40% of the global population live in coastal cities. This figure is predicted to increase as inland populations migrate to coastal areas. This is due to land availability steadily decreasing due to several factors such as drought and increasing requisite cropland stemming from growing populations and the growth of the biofuels market. Simultaneously, sea level rise (SLR) means that further strain will be put on land availability in coastal areas. It is important to have solutions prepared for this event. One such potential solution is the use of Modular Floating Structures (MFS) as dynamic platforms for floating urbanisations. An MFS consists of a floating substructure on top of which a superstructure (buildings) can be built. While MFS technology is not currently used anywhere in the world the technical feasibility and use case potential of MFS has been demonstrated in previous research. However the environmental impacts of MFS technology had previously not been investigated. A prospective life cycle assessment (LCA) of potential MFS substructure designs was devised in order to quantify and compare the potential environmental impacts and to find potential for optimisation of the environmental performance of the substructures. A case study of Taranto, Italy, a city which has the potential for using MFS technology was taken for this research. In the baseline case it was assumed that the substructures would have to travel 1898 km across the sea throughout their lifetime. All cradle-to-grave life cycle inputs were included, excepting cut-offs. Only two main materials were considered suitable for use in substructure construction: steel and reinforced-concrete. However four designs were deployed. Virgin steel (VS), recycled steel (RS), 35 MPa Portland concrete (35 PC) and 50 MPa Portland concrete (50 PC). The concrete designs did not have equivalent buoyancy to the steel designs given the default geometric dimensions so these dimensions were altered considering several factors (bending moment, stress, second moments of area, draft). A holistic approach was taken and all impact categories were considered, however particular attention was given to the impact categories of climate change and marine ecotoxicity. For all designs the raw materials used and the transport across the ocean held the greatest share of the impacts. In the concrete designs a surprising finding was that the stainless steel rebars used in their design held a larger share of the impacts across the board. The 50 PC design performed by far the best overall however due to issues with working with concrete of higher strengths it is unclear if the 50 MPa Portland concrete can be easily applied to MFS substructure construction in the present day. In terms of present-day usability and environmental performance the two forerunning designs were the RS design and the 35 PC design. Trade-offs were found between design choices. The RS design had significantly lower CO₂-Eq emissions with 7,292 t compared to the 35 MPa concrete design, which had 9,770 t CO₂-Eq. However, the 35 MPa design had much lower marine ecotoxicity with 984 t DCB-Eq, than the RS design 4,256 t DCB-Eq. This higher marine ecotoxicity for the RS design was largely (63%) due to the local environmental impact of sacrificial zinc anodes corroding into the sea. These are used as cathodic protection for the steel. Altering the cathodic protection is a possibility for reducing the marine ecotoxicity impacts of the steel designs. Normalisation and even weighting were applied to compare the overall impacts of the designs. When total oceanic transport distance of the substructures is 482 km, in terms of overall environmental impacts the 35 PC design outperforms the RS design by 22%. At 1898 km this decreases to 3%. At longer distances the RS design performs better due to its lighter mass and subsequent lower impacts from oceanic transport. At 2467 km the RS design outperforms the 35 PC design by 2%. In the context of the EU, the legislation surrounding deconstruction of end of life (EoL) ships and offshore installations is a key factor to consider. For offshore instalments which are to be recycled, the deconstruction can only take place in a limited number of shipbreaking yards. This directly affects the distance which the substructures must travel across the sea. And this is a main influencing factor which can lead to MFS substructure optimisation, in terms of environmental impacts. Finally, environmental impacts are not the only considerations important to design choice. Other factors should be considered, namely the material availability in a given region and the disparity in recyclability between steel and concrete. Further research can develop these findings and investigate other highlighted potential concrete reinforcement material options such as epoxy coated steel and plastic fibre reinforcements. Furthermore, this research can be used as a building block for comparing the environmental impacts of MFS technology to coastal urban expansion alternatives such as land reclamation.