Background material and considerations of sea-state limitations for helicopter landing and take-off on passenger ships and ferries are undertaken.

Transcription

1 1 Annex 7 Weather Conditions and Helicopter Landing Background material and considerations of sea-state limitations for helicopter landing and take-off on passenger ships and ferries are undertaken. 1 Sea-state Limitations The availability of ships decks for landing is reduced by wave induced motions among other factors. As an indication of magnitude one has considered a special case with characteristics: Ship type: Ferry Ship length: 158 m Greatest beam: 24 m Ship speed: 0 knots Helipad location: Midship or 70 m fore or aft Landing criterion: Max. short-term vertical motion amplitude 1.0 m/s Nautic area: World wide trade Short term response for heave and pitch are shown in Figure 1 and 2 respectively. Values are given in terms of significant response per unit significant wave height (4rms/Hs). Short term (1-3 hours) extreme amplitude is set equal to the significant response which gives somewhat conservative results. The sea-state limitations are sensitive to the helipad location. By location midship, the limiting vertical velocity is mainly set by the heave motion. By location far forward or far aft, the limiting vertical velocity is mainly set by the pitch motion. (Heave and pitch are considered 90 degrees out of phase.) Calculations are carried out in Table 1. Table1: Sea-state limitations by Max Vertical Velocity 1.0 m/s Period Midship: Fore/Aft: Tz 4rms/Hs Hs-max (m) 4rms/Hs Hs-max (m) The limiting sea-states are indicated on the scatter diagram for world wide trade from [1]. Midship location is shown in Figure 3 and Fore/Aft location is shown in Figure 4 respectively.

2 2 Counting the sea-states where helicopter landing is permissible, indicates that availability is: Helipad midship: 87% Helipad fore or aft: 71% The values are somewhat conservative because the limiting vertical amplitude is taken as the short-term extreme value. The values yield this particular ferry in the world trade sea-states. Bigger ships are expected to offer greater availability. Reducing the nautic areas to the typical cruise locations will offer still higher availability. 2 Cruise Areas With references to the Marsden chart, Figure 5, typical trading routes are indicated by solid lines. More details may be found in [2]. The typical cruise areas are indicated by shaded areas. Characteristic wave and wind data are given in the table below. Table 2: Characteristic wave and wind data in typical cruise areas Marsden Weibull wave data Wind data Area code j B Hs,20 V10hrs Gust Caribbean NE S Mediterranean W E North Sea Australia W S Tas NZ Spitsbergen S Alaska coast ocean S Long-term Weibull parameters j and B are taken from Classification Note 30.5 [3]. 20-years extreme Hs is calculated for 3 hours time intervals. Since wind statistics have not been found for all areas, winds are calculated from sea-state by the formula: H s g V = 0.18 This formula yields fully developed seas, and is associated with 10 hours mean wind. Transition to 3 seconds gust is done by the formula: 3 V ( 3s gust) = V (10hours mean) ( ln ) = 1.44 V (10hours mean)

3 3 All data yield all seasons. Data for Spitsbergen and Alaska are thus not representative because cruise traffic is most relevant in summer. 3 - References [1] Cramer, E. et al: Fatigue Assessment of Ship Structures. Det Norske Veritas, Report No Revision 6, 1 st September [2] Fearnleys, ISSN [3] Environmental Conditions and Environmental Loads. DNV Classification Note No March Figure 1 Short term response for heave

4 4 Figure 2 Short term response for pitch Figure 3 Scatter diagram for world wide trade with permissible sea states for helideck midship. Availability is 87%.

5 5 Figure 4 Permissible sea-states for landing on helideck. Availability is 71%. Figure 5 The world main trade lines and cruise areas.