The random nature of the ocean, wave conditions, a wide variety of shipping operations, indiscriminate loading conditions, arbitrary shipping routes and decisions by the crew are all contributing factors to the safety of a vessel and its seafarers, writes Dr Jan Jankowski, PRS (Polish Register of Shipping). The case of VLOCs (very large ore carriers), another major factor is probably the impact of high loading rates on structural safety. This can affect the structure in ports, and wave loads can often cause further failure of the structure, leading on occasion to marine casualties.
The traditional approach to ship safety criteria is often such that new safety regulations are triggered by casualties, which can result in the proliferation of regulations.
In reaction to this regulation culture, the IMO (International Maritime Organization) Maritime Safety Committee has developed its Goal Based Standards (GBS), which deal with the goals and functional requirements that define the required safety level for bulk carriers and oil tankers. The role of classification societies within the regime of GBS is to develop rules that transpose the functional requirement into ships.
The classification societies associated in IACS (International Association of Classification Societies) developed such rules — the Common Structural Rules for Bulk Carriers and Oil Tankers (CSR), which significantly increase the required safety level of these ships structures and which will significantly reduce the bulk carrier loses due to the hull structure failure in the future.
The CSR also provide criteria for the allowable thickness diminution of ships’ hull structures which require to:
* assess local and global corrosion during the operational life of ships (the span life of 25 years is the functional requirement of GBS); and
* measure the hull structures thickness which should be assessed against the new-building requirements incorporating corrosion additions.
The assessment is not required during the operational life of ships, provided that the measured thickness of any structural members remain greater than the renewal thickness specified in the Rules.
This approach, determined by CSR, improves the safety of dry bulk carriers during their operation due to the appropriate regime imposed on the vessels maintenance.
Although current rules provide a high level of safety for hull structures, there are still some issues that need to be improved. The most important is the probability distribution of sea state occurrence in the North Atlantic (the North Atlantic wave environment is the functional requirement of GBS). The distribution was develop by IACS for the northernmost zones of North Atlantic through:
* fitting the theoretical distributions to the average data of zones 8, 9, 15 and 16 of Global Wave Statistics (British Marine Technology. 1986), in the first step; and then
* the use of the fitted distribution to determine the probability distribution of sea state occurrence (presented in ‘IACS Rec. 34, Standard wave data’).
The recommendation of International Ship Structures Congress (Lyngby, 1988) that the observed, measured or hindcasted wave data should be smoothed and extrapolated to more extreme, less frequently occurring conditions, was incorporated into the fitting the theoretical distributions.
The Global Wave Statistics wave data is based on visual observations of waves collected from ships in normal service; however, the “eye concentrates on the near, steeper waves, while the visually observed wave periods tend to be shorter than instrumentally observed periods” (WMO Guide to Wave Analysis and Forecasting).
The visual observations of waves and their extrapolation causes that long term predictions of vertical bending moments are greater by about 15% than required by “IACS Unified Requirement S11 Longitudinal Strength Standard” which well assures longitudinal strength of hull girder.
This conclusion implies that Rec 34 should be updated to reflect the reality. However, there are problems how to do that because satellite data is not fully accepted due to lack of knowledge about their accuracy. The wave hindcast data generated by the wave spectral model are global today, but there are large discrepancies in prediction offered by different databases, the measured wave data limited to the coastal areas etc.
The wave hindcast data generated by the wave spectral model seems the only way to solve the problem of wave data. However, to determine the nonlinear energy transfer through the wave spectrum it is necessary to solve strongly non-linear wave problem derived by prof. Zakharov in 1968.
Simplifying assumptions introduced allowed for better understanding of the physics at the cost of a strict mathematical solution. Despite the simplifications introduced, the evaluation of the nonlinear energy transfer, caused by four resonantly interacting waves, requires an enormous amount of computation of integrals in six-dimensional space. Therefore, the proper solution of this problem still is a challenge for the industry.
PRS collaborates with professors of mathematics from Gdansk University in solving the problem in strict mathematical manner. At this stage, PRS is testing the solution of the problem for first, second, third,…, order of nonlinearities.