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Progress

This page details the progress of Deterministic Sea Wave Prediction under the following headings;

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Progress to Date 

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Sea Surface Measurements

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Issues Raised by the Initial Studies

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Viable Sea Types

The numbers in brackets relate to references listed on the References page.

Progress to Date

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. These articles are listed under the references page.

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.

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Sea Surface Measurements  

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].

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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:

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Fundamental prediction questions, [1,3,4,4a,5], considered initially under the grant GR/F 32165 (SHP54).

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Remote sensing metrology questions, were explored initially under grant GR/J51191 (SHP 171), and concerned a feasibility study of the issues involved in a shallow incidence laser-based wave-slope sensor (for moving vessel work). This study included electronic, optical and computational control issues together with a variety of novel signal processing and artificial intelligence topics, [8,9,10], which stemmed from the very unusual nature of the metrology problem. Open literature publication of some of the instrumentation findings has been delayed until now pending patent application (No., GB 9704867.2). However an EPSRC report has been available under confidentiality agreements. There has also been more applied work, aimed at applications of DSWP in the offshore industry, supported under the DTI Seasense Initiative.  

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Operational questions, have been examined under the grant GR/G58267 (SHP98), which concerned the exploitation and specialist ship modelling requirements that arise when DSWP is available, [11,12,13,14]. This is linked to work on ship models with time varying coefficients [15,16,17]

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Viable Sea Types

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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© Copyright 2003.
For problems or questions regarding this web contact [S.E.Adam@ex.ac.uk].
Last updated: March 18, 2003.