Tanks & pipes

Design and product quality

Mechanical properties
The mechanical properties of composites depend on the production technique, the product design (e.g. fibre orientation) and compatibility of base materials (glass, finish and resin). Ageing is a property of any plastic material. The behaviour of plastics depends on time and temperature, resulting in a regression of the original properties. This ageing effect can be influenced by:
- Environmental circumstances (weather)
- Medium
- Stresses
It differs for each plastic material and / or composite composition. This complicates the comparability of the different variants of this material and for this reason, long term (regression) testing acc ASTM D 2992 is required for pressurized pipe systems. An indication of the long-term behaviour of variants can also be obtained by determining the Ultimate Elastic Wall Stress and comparing the UEWS of the variant with the UEWS of the component that is qualified by regression analysis acc ASTM D 2992-b. See also section “Ultimate Elastic Wall Stress”.

Joints
Although the joints of GRP can be a weak spot, the number of defective joints per kilometre /per year are for steel pipe systems, some 4 -5 times higher than GRP pipe joints. The reason for this is corrosion. (Source: Veritec Offshore Technology).
Specific GRP jointing systems:
- Adhesive bonded joints
- (Integral) Mechanical (rubber sealed) joints; either tensile resistant or non-tensile resistant
- Laminated joints
- Threaded joints
- Flanged joints
- Third party mechanical joints
Each joint has its own specific strength and weakness and can be designed either to endorse full end thrust loads or with limited or no axial load ability. Making a reliable joint in GRP requires –as with any other material – requires a certain level of skill, knowledge and good workmanship. Fabricating a joint in GRP should be no more difficult than with traditional materials.

Qualification and Design of GRP pipes.
Due to the diversity of GRP (base material combinations and production techniques), the subject of design becomes quite complicated. Extra complexity comes from the fact that the properties differ in time. The ageing effect results in loss of strength and stiffness properties.
This regression depends largely on the quality of the combination of base materials and their compatibility and production technique. Therefore it is very important to determine the design parameters of each variant, not only initially, but also in the context of the service life. In December 2002, a new ISO standard was published, dealing with this subject in combination with qualification of components, system design, installation and quality assurance issues: ISO 14692. The scope of this document refers to offshore applications, but the standard can also be used as a basis for onshore applications. The main principle for this document is the link between the properties of the specific GRP product and the safety in the installed pipe system.
The qualification involves a test program that involves full-scale hydrostatic performance tests, establishing a long-term design basis, not only for pipes, but also for the system as a whole including joints and fittings.

Verification of the long-term performance of pipes, fittings and joints can is done by means of: a. Regression analysis acc. ASTM D 2992-B at the max design temperature on pipes b. Medium term testing on joints and fittings. For this test, two representative samples have to be tested for 1000 hrs at maximum temperature and at a test pressure exceeding the pressure level at 1000 hrs resulting from the regression analysis performed on pipes. In practice this will be ± 2.5-5 times the nominal pressure rating. This figure differs from each manufacturer, depending on the type of product, manufacturing procedures etc. The manufacturer can carry out qualification testing provided it is witnessed and certified by a recognized independent authority. Alternatively, testing and certification may be carried out by an independent testing organization. This shall be confirmed by submitting a certificate stating the test results. Hydrostatic design basis To achieve a safe allowable design basis, it is necessary to consider the behaviour of GRP material in a pressurized condition. The burst strength is not to be considered as a good basis for determining an allowable stress. Another point that can be distinguished when pressurizing a component is weeping. The weeping phenomenon is caused by inelastic deformation in the material. Weeping starts at pressure levels clearly exceeding those governed by the UEWS. Bursting pressure and weeping pressure are generally derived from short-term destructive test results; but these test results have only secondary value since there is no fixed ratio between the short-term value and the long-term load ability. The right ratio can be determined by testing a component acc ASTM D 2992 and determine the hydrostatic design basis/stress. Subsequently test 5 additional samples acc ASTM D 1599. Generally, the following variety is seen for these ratios: Bursting stress/design stress = 8-12 times the design pressure Weeping stress/design stress = 3-6 times the design pressure UEWS / hydrostatic design stress = 1.5-3 times the design pressure The design stress has to be relative to the long-term properties. Please note that the regression analysis and UEWS are in general determined on pipes only. The suitability of the hydrostatic design basis for the other components in a system can then be demonstrated by 1000 hr medium term testing acc ISO 14692. Ultimate Elastic Wall Stress Executing such an extensive test program limits the development of new materials. In cooperation with institutes and the industry, DSM has participated in the development of a new, fast and reliable method called the Ultimate Elastic Wall Stress (UEWS) for bi-axially loaded pipes. UEWS represents the highest stress at which the strain is reversible (comparable to the yield-point of steel). This test offers a comfortable method to determine the performance level at short notice and can be performed fully automated. DSM has implemented this new, fast and reliable test method to determine applicability of test data for new material combinations. The Ultimate Elastic Wall Stress method links properties of new material combinations to the existing full-scale test program. DSM will be happy to elaborate on this subject and to offer support in this matter.

Tanks & Pipes