The move towards all-electric subsea facilities will precipitate an evolution in umbilical design. Alan Dobson outlines the challenges and what is already available for the market.
Technip Umbilicals R&D Center. Photos from Technip.
As the subsea oil and gas industry has been adjusting to a “lower for longer” oil price, it has brought into focus the potential costing saving opportunities subsea processing technologies could bring to future projects.
By treating produced fluids on the seabed rather than on the topside, longer distance transportation to either an existing host facility or to an onshore facility could make future projects economically viable. Some operators are facilitating development of this concept through research and development initiatives, for example, Total’s DEPTH (Deep Export Production Treatment Hub) and Statoil’s Subsea Factory. Class society DNV GL released its All Subsea study in order to develop, integrate and qualify key technologies.
Certain technology building blocks, either in isolation or combinations, are required:
- All-electric control
- Heated pipelines
- Subsea separation (water/oil, and gas/liquid)
- Compression/multiphase boosting
- Seawater treatment and reinjection
- Electrical power transmission
In terms of controlling and supplying essential fluids and power to the additional subsea equipment, the control umbilical must evolve to incorporate new functionality required by electric control systems and provide greater levels of electrical power transmission.
A combined steel and electric cable umbilical.
Impact of all-electric control on the umbilical
The first subsea trees with all-electrical control were installed in the Dutch North Sea in 2007. Although in this application the hydraulic downhole safety valve was retained, all other actuators on the trees were electrically powered. Future deployment of all-electric systems will more likely use electric downhole safety valves, such as those now being trialed in the field (Read more: Electrifying).
The fluid functionality of the umbilical will be simplified as the hydraulic power supply and return is no longer needed. This will benefit extremely long tieback umbilicals because hydraulic power efficiency diminishes as lines get longer, leading to higher costs for larger bore tubes working at a higher operating pressure.
Although electrical transmission also suffers losses over longer distances, this can be eased by switching from conventional AC (alternating current) power supply to a DC (direct current). This change opens up the choice of cable construction to include coaxial designs and umbilicals have been supplied incorporating this type of DC coaxial cable.
As future deployment of all-electric control systems is likely to involve very long tiebacks, the control umbilical will remain a critical component that must be engineered for the operating environment.
The design and analysis tools developed by the industry for conventional AC cables have been adapted for DC cables and successfully proven through project experience. Subsequent deployment on further projects mean it can be considered a mature technology. Umbilicals will also retain fluid conduits to provide service fluids such as scale and hydrate inhibitors, methanol injection and also barrier fluids for pump motors. The cost of these conduits can be optimized by utilizing alternative construction methods, materials and efficient manufacture.
Assembly of these extra-long length umbilicals requires specialist assembly machines with large capacity bobbins to maximize component lengths, minimizing the number of welds and splices, leading to greater reliability. However, the ultimate length limit of a continuous umbilical is generally driven by the installation vessel carousel capacity and potential weather window restrictions.
Field proven factory fitted inline joints allow lengths of umbilical delivered on reels to be efficiently joined during offshore installation into a long-length tieback. They are fully factory tested before deployment, ensuring reliability and enable schedule flexibility during the installation campaign where partial lengths can be laid, maximizing available weather windows.
Of the other key technologies, heated pipes, pumps and compressors all consume high levels of electrical power, meaning increasing numbers of medium voltage three-phase power cables in the umbilical. This brought additional design challenges in terms of electromagnetic balance, temperature and structural integrity, for which solutions have been developed and qualified in readiness.
High power three-phase cables generate heat and all umbilical components need to be designed and materials selected to handle continuous operation at elevated temperatures. This could impact material yield strength, fatigue capacity or accelerate long-term aging. Thermal analysis models have been developed and validated, which enable accurate assessment of in-service conditions. Coupled with knowledge of long-term material behavior at temperature, this ensures design life reliability.
The electromagnetic radiation emitted by three-phase power cables needs to be carefully understood, again through detailed analysis, and components positioned accordingly to minimize electrical losses and induced voltages on metallic components. If neglected it could result in high transmission losses, accelerated corrosion of metallic components or cross talk of signals onto control cables. Recent advances in higher pressure, deeper water thermoplastic hoses, that can operate continuously at higher temperatures, offers an alternative solution to steel tubes without the thermal and induced voltage concerns.
With increasing amounts of copper cables being required, the weight and strength of the umbilical assembly becomes a concern. Copper has a high density and low mechanical strength, which can pose challenges during installation, particularly in deepwater. Aluminum cables offer a solution, where the reduced weight and greater strength significantly increase the installability of the umbilical and reduce material cost.
Umbilicals will continue to be a key component of the subsea infrastructure as the industry collaborates on placing more subsea equipment on the sea floor. Cables to support all-electric control have been designed and proven. Solutions are available to competently transmit greater levels of electrical power. Efficient manufacture of extra-long length umbilicals can be reliably performed and use of factory fitted mid-line joints can provide cost benefits during installation.
Alan Dobson is vice president of research and technology with Technip Umbilicals. He has worked in subsea technology for over 20 years, developing subsea robotic systems, cable and fiber-optic trenching technologies and, in his current role, several umbilical technologies focused around subsea power, chemical injection and control. Dobson has a PhD in subsea control and automation from Newcastle University. He is also a chartered mechanical engineer.