Tension Leg Platform (TLP) is recognized to be an aseismatic structure and earthquake has never been considered as an external force in its design. However, the vertical component of seismic force is an important item to take into consideration, because it is directly superposed to pretension of tendons1,2). The purpose of this report is to set up an analytical model for determining response of a TLP under unbalanced initial tension when the TLP is subject to vertical seismic excitation.
Hull of a TLP is moored to the seabed by four tendons. Tendon tension acts equally to the four tendons. As TLP receives various environment forces such as wind force, wave force and current force, it moves to the horizontal direction. The horizontal displacement is called "offset". When hull is under offset condition, the tendon tensions become unbalance. Earthquake response of TLP under unbalanced tensions is different from that of balanced tensions. Researches on snap load and tendon failure of TLP are already reported3,4). However, they do not have dealt with response of TLP under unbalanced initial tensions. In this research, we have constructed a 3-dimensional analytical model which is able to simulate the unbalance state of a prototype of the TLP. By using it, we have carried out numerical calculation in order to st.udy basic response of the TLP due to seismic excitation under unbalanced initial tensions.
Analytical model and condition A prototype TLP is shown in Fig.4. The particulars of the TLP are shown in Table 16). The earthquake introduced at bottom of tendons is vertical motion of EI-Centro wave. The initial length of tendons is 469.5 m. Due to initial tension(=29.4 MN), tendon length becomes 470.0 m. Unbalanced state of initial tension occurs due to the change of the tendon length.
