Offshore wind power is currently the fastest growing energy sector worldwide. Alongside with the demand growth for renewable energy, a proportionate increase in the wind turbines (WT) size is observed. The current WT design trend leads to increasingly larger rotors, and slender and higher towers. The offshore wind turbines (OWT) are constantly subjected to offshore environmental loads, which include the combined effects of wind and sea waves. This loading generates excessive structural vibrations and fatigue phenomena to the OWT tower, which in turn may lead to structural failure and jeopardise the turbines' contribution to the energy network. To ensure the OWT sustainability, it is essential to mitigate the structural vibrations and to improve the OWT design. In this research the vibration mitigation of a monopile OWT using a variation of the KDamper is investigated. In the Extended KDamper (EKD), the additional mass is connected to the nacelle with the NS element and the artificial damper and to the tower with the positive stiffness element. The OWT tower is modelled using variable cross section beam elements accounting for second order phenomena. The monopile soil-structure interaction (SSI) is modelled using a prismatic beam on elastic foundation with the corresponding springs and dashpots along the pile's length. The monopile OWT is subjected to the combined action of wind and sea wave environmental loading. The wind load time histories are produced using the Blade Element Momentum theory, while the influence of the sea wave excitation is studied using an integrated Computational Fluid Dynamics (CFD) approach. A multi-objective design optimization approach is also performed, to design a feasible EKD by taking into consideration factors and criteria such as engineering requirements, manufacturing specifications, and realistic uncertainties of the structure. An extensive case study is carried out on a monopile OWT providing insight to the structural dynamics and illustrating the viability of the VCS on the offshore wind industry. It is shown that vibration control can extend the lifetime of the structure, increasing the OWTs reliability and sustainability.