This paper expands on work previously presented [1] and describes riser fatigue methodologies. As a lead in, this paper concentrates on the processes of understanding the materials under corrosion fatigue conditions and the development of material design curves.
Although fatigue and corrosion fatigue testing of flexible pipe armour wires has been well documented over the years, little has been published to describe the effect of different test protocols and test set-ups. This paper goes some way to redress this with descriptions of testing regimes, the effects of different test set-ups and controls on the resulting s-n curves and how these increase or reduce conservatism in service life analysis.
The analysis of test results will also be discussed: descriptions of the different analysis methodologies and techniques which have to be employed to interpret raw data (including the use of finite element analysis to determine local stresses in the wires) will be presented along with details of their applications and results.
In this paper the authors will describe both the testing methodologies and some of the analysis processes which are respectively undertaken to define material fatigue properties which are then used to assess service life of carbon steel flexible pipe armouring wires.
The first part of the paper will discuss different testing methodologies, describing the advantages and disadvantages of each, and will make a comparison of results from some of the testing modes.
The second part of the paper will introduce, in the form of a case study, one of the kinds of analyses which are required to interpret the raw test data from a nub-nub fatigue test and put it into a useable format for fatigue curve generation and service life analysis calculations
The generic structure of a flexible pipe can be seen in Figure 1.
Figure 1: Flexible Pipe Structure (available in full paper)
The pressure armour (labeled as Flexlok in the above figure) provides the primary support for the pressure retaining polymer layer in the pipe (Flexbarrier) and continues to resist hydrostatic collapse of the pipe.
The tensile armour, labelled above as Flextensile, provides pipe tensile strength for installation and to resist end cap loading in the pipe due to internal pressure.
The typical cross section profiles of the reinforcing pressure and tensile armour wires can be seen respectively in Figures 2 and 3.
Figure 2: Pressure Armour Profile(available in full paper)
The complex profile of the pressure armour is required to maintain an interlock of one wire helix wrap around the pipe to the subsequent wrap. Specific pressure armour wire designs vary manufacturer to manufacturer; information from one wire type may not necessarily be applicable to another.
Figure 3: Tensile Armour Profile (available in full paper)
The flat tensile armour wire shape optimizes the fraction fill of the wires around circumference of the pipe (making it far more efficient than were a round wire to be used).
