Norwegian injection method uses separate line to wellhead

System removes injection from drilling process

The question of what to do with contaminated drill cuttings from drilling with oil-based mud has become an increasingly pressing issue in Norway, the UK, and anywhere else where environmental pressures require an alternative to partial treatment and dumping.

Re-injection from semisubmersible rigs has not been possible and for drilling done this way there has been no solution other than to transport well debris to shore as landfill. However, a new well re-injection system developed by Coflexip Stena Offshore, one based on flexible pipe technology and designed by the drilling and refining department specifically for semisubmersible drilling units, appears to solve the problem. System 001 and 002 have already been installed on two semis operating off Norway, where the problem of drill cuttings disposal has been greatest.

Background

In the last three to four years, drilling by semis in the Norwegian North Sea has increased by around 300% as the area matures and subsea wells become more common. This has increased problems for operators, which for the last six years have been required by law to transport well debris to shore if it could not be re-injected.

"Lack of process capacity onshore has resulted in a rapidly increasing storage problem," explains Bjorn Oftedal, a Statoil engineer specializing in well re-injection. In addition, he says, the process is costly and cumbersome and fails to solve to problem in that the waste remains contaminated. The only way to deal with the debris properly is to incinerate it, which makes disposal even more costly.

In the UK, pressure from the Department of Trade and Industry (DTI) has been mounting. All UKCS operators are required to have a phase-out schedule which will reduce the discharge of mineral oil-based cuttings to zero by December 31, 2000. Some have sought to effect this by using synthetic fluids, but DTI studies suggests the biodegradation of these fluids is no better than for mineral oils, so they too have to be phased out.

Only ester fluids are not subject to the reduction process, but the DTI has stated: "The UK will continue to monitor the impact of all discharges by seabed survey. If, in the future, results indicate ester systems or any substitute have an unacceptable impact, they will also be controlled as appropriate.

For the present, the drive to develop an alternative to dumping has come from Norway where Statoil has been seeking a solution for the last few years. Coflexip Stena Offshore, with its knowledge of flexible pipe manufacture and installation and also ROV technology, says it was approached recently for a solution.

"The system for grinding up cuttings is well established and we felt it would be possible to run a separate line to the wellhead,"explains Oftedal, "but the subsea remote connection at the wellhead seemed to pose the most difficulties. We wanted a connection which would be independent of the riser so that it did not interfere with drilling, so that re-injection could be continued even when drilling was not taking place."

Statoil initiative

The adopted process begins with drill cuttings coming back to the rig via the riser. They are then separated out from the mud, which is re-used. The drill cuttings are pulverized and mixed with seawater before being pumped at high pressure from a SMACC (Statoil Method for Octogeneous Crushing & Classification of Cuttings) unit into the re-injection guide base via a pliant wave-configured flexible riser. These aspects of the process are provided by Procon, a partner of Coflexip Stena Offshore in this project.

A suitable flexible line is deployed to the drilling template with interface buoyancy modules, clump weights and subsea stab connector attached. Deployment of the line is continued until the remote subsea stab is positioned above the template. At this point, the rig ROV locates the stab and assists with the connection. The ROV then monitors the line as it is being deployed. Topside interface is via a short flexible line between the slurrification unit and the riser. In the event of an emergency, deployment/abandonment of the line can be completed within 10 minutes. Once the emergency has passed, the line can be recovered by the ROV.

The task was accomplished within seven months. The system has now been installed, refined and proven on the Transocean Wildcat and Transocean Winner semisubmersibles working in Norwegian North Sea fields. "In addition, similar installations are being evaluated for use in new subsea fields,"says Oftedal. "The system will have other uses - it could carry fluids such as contaminated brine."

According to Coflexip Stena's Paul McQueen: "Taking into consideration that an average North Sea well may produce between 13,000-18,000 tons of cuttings waste, this system and process, which is presently being cost-evaluated, is not only environmentally friendly, but through its simplicity, also a cheaper option for transporting and treatment of debris elsewhere." The system can be also be used with jack-ups and drillships.