The Maritime Engineering Reference Book

A Guide to Ship Design, Construction and Operation

Anthony F. Molland

• Publisher: Butterworth-Heinemann
• Year: 2011
• ISBN:
  9780080560090
• Topic: Technology & Engineering
• Subtopic: Military Science & Technology
• Index: Technology & Engineering

The Maritime Engineering Reference Book is a one-stop source for engineers involved in marine engineering and naval architecture. In this essential reference, Anthony F. Molland has brought together the work of a number of the world's leading writers in the field to create an inclusive volume for a wide audience of marine engineers, naval architects and those involved in marine operations, insurance and other related fields. Coverage ranges from the basics to more advanced topics in ship design, construction and operation. All the key areas are covered, including ship flotation and stability, ship structures, propulsion, seakeeping and maneuvering. The marine environment and maritime safety are explored as well as new technologies, such as computer aided ship design and remotely operated vehicles (ROVs).Facts, figures and data from world-leading experts makes this an invaluable ready-reference for those involved in the field of maritime engineering.Professor A.F. Molland, BSc, MSc, PhD, CEng, FRINA. is Emeritus Professor of Ship Design at the University of Southampton, UK. He has lectured ship design and operation for many years. He has carried out extensive research and published widely on ship design and various aspects of ship hydrodynamics.* A comprehensive overview from best-selling authors including Bryan Barrass, Rawson and Tupper, and David Eyres* Covers basic and advanced material on marine engineering and Naval Architecture topics* Have key facts, figures and data to hand in one complete reference book

Chapter 1 The Marine Environment

1.1 The Ship in the Marine Environment

A ship or any ocean vehicle or structure is exposed to the marine environment. It is a complicated and often hostile environment. Environmental forces at sea come from wind, seaway, current, tidal waves, and waves from earthquakes (tsunamis). From the practical point of view, the seafarer has to cope with wind and seaway. Generally, seaway is generated by the wind at the sea surface.

The occurrence and magnitude of wind and seaway depend on the sea area and on the time of the year. Wind and seaway vary randomly and can be described by statistical methods based on probability theory. In detail, we look at the rate of occurrence, the magnitude, and the time variations of wind and seaway.

It is convenient to make a distinction between long-term (in terms of days up to years) and short-term time (in terms of hours) variations of the seaway. While the long-term approach allows for the rate of occurrence and the severity of the seaway, the short-term time variations are important for the dynamic ship response in a particular seaway of constant energy. Seaway is represented by gravity waves of the water at the sea surface. The exciting wave forces vary in time. The ship responds to the oscillating external forces as a dynamic system.

Wind and wave data have been assembled by observation, by measurement, and by mathematical description. Goals of the near future are, for example, to apply the non-linear pattern of extreme irregular seas in ship operation, and to have sea on-line data on the bridge. The literature on the sea environment is abundant. This chapter gives a general insight into the physical features of the marine environment.

1.2 Wind

By tradition, the magnitude of the wind is defined by the Beaufort Scale (Admiral Beaufort, England, 1806). The Beaufort wind scale is based on observation of the sea, by way of a rough grouping from 1 to 12 Bft. The observed wave pattern in deep sea is related to the generating wind force. Storm at Bft. 11 is described as ‘Waves are so high that ships within sight are hidden in the troughs; visibility poor’. Beaufort 12 means a hurricane, with the deep sea criterion describing the sea status as follows, de Beurs (1957): ‘The sea is white with streaky foam as covered by a dense white curtain; air filled with spray; visibility very poor’.

The Beaufort numbers also correspond to a rating of wind according to ascending wind velocity. Each Bft. number relates to a range of wind velocities. Any wind above 32.5 m/s (63.2 kn.) is Bft. 12. The Beaufort scale is given in Table 1.1 where wind velocity is given at a height of z₁=6 m. The scale is also depicted in Figure 1.1.

Table 1.1 The Beaufort scale

Figure 1.1 Range of wind velocity at Beaufort scale.

The upper and lower limits of the Beaufort wind regions are approximated by a quadratic polynomial function, with n as Bft. number, v_w1 upper limit and v_w2 lower limit of the velocity range:

(1.1)

(1.2)

The Bft. wind velocities are average values of the horizontal wind at sea. A detailed analysis of the wind profile above the sea surface shows an increase of wind velocity with respect to the vertical distance from the sea surface, see Figure 1.2. The wind velocity at z₀=10 m above sea level has been used as a reference or characteristic wind speed, van Koten (1976).

Figure 1.2 Wind velocity profile.

Only for detailed analysis and calculation of wind forces the vertical wind distribution must be taken into account. The wind profile is approximated by

(1.3)

The exponent α is 0.12 for wind at sea surface.

In order to look at the time variation of the wind speed, we can plot the mean energy versus the average occurrence cycle in time. Figure 1.3 gives an example of the so-called spectrum of the ocean wind velocity, data taken from van Koten (1976), see also Price and Bishop (1974). We see four energy peaks, which define four distinctly different ranges of wind energy with respect to their time variation:

(1) In the first peak, the repetition cycle of the wind is only a few minutes and less than a minute (about 0.5 to 3 minutes). This shorttest time variation of the wind is of interest for the wind action on the ship and her dynamic response. A wind with its rapid time varation taken into account is called a “gust”. In gusts, the maximum wind speed can be about 50% more than the mean wind speed. Daven...