Arctic-ready facilities: meeting the engineering challenge for a new era of offshore exploration. Mark Manton explains.
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The Sable South Centre module being loaded ready for installation at the Sable Offshore Energy Tier II Compression Project, offshore Nova Scotia. |
The world’s energy demand is expected to continue to rise, driven by growth in developing markets. To meet this demand, the energy industry has had to shift to difficult-to-access resources, including those in the Arctic.
The area is largely unexplored and drilling has associated risks due to the Arctic’s extreme and environmentally sensitive location. The oil and gas industry recently invested heavily in the technology and preparatory work required to make production in the Arctic more economic. Challenges include designing for powerful seas, extreme low temperatures, deepwater, high winds and, in some instances, all four at once, in the case of “Polar lows.”
The key to success of new projects depends upon a greater understanding of harsh environments resulting in improved reliability of the installed equipment.
Practical considerations
Engineers must be aware of environmental cooling mechanisms. The wind chill effect is hugely significant when calculating thermal loss. Classically, ~300W/sq m of heat tracing has been used to protect static equipment and pipework. This prescriptive power demand does not account for uninsulated surfaces, which are exposed to wind chill, e.g. emergency walkways. Emergency walkways must be maintained at +3°C minimum. No heat tracing systems can maintain a positive body temperature at these extreme ambient temperatures when subjected to wind chill, without expending an unfeasible amount of power. For example, it would barely take a 4-5m/s cross wind to make 700W/sq m heat tracing ineffectual across a walkway. The heat tracing power to overcome even a modest cross wind can result in insufficient power available onboard to meet all demands.
With this in mind engineers must ensure a thorough risk assessment is performed to evaluate wind chill effect. Mitigation solutions may include steam systems required for steam lancing and/or mechanical methods of breaking up ice formation but these need to individually assessed. The safety of the operators remains paramount.
When protecting static mechanical equipment and pipework, insulation is critical. If specified, installed and maintained correctly, it can reduce power consumption. As a rule of thumb, it takes ~250/300g of fuel to generate 1kWh on board a typical platform, therefore operators will be looking for winterization solutions which reduce power/fuel consumption.
Poorly specified, damaged or incorrectly installed insulation will not return the energy savings intended. Insulation materials can be crushed when installed or during maintenance which will significantly degrade performance. It is essential therefore that insulation is correctly applied in accordance with IEEE 515 or equivalent.
Many of the concerns associated with operational and unplanned maintenance in cold climates can be addressed at the conceptual design phase of a project. Engineering design contractors, such as SNC-Lavalin, must establish the client’s fundamental needs during this phase and engineers must be shrewd about their approach to winterization. Adopting a “heat trace everything” approach will be uneconomic. Possible solutions include:
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Offshore Energy Tier II Compression Project, Offshore Nova Scotia. |
Accommodating short periods of downtime and therefore circumventing the 100-year minimum design temperature, reducing CAPEX. Equipment must be specified to survive such shutdowns without causing damage.
Installing smart heat tracing control systems capable of minimizing power demand by factoring in wind speed and direction - the cold face of equipment remains heat traced whilst the sheltered side may be left unprotected, reducing OPEX.
Limit the use of systems designed to prevent or remove sea ice forming around equipment and walkways. When seas are frozen, there will be no sea spray to blow up on deck. Sea spray is only expected when wind speeds exceed 10m/s. However the potential for atmospheric ice and snow may need to factored in.
Install waste heat recovery units to generation trains and recycle exhaust from HVAC systems.
Naturally ventilating, partial or fully enclosed module design
Det Norske Veritas (now part of the DNV GL group) recently issued the Offshore Standard for Winterisation for Cold Climate Operation, DNV-OS-A201. DNV acknowledges that problems encountered in polar operations cannot be easily solved by prescriptive instruction and now favor a more functional approach to engineering design. The key is preparing a detailed winterization philosophy early in the design process. It should be agreed between the operator and contractor and cascaded through to the equipment and heat tracing suppliers.
Equipment layout and modularization is a key decision. It is sensible to locate safety and process critical equipment inboard, where the brunt of the wind and weather will be least felt. Shielding the equipment from the elements may however be contrary to allowing leaked hydrocarbons to escape safely and swiftly. Conventional designs are self-venting, where the process decks are open to the environment on all sides. But, this approach focuses on mitigating the risk associated with leaked gas and does not consider the human factors involved with operation and maintenance.According to NORSOK, the percentage of time that an individual employee is exposed to a wind chill index (WCI) above 1000W/sq m should be reduced as far as reasonably practicable. For arctic installations, outdoor operations should be identified and reduced to a minimum. Shielding workers from the wind will have a dramatic impact on their availability to carry out work.
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Husky White Rose FPSO, Offshore Newfoundland & Labrador. |
Equipment inspection and planned / unplanned maintenance will be easier if equipment trains are enclosed or semi-enclosed in heated modules. Conversely, modules will inevitably require more inter-connections as pipework will need to pass between bulkheads. So while enabling more time for maintenance, there could be a greater number of leak paths introduced.
The table shows naturally ventilated vs. enclosed modular platform design. Modern computational fluid dynamic (CFD) modeling is an invaluable tool engineers have at their disposal to understand the effects wind has on platform layouts and informing decisions on whether to add weather / wind shields or enclose modules entirely.
Ultimately, the control of energy consumption by a facility will determine its profitability. There is little point extracting these hard to reach hydrocarbons if the majority of them are going to be burnt in the process.
The cost for successfully winterizing an offshore arctic platform will be a significant part of a project’s overall CAPEX and OPEX. The combination of both freezing temperatures and wind chill must be considered when specifying and laying out mechanical and safety essential equipment. CFD modeling can assist with layout decisions and can establish whether enclosing modules is necessary for operations.
Electing to fully modularize mechanical equipment into small enclosures or leaving a platform open to naturally ventilate is a difficult and complex decision. It is therefore essential to ensure a detailed, useful and robust winterization philosophy is established with the operator at the very beginning of a project concept. Wherever possible waste heat must be utilized to minimize energy expenditure. SNC-Lavalin believes all technical challenges associated with operating in the Arctic can be overcome through intelligent engineering design if identified early enough.
References
[1] US Geological Survey, Circum-Arctic Resource Appraisal: Estimates of Undiscovered Oil and Gas North of the Arctic Circle. 2008.
[2] Det Norske Veritas AS (DNV), Ships for Navigation in Ice, July 2011
[3] Winterisation of Offshore Production Facilities Seminar, IBC Energy, February 2014
[4] NORSOK Standard Working Environment, S-002, November 1997
Mark Manton is a lead mechanical engineer at SNC-Lavalin. Manton graduated from Nottingham University with a BEng with honors in mechanical design, materials and manufacture. Since then, Manton has completed eight years with SNC-Lavalin, working as a design, project and mechanical engineer on several key onshore and offshore projects.
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