The project involved connecting a drilling and production platform to an onshore processing facility. DrillTec was awarded the contract for HDD and excavation works, which were integral to the success of the project and involved an overall drilling length of approximately 15 km.
German petroleum company RWE Dea and Germany’s largest oil and natural gas producer Wintershall make up the Mittelplate Consortium that is responsible for operating the oilfield. The Mittelplate deposit is located in the Schleswig-Holstein National Park on the North Sea coast, where production to date has been restricted by tidal and weather conditions for the shipment of oil to shore.
The only viable option for increasing production, reducing the risk of an oil spillage, and optimising the exploration potential of the oilfield was to install a direct pipeline connection to shore. Recent feasibility studies indicated that the expected reserves in the region were a great deal larger than original estimates.
Previous transport capabilities were limited to a maximum of 900,000 t/a. Current total production volume from the Mittelplate deposit exceeds 2 MMt/a through the modification of facilities and the pipeline connection. The environmental permits required for the construction of the pipeline were only granted under the most stringent conditions, as the proposed pipeline alignment ran unavoidably through a national park of huge environmental importance. Therefore, the government approvals and ultimate success of the project hinged on the adherence to stringent requirements and continuous monitoring.
The horizontal drilling and civil engineering works were awarded to DrillTec in September 2004, after which an intense pre-engineering phase followed. Two pipelines were to be laid, a 10 inch oil line and a 6 inch water reinjection line that ran parallel to each other in separate boreholes for 7.5 km. The alignment was split into 6 sections that varied in length between 1,100 m and 1,400 m. Environmental restrictions demanded a rapid drilling program to complete the twelve HDD boreholes. To fulfil these requirements logistically in such a tight timeframe, five rig spreads were mobilised for the project.
Site setup began in February 2005 for the six specially designed and constructed cofferdams. Towing operations of the barges to sea was delayed due to unseasonably low tides, thereby delaying the planned start of the offshore works. Work schedules and drilling plans had to be changed at short notice to make up for the lost time, which was achieved by the outstanding cooperation of all involved.
Once the pontoons were at sea and positioned, the excavation works began to construct a sheet pile excavation pit 6 m wide and 80 m long. These pits served as the entry and exit points for the HDD bores and allowed the welding works to be carried out 5 m below surface for the pipeline tie-ins.
Upon installation of the sheet piles and dewatering pumps, a custom built drilling platform was constructed to bridge the two barges so that the HDD rig could be set up directly above the pit and drilling could begin.
For each section, the 6 inch pipe was installed first in order to provide a mud return line. This method of transporting recycled mud between rig and pipe sites as well as between the various construction pits, greatly reduced mud consumption and waste.
The product pipe strings were prefabricated onshore at Friedrichskoog and towed along purpose built piperollers ready for installation into the HDD boreholes. The piperollers were mounted on top of steel piles installed into the seabed, which made pipe pullback operations independent of the 2 m tides. Pipe strings were towed into position at the appropriate pipe site using winches mounted at each pontoon.
The rig platforms at each site were approximately 6 m above the seabed.
For this reason casing was used to support and guide the drill pipe from the rig to the entry point at the base of the pit. Due to expected borehole hydraulics, a 12¼” bit and 6″ drill pipe were selected for use. The combination kept borehole pressures at low levels, thereby minimising the risk of drilling fluid breaking from the soft geological formation to the seabed surface.
Due to the soft formation the coverage depth was chosen to be 25 m. Additionally, pressure sensors built into the bottom hole assembly provided real-time monitoring of downhole mud pressures to mitigate the risk of fracturing. The narrow target window for the exit point brought an extra challenge to the project.
A target space approximately 1 m wide and 10 m long had to be hit leaving little room for error or second attempts. The main complication to delivering a highly accurate steering solution over such a long length was the strong magnetic interference from the steel sheet piles and barges.
However, after resolving initial steering difficulties, all boreholes accurately hit their targets using a combination of HDD survey methods. Pullback forces during pipe installation were kept to an absolute minimum by using a specially designed swivel reamer, and the highly successful application of saltwater-based drilling fluid.
Despite the shortened construction period the last of the twelve pipe strings was pulled-back on 19 May 2005. Simultaneously work at the other sites was carried out, excavating the pits deeper for the pipeline welding works. Upon completion of pipeline testing and tie-ins, the pits were filled in and the demobilisation of the barges began.
The Mittelplate pipeline was without doubt the largest HDD project in Europe for 2005. It highlights the importance of DrillTec’s thorough approach to planning, its experienced personnel and reliable equipment in achieving practical completion on-time despite the challenging offshore conditions. Commissioning of the pipeline and completion of the drilling platform upgrade has enabled the Mittelplate Consortium to increase production from Germany’s largest oilfield.