The various environmental phenomena which should be considered for investigation at a coastal site and methods of data collection are included in section 2 of BS Information on sea states is contained in section 4 of BS and general information on loads, movements and vibrations is contained in section 5 of BS The purpose of this section is to provide information on wave, wind and current loads which is specific to vessels and floating structures. For ships moored alongside a quay or jetty the information contained in clauses 30, 31 and 42 of BS should be used. The moorings for a vessel or floating structure may be designed for conditions having an average return period of less than 50 years provided that there are contingencies which allow the vessel or structure to be safely removed before the design limits of the mooring are reached. Combinations of wind, current and waves should be considered for all directions.
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The various environmental phenomena which should be considered for investigation at a coastal site and methods of data collection are included in section 2 of BS Information on sea states is contained in section 4 of BS and general information on loads, movements and vibrations is contained in section 5 of BS The purpose of this section is to provide information on wave, wind and current loads which is specific to vessels and floating structures.
For ships moored alongside a quay or jetty the information contained in clauses 30, 31 and 42 of BS should be used. The moorings for a vessel or floating structure may be designed for conditions having an average return period of less than 50 years provided that there are contingencies which allow the vessel or structure to be safely removed before the design limits of the mooring are reached.
Combinations of wind, current and waves should be considered for all directions. The wave climate and design wave properties should be estimated in accordance with section 4 of BS The range of conditions considered should reflect the characteristics of the vessel or structure, and the moorings.
It should be noted that the response of moored vessels and structures to waves is highly dependent on wave period and wave length. For example, maximum response may occur when a wave period coincides with the natural frequency of motion of a structure, or when a wave length coincides with the structure length.
They are usually defined as first order forces, which are proportional to wave amplitude. In addition to the oscillatory forces there are slowly varying drift forces which act on the vessel or structure and are primarily due to non-linear second order terms in the pressure field associated with the waves. These drift forces are proportional to the square of the wave amplitude and have a much smaller frequency than the first order forces.
The magnitude of the forces is small compared to first order forces. There is a resulting mean value, commonly called the mean drift force. This mean drift force is similar to the wave set-up observed when a wave reflects off a fixed wall or shoreline. Most floating structures are of sufficient size to cause disturbance of the waves incident on the structure. Physical models, and computational models based on diffraction theory can be used to simulate wave disturbances and hence the forces and resulting motions of a structure.
Simple estimates of wave loading can be made for the two limiting cases of: a no disturbance of the waves; b complete reflection of the waves. Vertical motions pitch, roll and heave cannot normally be suppressed by a mooring or restraining system and should be allowed to exist unimpeded. However, the effect of gusts should be checked for taut line mooring systems, small structures and small vessels. However, significant wave action of the order of 2 m height or over will cause large first order forces.
These forces can be avoided by providing a soft system, e. In the case of a stiff restraining system, e. However, in practice the softness or slackness of guide systems and fendering will allow motion which will generally be erratic and unpredictable. As a consequence, a stiff restraining system should be designed with an allowance for impact load added to the force required to hold the structure rigid.
For sheltered locations it may be sufficient to make simple estimates of forces and motions, as outlined in 2. The use of physical or mathematical models should be considered for exposed locations. The mean drift force acting on a vessel or structure should be considered as a steady force which acts in combination with steady forces due to wind and current.
For locations where the significant wave height is less than 2 m, it is sufficient to make a simple estimate of mean drift force as outlined in 2. Slowly varying drift forces and resultant motions may be neglected provided that the mean drift force is shown to be small.
For exposed locations, or locations where long waves are known to exist, mean and slowly varying drift forces should be estimated by taking account of the wave climate, the motions of the structure and the stiffness of the moorings.
Physical or computational models which input irregular waves can be used to obtain estimates. When a vessel or structure is small or slender it will have little effect on wave form, and forces may be estimated by the use of Morisons equation as described in When a vessel or structure is of a shape such that waves impingeing on it are reflected, forces can be estimated as for a standing wave.
Equation 1 neglects the set-up of the wave due to the presence of the vessel or structure. The response of a vessel or floating structure to regular waves can be described in terms of response amplitude operators RAOs which give the ratio amplitude of motion of vessel or structure amplitude of wave motion for a defined wave frequency or period, or length.
Simple estimates of RAOs can either be made from known estimates for similar vessels or structures, or recourse can be made to physical or mathematical modelling. In deep water the horizontal amplitude of motion of a wave particle at the surface is equal to the vertical amplitude. As the water depth decreases, horizontal particle motions increase, as indicated in Figure 1.
This increase should be allowed for when assessing horizontal motions. Physical and mathematical models have been used to determine values of drift force coefficient R2 f for tankers, semi-submersible vessels and barges [1, 2, 3, 4]. Various definitions of drift force coefficient exist, and care should be taken when interpreting results.
Daijas Also, loading situations may be impossible with other types of by grab rather than by pump greatly reduces the dredger, unless aided by a bed-leveller see 4. Care should be exercised to ensure that Collision Regulations Ships and Seaplanes on the the pipeline is routed in such a way as to minimize Water and Signals of Ships Order Strength of rock material However, especially in relation to dredging works, where the It is valuable to relate the strength of rock bw obtained in the rock to be excavated is underwater, it is usually necessary to uniaxial compression test to a general scale of strength as follows. Hence trial dredging soft material. Due to these limitations the The performance of the dredger and of the dredged method should only be used to complement other formation should be properly monitored throughout and better investigative methods. Pipeline decided upon, in which case each intermediate lift of for shore bx are discussed in the fill area elevation should be confined to greater detail in section 8.
BS 6349-6 PDF
BS 6349-6 Inshore Moorings and Floating Structures