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Fatigue endurance testing in seawater at TWI

TWI carried out a project for Pipeline Technique Ltd (PTL) to determine the fatigue endurance of girth welds in lined pipe in a simulated seawater environment with cathodic protection. The tests showed that the fatigue endurance of the girth welds was twice that of the DNV Class D air design curve, giving reassurance that the girth welds’ performance more than met the design requirements.

New weld procedure qualification

Pipelines and risers have redundancy inherent in their design but currents, waves and vortex-induced vibration are all sources of cyclic loading that these structures are subject to in the offshore environment. The welds in pipelines and risers are the weak spots in terms of fatigue and the corrosive effect of seawater can reduce fatigue endurance further. Since failure of a riser in operation would have severe consequences and be costly to repair it is important for designers to know that welds made using new procedures will have sufficient fatigue performance.

As part of their involvement with a high-profile project in Australian waters, PTL were required to qualify a number of new welding procedures to ensure that the fatigue endurance of resulting welds would meet design requirements.

PTL commissioned a fatigue testing project at TWI to determine the ‘knockdown factor’ on fatigue life when the welds were exposed to a seawater environment with cathodic protection.

Objectives

The project had two key objectives:

  • Carry out fatigue tests, in a seawater environment with cathodic protection, on strip specimens extracted from girth welds made using a new welding procedure to determine their fatigue endurance.
  • Compare the fatigue lives obtained to the DNV Class D (air) design curve [1] to determine the environmental knockdown factor for the seawater environment.
Figure 1 Strip specimen layout and a photograph of a specimen before the liner was removed
Figure 1 Strip specimen layout and a photograph of a specimen before the liner was removed

Specimen supply and preparation

The pipe of interest was 18in diameter lined pipe. PTL provided two 1m lengths each with a girth weld at mid-length. TWI then extracted and prepared for testing six waisted strips from these rings.

In service, the seawater would affect the outer surface of the pipe, and so the corrosion fatigue behaviour of the weld cap was of interest. TWI therefore removed the liner and ground flush the weld root to promote cracking from the cap side of specimens during testing. Figure 1 shows the specimen layout and a photograph of the root side of one strip specimen before the liner was removed.

Strain gauges were applied close to the girth weld on the cap side and ‘static tests’ carried out to determine the required loads for the tests. The strain gauges were then removed before the seawater cell was installed around the specimen.

Testing

The specimens were located inside a cell containing simulated seawater (to ASTM-D 1141 [2]). A separate reservoir was used to allow the seawater to be recirculated during the tests. Cathodic protection was set up to apply voltage of -1050mV Ag:AgCl to the specimen.

Tests were carried out in one of TWI’s 500kN servohydraulic fatigue test machines, as shown in Figure 2. Since the tests were being used to determine corrosion fatigue strength, the test frequency was an important parameter – it needed to be the same as occurs in service, to ensure that a representative amount of corrosion could occur in each cycle. The relevant loading frequency for this project was 0.1Hz.

Tests were continued until cracking occurred or a runout life was reached. Test durations ranged from seven to 60 days.

Results

Two of the six specimens failed by fatigue. Cracking initiated at weld cap toe (which is a common location for fatigue failures in welded joints due to the stress concentration and presence of small flaws introduced by the arc welding process).

The fatigue endurance of the cracked specimens was actually twice that of the DNV Class D (air) design curve, and so in this case, there was no reduction in fatigue life associated with the seawater environment.

For more information on our services to the offshore industry, email contactus@twi.co.uk.

Figure 2 A strip specimen inside the seawater cell, located in one of TWI's servohydraulic fatigue test machines. Upper image is a close-up showing the CP system and test specimen. Lower image is an overview
Figure 2 A strip specimen inside the seawater cell, located in one of TWI's servohydraulic fatigue test machines. Upper image is a close-up showing the CP system and test specimen. Lower image is an overview
Avatar Dr Carol Johnston Chief Engineer, Fatigue and Fracture Integrity Management

Dr Carol Johnston joined TWI in 2009. She is now a Chief Engineer in the Fatigue and Fracture Integrity Management section. Carol runs TWI’s resonance fatigue testing offering, and so has many years of experience in running projects to qualify girth welds made using new welding procedures, and of providing consultancy on fatigue design. She has carried out research and published papers on topics including the fatigue performance of girth welds and mechanically lined pipe used in the oil and gas industry, and of electron beam welds and large bolts used in the wind industry.

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