| dc.description.abstract | Over the few last decades, numerous mooring concepts have been developed, one of these being the dart/ torpedo anchor. A torpedo anchor is shaped like an arrow and is equipped with tail fins to provide good directional and non-rotational stability. The anchor is released from a predefined height above the seabed and penetrates into the soil using the kinetic energy accumulated during free fall. Hence, no external forces are required.
This thesis presents three torpedo anchor concepts:
Deep Penetrating Anchor (DPA)
OMNI-Max Anchor
Petrobras Torpedo Anchor
However, the thesis will focus on the Deep Penetrating Anchor concept. The scope of the thesis is to simulate a torpedo anchor installation, using data for an 80tonne DPA, and perform parameter studies on the complete model.
Two models (Model 1 and Model 2) are created in RIFLEX, which is a computer program developed for analysis of flexible marine riser systems and slender marine structures. Model 1 is used to find the static equilibrium position of the anchor, that is, the position of the anchor before it is dropped. Model 2 is used to investigate the dynamic behavior of the anchor and mooring line during free fall.
In order to verify if the RIFLEX model provides realistic results, the velocity and tilt angle when the anchor hits the seabed is compared to previous experimental test results and calculations. It is found that a touchdown velocity of 26m/sec agrees with the experimental results when dropped from approximately 70m. In addition to this, all observed tilt angles are within the design criteria of 5⁰.
The effect on the free fall velocity and tilt angle of the anchor as well as the dynamic behavior of the chain loop (mooring line) is investigated in simulations
With and without current
With varying distances between the two installation vessels in 500m water depth
With decreasing vertical chain length behind the anchor
The current only affects the static equilibrium position of the anchor and can be neglected during free fall. These observations applies to all of the water depths used throughout the simulations.
With increasing distance between the vessels, the anchor velocity decreases slightly, the tilt angle remains unaltered while the free fall velocity and the amount of curls present in the chain loop (mooring line) decreases.
The vertical chain length behind the anchor is decreased incrementally from 50m to 10m. Two water depths are considered, each with two different horizontal distances between the installation vessels. The results showed that there is a relatively small influence on the touchdown velocity when varying the vertical chain length and distance between the installation vessels, while the tilt angle of the anchor increases significantly with increasing distance between installation vessels for both water depths.
One recommendation is to investigate the soil penetration phase of the anchor, perform analyses including dynamic effects in the wire due to vessel motions and investigate the lateral drag coefficient of a vertical free falling chain. | |
