Trans RINA, Vol 157, Part C1, Intl J Marine Design, Jan –Dec 2015
4 1
.1 WFSV LIFTING P ROCEDURE .
2.
The sea height should be checked and carefullly monitored to see it does not exceed 2.5m Hs. This is the design limit of the system. Once the operation has been given the go ahead by the party chief the transfer vessel drives onto the rear bollard structure at up to 100% throttle to restrict the relative motion before the lifting line is connected. At 100% throttle the bollard structure maintains a safety factor of 2.5. To help stabilise
the motion of the WFSV
during lifting lines are fixed to either side of the bow and attached to low capacity winches within the A-frame. The lifting line is connected using a robotic mounted at the top of th Ae A- frame.
3. 4.
The WFSV is lifted clear of the water by a winch
located fully longitudinally between on. the
WFSV storage frames, with control of the liftiing lines on the bow used to stabilise vessel motio The A-frame is retracted over the deck where fendered v ertically mounted bollards protect the other vessel from damage due to the n of the WFSV in high sea states. The 12 deck mounted bollards have been designed to withstand the load of the WFSV at 17deg roll angle with a design factor of 4.8. The bollards allow the transfer vessel to move freely in the fore/a
motion aft fenders. 5.
Once set down on the deck the lifting operation is complete and the lifting line can be detached in preparation for lifting another WFSV on the other side of the OSV . The lifting point on the A-frame can be moved along the crossbeam using a gantry trol ley system, shown in Figure 30. The motion will be powered by an electtrric motor driving a rack and pinion gearing system. The motion will be started manually, limit switches will stop the motion where the gantry is to be locked in place using solenoid actuated pins.
6 .
The winch is also fitted to trolley system on the deck where it is translated transversely to aliign with the lifting position of the next WFSV. This also enables bothWFSVs on the same side to be moved as longitudinally the winch sits between them.
7 .
To launch the second vessel on each side the boom crane lifts the empty storage frame over the WFSV as it moves on the motorised storage frame towards the A-frame loading position at the transom.
direction using free rotating rubber
Figure 30: Pulleywith linear bearing 4.2
FEA OFA-FRAME
The load cases were defined in accordance with Lloyd’s Register Code for Lifting Appliances in a Marine Environment. Through exa
amining the
conditions the relevant design factors were spreadsheet was created to layout the vessel and to calculate the various loa
ad cases. The operating
specified. A information two loading
scenarios examined were a 71.85tonne live load in 2.5mHs and a 91.85tonne load in 1.5mH s. The first represents the unloaded WFSV, while the second represents the WFSV loaded wit th a wind turbine gearbox weighing 30tonne. The combination of heel and pitch accelerations caused by the respective wave motions were calculated by implementing the Lloyd’s calculation tables into the excel sheet. This gave load cases normal to the deck and parallel transverse/longitudinal to the deck for maximum heel, maximum pitch and maximum combined vessel motion cases. These are summarised in table 8, where
the static component associated pitch and roll angles h loads for the have been combined with the dynamic loads from the pitch and roll accelerations.
Combination cases (kN) Normal to deck
Parallel
Rolling motion Pitching motion
Combined motion
Table 8: Load cases
To simulate the load of the loaded sheave axle in FEA, these loads were implemented a
as bearing loads applied to
the sheave block. The results from the analysis are shown in Figure 31 and Figure 32 for the 71.85tonne load in 2.5mHs and the 91.85 tonne load in 1.5mHs respectively. Where the safety fa
actors achieved are 2.24 and 1.8.
Mainprize 71.85tonnes 2.5mHs Heel = 6.53deg, trim = 3.26deg A-frame angle = 45deg
932
1587 1607
transverse 613
0 526 Parallel
Longitudinal 0
93 162
C-94
© 2015: The Royal Instittu
ution of Naval Architects
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