Physical Modelling Of The Entire Installation Process Of The Hybrid Dynamically Installed Anchor

The OMNI-Max anchor,which is used to anchor floating facilities,combines the advantages of the dynamic installation of dynamically installed anchors and high capacity efficiency of plate anchors.The installation process of the OMNI-Max anchor is divided into two sub-stages:free fall process in water and dynamic penetration process within the seabed.The anchor is embedded itself within the seabed through its kinetic energy during free fall in water and gravitational energy relative to its final position within the seabed.After installation,the chain connected at the padeye is tensed,allowing the anchor to be keyed to a proper position to bearing uplift resistance.Scientific issues associated with the OMNI-Max anchor include:the hydrodynamic characteristics(drag coefficient,fall velocity,directional stability,etc.)of the anchor during free fall in water,the mechanism of anchor-soil-water coupling during the dynamic penetration process of the anchor within the seabed,and the mechanism of anchor-chain-soil interaction during the keying process of the anchor within the seabed.It is necessary to address the issues above and thus to establish theoretical models to predict the anchor impact velocity in water and penetration depth within the seabed,and to establish analytical method to calculate the anchor capacity during the keying process.The related scientific issues in the present study are also beneficial in further developing the mechanism of structure-soil-water interaction and in other similar offshore geotechnical problems.Previous works about the performance of the OMNI-Max anchor are primarily based on numerical simulations.By performing numerical simulations,the anchor dynamic penetration and keying process within the seabed are illustrated.However,physical modelling associated with the anchor performance in seawater and seabed is relatively limited.Therefore,the current study conducted model tests to systematically study the anchor dynamic installation and keying process.In order to perform model tests,a series of load cell,accelerometer and velocity transducer with mini size and high accuracy are designed independently,and matching testing devices,including the water tank,soil box,actuator with large range,are designed and fabricated.Based on the testing results,theoretical and analytical models using to predict the anchor impact velocity,penetration depth,and capacity are proposed.The contents and primary conclusions of the current study are summarized as following.(1)The similarity relationships are deduced in order to simulate the anchor free fall in water and dynamic penetration within the seabed by conducting model tests.The reasonableness and reliability of the similarity relationships are validated.For a free fall projectile in water,the drag coefficient is approximately independent of the Reynolds number when the total drag force on the projectile is primarily attributed to the pressure drag.Therefore,the drag coefficient derived from model tests can be directly used to represent that in prototype.For the dynamic penetration of a projectile(for instance,dynamically installed anchor,free fall penetrometer)within the seabed,the penetration depth derived from model tests can be extended to prototype by multiplying the scale factor once the stress ratio in model tests is equal to that in prototype.(2)An anchor booster is designed and aims to improve the directional stability and increase the penetration depth of the OMNI-Max anchor.The booster can be connected to the rear of the OMNI-Max anchor using a shear pin.The total energy of the hybrid anchor(i.e.the assembled booster and OMNI-Max anchor)is increased to a great extent due to the added weight of the booster,and hence the penetration depth of the OMNI-Max anchor is increased notably.After installation,the booster is retrieved and can be used to install other anchors.Overall,the booster can be considered as an auxiliary equipment for the OMNI-Max anchor,which is beneficial both in improving the anchor directional stability in water and penetration depth within the seabed without affecting the high efficient installation of the anchor.As the capacity efficiency of the OMNI-Max anchor is improved attributed to the booster,the acquired number of OMNI-Max anchor is reduced for a certain project.Therefore,the booster helps to reduce the costs of anchor fabrication and transportation,and improve the anchor installation efficiency.(3)The free fall process of the OMNI-Max anchor and hybrid anchor in water was systematically simulated in a water tank.The anchor fall history and tilt(i.e.the included angle between anchor axis and vertical direction)in water was recorded using a 6-axis accelerometer and gyroscope.The effect of the booster on the hydrodynamic characteristics of the hybrid anchor was illustrated.Moreover,theoretical model was established to predict the anchor fall process and impact velocity during its free fall in water.The testing results indicated that,with a tilt angle of 3°,the fall velocity of the OMNI-Max anchor is 22.8 m/s,which is lower than its terminal velocity of 24.6 m/s;while the hybrid anchor H1.5-R1.5(the weight ratio of booster to anchor is 1.5:1,the ratio of the ring radius of booster to the fluke width of OMNI-Max anchor is 1.5:1)has achieved its terminal velocity of 31 m/s.Therefore,the booster is beneficial both in improving the directional stability and increasing the terminal velocity of the hybrid anchor.(4)Model tests were conducted to simulate the dynamic penetration process of the OMNI-Max anchor and hybrid anchor both in normally consolidated and overconsolidated clay.The effects of booster weight,soil strength,impact velocity,initial inclined angle,and water entrainment on the penetration depth of the anchor were investigated.Based on the testing results,two theoretical models,which are based on the anchor total energy and based on the anchor motion differential equation,respectively,are proposed to estimate the anchor penetration depth.The testing results indicated that the penetration depth of the OMNI-Max anchor is increased notably by adding a booster as the booster increase the total energy of the hybrid anchor.When the initial inclined angle from the anchor axis to the vertical direction is less than 9°,its effect on the anchor penetration depth is limited.Moreover,the booster can help to release the effect of initial inclined angle on the penetration depth of the hybrid anchor.(5)The anchor motion trajectory in the soil during the keying process can be determined using a half anchor model,which can move against the inner side of the glass window of the testing box.A series of factors influencing the anchor capacity,embedment loss,and diving potential during the keying process were systematically investigated.These factors include the padeye offset angle,the uplift angle at the mudline/padeye,the thickness of the anchor fluke,the anchor initial embedment depth,the soil strength characterization,and so forrth.It is concluded from the model tests that the padeye offset angle and uplift angle at the padeye are two key factors dominating the anchor diving potential.When the padeye offset angle is in the range of 24-30°,the OMNI-Max anchor behaves high diving potential and efficient capacity at the same time.The anchor-soil contact is influenced by the dimensionless stress level of the soil around the anchor.A gap between the anchor and soil is formed during the anchor keying process if the dimensionless stress level is relative low,resulting in a reduction in anchor capacity factor.(6)By considering the chain yield envelope,the predicting accuracy of the chain inverse catenary profile within the seabed using the chain equation can be improved.This study carried out model tests to establish the yield envelope for both chain and rope.Moreover,this study proposed a modified method to estimate the anchor motion trajectory in the seabed based on the chain equation and chain yield envelope.A testing device was designed and model tests were conducted to validate the modified method.The testing results showed that estimated motion trajectory of the anchor consisted well with the actual one,indicating the present modified method is reasonable and reliable.The testing device using to predict the anchor motion trajectory is also helpful in investigating the anchor capacity mobilization versus anchor motion traj ectory,illustrating the mechanism of anchor-chain-soil interaction.The results and conclusions summarized from the current study are useful to illustrate the mechanical characteristics of the deepsea soil,the mechanism of the anchor-chain-soil interaction.As for practical engineering,this study is helpful to provide theoretical and design references for offshore dynamically installed anchors.