Estimation of pull out capacity of suction anchors in soft clays using numerical modelling

No specific design practices are recognized by the industry today for the design and installation of suction anchors. Only geotechnical consultants used design procedures. Some studies are in progress through API RP. These studies aimed at developing a state of the art and guidance regarding the design and installation of suction anchors to be included in future versions of API RP. The results of recent installation of suction anchors in highly plastic clays demonstrated that some of the design procedures used by the geotechnical consultants are to be improved. This paper summarises how to improve them. Case histories are presented to illustrate such improvement. Introduction A suction anchors or caisson is a cylindrical unit with an open bottom and equipped with valve at its top. In a first step suction anchor penetrates under its self-weight, with free evacuation of the water located inside the skirts. In a second step, an additional driving force is created by pumping out the water entrapped between the anchor top and the soil plug. After reaching the final penetration the valve is generally closed in order to increase the pullout resistance (Fig.1 and 2). The suction anchors are generally used as main foundation and anchoring system in deep water fields. The suction anchors are currently selected due to the following advantages (Colliat, 2002):fixed location on seabed which is important in congested sub-sea development,simple installation procedures with no need for proof load testing at the site,there is no particle limitation in water depth or installation,the capacity of suction anchors can be defined more precisely than for drag anchors. Fig.1 Forces acting on the suction anchor during penetration (AVAIALBLE IN FULL PAPER) Fig.2 Girassol FPSO anchors on barge (Colliat and Dendani, 2002)(AVAILABLE IN FULL PAPER) The paper presents the following critical aspects for the installations and design of suction anchors:The soil data should be well investigated. An integrated study including geohazards evaluation, geotechnical in-situ tests, high quality sampling, advanced laboratory tests should be conducted in order to select the location of the anchors and to establish the geotechnical design parameters.Particular attention should be taken concerning the effect of the stiffeners on the skirt penetration resistance. The stiffeners may reduce the penetration resistance by reducing the interface strength along the skirt wall above the stiffeners. Based on installation results a simplified approach is proposed in this paper to take into account the stiffeners effect.Suction anchors are now commonly used as anchoring system for TLP, riser towers and in taut-leg moorings where the vertical component is predominant. Such anchors are subjected to vertical long-term tension loading. The evaluation of their long term vertical capacity should take in-to account:the behaviour of the steel soil interface during shearing,the rate effect for long term loading,the stiffeners configuration. Finite element analyses are necessary to assess the stresses and displacements of the anchors under long-term tension loads (or sustained loads).

The soil conditions within the array of the Mad Dog suction anchor locations vary significantly. One anchor Cluster is located in a slump unit where both soft and stiff clay as well as sand layers are encountered. Due to the highly variable soil conditions five different anchor dimensions were used. The paper presents relevant installation considerations and field performance results. Predicted and observed embedment pressures, tilt and twist as the anchor penetrated are presented. Introduction The Mad Dog prospect is located in the Green Canyon Area of the Gulf of Mexico. The water depth at the site varies from about 1370 m (4500ft) on top of the Escarpment to about 1680 m (5500 ft) below. The field architecture consists of a truss Spar platform moored by a taut leg system consisting 11 polyester lines distributed in three Clusters with suction anchors at the end to provide holding capacity. Suctions anchors were selected as the preferred option due tohigh positioning accuracy which is important in a slump deposit where soil conditions may change significantly within short distancesrobustness to varying/uncertain soil conditions can be accommodated by using different anchor diameters and depth to diameter ratios andproven technology with more than 500 suction anchors installed worldwide at more than 50 locations per 2003 (Andersen et al (2005). Soil investigation The soil investigation was planned and performed following an integrated approach. Geophysical and geological analysis were performed prior to the main soil investigation and concluded that there was a high degree of confidence of homogeneous sediments at all anchors locations in Clusters 1 and 3. Therefore only one boring and one (two) CPTs were performed at each of these Clusters. The conclusion from reviewing geophysical data at Cluster 2 was that there was low confidence of homogeneous sediments at each anchor location and therefore the geotechnical field program consisted of one boring and one CPT at each anchor location (in total 4 borings and 4 CPTs were performed at Cluster 2), see Field Architecture shown on Figure 1 below. Figure 1. Mad Dog field architecture with the 3 anchor clusters (available in fullpaper) Penetration analysis The penetration analysis includes calculation ofskirt penetration resistanceunderpressure needed to reach target depthallowable underpressure limited to avoid large soil Heavesoil heave inside the anchor The penetration resistance, Qtot, is calculated as the sum of wall friction, Qside, and skirt tip bearing capacity, Qtip:Mathematical equation (Available in full paper) whereAside = total side areaAtip = total tip area?= remolding factor, assumed equal to 1/St, where St is the sensitivityz = penetration depthsuDSS = characteristic direct simple shear strengthsu,tipAV = characteristic average shear strength at skirt tip level

Continued demand for cost effective methods of installing mooring systems for both FPSO's and MODU's has created a special market for marine contractors. The market pertains to development of innovative methods for installing mooring systems for a variety of water depths and soil conditions. Currently, suction anchors are being utilized mostly in permanent moorings due to the cost of installation, installation techniques and the availability of suitable installation vessels. Installation method development for suction anchors has generally been adapted to installation by crane-type installation vessels. Further development of installation methods will increase the pool of available installation vessels to include standard anchor handlers because of suitable deck area and the existing anchor handling winch. Methods are now being developed to utilize anchor handlers to both install and recover suction anchors by application of standard anchor handling techniques. This will allow suction anchors to be used more frequently in both temporary and permanent mooring systems. This paper will present the innovative design and installation method of the Schiehallion FPSO suction anchors. The paper will address the impact on suction anchor design caused by the short time span from contract award to anchor installation. This short time span demanded focus on simplicity in anchor design, fabrication, transport logistics and installation. Other aspects we will address that impacted anchor design include the large variations in soil conditions at each anchor location. Finally, we discuss the anchor design choices made and their impact on installation method and the use of a very simple anchor launch method. Introduction The mooring system for the Schiehallion FPSO, BP's offshore development West of Shetland. is located in 400 meters of water and was installed in the summer of 1997. The mooring system was initially designed with suction anchors for holding power. However, after additional soil data became available in the late spring of 1996, it became uncertain whether the previously designed suction anchors could be penetrated with suction. The- mooring system designer consequently changed the anchor design from suction to hammer driven pile anchors. A proposal was submitted wherein AkerlMaersk offered a solution with suction anchors designed for the new soil conditions. The proposal also included an innovative suction anchor launch method using standard AHV's, providing BP with a cost-effective solution to the hammered piles. Development of the launch method for the Schiehallion suction anchors was based on a pre-existing launch procedure developed by Aker Marine, Inc. for Shell. (I) In November 1996 BP awarded a contract to Aker Marine for the design and fabrication of 14 suction anchors. The mooring installation portion of the project was to be executed in 1997 by Aker/Maersk, a joint venture company. The weather West of Shetland is generally very harsh for most of the year and not very suitable for installation activities. In order to benefit from the calm weather AkerlMaersk elected to start the installation on July 1st. The late summer installation start date ret1ected the availability of installation vessels most suitable for the project. Maersk vessels were engaged in other projects, and consequently the vessels were not available to start the project before this date. The outcome of selecting the late start date resulted in having to complete the anchor design by early February, anchor construction by June, and finally transporting the anchors to the shore base by July 1st. This required a very aggressive planning and engineering sch

Suction anchor piles are the main foundation and anchoring system for the Girassol development in 1,400m water depth, including the FPSO, the offloading export buoy, the riser towers and the subsea manifolds. This paper summarizes the main assumptions in the geotechnical design analysis, together with key features of installation behavior. Based on installation results, a revised method for the penetration analysis is proposed. The Girassol experience will be relevant for future applications of suction anchors at other deepwater locations in West Africa. Introduction For the permanent mooring of production floaters in waters over 1,000m, taut leg moorings are used, i.e. at an angle ranging between 35° and 45°at mudline, with steel wire cables or fibre ropes and with anchors having a vertical loading capability. In the soft clays generally found at deepwater sites, suction anchors and vertically loaded plate anchors (VLA) have become the two preferred options. A significant number of applications for the suction anchor have been found due to the following advantages [1]:fixed location on seabed, which is important in congested subsea field developments;simple installation procedure with no need for proof-load testing; andreliable design methods for ultimate capacity calculation, providing a larger confidence in their long term pull-out capacity. The Girassol field, the deepest offshore development off West Africa to date, is located in about 1400m of water in Block 17 operated by TotalFinaElf EP offshore Angola. The Girassol FPSO is connected to 40 subsea wells by means of three-riser towers and pipeline bundles [2]. In relation to the suction anchors, the installation operations took place between March and August 2001 and were carried out by Alto Mar Girassol (AMG) and Mar Profundo Girassol (MPG)), two consortia of Stolt Offshore and Bouygues Offshore Serviices, and SBM, from the crane barge Seaway Polaris and the vessel Seaway Eagle. Girassol Seabed and Soil Conditions From high-resolution 3-D seismic and shallow geophysical data, the Girassol seabed appears as sloping gently to the southwest, with a gradient of about 1:50 (1.2°). The soil investigation campaign carried out at the Girassol field comprised:four borings taken down to 30m with alternative push sampling and cone penetrometer testing (CPTU), with one boring at the site of each group of four FPSO anchors;three 30m deep borings, with combined sampling and CPTU testing, covering the subsea well locations over an area of about 8km by 7km;one 100m deep central boring for designing the penetration depth of the well conductors;two 30m deep continuous CPTU profiles, performed at a riser tower and FPSO anchor location; anda number of 5m deep CPTU profiles, aiming at improving the knowledge of seabed conditions for the pipelines and manifold foundations. The soil conditions, consistent and uniform over the whole Girassol field, are composed of soft organic clay of high plasticity.

Suction anchors are among the popular type of anchors used for mooring/position keeping of floating vessels in deep waters. It comprises a cylindrical anchor, closed at the top and achieves penetration into the seabed by applying vacuum pressure inside the anchor. In order to arrive at guidelines for design of such anchors, field tests have been conducted on suction anchors with varying diameters and lengths for evaluating the pullout behaviour under various loading conditions. The tests have been carried out in marine conditions within breakwaters where the impact of environmental loadings from winds and waves is low. Since suction anchors are subjected to cyclic loading in the offshore areas due to effect of waves and winds, tests for static and cyclic pullout have been carried out and comparison has been made. It is found that anchor geometry, angle of pullout and nature of pull (static/cyclic) have a significant influence on the response to pullout. As angle of pullout changes from vertical to horizontal the reaction offered by the suction anchor changes from skin friction to passive soil resistance. Resistance offered by the internal plug of soil is found to vary according to dimension of the anchors. Theoretical estimates are found to reasonably predict static mooring capacity of suction anchor. A reduction of 35 % in mooring capacity is observed under dynamic response for a frequency of 6 s.

1 Abstract This paper describes a full scale field trial of taut leg mooring performed by Saga Petroleum ASA in August/September 1996 offshore Norway, using suction anchors and two types of fiber ropes attached to a semi-submercible drilling unit. The purpose of the field trial was to qualify taut leg mooring as an effective mooring system for floating drilling units. The subject drilling unit was conventionally moored by 8 chain/wire lines. The field trial was based on installing two additional mooring lines consisting of one polyethylene(Dyneema) and one polyester fiber rope in a taut leg mode using suction anchors. The operation was performed in 200 m water depth. The two large suction anchors were installed and removed from a standard AHTS vessel by use of a purpose built launching frame. This operation is normally performed by large, heavy lift vessels or crane barges. The tension and elongation of two fiber ropes were continuously monitored and stored by a data acquisition unit at site during the trial. Additional laboratory tests of the fiber ropes were carried out before and after the field trial. Saga Petroleum's field trial has demonstrated that taut leg mooring using fiber ropes and suction anchors is a viable and effective alternative to conventionally moored and dynamically positioned drilling units, especially in congested seafloor areas and in deep water. 2 Introduction About 5 – 6 years ago the industry in Norway started to look into exploration drilling in deep waters (800 – 1500 m water depth). This was initiated in preparation for exploring the deep prospects offshore Mid-Norway (Mere and Vering Basins). Saga Petroleum has been working with a simplified rig concept for this application, called "The EfEx Rig" (Effective Exploration). In autumn 1995, taut leg mooring using suction anchors and fiber ropes was raised as an alternative mooring system for this rig concept. Taut leg mooring using suction anchors and fiber ropes soon developed into a project of its own. As conventional installation of suction anchors so far has been done with expensive heavy lift vessels or barges, it was imperative to develop a more cost effective installation method. In a cooperation between a small engineering company, named Structural Engineering, and Saga Petroleum, a method of deploying suction anchors from a standard anchor handling (AHTS) vessel was developed. This was accomplished by designing a special launching frame. Saga Petroleum found that in order to qualify this deployment of suction anchors and to evaluate the behaviour of fiber ropes in a taut leg mode, a full scale field trial would be essential. The semi-submercible drilling unit, Deepsea Bergen, was scheduled to drill a well for Saga Petroleum at the Tordis Field in The North Sea, summer 1996. Even though the water depth was only 200 m, this offered a good opportunity for afield trial as Deepsea Bergen was only to be moored by 8 out of possible 10 lines.

Recently, the offshore wind industry is transitioning from the fixed-bottom in shallow waters to the floating in deep waters. This shift requires station-keeping anchors to stabilize turbines. Maximizing the cost-effectiveness and installation efficiency of anchor foundations is critical for project success. Suction anchors offer an efficient installation method by creating an underpressure to penetrate the seabed without noise from hammering like driven pile anchors. This foundation concept, well-established in the oil and gas industry, holds promise for floating offshore wind applications. Traditionally, the top cap is fixed on the suction anchor after installation to provide a sealed condition inside for better vertical holding performance. However, in the case of a catenary mooring system in soft clay, where horizontal loads dominate, the conventional fixed top cap may not be necessary. Instead, a retrievable top cap can be designed, offering cost savings and ensuring full embedment of the anchor in the seabed, enhancing its capacity, and mobilizing more soil in deep areas. This study presents a series of numerical modelling to investigate the horizontal capability of the suction anchors in clay with retrievable top caps. Comprehensive numerical simulations were performed, which were validated first against the existing studies in literatures for traditional suction anchor with fixed top cap. A parametric study was then carried out to assess the effects of the retrievable top cap length, anchor length to diameter ratio (L/D), padeye location, and loading angle on the anchor horizontal capacity. Consistent with the existing studies, the failure mechanism of horizontally loaded suction anchor change with the padeye location, which transforms from a rotation failure to horizontal translation failure as the padeye location increase from seabed to around 0.6L (anchor length) and provides the highest horizontal capacity, although it should be noted that the padeye location is also affected by the loading angle. For the anchor with retrievable top cap, almost the same horizontal capacity was observed when the L/D ratio is larger than 1. In addition, when the retrievable top part extends to upper part of anchor, a higher capacity factor can be obtained due to the increased embedment and the mobilization of deep soil. Compared with the anchor with fixed top cap, the optimum padeye location of anchor with retrievable top part will also change to a shallower depth at around 0.5L.

Model tests and analysis method on the bearing capacity for suction anchors subjected to average and cyclic loads

The Paper presents the methodologies adopted for the design of mooring and riser anchors of the P43 and P48 FPSOs due to be installed in 2003 in deep water Campos Basin, offshore Brazil. It is shown how an efficient semi-analytical 3D finite element model can be used as a routine design tool for calculating the pull-out capacity of cylindrical suction anchors under combined horizontal and vertical loading. The speed of the FE model has allowed pull-out capacity contours to be developed which help in the selection of limiting installation tolerances. Fully coupled axisymmetric finite element consolidation analyses of the set-up phenomenon have been described in which the initial excess pore pressure field is established by simulating the final penetration stages of the skirts. Introduction The Barracuda and Caratinga fields are located in deepwater Campos Basin, some 150 km offshore Brazil. The average water depth is 825m at Barracuda and 1030m at Caratinga. A taut-leg spread mooring system has been adopted for both the Barracuda (P43) and Caratinga (P48) FPSOs. Each FPSO is moored by 18 lines, ten at the stern and eight at the bow. The mooring lines are made of polyester rope, combined with lengths of chain at the fairlead and seabed ends, Wibner1. Various types of anchor were considered in the initial phases of the project. Suction anchors were selected as the preferred option for the following reasons, Sparrevik2:Relatively accurate positioning on the seabed, which isimportant in fields congested with flow lines and subsea facilities,Economical and simple installation method, avoiding the need for pile driving in deepwater or dragging and proof loading.Reasonably well-established design methods compared to other types of anchor, even though no Standard Code of Practice is yet available. In addition to the 36 mooring suction anchors, 52 suction anchors in four different sizes were designed for restraint of the risers. The subject of this paper is the geotechnical in-place capacity of mooring anchors only and excludes installation aspects. The suction anchors were designed to comply with ABS Rules3 and current industry practice for 20 years service life. Site Investigation An integrated approach to SI was taken4,5. A comprehensive geophysical and geotechnical site investigation was performed in spring 2001 from the vessel Rockwater-1. Subsea7 (formerly Halliburton Subsea), performed the geophysical surveys consisting of swathe bathymetry, ROV high resolution sub-bottom profiling and side scan sonar imaging, while Fugro BV performed the following geotechnical SI from the same vessel:66 WHEELDRIVE Piezo-cone penetration testing (CPTU) to 30m penetration,12 WHEELDRIVE insitu vane tests up to 20m. One test every 0.4m and remoulded shear strength determination every second test,39 Push-in Piston cores up to 20m. The SI was followed by a programme of monotonic and cyclic laboratory testing both in Holland and the UK.

The Mad Dog Spar is moored by suction anchors arranged inthree Clusters. Two anchor Clusters lie on top of the Sigsbee Escarpment while one anchor Cluster lies in a slump unit below the Escarpment. The soil conditions at the 4 pile locations in the slump deposit vary significantly and are very different from the soil conditions on top of the Escarpment. This dictated the need to make five different designs. This paper describes how the undrained shear strength selected for design was determined by combining the various soil conditions detected by CPT with the results from advanced and index laboratory testing. The design procedures used to design the anchors are explained in this paper. Also presented in the paper are the governing load cases together with results from holding capacity calculations using an efficient 3 D FE program. Introduction The Mad Dog prospect is located in the Green Canyon Area of the Gulf of Mexico. The water depth at the site varies from about 1370 m (4500ft) on top of the Escarpment to about 1680 m (5500 ft) below. The field architecture consists of a truss Spar platform moored by a taut leg system consisting of 11 polyester lines distributed in three Clusters, with suction anchors providing holding capacity. Suctions anchors were selected as the preferred option because oftheir high degree of positioning accuracy, which is important in a slump deposit where soil conditions may change significantly within short distances,robustness to varying/uncertain soil conditions, accommodated by using different anchor diameters/depth to diameter ratios andthey are based on proven technology with more than 500 suction anchors installed worldwide at more than 50 locations as of 2003 (Andersen et al 2005). Soil investigation The soil investigation was planned and performed following an integrated approach. Reference is made to the accompanying papers (Berger et al 2006, Liedtke et al 2006 and Jeanjean et al 2006) for details about the considerations made. The practical consequence was that one boring and one or two CPTs were performed at each of the two anchor Clusters on top of the Sigsbee Escarpment, while one boring and one CPT were performed at each anchor location in the slump area at Cluster 2 (in total 4 borings and 4 CPTâ??s were performed at Cluster 2), see Field Architecture shown on Figure 1. Figure 1 Field architecture at Mad Dog field (available in fullpaper) General soil conditions and seabed topography The soil conditions at the anchor locations in Clusters 1 and 3 are relatively uniform and dominated by homogeneous, soft to stiff plastic clay, with essentially linearly increasing shear strength with depth.

This paper presents partial results of two series of centrifuge tests that were performed on suction anchors. The first series addressed the specific conditions of the Mad Dog Cluster 2 anchors on the Sigsbee Escarpment (Berger et al 2006, Jeanjean et al 2006, and Schroeder et al 2006). Tests were performed for soil conditions, anchor geometry, attachment points, and loading angles closely matching the Mad Dog design parameters. The results provided a means of calibrating and validating the design method and failure mechanisms. The second series of tests was performed on double-wall anchors and allowed the separation of the three components of capacity: outside friction, inside friction, and reverse end bearing. Results gave insights into the failure mechanism of vertically loaded anchors and suggest that the traditional procedures for determining outside skin friction appear to be conservative. Introduction When describing the centrifuge tests, all units in the text, tables and figures refer to model dimensions, unless otherwise noted. Purpose of the centrifuge tests This paper presents partial results from two separate series of centrifuge tests to investigate the holding capacity of suction anchors in soil profile representative of the Mad Dog Cluster 2 conditions (see Schroeder et al, 2006) and to separate the components of total vertical capacity: outside friction, inside friction, and end bearing. Centrifuge tests on stiff clays and layered soil profile As described in companion papers (Berger et al 2006, Jeanjean et al 2006, Liedtke et al 2006, and Schroeder et al 2006), the geotechnical conditions at the Mad Dog suction anchor sites for the four anchors in Cluster 2, the ones on the Sigsbee Escarpment, clearly fall outside the range of commonly encountered soil properties in the Gulf of Mexico. Centrifuge tests were therefore performed at the University of Colorado at Boulder with the purpose of obtaining load test data to verify the design methodology for soil conditions relevant to the Mad Dog Cluster 2 profiles. The failure mechanism of the anchor was also predicted. Centrifuge tests on double wall anchors in soft clays Secondly, a series of centrifuge tests were performed at C-CORE, in St Johnâ??s, Newfoundland, on a specially manufactured double-wall anchor. The anchor was loaded vertically and the intent of these tests was to separate the various components of vertical holding capacity: external friction, internal friction, and reverse end-bearing. It was suspected before the test that, contrary to what is commonly assumed when interpreting load test data, the inside friction and outside friction in the anchor might not be equal. The tests were also intended to provide measurements of the end bearing capacity factor, Nc, without having to assume an alpha factor, or speculate as to how much friction was generated inside the anchor, when pulled with the top closed.

This paper provides a methodology for optimizing a deepwater mooring layout with respect to ground conditions and gives limitations of the method. Specifically, this paper:describes development of a predictive soil model based on integration of AUV geophysical survey and geotechnical data in an area characterized by complex and discontinuous sand strata interbedded with clays;describes using the predictive model to help optimize a FPSO mooring layout to meet design criteria;presents as-installed results for the suction anchors and demonstrates reliability of the predictive soil model;describes learnings from this project; anddiscusses limitations of the methodology. Suction-anchor installation was completed successfully and no problems were encountered with respect to soil conditions. The as-installed results thus confirmed the general reliability of the predictive soil model. Sand strata as thin as about 30 cm thick were predicted by the model and their presence was confirmed by increased pump underpressures recorded during anchor installation. The fact that the 5-m-diameter suction anchors " felt?? such thin sands was instructive. We demonstrate that, by using appropriate geophysical survey data calibrated with geotechnical information, a model of soil conditions can be developed and used as a tool to predict soil conditions away from geotechnical control points. Confidence in the predictions depends on the amount, type, spatial distribution, and quality of the data available and on the geology of the site. This case history is useful as a model approach for optimizing mooring layout with respect to ground conditions or for other applications where shallow soil conditions are sufficiently complex and variable to affect layout of foundation elements or design. Integrated study of the geophysical/geotechnical data was concurrent with mooring system design in a collaborative effort that directly influenced design iterations. Use of the integrated and iterative approaches described here will help future project teams develop more reliable mooring or other foundation solutions faster and thereby reduce costs. Introduction Background. The Parque das Conchas development is located off the coast of Brazil in the Campos Basin in Block BC-10 (Figure 1). Shell is the operator of BC-10 with 50% equity share with co-venturers Petrobras (35%) and ONGC (15%). The site is on the continental slope and is characterized by hummocky topography (Figures 2 and 3a) caused by buried mass-transport deposits (MTDs). Water depths across the mooring spread range from about 1740 m to 1800 m (Figure 3a). Production facilities feature the FPSO Espirito Santo with a clustered 3x3 suction-anchor mooring system; the mooring layout is shown in Figure 3a. Mooring radius varies slightly from cluster-to-cluster and ranges between about 2500 to 2700 meters. More details and other aspects of the FPSO Espirito Santo are discussed by Howell and others (2010).

Suction anchors play a crucial role as marine supporting infrastructure within mooring systems. In engineering practice, the composite load comprising nonlinear waves and cyclic pull-out loads can have adverse effects on the seabed soil, posing a threat to the pull-out bearing capacity of the suction anchor. While existing research predominantly focuses on cyclic pull-out loads, the influence of nonlinear wave actions at the seabed surface remains overlooked. This study employs a two-dimensional integrated numerical model to investigate the dynamic soil response around a suction anchor under the influence of both nonlinear waves and cyclic pull-out loads, focusing on the mechanisms that lead to liquefaction and the deterioration of the interfacial friction due to the excess pore pressure buildup. The numerical results reveal that the cyclic pull-out load is the primary factor in the deterioration of the frictional resistance at the suction–soil interface, especially when the pull-out load is inclined with the suction anchor. Parametric studies indicate that the relative difference in frictional resistance deterioration between cases considering and excluding surface water waves becomes more pronounced in soils characterized by a small consolidation coefficient (Cv) and relative density (Dr).

This article presents a procedure to calculate the bearing capacity of suction anchors subjected to inclined average and cyclic loads at the optimal load attachment point using the undrained cyclic shear strength of soft clays based on the failure model of anchors proposed by Andersen et al. The constant average shear stress of each failure zone around an anchor is assumed and determined based on the static equilibrium condition for the procedure. The cyclic shear strength of each failure zone is determined based on the average shear stress. The cyclic bearing capacity is finally determined by limiting equilibrium analyses. Thirty-six model tests of suction anchors subjected to inclined average and cyclic loads were conducted, which include vertical and lateral failure modes. Model test results were predicted using the procedure to verify its feasibility. The average relative error between predicted and test results is 1.7%, which shows that the procedure can be used to calculate the cyclic bearing capacity of anchors with optimal loading. Test results also showed that the anchor was still in vertical failure mode under combined average and cyclic loads if an anchor was in vertical failure mode under static loads. The anchor failure would depend on the vertical resistance degradation under cyclic loads if an anchor was in lateral failure mode under static loads. Cyclic bearing capacities associated with the number of load cycles to failure of 1000 were about 75% and 80% of the static bearing capacity for vertical failure anchors and lateral failure anchors, respectively.

With the increasing utilization of suction anchors in mooring systems, field surveys have observed seabed trenches near suction anchors caused by anchor chain motion and soil erosion. The presence of these trenches significantly reduces the pull-out capacity of suction anchors, emphasizing the need for attention. This study investigates the impact of seabed trenches on the undrained pull-out capacity of suction anchors at an optimal padeye depth through a series of finite element analyses. Rectangular cross section trenches are considered in this study. The effects of trench size, trench distance, trench shape and local scour on reducing the pull-out capacity of suction anchors are systematically examined. Furthermore, we present an optimal design approach for suction anchors considering seabed trenches that addressed two aspects: (a) determining whether higher pull-out capacity can be achieved by relocating the anchor padeye to a shallower depth and reducing trench depth; and (b) identifying the optimal anchor aspect ratio considering material cost and seabed trench conditions. These analyses provide valuable insights for practical design considerations when dealing with trench-related issues encountered by suction anchors.