Vertical Loads on Drag Embedment Anchors

Abstract

ABSTRACT As industry moves into deeper water and economical solutions for mooring systems are sought, anchor systems capable of withstanding vertical loads are needed. Current industry standards and API recommendations constrain a mooring line to be tangent to the seabed when a drag embedment anchor is used. This paper shows that current High Holding Power (HHP) anchors such as the Bruce FFTS and Vryhof Stevpris can withstand significant vertical loads, By introducing a mooring line angle at the seabed, the vertical load at the anchor only increases slightly. The technical feasibility and economic benefits of this concept are described. New types of drag embedment anchors specifically designed to withstand vertical loads are also discussed. DRAG ANCHOR TYPES Drag anchors can be grouped in many different categories. For simplicity, they are grouped in this paper into three categories: "old" style "low" efficiency anchors (e.g., LWT, Danforth, Stockless, Bruce Cast), High Holding Power (HHP) anchors (e.g., Bruce FFTS, Vryhof Stevpris), and new generation Vertically Loaded Drag Embedment Anchors (VLA) (e.g., Bruce DENLA, Vryhof Stevmanta), see Figures 1 and 2. The discussion is limited to deeply embedded HHP and VLA anchors in soft cohesive soils. Many of the ideas presented are applicable to anchors in harder soils. It is shown that HHP anchors can resist loading of around 30-40 times their weight, including substantial vertical loads. It is also shown that VIA anchors, after they have been "tripped" or "keyed", can perhaps resist 100-200 times their weight at any angle. CURRENT DESIGN PRACTICES Current mooring design practices for drag anchors require the mooring line always to have tangential contact with the seabed. Reference 1 states "If drag anchors are used, the outboard mooring line length should be sufficient to allow the lines to come tangent to the ocean bottom at the anchor when the system reaches the maximum anticipated offset under the damaged condition". Reference 2 includes a very similar statement. The U.S. Navy"s mooring design manual, Reference 3, states "Drag-embedment anchors are designed to resist horizontal loading. A near-zero angle between the anchor shank and the seafloor (shank angle) is required to assure horizontal loading at the anchor.. .. As the shank angle increases from zero, the vertical load on the anchor increases and the holding power of the anchor decreases". However, one design office in the Department of the Navy typically uses a seabed line angle of 3° as a maximum with no anchor performance reduction. A recent departure from the traditional no uplift at the seabed approach has been ABS approval of a Floating Production System mooring designed to have a seabed line angle of 3.5° in the damaged condition (one-line damaged -100 year hurricane). ABS stated, however, they would not allow a seabed line angle in the intact condition (no damaged lines -100 year hurricane).