Modern Windships; Phase 2

Appendix 4. Wind speed calculations

Survival condition
Full sail condition

The calculations leading to the design wind speed are further described below.

Survival condition

The first case to be considered is case 3 from above, the survival case.

From ref.[4], DNV MOU, we have that the 10 min average wind need not be taken greater than 55 m/s. This is the assumed extreme wind speed at 10 m height above sea-level.

From ref.[13], DS 410 and ref.[10], SBI 158, we get the following terrain parameters for open sea:

z₀=0.01[m]
k_t=0.17
z_min = 2

Adjusting for the wind shearing at 45 m above sea-level we get:

This means that with 55 m/s at 10 m above sea-level we have:

m/s at sea level.

Which in turn gives the 10 min average wind speed near the mast-top as:

m/s.

In order to calculate the gust speed a simplified approach according to ref.[6], DNV no. 30.7, was used. Here one simply multiplies the 10 min average wind speed at 10m above sea-level with a factor (from table 2.1). The factor was calculated by DNV using a Weibull distribution. This gives a 3 sec. gust of:

m/s at 50 m above sea-level.

Full sail condition

Here the 10 min average wind at 10 m height was chosen to 25 m/s. Repeating the calculations from above we thereby have:

m/s at sea level.

This in turn gives the wind speed at 45 m height as:

m/s.

An average wind speed of 29.35 m/s was used for the basis speed when calculating loads affecting the mast and sandwich flaps. This value is based on averaging the wind speeds at 21.6, 32.3, 43.0 and 53.7 metres above sea-level.

The gust speed a 50 m height is:

m/s.

The gust speed at 10 m height is:

m/s.