Vibratory Installation Study of a Monopile in Dense Sand

The assessment of monopile installation by means vibratory driving is discussed for a platform located in the Dutch Sector of the North Sea. The vibratory driving analysis of the 4.7 m diameter monopile was performed using the “Hypervib” method in order to evaluate the performance of a 960 kg.m excentric moment vibrohammer. The results of the installation study indicated that the monopile could be installed up to a penetration depth of 31.5 m considering the full nominal power and frequency (23 Hz) of the specified vibrohammer. The frequency of the vibrator is shown to influence the projected penetration of the monopile. In particular, the “Hypervib” method indicated that refusal could be encountered if the effective frequency of the vibrator dropped below 20 Hz. As a consequence, monitoring measures were recommended to mitigate/characterize the risks of early refusal.

Effect of Large Deformation Analysis for Site Investigation Tool - CPT in Layered Soils

Cone penetration test (CPT) is regularly used during offshore site investigations to interpret soil stratification and soil characteristics due to its continuous penetration resistance profile. However, its use could be improved if better numerical methods to simulate its penetration could be developed. Finite element (FE) analysis, for instance, has the potential to provide insightful information on soil response and soil flow mechanisms. However, it is challenging to simulate CPT in layered soils, as the soil experiences extremely large strains around the cone and the simulation costs are high. In this study, the efficiency of using a partial large deformation FE (LDFE) approach was explored to examine the pre-embedment depth allowed for saving LDFE analysis cost. The LDFE analysis was conducted using the remeshing and interpolation technical with small strain (RITSS) method to model the large strain problem. Both soft-stiff-soft clays and clay-sand-clay soil were considered to study the thin stiff layer effect when it was sandwiched in soft clay. The LDFE/RITSS analysis compared a CPT penetrating from the soil surface with penetrations from a pre-embedded depth above the stiff layer. Pre-embedded small strain analysis was also conducted for comparison. The results show that the small strain analysis underestimated the resistance in both clay and sand. For the partial LDFE analysis with pre-embedment in the top clay layer, the CPT response in the middle stiff clay layer could be well captured regardless of the initial pre-embedment depth. However, for the middle medium dense sand layer (ID = 60%), the pre-embedment depth needs to have sufficient distance above it (10D, D is cone diameter) to capture the soil response in the sand layer correctly.

Study on Vertical Bearing Capacity of the Wide-Shallow Bucket Foundation in Two Layered Sand

The wide-shallow bucket foundation proposed by Tianjin University of China is a new type of offshore wind turbine foundation. In this paper, the vertical bearing capacity of wide-shallow bucket foundation embedded in two layered sand that contains an underlying medium strength sand layer and a weaker or stronger overlaying sand layer is studied. A parametric study for bearing capacity is carried out with the ratio of unit weight γ1/γ2 (where γ1 and γ2 are the unit weight of the upper and lower sand layers respectively), the ratio of internal friction angle φ1/φ2 (where φ1 and φ2 are the internal friction angle of the upper and lower sand layers respectively) and relative thickness of the top sand layer H1/B (where H1 and B are the thickness of the top sand layer and the bucket foundation diameter). All of the presents were performed by the Finite Element Method and the results show that the thickness of the top layer has a great influence on the vertical bearing capacity of the foundation. Specifically, the upper sand layer is stronger, the bearing capacity ascends with the increase of the thickness of the top layer, and on the contrary, the upper layer is weaker, and the bearing capacity decreases with the increase of the top layer thickness. In addition, the bearing capacity of the foundation also increases with the ratios of the effective unit weight and the internal friction angle.

Field Trials of Suction-Assisted Installation of Circular Steel Pipe Cofferdam in Silty Sand

The cofferdam is the temporary barrier to stop the flow of water from a construction site work such as a support column foundation at a river or offshore. It allows for working in the dry condition when the construction is done adjacent or within the waters. However, it is a major cause of delays and increased construction costs because additional works are required to stop the water flow. Recently, in order to overcome the limitations of the conventional cofferdam methods such as sheet pile or caisson tube cofferdams, a large-diameter steel pipe cofferdam method has been proposed which can be installed quickly using suction installation method. The steel pipe cofferdam method is characterized in that the top-lid of the steel pipe is located above the sea level in order to use it as a water barrier, unlike conventional suction buckets where the whole structures are submerged. In this study, the circular steel pipe cofferdam with a 5 m inner diameter was fabricated and the installation tests were conducted on silty sand at the Saemaguem test site. During the experiment, variations of suction pressure and inclination of the steel pipe cofferdam were measured and post-analyzed. This study verified the new steel pipe cofferdam method and confirmed that the suction installation method can be successfully used for various purposes on the offshore structures.

Coupled Discrete Element-Finite Difference Method Simulations of OMNI-Max Anchor Installation in Granular Seabed

The ocean resources development is becoming increasingly urgent as the depletion of land reserves combine to enhance demand. In some of the deep-water areas, terrigenous deposits are found near landmasses, which are formed from material eroded from the land surface. They are constituted of clay, silt, sand, and granular soil. More and more gravity installed anchors (GIA) are employed as a part of the offshore foundation systems in these deep-water areas. A kind of newly developed GIA, called OMNI-Max anchor, with a mooring arm located near the anchor tip that is free to rotate about the anchor length, are an effective approach for mooring marine devices to the ocean floor. While providing the reaction force, these anchors can maintain the stability of offshore facilities. The ability of the anchor to achieve these duties relies on its keying and diving behaviors after penetration. Both shallow and deep penetrations, including offshore foundations and anchors penetrations, in granular media are what long interests geotechnical and geophysical fields. The final penetration depth of GIA in the granular seabed is also influenced by quite a few factors, such as impact velocity, particle size distribution (PSD) and anchor surface friction. However, this kind of large-strain problem is not agreeable to typical continuum numerical methods. In the current work, we propose that the coupled discrete element method (DEM) and finite difference method (FDM) is a more proper and efficient tool to investigate the penetration of OMNI-Max anchors in granular soil. The effects of the factors mentioned above are considered in the coupled DEM-FDM simulations. The relative ultimate penetration depths for different penetration conditions are presented and quantified. The response of granular material during penetration is applied to provide insight into system response at the microscale. Energy dissipation in the assembly by both friction and collision at the particle scale is considered. Results show that anchor penetration depth grows with rising impact velocity, while it decreases with an increase of anchor surface friction. When the ratio of fluke width and median diameter of granular size is smaller than 5.6, even under a relatively loose state, the application of OMNI-Max anchor is not recommended because of difficulty in assuring the required penetration depth (about 1.3 anchor lengths, 11.90 m). Although, at the similar impact velocity, the GIA tip embedment in sand is quite shallower than that in clay, alternative GIA designs may realize higher penetrations in the sand, and prove to be viable anchoring solution for granular seabed sediments. Finally, the fabric characters after penetrations are presented to analyze and reveal the state of soil experienced drastic disturbance. The characteristics of these distributions tend to particular states depending on the relative position to the anchor, which have a significant influence on subsequent behaviors of OMNI-Max anchor during the keying process.

Numerical Investigation on the Global Behavior of Jack-Up Unit Next to Footprint With Rectangular-Shaped Spudcan

Footprints were left on the seabed due to previous operations, and the problem of the spudcan-footprint interactions gradually becomes one of the major concerns of jack-up unit installation. The spudcan is subjected to eccentric loading conditions, which can lead to structural failure and the overturn of the jack-up unit. This paper reports the global behavior of the three-legged jack-up unit with a novel rectangular-shaped spudcan when it needs to reinstall near an existing footprint. The coupled Eulerian-Lagrangian (CEL) approach was used. A simplified three-legged jack-up unit model was developed and the flexible stiffness between the hull and legs was considered. The free-rotation heading and force-controlled loading conditions were also adopted. The bending moment distribution along the legs, horizontal force and the overturn of the jack-up unit are discussed to assess the potential of the jack-up unit sliding toward the footprint center. The critical offset distance was found at 0.3D and an offset distance of at least 1 times of the spudcan diameter is able to diminish influence of the footprint. The ability of the novel rectangular large spudcan with a flat base is shown to be effective at easing spudcan-footprint interactions, compared with generic circular spudcan.

CFD Simulation on Dynamic Installation of the Light-Weight Gravity Installed Plate Anchor in Clay

A light-weight gravity installed plate anchor (L-GIPLA) is put forward in the present study to secure offshore floating structures, such as floating wind turbines, floating net-cages, wave and tidal energy converters, and floating oil and gas platforms. The L-GIPLA is installed with the aid of a booster, which can be connected at the tail of the anchor during dynamically installation and retrieved after installation. The L-GIPLA owns the advantages of dynamically installation, deep penetration depth and high capacity efficiency. In the present study, the dynamic installation process of the L-GIPLA with a booster in clay is modelled by conducting three-dimensional large deformation analysis in commercial software ANSYS CFX, which is a computational fluid dynamics (CFD) software based on the finite volume method (FVM). Non-Newtonian fluid model, incorporating the strain-rate and strain-softening effects, is used to simulate the undrained clayey soil. Various factors influencing the final penetration depth of the anchor have been studied. These factors include the soil strength characterizations (strain-rate and strain-softening effects), and impact velocity. The results show that the L-GIPLA can obtain a relatively deep penetration depth in the seabed, indicating the L-GIPLA is an efficient alternative in offshore engineering.

Numerical Investigation on Suction Pile’s Holding Capacity Installed in Carbonate-Type Soils

Suction piles have not been widely used in carbonate-type soils (i.e., muds/silts) because the pile skin frictions in this type of soils are only about 5% of that in normal clayey soils. The holding capacity of a suction pile installed in these types of soils may be affected by its lower friction. Moreover, pile designers have concerns not only on the development of the Reverse End Bearing (REB) but also on how long the REB can sustain. This paper presents the development of a three-Dimensional Finite Element Analysis (3D FEA) model and the analysis results to investigate the behavior of suction pile for different levels of skin frictions. Firstly, the FEA model is used to investigate the development of the Reverse End Bearing (REB) of a suction pile by assigning two different levels of pile external skin frictions, i.e., 5% and 100% (full skin friction). A vertical load is applied at the center of the pile top. Secondly, the FEA model is used to investigate the behavior of a suction pile for a very low level of pile skin friction (i.e., 5% skin friction). An inclined load with various load angles from horizontal is applied at the padeye (i.e., 16m below seabed). Thirdly, the load carrying (failure) mechanism has been checked by examining the total displacement vectors of soil masses around the pile. Fourthly, a sensitivity study is carried out to investigate the capacity of a suction pile for different usage factors of REB. Finally, suction pile design requirements for carbonate-type soils (i.e., low level of pile skin frictions) are recommended.

New Approach for Offshore Pile Decommissioning With Hydraulic Presses and Floating Panels

Renewable Energies become more and more important in industries and society all over the world. In Germany, offshore wind farms generated 49 % of the renewable energies in 2018. Monopiles are the preferred system for the foundation of offshore wind turbines in water depths up to 40 m. They are authorized by the competent authority for 25 years. When reaching the end of lifetime, the structure inclusive the foundation must be decommissioned. The decommissioning of monopiles will be challenging in the future and can lead to unexpected costs and risks for the owners. Removing the monopiles in it’s entirely ensures the opportunity to reuse the space for new offshore wind farms. The Institute of Geomechanics and Geotechnics of the Technische Universität Braunschweig (IGG-TUBS) obtained the funding for the research program on technical solutions with large-scale tests for decommissioning of offshore monopiles named DeCoMP. Several decommissioning methods such as vibratory extraction, internal dredging, external jet drilling, decommissioning with overpressure and the use of buoyancy force are investigated. The proposed paper will present technical opportunities and issues for extracting the pile with hydraulic presses in combination with a steel framework. Hydraulic presses brace the steel framework with the monopile. Further hydraulic presses, positioned at a certain distance to the pile on the framework, use the seabed as abutments to push out the monopile. In addition, results of a feasibility study to remove monopiles with floatation panels are presented in this paper. This method is based on floating panels, which are attached to the monopile above the mud line. These panels are inflated with air pressure to reach the required amount of buoyancy to overcome the pullout resistance. The decommissioning solutions are compared to point out possible combinations.

Optimization of the Trench Dimensions for Bahrain LNG Import Terminal Project

Large deformation finite element analyses were carried out to investigate the optimized trench geometry for 24 inch (610 mm) diameter subsea pipelines and power cables (33 kV; outer diameter = 186 mm) used for transporting gas in shallow water depth (approximately 15 m), with an emphasis being on pipe-soil interaction assessment. An extensive parametric investigation was performed varying the relative density of the backfill sand (35%, 54% and 78%) and buried depth (1.89 and 3.14 diameters). Upward and lateral soil failure mec hanisms around a pipe uplifting in a backfill sand layer were compared. The failure mechanisms were manifested by shear band formation and subsequent propagation. It was shown that the trench dimensions originally designed based on conventional conservative approaches can be reduced by up to 10.6% in loose backfill sand (ID = 35%) and 8.7% in dense backfill sand (ID = 78%), leading to minimize cost and time of trenching and backfilling.