Section 2 Wave environment

2.1.1 Generally ships of Naval Ship Groups 1 and 2, NS1 and NS2, see Vol 1, Pt 1, Ch 2, 2.1 Applicable ship types will be designed for unrestricted world-wide operation. Ships in group 3, NS3, may also be designed for world-wide operation but typically will be designed for more specific roles within clearly defined areas of operation, e.g. coastal patrol craft, landing craft, harbour vessels, tugs, etc.

2.1.2 The procedure for deriving the design factors and the wave environment used for the assessment of local and global loads is illustrated in Figure 2.2.1 Procedure for the specification of environmental conditions.

2.1.3 The following definitions are applicable:

Service area

1. A service area refers to a collective group of sea areas. The service area specifies the limits of the ship’s operational area.

Sea area

1. A sea area is small area of the world’s oceans for which statistical wave data has been collected, the sea areas are shown in Figure 2.2.2 Sea areas.

2.2.1 All ships classed under the Rules will be assigned a service area notation SA followed by a number or letter, e.g. SA1.

2.2.2 The service area notations listed below are available. The definitive extents of the service areas are shown in Figure 2.2.2 Sea areas and Table 2.2.3 Environmental wave data for individual sea area. The chart shows the minimum service area requirement for operating in different areas of the world.

2.2.3 For all ships that are designed for specific areas of operation, the designer may take advantage of reduced wave loadings that are likely to be encountered. This covers all ships which are assigned a service area notation SA2, SA3, SA4 or SAR.

2.2.4 For all cases where a SAR service area notation (Service Area Restricted) is required, the extents of the restricted area will be specified after the SAR service area notation. The service area factor, f _s, and the wave environment characteristics for the ship will be specially considered. Where the geographical limits of the intended service can be satisfied by a single or group of contiguous sea areas then the service area factor, f _s, and the wave environment characteristics may be derived using the methods given in Vol 1, Pt 5, Ch 2, 2.4 Service area factors and Vol 1, Pt 5, Ch 2, 2.5 Derivation of wave statistics for a combination of sea areas.

2.2.5 Under normal circumstances, a ship which is assigned a service area notation SA2, SA3, SA4 or SAR is to operate in solely the designated area and is not transit to other areas of the world, see Vol 1, Pt 1, Ch 2, 3.5 Ship type notations. Due allowance is to be made for the ship’s trials, work-up period and delivery voyages in the assignment of a service area notation. However special consideration may be appropriate to these periods in order to ensure that the ship is not subjected to dynamic loads which might impair the structural working life of the ship.

2.2.6 The wave environmental data for service areas SA1, SA2, SA3 and SA4 are specified in Vol 1, Pt 5, Ch 2, 2.3 Wave environment.

2.2.7 It is the responsibility of the Owner to determine that the chosen Service Area, the service area factor, f _s, and the wave environment characteristics as defined in Vol 1, Pt 5, Ch 2, 2.3 Wave environment are appropriate for the intended areas of operation.

2.2.8 The allocation of a service area notation to a ship does not remove the responsibility of the Master or commanding officer to take suitable measures to avoid typhoon, hurricane and other extreme weather conditions, as appropriate.

2.2.9 The requirements for ships which are required to maintain station or operate in typhoon, hurricane and other extreme weather conditions will be specially considered.

2.3.1 The environmental wave data for each service area notation is given in Table 2.2.1 Environmental wave data for each service area Table 2.2.4 Environmental wave data for each sea area gives the environmental wave data for each individual sea area.

2.3.2 The definitions of wave height, wave period and wave period range given below are to be used in the determination of the environmental loads acting on the ship.

2.3.3 Wave environmental design criteria for normal design assessment. These design parameters have been used to derive the standard local and global environmental loadings in Vol 1, Pt 5, Ch 3 Local Design Loads and Vol 1, Pt 5, Ch 4 Global Design Loads. All direct calculations and model tests required to supplement these loads are to use these environmental loadings.

Design wave height, H _dw

1. Average of the one hundredth highest observed wave heights for the service area. To be taken as:

   H _dw = 1,67H _s m

Design wave period, T _dw

1. Average zero crossing period of all sea states in the service area

   T _dw = T _z seconds

Standard deviation of wave period, T _dsd

1. Standard deviation of the zero crossing periods for the service area
   T _dsd = T _sd seconds

Design wave period range, T _drange

1. To be taken as the design wave period plus and minus 2 standard deviations of the zero crossing period, i.e.

   T _drange is T _dw – 2T _dsd to T _dw + 2T _dsd seconds

H _s, T _z and T _sd are given in Vol 1, Pt 5, Ch 2, 2.3 Wave environment 2.3.6.

2.3.4 Wave environmental design criteria for extreme design analysis, ESA notations. These design parameters have been used to derive the global environmental loadings in Vol 1, Pt 5, Ch 4 Global Design Loads which are used for the extreme strength assessment notation. All direct calculations and model tests required to supplement these loads are to use these environmental loadings.

Extreme wave height, H _xw

1. To be taken as the significant wave height that has a probability of 5 x 10^-5 of being exceeded.

   H _xw = H _x m

Extreme wave period, T _xw

1. To be taken as the design wave period plus one standard deviation

   T _xw = T _dw + T _dsd seconds

Extreme wave period range, T _xrange

1. To be taken as the extreme design wave period plus and minus 1,5 standard deviations

   T _xrange is T _xw – 1,5T _dsd to T _xw + 1,5T _dsd seconds

Duration of extreme storm

1. It is to be assumed that extreme storm events are to persist for three hours.

T _sd and H _x are given in Vol 1, Pt 5, Ch 2, 2.3 Wave environment 2.3.6

T _dw and T _dsd is given in Vol 1, Pt 5, Ch 2, 2.3 Wave environment 2.3.3.

2.3.5 Wave environmental design criteria for residual strength analysis, RSA notations. These design parameters have been used to derive the global and local environmental loadings in Vol 1, Pt 5, Ch 3 Local Design Loads and Vol 1, Pt 5, Ch 4 Global Design Loads which are used for the residual strength assessment notation. All direct calculations and model tests required to supplement these loads are to use these environmental loadings.

Residual wave height, H _rw

1. To be taken as the significant wave height that has a 20 per cent probability of being exceeded for the service area
   H _rw = 0,90H _s m

Residual wave period, T _rw

1. To be taken as the standard design wave period

   T _dw = T _z seconds

Residual wave period range, T _rrange

1. To be taken as the standard design wave period range

   T _rrange = T _drange seconds

Duration of sea state

1. It is to be assumed that the duration of sea states of this magnitude is 12 hours.

H _s and T _z are given in Vol 1, Pt 5, Ch 2, 2.3 Wave environment 2.3.6

T _drange is given in Vol 1, Pt 5, Ch 2, 2.3 Wave environment 2.3.3.

2.3.6 The parameters used in above equations are given in Table 2.2.1 Environmental wave data for each service area for service area notations SA1, SA2, SA3 and SA4. For the restricted service area notation SAR, the values of H _s, T _z, T _sd and H _x will need to be derived, see Vol 1, Pt 5, Ch 2, 2.5 Derivation of wave statistics for a combination of sea areas.

2.4.1 The service area factor, f_s, is used to derive the global hull girder loads in Vol 1, Pt 5, Ch 4 Global Design Loads. The service area factor applicable for Service Area notations SA1, SA2, SA3 and SA4 is given by:

where

1. f_s is to have a minimum value of 0,5 and normally a maximum value of 1,0 and is to be rounded up to the nearest 0,05

   f_sl is the service life factor as defined in Vol 1, Pt 5, Ch 2, 2.4 Service area factors 2.4.2

   f₁ and f ₂ are given in Table 2.2.2 Environmental wave data for service area

   L_WL is defined in Vol 1, Pt 3, Ch 1, 5.2 Principal particulars.

2.4.2 The service life factor, f_sl, is based on a ship’s operational life. This factor may need to be increased for service lives which are predicted to require significantly more wave encounters. The service life factor is to be taken as follows:

2.4.3 For unrestricted sea-going service, i.e. service area notation SA1, the service area factor is to be taken as 1,0fsl.

2.4.4 Alternatively the service area factor may be derived by direct calculation of the long term hull girder loads, see Vol 1, Pt 5, Ch 4, 1.3 Direct calculation procedures 1.3.1

2.4.5 For restricted Service Area Notation SAR the service area factor is to be derived by combining the service areas factors for each sea area using the following formula:

where

f _1i and f_2i are the individual sea area factors given in Table 2.2.3 Environmental wave data for individual sea area

ln ( ) is the natural log

P_i is defined in Vol 1, Pt 5, Ch 2, 2.5 Derivation of wave statistics for a combination of sea areas 2.5.3

2.5.1 For the SAR restricted service area notation, it is necessary to derive the environmental wave data for the required area of operation. This data may be determined using statistical methods as specified in Vol 1, Pt 5, Ch 2, 2.6 Direct calculations or using the information given in this Section.

2.5.2 The following formulae may be used to derive the design wave statistics for a ship designed to operate with a particular service area restriction. These formulae enable the environmental wave data from a set of individual sea areas to be combined to give the appropriate design wave statistics for the SAR restricted service area notation, see also Vol 1, Pt 5, Ch 2, 2.2 Service areas.

2.5.3 The environmental wave data for each sea area is given in Table 2.2.4 Environmental wave data for each sea area The extents of each sea area are shown in Figure 2.2.2 Sea areas

Wave height, H _s

Design wave period, T _dw

Standard deviation of wave period, T _sd

Extreme wave height, H _x

2.5.4 The designer/Builder is to supply full details of the SAR notation required together with full supporting calculations. All transit voyages and ship work-up/trial periods should be included in the list of sea areas if their inclusion results in a more severe wave environment, see also Vol 1, Pt 5, Ch 2, 2.2 Service areas 2.2.5

2.6.1 The wave environmental parameters may be derived by direct calculation using long term statistical wave data for the selected area(s) of operation based on hindcast data or similar analysis. This data is to represent accurately the sea environment in the intended area of operation over a long period and enable sound long term and extreme short term statistics to be derived. Depending on the area of operation, due account is to be taken of typhoon, hurricane and other seasonal extremes, see also Vol 1, Pt 5, Ch 2, 2.2 Service areas 2.2.8

2.6.2 The derivation of the wave environmental parameters is to be in accordance with the data specification and methods given in Vol 1, Pt 5, Ch 2, 2.3 Wave environment. The areas used for the direct calculation will then be specified after the SAR service area restriction notation.