The Importance Of Static Stability Of The AHV

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Static stability assessment with existing IMO A 469 criteria
The primary objective of this section is to assess the static stability of the AHV during AHOs. Intact vessel capsizing prevention is a fundamental principle of fulfilling the ship stability requirement. The stability of a vessel is universally presented with a static stability curve, which describes the overturning moment required to incline a vessel to various angles for a given loading condition. This curve in principle similar to the load deflection curve of a mechanical system and is normally represented in terms of righting arm versus the angle of the vessel inclination (heeling angle). This curve constitutes as a basis for the traditional ship stability criteria. Existing
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The chosen vessel’s bollard pull and the winch capacity can be different. As mentioned in the Section 2.1.6 for modern AHV these loads are in the range of 65 to 650 tons. Generally, in view of stability, these loads are difficult to handle by the AHV if it is acted away from the centreline (not in line with the centreline). Therefore, it is essential to define the allowable mooring load for a chosen vessel in the design phase itself. If the magnitude of the mooring load (F_(ML,XYZ) ) is higher than the allowable mooring load, then the chances of the vessel capsizing for small environmental disturbances can be higher. Besides this the AHV freeboard at the aft end is smaller. As a result, if the vessel is subjected to higher magnitude of F_(ML,XYZ) then the freeboard reduces further. At the same time, if the sea comes along the astern then the water can engross easily onto the deck of the vessel. Without a doubt, these effects can bring down the vessel reserve stability in two ways: (a) increasing draft and vertical rise of the centre of gravity, and (b) free surface effect of the water on the deck. The effect of the water on the deck is not considered in this analysis. Figure 4.7 shows that the effect of F_(ML,XYZ) on the righting lever curve is …show more content…
The magnitude of F_(ML,XYZ) is one of the primary influencing parameters on the vessel static stability. Latter, the sequence of influencing parameters are β and α. If the F_(ML,XYZ) increases, the vessel’s initial list changes rapidly for the same set of β and α as a result the range of stability reduces significantly. This leads to the scenario the vessel can capsizing quite rapidly and leaves no time for remedial actions. The β is the next higher level influencing parameter. The angle β is primarily dependant on the vessel position and bearing, with respect to the rig position and angle at the fairlead, and, further, it depends on the tow pin position. The variation in the overturning moment due to the vertical component of the mooring load (F_(ML,Z) ) depends on the lever. The lever depends on the angle β until the line reaches the obstruction at the transom. As a result, the heeling moment due to the F_(ML,XYZ) increases with respect to the angle β. Accordingly, the vessel’s static heeling angle increases and vanishing angle decreases. The vessel stable and unstable zone, and vanishing angle for a range of mooring parameters (F_(ML,XYZ), β and α) are listed in Table 2 and Table 3, respectively. From these tables it can be concludes that by increasing F_(ML,XYZ) and β the vessel capsizing phenomenon occurs (loses its stability) without any

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