Specifying, Monitoring, and Verifying Quality and Reliability of Flexible Pipe
Authors: Bouvard Michel , Mallard Marc , Rigaud Jean 
Published: October 2008

Great progress has been made during recent years in the oil & gas industry. This progress has resulted directly from improved design, manufacturing, and installation methods, tools, and equipment. Virtually all segments of the industry have participated in these developments.

Flexible pipe technology has progressed rapidly since the early 1980's. Progress has benefited from improvements in a multitude of domains, such as computational tools, engineering knowledge I operational experience, material technology, factory automation, installation techniques, and total quality assurance.

This paper presents the current state of flexible pipe technology. Vital insights are revealed that concern the most critical issues every user and certifying authority should know when specifying, evaluating, or approving the use of flexible pipe.

The paper also focuses on fundamental criteria that are essential in achieving consistent, reputable flexible pipe quality and reliability.

INTRODUCTION

Flexible pipe is increasingly being used for offshore oil & gas development. Many of these projects are technically challenging; thus, the pipe is usually subjected to severe operational conditions. Over 2,500 kms of flexible pipe have been manufactured and installed to date in water depths reaching 800 m; approximately half this amount has been in service for nearly 10 years. 700 kms of pipe have been retrieved and reused.

Since subsea flowlines are a significant component of field facilities, pipe quality and reliability are of great importance With respect to project operational economics and, especially, the safety of personnel, equipment, and the environment. This quality and reliability can only be achieved if all the stages from design to installation are properly mastered through the correct definition of material requirements, manufacturing and quality control.

FLEXIBLE PIPE DESIGN REOUIREMENTS

Flexible pipe design principles are now widely known and include basically 4 different types of pipe structure, one smooth bore and three rough bore (see figures Ia to ld).

All unbonded flexible pipe structures are a mix. of plastic layer to ensure leak free barriers and steel layers to proV1de the mechanical strength.

The equations governing the mechanical behavior of this combination of complimenting layers when subjected to external forces (internal pressure, axial force or torque) can be theoretically studied. Due to the distinct function of each layer, stress and deformation of each layer can be individually computed either by equations of static equilibrium or by the Castigliano theorem.

In addition to the application of these equations individually to each layer of the flexible pipe, some assumptions relating to the pipe as a whole must be considered. Typical assumptions can be as follows:that the pipe is straight while subject to various forces;that friction does not exist between the various layers ;that strain and torsion are equal in all the layers, a condition in fact imposed by the end-fitting of the pipe ;that all calculations are made 10 the elastic domain with no buckling or plastic deformation of any of the layers considered.