# FPSO Hull Structures with Sandwich Plate System in Cargo Tanks
## 7 Conclusions  
This work focuses on the research for an initial design and construction of hulls for FPSOs with sandwich plates without stiffeners in the bottom plate and longitudinal bulkheads of cargo tanks. The main idea is to have the walls and bottom of the cargo tanks free of stiffeners to allow remote cleaning and thickness gauging of bottom and bulkhead plates using autonomous equipment.  
The initial design of the traditional steel arrangement was made by using the results of this study and the guidelines of the ABS FPI rules [14]. This design was also compared to the DNV rule to check possible changes, and the total strength assessment was made. In general, the design obtained passed all the criteria of yielding (static and dynamic load case), buckling and ultimate strength proposed. The only exception was the deck plates at the static load case, which obtained a factor of 0.62 from the yield stress. This design was considered acceptable because it is close to the maximum safety factor (0.6 for the static load case).  
The design of the hull with SPS panels was made by using the steel panels' dimensions and properties with the guidelines of DNVGL [16]. The SPS panels can withstand both static and dynamic load cases as the steel arrangement, but with a lighter structure.  
Particular emphasis is given to the construction of the FE model for analyzing the ultimate strength of both arrangements: traditional steel and SPS panels. The static load case was used as a load condition in order to validate the models using as reference the results from beam theory. Although both steel and SPS models achieved results similar to the ones predicted, it is important to note that this analysis was a simplistic one. This is due to the use of the equivalent plate instead of stiffened panels. Thus, the buckling behavior can be slightly different. Additionally, a more refined mesh would be interesting for further analysis.  
The FPSO's hull weight was calculated for both types of arrangements, and it was observed that the SPS one was overall 2.8% lighter than the steel one, primarily due to the bottom structure. This happens because the bottom structure has the plates with the highest thickness and with a more robust stiffener than the traditional hull structural arrangement.  
Therefore, to summarize, the SPS design demonstrated superior structural performance over the traditional steel one. However, further research in the following areas would be interesting:  
* Perform a local ultimate analysis for the SPS plates and stiffened panels with more details. This would provide more insights into the buckling and ultimate strength of the panels.
* Study the efficiency of the fabrication process of the SPS panels.
* Research the financial attractiveness of this new panel both during the building and operation steps in order to know how different kinds of costs vary.
* Research different filling materials and methods for polyurethane in order to have alternatives.
* Research the impact of this new technology during the operational life of the FPSO, converting inspection requirements, maintenance and repair work.  
**Author Contributions:** Conceptualization, B.-Q.C. and P.M.V.; methodology, J.P.S.; formal analysis, J.P.S.; writing--original draft preparation, J.P.S.; writing--review and editing, B.-Q.C. and P.M.V.; visualization, J.P.S.; supervision, B.-Q.C. and P.M.V. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This study contributes to the Strategic Research Plan of the Centre for Marine Technology and Ocean Engineering, which is financed by the Portuguese Foundation for Science and Technology (FCT), under contract UIDB/UIDP/00134/2020.  
**Institutional Review Board Statement:** Not applicable.  
**Data Availability Statement:** Not applicable.  
**Conflicts of Interest:** The authors declare no conflict of interest.