With the help of experimental results, this paper first describes the behaviour of embedded mooring chains connected to a buried anchor point. The tests were carried out on three different sizes of chain in two clays with different undrained shear strengths. Subsequently, the effect of changing the effective chain widths (for calculating soil resistances) on the chain forces at the anchor point has been discussed with certain anal~tical results. The studies are believed to be useful to the practicing engineers.
The authors have presented a summary of the various methods that are available to determine the holding capacity and trajectory of drag embedment anchors. One of the drawbacks of the methods that are available today is that these are generally only suited for a single type of soil. The authors are currently working on a methodology that will predict the performance of anchors in varying soil conditions. The initial findings and the methodology are presented. Introduction Drag embedment anchors are a common method of fixing floating structures to the seabed. Over the years numerous methods have been developed to determine the holding capacity and trajectory of the anchor in the seabed. The most common of these, are the design graphs developed by the various anchor manufacturers. More recent developments are the development of methods which can predict the anchor holding capacity and trajectory based on geotechnical methods. The prediction of the anchor holding capacity and trajectory can be split into a number of distinct parts, being:The static holding capacity of an anchor penetrated to a certain depth into the seabed, both in cohesive and cohesionless soils.The profile that the mooring line makes in the seabed (the inverse catenary).The kinematic behavior of the anchor and mooring line in the seabed. To accurately predict the anchor performance, these 3 parts should be combined into a single methodology. This paper will discuss the various methods that are available to determine the performance of the drag embedment anchor. The design graphs, static holding capacity, mooring line profile and anchor kinematics will be presented in details showing the various methods that have been developed. The final part of the paper will discuss a method that the authors are working on, which in their opinion gives an accurate method of predicting the anchor performance. While the authors have tried to give a complete view of the material that is available, they do not claim that the material presented in this paper is complete. Design graphs The most common method of predicting the holding capacity of drag embedment anchors is the use of design graphs. Typically these graphs are shown on double logarithmic scale, with the weight of the anchor on the horizontal axis and the holding capacity of the anchor on the vertical axis. Holding capacity curves are generally presented for sands and soft clays. The basis of these design graphs for a specific anchor type is the tests performed with a number of small scale anchors (typically up to a weight of 3 metric tons) which have been tested up to the ultimate holding capacity. The ultimate holding capacity being defined as the maximum horizontal steady pull which can be resisted by the anchor at continuous drag. Based on the tests with the small scale anchors, the performance of the larger size anchors has been extrapolated. These design graphs are supplied by the various anchor manufacturers for their specific anchors.
This paper presents a method for the installation of anchors for deep water moorings, based on the use of a subsea tensioning device to generate the high installation loads required. Subsea tensioning devices have been used in waterdepths of up to approximately 200 m for the installation of conventional drag embedment anchors and pre-tensioning of piles, but have not yet been used for a deep water application. The proposed method of installation will be presented, and is based on data gained from full scale tests with vertically loaded anchors (VLAs) in approximately 1000 m of water offshore Brazil and also on simulations performed on the computer. The results of the computer simulations and the field tests show that using a subsea tensioning device in deep water can significantly reduce the required time to install the mooring system as the installation and pre-tensioning of the anchors can be performed with two anchors simultaneously, instead of installing them one by one. The use of the subsea tensioning device also requires less powerful vessels for the installation as only vertical pulling capacity is required and not bollard pull. The simulations and field tests show that by using a subsea tensioning device a vertical pulling capacity of approximately 40% to 50% of the required anchor installation tension is required, meaning that the winches of current anchor handling vessels are quite sufficient for deepwater installation of anchors. Introduction The use of catenary moorings systems in deep and ultra deep water requires long and heavy mooring lines. As a consequence the floater has to support a large weight and the mooring footprint becomes large and may interfere with neighbouring infrastructure. Installation of these mooring systems becomes very expensive due to the vessels that are required. For deep and ultra deep water mooring systems the more appropriate moorings system is the taut leg mooring, using shorter and lighter mooring lines. The mooring loads arrive at an angle between 30 and 45 degrees with the bottom, requiring the anchor points to resist not only horizontal, but also considerable vertical loads. As conventional drag embedment anchors are not capable of withstanding large vertical loads, a new type of anchor had to be developed with the capability of withstanding the large vertical loads associated with a taut leg mooring system. Research in finding a solution for a Vertical Loaded Anchor (VLA) capable of resisting large uplift loads started with laboratory testing of models in sand, laponite, bentonite and mud. This resulted in the fine tuning of an anchor shape that is stable in the soil during penetration, able to resist large normal (vertical) loads and easy to handle. These model tests were followed by large-scale tests in the Gulf of Mexico and offshore Brazil. Petrobras initiated a test program on several deep-water sites in Campos Basin where the handling, installation and pullout capacities of VLA anchors have been verified.
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