The technical challenge associated with building a deepwater pipeline is often measured by the combination of pipe diameter and water depth. In the past, projects like the Blue Stream Pipeline (24 inch diameter and 2,150 m depth) and Mardi Gras Pipeline (28 inch diameter and 2,200 m depth) set the frontier for such a challenge. However, recently proposed projects, such as the South Stream Pipeline (32 inch diameter and 2,200 m depth) are setting new records.
For a given throughput and outside diameter combination, wall thickness design of deepwater pipelines is often governed by collapse (local buckling) during installation, when the combination of external pressure and bending reaches the most extreme condition. The thick wall required for projects like South Stream is near the limit for pipe mills to form pipe and to control ductility and toughness in the weld area. A small reduction in wall thickness will significantly improve the manufacturability and constructability.
The key to achieving an optimal wall thickness design is to accurately quantify collapse pressure under the combination of bending and external pressure. For pipes with small diameter-to-thickness ratios that collapse inelastically, factors that are most influential to the collapse pressure include the stress-strain relationship, residual stresses, ovality, and the thermal treatment process associated with applying fusion bond epoxy coating. The benefit of thermal treatment has been examined by several full-scale testing programs (e.g. Oman – India, Blue Stream and Mardi Gras), which have shown that collapse pressure of heat-treated pipes could be 20-30 per cent higher than those without heat treatment (i.e. as-received pipes).
Finite element models based on stress-strain curves obtained from coupon tests have achieved some success in predicting collapse pressure; nevertheless they cannot provide the consistent accuracy required for an optimal wall thickness design.
Among many explanations for the discrepancies between finite element predictions and test results, it is mentioned that current techniques of coupon tests and residual strain measurement do not allow the models to fully capture the through-wall thickness variation of stress-strain behaviour and residual stresses. Furthermore, it remains a challenge for finite element models to take full advantage of the beneficial effect due to heat treatment, due to the complexity of its effect on pipe materials (past studies have shown that heat treatment leads to an increase in yield stress and residual stresses).
As a result, finite element predictions need to be verified and calibrated by full-scale tests, even though the models can be used as an effective tool to preliminarily select wall thickness and study parametric sensitivity.
Full-scale collapse and pressure-bend tests were an integral part of the wall thickness design for several deepwater projects (e.g. Oman – India, Blue Stream, Mardi Gras and South Stream). These tests are typically carried out in a deepwater experimental chamber that is equipped with a bending apparatus, such that external pressure and bending moment can be applied to full-size pipe samples of sufficient length.
Experiences with past testing programs have shown that the following components are essential for achieving an optimal wall-thickness design:
- Test specimens of both heat-treated and as-received pipes;
- Pipe samples from various suppliers;
- A number of pressure-only collapse tests;
- A number of pressure-bend tests with different pressure levels and failure bending strains to define the failure envelope under combined loading;
- Tensile and compression tests of axial and circumferential coupons;
- Measurement of circumferential residual stresses; and,
Detailed geometric measurements of diameter, wall thickness and ovality.