Anchor loads and safety factors

The loads in a mooring system are caused by the wind, waves and current acting on the floater. Depending on the location of the floater in the world, different metocean conditions will prevail. In the table below, some extreme metocean conditions are presented for different areas.

The loads induced in the mooring system can be divided into quasi-static loads and total dynamic loads. The quasi static load is the load due to the swell, wind, current and the frequency of the system. For quasi-static loads, the systems tend to move at a low frequency, generally with a period of 140 to 200 seconds.

On top of this quasi-static load there are the individual wave forces causing a high frequency motion. The high frequency motion causes dynamic shock loads with a period of 10 to 14 seconds due to the rolling of the vessel and the movements of the anchor lines through the water. The quasi-static load plus the individual wave forces is called the total dynamic load. Generally the quasi-static loads will be equal to 50% to 90% of the total dynamic load. See Fig. 2-15 for an example of the difference between the quasi-static load and the total dynamic load.

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The quasi-static and total dynamic loads are generally calculated for the intact and damaged load condition. The intact load condition is the condition in which all the mooring lines are intact. The damaged load conditions is the condition in which one of the mooring lines has broken.

From the quasi-static load and the total dynamic load, the required holding capacity of the anchor can be calculated. This is called the ultimate holding capacity (UHC) for drag embedment anchors and the ultimate pull-out capacity (UPC) for VLAs. The required holding capacity is calculated by applying the factors of safety specified by the classification societies.

In the tables G and H, the factors of safety are pre-

36sented for the different load conditions for drag embedment anchors (for instance the Stevpris Mk5

anchor), according to API RP 2SK. The factors of safety used by the major classification societies are generally similar to those given in API RP 2SK (2nd edition, 1996).

For VLAs, the recently used factors of safety suggested by ABS, are presented in table I.

The factors of safety for VLAs are higher than the factors of safety required for drag embedment anchors, due to the difference in failure mechanisms. When a drag embedment anchor reaches its ultimate holding capacity, it will continuously drag through the soil without generating additional holding capacity, i.e. the load will stay equal to the UHC. When a VLA exceeds its ultimate pullout capacity, it will slowly be pulled out of the soil.

table I

The rate effect

An increased rate of loading increases the soil resistance, consequently the anchor holding capacity increases. This must be taken into account with respect to total dynamic loads. For anchor behaviour the rate effect factor indicates how much higher the dynamic high frequency load may be without causing extra movement of the anchor once installed at the installation load. The rate of loading influences pore pressure variations, viscous inter-granular forces and inertia forces. Typical rate effect factors are 1.1 to 1.3 for total dynamic loads, see Fig. 2-16 where the rate effect is presented for two different soil conditions (Su = 10 kPa and Su = 50 kPa).

Using the rate effect and set-up factors, the beha-

38viour of the anchor after installation can be predicted more accurately.

Vertical Load Anchors

A VLA is installed just like a conventional drag embedment anchor. During installation (pull-in mode) the load arrives at an angle of approximately 45 to 500 to the fluke. After triggering the anchor to the normal load position, the load always arrives perpendicular to the fluke. This change in load direction generates 2.5 to 3 times more holding capacity in relation to the installation load. This means that once the required UPC of the VLA is known, the required installation load for the VLA is also known, being 33% to 40% of the required UPC.

As a VLA is deeply embedded and always loaded in a direction normal to the fluke, the load can be applied in any direction. Consequently the anchor is ideal for taut-leg mooring systems, where generally the load angle varies from 25 to 450.