Offshore Technology Research Center


Abstract ID# :


Report Title:

Deepwater Anchor Design Practice Phase I and II (Report to API/Deepstar)


Knut Anderson, NGI, J. Don Murff, Texas A&M University, Mark Randolph, COFS

Report Date:

December, 2003

This report documents the work accomplished in Phase II of a two-phase study on the design and analysis of deepwater anchors. The study is limited to suction caissons and drag-embedment anchors in soft clays, subjected to inclined loading, i.e., loading with a vertical component. The first year of the study focused on collection of references, prediction methods, and data on actual applications, field tests, and experimental studies related to these two anchor types. It was an attempt to establish a baseline of data and prediction methods for deepwater anchors. This baseline, in turn, served as a “jumping off point” for Phase II which is aimed at evaluating the general methods in use by Industry for analysis and design of deepwater anchors. The evaluation includes an assessment of each method’s simplicity, completeness, sensitivity, practicality, and generality.

A suction caisson anchor is a large diameter, cylinder (either steel or concrete) open-ended at the bottom and closed at the top. Mooring loads are applied by an anchor line attached to the side of the caisson. The length to diameter ratio of the caisson is typically six or less. Once installed, the caisson acts much like a short rigid pile and is capable of resisting both lateral and axial load. The suction caisson gets its name from the fact that it is usually installed by applying under-pressure (“suction”) to its interior after it is allowed to penetrate under its own weight. Since the caisson’s interior is sealed from the seafloor by the soil, vertical loading creates an internal draw-down pressure which in turn mobilizes the end bearing resistance of the soil at the caisson tip. Of particular interest to operators and contractors is an assessment of the bias and uncertainty in predicting the anchor’s installation performance and holding capacity.

A vertically loaded drag anchor (VLA) typically derives its capacity from a large bearing plate called a fluke. During installation and design loading, force is applied to the plate through an anchor line attached to the fluke either by a rigid shank or bridle arrangement. As the anchor is dragged horizontally, it cuts into the soil eventually becoming seated well below the mudline. Anchors designed specifically for significant vertical loading (VLAs) can be subsequently rotated such that the fluke is essentially perpendicular to the load applied by the anchor line. Of particular interest to operators and contractors is a reliable means for predicting the anchor’s installation performance and holding capacity.

In Phase I references on suction caissons (>125) and on VLAs (>80) were identified and a number of prediction methods and data for both anchor concepts related to installation performance and holding capacity were identified and summarized. The results are summarized in a three volume report issued in November 2001 and updated in December 2003.

In Phase II, the current practices for predicting the installation performance and capacity of suction caissons and VLAs identified in Phase I were evaluated, including an assessment of their simplicity, completeness, sensitivity, practicality, and generality. Research topics with the potential for improving current practice were identified. The basis of the evaluation was a comparison of predictions of hypothetical cases among various simplified methods and predictions using these methods with ‘ground truth’ data from either rigorous numerical analyses (FEM results) or field/experimental tests where available. The results of Phase II are contained in a nine volume report, dated December 2003, documenting the various studies conducted as well as the findings and recommendations stemming from those studies.

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