Section 4 Environmental considerations

4.1 General

4.1.1 The Owner or designer is to specify the environmental criteria for which the riser system is to be approved. The extreme environmental conditions applicable to the location are to be defined, together with all relevant operating environmental limits. Full particulars are to be submitted with sufficient supporting information to demonstrate the validity of the environmental parameters.

4.1.2 The extreme environmental criteria to be taken into account in the riser system design are, in general, to be based on a return period of:

50 years for Mobile Offshore Units.
100 years for Floating Offshore Installations at a Fixed Location.

For sites susceptible to cyclones, the design of riser systems for Floating Offshore Installations at a Fixed Location which remain on-station during cyclones is to be based upon the most onerous of the cyclonic and non-cyclonic environments determined at a return period of 100 years. For locations subject to squalls events, 100-year return period squalls are to be considered, see also Pt 3, Ch 10, 3.4 Environmental parameters 3.4.2

4.2 Environmental factors

4.2.1 The following environmental factors are to be considered in the design of the riser system:

Air and sea temperatures.
Current.
Fouling.
Ice.
Water depth.
Wave.
Wind.

4.2.2 Environmental factors to be accounted for in the design loadings are contained in Pt 4, Ch 3, 4 Structural design loads together with the additional considerations below.

4.3 Waves

4.3.1 When using acceptable wave theories to determine local wave velocities for smooth cylindrical members, appropriate hydrodynamic coefficients should be used. These values should be modified to account for marine growth, for proximity to the sea bed, or structural members on the unit.

4.4 Current

4.4.1 Where a current acts simultaneously with waves, the effect of the current is to be included. The current velocity is to be added vectorially to the wave particle velocity. The resultant velocity is to be used to compute the total force.

4.4.2 In the absence of more detailed information, the distribution of current velocity with depth may be assumed to vary according to the 1/7th power law.

4.5 Vortex shedding

4.5.1 Consideration is to be given to the possibility of vibration of structural members due to von Karman vortex shedding. (This is to apply to wind on exposed risers, and to wave and current on immersed risers).

4.6 Ice

4.6.1 Riser systems intended for operation in ice are to be designed to minimise the effect of ice loading. Proposals are to be submitted for consideration.