Reliability Methods for Deepwater Position-Mooring Design and Analysis
Authors: Bai Yong 
Published: June 2014

ABSTRACT
This paper describes the reliability methods for design and analysis of position mooring systems with various degrees of complexity and with a particular emphasis on deepwater applications. The limit state design and safety factor formats for position mooring systems of various floating installations are first addressed. Then the technical issues of both passive and active mooring systems including winch or thruster assisted and dynamic positioning systems are discussed. Modeling uncertainties of numerical analysis, mooring components, environmental conditions and reliability methods are also addressed. A logical approach is presented to perform reliability analysis for the calibration of partial safety factors for both passive and active mooring systems.

INTRODUCTION
As the development of offshore oil and gas is moving into deeper waters, floating installations are used for drilling and/or production operations. For these operations, position mooring systems are required in order to keep the floating installations on station under the design environmental criteria of wind, current and waves. Various types of floating installations and mooring systems can be considered for offshore applications. For mooring strength limit states, partial safety factor formats have been proposed recently in the offshore industry. It is important, however, to fully understand how to use the reliability methods to calibrate the partial safety factors selected. The subject matter is significant to the offshore industry for enhancing the reliability of position mooring systems with cost-effective designs. This is especially important for deepwater applications because the consequences of position mooring system failure would be much more costly than in relatively shallow waters.

LIMIT STATES OF MOORING DESIGN
Limit state design methods are described in this section with particular reference to the design of position mooring systems for floating installations. The limit state design philosophy may be used to provide a rational framework for the design of safe and serviceable structures or structural components, by accounting for uncertainties and variabilities in the basic variables affecting the design, [1, 2]. This is achieved by describing these uncertainties and variabilities statistically, using data from offshore practice, and calibrating the limit state formulation, including the deterministic and reliability analysis methods used in evaluating the probability that the limit state will be exceeded, to ensure that existing accepted safety levels are achieved.
The performance of a structural system, or a component of the structure, may be described by a set of limit states (limiting conditions) beyond which the structure, or component, is no longer deemed to satisfy the design requirements. Limit states can be regarded as a discrete representation of a more general continuous loss function. In general, the failure of a structural system or component to satisfy the design requirements may be represented by an inequality of the form:(1)g X1,..., Xn, C?0
Where, g is the limit state function, X1,...., Xn are the basic variables associated with the limit state, and C are constraints describing the limits of acceptable accelerations, offsets, clearances, etc.