Hydraulic and earth engineering – Bank – shore – or bed protection – Wave or flow dissipation
Patent
1985-09-30
1988-02-02
Scanlan, Jr., Richard J.
Hydraulic and earth engineering
Bank, shore, or bed protection
Wave or flow dissipation
405 24, 405 21, E02B 304
Patent
active
047226390
DESCRIPTION:
BRIEF SUMMARY
Previous attempts to combat erosion of the sea bed around structures such as offshore oil or gas pipelines have included the planting of a bed of artificial reeds or `fronds` alongside the structure. Such beds have also been planted in coastal waters to combat erosion of the coast by creating artificial sandbanks or stabilizing existing sandbanks.
In U.S. Pat. No. 4,337,007, for example, it is suggested that the fronds or reeds are first secured in separate bunches to a matting, and this matting is then staked to the sea bed. One obvious disadvantage of this system is that such matting is expensive and subject to secondary scour, particularly when using dead-weight anchoring such as chains and concrete weights.
A further disadvantage is that each bunch of fronds must be individually tied to the matting, and the gaps between the bunches can produce undesirable undulations in the sea bed beneath the matting. Over a period of time local stresses produced by viscous drag on each individual bunch of fronds can be sufficient to detach the bunches from the matting.
In one alternative system as described, for example, in U.S. Pat. No. 3,299,640, a continuous line or row of buoyant synthetic filaments, threads or tapes are secured to a common anchor line which rests on the river or sea bed. It is suggested that several rows of filaments may then be positioned one behind the other.
In practice, however, systems such as those described in U.S. Pat. No. 3,299,640 have failed to work satisfactorily for a number of reasons.
Firstly, it has been found that the buoyant filaments are at least partially flattened by underwater currents, particularly in deep waters, and therefore produce only a low order viscous drag for a short height above the base of the filaments. At best, therefore, a low profile non-permanent sand bank is created around the base. Even this bank is liable to be swept away by following currents of marginally higher velocity.
Experience has shown therefore that such frond lines retain only a small bulk of particles and cannot provide a consolidated sandbank.
Secondly, no satisfactory method has yet been found for laying these lines and anchoring them to the sea or river bed. In U.S. Pat. No. 3,299,640 it is suggested that the lines may comprise either dead-weight anchor lines formed of concrete, lead or steel, or flexible anchor lines consisting of rope or cord provided with dead-weight sinkers.
Both these arrangements would suffer from secondary scour around the line and/or the sinkers, and none of the described arrangements would be capable of holding down the lines against the enormous buoyancy forces and currents which would be experienced in deep waters.
Thirdly, no satisfactory method has yet been devised for attaching filaments or tapes to flexible anchor lines. In particular the suggestion in U.S. Pat. No. 3,299,640 that the filaments or tapes could be lashed, woven or braided into an anchor rope would be a tedious and time-consuming process unsuited for the high speed bulk production which is necessary if sufficient quantities are to be produced for covering large areas such as those surrounding oil or gas pipelines.
An object of the present invention is to provide a practical erosion control system which overcomes these drawbacks, the system including frond lines capable of withstanding the strong underwater currents experienced in deep waters and thereby creating consolidated permanent sandbanks.
In accordance with one aspect of the present invention, there is provided a frond line for filtering particulate material from currents flowing over a river or sea bed, the line comprising a substantially continuous curtain of elongate buoyant elements or artificial reeds extending transversely in a common direction from a flexible anchor line, the elements at least partially overlapping one another to provide the required density of elements along the line.
Each element preferably comprises a strand or filament in a strip having a contracted width, a plurality of the strips being closely spaced along
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Knight Anthony
Scanlan, Jr. Richard J.
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