Offshore triangular tension leg platform earthquake motion analysis under distinctly high sea waves

Abstract

TLPs are compliant structures designed to withstand moderate loads without damage and severe loads without seriously endangering the occupants. Dynamic behavior of TLPs under distinctly high sea waves in the presence of both horizontal and vertical seismic excitations is examined and method of analysis is discussed. Seismic forces imposed at tether bottom make tether tension unbalanced when the hull is under offset condition. Tether tension varies nonlinearly under vertical seismic excitations generated using Kanai-Tajimi ground acceleration spectrum. Analytical studies conducted on triangular TLPs show that this tension variation is much higher than the regulation values indicating the necessity for examining them for seismic safety. Clearly, the peaks seen in the response of all active degrees-of-freedom occurring near to the average sum frequencies of waves and input ground motion is a significant influence of seismic excitations on TLP tethers under high sea waves. The numerical results obtained also verify/establish the fact that TLPs built in deeper sea locations show significantly lesser response to the combined wave and earthquake loading.

Keywords:

• horizontal and vertical seismic excitation
• high sea waves
• Kanai–Tajimi spectrum
• random ground motion
• tether tension variation
• triangular tension leg platform