Eivind Grøv, Chief Scientist, SINTEF/Professor, NTNU, Trondheim, Norway
Bjørn Nilsen and Amund Bruland, Professors, NTNU, Trondheim, Norway
Oil spills on the ocean are among the most damaging of environmental disasters. In 2009, the oil drilling industry suffered the particularly severe setback in the Gulf of Mexico, which heightened public awareness of the environmental impact of such accidents. Eivind Grøv, Bjørn Nilsen and Amund Bruland in Norway, one of the world's major oil and gas producing nations, have developed a new design concept for using subsea tunnels to access offshore oilfields to minimize the environmental risk of ocean oil spills and improve the utilization and production of the drilling processes compared with traditional solutions. A base study evaluates the feasibility of the concept, includes time and cost estimates, and presents the need for breaking new frontiers on the technological level.
- There are many subsea tunnels around the world for road and railway links and utility underpasses. Tunnels have also been used by the oil and gas industry as landfall tunnels for pipelines, but never yet for oil or gas production. Previous plans to use subsea tunnels for petroleum development have suffered from long construction schedules that result in higher costs compared with conventional solutions and safety concerns such as managing evacuation and the potential for gas explosions and blowouts.
Fig 2. Schematic sketch of base case for the study
- Despite this, a recent study carried out by Acona Wellpro in cooperation with the Norwegian University of Science and Technology and the research institute SINTEF of Norway found that using subsea tunnels combined with directional drilling for production wells could be a realistic alternative for future petroleum activities in coastal areas between 25km to 50km from shore. The concept is based on two or three subsea parallel tunnels descending from an onshore terminal facility to a base station at the low point and parallel TBM bored tunnels connecting to large production caverns established to house oil drilling and operational facilities (Fig 1).
- To explore the potential of the concept, a prospective oilfield located in the Lofoten-Vesterålen area of Northern Norway was chosen as a base case study. A tunnel of approximately 27km long would be required to reach the oilfield location with the first 7km to 9km passing through hard crystalline bedrock of up to 300PMa and the remaining 18km to 20km in moderately to poorly consolidated Cretaceous rocks including shale, mudstone, siltstone and sandstone.
- With the first part of the system on a down gradient of 1:12.5 to the base station and the major part of the continuing tunnels to the production caverns at a 0.3% incline, the lowest part of the system would be about 365m below sea level. At the outer end of the system, and with access into the top of the production cavern, the tunnel would be about 300m below sea level comprising a maximum sea depth of 150m, with a further 100m of Quaternary sediments and a rock cover of 50m (Fig 2). The rock cover complies with minimum design criteria for current Norwegian subsea road tunnels and, at these depths, the oilfield base study tunnels are about 70m deeper than the deepest Norwegian subsea road tunnel to date (Eiksund at 287m) and slightly less than the planned Rogfast road tunnel at approximately 400m.
Considering escape and evacuation requirements, as well as ventilation and efficiency of construction, the concept is designed with a minimum of two parallel running tunnels with rail transportation suggested for travelling from the base station to the production caverns. One tunnel would be a transportation tunnel for typical container sized loads and the other an escape/evacuation tunnel also used for transporting personnel. All transport of hydrocarbons and other utilities would be housed in the transport tunnel, and redundant power supply and independent ventilation for the escape system would also be installed in the transport tunnel for operation of the escape/evacuation tunnel.
Fig 3. Parallel tunnels for equipment and hydrocarbon transportation, emergency/evacuation and personnel transportation and gas emission relief
- A minimum of two caverns of approximately 30m high, 15m wide and 75m long would be excavated at the end of the TBM tunnels in a top heading and bench sequence with two or three benches.
- Of the various configurations, a concept with three parallel tunnels is preferred (Fig 3). The third tunnel would be used as a relief tunnel for any gas emissions in abnormal situations. This would provide the ability to fully isolate a production cavern with any gas from leaks or blowouts directed to the relief tunnel. In an emergency, the transportation tunnel would remain open for recovery of an incident or for utilizing another cavern for relief-well drilling.
- The three-tunnel concept costs more, but is superior in other associated aspects. It provides early access to the caverns, enabling their excavation to start as soon as possible. The central evacuation tunnel may also act as a pilot tunnel and be used for pre-grouting the rock mass ahead of the parallel TBM transport tunnel headings. Further, this solution provides three available working faces, which increases flexibility. A three-tunnel solution is safer during construction and operation and different activities can be allocated to each of the tunnels.
Geological conditions and excavation plans
The tunnel system for the base case study would likely intersect several faults in the Cretaceous layers. Based on completed shore approach tunnels on the Norwegian coast, major faults are also likely to be encountered in the crystalline rocks before reaching the young, sedimentary bedrock. The risk of shallow gas influx needs to be considered, particularly in sandstone layers and at faults crossing the tunnel alignment.
- Excavation by drill+blast is considered the best alternative for the access decline from the surface to the base station and for crossing the transition between the crystalline and Cretaceous rocks. It provides a good overview of the conditions at the tunnel face and great flexibility in applying various types of equipment. This improves the basis for decision-making and performance of rock support and grouting ahead of the tunnel face.
- One main challenge during the drill+blast excavation of deep subsea tunnels and caverns is the potential for large water ingress at high pressures. In all subsea tunnelling, systematic probe drilling ahead of the tunnel face using percussive drilling and/or core drilling when needed is one important contingency to ensure safety. For very high water pressures in soil-like conditions, the tunnel excavation has to be protected against water and debris inflow, and a blowout preventer must be installed at the drilling equipment.
- Regarding rock support for drill+blast tunnels in the crystalline bedrock, a combination of fibre-reinforced shotcrete, rock bolting and reinforced shotcrete ribs is considered sufficient (Fig 4). Experience from subsea tunnelling in Norway demonstrates that if the thickness of the sprayed concrete is more than 60mm to 70mm and of high quality concrete, degradation of the shotcrete is slow. The harsh saline environment would require rock bolts to be fully grouted and double corrosion protected.
- Once into the Cretaceous bedrock, excavation by TBM is recommended as the most viable tunnelling method. It is faster in weak rock than drill+blast and there are fewer ventilation requirements for a TBM. Scheduling would require a TBM in each of the parallel tunnels to operate simultaneously and at high advance rates, working around the clock and for several years through challenging ground conditions. Of the different types of machines, shielded TBMs of about 6.6m o.d. would need to be designed specifically for the rock mass conditions and based on thorough site investigations. Contingencies need to be established to handle unforeseen ground with the main challenges expected to be:
• very weak and unstable rock masses, including running ground
• stress related problems including squeezing and rock bursting
• large water ingress at high pressures
• gas pockets/shallow gas
• mixed face conditions
- As of today, TBMs may not have been applied in similar situations, so this project is likely to push the state of the art of TBM tunnelling. The possibility of operating the TBMs in a kind of EPB mode, to be able to close the face in case of running ground, may also be specified. In such cases the machines would need to operate in closed mode against a water head of 300m pressure. This represents a substantial improvement in machine capability compared to current TBM designs.
- The need for water sealing is based on results from probe drilling and is carried out when the inflow of the probe drill hole exceeds a pre-set limit. Sealing off of excessive water ingress is carried out preferably as pre-grouting with cement-based grouts and injected under high pressures of up to 10MPa. Pre-grouting is likely to be needed for a considerable part of the tunnels. Consequently, any TBM used for this project must be equipped with drilling units that can secure probing and grout hole drilling ahead of the tunnel face (Fig 5). For appropriate pre-grouting, drilling through the cutterhead would be required. Additionally, an adequate water discharge and pumping system would need to be designed and installed.
Fig 5. Probe drilling and pre-grouting ahead of a TBM face
- In the sedimentary rocks of the outer parts of the tunnels, a continuous lining installed as pre-fabricated segments is expected to be required for the complete tunnel length. Design of the concrete mixes would consider the difficult environment. In order to achieve a high advance rate, the discharge of muck from the tunnel, supplying lining segments and materials to the tunnel face must be addressed. Transport of support measures and grout is a priority for stabilizing the tunnel at the face.
- Roadheaders are the best alternative for excavation of the caverns. Excavation with roadheaders is usually slower than drill+blast, but has the advantage of great flexibility, and several units may be used simultaneously to achieve a satisfactory excavation rate. The ventilation requirement is also less extensive than for drill+blast, which is important because methane is a potential problem and needs to be handled appropriately. While larger caverns have been successfully excavated in a wide range of ground conditions, the planned dimensions may be a considerable challenge in the weak, sedimentary rocks expected in this case.
Time and cost estimates
Scheduling of the project concept described requires high utilization and capacity of both drill+blast and TBM excavation to be competitive and viable for the oil industry. Cost and time estimates are based on extensive experience from Norwegian tunnelling, and do not include time for geo-investigations. There is vast experience of drill+blast excavation in crystalline rocks in Norway and a weekly advance rate of up to 175m in one heading has been achieved in good quality rock. For TBM operations, the following parameters were defined for the headings: length = 22,000m, TBM diameter = 6.6m, DRI = 65, CLI = 20, angle of rock mass planes of weakness = 0 degrees, and spacing between planes of weakness = 7.5cm. The results from the Fullprof software, applying the NTNU Prognosis model, are: an advance rate of 283m/week with an excavation cost, excluding rock support, of 12,666 NOK/m. This is based on working 24hr/day, 7 days/week for 50 weeks/year. The figure of most interest here is the advance rate of 283m/week. This is by far world record speed. To allow this project to be viable, it is strictly required that these machines are robust and able to operate at this speed throughout the entire construction phase.
- The total cost for excavating the tunnels and caverns for the base case study has been estimated at 8.75 billion NOK (approximately US$1.5 billion) and the construction time is calculated at about 210 weeks.
- Despite the challenges discussed, which require close attention, there is nothing to indicate that the case presented here is not feasible to construct. The cost and time schedules of this tunnel concept are competitive with traditional oilfield development strategies for fields within 30km from the shore. Based on the results of this project in Norway, this concept may allow for the possibility of petroleum development at the sea floor in places where it has been previously restricted due to environmental or climatic restrictions.
1. Vassmyr, Nilsen, Grøv & Bruland, 2011. Innovative approach for oil production based on subsea tunnels and caverns. Proc. 2011 RETC Conference, San Francisco.
2. Grøv, Nilsen & Bruland, 2013. Sub sea tunnels to oil field developments in Northern Norway. Strait Crossings 2013, Bergen, Norway.
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