As reported by Opex360, at the 2025 International Maritime Defense Industry Exhibition (MADEX), held from May 28 to 31 in Busan, South Korea, HD Hyundai Heavy Industries (HD HHI) signed a memorandum of understanding with the Portuguese Navy aimed at establishing a strategic partnership in naval ship construction, maintenance, repair, and overhaul (MRO), as well as the joint development of a small submarine model. The agreement was formalized at HD HHI’s booth at BEXCO on May 30 and reflects Portugal’s search for additional submarine capabilities amid recurring availability issues with its existing Tridente-class fleet.
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The NRP Tridente has been undergoing a major overhaul since 2022, while in March 2025, impurities found in the Arpão’s hydraulic system temporarily left the Portuguese Navy without any functioning submarine. (Picture source: Portuguese Navy)

The Portuguese Navy currently operates two Tridente-class submarines, the NRP Tridente and NRP Arpão, based on the German Type 214 platform and equipped with fuel-cell AIP propulsion, the ISUS 90 combat system, KH-1007 navigation radar, and 533 mm torpedo tubes capable of launching Sub-Harpoon missiles. These submarines, commissioned in 2010, replaced the Albacora class and form the core of Portugal’s undersea warfare capabilities. However, in recent years, operational availability has declined. The NRP Tridente has been undergoing a major overhaul since 2022, while in March 2025, impurities found in the Arpão’s hydraulic system temporarily left the Portuguese Navy without any functioning submarine. These constraints, combined with concerns about Russian naval activity off the Portuguese coast and in the Mediterranean, have led the Portuguese Navy to consider acquiring either a third submarine of similar tonnage or two smaller submarines in the 800 to 1,300-ton range.

In response to these developments, Admiral Jorge Nobre de Sousa, Chief of the Portuguese Navy, has indicated a preference for acquiring smaller units. This approach was reaffirmed during an engagement at MADEX 2025, where Admiral Fernando Pires, head of the Portuguese Navy’s logistics command, signed the MOU with HD HHI’s special ship division head, Ju Won-ho. The agreement does not specify a final design but refers to the joint development of a small submarine model based on mutual trust and strategic cooperation. HD HHI is currently developing three export-oriented submarine models: the HDS-2300, HDS-1500, and HDS-800. All three models were displayed at MADEX 2025. The HDS-2300 is the largest, intended for blue-water operations, and has received Approval in Principle (AIP) from DNV. The HDS-1500, developed in partnership with Peru’s SIMA, is 65 meters long, 6.5 meters wide, features lithium-ion battery propulsion, requires a crew of 25, and has also received DNV’s AIP certification. The HDS-800, the smallest variant, is intended for coastal operations. It is expected to offer a compact and lower-cost option for navies with littoral security requirements.

Although Portugal has not officially confirmed a specific model, the Portuguese Navy may consider leveraging ongoing work on the HDS-1500, particularly given the displacement, propulsion configuration, and crew size align with the needs expressed in recent public statements. The HDS-1500’s open-architecture design enables the integration of country-specific combat systems, sensors, and weapons, offering a potentially adaptable solution. The agreement signed at MADEX reflects not only Portugal’s intention to strengthen its submarine fleet but also HD HHI’s strategy to promote its submarine designs in the global market through cooperative development and export-oriented customization. Portugal’s current Tridente-class submarines, the NRP Tridente and NRP Arpão, displace over 2,000 tons submerged, with a maximum test depth exceeding 300 meters, and endurance of up to 60 days. However, maintenance challenges have limited their availability for sustained deployments.

Alongside this bilateral agreement, HD HHI also signed MOUs with several other international and Korean defense companies, including POSCO, Leonardo, Thales, Korea Aerospace Industries (KAI), and LIG Nex1, as part of a broader initiative to develop HD HHI’s production and R&D capabilities. On May 28, HD HHI signed an agreement with POSCO to jointly develop new materials for use in next-generation naval hulls. In addition, HD HHI signed MOUs with Italian firm Leonardo and French company Thales for collaboration in the export of naval vessels and systems. These agreements are intended to support integrated platform development, combat system alignment, and sensor suite compatibility in future joint projects. The participation of European firms suggests a deliberate move to increase HD HHI’s appeal to NATO-aligned markets. The collaboration also strengthens Korea’s domestic defense-industrial base through material innovation and system integration.

The agreements signed at MADEX 2025, including the submarine cooperation with Portugal and joint initiatives with global defense companies, reinforce HD HHI’s strategic objective to increase its role in the international naval market. By aligning with smaller navies that require modular, flexible, and cost-efficient underwater platforms, HD HHI is targeting operational needs not addressed by large-scale submarine builders. For Portugal, the initiative provides a path to restoring its undersea presence while potentially benefiting from industrial return mechanisms similar to those included in previous procurement contracts. For HD HHI, the engagement may serve as a case study for future submarine co-development projects with other allied or NATO-affiliated nations.