## 1 Introduction  
Offshore wind energy has been widely recognized for its important role in reducing the greenhouse gas emission and satisfying the increasing energy demand. Currently almost all offshore wind turbines around the world are bottom fixed structures. As the water depth increases, the construction difficulty and the cost of the fixed foundation offshore wind turbine rise rapidly. It has been shown that the traditional fixed foundations, such as monopile, jacket, tripod, etc., would not be the best choice for water depth beyond 50 m for the economic consideration [1; 2]. Therefore, the floating foundation concept comes into an alternative option for offshore wind turbine in deep waters due to a number of benefits [3]. The concept of floating offshore wind turbines (FOWTs) was proposed by [PERSON] [4] in 1972. However, restricted by immature technology, it is only in the mid 1990's that the FOWT started to become a widespread concept. Since then, several configurations of floating support platform have been developed for offshore wind turbines and the performance of these concepts has been tested through experimental and numerical methods. In terms of the floating principles to achieve static stability [5; 6], FOWTs can be classified into three categories: spar type [7; 8; 9], semisubmersible type [10; 11; 12; 13; 14] and tension leg platform (TLP) type [15; 16]. The Hywind concept developed by Statoli [7] whose foundation is a slender deep draft substructure is a typical spar type floating offshore wind turbine. It has been not only studied numerically [17; 18; 19; 20], but also experimentally using scale models [21; 22; 23] and a long-term prototype test [24]. Due to spar buoy's excellent stability and after continuous theoretical studies and site monitoring, the Hywind concept has eventually been commercialized in the world's first floating wind farm: Hywind Scotland [25], in 2017, which is a milestone in the offshore wind turbine history. [PERSON] [26; 27; 28] also proposed a hybrid spar type FOWT. The lower part of the spar platform was made of prestressed concrete composed of precast segments and the upper part was made of steel in order to reduce the platform construction cost. The semisubmersible foundation is another versatile concept for FOWTs. It relies on large columns linked by braces or pontoons and ballasting to maintain stability. [PERSON] et al. [11] proposed a column-stabilized offshore wind turbine named WindFloat. It comprised three columns with water-entrapment plates at the end of each column to mitigate motions, and the wind turbine tower was positioned directly above one of the stabilized columns. Principle Power built a 2-Megawatt (MW) prototype using this concept in Portugal in 2011 [29]. For the sake of improving fatigue performance and economic efficiency of semisubmersible FOWT, Mitsubishi Heavy Industries developed a V-shaped braceless semisubmersible offshore wind turbine and the prototype equipped with a 7 MW wind turbine was installed at Onahama Port in Fukushima Prefecture in 2015 [30]. The tension-leg-platform (TLP) -style foundation, although having been used in offshore oil and gas industry for years, has the disadvantages of tougher mooring installation and incurred higher marine operation cost when compared to spar and semisubmersible concepts. The commercialization progress of TLP-FOWT is relatively dim. Nonetheless, in 2009 GICON and its partners declared that they have developed an improved manufacturing and installation TLP concept GICON(r)-SOF to drastically reduce the levelized costs of energy (LCOE) [15]. Different from the conventional TLP concepts with only vertical tendons to restrict heave motions, GICON(r)-SOF is unique for the additionally angled mooring lines which limit the platform horizontal movement as well as the nacelle in its entirety. The GICON(r)-SOF's full-scale transport and installation processes simulation are scheduled to take place in 2018 [31]. In the design of FOWT, safety and economy are two most important considerations.