Yang, Chan-Kyu (Korea Research Institute of Ships and Ocean Engineering) | Hong, Keyyong (Korea Research Institute of Ships and Ocean Engineering) | Choi, Hark-Sun (Korea Research Institute of Ships and Ocean Engineering)
Literature on breaking waves is reviewed in the context of offshore structure design. Particular attention is paid to deep and intermediate water. The effects of three-dimensionality and randomness on the water particle kinematics are considered. Recommendations are made for alteration of the design wave elevation/height ratio and deep water steepness limits.
The study of breaking waves has proceeded on a wide range of fronts: analytical, numerical, experimental, and full scale research from shore and ship-based to satellite photography. The investigators approach the problem from a wide range of backgrounds: mathematicians, engineers, physicists and oceanographers have all contributed to furthering our knowledge of wave breaking. Additionally, these investigators have often been looking at particular aspects of breaking with a specific purpose in mind. The transfer of knowledge between, say, the oceanographer and the engineer is limited, and often negligible, as the list of references appended to learned papers in these two fields demonstrates. To bring together all of the research on wave breaking has become an almost impossible task, so this study will concentrate on breaking wave probability and breaking wave limits and wave kinematics in the context of offshore wave loading. A fuller review of forces and shallow water effects is published in Easson (1996). Banner and Peregrine (1993) provide a more general introduction to deep water breaking waves, covering experimental and theoretical developments for probability and kinematics. The breaking wave is only one example of the limiting wave and for this reason both large non-breaking and breaking waves are considered. Small breaking waves are of limited interest to a design engineer calculating extreme loading and, as the following sections demonstrate, large non-breaking waves are more frequent than large breaking waves.
Regular wave limits
The nonlinear response of a coupled ISSC-TLP system in a design sea state is simulated by TIOSTAMU. The low-, wave- and high-frequency responses are systematically investigated and statistical analyses of the TLP response are performed. The objective of this study is to investigate the effects of numerically simulated and experimentally measured forces on the nonlinear responses including both springing and ringing. The present study reveals that springing is due to weak asymmetric waves while ringing due to strong asymmetric waves.
The TLP system is one of the best designs for oil production in deep water, since the construction cost does not increase dramatically with increasing depth of water, and the resonant motions are detuned from the frequencies of dominant wave energy spectra. However the lowand high-frequency forces excite the low- and high-frequency resonant motions, while the mean forces create the offset and setdown. These are associated with the key design parameters, i.e. air gap, tether fatigue and extreme loading. Therefore we need to develop reliable and sound techniques for the dynamic analysis of TLPs. The waves, wind and current are the important environmental elements to yield the loads on the TLP system. However the present investigation considers only the wave loads in order to compare the effects of theoretical and experimental wave loads. The low- and high-frequency wave forces are clue to the nonlinear interactions of the linear responses. The second-order diffraction theory (Molin, 1979: Kim and Yue, 1989, 1990; Chau and Eatock Taylor, 1992) gives the interaction terms up to the second-order. Third-order diffraction theory has recently been proposed by Faltinsen et al (FNV, 1995) and Malenica and Molin (MM, 1995) to compute ringing wave loads. Rainey (1995) argues that the FNV assumption of wave amplitude and cylinder radius being of the same order could lead to divergence of the Stokes'' expansion.
The European Union called for proposals for "the assessment of any possible risk likely to affect the marine environment in association with research, monitoring and surveying in marine sciences and technologies". An assessment was made by a small group of deepsea ecologists, stating that normal scale research would impact the environment to a negligible extent. Future requirements for ecological deep-ocean research related to the use of the deep for waste disposal and mining of metalrich ores were also presented. Experimental large-scale research for commercial-scale impact assessment as well as pre-commercial pilot operations are discussed. It is concluded that studies of this type are still of an order of magnitude that is acceptable and unlikely to cause significant impacts.
Serious research in the open ocean began in the 1870s with the circumnavigation of the British naval surveying vessel HMS Challenger. The sample and data gathering techniques used at that time were, of course, very different from those used today. But the scale of sampling and the broad approach adopted during the Challenger Expedition were not dramatically different from those of the late twentieth century. Admittedly, the spatial sampling regime during the Challenger and many subsequent research cruises, in which individual sampling localities were separated by tens or even hundreds of kilometres, is not typical of modem cruises. In the last thirty or forty years, the increasing realisation that the deep ocean is much more variable, physically, chemically, geologically and biologically, and at a wide range of spatial and temporal scales, has led to multiple sampling within restricted areas and repeated visits to the same locality at different times. Nevertheless, because of the small size of individual samples in relation to the size of the environment studied, the environmental impact of this conventional research must be trivial. For example, the total area of seafloor sampled "destructively" by all the gears used by oceanographers in the 125 years since the Challenger, trawls, dredges, corers and grabs, amounts to no more than a few square kilometres, a tiny fraction of one percent of the more than 300 million square kilometres of the deep ocean.
Most offshore structures have been installed with assistance of derrick crane vessels. However as the size of offshore structure increases, the barge mounting method (sometime called as IFO technique but it will be called as BM method m this paper for convenience) has been introduced and applied. And this method has proved that without heavy crane vessel s , large offshore structures can be installed safely and economically. The key of the BM method is the control of barge motion during docking and mating operations. Also to absorb impact loads and to help with load transfer during mating between substructure and deck, several damping mechanisms have been developed in the past. This method is being planned for the application of a bridge installation in Korea because it is found that the BM technique is not Only attractive to heavy offshore structures but also can be effectively applied to the bridge installation: The most critical area of concern is the safe docking and mating operation of the bridge being mounted on high level piers. To overcome these difficulties, a strand jacking system is applied and introduced for the installation of bridges on piers. Docking can be assisted by tug boats and mooring lines secured to the existing substructures. Mating will be achieved by a jacking system together with a ballasting of transportation barge. Since the motion of barge is also very" critical in this operation, the acceptable sea states and time are to be carefully investigated. This paper describes the technical considerations and installation procedures illustrating detail sequence from docking ''to mating operation of a 4,500 ton bridge construction m Busan, Korea and also introduces the engineering analysis of the BM method for the specified bridge installation.