Wave-induced hull vibration for the assessment of hull girder collapse characteristics. DNV GL has developed a computational approach that takes into account the individual hull shape and propulsion power and is seamlessly embedded in the structural design process.
It is well known that hull girder vibration induced by bow slamming impacts, so called whipping, may considerably increase the still-water and wave-bending moments acting on the hull girder. This is most critical for hogging hull deflection patterns when the double bottom is exposed to high compressive stresses. If these exceed the buckling capacity of some shell or double bottom plating, this may trigger the progressive collapse of larger structures and, ultimately, of the whole hull.
Conventionally, the amplification of structural stresses due to whipping is covered by implicit safety margins in the design rules. However, this has become questionable in view of the rapid development of new container ship designs. Many ship owners are concerned about whether their ships are strong enough to withstand loads associated with severe and violent sea conditions. Large and ultra-large container carriers are the focus of these concerns because of their exposure to very high slamming loads.
Analysis concept
DNV GL developed the computational approach under the premise that the deadlines for the submission of the final key structural drawings for plan approval and steel ordering must not be delayed by the extensive required computational times. This was accomplished by computing the extreme whipping loads and ultimate hull girder capacity in parallel to conducting the strength analyses typical for large container vessels.
Whipping involves highly nonlinear effects and thus requires the application of high-fidelity numerical methods. DNV GL uses CFD, based on the numerical solution of the Reynolds-Averaged Navier-Stokes (RANS) equations, combined with the simultaneous calculation of ship motions and deflections. This is the most accurate numerical method available today, and allows the explicit definition of hydrodynamic loads without the need for ad-hoc models or additional safety margins.
Loads due to slamming and consecutive whipping strongly depend on the forward speed of a ship. Assuming a constant speed is unrealistic, particularly for whipping loads: it is excessively conservative in severe sea conditions (where the ship cannot maintain such a speed) but too non-conservative in moderate conditions (when the ship may sail at a higher speed). DNV GL’s approach accounts for involuntary speed reduction due to added resistance in waves, based on the vessel’s individual hull shape and propulsion system and defined individually for each sea state.
Predicting slamming is not a trivial task
The use of CFD is already widely accepted for hull resistance predictions, and there is growing interest in its application for seakeeping and load analysis. The inherent supremacy of CFD for wave impact load analysis is due to the implicit account taken of strongly nonlinear effects: wave breaking, splash and green water effects are directly computed and do not need ad-hoc models or empirical calibrations.
DNV GL has pioneered developments in this field and has routinely used CFD for wave load predictions for more than a decade. This has required significant work to couple CFD solvers with ship motions and flexible deformations so that the interaction of fluids and structures is part of the computed solution. Many years of experience in using coupled solvers in whipping analyses and extensive validation work result in strong confidence in our results.
What differentiates the DNV GL approach?
Thanks s to substantial research and practical experience, DNV GL is able to offer a unique hydrodynamic assessment procedure, combining the ship- and sea-state-specific maximum achievable ship speed, high-fidelity CFD methods and a comprehensive nonlinear statistical analysis concept.
Unlike widely used simpler approaches to estimate extreme loads, the DNV GL method accounts for all sea state conditions the vessel might encounter. This includes not only extreme sea states at slow sailing speeds but also moderate seas, when strong slamming impacts can be induced due to high ship speeds. This allows the whole wave scatter table to be covered so that the effect of wave-induced vibration on load amplification can be accurately predicted
Assumptions and simplifications, unavoidable in any theoretical analysis, are made on the safe side. The wave climate of the North Atlantic is used according to IACS Rec. 34, without including weather routing effects. The 100% still-water bending moment and 100% wave-induced bending moment are summed up without any reduction factors, although such a combination is extremely unlikely to occur. Moreover, the maximum achievable speed in the seaway is used in the analysis, i.e. voluntary speed reduction is ignored. Further assumptions concerning the wave heading distribution and the dependency of whipping loads on the wave heading are on the conservative side.
Although the procedure is on the safe side, several commercial projects have proven that the results agree with design experience and are not overly conservative. This is important to note as any unexpected results may lead to additional design loops.
high risk of severe whipping
The way ahead
By statistically evaluating AIS data in combination with weather hind-cast data, DNV GL confirmed that severe storms are typically avoided by ship masters through routing. Such statistical observations combined with the enhanced use of hull monitoring systems will enable more realistic assumptions about the environmental and operational conditions a vessel will experience during its service life. This represents the basis for the continuous adaptation of DNV GL’s design rules and methods to take account of technological developments.
Written by Holger Mumm
Global Head of Practice Structures, DNV GL – Maritime Advisory
Above article has been initially published in DNV GL’s publication Container Ship Update No. 01 2016 and is reproduced here with author’s kind permission
The views presented hereabove are only those of the author and not necessarily those of SAFETY4SEA and are for information sharing and discussion purposes only.
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About Holger Mumm
Mr. Holger Mumm specializes in the structural analysis of ships, including static and dynamic hull response as well as fluid structure interaction phenomena. He has worked for DNV GL’s Technical Advisory Services for more than 25 years, consulting customers from the maritime industry on the development of novel ship designs. Mumm’s background in numerical analysis and full scale measurements enables him to develop simulation methods and assessment criteria for wave induced hull vibration with a strong focus on practical applicability.
