Stability Assessment of Anchor Handling Vessel during Operation Considering Wind Loads and Wave Induced Roll Motions

The objective of this paper is to apply IMO “severe wind and rolling criterion (weather criterion)” for anchor handling vessels during operation. So far, this criterion is used for merchant ships greater than a certain size and of conventional hull forms but it is not mandatory for special purpose ships like anchor handling vessels. It is considered inappropriate to be used in its present form due to the large initial heel involved in anchor handling vessel and vessel not subjected to severe wind and roll. A slightly modified weather criterion is proposed in this paper. A quasi dynamic criterion is used for studying wind and wave effects on vessel stability. The vessel stability depends primarily on the magnitude of mooring load, angle between vertical axis and mooring load, angle of attack, and weather conditions. The analysis is carried out for loading conditions while the vessel is in operation. The quasi static approach is considered for incorporating the influence of mooring parameters in the loading condition itself. Due to the overturning moment coming from the mooring load, vessel changes to new static equilibrium position. In this state the vessel is subjected to an initial heel angle, at the same time the vessel range of stability is reduced. The vessel loses its range of stability due to the overturning moment coming from the mooring load. Therefore, the area under the righting moment curve is drastically reduced. The reduction of dynamic transverse stability limits the allowable roll angle influenced by the waves for safe operations. Hence, it is essential to study the influence of mooring parameters, and wind speed on the vessel’s dynamic stability to determine the allowable sea state. Alternatively, for a given sea state and wind speed the limitation of mooring load can be determined. The modified method can be useful during the planning of operating guidelines regarding the stability of the anchor handling vessel.