The designer of Floating Production Systems need to determine the safe working limits for a floating platform over a period of many years. One of the most important parameters is the loads experienced by the mooring system over the life of the system or between inspections. This paper studies one such system and probabilistic description of the mooring loads. The technique depends on separating the slowly varying component from the wave frequency motions and making several assumptions concerning the statistical independence and the nature of the resulting distributions. The paper differentiates between line dynamics and mooring dynamics. Line dynamics are considered as a separate phenomena that is due to the dynamic oscillations within the mooring line while mooring dynamics are considered as the global motions of the system. Here we will not consider line dynamics, this does not imply that the line dynamics are unimportant. On the contrary, we believe that under certain circumstances they are very important and should be included in any final design evaluation, but rather that they are beyond the scope of the present investigation.
The system modeled is a turret moored tanker with an eight point chain mooring pattern. Modeled at a scale of 1 : 53 using Froude scaling. The prototype system consisted of a 200,000 DWT tanker moored with 5000 ft of 5 1/4" chain in 1476 ft of water. The system is described in Table 1. Figure 1 gives a detailed layout of the system. A stock tanker model was outfitted as an FPSO with facsimiles of the production equipment decks in addition to the deckhouses to stimulate the wind loads. A Turret was modeled and located just aft of the watertight bulkhead between tanks 1 and 2 for a typical tanker. The breakout torque of the turret was modeled to permit weathervaning.
Because of the impracticability of modeling the actual mooring system at a reasonable scale a horizontal mooring configuration was modeled (see figure 2) and the data extrapolated using STAMOOR (ref 1) to determine the actual mooring line tension. This procedure was shown to be accurate as shown in Figure 3. This is a plot of mooring line tension against surge displacement with the spring stiffness plotted through the data. It can be related directly to the displacement through the non-linear spring stiffness. At the lower turret exit a simulated flexible riser system was attached. Modeled for visual inspection only the risers were connected in a J configuration to a submerged buoy platform. This platform supported the risers connected to the bottom of the test tank. Table 2 gives the complete environmental parameters for the tests performed.
The tests analyzed in this paper consisted of collinear wind, wave and current parameters were set independently and superimposed upon one another to arrive at the complete environment. This is perhaps not the most realistic but it does have the advantage of simplicity and allows each individual component of the environment to be measured accurately. The principal effect of the wind and current was to add a large mean offset to the system on the order of 30 feet. Due to the predominance of the slowly varying response of the system an investigation was carried out on the effect of test length on the results.
