Study on the influence of entrapped air in a suction anchor during installation

Abstract

In the past few years suction anchors have been increasingly used as a foundation for different sub-sea installations and for mooring of different floating structures. A typical suction anchor is a hollow steel cylinder with a closed top which uses under pressure to penetrate into the seabed. The top plate comes with a vent valve to offer ventilation during lowering and installation. The installation of suction anchor requires examination and control of dynamic loads and displacements. When a suction anchor is lowered through the splash zone, insufficient air ventilation through the vent valve will create an upward force which might cause the lifting wire to become slack. This can result in snap forces on the lifting wire when struck by a wave the next moment. Many of the suction anchors have been installed using simplified rules and models. But these models do not capture the entrapped air effect. A suction anchor analysis tries to maximize the achievable weather windows by assessing the hydrodynamic forces and many people are doing so using these simple models. But most people are excluding the entrapped air effect. Therefore, it is important to investigate it.

This master thesis presents an investigation on the entrapped air effect as well as hydrodynamic effects and loads acting on a suction anchor during installation through scaled down experiments and numerical analysis. An experimental setup consisting of several electronic sensors, micro controllers and mechanical devices was designed and developed to measure different parameters that determine the behavior of suction anchor while lowering. The experimental setup facilitated for an integrated and synchronous recording of sensor data which provides the force, pressure and displacement information throughout the lowering process.

The total lowering process was decomposed into different test cases that isolate each physical effect. Scaled down experiments were conducted for each test case and the measurement data was recorded. Simultaneously, a mathematical model which is an ordinary differential equation (ODE) was formulated for the lowering process and simulated using time integration method. The experimental results were used to validate the mathematical model developed. Comparing the simulation results with the experiment data, it was observed that the experimental results had a good match with the mathematical model results for static test cases. For dynamic cases, some mismatches were observed in the pressure and force data during the transient phase. The mathematical model used approximation to calculate the hydrodynamic forces and the pressure drop due to air ventilation was calculated using orifice flow equation. The possible reason for the mismatch could be that the mathematical model failed to capture the ventilation effect or there might be some other effects that occur during lowering which were not accounted in the mathematical model. This was observed in the late stage of the thesis work and error correction has not been achieved. The mathematical model should be extended or repeated experiments have to be conducted to better capture the dynamic effects int the transient phases.

Finally, in order to investigate the potential improvement of the suction anchor lowering process, a parametric study for different lowering velocity and vent hole size was performed using the developed experimental setup. The results were used to analyze the influence of each parameter on the behavior of suction anchor and recommendations to improve the lowering operation were discussed.

In the past few years suction anchors have been increasingly used as a foundation for different sub-sea installations and for mooring of different floating structures. A typical suction anchor is a hollow steel cylinder with a closed top which uses under pressure to penetrate into the seabed. The top plate comes with a vent valve to offer ventilation during lowering and installation. The installation of suction anchor requires examination and control of dynamic loads and displacements. When a suction anchor is lowered through the splash zone, insufficient air ventilation through the vent valve will create an upward force which might cause the lifting wire to become slack. This can result in snap forces on the lifting wire when struck by a wave the next moment. Many of the suction anchors have been installed using simplified rules and models. But these models do not capture the entrapped air effect. A suction anchor analysis tries to maximize the achievable weather windows by assessing the hydrodynamic forces and many people are doing so using these simple models. But most people are excluding the entrapped air effect. Therefore, it is important to investigate it.

This master thesis presents an investigation on the entrapped air effect as well as hydrodynamic effects and loads acting on a suction anchor during installation through scaled down experiments and numerical analysis. An experimental setup consisting of several electronic sensors, micro controllers and mechanical devices was designed and developed to measure different parameters that determine the behavior of suction anchor while lowering. The experimental setup facilitated for an integrated and synchronous recording of sensor data which provides the force, pressure and displacement information throughout the lowering process.

The total lowering process was decomposed into different test cases that isolate each physical effect. Scaled down experiments were conducted for each test case and the measurement data was recorded. Simultaneously, a mathematical model which is an ordinary differential equation (ODE) was formulated for the lowering process and simulated using time integration method. The experimental results were used to validate the mathematical model developed. Comparing the simulation results with the experiment data, it was observed that the experimental results had a good match with the mathematical model results for static test cases. For dynamic cases, some mismatches were observed in the pressure and force data during the transient phase. The mathematical model used approximation to calculate the hydrodynamic forces and the pressure drop due to air ventilation was calculated using orifice flow equation. The possible reason for the mismatch could be that the mathematical model failed to capture the ventilation effect or there might be some other effects that occur during lowering which were not accounted in the mathematical model. This was observed in the late stage of the thesis work and error correction has not been achieved. The mathematical model should be extended or repeated experiments have to be conducted to better capture the dynamic effects int the transient phases.

Finally, in order to investigate the potential improvement of the suction anchor lowering process, a parametric study for different lowering velocity and vent hole size was performed using the developed experimental setup. The results were used to analyze the influence of each parameter on the behavior of suction anchor and recommendations to improve the lowering operation were discussed.