Section 2 Environmental criteria - Forces and motions
Clasification Society 2024 - Version 9.40
Clasifications Register Rules and Regulations - Rules and Regulations for the Classification of Ships, July 2022 - Part 7 Other Ship Types and Systems - Chapter 8 Positional Mooring and Thruster-Assisted Positional Mooring Systems - Section 2 Environmental criteria - Forces and motions

Section 2 Environmental criteria - Forces and motions

Parent topic: Chapter 8 Positional Mooring and Thruster-Assisted Positional Mooring Systems

2.1 Limiting environmental criteria

2.1.1 Limiting criteria in the form of maximum operating and survival environmental conditions are to be specified by the Owner or Designer.

2.1.2 The limiting criteria are to be defined in terms of wind and current speeds, wave heights and periods, and water depth range.

2.1.3 As water depth will have a large influence on a ship's mooring capability, the limiting environmental criteria may be varied according to water depth.

2.1.4  Maximum operating conditions will be those in which the ship is able to carry out its primary operational activities, while anchor line tensions remain within designated operating limits. See Pt 7, Ch 8, 3.2 Design cases and factors of safety for required factors of safety in operating conditions.

2.1.5  Survival conditions are to be based on an average recurrence period of not less than 50 years. The mooring system is to be such that maximum line tensions will be limited in these conditions to designated survival levels. See Pt 7, Ch 8, 3.2 Design cases and factors of safety for required factors of safety.

2.2 Design environmental criteria

2.2.1  Wind. The design wind speed for wind force determination can normally be taken as the one-hour mean value referenced to 10 m above sea level. Account is to be taken of the variation of wind speed with height above sea level. The wind velocity gradient can be calculated from the following:

V H = V 10 0,125
where
V H = wind velocity at H height above sea level
V 10 = 1 hour mean wind speed referenced to 10 m above sea level.

2.2.2  Current. The design current speed is to be taken as the sum of wind-induced and tidal current velocities. For calculation purposes, tidal current velocity can be assumed constant over water depth, and wind-induced current velocity can be taken to reduce linearly from its maximum value at the surface to zero at 50 m below sea level.

2.2.3  Wave. The significant wave height and period range is to be defined for each relevant design case.

2.3 Environmental forces

2.3.1 In determining environmental forces, it is to be assumed that the defined limiting environment of wind, waves and current will act concurrently. For fixed azimuth mooring systems, these forces are considered to act in the same direction.

2.3.2 Environmental loading on the ship (and the corresponding catenary system motions analysis to determine anchor loads and line tensions, etc.) will require to be investigated for a sufficient number of directions to establish the critical cases.

2.3.3 It is generally to be assumed that the maximum specified environmental conditions can come from any direction relative to the ship's heading. However, in cases where a ship is to be restricted to specific defined locations, consideration will be given to the acceptance of an environmental rosette (allowing the ship to be headed in the most favourable direction with respect to mooring loads).

2.3.4 Where quasi-static methods of analysis are adopted (see Pt 7, Ch 8, 3.1 General 3.1.2), at least the wind, current, and mean wave drift forces acting on the ship in the various relevant design conditions are to be calculated or determined. In addition, any significant yawing moments induced by these effects are to be taken into account when carrying out the mooring system motions analysis.

2.3.5 Environmental forces and moments can be determined by suitable methods of direct calculation or by model testing. In either case, account must be taken of all significant load generating structural elements or equipment. In the case of wind force and moment determination, all deck structures, fittings, cranes, towers, superstructures, etc. are to be considered, and for current force, account is to be taken of thrusters, nozzles, propellers, etc.

2.3.6 First order wave motions − the oscillatory motions of the ship − are to be determined. Surge and sway are the most relevant motions in terms of quasi-static mooring analysis, see Pt 7, Ch 8, 3.1 General 3.1.2 and Figure 8.3.1 Quasi-static analysis.

2.3.7 The first order wave motions of the ship are to be determined from appropriate wave spectra, either by use of tank testing or by suitable direct calculation methods.

2.3.8 Account is to be taken of the effects of shallow water on the ship's first order wave motions.

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