Integrity Management of Flexible Pipe: Chasing Failure Mechanisms

Abstract

Integrity management of flexible pipe has been a tenuous undertaking ever since flexible pipe was introduced, notwithstanding the substantial effort that has since been devoted to develop inspection and monitoring techniques. This is partly because flexible pipe as a composite structure has many failure processes and some of them are deeply buried. It is partly because the application boundaries are ever advancing and new failure modes manifest themselves. The paper examines some of the new failure mechanisms, such as those associated with gas applications at high pressure, temperature and velocity (erosion, Flow Induced Pulsations, PVDF applications near the design limits.) It further examines the current understanding of the risks that needs to be managed and what the means are to fulfill this task. These means start with standards and proper design specification, continue with the requirements of QA/ QC during manufacturing, monitoring and inspection during installation and finally during the service life. The safeguarding and monitoring methods and inspection techniques that significantly reduce the risk are discussed and so are the gaps that remain. Inspection only reduces the risk if the results can be interpreted well and the resulting course of action is clear. Corrosion and corrosion-fatigue of the steel reinforcing are highly uncertain processes, however. The consequences of this are treated. Finally, Life Time Extension (LTE) of flexible pipe is becoming topical. The paper addresses what should be demonstrated during LTE, what can be demonstrated and how any gaps should be addressed.

Integrity Management of Flexible Pipes

“…These curves may not be available, and curves for a roughly comparable environment may be used. If the calculated corrosion-fatigue life is critical, especially if not conservative, it is advised to cover this by commissioning the S-N curves for corrosion fatigue for the particular application [6]. Erosion Figure 5.10 shows picture of erosion of a collapsed carcass.…”

“…From the statistical results of failure and damage incidents worldwide for flexible pipe in operation, damage of the external sheath is the most common failure mode. Table 1 of the document by Out [6] presents a summary of the failure modes likely to lead to bursting or leaking of flexible pipe. The details of these failure modes are described in the catalogue of components of unbonded flexible pipes in the following sections.…”

“…For dynamic flexible risers, the sheath is made of either PA-11/12 (polyamide or nylon), PVDF, HDPE, or XLPE. Specifying a design temperature too high, for example, may lead to deselection of PA-11/-12 or XLPE as the pressure sheath material and unnecessarily in favor of PVDF, in spite of PVDF's higher cost and relative vulnerability [6].…”

“…Traditionally, a velocity limit of 20 m/s is used, but the basis of this is unclear in the context of sand erosion. The chief parameters for erosion of the carcass of a flexible pipe include [6] • Gas flow velocity. …”

“…The integrity management of flexible pipe starts with the design specifications [6]. It is necessary to emphasize that a flexible pipe is a pipeline that contains nonmetals and is built up in a number of layers.…”