An Innovative Construction Method for Concrete Spars Using Steel-Concrete-Steel Composite Systems
Authors: Akinci O. , Kalpakidis I. , Udoh I. , Huber D. 
Published: April 2024

Abstract
Floating wind energy is a very promising source of energy, with governments across the globe embarking on ambitious plans to tap into wind energy offshore over the coming decades. There are clear advantages of offshore wind over onshore, such as higher sustained wind speeds and more locations for new developments, including deep water (i.e., water depths exceeding 200 ft). We performed a concept development study and preliminary analysis to test the feasibility of a proposed floating offshore wind spar concept using an innovative steel-concrete-steel ‘"sandwich" composite system, with emphasis on efficient, cost-effective fabrication for a low Capital Expenditure (CapEx). This type of composite modular system has been successfully utilized and implemented into both safety-related nuclear facilities for use as containment structures and multi-story commercial structures as lateral force resisting core walls. We developed our spar concept design considering properties of the 5 MW and 15 MW NREL wind turbine available in public domain. The objective of this study is to demonstrate the effectiveness of an innovative construction method to lower the levelized cost of energy (LCOE) of floating offshore wind turbine (FOWT) support structures by utilizing local resources and existing infrastructure. This approach allows for modular and horizontal fabrication and assembly of a hull system that utilizes exterior steel "face" plates that act as permanent formwork and reinforcing steel to an inner concrete core – where the faceplates are anchored to the concrete using steel anchors (shear studs or similar) and to each other using steel ties (round bar being the most common type of tie) that are typically welded to the faceplates. The resulting structure behaves similarly to proven concrete spar designs but can be constructed in a more expedited and more economical way compared to any other hull system capable of supporting large wind turbines and associated infrastructure. A steel-plate composite (or SC) construction methodology for spar hulls has several advantages compared to traditional hull construction, with one of the key benefits being an accelerated construction timeline due to being able to fabricate individual steel sub-modules in a controlled environment. These sub-modules are then interconnected into larger steel modules in a planned sequence horizontally over the length of the spar, with concrete placement occurring as the larger steel modules are completed. Similar concrete spar structures are typically built in a vertical arrangement using labor intensive and time-consuming slip forming methodologies requiring specialized dry dock facilities. An SC spar system constructed as described would also be better suited for local (North American) port infrastructure and shipyards to be able to fabricate and assemble. In our study, we ran preliminary calculations using first principles to estimate the dimensions and weights of the SC spar system. Once detailed sizing analysis of the platform is performed, design parameters such as wall thickness and weight can be further optimized. Additional studies and testing shall be performed at a later stage to improve the maturity of this concept.