OMNI-Max Anchors Research Articles

The pullout capacity of OMNI-Max anchors in sand

As the latest development and benchmark of a gravity installed anchor (GIA), the OMNI-Max anchor stands as a cutting-edge achievement and benchmark, finding increasingly widespread use within mooring systems due to its exceptional operational performance and adaptability. Notably, while investigations into the pullout capacity of OMNI-Max anchors have been conducted extensively in clay, the relevant studies are seldom observed in sand. Actually, the mechanical properties of sand are quite different from those of clay, and sand is also widely distributed in seabed soil. Full knowledge of OMNI-Max anchors not only in clay but also in sand is necessary to a wider application of the anchors. In the present work, the large deformation finite element (LDFE) method is adopted combined with the coupled Eulerian-Lagrangian (CEL) technique to study the end-bearing characteristics of the OMNI-Max anchor in sand seabeds. A bounding-surface plasticity model is taken as the constitutive model to capture the characteristics of sand. Through investigation and analysis, OMNI-Max anchors are closely related to the anchor embedment depth, the soil relative density, the anchor orientation, the loading angle and the bearing area, so the working conditions related to these five factors are designed and calculated. An explicit expression of the end-bearing capacity factor is finally derived to provide a simple and fast tool of evaluating the pullout capacity of the anchor in sand under multiple factors. Validation cases and orthogonal tests have confirmed the effectiveness and applicability of the explicit expression.

Numerical study on keying capacity of gravity installed anchors in sand

Abstract Being the latest representative of GIAs, the OMNI-Max anchor performs comprehensive behaviors in the seabed, like keying and diving, by adjusting orientation and position to derive higher capacity and therefore to avoid anchor failure. Current research of OMNI-Max anchors is concentrated on clay, while there is a notable gap of the work on sand. During the keying process of GIAs, many factors may influence the keying capacity, such as the embedment depth, the anchor orientation, the rotational center, the bearing area and the soil strength. Large deformation finite element analyses combined with a bounding-surface plasticity constitutive model are performed to investigate systematically the keying behavior of GIAs. A series of analytical cases involving multiple factors are designed and analyzed to explore the effects of various factors on the keying capacity of GIAs, defined by the soil resistance coefficient during keying. The soil resistance coefficient increases with increasing soil density, while it tends to be stable with the increase of the embedment depth. The closer the rotational center approaches to the two ends of the anchor, the greater the soil resistance coefficient becomes. An explicit expression of the soil resistance coefficient during keying is derived to quantify the effects of various factors. These findings are helpful to understand further the comprehensive anchor behaviors and to promote the application of GIAs in offshore engineering.

Loading Rate Effect on the Pullout Capacity of OMNI-Max Anchors in Clay Coupled With Multiple Factors

Abstract As the latest development of gravity installed anchors (GIAs), the OMNI-Max anchor has drawn much attention from worldwide due to its unique behavior in the seabed. The pullout capacity of OMNI-Max anchors is a key index in engineering. However, most of the relevant studies were carried out under a quasi-static condition, which do not actually meet the installation and operation requirements. In practice, the anchor may be subjected to both long-term and short-term sharp loading during mooring. As an important environmental variable, it is essential to evaluate the effect of loading rate on the pullout capacity. Since the bearing capacity of OMNI-Max anchors is affected by many factors, it is also essential to explore systematically the coupling effects of the loading rate and other factors, including the anchor embedment depth, the anchor orientation, the bearing area, the loading angle, and the soil strength. Based on the coupled Eulerian–Lagrangian (CEL) technique, numerous analytical cases are designed and calculated by the large deformation finite element (LDFE) method. The loading rates span four orders of magnitude from the quasi-static velocity to 10 m/s (about one anchor length per second), covering a wider range in pulling out of GIAs. The end-bearing capacity factor changes remarkably with the pullout velocity for OMNI-Max anchors, and the increase can even reach more than twice of that in a quasi-static condition. As a result, a succinct explicit expression is constructed in terms of the loading rate and multiple factors, which can be effectively utilized to calculate the end-bearing capacity factor of OMNI-Max anchors in clay under complex conditions.

Applicability analyses on the end-bearing capacity of gravity installed anchors in clay

As an efficient solution for deepwater moorings, gravity installed anchors (GIAs) have many attractive attributes and play a remarkable role in various deepwater applications. The end-bearing capacity factor offers an efficient method to evaluate GIA end bearing under complex conditions involving multiple factors. It can also assess total anchor bearing capacity. Based on numerous large deformation finite element (LDFE) analyses, two formulas have been developed to calculate the end-bearing capacity factors for GIAs in clay under quasi-static or dynamic conditions. Through rigorous statistical analysis, it is demonstrated that the formulas provide accurate predictions of the end-bearing capacity for various GIAs, particularly those with loading characteristics similar to OMNI-Max anchors, i.e., the padeye location and the loading angle. The contribution of the end bearing to the overall bearing capacity of GIAs is analyzed under various loading conditions. Furthermore, a comparative study proves that the formula accounting for the loading rate effect can be degenerated to evaluate the end-bearing capacity factor of the anchors under quasi-static conditions. The results provide a comprehensive understanding of the end bearing and offer an efficient solution for determining the total bearing capacity of various GIAs in clay under complex conditions. These findings contribute to the advancement of design and performance assessment of GIAs, enhancing their reliability and effectiveness in applications.

Numerical study on the performance of a novel gravity installed anchor with ring fins in clay

A novel gravity installed anchor with ring fins (RGIA) is proposed in the present work, which combines features of the conventional gravity installed anchor (GIA) and the drag embedment plate anchor, with two soft shanks attached to the anchor body. A systematic study on the performance of the RGIA in clay is performed by large deformation finite element analyses using the coupled Eulerian-Lagrangian technique (CEL), with emphasis on the initial penetration, the behaviors of keying, diving and pulling out, the optimization of the shackle point, and the effect of loading rates on the maximum embedment loss (MEL). The initial penetration of the anchor is simulated under various impact velocities, from which the maximum initial embedment depth in the seabed is acquired. Two factors of the shackle point, en and eh, are defined and the detailed criteria for optimizing the shackle point are put forward. A series of analytical cases are designed to thoroughly explore the effects of the shackle point on the performance of the RGIA in clay, focusing on the keying and diving behaviors and the pullout capacity. The formulas for calculating the diving angle and the MEL are then constructed with high precision, by taking into account the effect of the shackle point, and can be utilized to optimize the position of the shackle point. Comprehensive drag effects under multiple factors, especially the high loading rate, on the performance of the RGIA in clay are quantitatively investigated by performing six groups of cases, from which the effects of drag velocity, initial embedment depth, gradient of shear strength of clay, and cable parameters can be known. The formula for calculating the MEL of the RGIA in clay is also constructed with high precision by taking into account comprehensive drag effects. The numerical study demonstrates that, compared with the OMNI-Max anchor, the RGIA achieves deeper initial penetration depth during installation, requires shallower initial penetration depth that ensures subsequent keying and diving, and possesses stronger ability of resisting dynamic loading with high loading rates.

The failure induced by embedment loss of gravity installed anchors in clay

The reliability of mooring systems is a critical issue for deepwater floating structures. Gravity installed anchors (GIAs) have been effectively employed in a significant number of mooring systems, whose embedment loss and kinematic behaviors under the cable loading conditions also attract attention to the reliability of the anchors during service. Particularly, the embedment loss of the anchor may result in the failure of the anchor. The present work focuses on systematically investigating the failure state of anchors in clay induced by embedment loss under various complex conditions. A theoretical model for analyzing the comprehensive behaviors of the anchor is constructed, which is validated by previous centrifuge tests and 1g model tests. A rational criterion for determining the state of failure induced by embedment loss is proposed and applied to the analytical results of the theoretical model. The factors that have a substantial impact on embedment loss of the anchors, including the anchor embedment depth, anchor orientation, cable loading angle at the seafloor, soil adhesion factor, soil strength, and cable properties are investigated in the analytical cases. OMNI-Max anchors, being one of the most remarkable GIAs and having complex kinematic behaviors, are selected to reveal the influences of these factors on the failure state of the anchors. Based on the results of numerous analytical cases, the failure surfaces in the three-dimensional spatial data field are presented, which reveal the critical state of the anchor under different conditions. Furthermore, these failure surfaces are stated using a surface function. The function can be used as a simple judgment condition for the state of failure owing to anchor embedment loss. The findings of earlier investigations support the applicability of the surface function. In addition, the promising application of the surface function in practice is highlighted.

Theoretical and numerical approaches to analyze gravity installed anchors in multi-layered clays

Gravity installed anchors (GIAs) are designed to dive into deeper seabed under cable loading to seek higher capacity, typically for OMNI-Max anchors. This feature makes the anchors more likely to penetrate through different soil layers, leading to more complicated behaviors than in single-layered soil. Furthermore, the behavior of anchor lines in multi-layered soils becomes more complicated and significantly affects the anchor behaviors. All of these bring challenges to the trajectory prediction of GIAs in the multi-layered seabed. The present work introduces theoretical and numerical approaches to predict the comprehensive behaviors of GIAs by dealing with the coupled effect of anchor lines in multi-layered clays. The theoretical and numerical models are verified by comparing with the published data of centrifuge tests and numerical simulations. The comparative study indicates that both the models have the ability to explore the effects of different factors, such as the embedment depth, the soil strength, the thickness of layers and the distribution of multi-layered soil. The complexity and variety of anchor behaviors in multi-layered clays are revealed by typical application cases, further confirming the ability and potential of the developed models.

The diving and keying capacities of OMNI-Max anchors in clay

Different to the conventional gravity installed anchors (GIAs), the OMNI-Max anchor is capable of deriving higher capacity through the diving behavior (obtaining deeper embedment depth) after the keying process (usually accompanied by the loss of embedment depth) subjected to excessive loading, due to its three-dimensional structure and the various way to bear applied loading. The capacity properties of the anchor during keying and diving not only determine the keying and diving behaviors of the anchor, but also influence the other anchor behaviors, such as the embedment loss, pulling out and kinematic trajectory. Existing knowledge of the diving and keying behaviors is rather rare, stimulating further exploration of diving and keying capacities for OMNI-Max anchors. A systematic and quantitative study is performed on the complex capacities of OMNI-Max anchors during diving and keying in clay under multiple factors, including the anchor embedment depth, the anchor orientation, the interface friction, the loading direction, the bearing area of the anchor, the rotational center of the anchor, and the property of soil. Numerous cases of large deformation finite element (LDFE) analyses are conducted by employing the technique of coupled Eulerian-Lagrangian (CEL), through which the impacts of different factors can be analyzed. The position of the rotational center affects most the moment capacity factor among all the factors, while the anchor orientation has only a tiny influence on the moment capacity factor. The minimum value of the moment capacity factor results from the rotational center located at approximately one-third to half of the shaft length away from the anchor rear. The end bearing during diving when the movement is parallel to the shaft, is about one-quarter to one-third of that during pulling out when the movement is perpendicular to the shaft. Explicit expressions of bearing capacity factors composed of multiple factors are eventually formulated, which are validated by orthogonal tests and published data, and can be used conveniently not only to quantify the effects of different factors but also to evaluate the diving and keying capacities of OMNI-Max anchors.

Numerical investigation on hydrodynamic characteristics of a novel gravity installed anchor with ring fins

A novel gravity installed anchor (GIA) is proposed in the present work, which combines features of the GIA and the drag embedment plate anchor, with a ring fin attached to the anchor tail during free fall to enhance the directional stability and separated from the anchor body when penetrating the seabed. A systematic numerical investigation on hydrodynamic characteristics of the novel GIA is performed by the computational fluid dynamics (CFD) approach to clarify the effects of the ring fin. First, different anchor shapes for the novel GIA are compared to acquire the optimum anchor body, focusing on the drag coefficient, terminal velocity and directional stability. Second, a set of orthogonal tests and three groups of cases are designed and performed to thoroughly investigate the effects of ring fin on the hydrodynamic characteristics, from which the effects of the radius, height and thickness of the ring fin can be known on the terminal velocity and the directional stability of the anchor. Finally, empirical formulas for calculating the restoration moment coefficient and the terminal velocity of the novel GIA are constructed with high precision and efficiency, by taking into account the effects of the ring fin, which can be used to determine the optimum dimensions of the ring fin. The numerical investigation confirms that the novel GIA possesses better directional stability and higher terminal velocity compared to the OMNI-Max anchor.

Fish anchor diving behaviour: confirmation through field tests in the Swan River

This paper reports the results from field tests on a 1/15th scaled fish anchor. The tests were conducted at three locations in the Swan River, Perth, Australia. Two series of tests were performed from the Burswood and Maylands jetties with water depths between 1·1 and 1·9 m. The final series of tests were undertaken in deeper (2·6 m) waters from a barge. The riverbed at the Burswood jetty and barge test location consisted of clay, and that at the Maylands jetty comprised sandy silt. The tip embedment depths of the scaled fish anchor, with dry weight of 0·304 kN and impact velocity of 5·89–9·55 m/s, in soft clay were 1·17–2·40 times the anchor length. For similar impact velocities, the embedment depths in sandy silt were 30–60% shallower. By comparing the field test data in clay, the fish anchor achieved normalised embedment depths similar to those of the torpedo and OMNI-Max anchors under half or less impact velocity. Importantly, the field tests confirmed the diving behaviour of the fish anchor under loading with mudline inclination of 20° and 25°, with the second peak dictated by the capacity. The ultimate capacity was 5–7 times that of the anchor submerged weight in water.

Innovative Booster for Dynamic Installation of OMNI-Max Anchor in Clay: Numerical Modeling

Abstract An innovative booster is proposed with the aim of increasing the final penetration depth of the OMNI-Max anchor in the clayey seabed with high strength gradient. The booster is attached to ...

A Unified Model for Analyzing Comprehensive Behaviors of Deepwater Anchors

Anchors may exhibit various complicated behaviors in the seabed, especially for deepwater anchors including gravity installed anchors (GIAs) and drag embedment plate anchors (drag anchors), stimulating the development of an efficient analytical tool that applies to a variety of anchors. The present paper introduces a unified model for analyzing different anchor behaviors in both clay and sand, consisting of unified concepts, mechanical models, and analytical procedure. The kinematic behaviors of the anchors are classified uniformly as three types, i.e., diving, pulling out, and keying. By utilizing the least-force principle, various anchor properties, such as the ultimate pullout capacity (UPC), failure mode, movement direction, embedment loss, and kinematic trajectory, can all be determined by the combination and analysis of the three behaviors. Applications of the model are demonstrated summarily, by solving the UPC and the failure mode of anchor piles and suction anchors, the kinematic trajectory of drag anchors in a single soil layer or layered soils, the maximum embedment loss (MEL) of suction embedded plate anchors (SEPLAs) and OMNI-Max anchors, and the kinematic behavior of OMNI-Max anchors. Compared to existing theoretical methods, this unified model shows strong applicability and potentiality in solving a variety of behaviors and properties of different anchors under complicated seabed conditions.

A unified explicit formula for calculating the maximum embedment loss of deepwater anchors in clay

Deepwater anchors, typically the suction embedded plate anchor (SEPLA) and the OMNI-Max anchor, would encounter the phenomenon of embedment loss after initial penetration, which remarkably reduces the pullout capacity and affects the subsequent behavior of the anchor. Therefore, the maximum embedment loss (MEL) becomes a key index for design and engineering practice. Based on the mechanical mechanism and kinematic property of anchors, a unified explicit formula for calculating the MEL of SEPLAs and OMNI-Max anchors in clay, in terms of the anchor geometry, interactional property, soil condition, anchor line property and applied loading, is developed by combining theoretical models and nonlinear fitting analyses. The efficiency and precision of the formula are validated by experiments and other available methods. Compared to existing methods, the proposed formula provides a simple tool for fast assessing the MEL of both SEPLAs and OMNI-Max anchors in clay, and performs superiorly in easiness, efficiency and applicability.

The pullout capacity of OMNI-Max anchors in clay

Being an impressive representative of gravity installed anchors (GIAs), the OMNI-Max anchor has many attractive attributes and becomes more and more a conspicuous role in recent deepwater applications. However, the pullout capacity property of OMNI-Max anchors has not been well understood due to their three-dimensional structure and the loading way in variational seabed conditions. Full knowledge of the pullout capacity property is beneficial to understand not only the working performance but also the comprehensive kinematic behaviors of OMNI-Max anchors in the seabed. The present work aims to investigate the complicated pullout capacity property of OMNI-Max anchors in clay under multiple factors, including the anchor embedment depth, the anchor orientation, the bearing area, the loading angle, and the soil strength. Large deformation finite element analyses are performed using the coupled Eulerian-Lagrangian (CEL) technique. Numerous cases of multiple factors are designed and analyzed to acquire the effects of different influential parameters. An explicit expression of the end-bearing capacity factor of OMNI-Max anchors is finally formulated in terms of multiple factors, whose effectiveness and applicability are validated by orthogonal tests and available published data. This expression can be simply utilized to evaluate the pullout capacity of OMNI-Max anchors under complicated conditions.

Loading performance of fish and OMNI-Max anchors in crust-over-soft clays

This paper reports the results from three-dimensional dynamic finite element analysis undertaken to provide insight into the behaviour of the fish and OMNI-max dynamically installed anchors during loading in crust-over-soft clay sediments. Particular attention was focused on the situations where the anchor is embedded to a shallow depth during dynamic installation due to the strong crust layer. Large deformation finite element analyses were carried out using the coupled Eulerian-Lagrangian approach, incoporating the anchor chain effect. Parametric analyses were undertaken varying the initial embedment depth, anchor shape, loading angle, strength ratio between the top and bottom layers. The tracked anchor trajectory confirmed that the diving potential of the fish and OMNI-Max anchors were enhanced by the presence of the crust layer as that somewhat restircted the upward movement. This will be beneficial for many hydrocarbon active regions with layered seabed sediments where the anchor embedment depths during dynamic installation are expected to be low.

CFD analysis on the directional stability and terminal velocity of the OMNI-Max anchor with a booster

The OMNI-Max anchor, a newly developed gravity installed anchor, offers a cost effective anchoring solution with improved reliability for deep-water mooring facilities. However, the anchor final penetration depth is relatively shallow in the soil with high strength gradient. A booster, which can be attached at the tail of the OMNI-Max anchor, is designed with the aim to improve both the anchor directional stability and terminal velocity, such that the anchor can penetrate deeper into the seabed and consequently gain higher holding capacity. The objective of this paper is to investigate the directional stability and terminal velocity of the OMNI-Max anchor and hybrid anchor (i.e. an OMNI-Max anchor with a booster) based on the computational fluid dynamics (CFD) approach. The numerical simulation results demonstrated that the booster is beneficial in moving the position of the hydrodynamic centre towards the anchor rear, hence the directional stability of the hybrid anchor is improved. Moreover, the anchor terminal velocity was increased from 22.44 m/s to 32.01 m/s by the aid of a booster. Finally, an optimised design on the booster geometry was performed to ensure that the hybrid anchor can obtain high terminal velocity and maintain directional stability at the same time.

An innovative booster for dynamic installation of OMNI-Max anchors in clay: Physical modelling

An innovative booster concept is proposed to increase both the kinetic and potential energies and hence to increase the penetration depth of the OMNI-Max anchor within the clayey soil with high strength gradient. This paper carried out 1g model tests to investigate the working efficiency of the booster on the anchor penetration depth both in normally consolidated (NC) and lightly over-consolidated (LOC) soils. The similarity relationships of the anchor dynamic penetration within the soil were first deduced, and then a series of dynamic penetration tests were performed to investigate the parameters that influence the anchor penetration depth. The parameters include the soil strength characterisation, impact velocity, booster weight, inclined impacting angle, and water drag and entrainment effects. The anchor dynamic penetration process within the soil was measured by a micro-electromechanical systems (MEMS) accelerometer, through which the soil strain-rate effect and the hydrodynamic aspects including the water drag force and potential water entrainment along the anchor-soil interface were analysed. Finally, energy-based and force-based prediction models were put forward to estimate the penetration depths of OMNI-Max anchors and hybrid anchors (i.e. an OMNI-Max anchor with a booster).

An innovative concept of booster for OMNI-Max anchor

OMNI-Max anchors are newly developed dynamically installed anchors for deep water mooring systems. However, the final penetration depth of the anchor after its dynamic installation can be limited, especially in a strong soil seabed with high strength gradient, due to its limited release height in water and large contact areas between the anchor flukes and the adjacent soil. To increase the anchor final penetration depth in seabed, an innovative concept of anchor booster is proposed in this paper. The booster, comprised of a cylindrical shaft with three rear fins, is attached to the anchor tail during its dynamic installation and then is retrieved after installation for reuse. The boosters with three different sizes, hence different masses, are designed and tested. Three dimensional computational fluid dynamics (CFD) simulations were conducted to investigate the working efficiency of a booster on the final penetration depth of the OMNI-Max anchor in a strong soil seabed. Moreover, the entire process of the dynamic installation of the hybrid anchors (an OMNI-Max anchor with a booster) from water to seabed is predicted by a theoretical model. The results demonstrate that the booster is beneficial both in improving the directional stability and impact velocity for the OMNI-Max anchor during its free fall in water. Relative to the OMNI-Max anchor, the final penetration depth of a hybrid anchor increases significantly (from 23% to 115%) due to the addition of the booster, which helps the anchor to gain higher holding capacity. This preliminary study is only to prove effectiveness of the booster concept without a complete investigation of the booster. More detailed studies are planned in the pipeline.

Keying Process of the OmniMax Anchor Shallowly Embedded in Undrained Normally Consolidated Clay

Keying Process of the OmniMax Anchor Shallowly Embedded in Undrained Normally Consolidated Clay

Experimental investigation of the keying process of OMNI-Max anchor

ABSTRACTThe OMNI-Max anchors are newly developed dynamically installed anchors for deep water mooring systems. After installation, the anchor is keyed to a new orientation and position by tensing the attached mooring chain, which is known as the “keying process”. This study conducted 1g model tests to study the trajectories and capacity developments of OMNI-Max anchors in homogeneous and lightly overconsolidated (LOC) clays. A testing arrangement was designed to simulate the anchor keying process with a constant pullout angle at the mudline. A half model anchor which could move against the box glass was used to determine the anchor trajectory in the soil. The effects of padeye offset angle, uplift angle at the mudline, anchor fluke thickness, anchor initial embedment depth, and soil strength on the anchor trajectory and capacity were systematically investigated. Moreover, the critical uplift angle at the padeye and the anchor critical initial embedment depth were discussed. The results indicate that the anchor can dive both in homogeneous and LOC clays under certain conditions. A padeye offset angle of 24–30° is recommended for the OMNI-Max anchor to maintain high capacity and diving trend simultaneously. Besides, the anchor diving trend can be improved with small uplift angles at the mudline and with thick anchor flukes. A critical initial embedment depth of 1.3 times the anchor length is recommended to preclude the anchor from being pulled out.