Abstract

The deep and ultra deep water mooring locations typical for many Brazilian offshore developments require anchor points suitable for both horizontal and vertical loads. Traditionally most designers opt for the use of suctions piles, although in recent years a number floating production units and MODUs have been moored using vertical loaded anchors (VLAs). Using the recently developed anchor data acquisition system will provide additional confidence in the installation and use of VLAs.

The anchor data acquisition system allows the installation parameters to be measured directly at the anchors. This will give the user direct feedback as to the orientation of the anchor as well as the applied load and provides confirmation that the anchor has embedded to the required depth and that it can generate the required capacity in the soil.

The application of vertically loaded anchors in combination with the data acquisition system is described and shows how this can provide technical and economical benefits for deep water applications. Additionally the results of actual (monitored) anchor installations are presented and compared to the values predicted beforehand.

Introduction

For deep water mooring systems the availability of real time anchor installation data can be of importance with regards to the actual anchor installation. A common anchor for deep water mooring systems is the vertically loaded anchor (VLA). The VLA is decribed together with a newly developed anchor data acquisition system. Integration of the two technologies gives significant benefits. Testing of the system has shown that it gives reliable and repeatable results.

Vertical Loaded Anchor (VLA)

To meet industry demands for an anchor suitable for taut leg mooring systems, an extensive testing program has been completed which has led to a new type of anchor, the Stevmanta VLA (Figure 1), where a traditionally rigid shank has been replaced by a system of wires connected to a plate. The anchor is designed to accept vertical (or normal) loads and is installed as a conventional drag embedment anchor with a horizontal load at the mudline to obtain the deepest penetration possible. By changing the point of pulling at the anchor, vertical (or normal) loading of the fluke is obtained thus mobilizing the maximum possible soil resistance.

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 45 degrees. The angle adjuster changes the mode of the anchor from pull-in mode to vertical (or normal) mode.

During installation (pull-in mode) the load arrives at an angle of approximately 45 to 50 degrees to the fluke. After triggering the anchor with the angle-adjuster to it's normal load position, the load always arrives perpendicular to the fluke. This change in load direction generates 2.5 to 3.5 times more holding capacity in relation to the installation load, depending on soil, mooring line and anchor parameters. In other words, if a Stevmanta VLA anchor is installed with a horizontal load of 1900 kN, the anchor will resist a normal (or vertical) load of 4750 kN to 6650 kN before breaking out.

Due to the construction of the shank of the Stevmanta VLA, i.e. flexible cables instead of a rigid shank, the Stevmanta VLA can be loaded in any direction in the horizontal plane after installation. If after installation the mooring load is applied in a different direction, this will cause the anchor to rotate in the soil until the load is again perpendicular to the fluke plate. This means that the installation direction of the Stevmanta VLA can be chosen in any direction to suit the requirements of the field, for example installing the anchor perpendicular to the final loading direction.

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