Wave-in-deck loading

The mean value of Pi obtained from repeated runs was normalised by the reference dynamic pressure of a wave event and reported for all test conditions (P* = Pi/0.5 C2), see Table 3-6.

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The average normalised values (P*) corresponding to wave events WE#1 and WE#2 are shown in Figure 3-19. All plotted values associated with WE#1 show that the aft column region experienced large impact pressures at the different deck clearances. At lower deck clearance (a0 = 110 mm or a0 = 100 mm), the model experienced lower impact pressures at the aft column region (PT#14 – PT#16) than those measured at the original deck clearance, a0 = 120 mm. This confirms the finding from the analysis of the global wave impact force Fz(+) versus deck clearance, see Figure 3-18. In contrast, WE#2 against a0 showed an increase in the localised impact pressures (the same finding was reached with Fz(+), see Figure 3-18) as the deck clearance reduces.

Figure 3-19: Average normalised impact pressures [P* = Pi/0.5ρC2] measured by sixteen pressure

transducers (PT) at different deck clearances: wave event WE#1 (left); wave event WE#2 (right). Pairwise comparisons can be made between Fz(+) and P* shown in Figures 18(a) 20 and 21. The findings in pressure measurements versus deck clearance are in-line with Fz(+) against deck clearance. The observed changes in P* values versus deck clearance are consistent with those for Fz(+). For instance, WE#4 (S = 0.10) shown in Figure 3-20 and WE#5 (S = 0.06) shown in Figure 3-21 indicates that there was found an effect from reducing the deck clearance on the pressure magnitudes (localised in a certain area). Despite this, no clear correlation can be seen between P* and a0.

The fact that the reduction in the deck clearance does not result in the increased vertical force Fz(+) and localised slamming pressures may appear counterintuitive and

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deserves further investigation. The following considerations may provide at least partial explanations for the observed effects:

 First, when a0 = 110 mm the model draft is increased with the model being sunk deeper against the oncoming waves, which will increase the distortion and reflection of the waves caused by the forward columns. With an additional effect of the column overtopping, the amount of wave energy reaching into the underdeck region will be decreased and the underside of the deck will, therefore, experience decreased vertical force (Fz) and/or lower localised impact pressures.

 Second, when a0 = 100 mm the contribution of the submerged pontoons into the vertical wave-in-deck impact force is less significant than at a0 = 120 mm (pontoons are further from the still-water level and, therefore, attract less wave force contributing less to wave entrapment under the deck).

Figure 3-20: Average normalised impact pressures [P* = Pi/0.5ρC 2

] measured by sixteen pressure transducers (PT) at different deck clearances: wave event WE#3 (left); wave event WE#4 (right).

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Figure 3-21: Average normalised impact pressures [P* = Pi/0.5ρC2] measured by sixteen pressure

transducers (PT) at different deck clearances: wave event WE#5 (left); wave event WE#6 (right).

The results for local slamming pressure for all test conditions are combined in Figure 3-22 so that the effect of deck clearance reduction on the wave-in-deck slamming pressures at the areas of interest, i.e., the forward and aft columns can be further explained. The wave steepness, S, was used to establish the effect of deck clearance reduction on the average normalised impact pressures, P*. Only with WE#2 (S = 0.09) the deck clearance reduction was found to have a consistent effect on the P* values at all pressure transducers, except for PT#3 (near the side edge).

Since PT#2 and PT#15 are symmetric along the diagonal of the deck underside, when PT#2 measured larger pressures at S = 0.09 (short wave), PT#15 measured much lower pressures. At S = 0.04 (longer wave) PT#2 measured lower pressures but PT#15 measured much larger pressures. Same pairwise comparisons can be made on PT#1 versus PT#16 and PT#3 versus PT#14. This finding indicates that when the area around the forward columns experienced large slamming pressures, the area around aft columns received lower pressures and vice versa.

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Figure 3-22: Average normalised impact pressures [P* = Pi/0.5ρC2] versus wave steepness at different

deck clearances: deck-forward column region (left); deck-aft column region (right). Legend: ▲ a0 = 120

mm, ■ a0 = 110 mm, ▼ a0 = 100 mm.

3.7.

Summary

On the basis of the findings reported in this chapter, the following conclusions can be drawn:

 Data analyses conducted showed that extreme wave events and the associated global horizontal forces acting on the TLP model could be generated with very good repeatability. However, the global vertical forces were found to be significantly corrupted by the structural dynamic response as observed in the force and acceleration time series and thus, this undesired effect was removed by using a low-pass filter with a carefully selected cut-off frequency.

 For local pressures, the deck-column intersection areas were found to experience large wave-in-deck slamming pressures, in particular around the aft columns. The impact pressure was found to be extremely variable in magnitude and duration, particularly at regions experience strong wave slams; large local pressures were found to be associated with high standard deviation.

 In all conditions tested, the magnitude of horizontal forces acting on the TLP model was found to be clearly affected by a small reduction in the deck clearance of 10 mm and 20 mm (1.25 m and 2.5 m full-scale, ≈ 8 %– 17% of the original deck clearance) such that the force magnitudes may be amplified by 7% to 22%.

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 Conversely, the global vertical forces and local wave-in-deck slamming pressures did not show the straightforward increase when the deck clearance reduced. With an additional effect of the water reflection and the column overtopping at lower deck clearance, the amount of wave energy reaching into the underdeck region may be decreased. Further work is required to fully understand this effect.

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