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.
The past few years have seen the updating of codes for the use of drag embedment anchors (fluke and plate anchors). Examples are the specific requirements for drag embedment anchors in the Gulf of Mexico due to the hurricanes and the updating of anchor codes in Norway. This paper will summarize the current rules and recommended practices for drag embedment anchors and their application. It will then show the effect on drag embedment anchors and the application for MODU moorings. Introduction Most floating MODUs are moored using drag embedment anchors, either fluke or plate anchors. Setting of the anchors using conventional guidelines has generally been sufficient to ensure a safe mooring. However recent hurricanes in the Gulf of Mexico resulted in a number of mooring failures. Storms in the North Sea have also resulted in a number of mooring failures. These mooring failures and specifically anchor failures have resulted in new guidelines and regulations being developed for among others the Gulf of Mexico region and the Norwegian section of the North Sea. This paper will focus on these guidelines / regulations with regards to the use of drag embedment anchors for MODU operations. Background Gulf of Mexico In the period 2002 to 2008 a number of severe hurricanes passed through the Gulf of Mexico region causing severe damage to offshore infrastructure. Damage consisted of floating MODUs drifting off station due to mooring line failures and damage to fixed platforms and jack-ups. During hurricanes Ivan (2004), Katrina and Rita (both 2005) a total of 17 moored MODUs broke free from their moorings and drifted off-station for a distance larger than 1 mile. Of these, only four cases reported the drag embedment anchors dragging for a significant distance. The damage that occurred to the platforms has led to the development of interim guidance for the 2006 and 2007 hurricane seasons (API 2006) and an amendment to API RP 2SK (API 2008). North Sea The regulation of offshore safety in UK Continental Shelf (UKCS) is the role of Health and Safety Executive (HSE) and Department of Energy. In Norwegian Continental Shelf this role is shared by Norwegian Maritime Directorate (NMD), Petroleum Safety Authority (PSA) and Norwegian Petroleum Directorate (NPD). For the UK continental shelf the audits and accident statistics between 1981 and 2003 are published by Health and Safety Executive (HSE 2005). Within this period, among 3485 reported incidents on all moored units (permanent and mobile), 240 incidents relate to mooring system failures (a failure frequency of about 0.2). The mooring system failures include problems with mooring lines, mooring devices, winching equipment or fairleads, and anchors. Among the 240 mooring incidents only 34 incidents are categorized as accidents, the rest are incidents of near misses or insignificant situations. Only one of the 34 incidents categorized as accident relates to anchor drag due to one line being damaged and the tension control on the winches could not be implemented.
The deep and ultra deep water mooring locations typical for many Brazilian offshore developments require anchor points suitable for both horizontal and vertical loads. Traditionally most designers opt for the use of suctions piles, although in recent years a number floating production units and MODUs have been moored using vertical loaded anchors (VLAs). Using the recently developed anchor data acquisition system will provide additional confidence in the installation and use of VLAs.The anchor data acquisition system allows the installation parameters to be measured directly at the anchors. This will give the user direct feedback as to the orientation of the anchor as well as the applied load and provides confirmation that the anchor has embedded to the required depth and that it can generate the required capacity in the soil.The application of vertically loaded anchors in combination with the data acquisition system is described and shows how this can provide technical and economical benefits for deep water applications. Additionally the results of actual (monitored) anchor installations are presented and compared to the values predicted beforehand.
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