NEWS AND COMMENT
NEWS AND COMMENT |
|
|
- |
Preset installation of a mooring system for a MODU has become established as a cost-effective alternative to upgrading its mooring system for deepwater applications. More recently taut-leg preset moorings, using synthetic rope, have enabled shallow water MODUs to operate in deep water at significantly less cost than a deepwater moored MODU or a dynamically positioned MODU.
The anchor with the longest track record for preset moorings (for permanent applications) is the suction pile, but it is large and bulky and an AHV cannot carry a full mooring spread on one trip. Multiple trips or multiple AHVs are therefore needed to bring suction piles to site, which make them cost ineffective for MODU applications where a rapid cycle of installation, recovery, and reinstallation at a new location is necessary.
The main alternatives to suction piles are the vertical load anchor (VLA, more correctly termed 'normal load' anchor or NLA), the suction embedded plate anchor (SEPLA), the free falling torpedo anchor (FFTA) and the Near Normal Load Anchor (NNLA).
The VLA is a special design of drag embedment anchor that 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 Normal Load Anchors or NLAs, i.e., anchors having a final centroid angle of 90°.
The SEPLA is an NLA embedded vertically with a suction pile and subsequently pulled to its operating attitude after removal of the pile. This approach to NLA embedment and orientation encounters two problems: loss of depth and holding capacity due to keying in soil disturbed by the suction pile, and the need for high bollard pulls to turn the anchor to its final attitude in the soil.
The FFTA in its various forms has a large size and weight for a given load capacity, requires a ROV to view penetration marking on the mooring line to establish depth of embedment, and is not easy to recover. It develops high recovery loads on reconsolidation of the soil following embedment. Although FFTAs have been used for permanent and MODU preset moorings, verification of verticality of the inserted anchor remains an important issue affecting holding capacity.
These features of FFTAs preclude a rapid cycle of installation, recovery, and reinstallation of deepwater preset taut-leg moorings for MODUs. Like any other anchoring means, the installed FFTA also requires proof loading of the mooring line and so does not escape bollard pull requirements.
The NNLA, developed by Bruce Anchor Limited, is an anchor for taut leg mooring that has been designed for high performance and ease of recovery, features which have been proven in extensive offshore use. 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 significant advantages. Once the Dennla has embedded along a conventional trajectory to a depth greater than two fluke lengths, a final centroid angle of about 80° is established and further loading causes the anchor to embed deeper along a new steeper trajectory to give increasing holding capacity until a maximum is reached at the bottom of the new trajectory.
This steeper trajectory is a feature unique to the NNLA and in effect makes it 'fail safe': it either embeds deeper when overloaded, giving increasing capacity, or, if already at the bottom of its trajectory, it drags horizontally at constant load like a conventional drag embedment anchor.
Figure 1 shows an embedment trajectory of a fully instrumented 12 mē 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 NNLA geometry reconfigured to give a final centroid angle of 79°, enabling the anchor to enter its second, steeper, trajectory. The final line load during embedment was restricted to 300 tonnes to prevent overload damage to the mooring line. This meant that the embedment trajectory of the Dennla could not be explored to its limit. However, Figure 1 confirms that the NNLA enters a steeper trajectory when its centroid angle is changed to 'Near Normal' at the trigger point.
The Dennla is also recoverable backwards at consistently low loads (typically about half of the installation load) following reconsolidation of the soil after installation. The anchor negotiates stern rollers easily and can be turned around on deck and made ready for redeployment within about fifteen minutes. As a result, the cycle of installation, recovery, and reinstallation of a preset deepwater mooring spread can be achieved consistently in a significantly shorter time than with suction piles, VLAs, SEPLAs, or FFTAs.
The Dennla can also be racked on bolster bars, if required for deployment from a MODU, making it the most versatile of deepwater taut-leg mooring anchors.
Since 2002 the Dennla has been proven for deepwater preset moorings for MODUs offshore Borneo, Angola, Mauritania, and in the Gulf of Mexico, a track record which has led to it being introduced for North Sea applications in 2007 and for the Ivory Coast in 2008.
In response to a demand for extending the anchor's performance to sands and stiff clays, the Dennla Mk 4 was introduced in 2008 with a shank lock down feature which enables it to be used also as a conventional MODU anchor. This makes the Dennla Mk 4 uniquely versatile, in effect two anchors in one.
For Dennlas for your application, contact us at: sales@bruceanchor.co.uk
For video demos click on: