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Geoprober drilling – light weight deepwater drilling

Thursday, June 17, 2010

Geoprober Drilling of the UK is developing a method to drill in deepwater at 50 per cent less cost than conventional drilling, using lighter drilling equipment, but which will actually improve safety and reduce environmental impact. Technical director Tony Bamford explained how it works at the May 26 Finding Petroleum deepwater forum

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Suspending the blow out preventer from a vessel

Geoprober Drilling of the UK is developing a method to drill in deepwater at 50 per cent less cost, using lighter drilling equipment, but which will improve safety and reduce environmental impact including fuel consumption, the company claims.

Technical director Tony Bamford explained how it works at the May 26 Finding Petroleum deepwater forum.

The company has been working on the project for 7 years, supported by Chevron and StatoilHydro, but would like $50m investment in order to develop the complete system including the deployment equipment.  The company plans a test of the lower part of the system with Statoil in the North Sea in 2011.

“We’re probably about 2.5 years away from fully testing the system - if we found the money tomorrow,” he said.

On 2004 data, the company estimates that the cost of a well drilled like this could be $5.73m, compared to $13.9m for a conventional Gulf of Mexico well.

The system has much less equipment on the seabed than with conventional drilling, and it can be installed in a single trip. The seabed equipment weighs 30 tons.

The conductor is combined with the casing hanger and wellhead is connected to dual shear rams. The assembly is run to the seafloor on 7-5/8” casing and jetted in. Once installed on the seabed, drilling can commence straight away. “You’re in the reservoir in perhaps 12 days,” he said.

The idea is to drill to the first pressure containing point, then connect the casing on the seabed to the blow out protector suspended just beneath the vessel (ie so the surface casing and riser are the same diameter all the way to the surface.  The 7 5/8inch casing is then cemented in place. The next hole size is 6-1/2” where a 5-1/2” liner is set. Finally drilling into the prospective reservoir continues with a 4 ¾ inch bit.

This section  is all drilled by 2 7/8 inch coiled tubing. The coiled tubing is equipped with a data cable and it can be rapidly spooled through the water column  to run a variety of slim hole formation evaluation tools.

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The seabed mini blow out protector

Well control protection is provided by a “Near Surface”  blow out protector which  is suspended below the vessel, and the high pressure casing riser from the seabed to this blow out protector is strong enough to hold the pressure of the reservoir (up to 680   bar).

 A riser which can hold the reservoir pressure is more easily achieved  with this system, because the riser is only 7 5/8 inch diameter, compared to a 21 inch standard deepwater riser.

The system is ideal for reservoirs which are in deepwater, but at relatively shallow depths beneath the seabed. However if a series of  expandable liners are used , , Mr Bamford reckons the system could drill to 6000m below the seabed.

“We’ve  got a well architecture and asking you, have you got the geology that fits it,” he said.

“We think - if you could drill these wells for $5m a pop - how many more targets would you be able to reach?”

On the topic of safety, commenting on  the Gulf of Mexico disaster, Mr Bamford said on a conventional subsea drilling rig there was only one point where the hydrocarbons could be controlled after they got into the well, that was at the the subsea blow out preventer on the seafloor. “Once gas  got past that and entered the riser, there was nothing that could be done with them except divert the gas overboard

With the Geoprober system, there is an additional blow out preventer just below the vessel.
“We believe it is safer than existing technology,” he said.

Seabed apparatus

The equipment on the seabed is much reduced – instead of a conventional 300-400 ton subsea blow out preventer tor, there is a 30 ton mini blow out preventer  – This has a much lighter impact on the soft subsea soils.

Geoprober has put most of its design efforts into the seabed equipment – because it wanted a system which would provide all the necessary functionality, but which could be installed from the vessel in one trip.

All of the equipment on the seabed, including the template, conductor and a mini blow out protector, can be lowered to the seabed in a single operation, together with  the drillbit.

 By drilling a small hole with a narrow annular clearance between the drilled hole, drilling through shallow gas zones can be less hazardous. This is because mud circulating rate can be adjusted to provide the right back pressure to limit the flow from shallow gas sands. 
Geoprober did a lot of research into a special gripper system with dual seals, to ensure that nothing can leak out from around the casing into the ocean.

As part of the development, Geoprober studied  the performance of  conventional blow out preventers.
They analysed the way control systems are put together in the aerospace industry for examples of super reliable control systems, based on  triple modular redundancy principles.

The shear ram of the blow out preventer  needs to be able to close in under 45 seconds to meet the API certification requirements. This was proven through a practical demonstration.

The system has 3 separate control pods, and can communicate signals via acoustics or through the submarine vehicle (ROV).

The blow out preventer uses lead acid batteries for a seabed power source.  The batteries enable a much lower weight of equipment to be installed on the seabed. With conventional blow out preventers, energy is stored in liquid under pressure in accumulators.

The charge in the batteries is maintained with a steady flow of power from the vessel.
“The amount of power needed to keep batteries fully charge is very small indeed,” he said.

Vessel

The vessel required on the surface is a single hulled vessel, dynamically positioned, size approx 100m long by 22m across – Mr Bamford says that such vessels are currently available on the market for around $50,000 a day.

The vessel has a moonpool in the centre (a hole through which equipment can be lowered).

To distribute the weight of the subsea equipment and riser loads,, Geoprober has devised  a tensioning system, which spreads these loads around a large area of the deck, rather than suspending the load from the top of the drilling derrick.

Even with a slimmed down system, the total weight of equipment being sent to the seabed, including the riser can be 300 tonnes, so the vessel needs to be strong enough to handle it.

The equipment on the vessel has a “heave compensator” – which means that as the vessel moves up and down on the waves, the drillbit and any equipment it is lowering to the seabed stays in the same place.



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» Geoprober Drilling

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