A single mooring cable system using freefall or anchor-last mooring technique is investigated to observe the dynamic stress and motion of the system. The cable system consists of a top buoy and several intermediate buoys such that it is floated on the sea surface before the anchor is pay out. The weight of the anchor then pulls the mooring system into position with the help of a ship at the buoy end of the cable which controls the final positioning by changing the pulling force or following a certain predetermined course.
A set of nonlinear partial differential equations are set up for the mooring system and then solved by finite-difference technique. Because of the presence of current, three dimensional finite displacements and hydrodynamic forces are under consideration.
A digital computation program was executed to solve this set of equations. Various effects on the cable system are investigated, such as the pulling force applied by the towing ship, the length of each cable section between I buoys, the weight of the anchor and the total length of the mooring system. The results indicate that the anchor-last anchoring technique is practical even for a large cable system. The computational technique can also be used in related underwater applications in towing and wave-excited motions.
In the search for a reliable and easy mooring system for use in deep ocean activities, the free-fall or anchor-last anchoring technique stands out as a feasible solution. This method of mooring provides a fast deployment of anchor and thus eliminates the undesirable effects of surface waves. In fact, several free-fall anchoring systems have been developed successfully for use in deep water. However, these systems consist of simple light loads and mechanical mooring lines of small sizes. They are empirical cases with little attention given to detailed dynamic motions.
This paper presents an analytical method of solution which may be used in free-fall mooring and other related underwater cable dynamics motion problems. We consider here the effects of the drag and inertia forces on the cable system. The external forces are the weight, buoyancy and wave forces.
The physical system under investigation consists of a riser cable and an array cable. The riser cable with four riser buoys on it can be moored by the traditional anchor-first method without difficulty. However, due to its size and other functional requirements, the array cable, which is connected with the upper end of the riser cable by the array anchor set, has to be implanted with the use of the anchor last technique to reduce the risk of long exposure to wave motions. In addition to the subsurface main buoy, there are 15 modules and 15 intermediate buoys on the array cable.