What are the subsea cable challenges for an Asia-Pacific HVDC interconnector? Jeremy Gordonnat and James Hunt explain.
With high insolation levels, favourable conditions for wind farms and immense availability of land, Australia offers an attractive environment for large utility-scale renewable developments.
Recently, developers have proposed projects to leverage this renewable energy potential to generate and export clean electricity to Southeast Asia, and Singapore in particular, through a subsea High Voltage Direct Current (HVDC) interconnector.
This prospective project requires a unique integrated contracting strategy involving multiple HVDC cable suppliers, marine heavy transport companies and cable installation contractors to be delivered within a sensible timeframe effectively, safely and sustainably.
Although relatively ambitious, a study by global energy consultancy Xodus demonstrated the technical feasibility and quantified the environmental benefits.
It is one of the few credible options to help move the region towards a net zero future and, in the meantime, strengthen its energy security. The tri-party power link model is currently being implemented by the EuroAsia interconnector project 1 between Israel, Cyprus and Greece.
To connect Australia to Singapore, Aberdeen-headquartered Xodus identified a potentially suitable marine route estimated to be 3,200km long with three main distinct sections.
The first 400km and last 1,600km, sections A and C respectively in Figure 1, lie in relatively shallow waters, typically 100m to 200m deep, with a flat profile.
Over these sections, the cable would need to be trenched to protect them from external hazards in addition to wave actions and sediment movements.
Conversely, the water depth along section B varies significantly with alternate deep and shallow sections and high gradients. The deepest section is at the Timor Trough and reaches water depths of 1,900m and high slopes.
The structure of the proposed cable includes a central conductor surrounded by an insulation, armouring and external sheath.
Adopting the current technology, the interconnector will most likely be configured in a bipolar mode to offer 2 to 3GW power capacity, using two individual and identical HVDC underwater cables.
In the event of one cable failure, this arrangement allows using the intact cable into monopolar mode with earth return over the repair duration, enabling half of the electricity capacity to be transmitted.
Typical linear weights of these subsea HVDC cables range from 40 kg/m to 60 kg/m with diameters in the order of 150 mm and a capacity exceeding 1GW. It is estimated that the total cable weight of this interconnector (two off 3,200km cables) would reach 300,000 to 400,000 tonnes.
The cable will be transported into basket carousels using heavylift vessels (HLVs) or barges with cable lengths split into manageable sections of about 100-120km long.
Given the route length and variability, it is anticipated that various cable designs will be adopted to meet the diverse requirements.
The cable design features include different conductor materials, insulation technologies and armouring arrangements to account for constraints related to water depth, laying operations, seabed on-bottom stability, and protection against external damage.
The electrical loss through HVDC cable is approximately 3% per 1,000km resulting in a 10% total electrical loss along this interconnector.
Supply, transportation and installation
HVDC cable manufacturing requires specific know-how and capital-intensive power plants with facilities located in areas with well-established logistical infrastructures and marine access facilities.
The location of the HVDC cable suppliers is illustrated in Figure 2 (Surabaya being the midpoint along the marine route).
Most long subsea HVDC interconnector projects executed to date were located within 500 to 1,000km from the HVDC cable manufacturing facilities. This enables the cable lay vessel (CLV) to act as the transport and installation vessel, transiting back and forth from the offshore cable route to the plant for reloading each cable section.
Typical CLV transit durations from Indonesia to Northern Europe, Southern Europe and Northeast Asia were estimated at 37 days, 26 days and 11 days, respectively (one way).
Based on a CLV capacity of 10,000 tonnes (equivalent to approximately 200km of cable), a total of 32 vessel trips will be required to complete the entire interconnector offshore installation scope.
Depending on the number of simultaneous CLVs used for both transportation and installation (assumed one to three vessels) and the various plant locations, the overall cable installation campaign would last between two years (three CLVs and plants in Northeast Asia) and 11 years (one CLV and plants in Northern Europe), continuously.
It was concluded that an integrated approach combining CLVs and a fleet of transport vessels, fitted with high-capacity basket carousels, will most likely be required to achieve an acceptable project delivery duration.
In this situation, the CLVs would remain along the cable route vicinity during the entire installation campaign, whilst transport vessels journey back and forth from manufacturing plants to CLVs to ensure operational continuity and avoid excessive stand-by durations.
Carbon tax and emissions
To fairly estimate the actual net greenhouse gas emissions of this type of project, a carbon lifecycle analysis of the embodied carbon of HVDC cables as well as the carbon emissions related to transportation, installation, operations and decommissioning, were undertaken.
Assuming an interconnector capacity of 2.4GW associated with a 70% factor of charge (with adequate storage capacity, excluded from this assessment), approximately 15TWh per annum of power generated by Singapore-based gas-fired plants can by replaced by clean Australian electricity, which corresponds to about 30% of Singapore’s annual electricity consumption.
The avoided emissions from Singapore’s gas-fired plants were estimated to reach 320 MteCO2e over 50 years – the typical design life of interconnectors. The carbon emissions of a 10GW solar plant coupled with an interconnector were evaluated to 40 MteCO2e, resulting in a net saving of approximately 280 MteCO2e over the course of its service life as described in Table 1.
The cost of utility PV solar has decreased significantly over the past decade and this trend is expected to continue in the short and medium terms.
As of today, the capital expenditure of a 10GW solar farm with an intercontinental power link is estimated at $20 billion with one third of the cost allocated to the subsea interconnector.
It is difficult to predict the carbon tax evolution over the next 50 years and a more refined cost model would be required to estimate the present value of this complex development.
However, if the carbon tax in Singapore, currently set at approximately $4 per tonne of CO2e, is expected to reach $10–20 per tonne of CO2e by 2030, this could result in $3–6 billion savings if 280 MteCO2e are not emitted from gas-fired plants.
This simplistic calculation illustrates that carbon tax savings could, at least partially if not entirely, offset the cost of an intercontinental power link over its design life.
Although relatively ambitious, interconnector projects such as an Australia-Singapore power link are technically feasible and can potentially offer valuable benefits, not only to both exporting and receiving countries, but also to the entire region collectively in the long term.
Most notably, over its 50-year design life, the link can reduce the carbon footprint of a country’s power generation system, traditionally dominated by fossil-fuel, by a few hundred million tonnes of CO2.
Further, projects of this nature will foster renewable energy integration at a continent level and enhance country energy security. Ultimately, increasing climate concerns and carbon tax are likely to create credible business cases for intercontinental power links soon.
ABOUT THE AUTHORS
Jeremy Gordonnat is a Consultant Engineer with Xodus Group. He graduated with a BSc in Mechanical Engineering and MSc in Ocean Engineering, is a Chartered Engineer, member of RINA and Project Management Professional (PMP) certified.
James Hunt is Interconnectors & Cables Lead at Xodus Group. He has a BEng in Mechanical Engineering, an MSc in Physical Oceanography and an MBA. He has 33 years of offshore experience and, in the past 17 years, has been involved in significant projects in the HV power and renewable energy sectors from early-stage feasibility through development, planning, construction and O&M.