Use of Spectral Techniques for Operationally Forecasting Vessel Motions
Authors: Silveria William A. 
Published: December 2007

A computerized system is discussed that permits at-sea managers to anticipate best modes of operation in advance and thereby schedule accordingly. This approach employs spectral techniques to forecast motions of vessels induced by the seaway. Use of this system has permitted tighter operational control of semisubmersibles, drillships, and barges.

Introduction
All semisubmersibles, ship shapes, and barges respond to the kinematics of the random seaway. Indeed, the irregular vessel motions induced by ocean waves often dictate the ability of vessels to operate.The ocean varies between benign and extremely hostile, when dangerous wave conditions disrupt or prevent work. Weather-sensitive operations require prevent work. Weather-sensitive operations require that waves be within certain limits during the entire duration of a given undertaking, whether it be the placing of a construction module on a jacket, the placing of a construction module on a jacket, the raising of a blowout-prevention stack, or the handling of anchors. In these and other circumstances, the go/no-go decision depends on knowledge of the upcoming wave state and, as such, is predictive in nature.An offshore operator's problem with planning upcoming mark can be viewed as being threefold:to predict properly wave conditions in advance of occurence,to estimate the resulting vessel motions induced by the forecast waves, andto evaluate what effects these motions will have on scheduled work.
All three steps are integral components of the operator's decision-making process.An operator's problem of planning work at sea never has been easy, nor has it ever been approached in a totally objective manner. With regard to the first step of the decision process, predictions of ocean waves have been recognized for a long time as often being inaccurate. This situation has been true particularly for forecasts of long-period swells that particularly for forecasts of long-period swells that seriously affect offshore operations. The problem also has been compounded by the wave forecasts usually being given only in terms of characteristic parameters, (i.e., significant heights and periods), parameters, (i.e., significant heights and periods), which has limited the utility of these forecasts in quantifying vessel motions. In short, the prediction techniques and general use have failed to keep abreast with new knowledge of wave generation, propagation, and dissipation and with advancements propagation, and dissipation and with advancements in the fields of computer simulation and the dynamic analysis of floating structures.Application of spectral wave models can help overcome these deficiencies. Why use the spectral approach? There are two basic reasons. First, if properly done, the spectral approach is the most properly done, the spectral approach is the most accurate technique for forecasting wave conditions, giving two-dimensional details of wave states that are unobtainable otherwise. Second, the spectral approach bridges the gap between wave descriptions and wave-induced loads and motions in a way consistent with modern dynamical techniques.The concept of incorporating the spectral approach into a practical wave forecasting method originated in 1953 with the work of Pierson, Neumann, and James.
JPT
P. 555