Transportation studies in unidirectional waves are usually thought to provide a conservative approach to the design of jacket structures and their sea-fastenings. Model tests in multidirectional waves, however, have shown that the presence of directional spread sometimes increases motions, accelerations and loads. This conclusion is supported by computer calculations using the NMIWAVE wave diffraction and motions program, and can largely be e:ll;p1ained by linear sea keeping theory. Several distinct causes have been identified.
Damage may be caused to a jacket structure if a severe storm occurs during transportation from the fabrication site. Large accelerations, particularly at extremities of the structure, may impose severe inertial loads, and parts of the structure overhanging the barge may sustain wave impact damage. Sea-fastenings also have to be designed to keep the vessel and structure intact during the voyage.
The design of a North Sea jacket structure normally involves both computer analyses and model tests in a wave basin. In both cases the waves are usually represented as unidirectional - i.e. all wave components come from a single direction. Waves in the real sea are short crested, with components coming from many directions simultaneously. The difference between uni- and multidirectional waves is illustrated in figure 1. There is little systematic published data showing how wave directional spread affects vessel response, due partly to a lack of knowledge about directional spread in sea waves, coupled with the complexity of simulating short-crested waves both on the computer and in the laboratory. It is generally assumed that the use of unidirectional waves in design is conservative, and will result in larger motions, accelerations and loads than using a more realistic short-crested sea.
Most of the observed differences between uni and multidirectional responses can be explained with the use of a computer model. British Maritime Technology?s NMIWAVE program is based on three-dimensional linear wave diffraction theory, and is described in detail in [1] and [2]. NMIWAVE has been validated for application to a wide range of structures including semi submersibles, tension leg platforms [3], gravity platforms [4], barges and ships [5].
The wave height and vessel motions are assumed small, so that the equations describing motions of the fluid and vessel can be 1inearised (retaining only terms proportional to wave height). The flow is described in terms of a velocity potential, and is assumed inviscid. The incident waves are assumed initially to be unidirectional and sinusoidal, at one frequency velocity potentials associated with wave diffraction and radiation by the vessel are calculated using a numerical source/sink method. The vessel's underwater surface is divided into a large number of plane area elements. A fluid source, pulsating at the incident wave frequency, is placed at the centre of each element, and source strengths and relative phases are calculated to satisfy the hull surface boundary condition: that there is no flow normal to this surface. Having first obtained the source strengths, NMIWAVE then derives the pressure distribution over the hull's surface, thence the fluid forces and moments, added masses and damping coefficients.
