To avoid or manage the consequences of dragging anchor when being installed on a congested seabed, particularly where the soil is layered, the anchor behaviour must be reliably predicted. To deal with this, DNV is launching DeepC for anchor analysis, DIGIN, a powerful fluke anchor design tool with a state-of-the-art graphical user interface.

The devastating consequences of the hurricanes Ivan in 2004 and Katrina and Rita in 2005, together with other weather-related catastrophes, have led to a sharper focus on the reliability of the mooring systems of mobile drilling units. Says Rune Dahlberg, DNV’s project manager: “In the Gulf of Mexico, for example, many rigs are now increasing their number of mooring lines and having their anchors upgraded. In Norway, the Petroleum Safety Authority has increased its focus on the anchor installation tension at locations where anchor drag should be avoided because of congested seabeds for example pipelines.”

Joint development of DIGIN
Related to this reality, oil companies have shown great interest in an existing but internal DNV version called DIGIN, a powerful design tool for fluke and plate anchors in clay. To make this tool available also to the industry, Dahlberg explains that a DNV Joint Industry Project was completed in early 2008. The results provided an external version of the existing DIGIN tool and linked that to DeepC, providing a number of new advantages. DeepC for anchor analysis is a subset of the DeepC software and DIGIN. Using the modelling and program logistics facilities in DeepC, the fluke anchor design tool now has an efficient graphical user interface for program input, execution, output and results in a ready-to-report format. The complete version of the DeepC software performs coupled analyses of deepwater floating production systems.

Heavy requirements
The requirements when designing the tool were to:

• Predict the drag distance for selected anchors and check that the project tolerance level is met
• Specify a required anchor installation tension which satisfies the governing safety criteria
• Identify and communicate the significance of different combinations of anchor size, fluke angle and installation tension in relation to the actual soil conditions.

In answer to these requirements, the new DIGIN program provides a scaleable anchor geometry database with models of the most common fluke anchors. The models have been calibrated through back-fitting analyses of high-quality instrumented full-scale tests. It also includes models of the most recent types of deepwater anchors used by the industry, e.g. the Vryhof Stevpris NG and the Bruce Dennla Mk3.

Broad feature list
DeepC for anchor analysis investigates the behaviour of the anchors for a plane condition. It performs a stepwise penetration where equilibrium for each of the depth increments is calculated independently of the previous step. The penetration direction of the anchor at each depth increment is the basis for the penetration path. The tool calculates the anchor line geometry from the fairlead to the anchor, which can either be a fixed point (padeye of a pile, suction anchor, plate anchor) or a fluke anchor. The anchor line can be divided into a number of segments with different properties.