Optimizing offshore substation power supply design
A call for industry collaboration ensuring that DNV’s Offshore Substation standard continues to evolve in line with industry needs and safety requirements. Update of industry standard DNV-ST-0145 distributed for hearing soon.
For over 15 years, DNV has been at the forefront of certifying the design and safety of offshore substations (OSS), including the electrical systems and structural components that are critical for offshore wind projects' success.
One of the most important technical standards in offshore substation design is the DNV-ST-0145[1]. This standard provides a comprehensive framework for designing offshore substations, ensuring they meet high safety and operational benchmarks. DNV-ST-0145 was originally developed in 2008 through a Joint Industry Project (JIP), and it has been regularly updated to reflect new knowledge, technologies, and industry developments. The most recent edition was published in 2020, with a revision in 2021.
Despite these advancements, some recurring challenges remain when it comes to certifying OSS designs. One key topic of ongoing discussion is the design of the auxiliary power system and its emergency backup power supply. This is why DNV is set to release a new edition in 2025, with a public hearing for stakeholders to contribute to the draft version.
Current Industry practice for auxiliary power supply
The OSS auxiliary power supply and back-up system requirements as given in the latest edition of DNV-ST-0145 can be traced back to IMO conventions for the Safety of Life at Sea (SOLAS)[2] defining safety rules for vessels or the IMO MODU code[3] applicable for mobile drilling units, but with some small modifications. Further publications such as standard IEC 61892-2, or document API 14F[4] issued by the American Petroleum Institute refer to these codes as well.
They all have one thing in common, they share the same philosophy concerning the auxiliary power system design and availability of emergency services/safety systems considering different operational conditions and scenarios.
During normal operation, the low-voltage auxiliary power system shall be supplied by a source, the main source of auxiliary power. This can be established by a connection to the onshore grid or by an auxiliary diesel generator on the platform. The purpose is to maintain normal operational and habitable conditions. In case this main source not being available any longer (outage, failure, or any other contingency), a back-up source defined as emergency source of power shall secure uninterrupted power supply to at least the systems safeguarding personnel and the installation. The emergency source shall facilitate safe evacuation of service personnel from the platform. Islanded condition as an interims operational state is disregarded and not discussed further herein.
Figure 1 shows a generic and simplified schematic to illustrate the setup. During normal operation, the emergency switchboard, supplying power to emergency services, is fed by the grid-connected main switchboard. In case of an accidental event or contingency forcing a shutdown of the main power system (high-voltage power transmission infrastructure), uninterrupted power is provided to emergency consumers by a combination of UPS (transitional source, bridging the time until the emergency generator started and is capable of taking the load) and the emergency source of power (emergency generator). It is important that these sources (main source and emergency source) and related distribution are independent from each other and are not installed in the same room or location, so that they are not affected by the same contingency or accidental event. There are additional conditions and requirements, e.g., concerning starting energy of generators, starting times etc. that are important, but will not be further discussed herein as the focus is on topology and availability of power to emergency consumers only.
Figure 1: Simplified schematic with an emergency diesel and a dedicated emergency switchboard
Another acceptable option is the so-called meshed system and is shown in Figure 2. This topology does not consist of a dedicated emergency source of power and emergency switchboard but instead is based on the redundancy concept asking for two independent and separated power sources (2x100%) that guarantees supply of normal and emergency services even during a contingency. Such a system relies on multiple possibilities of connecting a switchboard, specific systems or consumers to the available power sources.
Independent of the above power supply strategies, evacuation of personnel shall be completed within 30 minutes as a fundamental criterion in the DNV standard. Uninterrupted power shall be provided to secure operation of emergency services such as emergency/escape lighting or fire detection, fire alarm and active firefighting systems for at least duration according to table 5-2 of the standard DNV-ST-0145. An additional power source referred to as transitional power source is required to supply emergency loads during the generator's staring sequence for both concepts.
The future of offshore substation certification
It has been recognized in recent years that developers and their designers were leaving the pathway of the well-defined prescriptive requirements outlined above, which have safe-guarded installation and personnel in the maritime world for decades. The auxiliary power system design of an unattended installation (OSS Type A) seen in certification projects nowadays has changed to a mixture of the two design options offered in DNV-ST-0145, not compliant with the requirements set for one or the other. The risk-based approach offered in DNV-ST-0145 Appendix A and B must be applied more often to justify deviations from standard and prescriptive requirements. The methodology is characterized by a thorough assessment of threats, failure modes and probabilities (“what if...”) showing that the risk is reduced to an acceptable level (ALARP principle - as low as reasonably practicable) although the emergency power supply design and required durations are deviating from those listed in table 5-2 of the DNV standard. Following the trend in the industry, requirements related to the emergency power supply system design will be adapted in an update of DNV-ST-0145.
Taking the specific risk profile of an unattended installation into account, a reduced minimum emergency back-up supply duration is now introduced based on the provisions of NORSOK-S-001:2020+AC.2021[5]. This and many other changes or additions have been made, e.g. a totally revised section for offshore HVDC stations or added requirements for Lithium batteries, to defend the position of DNV-ST-0145 as the only all-embracing state-of-the-art technical standard for offshore substations worldwide. This approach will enable designers and developers to pursue a more economical design, resulting in a reduction to CAPEX and OPEX while maintaining a code compliant design ensuring a consistent level of safety.
A call for industry collaboration
As DNV prepares to release the updated edition of DNV-ST-0145, the company invites all stakeholders—designers, developers, and industry experts—to contribute to the public hearing of the draft version. This collaboration is crucial to ensuring that the standard continues to evolve in line with industry needs and safety requirements.
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References
[1] DNV-ST-0145: Offshore Substations, Edition October 2020, amended September 2021
[2] SOLAS: International Convention for the Safety of Life at Sea (SOLAS), 1974
[3] MODU Code: Code for the Construction and Equipment of Mobile Offshore Drilling Units, 2009, amendment January 1, 2024
[4] API 15F: Recommended Practice for Design, Installation, and Maintenance of Electrical Systems for Fixed and Floating Offshore Petroleum Facillities for Unclassified and Class I, Division 1 and Division 2 Locations, API recommended practice.
14F Ed, October 6 2018
[5] NORSOK-S-001:2020 + AC:2021: NORSOK Standards
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