Pasha Bulker 243 - Independent investigation into the grounding of ...

In comparison, Sea Confidence and Betis had deployed a scope of cable of a little
over three and four, respectively. Neither had deployed the Admiralty
recommended minimum length of cable. Most ships in the anchorage were
anchored further offshore than Pasha Bulker. Consequently, few, if any, achieved a
scope of six or more.
Therefore, the parts of Newcastle anchorage where an acceptable scope of cable,
according to the IACS guidance, can be achieved are about three miles or less from
the coast. Even the Admiralty recommended minimum cable length for calm
weather is not achievable further than about five miles from the coast as the water
becomes deeper than 60 m. As a result, a lee shore in adverse weather can be
particularly close for anchored ships.
Increasing the distance to a lee shore by anchoring further away from the coast has,
besides reducing the achievable scope of anchor cable, other limitations. Anchoring
more than five miles off the coast is impractical as the water depths exceed 80 m.
This is because an anchor windlass, in good condition, can generally only be
expected to lift the dead weight of three shackles (82.5 m) of chain cable with the
additional weight of the anchor hanging on it. Furthermore, in adverse weather,
deep water is particularly unsuitable due to the increased risks of the anchor
dragging and windlass failure.
The holding power of an anchor depends to a large extent on the nature of the seabed,
or holding ground. Certain types of mud and clay provide the best holding
ground while rock provides the worst. Newcastle anchorage has sand on the sea-bed
which provides relatively good holding ground. However, this must be balanced
with other factors, such as the water depth and the prevailing weather conditions.
In strong winds, an anchored ship will yaw and at higher wind speeds it is
increasingly likely to surge rapidly from one extremity of the yaw to the other. This
can place a shock load on the anchor cable, breaking the anchor’s hold in the seabed.
Increased yawing in strong winds, therefore, increases the risk of the anchor
dragging.
Because of the large number of variables, the holding power of an anchor can vary
significantly and, therefore, can only be estimated. Masters are not normally
provided with information on the holding power of their anchors. However, the
master of Betis did have, on board the ship, a sample calculation to estimate the
wind speed that would cause an anchor to drag.
The sample calculation presented the various formulae and estimates used and
noted that factors such as swell and waves were not taken into account. The sample
ship data was not for Betis, but a much larger bulk carrier. The calculation was
based on the principle that an anchor will drag when the tension in the anchor cable
exceeded the total holding power of the anchor and the cable.
The holding power coefficient of an AC 14 type anchor in a holding ground of
sand, according to the sample calculations, is seven. The anchor should, therefore,
impart seven times its own weight to the total holding power in sand. Similarly, the
coefficient for the cable is 0.75 in sand and its contribution to the total holding
power should be three quarters of the weight of the cable that lies along the sea-bed.
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