API REPORT 90-60
Cyclic Pressure Fatique Evaluation of High Pressure Fiberglass Pipe
|Publication Date:||19 November 1992|
Currently the American Petroleum Institute Committee 15 on Plastic Pipe is developing specifications for test methods to assess the cyclic pressure fatigue characteristics of high pressure fiberglass pipe that models the oil-field environment to which it is subjected. The oil-field environment often involves high mean pressures and relatively high frequency cyclic surges due to pump action. This load history differs significantly from conditions specified in standard test practices for low pressure pipe in ASTM D2143 that call for low mean pressure and low frequency testing. Consequently, Committee 15 is responding to the need of pipeline engineers and pipe manufacturers with a new methodology for fatigue design of high pressure fiberglass pipe.
The standard method for cyclic pressure testing of fiberglass pipe is given in ASTM D2143, "Cyclic Pressure Strength of Reinforced Thermosetting Plastic Pipe". This specification requires the application of cyclic internal pressures from zero pressure to the test pressure and back at a frequency of 25 cycles per minute for up to 15,000,000 cycles. Low pressure (less than 1,000 psi) line pipe, for which the test method was originally developed, is often used in situations with pump jacks that produce similar load histories. The data produced in these standard tests are extrapolated one log cycle to estimate the fatigue performance for 150,000,000 cycles. This represents approximately 28 years of service life for a product when cycled at 10 cycles per minute.
In contrast, high pressure fiberglass pipe is often used as line pipe near triplex injection pumps that produce significantly different pressure fluctuations than that of pump jacks. Cyclic pressure frequencies of 100 cycles per minute or greater are common, and the pressure fluctuations do not involve a drop to zero pressure; cyclic minimum pressures are commonly more than 80 percent of the cyclic maximum pressure. In 20 years this pipe may experience more than 1,000,000,000 fatigue cycles. The fatigue load history for high pressure fiberglass pipe then differs from that of low pressure pipe in three principal parameters:
• Cyclic frequency
• Maximum cycles to failure of practical interest
• Load ratio, R (minimum pressure divided by maximum pressure).
In prior work¹,²,³ the membership of API Committee 15 asked Battelle to undertake a group industry program to develop a method that can be used to perform cyclic pressure fatigue testing of fiberglass pipe under conditions that more closely represent high pressure service applications. This program was completed in 1990, and achieved the primary research goals and objectives that include:
• The design, construction, and evaluation of a test system for evaluating cyclic pressure fatigue of high pressure fiberglass pipe according to a new test method proposed by Committee 15
• The performance of a series of cyclic pressure fatigue tests on pipe from the six participating fiberglass pipe manufacturers to evaluate both the test method and the specialized fatigue equipment.
The members of this group program released, to the API committee, research results for use in establishing an industry-wide specification for the performance, design, and testing of fiberglass line pipe.