Deployed on floating bodies or along cables, offshore energy harvesters can convert wave, solar, tidal, ocean currents, and other renewable energy sources to stable electrical energy [1]. Creating hybrids with wind electricity generation would reduce the currently significant operations and maintenance (O&M) of wind turbines (WT), which is around 10–25% of the total cost of electricity, and a lower transmission cost ^[2][3][2,3]. By bringing together two marine renewable technologies with considerable synergies, the combined harnessing of offshore energies creates excellent potential for development. This is corroborated by some recent European Union (EU)-funded projects: MARINA, ORECCA, TROPOS, MERMAID, and H2OCEAN [4]. MARINA classifies combined wave–wind systems according to the technology, water depth (shallow, transition, or deep water), or location relative to the shoreline (shoreline, nearshore, offshore). ORECCA analyses the offshore renewable energies (ORE) combined resources in Europe. Looking particularly at Europe’s combined wave–wind resource, this can be divided into three main sea basins: the Mediterranean Sea, the North and Baltic Seas, and the Atlantic Ocean. TROPOS is aimed at developing a floating multi-purpose platform system for deep water [4]. The MERMAID project seeks to develop concepts for the next generation of offshore activities for multi-use ocean space. It proposes new design concepts for combining offshore activities, such as energy extraction, aquaculture, and platform-related transport at various ocean areas [5]. H2OCEAN is developing a wind–wave power open-sea platform for hydrogen generation with support for multiple energy users [6].

The main projects installed in the previous decade (2010–2019) were the 2.3 MW Hywind in Norway in 2009, the 2 MW Principle Power in Portugal in 2011, and the MOE project in Japan with capacities of 100-kW half-scale model in 2012, and 2 MW full scale in 2013 [7].

Da et al. (2009) propose a control scheme for a hybrid system. Adjusting the generator’s rotation speed can maximize the system’s output power under fluctuating wind or tidal currents [8]. Li et al. (2017) show the integration of floating wind turbines with a wave energy converter and tidal turbines increases power production by 22–45% [9]. Lande et al. (2019) modelled the co-location of a wind turbine with an array of tidal stream turbines in the MeyGen site located in Pentland Firth, UK. It will increase energy yield by around 11% and decrease the levelized cost by 10% [10]. Nichita et al. present the “accelerated simulation time” method and its experimental validation. Wind or tidal turbine characteristics are obtained using the simulation approach developed at the GREAH lab and are validated with an actual ocean turbine installed in the Circulating Water Channel at Inha University Ocean Engineering Laboratory, South Korea [11]. Phurailatpam et al. [12] present a DC microgrid for rural applications in India using wind turbines (WT) and photovoltaic panels (PV). Azaza et al. give some insight and techno-economic analysis of microgrid deployment in different Swedish regions using PV/WT/DG, a battery bank, and an energy management system to identify the optimal system size and configuration [13]. Thakur et al. designed, constructed, and tested a new physical simulator under different operating conditions in a real microgrid environment. The simulator replicates the behaviour of a designed wind turbine. The experiments have also shown that the designed wind turbine can work in harmony with PV power modules and battery storage in response to weather and load variations in an island microgrid environment [14]. Wang et al. analysed the stability of a microgrid system containing an offshore wind farm (OWF), an offshore tidal farm (OTF), and a seashore wave farm (SWF) fed to an onshore power grid through a high-voltage direct current (HVDC) link based on a voltage-source converter (VSC) [15]. Adetunji et al. proposed an optimized grid-connected microgrid for South Africa using photovoltaic panels (PV) and a supporting lead-acid battery for downtime [16]. Kitson et al. present a DC microgrid system, interfacing wind and solar using a power electronic interface with droop functions. A case study site in Nepal is simulated to demonstrate the system’s performance to variable generation and loads [17]. Oulis Rousis et al. designed an off-grid system in Greece relying on PV, diesel generators, and batteries for energy storage [18]. Phurailatpam et al. compared different scenarios of DC microgrids in the Indian context using wind and photovoltaic panels for India’s rural and urban power supply [19]. Faridnia et al. designed a grid-connected microgrid for a tidal farm near Darwin, in the north of Australia, including tidal power as the main supply, a pumped hydro system (PHS) with 1000 kWh capacity as the long-term storage system, and a micro-turbine (MT) to minimize the operating cost [20]. Colombo et al. added Photovoltaic (PV) to power-to-gas (P2G) to reduce emissions [21].

Over the recent decades, offshore wind farms have attracted more investment [22]. It is estimated that onshore and offshore wind power will generate more than a third of the total electricity needed in the medium term, becoming the primary generation source by 2050 [23]. Compared with onshore wind energy resources, offshore wind fields have many advantages, such as persistent wind, faster-flowing speed, higher uniformity, and longer available time per year, flat sea surface, and low turbulence intensity, which promotes the vigorous development of the offshore wind power industry ^[24][25][24,25]. More importantly, installing wind turbines in the ocean can protect the environment [26] and save land resources [27]. The vast ocean area provides good conditions for developing large-scale wind farms and turbines [28]. The power generation by the identical turbines in the offshore area is 50–100% higher than in the onshore area [29]. However, the main issue for investors is capital cost which results in increasing the electricity cost for customers. The most expensive component of an offshore wind turbine is the foundation, accounting for 19% of the capital cost. In addition, foundation installation with 6% of the capital cost is the highest cost compared with other parts [30]. However, the cost of electricity using offshore wind is still high [31]. As another offshore energy source, installing tidal turbines has attracted less investment because tidal turbines are exposed to harsh currents. Their lifetime is low, and foundation design is complex in most cases, where the water depth is more than 30 metres [32]. However, tidal turbines can produce an enormous amount of electricity, more than four times per square meter of the rotor than wind turbines [33]. Integration of both wind and tidal turbines with the same foundation may be a way to reduce the cost of electricity [7] and enable predictable power generation from two different energy sources.

Although New Zealand is surrounded by water and has good potential for offshore energy, it has not yet been used for power generation. In recent years, several reports indicated the annual demand of electricity increases from current demand of 40 TWh to 70 TWh by 2050 [34]. Therefore, looking for new sources of harvesting power generation is essential.