ANCHORS FOR MODU TAUT-LEG PRESET MOORING (Cost-efficiency Considerations)

Taut-leg preset mooring is now well established for deepwater applications. However, factors that determine the cost-effectiveness of the anchors used are often overlooked.

When considering cost-effectiveness of an anchor, in addition to speed of installation and recovery, ease of handling on deck, and reliability - all of which influence operational costs - it is important to consider any extra cost specific to the anchor, which may be overlooked at the anchor procurement stage but can later add considerably to the over-all cost of mooring operations.

For example, if a ROV equipped anchor handling vessel has to be hired for installation and recovery of the anchor, this can add an extra cost of up to $10,000,000 per year to a busy drilling schedule in deep water compared to using anchors that do not need a ROV.

Even where the ROV equipped vessel is also used for non-mooring applications, the cost of its use for anchor installation and recovery does not go away and should be borne in mind as an extra mooring cost.

Suction piles and most free-fall anchors on the market require a ROV. They are therefore subject to such extra mooring costs, which can far exceed the cost of the entire anchor spread - each and every year. In contrast, if anchors that do not need a ROV are used, mooring costs either decrease with time as anchor costs are amortised or they remain at market rates if the anchors are rented.

The suction pile has the longest track record for pre-set mooring for permanent applications. Its large size increases the cost for transporting a mooring spread to site compared to alternative anchors and it is relatively slow to install. These features, apart from its need for a ROV, make it uncompetitive for MODU applications.

The main alternatives to the suction pile are the VLA (Vertical Load Anchor), the suction embedded plate anchor, various free-fall anchors, the Dennla (Drag Embedment Near Normal load Anchor) and the Pennla (Pile Embedment Near Normal Load Anchor).

VERTICAL LOAD ANCHORS (VLAs)

The VLA is a special design of anchor that, in drag embedment form, can be triggered so that the angle of the load line through the centroid of its fluke (the centroid angle) increases to a final angle of 90°, i.e., 'normal', to its fluke. When this angle is reached, the anchor is at its ultimate holding capacity for a given depth of embedment and further loading will cause it to fail towards the seabed surface. This is a characteristic of all VLAs.

Although the drag embedment VLA does not need a ROV for installation and recovery, such anchors have a track record of difficulty in breaking out after reconsolidation of the seabed soil. This increases the time for the cycle of installation, recovery, and reinstallation at a new location and increases mooring costs.

SUCTION EMBEDDED PLATE ANCHORS

The suction embedded plate anchor is a VLA embedded with a suction pile and subsequently pulled to its operating attitude after removal of the pile. This type of anchor is difficult to break out after soil reconsolidation because it is recovered by a pendant line that has to break out the embedded mooring line as well as the anchor.

The suction embedment approach to VLA installation has further cost-increasing features: loss of depth and holding capacity due to keying in soil disturbed by the suction pile, and a need for high bollard pulls to turn the anchor to its final attitude in the soil. Like the suction pile, the anchor also needs a ROV for installation and recovery.

All of these requirements, together with time-consuming deck handling, make the cycle of installation and recovery a relatively slow procedure that attracts extra mooring costs of the order already noted.

FREE-FALL ANCHORS

Free-fall anchors are dropped from a specified height above the seabed and depend on terminal momentum for seabed penetration. Such anchors have a large size and weight for a given holding capacity. They need a ROV for installation and recovery and for viewing penetration markings on the mooring line to establish depth of embedment. This, together with the time needed for deck handling due to the size and weight of the anchor, extends its installation time and results in extra mooring costs similar to those of the suction embedded plate anchor.

The free-fall anchor may incur further operational costs if resetting is needed because of verticality issues or penetration issues due to encounters with sand layers.

Verticality of the embedded free-fall anchor is always uncertain because a ROV cannot confirm anchor orientation below the mud line. Should the anchor be offset from the vertical and leaning towards the MODU, proof loading (or later overload) may induce pullout to the surface with a sudden loss of holding capacity requiring resetting of the anchor.

Layers of sand can also extend installation time by requiring anchor repositioning as a result of impaired penetration due to the dynamic effect of the anchor's sudden impact on such layers. Enormous negative pore pressures induced by rapid disturbance of a compact sand layer produce transient penetration resistances similar to that of rock. This can abruptly halt embedment of a free-fall anchor.

Hurricane experience in the Gulf of Mexico in recent years has focussed attention on the effect of out-of-plane loading on various types of anchor, which led to a claim that the free-fall anchor had an advantage over conventional anchors by being able to accommodate out-of-plane loading. However, all anchors that have a shank self-correct for out-of-plane loading by veering into alignment with the direction of loading until the load comes back in-plane.

DENNLAS

The near normal load anchor, or NNLA, was developed by Bruce Anchor Limited as a solution to the problem of overload-failure of VLAs and difficulty in recovering them after soil reconsolidation. Importantly, the NNLA does not need a ROV for installation or recovery, so it does not attract the extra costs of suction embedded plate anchors and the various types of free-fall anchors.

In its drag-embedment form, it is termed Dennla (Drag Embedment Near Normal Load Anchor) because its final centroid angle is about 80° or 'Near Normal'. This constraint of the final centroid angle has a significant advantage. Once the Dennla has embedded to a depth greater than two fluke lengths, and its final centroid angle has been established by parting a shear pin, further loading causes the anchor to embed deeper to give increasing holding capacity.

Figure 1 shows an embedment trajectory of a fully instrumented 12 m² Dennla offshore Borneo in December 2003 using two AHVs pulling in tandem in a water depth of 1200 metres. The trigger point indicated is the point where the shear pin parted, enabling the NNLA geometry to change to give a final centroid angle of 79°. The final mooring line load during embedment was restricted to 300 tonnes to prevent overload damage to the line. This meant that the embedment trajectory of the Dennla could not be explored to its limit. However, Figure 1 confirms that the NNLA embeds deeper when its centroid angle is changed to 'Near Normal' at the trigger point.

A drag embedment anchor follows a curved embedment path in the seabed soil. Soil resistance on the anchor cable induces a moment on the anchor fluke, via the anchor shank, in proportion to the length of the shank that acts as a lever. This rotates the fluke progressively as cable is pulled below the seabed surface until the curved path becomes horizontal to give the Ultimate Holding Capacity (UHC) of the anchor and cable system when the anchor cable is pulled horizontally at the seabed. The length of the shank determines the depth at which the curved path becomes horizontal and so determines the UHC.

In conventional drag embedment anchors, the length of the shank is determined by the conflicting requirement for the shank to be sufficiently long to promote penetration of the seabed surface while at the same time being sufficiently short to facilitate deeper embedment. The result is a compromise in shank length that limits the depth that the anchor can reach and hence limits its UHC.

The Dennla avoids this problem by having a variable shank length that enables it to reach embedment depths unattainable by conventional anchors. To achieve this, the load application point on the Dennla is transferred from its shank shackle pin to the shank pivot pin when a shear pin is parted following penetration beneath the seabed surface exceeding two fluke lengths. This reduces the effective length of the shank by a factor of 5. Consequently, in a deeply penetrable seabed, the embedment path may not become horizontal until a depth is reached that would produce a UHC in excess of the structural strength of the anchor. Accordingly, for a Dennla with a wire forerunner for minimising penetration resistance, the UHC can be regarded as the load at the anchor shackle that induces first onset of yielding in the structure of the anchor. For a Dennla with a fluke area, A, in the range 10m² to 20m², this gives the approximate relationship: effective UHC = 81A tonnes.

In addition to its shank shortening feature, the Dennla has a low penetration profile which optimises embedment capacity. Its shank can slide to the rear of the anchor to exploit its low profile for ease of recovery at loads typically about half of the installation load.

A Dennla negotiating the stern roller of an AHV

The anchor negotiates stern rollers easily, it can be turned around on deck and made ready for redeployment within about fifteen minutes, and an anchor handling vessel can carry a full mooring spread in one trip. As a result, the cycle of installation, recovery, and reinstallation of a preset deepwater mooring can be achieved in significantly less time than with VLAs and in considerably less time than with suction piles, suction embedded plate anchors, and free-fall anchors. This translates into a saving of two to three days, or more, per rig-move compared with such anchors - in addition to very large cost-savings from avoiding the need for a ROV.

Since 2002 the Dennla's capacity for rapid-turn-around for deepwater presets has been proven offshore Borneo, Angola, Mauritania, and in the Gulf of Mexico, a track record which led to it being supplied for North Sea applications in 2007 and for the Ivory Coast in 2008.

For seabeds consisting of sand or stiff clay, the Dennla Mk4 has a lock down mechanism that fixes the centroid angle at 36° and allows the anchor to perform like a convention MODU anchor. In such seabeds this enables the 12m² and 14m² Dennla Mk4 anchors, for example, to respectively the performance of a 12,000kg and 15,000kg Bruce FFTS Mk4 anchor whose holding capacities in sand and stiff clay are given by the formula HC = 46.86w^0.94 tonnes.

Unlike VLAs, the Dennla can be racked on bolster bars for deployment from a MODU if required, which makes it the most versatile of taut-leg mooring anchors.

PENNLAS

The Pennla (Pile embedded Near Normal Load Anchor) is an ongoing development of the Dennla for precision placement in crowded seabeds. Like the Dennla, the Pennla does not need a ROV for installation and recovery so it is not subject to the extra mooring costs of anchors which need a ROV.

The Pennla is embedded by a gravity pile of modular construction, with each module weighing less than 25 tonnes for easy shipping. The module at the lowest end of the pile holds the Pennla in place for installation and automatically supplies a lubricant that reduces skin friction to assist embedment. A mechanism on the pile enables the Pennla to be rotated through 45° about a fulcrum on the pile before recovery of the pile, so the anchor does not lose depth and holding capacity due to keying. Final proof loading can be applied at uplift in excess of 45°. If loaded in excess of its installed capacity, the Pennla embeds further with increasing holding capacity in the same manner as the Dennla. Similarly, with a wire forerunner for minimising penetration resistance in deeply penetrable seabeds, the effective UHC for the Pennla with a fluke area, A, in the range 10m² to 20m², is approximately 81A tonnes.

Like the Dennla, the Pennla has a low penetration profile that optimises embedment capacity and its shank can slide to the rear of the anchor to exploit its low profile for ease of recovery.

Like all anchors with a shank, the Pennla self corrects for out-of-plane loading by veering into alignment with the direction of loading until the load is back in-plane.

CONCLUSION

For optimal cost-efficiency when choosing an anchor for either MODU or permanent taut-leg mooring, the anchor should meet the following criteria:

1. It does not need a ROV.
2. It is easy to recover from the seabed and bring on deck.
3. It has minimal turnaround time on deck.
4. A full rig set can be shipped on one AHV.