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7 – Operational Considerations
7.2.1.3 Skew
Figure
7-3(a) shows the ideal aspect between the coil array and the target.
(a)
Motion of
ROV
Coil separation
distance
Figure 7-3: ROV skew errors
The accuracy of vertical range and lateral offset measurements may degrade if large angles of skew
exist between the coil array and the target. This is because the effective coil separation distance
decreases as the angle opens as shown in
If there is a slight crosscurrent in the survey area, it may be possible to perform the survey only with a
small angle of skew present.
Under these circumstances, the System will continue to supply valid data with skew angles up to ±15°.
If you know that this condition will prevail in the survey area, assess the degree of error by conducting
dry-land test measurements on a sample of the target with applied skew. See
Scaling" on page 6-33
for instructions.
Summary:
1. Avoid operating the ROV with angles of skew greater than ±15°.
2. Sometimes you may conduct a survey only with an angle of skew present (for example,
because of crosscurrents in the survey area). If this is the case, perform a series of dry-land
tests to determine the effect that the predicted angle of skew will have on measurement accu-
racy.
7.2.1.4 Altimeter Positioning
A simple altimeter measures the ROV altitude above a single point on the seabed. Unless the local
seabed topography is essentially flat, this measurement may not be precisely the same as the true
altitude of the ROV above the mean seabed level.
Figure 7-4 shows an ROV with four altimeters mounted at various points on its body. In this example,
the ROV is surveying a partially covered pipe at the bottom of a shallow trench. Low spoil heaps
created by the action of a trenching plough mark the two sides of the trench.
7-4
15°
(b)
Effective coil
separation distance
Figure
7-3(b).
© Teledyne TSS
ROV
motion
section 6.5 "Target
DPN 402196 Issue 4.1
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