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Progress This page details the progress of Deterministic Sea Wave Prediction under the following headings; The numbers in brackets relate to references listed on the References page.
To
address the needs of the applications described in the
Technical
section, the
Exeter Marine Dynamics Group and their various collaborators introduced and
explored the discipline of Deterministic Sea-Wave Prediction. This has been
under SERC/Industrial/MOD/DTI support, with three jointly funded EPSRC/MOD
grants, a DTI Link Scheme programme and smaller regular inputs from DERA Bedford
and MOD (Future Projects Flying). A range of large and small commercial concerns
have collaborated in the work: Shell International Trading and Shipping, TSS,
Vosper Thornycroft, Nortel, Vickers, Basys Marine, and Valeport.
The results are contained in 16 articles plus EPSRC reports and currently
one patent. An initial feasibility study funded under grant number GR/F 32165 (SHP54), assessed the viability of DSWP and the conditions which must be satisfied for such a prediction. This included theoretical, computational, instrumentation and signal processing aspects, and was set against the prevailing wisdom that the sea could only be modelled in statistical terms [2]. The broad conclusions were that given appropriate instrumentation, a useful prediction of future sea surface shape could be achieved to within 10% error, [1,3,4,4a,5], over at least the 20 seconds period needed for both the Offshore Oil and Gas and the Naval applications.
DSWP involves building a short term prediction model of the sea using
appropriate measurements of the sea surface profile. In the most general case it
has been shown, [1,3,4,4a,5], that these measurements can be made over a finite
time in a given region around the site of interest. This can be achieved with
the remote sensing laser interferometer instrument, which forms the basis of this
proposal. However, for fixed locations such as offshore oil/gas rigs, support
vessels deploying submersibles and SPSO's/shuttle tankers, it is possible to
exploit a special case of DSWP where measurements are made at a set of fixed
spatial locations over finite time interval (THE FIXED POINT METHOD, [3,4,4a]).
This allows the use of developments of traditional wave-slope buoys of the type
currently being produced at Exeter under a DTI Link scheme programme.
The wave-buoy approach is clearly not viable for moving vessels, which require the proposed ship based remote sensing system. Clearly when available this will also satisfy the needs of fixed site applications having the added advantage of dispensing with the moorings associated with the wave-slope buoys. A users overview of the various types of DSWP and its applications has been published jointly with Shell, [6]. Issues Raised by the Initial Studies
As would be anticipated, the initial feasibility studies of DSWP raised a whole
set of highly interdisciplinary issues, on which the Exeter Marine Dynamics
Group have been subsequently working. These fall into three main areas:
The theory of DSWP developed by the applicants shows that for a realistic DSWP system the total time available for a complete sea-surface profile estimate from the commencement of sea surface data collection, through building the prediction model to the final prediction is typically a few tens of seconds (this includes the prediction time itself). Thus, if sea surface shape estimates are needed 20 seconds ahead, the measurement time and sea modelling computational effort must be modest. Strongly non-linear, wind-wave descriptions cannot be built in the time available, only swell sea models can be identified quickly enough. Fortunately, for vessels of the sizes of interest, large swell seas are typically the most relevant for operations. If the local wind waves were large enough to be important, then the local weather conditions would be so severe as to curtail activities. However, large long and short crested swells often exist under otherwise good operating conditions. During operationally useful prediction time intervals, i.e. a few tens of seconds, swell seas are sufficiently linear to neglect the harmonic generation mechanisms [18,19]. Thus, the prediction process involves so called short-crested swells, i.e., the sum of a set of long crested (one dimensional) swell waves. |
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