3.5.1 All involved in the conduct of the risk assessment should be aware that Flight Manuals include speed, wind and crosswind limitations for hoisting/winching operations which must be observed.
This is critically important because the combination of a crosswind component and excessive roll rate could take a specific helicopter out of safe operating limits. Because BW operations are restricted in the direction and orientation of the helicopter to the hoisting/winching site (see Figure 9), the ship's course, speed and the relative wind become critical factors in the ability to operate safely - i.e. there will be some conditions which will render hoist operations unsafe unless the ship is orientated in a certain direction with regard to the wind.
Control: The ship should be aware that in some wind conditions, operations to a specific site might not be possible. This might necessitate a change of course to bring the site back into operational use.
3.5.2 One risk resulting from excessive heave or roll is that the helicopter could strike an obstacle, either below or to the starboard side of the helicopter. Another is that the marine pilot could sustain an injury during lowering or lifting if he/she misses the reduced hoisting/winching area, - i.e. the width of the BW. These risks should be minimised/mitigated by provision of, and adherence to, the movement limits discussed below.
3.5.3 Due to the critical nature of movement at the BW site, it is important that accurate measurement of the ship’s movement is carried out. A package of accelerometers with associated hardware/
software is used on most mobile installations/ships operating in the offshore oil industry. Such equipment can provide not only real time output but an averaged figure over an operational window so that the probability of completing the task within the window can be predicted. (See Section 4.2.3 of the main guide on wave motion effects.)
3.5.4 Angular movement of the ship has to be converted to linear displacement at the BW level in order to assess whether obstacles will penetrate the helicopter manoeuvring zone. As the manoeuvring zone is dependent upon the size of the helicopter, it will not be possible to set absolute limits of operability for the ship, only for the helicopter. It is important not only that critical obstacles are identified on the ship’s template, but also the upper height limit on the BW is stated in order that linear displacement of these critical obstacles can be mapped and limits established (see a partially annotated diagram with penetration limits marked for the EC155 in Figure 9).
Control: Ships’ operational templates should contain an annotated diagram indicating significant obstacles and their distance from the centre of the hoisting/winching clear zone; this could be achieved by having a series of concentric circles around the centre of the zone. The height of the BW should be indicated.
3.5.5 Serious consideration should be given to the fitting of an electronic means of ascertaining pitch/roll/heave motions at the BW location (see Section 4.2.3 of the main guide).
Control: Ships which conduct BW operations should give serious consideration to the fitting of electronic motion recording devices.
3.5.6 Heave Limits: there are two risks that result from excessive heave, (a) striking obstacles below the helicopter, the BW itself or any obstacle of a height up to 3 m in the manoeuvring area and (b) injury to the marine pilot if the deck rises while he/she is being lowered. Because of the reduced size of the hoisting/winching site and the reduction of visual cues due to the height of the BW above the deck, it is recommended that the operating height be based upon the visual environment and the width of the clear zone. In a reduced visual cue environment and narrow clear zone, this could result in an operating height of 4 m; where there are good visual cues and a 5 m wide clear zone, the operating height might be increased to 8 - 10 m. The limit of heave should be based upon a vertical clearance from obstacles of 2 m, at the top of the heave cycle, and a heave rate that would not result in injury to the marine pilot during the lift.
Control: The operational height should be based upon the visual cue environment and the width of the clear zone. The heave limit should be based upon the operational height, a vertical clearance from obstacles of 2 m at the top of the heave cycle, and a heave rate that would not result in injury to the marine pilot.
3.5.7 Roll Limits: as with heave, there are two risks: obstacles moving towards the helicopter; and injury to the marine pilot caused by lowering outboard of the bridge wing. The limit of operation should be the smaller of either the angle of roll, converted to a linear distance at the BW that preserves the margin of 0.5 RD (rotor diameter) between the rotor disc and the closest obstacle (thus meeting the intent of clearance from obstacles of 2 D in the manoeuvring zone) or the angle of roll which permits the marine pilot to be landed within the extent of the clear zone.
Note: There will be an optimum lateral position for hoisting/winching which should reflect the balance of the amount of space in the clear zone and the distance to the nearest significant obstacle. In Figure 9, the clear zone is established at the end of the BW.
While this does maximise the distance to the nearest significant obstacle (about 8 m to the edge of the no penetration manoeuvring zone - permitting a roll of 10º to port with the EC155 - shown for demonstration purposes), a roll of 10º to starboard would position the marine pilot 6 m off the end of the BW. Such considerations should be taken into account when the operator establishes the limit of operation for the specific
ship/helicopter combination.
Control: The roll limit should be the smaller of either the angle that preserves a margin of 0.5 RD between the nearest significant obstacles and the helicopter rotor disc, or the angle that permits the marine pilot to be landed within the extent of the clear zone.
Control: If an accurate electronic motion recording device is not fitted, the basis of measurement of the ship’s movement should be stated on the ship’s template. If accurate measurement is not considered possible, a reduction in the roll limit, based on the operator’s assessment of the risk, should be applied.
3.5.8 Pitch Limits: one risk associated with pitch is injury to the marine pilot. Because of this, the limit of pitch should be 2.5 x the width of the hoisting/winching site (BW) or 5 m - whichever is the lesser (because of the longer lever arm, it is likely that the pilot will be able to follow any fore and aft movement of the BW, the so called “flying the hook”).
Control: The pitch limit should be the smaller of either the angle that produces a linear distance of no more than 2.5 x the width of the hoisting/winching site, or 5 m. (See Section 4.2.3 of the main guide).
Figure 9 - Partially annotated diagram (using an EC155 scaled model)