around the drill pipe to the surface, the return flow is intercepted at the seabed and pumped to the surface as if the drilling had been moved to seabed level.

What attracts drilling prospects to the new technology is the ability to more closely mirror natural pressure conditions found in the borehole as it is steadily extended to full depth. A suction module is mounted at the top of the well and a nearby subsea pump sends mud and cuttings back to the surface drilling rig via an eight-inch hose, which enables the use of heavy mud compositions to overcome soft soil conditions.

The engineered fluid system relies upon a combination of specialized drilling fluids incompatible with the sea environment or too expensive to discharge to the seafloor. By applying greater overbalance than the mud in the hole at the time would impart via a mechanical means as opposed to mud weight hydrostatic alone, the riserless approach offers the potential to re-circulate the mud and ultimately extend the capacity for handling mud with high gravities. With the RMR system, it is now possible to handle mud with a specific gravity of up to 1.5 rather than the 1.1 limitation associated with a riser approach.

The special RMR system originated when AGR was exploring new methodology for moving drill cuttings at seabed level as an alternative to ‘pump and dump’ top hole drilling. After AGR installed their first subsea mud pump in 1997, they developed a patented cuttings transport system (CTS), which went into commercial use in 1998. The RMR technology evolved from this original cuttings system after seven years of development work.

The Caspian Sea was the first regional candidate RMR technology exerted its influence upon as the area is well known for its shallow geohazards, which require the special technology to overcome the condition difficulties. The world’s first commercial application of riserless drilling technology occurred there on the Azeri Field for BP in 2003. Since that time, a total of 15 wells have been successfully completed on the field using the RMR technology.

The RMR system has proven to offer a host of economic and environmental benefits. By enabling use of engineered fluid systems to displace the common ‘pump and dump’ mechanism, the system eliminates casing strings, improves wellbore stability and significantly reduces discharge to sea. In turn, RMR drastically improves drilling efficiency and offers considerable savings. The benefits have been hailed as substantial as a result of considerable savings in mud costs and logistics support costs to transport large fluid volumes to the rig site.

Over the course of the past three years, RMR has significantly penetrated the offshore drilling landscape and established itself as a viable and preferred alternative to traditional drilling approaches. RMR technology has emerged in the industry at an opportune time and promises to be a significant step in the evolution of deep-water drilling technology. Advancements that were once deemed impossible are now becoming tangible in light of the new technology.