To avoid or manage the consequences of dragging anchor when being installed on a congested seabed, particularly where the soil is layered, the anchor behavior must be reliably predicted. To deal with this, DNV is launching 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.
"In the Gulf of Mexico, for example, many rigs are increasing their number of mooring lines and having their anchors upgraded. In Norway, the Petroleum Safety Authority (PSA) has increased its focus on the anchor installation tension at locations where anchor drag should be avoided because of congested seabeds (e.g. pipelines)," says Rune Dahlberg, senior principal engineer at DNV.
Joint development of DIGIN
Based on this reality, many companies in the oil industry have shown great interest in an existing, but internal, DNV version of the DIGIN software, 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 result is now providing an external version of the existing DIGIN tool.
DIGIN is integrated with DNV's DeepC software package, which provides a number of new advantages. Using the modelling and program logistics facilities in this package, DIGIN now has an efficient graphical user interface for program input, execution, output and results in a ready-to-report format.
The requirements when designing and installing fluke anchors are 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 scalable anchor database with 'calibrated' models of the most common fluke 'and drag-in plate' anchors, including the most recent types of deepwater anchors used by the industry, e.g. Vryhof Stevpris NG and Bruce Dennla Mk3.
Broad feature list
DIGIN investigates the behavior 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.
The anchor is modelled as members further divided into elements for the application of interaction stresses and integration of these up to forces. The equilibrium for forces and moments is sought between anchor and anchor line. The anchor models in DIGIN were calibrated through back-fitting analysis of instrumented full-scale anchor tests, which also served as a basis for validation in the program.
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