IACS: wave load, fatigue, and seakeeping calculations

A new scatter diagram and wave spectra defining design environmental conditions will be used for wave load, fatigue, and seakeeping calculations, the International Association of Classification Societies (IACS) said in its press release.

As part of IACS’ ongoing commitment to safe ships and clean seas, a long-term review of wave data has now concluded with the publication of a revised version (Rev.2) of IACS Recommendation No. 34 which provides advice on sea states as well as wave spectrum, spreading, heading distribution, and vessel speed.

Accurate Wave data remains of paramount importance as this data is used to represent the ocean environment, underpinning wave load prescription, which in turn, greatly impacts hull structural requirements.

IACS Recommendation No. 34 describes wave statistics intended for the design of sea-going ships above 90 meters including the effect of bad weather avoidance.

It is based on North Atlantic trade, which represents the most severe conditions ships tend to operate in.

Following indications that the representation of North Atlantic waves in the existing IACS Recommendation No. 34 may have become outdated, IACS began work in 2016 on a long-term review of wave data tasked with investigating if and how Recommendation No. 34 could be improved using more recent data sources, with modern data showing both an increase in mean significant wave height for the North Atlantic and that more extreme weather is being experienced in recent years, including the existence of rogue waves and the possible effects of climate change.

Several sources of wave data, including altimetry (measurements from satellite), hindcast model (re-analysis of past weather), and wave buoys were used to derive the scatter diagram from a combination of vessel tracks and hindcast wave data.

These new, more modern, data sources represent a significant improvement in the quality of data, given that previous wave data were collected in the second half of the 20th century from visual observations on board ships.

The IACS Project team Global hindcast datasets are built on global coverage over an extended period of time and have been analysed and validated through the use of measurements from buoys and altimeters.

By taking into consideration publicly available AIS ship position data, this allowed the wave data to be mapped to actual ship position & time when generating the corresponding statistics whose analysis showed that bad weather avoidance had a significant impact on the wave statistics of the sea states encountered.

The resulting updated simple scatter diagram, using validated datasets of wave data and ship positions will facilitate a more accurate estimation of design loads such as pressures, motions, accelerations, hull girder loads, all contributing to the improved standardisation of safety levels of the fleet.

This data will be used by individual Classification Societies when reviewing their current rule requirements including, by IACS Members, for Common Structural Rules.

This significant new data source will also be of value to other industry stakeholders who use wave data for individual projects.

Commenting on the publication of this latest recommendation, IACS Secretary General, Robert Ashdown, said:

"This extensive, long-term project to update and improve one of the key data sources that go into modern ship design is indicative of IACS’ ongoing commitment to safer shipping and provides a valuable tool for all other stakeholders who rely on accurate wave data in their work."

LINK TO THE ORIGINAL ARTICLE