Flexible risers, heavyweight hoses from production equipment on the ocean floor, are a critical link up to a surface vessel that produces, processes, stores and offloads oil from nearby fields on to tankers.
Hess Corporation was the first energy company to use a new ultrasound method to help test the integrity of flexible risers, heavyweight hoses carrying thousands of barrels of oil a day from production equipment on the ocean floor to an FPSO, a vessel that produces, processes, stores and offloads oil from nearby fields on to tankers.
Ultrasound testing at the Hess-operated Triton FPSO is part of a wider improvement program by the company to confirm the integrity of the flexible risers on all Hess-operated North Sea assets. Early detection of potential weaknesses in risers is an additional safeguard against safety and environmental problems.
Hess began its program of integrity management earlier than most in the industry. UK regulators are now instructing North Sea operators to institute more robust programs to ensure the soundness of their flexible risers.
The Triton risers are each approximately 700 feet long and installed in water that is 300 feet deep. They are constructed of multiple layers designed to contain the oil, gas and water that flow through them and withstand the hostile North Sea environment. The focus of Hess’ assessments was to establish if the outer sheath had been breached and if so, to assess the condition of the layers of steel armor wire that provide the main structural strength of the flexible risers.
The layers between the internal-pressure-retaining plastic sheath and the external plastic outer sheath are known as the riser annulus. Within the annulus the steel wire layers serve much the same function as the steel belts inside a car tire. The ingress of seawater and the flooding of the annulus can lead to corrosion, both chemical and mechanical, either of which can damage the steel and significantly shorten the life of the riser.
The first step in determining the integrity of the risers is a vacuum test that is similar to sucking on a straw that has been sealed at one end. Any damage along the length of the riser breaks the seal and a vacuum cannot be created. The volume of gas, and in some cases water, that is drawn off provides a strong indication of whether the annulus is flooded.
To test further, Hess relied on a new technology that uses ultrasound to bounce high-frequency sound waves off the object that is being tested. This requires that there be fluid in the area examined. If no fluid is in the annulus, no image is created, which is good news for the operator.
If an image is created, the scanning system can be set to establish the thickness of the visible layer within the annulus. This can provide an indication of how much steel has been lost due to corrosion in a fully submerged riser section.
After onshore trials, the testing device was mounted on Triton’s ROV system and launched from the FPSO. It was taken down to a section of riser about 120 feet underwater. In a short time it examined five risers.
The scanning provided some good news; following a failed vacuum test, scanning the natural gas export line showed water had not seeped into the annulus and replacement was not required.
Other risers will need to be replaced over time and four new ones have been ordered – three replacements and a spare – for delivery later this year.
When they are replaced, the recovered risers will be examined to see if further information can be gained about corrosion and possible failure, and the condition of the risers after more than 10 years service. That data will be shared with others in the industry.