Trim Capabilities

7.2.1 The Equilibrium Polygon

The equilibrium polygon is a graphical illustration of the trim capabilities of the vessel. The displacement is plotted on the y-axis and longitudinal moment on the x-axis. A positive moment would lower the bow relatively to the stern. All combinations of displacement and net moment that lie within the trim polygon can be compensated for by the trim and compensating (T & C) tanks. This can also be used to dimension the T & C tanks. The sign convention used in moment calculations and thus in the equilibrium polygon is as follows:

 The loading conditions present the weight of the vessel, and a positive net longitudinal moment in Table 54 would cause the stern to rise and bow to sink.

 The equilibrium polygon itself is an expression for the buoyancy capabilities of the vessel, hence a positive moment would cause the bow to rise and stern to sink.

The approach when dimensioning the T & C tanks is fairly simple:

1. Determine which weights that are constant and which that are variable. The same is done with the buoyancy (the airlock can for instance be viewed as a section with variable buoyancy).

2. Determine different loading conditions by varying the variable weights and buoyant sections.

The density of the water can also be varied. The loading conditions are plotted in the diagram below and form a minimum requirement for the T & C tanks.

3. The trim polygon is established by first emptying the trim tanks successively, starting with the forward trim tank, plotting the resulting displacement and moment. Once all tanks are emptied the tanks are successively filled again, starting with the forward tank. The

equilibrium polygon is formed by connecting these points. The size and location of the T & C tanks can then be established through an iterative process.

7.2.2 Loading Conditions

The vessel has four basic operational modes which were divided into three sub-conditions; standard, light and heavy. In the standard condition the vessel is fully stocked with consumables and floating in seawater with a density of 1025 kg/m3. In the extreme light condition all consumables are spent and the vessel is floating in dense water (1035 kg/m³). Reversely the vessel retains all consumables while floating in low density water (1015 kg/m³) in the heavy condition. The standard condition is only applied to the normal mode and payload mode as they closely resemble the loading condition at the start of any mission. Inspection mode and installation mode will however only be entered after transit to the field, hence the vessel will be somewhere between the extreme light and heavy conditions. The standard condition is therefore not applied to those operational modes. The respective masses, buoyancies and moments of the different loading conditions are listed in Table 54.

76 The different loading conditions are

 Normal mode: The majority of the missions are expected to be pure inspection missions; the normal mode is therefore recognized as a vessel without any payload in the cargo hold and an airlock that is free of water.

 Payload mode: The payload mode is equal to the normal mode with one exception; a payload of 20 Te is included. The added mass and drag of the crane design load bar is also included.

 Inspection mode: This is the expected state while performing structural inspection. The airlock is flooded and the ROV outside inspecting.

 Installation mode: This mode is equal to inspection mode with one exception; a payload of 20 Te is included. The added mass and drag of the crane design load bar is also included.

Table 54 Loading conditions summary

Loading condition Vessel mass [Te] Vessel buoyancy [Te] Net longitudinal moment [Te∙m]

7.2.3 Trim & Compensation Tanks

The vessel is fitted with seven trim tanks; a forward trim tank, two central compensation tanks, three cargo compensation tanks and an aft trim tank. Only the forward trim tank and the central

compensation tanks are directly connected to deballasting pumps, while the trim ballast transfer pump handles the flow in and out of the aft tank and cargo compensation tanks. The forward and aft trim tanks and the bottom cargo compensation tank are placed on the centre line, and can therefore only be used to change the trim of the vessel. Two of the cargo compensation tanks and the central compensation tanks are however placed to the sides of the vessel, and can therefore also

compensate to a certain degree for heeling moments as well. Positioning components in the cargo hold so that their centres of gravity are centred perfectly on the centre line is unrealistic; hence some heeling moment must be expected. Other imbalances might occur due to damage. With the current T

& C tanks such imbalances can be countered without the use of thrusters while stationary or hydroplanes while moving. The layout of the tanks is shown in Figure 30 and Figure 31 while their sizes are listed in Table 55.

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Figure 30 T & C tanks shape and placement seen from the side. Trim tanks are marked in green, cargo compensation tanks in blue.

Figure 31 T & C tanks shape and placement as seen from aft. Trim tanks and cargo compensation tanks are again shown in green and blue.

Table 55 T & C tanks summary

Tank type Longitudinal distance

from centre point [m]

Volume [m³]

Forward trim tank 28,5 60

Central compensation tanks 2 100

Bottom cargo compensation tank -19,6 10,2

Lateral cargo compensation tanks -20,5 49,64

Aft trim tank -26,6 6,3

The loading conditions and equilibrium polygon are plotted in Figure 32. The chosen trim tank configuration is able to compensate for even the most extreme conditions envisioned. There is also a fairly large margin of safety.

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Figure 32 Equilibrium polygon