DNV Houston is providing consultancy and verification services for the development of a dual gradient drilling system for DeepVision, a joint venture between Transocean and Baker Hughes Inc. The project aims to qualify a drilling system in which a pump is placed on the lower end of the riser so that drilling mud can be circulated separately, avoiding the weight of mud in the riser. Installing casings in deepwater bores fractured by mud weight is costly, reduces the bore diameter and thus limits production rates.
DeepVision project manager Geir Sjøberg explains ‘Our solution to this problem is the dual gradient drilling concept. Instead of having mud column connected all the way from the rig to the seabed, we have substituted the mud in the riser with sea water. This gives one pressure gradient from the surface down to the seabed, and another pressure gradient from the seabed down into the well. Sea water weighs less than drilling mud and the actual mud weight can be increased without increasing the overall pressure in the well. The drilled cuttings and mud will be brought up to the rig using a pump system located at the wellhead close to the seafloor, thus ensuring the necessary circulation in the well.’
Says Sjøberg, 'We want to develop technology that will allow companies to explore for oil and gas in areas so far regarded as difficult or impossible. Through the use of new technology, we will reduce the costs of drilling in deep waters by around a third compared with existing technology, and will reduce the risk associated with deepwater drilling.'
The financial risk is far greater in deep water compared to more traditional depths. The economic penalties for delays, lost production or the breakdown of equipment become greater.
Repairs and intervention in deep water are more expensive and take longer. The day rate for a deepwater rig could be in the region of $200,000 USD per day. The oil company's own costs including service companies and logistics etc. are in addition to this. The total costs can be around $350,000 USD per day.