A time domain analysis procedure is used to calculate maximum design tensions for the mooring system of a truss spar. Fairlead motions are used as input to calculate time histories of dynamic mooring line tensions. Several random simulations are used to derive design values of maximum tension. The variability from simulation to simulation in maximum tension and in the dynamic portion of the response is summarized. Alternative methods for estimating the maximum value are compared and the effect of the number of simulations on the uncertainty in the maximum is demonstrated. The dynamic characteristics of the mooring line tensions are examined by looking at the high frequency and low-frequency portions of the response. A comparison of mooring line tensions for a truss spar and a classic spar is also provided. In addition, maximum tensions from the time domain analysis are compared with results from a frequency domain analysis.
There are three production spars presently operating in the Gulf of Mexico. Those spars are deep draft, vertical steel cylinders and are referred to as "classic" spars. An alternative configuration is the truss spar, where the ring-stiffened shell of the midsection is replaced by a tubular steel truss (Figure 1). The improved structural efficiency of the truss reduces the weight of the hull, which reduces the cost and fabrication time. In many cases, the drag load on the open truss section is less than for the classic spar. This reduces the mooring line tensions in high current conditions. There are three truss spars presently under construction.
From a global motions perspective, one disadvantage of the truss spar is that there is no longer the large mass of water (both enclosed mass and added mass) associated with the midsection of the classic spar. The added mass and damping of the truss heave plates produces a heave response that is nearly the same as a classic spar. However, the reduction in lateral mass of the truss spar increases the surge/sway highfrequency response. This is because the lateral wave load on the truss spar is essentially the same as it is on a classic spar. Since the mass is less, the high-frequency response in surge/sway will be greater for a truss spar than for a classic spar.
Due to the increased high-frequency motion, it is important to accurately account for line dynamics in designing the mooring system of a truss spar. When using time domain simulations to evaluate maximum response, it is also important to understand the variability in the response and the uncertainty in the estimate of the maximum response.
The results presented herein are for a typical truss spar hull configuration and mooring system. The mooring tensions are also compared with those of a classic spar whose topside capacity and maximum pitch response matches those of the truss spar.