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Anchoring

Anchoring
An excerpt from “An Introduction to Practical Rig Moving”
by Roger C. Dutton

Gear

Bottom Types:

For an all wire anchor system as is used on jack ups, 10 times the water depth is considered prudent for the anchor scope. For hard bottoms this may need to be lengthened, for soft bottoms, it can be trimmed a little. It should be noted that setting an anchor in soft bottom with less than recommended scope will increase the force required to break the anchor free from the bottom. The type of anchor is not normally going to change for individual locations. But anchors are designed to perform for a certain bottom type. Mud anchors generally try to have increased horizontal forces and less digging forces. Hard bottom anchors will have the opposite. A good example is the Delta Flipper which has a cut-out to change back and forth. When the cut-out is removed, it allows the flukes to tilt to a greater angle. This will actually reduce the depth to which the anchor digs into the bottom. With the cut-out removed, the anchor will stop moving at a lesser depth.

Slipping Anchors:

Proper procedure while setting an anchor is very important to reduce the likelihood of slipping. If the anchor is just dropped on a hard bottom and then pulled in, it will likely slip. Technique for setting anchors will be discussed later. But if slipping occurs, try to reset using this procedure: Keep tension as the boat is lowering the anchor to bottom as is recommended in the anchor section, but haul on the anchor winch at the same time. Have the boat slack off fast enough to pierce the bottom with the fluke tip while the rig is hauling. The wire should come tight very fast, so beware. This technique is always worth a couple of tries before rigging up piggybacks.

Piggybacking:

If an anchor is not holding, it may be wise to put two anchors in tandem. The arrangement depends on what kind of anchors will be used together. In general, the first anchor is used to fairlead the second anchor down. If the only anchor available is a non-biting anchor such as a solid block (or clump weight), the weight should be the closest to the rig for fairleading the shank of the second anchor to the bottom. The pendant between the anchors needs to be as long as practicable. Placing a non-biting anchor behind a biting anchor can actually reduce the holding force of the biting anchor. If you have two biting anchors in tandem, the one with the most aggressive bite should go closest to the rig.

Orientation of Anchors

Anchors on Jack ups are run in order to better maintain control of the rig during positioning. The Rigmover must keep in mind that his goal is to maximize his options for longitudinal, transverse, and pivoting movement. In practice we find that, relative to the force required to pull the rig back toward the platform, it is easier to pull the rig forward, difficult to rotate the rig, and most difficult to translate the rig port or starboard. The reasons for this are varied, but relate to the natural function of the change in angle of the wires during pull in and bottom scarring as we drag legs close to final position. We must therefore set the anchor pattern to maximize the ability to move side to side, and rotate the rig. We (generally) are willing to sacrifice the ability to pull off the platform to achieve this goal.

If anchor positions were set symmetrically, one anchor would be redundant any time the rig is moved from the center position. (See Appendix V). Therefore, to best utilize the equipment, we must set the anchors asymmetrically. For control, it is best to set the bow anchors wide and the stern anchors slightly narrower, but both forward and aft anchors should have well more than 90 degrees enclosed angle. I prefer 140 degree spread on the bow, and a 120 degree spread on the stern. This prevents cross-alignment.

The argument against setting the bow anchors wider than the aft anchors is that we can tension the bow anchor wires above the maximum pull of the winches by applying maximum pull to the stern winches with the legs off bottom. [The aft force is greater for the same wire tension as the enclosed angle of the stern anchors is less (F=F1 cos θ)]. This is a valid argument and, if not recognized, it is possible to cause failure of the forward anchors. If we stall the bow anchor winches on set up to test the holding power of the anchor and the integrity of the wires, then do the same on the stern, we must recognize that if we simply lifted the legs clear of the bottom without slacking any wire, the force on the bow anchor wires will further increase as the rig will tend to move aft slightly.

Given the caveat of the above argument, we can ease off each anchor slightly and prevent the over-tensioning of the bow anchors. The good news is, that as the rig moves aft (toward the platform) the enclosed angle of the bow anchors decreases (and the enclosed angle of the stern anchors increases) making less and less likely to over-tension the bow wires. Done properly, I have always felt that the increased ability to maneuver the rig is well worth the extra care required.

Cross-alignment is when a straight line may be drawn from one anchor, through both fairleads, to the opposite corner anchor. (See Appendix V). If the rig experiences cross-alignment, it will be virtually impossible to change the rig heading without moving the rig position. The rig mover will want to be able to use the anchors to create a couple at all times.

The stern anchors must be set far enough aft of the final location so sufficient force may be exerted to pull the rig to the location against some nature force. The force exerted by the stern anchors on the rig will reduce as the rig moves aft. This is because the angle between the intended path and the anchor wire increases as you move aft. The force exerted is represented by the cosine of the angle between the anchor wire and the intended path. In other words, if you are trying to move aft, and the anchor is directly aft (angle=0), you would get 100% of the anchor tension pulling you in the direction you want to go. If the anchors are set off the quarters (as is normal) you get a percentage of the tension acting aft. As the anchors are set wider and wider, they eventually give no force aft. This is called tight-lining. This is the case when they are set up off the beam and the angle is 90 deg. (Cos 90 deg = 0).

Since we said that the bow anchors should be set wide, we have the same problem of limited force being exerted by the anchors. Due to the small percentage of the wire tension actually pulling the rig forward, we will not be able develop strong pulling forces in the forward direction. I feel this concession is worth the trade because;  1) In the case of the bow anchors, the angle of the anchor wire is increasing as you move aft. This will give you progressively more and more ability to pull off using the anchors; and 2) It is recommended that a tow boat be maintained, made fast to the bow, during positioning with anchors in case of emergency anyway. This boat can be used to hold additional force if required.

Also, experience has taught us that frequently we find ourselves at the correct distance off the platform for final position, but we need to move the rig directly sideways (transversely) to line up with a well or envelope position. By widening the angle of the bow and stern anchors, we are narrowing the relative angles as viewed transversely (port or starboard) thus maximizing our ability to move the rig side–to–side (F=F1 cos θ). As is the case with towing vectors, all the movements of the rig can be computed using the vector diagram. I am not suggesting we do an exact calculation on every pull of an anchor line, but a basic understanding of vectors is very helpful when doing the mental gymnastics of maneuvering.

Pinning Vs. Final Position

Anchors will be run using either a course heading and distance from the rig or using preset buoys. The Rigmover must keep in mind that the distance and bearing to the anchors will be different for each of three positions. It is best to start with a drawing which places the rig in the final position and draw in the required anchor positions. (See Appendix VI.a.) Two scenarios follow:

1. Buoys will be preset. In this case, a bearing and distance from the position of the proposed well bore should be computed and given to the surveyors. These will be different than the bearings and distance from the anchor wire fairleads. (See Appendix VI.b.). The Rigmover needs to communicate the position of the buoys to the survey personnel relative to a position they can reference. If the rig is to position in a field which has a metric grid system, this is often the best way to relate the required positions of the buoys.

2. Anchors will be run using bearings and distances from the rig. In this case, the anchor pattern should be drawn and headings calculated for about one rig length from location. This should put the bow wires almost abeam of the fairleads in medium depth waters. (See Appendix VI.c.).

Fairlead Position:

Most of the time, the forward fairleads are relatively close to the bow, but the aft fairleads can be nearly to amidships. Always keep this in mind when determining the anchor pattern.

Boats

Preparations:

The boat, or boats, that you are going to use for running anchors should be decided upon before the move. The Rigmover and the Captain should have a discussion on the step by step procedure to be followed. Normal operation is to rig up one length of pendant wire on each anchor prior to the move. The buoys, additional pendants, shackles and pins should be transferred to the boats. All of this should be done before the move. The additional pendants should be wrapped on the boat’s work drum as soon as they are transferred. Most Captains are willing to tow with buoys on deck if they are secured along the bulwarks. This should be part of the pre-move checklist.

Running the Anchor:

When the boat comes along side, for the anchor running job, the pendant should be passed down to the boat with the crane. Be sure the pendant is attached in a fashion so as to allow the boat crews to quickly disconnect the crane. Most likely, the boat crew will pass a tugger wire through the eye and pull the pendant into their wire jaw, then connected to the work wire. Next, the anchor should be lowered into the water by walking it out using the winch. With plenty of slack, it will pulled up to the stern roller of the anchor handling boat. Be sure the Captain knows the peculiarities of your winches, including the time it takes to release the brakes. After the anchor is on the roller, release the drum brake. Now is the time to get the boat to position himself to run the anchor. Take the time to get the boat on the correct heading before running out the wire. Often, a Captain will start to run the anchor, then come to the heading. Be aware of any current running, and have the Captain compensate with his vessels heading before running out any wire. Be sure the Captain understands that the requested bearing is the bearing to the anchor position, not just the ship’s heading. If the boat changes course continually on the way out, the anchor wire will be curved as it lays out. This can either sink in the mud, or catch on an obstruction on the sea bed. In either case, it can make a considerable difference in the lead of the wire. The change in the lead of the wire will change all the forces acting on the rig.

While the boat is on course, and running out to the location, keep slight tension on the wire using the brake. This will help keep the wire off the bottom and straight. If running to a buoy, have the Captain continually give you distances to the drop point.

Setting the Anchor on Bottom:

You should have already planned the power down sequence.

Have the boat come to minimum power before reaching location. Slowly tighten the brake as the boat further reduces to zero power. The boat may have to use some reverse power, if the winch will not stop it’s forward progress. But in any case, you will want the anchor relatively wire tight when the boat stops over the drop location.

Have the boat keep tension on the wire as he backs the anchor off the roller and connects the buoy. It is preferable to have medium tension on the anchor wire and pendant just before setting on bottom. Have the boat ease the anchor onto the seabed instead of dropping it. This will ensure that the flukes are pointed downward and the shank of the anchor is in line with the anchor wire.

After the anchor is on the seabed, tension the wire to dig the anchor in as the boat connects the buoy. The reason for this is two-fold. The dug-in anchor is less likely to be dragged by the boat if he drifts while making up the buoy, and if the anchor slips the boat can re-set without the need for retrieving the pendant and buoy.

Retrieval of Broken Pendant Wires:

Occasionally pendant wires will get broken while attempting to retrieve anchors. There are several tools which can be used to retrieve an anchor, and they all involve stripping over the anchor wire.

1. An 80 ton shackle can be put on anchor wire and attached to one pendant. Be sure that the pin goes through the eye of the pendant and the anchor wire will ride in the bend of the shackle. Pass the pendant to the boat and have him slowly run out to the anchor while slacking his work wire with the pendant attached. Keep light to medium strain on the anchor wire. Have the boat go well past the anchor and pull. The shackle will have run down to the shank, and will pull the anchor up vertically with the combined action of the pendant and anchor wire. This method is the most simple, but also puts the most strain on the wire at the poured socket attachment.

2. A chaser chain may be used much in the same manner as a shackle. The difference is that it is possible to work the loop of chain not only down the anchor wire,
but onto the shank of the anchor. This is much preferable, as it reduces the bending of
the wire line at the poured socket.

3. There are stripping hooks available and some chasers that have rollers available for the same type of operation. In most cases, the boat Captains will be familiar with their gear. But it is always a good idea to ask the Captain what gear he has during the pre-move meeting.

Anchor Pendants:

Anchor pendants should be 25% larger in diameter than the anchor wire. This is due to the need to free the anchors from deep penetrations. The water depth and expected bottom type needs to be determined in order to ensure that the anchor pendants are of the correct length. 120% of the water depth plus the expected sink of the anchor is a good estimate. With a Delta Flipper type anchor, 20% of the expected leg penetration is a good estimate of the anchor penetration. If the boat used to run anchors is available before the move, it may be possible to pre-arrange the pendants on the work drum of the boat well before the move. This will allow fast, safe deployment of pendants.

Appendix:

anchor_drawing1