The economics of offshore oil and gas operations have now changed and the industry is adapting to long-term lower oil prices below US$100/bbl. But even as prices climb back up, the industry will not simply return to the way things were. Operators will not just abandon efficiency and productivity measures adopted to lower break-even points and maintain viable margins.
Bringing fields online in remote, deep-water areas will become an increasingly important factor in the future operations of oil and gas operators. As always, the challenge is to bring these sites online in as safe, efficient and cost-effective a manner as possible.
All of which means that the concept of an all-subsea field development, including advanced subsea processing and storage is not so much an ideal scenario, but rather a fundamental necessity. The more remote the field, generally the greater the costs involved in conventional top-side or onshore processing.
If we look at processing functions like compression, pumping and separation for example, there are clear cost-efficiencies to be gained from subsea operations using these technologies. For example, energy consumption can be reduced by eliminating the need to lift oil, plus sand and water to be separated at the surface. Equally, subsea water injection is a more efficient means of exploiting mature reservoirs for a more cost-effective output.
However, the ability to realise these benefits is largely dependent on the ability to transmit and distribute greater amounts of power, and the ability to deploy electrical and/or hydraulic controls to seabed operations. In other words, it requires advanced cabling and umbilical design to accommodate and control the new consumers of energy on the seabed.
Fortunately, for cables and umbilicals, this is an area that has evolved significantly over the past 25 years, since early product introduction and technology proof of concept. After many years of improved testing, qualification and simulation for offshore environments, advancing cabling and umbilical options have become available on a sound commercial basis. This technology can help operators exploit their offshore reserves in a much more cost-effective fashion.
The past ten years have also seen increasing demands made on power delivery and capacity on the seabed. Processing equipment like pumps and compressors need a few megawatts of power; the technology that heats pipelines to reduce the risk of hydrate formation and improve flow requires even more. Many of the more recent installations of subsea power transmission and distribution applications have included medium voltage insulated cores within the umbilical component bundle that operate at voltages in the range of 6.6 kV to 36 kV.
A new technology being introduced is 72.5kV ‘wet-design’ cables. The 72.5kV ‘wet-design’ cable eliminates the need for a metallic radial moisture barrier used in conventional high voltage subsea cables. Typically, high voltage cables include an extruded circular lead sheathing, or other metallic barriers such as copper, resulting in a heavier product than those operating at medium-voltages. The ‘wet-design’ cables can therefore reduce the submerged weight of the higher voltage power cable or umbilical, significantly reducing the product load-out weight as well as the hang-off tension for deeper water applications.
At 72.5 kV operators will effectively be able to deliver double the amount of power using the same amount of conductor material (e.g. copper or aluminium), than current products at 36 kV. Although the higher voltage cables at 72.5 kV will have slightly larger diameter over medium-voltage alternatives, the cable will not be double in size or weight. For those operators looking to implement full subsea processing solutions, the possibility now exists to increase the power core voltage up to 72.5 kV – and with this comes even greater economies of scale and resource usage.
In addition to high-power, deepwater characteristics, this higher voltage cabling enables operators to lengthen step-outs and the distances between - for example, shore and field distribution hubs and subsea transformers that can distribute power out to the low-voltage subsea consumers on the seabed. Advanced design means that operators can run high-voltage cables from the shore to an offshore substation between 50 and 80 km away, where the power can then be distributed to different consumers with various voltage levels, different sizes and diverse locations.
This addresses one of the biggest challenges for bringing remote platforms online: the need for much larger amounts of power to be transmitted between offshore infrastructures and back to either a host platform or to onshore facilities. Until recently, the only real option has been to install a new platform or use a new floating vessel as an additional ‘node’ in the cabling network.
However, advances made in cabling and umbilical design mean that operators can expect to make greater use of tiebacks that employ existing platforms to connect different parts of subsea infrastructure. And as new fields are developed in deeper, more remote locations the need for fixed platforms will reduce. This means that running cable for longer distances between individual installations can save capital expenditure over adding new platforms or floating vessel infrastructure.
Operators could also be in a position in which a complex array of subsea cabling, buried or entrenched to avoid anchor damage or marine erosion, is supported across the seabed. The future could see oil and gas platforms, offshore wind farm substations and other energy infrastructure connected over long distances. And with the use of higher voltage cables operators will be able to reduce the impact of electrical losses and extend the distance over which power can be distributed.
Interestingly, the ‘wet design’ and long-distance, high-voltage capacity of 72.5kV cabling was initially developed to support the expansion and increase the availability of suitable locations for floating and fixed offshore wind farms (typically referred to as 66kV in this market). We are already seeing some examples of a distributed, hub-style design off the coast of Cornwall in the UK, this time for the wave energy installation ‘Wavehub’. A 6 core ‘export’ power cable runs underneath the beach in the village of St Ives and 25 km out into the Bristol Channel to a hub where a number of smaller cables split off to connect different wave-energy devices, allowing them to be tested and also to collect energy and transmit power back to the grid.
This kind of distributed system, also presents great opportunities to create synergies from using future renewable energy devices being connected back into oil and gas infrastructure. The DNV-led Win-Win JIP project is another example, where the industry is looking at the feasibility of using energy generated by a floating offshore wind turbine with battery storage and other equipment to power the pumping of water into an oil or gas reservoir.
The technology behind these more advanced cables and umbilicals is enabling organisations to optimise energy production from multiple sites and extend the life of others close to decommissioning. Increased use of power and control lines such as those described could enable operators to bring on line marginal sites. As more remote fields are progressed, advanced cables and umbilicals will be a key enabler to provide power to all-subsea field developments. The advanced cable and umbilical products being introduced now will play a vital role in connecting the offshore energy infrastructure of the future.
Article written by James Young, at JDR.
Read the article online at: https://www.oilfieldtechnology.com/drilling-and-production/21122016/powering-the-future/