Waves and current
Wind, wave and current data are often referred to as ‘met ocean data’. This can be based on:
- historic and observed data from wave rider buoys;
- observation from ships and platforms;
- calculated based on wind data; or,
- from current measuring devices.
Typically, it is measured over 3 to 6 months, but preferably for many years.
Design waves and currents are based on the statistical probability of a wave or current occurring once in each period. The period, referred to as a ‘return period’ and can be 1 year, 10 years, 50 years, or 100 years. Return periods are typically based on:
- three times the design life for the operational case; or,
- three times the installation period for the installation case.
In some regions, extreme events are due to irregular events such as loop currents, solitons and typhoons/hurricanes.
Waves and currents differ in magnitude depending on direction. Assuming waves and currents are at 90° to the pipeline is very conservative. A more accurate method is to use directional data from a rosette.
Wave theory is used to derive water velocities and accelerations close to the pipe on the seabed. These in turn are used to derive forces and to analyse the stability of the pipeline on the seabed. Available way theories are:
- Stokes 5th;
- Cnoidal; and,
The applicability of each wave theory is determined using a map which considers the height, period of the wave, and the water depth.
Currents are typically measured at the sea surface. Methods for extrapolating these close to the pipeline at the seabed are 1/7 power law and logarithmic profile. The logarithmic profile method takes account of seabed roughness.