Diseño del marcado

Rig heaves in anchor line, pulling AHV towards it. AHV keeps sufficient ten- sion in pendant, chaser remains in tight contact with anchor, anchor remains correctly oriented (fig. 3-44).

At some distance from the rig, AHV pays out winch wire while keeping suf- ficient bollard pull (at least 1.5 times anchor weight) to keep chaser on anchor head. Anchor flukes point towards the rig. Rig hauls, AHV veers while keeping some tension in the pendant line transferring the anchor to the bolster. The direction of the anchor cable must now be perpendicular to the rack (fig. 3-45).

When anchor arrives at bolster, reduce tension to 15 tons. As soon as anchor is resting on bolsters, slack pendant wire completely. If tension is not suffi- cient, anchor falls out of control of the chaser and might rotate and make racking difficult. If this occurs, bring anchor to the stern of the AHV, rotate anchor with fluke points directing outwards and keep chaser tight on the anchor (fig. 3-46).

Deploying Stevpris from the anchor rack

AHV receives pendant from rig and connects to AHV winch wire. AHV moves to a position at a good distance but less than the water depth (for instance 50 meter dependent on weather) from the rig. Stop winch and keep sufficient tension, 20 to 30 tons or more as required to maintain the chaser on the head of the anchor. Only now rig pays out cable while AHV hauls in on the winch. The AHV maintains sufficient tension while pulling the anchor to the stern roller. Reduce the power of the propeller as anchor passes the wash zone and bring anchor on roller for inspection and reacti- vate thrust (fig. 3-47).

fig. 3-44 keep tension ! D fig. 3-45 keep tension ! D fig. 3-46 wrong ! risk losing control over anchor orientation

fig. 3-47

keep tension !

Stevpris installation

Boarding the anchor in deep water

In deep water the weight of the anchor line becomes of predominant importance. For line loads larger than 8 times the anchor weight the anchor could be pulled against the chaser as illustrated, it could even position itself upside down! In such cases boarding the anchor is difficult and damage might occur (fig. 3-48).

The best and preferred solution is to pull the anchor from the bottom and have the rig haul the anchor line, allowing the boarding of the anchor near the rig where loads are smaller.

If this is not possible or allowed for some reason, another solution is to reduce the weight that is hanging from the anchor. This can be done by lifting the anchor line using a lock chaser or grapnel handled by a second vessel (fig. 3-49).

It is recommended to board the anchor with the chain between the fluke. The anchor fluke is generally designed to withstand loads up to 8 times the anchor weight (fig. 3-50).

It happens that the anchor is accidentally pulled over the roller on its side. Due to the large forces damage to shank and fluke might occur when the chain is hanging over the anchor (fig. 3-51).

fig. 3-48 anchor weight high tension fig. 3-49 lock chaser fig. 3-50 8 x anchor weight fig. 3-51 large weight

Stevpris installation

83

fig. 3-52

fig. 3-53

with ballast in fluke use chain forerunner

D chain

wire

If boarding the anchor on its side is inevitable, make sure that before boarding, the vessel is turned to free the anchor line from the anchor and haul gently. The chain will pass the stern roller next to the anchor. However, this situation should be avoided as damage may occur (fig. 3-52).

Ballast in fluke

Using a wire rope forerunner and ballast material placed inside the hollow fluke, the anchor may not topple over with the fluke points directed downwards. A wire anchor line might be too light to position the anchor correctly and the anchor may not topple over, the anchor could skid over the seabed and prevent penetration.

When the fluke is ballasted, the weight of a chain forerunner will cause the shackle to nose down and bring the fluke in penetration position (fig. 3-53).

Stevpris installation

Chaser equilibrium

To control the anchor, the chaser collar must always be on the anchor head. The tension in the anchor cable must be equal or larger than 1.5 times the weight of the anchor. If not, the anchor slides through the chaser and the orientation is not controlled (fig. 3-54).

Equilibrium forces determine if chaser is in contact with the anchor. Near bottom, the vertical load at the chaser from the anchor line Flv is small.

The chaser remains only in contact with the anchor if the bollard pull Fphis

larger than the horizontal line load Fl_hwhich in turn must be larger than

the anchor weight W (if not the anchor will slide down). The angle of the pendant line must be larger than 45° (fig. 3-55).

Recommendation: Bollard pull must always be equal or larger than the line tension, i.e. use a minimum bollard pull of 20 to 30 tons for a 12 to 15 ton anchor. Use a minimum pendant line length of 1.4 to 1.5 times the water depth in shallow water (100m) and 1.3 to 1.4 times the depth in deeper water (fig. 3-56).

fig. 3-54

pendant line force

anchor line tension

anchor weight fig. 3-55 Fp Fp_h Fp_v Fl_h Fl Fl_v W fig. 3-56 chaser