The assessment of mooring line damping for offshore structures

Student thesis: Doctoral Thesis

Abstract

With increasing water depth of oil and gas exploration, greater importance has been attached to the damping force from mooring systems. The effect is significantly important to slow drift motion of the floating structure, and it is also coupled with its motions. Coupled analysis is thus preferred to be applied to estimate the floating structure motions and to calculate the mooring system response, especially for offshore structures in deep-water.In this study, the aim is to achieve a better understanding of mooring line induced damping estimation. Drag forces normal to the mooring line due to the motion of the mooring line through the water, are the main source of hydrodynamic damping of the mooring line. A method of energy dissipation based on the mooring line dynamic response obtained by Orcaflex is developed. The validation is established through two types of mooring lines in shallow water and deep water.The present approach shows a good agreement with the published results, but with two exceptions. One is for the mooring line oscillated by a very slow LF motion in shallow water, when the hydrodynamic damping is very small. Another one is for the wire mooring line oscillated by WF motion in deep water, the result shows significant discrepancy with that from the quasi-static method.Then a non-dimensional analysis is completed, due to the strong complexity of the mooring line induced damping. The effects of the factors can be divided into three groups: first, the effects from pretension and scope are related to the geometry changes of the mooring line; second, the oscillation, current velocity and drag coefficient make contributions to the drag forces of mooring line directly; and the last, the effects of stiffness and seabed friction which, it was found, can be neglected.In order to experimentally investigate the chain behaviour moving in water, a series model tests for the damping characteristics of a single chain line is implemented through oscillation tests of various parameters. The drag coefficient ( C D ) variations with different Reynold ( Rn ) and KC numbers are investigated. The drag coefficients in this study range from 1.5 to 4.0, which is case-dependent, because both Reynold ( Rn ) and KC number affect them.With the increase of KC number, the drag coefficient shows a decrease with exceptions occurring in low KC cases. In addition, it is shown that the chain segments near the fairlead and touch down area are most sensitive to the drag coefficient, which is consistent with the velocity distribution along the mooring line.Finally, the validation is established by comparing the results of experimental tests and numerical simulations. Based on the assessment of drag coefficient from the scaled experimental investigation, numerical simulations of estimated drag coefficient are carried out within Orcaflex. A good agreement is achieved between the numerical calculations and experimental measurements, which illustrates that the present method can be applied for mooring line damping estimation. Meanwhile, the effects of the amplitude and frequency of the oscillation are studied.
Date of Award17 May 2019
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde
SupervisorNigel Barltrop (Supervisor) & Narakorn Srinil (Supervisor)

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