4.3 Spatial distribution of local ice loads around the hull
The origins of ice loads on different hull areas are different. When the ship proceeds straight ahead, only the bow waterline is breaking ice. Other areas of the hull encounter hits from broken ice pieces. The midship and aft areas can break ice when the ship is maneuvering (e.g. turning) or going astern. As the ice loads on different hull areas are of different origins, it is clear that the magnitudes of the loads are also different. This is taken into account in most ice rules by dividing the ship hull into regions and giving so called hull area factor for each region.
In FSICR the ship hull is divided into forward, midship and aft regions, among which the forward region (bow) has the highest hull area factor (i.e. 1.0). This is because the dominant operational mode of a ship is moving straight ahead. Accordingly the long-term ice loads at the bow should be considerably higher than the short-long-term ice loads.
The load level defined in the ice rules of course need to reflect the long-term ice loads.
But it has been suggested by the model tests (Izumiyama, 2006) and numerical calculations (Valanto, 2007) that in turning operation the ice loads at aft shoulder are pronounced and may be higher than at the bow area. Thus for the aft shoulder, the safety reserve in the ice rules may be lower than elsewhere. Information on the spatial distribution of local ice loads around the hull can thus be used for more reliable design of the ship which is intended to have a good turning ability in ice.
As shown in Fig. 4.16, thirty frames on different hull areas of MT Uikku are selected at 10 m intervals, the frame spacing is assumed to be same and equal to 0.35 m on each hull area. The spatial distribution of local ice loads is then investigated in both straight going and turning operations.
Figs. 4.17 and 4.18 show the simulated time histories of the ice-induced frame loads on two different hull areas when the ship is turning with a 30º steering angle (of the Azipod propulsion system). Since the spatial distribution of local ice loads around the hull is investigated in this simulation, the thickness and strength properties of the ice are thus assumed to be uniform. The frame in Fig. 4.17 is at the bow, the simulated ice loads on bow frames are similar in both straight going and turning operations. As shown in Fig.
4.17, the simulated ice loading process consists of numerous short duration spike-like peaks. It is quite similar to the normal pattern of local ice loads obtained from full-scale trials which are usually performed in straight going mode. The frame in Fig. 4.18 is at aft shoulder, where the ice loading history is quite different from the bow frames. This is because the frame at aft shoulder is close to vertical, accordingly the ice edge in the outside of the turning circle (the starboard side shown in Fig. 4.16) can be continuously crushed by the aft shoulder without bending failure. In this situation, the band-like load peaks may occur, and the mean ice load may increase considerably. It should be noted that the mean ice load may be overestimated, as the buckling or shearing failure of the ice is neglected in this simulation while it occasionally takes place in practice.
4.3 Spatial distribution of local ice loads around the hull
The origins of ice loads on different hull areas are different. When the ship proceeds straight ahead, only the bow waterline is breaking ice. Other areas of the hull encounter hits from broken ice pieces. The midship and aft areas can break ice when the ship is maneuvering (e.g. turning) or going astern. As the ice loads on different hull areas are of different origins, it is clear that the magnitudes of the loads are also different. This is taken into account in most ice rules by dividing the ship hull into regions and giving so called hull area factor for each region.
In FSICR the ship hull is divided into forward, midship and aft regions, among which the forward region (bow) has the highest hull area factor (i.e. 1.0). This is because the dominant operational mode of a ship is moving straight ahead. Accordingly the long-term ice loads at the bow should be considerably higher than the short-long-term ice loads.
The load level defined in the ice rules of course need to reflect the long-term ice loads.
But it has been suggested by the model tests (Izumiyama, 2006) and numerical calculations (Valanto, 2007) that in turning operation the ice loads at aft shoulder are pronounced and may be higher than at the bow area. Thus for the aft shoulder, the safety reserve in the ice rules may be lower than elsewhere. Information on the spatial distribution of local ice loads around the hull can thus be used for more reliable design of the ship which is intended to have a good turning ability in ice.
As shown in Fig. 4.16, thirty frames on different hull areas of MT Uikku are selected at 10 m intervals, the frame spacing is assumed to be same and equal to 0.35 m on each hull area. The spatial distribution of local ice loads is then investigated in both straight going and turning operations.
Figs. 4.17 and 4.18 show the simulated time histories of the ice-induced frame loads on two different hull areas when the ship is turning with a 30º steering angle (of the Azipod propulsion system). Since the spatial distribution of local ice loads around the hull is investigated in this simulation, the thickness and strength properties of the ice are thus assumed to be uniform. The frame in Fig. 4.17 is at the bow, the simulated ice loads on bow frames are similar in both straight going and turning operations. As shown in Fig.
4.17, the simulated ice loading process consists of numerous short duration spike-like peaks. It is quite similar to the normal pattern of local ice loads obtained from full-scale trials which are usually performed in straight going mode. The frame in Fig. 4.18 is at aft shoulder, where the ice loading history is quite different from the bow frames. This is because the frame at aft shoulder is close to vertical, accordingly the ice edge in the outside of the turning circle (the starboard side shown in Fig. 4.16) can be continuously crushed by the aft shoulder without bending failure. In this situation, the band-like load peaks may occur, and the mean ice load may increase considerably. It should be noted that the mean ice load may be overestimated, as the buckling or shearing failure of the ice is neglected in this simulation while it occasionally takes place in practice.
Probabilistic and Spatial Distribution of Local Ice Loads around the Hull 53
4.3 Spatial distribution of local ice loads around the hull
The origins of ice loads on different hull areas are different. When the ship proceeds straight ahead, only the bow waterline is breaking ice. Other areas of the hull encounter hits from broken ice pieces. The midship and aft areas can break ice when the ship is maneuvering (e.g. turning) or going astern. As the ice loads on different hull areas are of different origins, it is clear that the magnitudes of the loads are also different. This is taken into account in most ice rules by dividing the ship hull into regions and giving so called hull area factor for each region.
In FSICR the ship hull is divided into forward, midship and aft regions, among which the forward region (bow) has the highest hull area factor (i.e. 1.0). This is because the dominant operational mode of a ship is moving straight ahead. Accordingly the long-term ice loads at the bow should be considerably higher than the short-long-term ice loads.
The load level defined in the ice rules of course need to reflect the long-term ice loads.
But it has been suggested by the model tests (Izumiyama, 2006) and numerical calculations (Valanto, 2007) that in turning operation the ice loads at aft shoulder are pronounced and may be higher than at the bow area. Thus for the aft shoulder, the safety reserve in the ice rules may be lower than elsewhere. Information on the spatial distribution of local ice loads around the hull can thus be used for more reliable design of the ship which is intended to have a good turning ability in ice.
As shown in Fig. 4.16, thirty frames on different hull areas of MT Uikku are selected at 10 m intervals, the frame spacing is assumed to be same and equal to 0.35 m on each hull area. The spatial distribution of local ice loads is then investigated in both straight going and turning operations.
Figs. 4.17 and 4.18 show the simulated time histories of the ice-induced frame loads on two different hull areas when the ship is turning with a 30º steering angle (of the Azipod propulsion system). Since the spatial distribution of local ice loads around the hull is investigated in this simulation, the thickness and strength properties of the ice are thus assumed to be uniform. The frame in Fig. 4.17 is at the bow, the simulated ice loads on bow frames are similar in both straight going and turning operations. As shown in Fig.
4.17, the simulated ice loading process consists of numerous short duration spike-like peaks. It is quite similar to the normal pattern of local ice loads obtained from full-scale trials which are usually performed in straight going mode. The frame in Fig. 4.18 is at aft shoulder, where the ice loading history is quite different from the bow frames. This is because the frame at aft shoulder is close to vertical, accordingly the ice edge in the outside of the turning circle (the starboard side shown in Fig. 4.16) can be continuously crushed by the aft shoulder without bending failure. In this situation, the band-like load peaks may occur, and the mean ice load may increase considerably. It should be noted that the mean ice load may be overestimated, as the buckling or shearing failure of the ice is neglected in this simulation while it occasionally takes place in practice.
Probabilistic and Spatial Distribution of Local Ice Loads around the Hull 53
4.3 Spatial distribution of local ice loads around the hull
The origins of ice loads on different hull areas are different. When the ship proceeds straight ahead, only the bow waterline is breaking ice. Other areas of the hull encounter hits from broken ice pieces. The midship and aft areas can break ice when the ship is maneuvering (e.g. turning) or going astern. As the ice loads on different hull areas are of different origins, it is clear that the magnitudes of the loads are also different. This is taken into account in most ice rules by dividing the ship hull into regions and giving so called hull area factor for each region.
In FSICR the ship hull is divided into forward, midship and aft regions, among which the forward region (bow) has the highest hull area factor (i.e. 1.0). This is because the dominant operational mode of a ship is moving straight ahead. Accordingly the long-term ice loads at the bow should be considerably higher than the short-long-term ice loads.
The load level defined in the ice rules of course need to reflect the long-term ice loads.
But it has been suggested by the model tests (Izumiyama, 2006) and numerical calculations (Valanto, 2007) that in turning operation the ice loads at aft shoulder are pronounced and may be higher than at the bow area. Thus for the aft shoulder, the safety reserve in the ice rules may be lower than elsewhere. Information on the spatial distribution of local ice loads around the hull can thus be used for more reliable design of the ship which is intended to have a good turning ability in ice.
As shown in Fig. 4.16, thirty frames on different hull areas of MT Uikku are selected at 10 m intervals, the frame spacing is assumed to be same and equal to 0.35 m on each hull area. The spatial distribution of local ice loads is then investigated in both straight going and turning operations.
Figs. 4.17 and 4.18 show the simulated time histories of the ice-induced frame loads on two different hull areas when the ship is turning with a 30º steering angle (of the Azipod propulsion system). Since the spatial distribution of local ice loads around the hull is investigated in this simulation, the thickness and strength properties of the ice are thus assumed to be uniform. The frame in Fig. 4.17 is at the bow, the simulated ice loads on bow frames are similar in both straight going and turning operations. As shown in Fig.
4.17, the simulated ice loading process consists of numerous short duration spike-like peaks. It is quite similar to the normal pattern of local ice loads obtained from full-scale trials which are usually performed in straight going mode. The frame in Fig. 4.18 is at aft shoulder, where the ice loading history is quite different from the bow frames. This is because the frame at aft shoulder is close to vertical, accordingly the ice edge in the outside of the turning circle (the starboard side shown in Fig. 4.16) can be continuously crushed by the aft shoulder without bending failure. In this situation, the band-like load peaks may occur, and the mean ice load may increase considerably. It should be noted that the mean ice load may be overestimated, as the buckling or shearing failure of the ice is neglected in this simulation while it occasionally takes place in practice.
The peak values with a non-exceedance probability of 99% (refer to Izumiyama et al.
(2005)) and the time averages (mean ice loads) are then derived from the ice loading histories. Fig. 4.19 shows the spatial distribution of simulated peak loads on frames around the hull. Three different steering angles (5, 15 and 30 deg) are used for the simulation of turning operations, all of them can result in high loads at aft shoulder, which means the aft shoulder of MT Uikku may be equally or even more vulnerable to ice damage as compared to frames on the bow area, when this ship is turning in a considerably severe ice condition. It is also found that the steering angle has only a slight influence on the magnitude of high peak load at the aft shoulder, while it has a significant influence on the number of frames which are under this high load level. Fig.
4.20 shows the spatial distribution of simulated mean ice loads on frames around the hull. Herein the steering angle has a significant influence on the magnitude of the mean ice load at aft shoulder, and this magnitude can be much higher than that at the bow. It means that in turning operations the aft shoulder of MT Uikku may encounter much heavier turning resistance as compared with the bow area.
Fig. 4.16 Numbering of the frames on different hull areas of MT Uikku
Fig. 4.17 A simulated time history of the local ice loads on frame 13 (uniform ice thickness: 0.34 m, steering angle: 30 deg)
Fig. 4.18 A simulated time history of the local ice loads on frame 5 (uniform ice thickness: 0.34 m, steering angle: 30 deg)
The peak values with a non-exceedance probability of 99% (refer to Izumiyama et al.
(2005)) and the time averages (mean ice loads) are then derived from the ice loading histories. Fig. 4.19 shows the spatial distribution of simulated peak loads on frames around the hull. Three different steering angles (5, 15 and 30 deg) are used for the simulation of turning operations, all of them can result in high loads at aft shoulder, which means the aft shoulder of MT Uikku may be equally or even more vulnerable to ice damage as compared to frames on the bow area, when this ship is turning in a considerably severe ice condition. It is also found that the steering angle has only a slight influence on the magnitude of high peak load at the aft shoulder, while it has a significant influence on the number of frames which are under this high load level. Fig.
4.20 shows the spatial distribution of simulated mean ice loads on frames around the hull. Herein the steering angle has a significant influence on the magnitude of the mean ice load at aft shoulder, and this magnitude can be much higher than that at the bow. It means that in turning operations the aft shoulder of MT Uikku may encounter much heavier turning resistance as compared with the bow area.
Fig. 4.16 Numbering of the frames on different hull areas of MT Uikku
Fig. 4.17 A simulated time history of the local ice loads on frame 13 (uniform ice thickness: 0.34 m, steering angle: 30 deg)
Fig. 4.18 A simulated time history of the local ice loads on frame 5 (uniform ice thickness: 0.34 m, steering angle: 30 deg)
54 Chapter 4
The peak values with a non-exceedance probability of 99% (refer to Izumiyama et al.
(2005)) and the time averages (mean ice loads) are then derived from the ice loading histories. Fig. 4.19 shows the spatial distribution of simulated peak loads on frames around the hull. Three different steering angles (5, 15 and 30 deg) are used for the simulation of turning operations, all of them can result in high loads at aft shoulder, which means the aft shoulder of MT Uikku may be equally or even more vulnerable to ice damage as compared to frames on the bow area, when this ship is turning in a considerably severe ice condition. It is also found that the steering angle has only a slight influence on the magnitude of high peak load at the aft shoulder, while it has a significant influence on the number of frames which are under this high load level. Fig.
4.20 shows the spatial distribution of simulated mean ice loads on frames around the hull. Herein the steering angle has a significant influence on the magnitude of the mean ice load at aft shoulder, and this magnitude can be much higher than that at the bow. It means that in turning operations the aft shoulder of MT Uikku may encounter much heavier turning resistance as compared with the bow area.
Fig. 4.16 Numbering of the frames on different hull areas of MT Uikku
Fig. 4.17 A simulated time history of the local ice loads on frame 13 (uniform ice thickness: 0.34 m, steering angle: 30 deg)
Fig. 4.18 A simulated time history of the local ice loads on frame 5 (uniform ice thickness: 0.34 m, steering angle: 30 deg)
54 Chapter 4
The peak values with a non-exceedance probability of 99% (refer to Izumiyama et al.
(2005)) and the time averages (mean ice loads) are then derived from the ice loading histories. Fig. 4.19 shows the spatial distribution of simulated peak loads on frames around the hull. Three different steering angles (5, 15 and 30 deg) are used for the simulation of turning operations, all of them can result in high loads at aft shoulder, which means the aft shoulder of MT Uikku may be equally or even more vulnerable to
(2005)) and the time averages (mean ice loads) are then derived from the ice loading histories. Fig. 4.19 shows the spatial distribution of simulated peak loads on frames around the hull. Three different steering angles (5, 15 and 30 deg) are used for the simulation of turning operations, all of them can result in high loads at aft shoulder, which means the aft shoulder of MT Uikku may be equally or even more vulnerable to