Springing as a nautical term refers to global (vertical) resonant hull girder vibrations induced by continuous wave loading . When the global hull girder vibrations occur as a result of an impulsive wave loading, for example a wave slam at the bow (bow-slamming) or stern (stern-slamming), the phenomenon is denoted by the term whipping . Springing is a resonance phenomenon, and it can occur when the natural frequency of the 2-node vertical vibration of the ship equals the wave encounter frequency or a multiple therefrom. Whipping is a transient phenomenon of the same hull girder vibrations due to excessive impulsive loading in the bow or stern of the vessel. The 2-node natural frequency is the lowest and thereby the most dominant resonant mode leading to hull girder stress variations, though in theory higher vibration modes will be excited as well.
15-414: Springing induced vibrations can already be present in low or moderate sea states when resonant conditions occur between wave lengths present in the wave spectrum and the hull girder natural modes, while whipping typically requires rough sea states before the very local occurring slamming impact has sufficient energy to excite the global structural vibration modes. The hydrodynamic theory of springing
30-422: A certain location and moment. A sea state is characterized by statistics , including the wave height , period , and spectrum . The sea state varies with time, as the wind and swell conditions change. The sea state can be assessed either by an experienced observer (like a trained mariner) or by using instruments like weather buoys , wave radar or remote sensing satellites . In the case of buoy measurements,
45-459: Is a scale which measures the height of the waves and also measures the swell of the sea . The scale is very simple to follow and is expressed in one of 10 degrees. The Douglas sea scale, also called the "international sea and swell scale", was devised in 1921 by Captain H. P. Douglas, who later became vice admiral Sir Percy Douglas and hydrographer of the Royal Navy . Its purpose
60-469: Is not yet fully understood due to the complex description of the surface waves and structure interaction. It is, however, well known that larger ships with longer resonant periods are more susceptible to this type of vibration. Ships of this type include very large crude carriers and bulk carriers , but possibly also container vessels . The first experience with this phenomenon was related to fatigue cracking on 700 ft Great Lakes bulk carriers during
75-463: Is to estimate the roughness of the sea for navigation . The scale has two codes: one code is for estimating the sea state , the other code is for describing the swell of the sea. The Degree (D) value has an almost linear dependence on the square root of the average wave Height (H) above, i.e., D ≃ β + λ H {\textstyle D\simeq \beta +\lambda {\sqrt {H}}} . Using linear regression on
90-408: The 1950s. Later 1000 ft Great Lakes bulk carriers experienced the same problems even after strength specifications increased. The Great Lake bulk carriers are typically rather blunt and slender ships (length to width ratio of 10) sailing at shallow draft resulting in long natural periods of about 2 seconds. This mode can be excited by short waves in the wave spectrum. A rather complete overview of
105-631: The Degree can be approximated as the average between the low and high estimations, i.e.: D ≃ [ 1 2 ( λ L H L + λ H H H ) + 1 2 ( β L + β H ) ] {\displaystyle D\simeq \left[{\tfrac {1}{2}}\left(\lambda _{L}{\sqrt {H_{L}}}+\lambda _{H}{\sqrt {H_{H}}}\right)+{\tfrac {1}{2}}\left(\beta _{L}+\beta _{H}\right)\right]} where [.]
120-772: The bow however rarely exits from the water on such ships. Vibration from whipping may also increase the extreme loading of ships potentially resulting in vessels breaking in two in severe storms. In the extreme cases springing may cause severe fatigue cracking of critical structural details, especially in moderate to rough head seas with low peak periods. Vibration is normally more easily excited by waves in ballast condition than in cargo condition. The converse may also be true since some ships experience more head wind and waves in ballast conditions, while other ships may experience more head wind and waves in cargo condition, thereby vibrating less overall. Ocean-going ships have not had this problem until recently, when high tensile strength steel
135-422: The full scale experiences and relevant literature on springing can be found in references and. The container ships are more slender, have higher service speeds and have more pronounced bow flares. Container ships are also known to experience significant whipping ( transient ) vibrations from bow impacts. Blunt ships may also experience whipping especially with flat bottom impacts in the bow area. The bottom part of
150-454: The joint frequency table, and from the wave spectrum, the designer can find the most likely highest wave elevation in the most extreme sea states and predict the most likely highest loads on individual parts of the ship from the response amplitude operators of the ship. Surviving the once in 100 years or once in 1000 years sea state is a normal demand for design of ships and offshore structures. Douglas Sea Scale The Douglas sea scale
165-504: The sea state cannot be quickly and easily summarized, so simpler scales are used to give an approximate but concise description of conditions for reporting in a ship's log or similar record. The World Meteorological Organization (WMO) sea state code largely adopts the 'wind sea' definition of the Douglas Sea Scale . In engineering applications, sea states are often characterized by the following two parameters: In addition to
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#1732802398992180-416: The short-term wave statistics presented above, long-term sea state statistics are often given as a joint frequency table of the significant wave height and the mean wave period. From the long and short-term statistical distributions, it is possible to find the extreme values expected in the operating life of a ship. A ship designer can find the most extreme sea states (extreme values of H 1/3 and T 1 ) from
195-455: The statistics are determined for a time interval in which the sea state can be considered to be constant. This duration has to be much longer than the individual wave period, but shorter than the period in which the wind and swell conditions can be expected to vary significantly. Typically, records of one hundred to one thousand wave periods are used to determine the wave statistics. The large number of variables involved in creating and describing
210-440: The table above, the coefficients can be calculated for the low Height values ( λ L = 2.3236 , β L = 1.2551 {\textstyle \lambda _{L}=2.3236,\beta _{L}=1.2551} ) and for the high Height values ( λ H = 2.0872 , β H = 0.6091 {\textstyle \lambda _{H}=2.0872,\beta _{H}=0.6091} ). Then
225-436: Was introduced as a common material in the whole ship to reduce initial costs. This makes the ships less stiff and the nominal stress level higher. Today's ship specifications do not account for springing which may be the dominant fatigue factor for some vessels. Sea state In oceanography , sea state is the general condition of the free surface on a large body of water—with respect to wind waves and swell —at
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