Floating wind turbines (FWTs) have become the preferred structures to exploit offshore winds since a large part of the offshore wind resource is located in deep water zones. In the harsh marine environment, a spar-type substructure is the most promising substructure for FWTs with good structure stability and economic advantages. In this study, the surge, heave, and pitch motions of the spar-type substructure under combined action of wind, wave, and current are analyzed by using the AQWA module of ANSYS. Besides, the effects of different parameters, namely, the mooring line distribution form, fairlead position, and line number of mooring systems, on the spar-type substructure motion response are also investigated. Based on the calculation results, a new spar-type substructure with an optimized mooring system is put forward and analyzed. It can be found that both the dynamic response and the mooring tension of the new spar-type substructure are significantly improved relative to the non-optimized structure. The results of this paper could offer reference for further development of spar-type FWTs in real marine environments.

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As the main bearing components of double-arch tunnels, the stability of the middle wall is related to the construction safety of double-arch tunnels during construction and operation. Different construction methods of double-arch tunnel have great influence on the stability of middle wall, especially when the excavation span is large and the buried depth is very shallow. Based on an ultra-shallowly buried double-arch tunnel with extra-large spans in Xiamen, China, the mechanical response and deformation law of the middle wall are obtained. Furthermore, the performance of the CRD method and the double-side-drift method for optimizing the deformation and stress state of the middle wall are compared. The results can serve as initial guidelines for the for the selection of double-arch tunnel construction schemes.

An installation platform will disturb the natural seabed adjacent to a monopile and, consequently, affects the bearing capacity of the monopile foundation. Thus, consideration of the influence of installation disturbance is required in the monopile design, which can also save a fraction of the construction costs and avoid security risks. It was found in a sensitivity analysis, depending on the numerical simulations for an offshore monopile foundation installed using an installation platform, that the bearing capacities of the monopile were reduced by the penetration and extraction of the support legs of the installation platform, which were, a reduction of about 7% of the horizontal bearing capacity of the monopile (decreasing with a larger diameter of the monopile), about 2% of the ultimate bending moment, and almost no reduction of the vertical bearing capacity. For the monopile specifically subjected to the combined loads, it was found in SLS that horizontal displacement and rotation at the mudline increased by about 5.13% and 2.12%, respectively, and internal forces increased by about 3.29%–9.87%; and the horizontal displacement and the rotation at the mudline increased by about 4.56% and 2.79% in ULS, respectively, and the internal forces increased by about 3.49%–7.79% in the ULS. The study on the effects of the disturbance of the installation platform can be an important suggestion for the engineer in monopile design.