FEASIBILITY STUDY FOR DEEP WATER ANCHORING SYSTEM

Ziel

A theoretical and experimental study of the dynamics of mooring lines will be conducted, with particular emphasis on the effect of attached submerged buoys, and on the effect of automatically controlled winches. This will allow imposed designs of mooring systems for current water depths (up to 800 m) and, more importantly, the development of mooring systems for deep water applications (of the order of 3. 000 m).
The main results obtained in the various project tasks are summarized in the following.
(A) REGARDING THE STUDY OF THE EFFECT OF PRETENSION :
For extreme storms, elastic stiffness is the predominant mechanism for tension build-up and pretension must be set to a sufficiently low limit to guarantee that the maximum tension will not exceed the breaking strength of the cable.
The dynamic tension, is directly proportional to the pretension to breaking tension ratio. When the maximum tension (static plus dynamic) is considered, however, it is inversly proportional to the pretension and the total tension is influenced by a change in pretension more in shallow water than in deeper waters.
(B) REGARDING THE STUDY OF WIRE VERSUS CHAIN LENGTH :
The determination of the appropriate chain and wire lengths in a mooring line for a specific water depth and specific storm must follow an iterative procedure aimed at balancing the two conflicting attributes of chain :
reduction in the overall lengthof the line and in the amount of pretension, and increase it in the elastic stiffness of the line
.
(C) REGARDING THE STUDY OF THE EFFECT OF SUBMERGED BUOYS :
A large peak of the dynamic tension per unit length at higher frequencies and cancellation frequency in the dynamic tension which can serve as the basis for designing and optimizing mooring lines in deeper waters were ligtend.
(D) REGARDING THE EXPERIMENTAL STUDY :
The following general trends were deduced from the experiments : (a) The dynamic tension amplification obtained in the case of heave excitation is insignificant in comparison to the corresponding one for the surge excitation.
(b) Increasing the cable's pretension, results in a corresponding increase in the dynamic tension amplification, regardless of the presence of buoys and their location.
(c) For the case of one buoy, its location along the cable's length plays a significant role in the amount of reduction of the dynamic tension amplification.
(E) REGARDING THE COMPARISON OF EXPERIMENTAL AND NUMERICAL RESULTS :
(a) Very satisfactory agreement has been observed between the experimental data and the numerical.
(b) Nonlinearities in the cable's response which were observed in the computer output were not capatured during the experiments.
The main reason for this discrepancy is thought to be the limitations imposed on proper scaling by the finite depth of the towing tank. This requires further scaled experiment TH. 06.
Dec (88).
(c) Dynamic response of the mooring line is not influenced by drag coefficient of the cable and the buoy in low excitation frequency regions.
(F) REGARDING THE STUDY OF THE FEASIBILITY OF AUTOMATIC WINCHES : (a) Compensation poses excessive power requirements just to overcome the static tension.
(b) Alternative schemes, employing passive devices are desirable, however massive equipment or pneumatic devices are required.
(c) An alternative passive control scheme is the use of submerged buoys.
The duration of the project was two (2) years. As described in the contract, the tasks to be caried out within this project were the following :
(1) Study of the effect of pretension
(2) Study of wire versus chain length
(3) Study of the effect of submerged buoys
(4) Experimental study
(5) Comparison of experimental and mumerical results
(6) Study of the feasibility of automatic winches
The major findings from these tasks are described in paragraph 4.

Programm/Programme

ENG-HYDROCARB 2C - Programme (EEC) of support for technological development in the hydrocarbons sector, 1986-1989

Thema/Themen

1.3 - ENVIRONMENTAL INFLUENCE

Aufforderung zur Vorschlagseinreichung

Data not available

Finanzierungsplan

DEM - Demonstration contracts

Koordinator

NATIONAL TECHNICAL UNIVERSITY OF ATHENS (LABO)

EU-Beitrag

Keine Daten

Adresse

HEROON POLYTECHNIOU AVENUE 9
157 80 ZOGRAFOS
Griechenland

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