Ships – Anchoring arrangement
Patent
1997-10-22
1999-10-26
Sotelo, Jesus D.
Ships
Anchoring arrangement
73153, B63B 2124
Patent
active
059709014
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a mooring bed assessment apparatus and method.
When drag embodiment anchors are being assessed as anchoring elements in a spread mooring for deployment in a submerged marine mooring bed, it is desirable to have knowledge of the engineering properties of the mooring bed soil at co-ordinate points on the trajectory that each anchor is expected to follow as it buries in the mooring bed in response to a large pulling force applied substantially horizontally to its attached pulling cable.
In terms of anchor fluke lengths, drag embedment anchors of the most modern designs have been shown experimentally to follow a trajectory which penetrates as much as five fluke lengths below the surface of a frequently-occurring normally consolidated clay sea bed, of shear strength gradient 1.6 kPa/m, when pulled by a chain cable. When pulled by a wire rope cable of diameter one-third that of a notional circumscribing cylinder containing the chain cable, the trajectory penetrates as much as nine fluke lengths. For a drag embedment anchor of the largest size presently in use, having a fluke length of about 6 meters and pulled by a wire rope cable, the burial trajectory forms a curve that decreases progressively in inclination to the horizontal from 50.degree. at the initial penetration point until it becomes horizontal at a maximum anchor holding capacity of about 40 times anchor weight when the anchor has moved about 300 meters horizontally and buried about 54 meters vertically. Thus, it is advantageous to have engineering data for the mooring bed soil over a planar vertical area extending at least 300 meters along the sea bed surface and 54 meters below it at each of as many as twelve widely-separated locations in a spread mooring site to enable each trajectory to be predicted.
In the past, engineering data for deeply submerged mooring bed soils have been derived from remote shear vane tests and remote cone-penetrometer tests assessed in conjunction with laboratory tests performed on soil samples taken from a small number of boreholes drilled in the mooring bed at chosen locations on the site. These tests and samplings are performed at a range of depths to 60 meters or more below the surface of the seabed at each of the chosen locations to give a three dimensional array of soils data for the site. However, due to extremely high costs involved in performing such investigations, the number of chosen testing and sampling locations is invariably minimised. A disadvantage results in that interpolation of the small number of data points over a large site area leaves a large margin of uncertainty between the chosen locations. This, in turn, gives considerable uncertainty in predicting the performance and trajectory of a drag embedment anchor in the mooring bed soil between these locations.
In the past also, anchor burial trajectories have been measured. Horizontal co-ordinates on a trajectory have been determined by obtaining approximate horizontal displacements of the anchor by measuring the corresponding horizontal displacements of a designated point on a horizontal portion of its pulling cable which has not buried under the seabed surface. Corresponding vertical co-ordinates have been determined either directly, by using a thin cable attached to the anchor and pulled up vertically to measure its depth of penetration below the seabed surface, or indirectly, by using a pressure sensor mounted on the anchor to measure the static pressure of the column of water from the buried anchor to the sea surface via a flexible tube serving to conduct the water column from the seabed surface to the anchor. For the direct method, disadvantages of these past measurement methods include lack of certainty that the thin cable has been pulled up taut enough to become truly vertical in the soil without disturbing the buried anchor and the necessity for numerous repetitions of this operation. For the indirect method, disadvantages of these past measurement methods include uncertainty due to pressure fluctuations caused
REFERENCES:
patent: 3685479 (1972-08-01), Bruce
patent: 4651139 (1987-03-01), Oettli
patent: 5325714 (1994-07-01), Lende et al.
BruPat Limited
Sotelo Jesus D.
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