Deepwater Anchors

OVERVIEW: The Offshore Technology Research Center is undertaking a comprehensive research program on deepwater anchors with applications to drilling and production operations. This work has three general objectives:

1. Determine the currently available, best practices for analysis and design of suction caisson anchors (SCA's) and vertically loaded anchors (VLA's), the latter of which includes both drag embedment anchors (DEA's) and suction embedded plate anchors (SEPLA's).
2. Characterize deepwater environments and site investigation strategies relevant to deepwater anchor design.
3. Effect significant improvements in the accuracy and precision of installation and capacity predictions for SCA's and VLA's through theoretical and experimental studies.

 

OBJECTIVE 1 -- Define State of the Practice:
BACKGROUND: The API/ISO Geotechnical Resource Group/Panel (GRGP) has invited proposals

• "To update the existing API documents RP2FP1 and RP 2SK"
• with regard to guidelines for SCA's and VLA's. Current guidance is essentially non-existent in spite of a significant number of applications, especially in the past few years. The major design issues of interest are (1) installation and (2) extreme environmental loading.
• The SCA work and the VLA work will, for the most part, be conducted independently although the approaches will be similar in concept. The work is expected to be conducted over a two year period. The following is a discussion of the scope of work for this project component.

SCOPE: The first year's work will be directed toward collection of the relevant information (such as data sets where available), including operator interviews, literature searches, and collaboration with other research/industry groups. The previous JIP project by NGI will provide a useful starting point for this work. In addition, the information collected will be organized and reviewed so that potential "best" practices for design and analysis can be identified and data sets that might be useful for subsequent scaling or calibration of analysis methods can be tagged.

In the second year, focus will be on implementation and testing of selected design and analysis approaches that are deemed potential "best" practices. These methods will be examined/tested to identify major deficiencies and uncertainties. Implementation of the "best practices" will help to identify and prioritize issues not clearly recognized at the outset. Design procedures representing best available practice will then be selected and formulated in a probabilistic format with key design parameter statistics characterized as random variables. This work will complement the ongoing theoretical and experimental work detailed below. The probabilistic model will be exercised to provide a basis for selection of safety factors and/or LRFD coefficients by the API Geotechnical Resource Group. A comprehensive final report for the project will be issued.

OBJECTIVE 2 -- Characterize Deepwater Environment:

Geologic and Geotechnical Characterization
BACKGROUND: The continental slope off Texas and Louisiana is likely the most complex passive continental slope in the world. Salt tectonics has created geologic structures at the rate of several centimeters per year. Bed forms such as brine or slurry flow channels, furrows many kilometers in length, and the areas adjacent to the escarpment being devoid of recent sediments indicate the occurrence of high velocity, deep-water currents that have not been acknowledged heretofore. In addition, extensive, near-mudline sediment sections, 5 to 7 meters in thickness, consisting of 1 to 3 centimeter, varve-like layers of fine sand, silt and clay-sized quartz have been discovered.

Recently data have been collected across the northwestern continental slope of the Gulf of Mexico using long coring techniques with recovery of twenty plus meters and deep towed arrays. This data set, along with other data from site specific boreholes and DSDP core data provide a unique opportunity to better understand the complex geological and geotechnical nature of the region as well as to test the efficacy of high quality seismic data to interpolate and/or extrapolate conditions at significant distances from boreholes.

SCOPE: Analyze the data from recent surveys (over 700 cores from 4 to 20 meters long and relevant deep-tow high resolution, geophysical surveys) as well as from other data sources that can be identified and accessed. Determine the slope bathymetry, identify significant geological structures, and determine the strata and the geotechnical stratigraphy of the slope in this region. Integrate all data into a GIS format. This work will be carefully coordinated with the work on Optimization of Deepwater Geotechnical Investigations described below.

Optimization of Deepwater Geotechnical Investigations
BACKGROUND: The conventional investigation program for designing an offshore foundation may not always be practical for deepwater anchor systems, with regard to cost and/or schedule. To account for the unique requirements of deepwater site characterization, cost benefit analyses can be employed using a reliability-based approach. The essence of the approach is to provide consistent levels of reliability for a foundation, whether or not a site-specific boring is used in its design. The optimal investigation program involves a trade-off between the cost of the program and the cost required to account for uncertainty in the design of the foundation. This research effort will benefit significantly from previous research in this area recently completed at The University of Texas at Austin with funding from Mobil and Unocal.

SCOPE: Select a subset of representative offshore fields (study fields) to analyze. Where possible exploit data from the Geological and Geotechnical Characterization work described above. For each of these fields compile and analyze available geologic information, soil boring data, geophysical data, and pile-driving records. Develop field-specific models for spatial (both horizontal and vertical) trends and variability in design data. In addition, develop statistical correction factors and weights to relate data obtained from other sources to the representative data. Develop methods that relate partial safety factors or resistance factors to uncertainty in design data for different deepwater foundation types and configurations.

OBJECTIVE 3--Installation and Capacity Predictions for Suction Caissons and Vertically Loaded Anchors

Vertically Loaded Embedment Anchors BACKGROUND: In deepwater, traditional catenary moorings, which apply essentially horizontal loads on anchors, become very expensive and difficult to manage because of their wide anchor spacing. They have the further disadvantages that the amplitudes of platform motions can become larger and that the lines may encroach on nearby facilities or even on adjacent tracts. One way to reduce the length of lines and width of the anchor footprint is to use taut moorings. In this case, the anchors will be subjected to forces having a significant vertical component; hence, they are referred to as vertically loaded anchors. Embedment anchors being considered by the offshore industry for these applications include suction embedment plate anchors (SEPLA's) and drag embedment anchors. The former is the subject of a comprehensive research proposal (pending) to NSF (Aubeny and Rauch) as discussed below. If this work is not funded by NSF then the work proposed herein will be somewhat modified to include consideration of SEPLA's. Since it is assumed that the NSF work will be funded, the work described below considers only the drag embedment anchor.

Two basic issues arise in connection with drag embedment anchors: (1) the trajectory of the anchor during installation and (2) the holding capacity of the anchor under inclined loading. While industry has been using drag embedment anchors (with catenary mooring lines) for temporary moorings for many years, the practice is highly empirical, with installations being carried out by trial and error. Further, industry has little experience with vertically loaded anchors. For permanent moorings (e.g. production facilities) operators must have assurance prior to deployment that the requisite anchor capacity can be achieved for a particular design. Prediction and design methods must be significantly improved if vertically loaded drag embedment anchors are going to be used with confidence and routinely accepted by certification and permitting agencies.

SCOPE: An overview of the research approach for the development of analysis and design tools for drag embedment VLA's is outlined herein. However, because of the immature state of practice in this area, initiation of this work will be delayed until the state of the practice study has progressed somewhat. A preliminary literature review and interviews with industry representatives are needed prior to defining the details of a research plan for this topic.

Based on information collected during the early stages of the project, ranges of parameters (soil type, anchor geometry, etc) of primary interest will be selected. Initially analytical methods published in the literature will be implemented and tested for the selected conditions. This along with other information collected will provide a basis for selecting the most effective analytical framework (e.g. solid or viscous fluid soil characterization) for simulating the installation process and selecting the most appropriate constitutive equation(s). It is anticipated that the loading process will be modeled using both simple methods such as limit analysis and more sophisticated finite element methods with an advanced plasticity constitutive equation.

Experimental results will be needed to (1) provide insight and suggest mechanisms to be incorporated in analytical models and (2) to provide a basis for calibration of analytical models. Experimental data obtained during the State of the Practice phase of the project, if any, will be exploited to the extent possible. It is anticipated that additional tests will be required however. These tests may be 1-g experiments and/or centrifuge tests. The detailed analytical techniques and experimental results will be used to develop simplified analysis and design methods. These in turn will be formulated in a probabilistic format for use in codes and practices.

Performance of Suction Caissons Used to Anchor Structures in Very Deep Water

BACKGROUND: The offshore oil industry is currently designing and using suction caissons to anchor floating exploration and production platforms in the Gulf of Mexico. However, there is a paucity of performance data on suction caisson behavior and our current level of understanding is limited. This is especially true for long, slender caissons subjected to inclined loads as required for deepwater applications in soft cohesive sediments. The proposed research will yield detailed experimental data to clarify caisson behavior and to provide a basis for the development and validation of analysis and design methods. Because of the uncertainty involved conservative assumptions and relatively large factors of safety are currently needed to ensure adequate performance of these foundation systems.

SCOPE OF EXPERIMENTAL STUDIES: The experimental studies outlined herein will be conducted concurrently with the analytical work which is detailed separately below. The initial experimental studies will employ 1-g model testing and will address a number of issues that need resolution to improve the reliability of performance predictions for suction caissons. These issues include (1) the upper limit (if one exists) on the aspect ratio (length/diameter) for installation of a caisson using suction pressures, (2) the effect of the consolidation process on lateral load behavior, is unclear (3) the effects of pore water drainage rates on the load displacement behavior, (4) the effects of loading details on caisson behavior including loading geometry, sustained loading, and cyclic loading and (5) the effects of various soil types on caisson performance. This study will benefit from previous experimental work carried out at the University of Texas, however that limited study left many questions unanswered.

Clearly, experimental data is needed to answer a variety of questions surrounding the behavior of suction caissons. The approach proposed herein begins with a comprehensive investigation by testing scaled models of reasonable size. First, concentration will be on basic issues to develop an understanding of fundamental behavior, and then gradually this understanding will be exploited to deal with more complex conditions and, eventually, tests at scales closer to prototype such as larger field tests will be conducted. Consideration will also be given to other types of testing such as larger 1-g models or centrifuge testing as the issues become clearer.

The results of laboratory model tests can be useful in the field if appropriate analytical methods are developed that can adequately track the laboratory data. It is proposed that development of analytical methods will occur simultaneously with the experimental work as detailed below. This will ensure that all of the relevant data needed for calibrating the numerical analyses is obtained and it will greatly enhance our ability to interpret the experimental results.

SCOPE OF ANALYTICAL STUDIES: As part of a comprehensive research effort aimed at better understanding the behavior of suction piles, it is proposed to conduct a computational study addressing the effects of inclined loads, such as would be imposed by a taut anchor. The study will extend work currently nearing completion at the Offshore Technology Research Center on numerical modeling of suction-pile installation and pull-out behavior under axial loads.

It is expected that the proposed extension will make use of several capabilities already implemented for the purposes of the current study, specifically, modeling of (1) the installation process, (2) coupled flow-deformation for porous media, (3) effective loads at pile-soil interfaces, (4) bounding-surface plasticity theory for clayey soils. The proposed work will be focused on achieving robust and efficient modeling of the pile-soil interfaces. Upon implementation of a satisfactory interfacial treatment, the complete three-dimensional model involving a suction-pile under an inclined load will be calibrated using results of the experimental work described above.

Additional Studies and Other Considerations
Additional funding will be required to carry out the VLA and SCA experimental programs as described above. Possible funding sources for this work include independent, related research programs as described below and/or joint industry programs.

There are several other studies on aspects of deepwater anchors that are currently underway or in proposal stage. Two independent studies have been proposed by PI's for this study. Aubeny and Rauch have proposed a comprehensive three year NSF study on SEPLA's and Rauch and Olsen have proposed a two year study on Suction Caissons under the State of Texas' Advanced Technology Program. It is assumed herein that these programs will be funded.

Another initiative on anchors is underway at the University of Western Australia under the direction of Professor Mark Randolph. We have had preliminary discussions with him regarding cooperation between our two programs. He has indicated an interest in an informal collaboration in which our two organizations conduct complementary studies. We plan to pursue the possibility of such a collaboration by comparing plans for experimental and analytical work and reaching an informal agreement on an overall program. Because of the high cost of experimental studies and the large number of parameters involved in deepwater anchor design, the above collaborations will be particularly important. They have the potential for creating high leverage and accelerating schedules. Further, each group involved can serve as a useful sounding board for the others which should increase the likelihood of consensus findings. Finally other related activities world wide will be monitored and exploited to the extent possible.

DELIVERABLES: The research plan discussed above is designed to produce specific deliverables that should be useful to Industry in establishing design practices for deepwater anchors and consequently should be useful to regulatory and permitting agencies. A brief description of each deliverable is provided below.

1. State of the Practice Report: This report will detail findings from the state of the practice study on deepwater anchors including:

1. A summary of information collected including operator experience, results from previous experimental programs and analytical models, and existing design procedures.

   • A recommended list of best available practices.
   • A detailed description of simplified prediction models in a probabilistic format for installation and environmental loading.
2. Site Characterization Reports: Two separate but related reports will detail deepwater site conditions and site investigation strategies in the Gulf of Mexico as follows:

   • A report detailing the shallow geologic and geotechnical conditions and the efficacy of using high quality geophysical data for interpolating or extrapolating from boreholes on the northwestern continental slope in the Gulf of Mexico at depths relevant to SCA's and VLA's. Included will be a GIS formated database of the geological and geotechnical characteristics of the region and a catalog of interpreted high resolution subbottom profiles.
   • A report detailing a methodology for optimizing deepwater site investigations for deepwater anchors based on probabilistic methods.
3. Installation and Loading Prediction Reports: Two reports will be issued on the topics of installation and environmental loading of deepwater anchors.

   • A report describing critical installation and loading issues of drag embedded VLA's, describing and interpreting experimental and analytical results, recommending procedures for addressing critical analysis and design issues.
   • A report describing critical installation and loading issues of SCA's describing and interpreting experimental and analytical results and recommending procedures for addressing critical analysis and design issues.

PRINCIPAL INVESTIGATORS & OTHERS INVOLVED:
William Bryant, Robert Gilbert, Charles Aubeney, Roy Olsen, Allan Rauch, John Tassoulas, Don Murff