The management team at the INEOS Hydrocarbons Asset in Grangemouth, Scotland, approached Applus RTD about a particularly ‘interesting’ inspection project.
The Grangemouth refinery dates from 1927 and is the second oldest in the UK. It is also one of the largest, with a “nameplate” capacity to process over 200,000 barrels of oil a day. Applus RTD have provided inspection and NDT services to the refinery for many years but in November 2011 they presented us with a new sort of challenge concerning the inspection of the interior of large diameter spherical pressure vessels used for the storage of propylene (see figure 1).
This vessel is just over 22m in diameter and has the capacity to store over 5.7 million litres of propylene. Propylene (or propene) is an important feedstock in the petrochemical industry. It is the raw material for a wide variety of products including the plastic polypropylene, which accounts for nearly two thirds of its usage. Due to its chemical properties, producers take great care to ensure the integrity of the vessels storing it.
INEOS is required to undertake a statutory ten year inspection on the vessel to check for vessel integrity, including inspection of the welds. The vessel is coated with a thick layer of intumescent fire protection (firecrete). The only practical way to inspect the welds is from the inside. This means the vessel has to be taken out of commission and, traditionally, the inside would then have been scaffolded out in its entirety.
Now by any measures this vessel is huge. It is as almost the same size as the dome of St Paul’s Cathedral in London. Scaffolding the interior would take months and involve the use of a vast amount of labour and materials. And if that wasn’t enough, there is an additional dimension to add to the challenge in that every man, every pole and every plank has to go and out through a hatch that is only 600mm in diameter. You can just spot the tiny hatch at the bottom of figure 1.
When the last inspection was carried out some ten years previously (prior to INEOS taking ownership of the refinery) it was reported that the vessel was out of action for over a year. And this was the real challenge presented to Applus RTD: to reduce significantly the time out of service and complete the works to the highest standards and deliver excellence in Health and Safety.
Given the complexities of the task, the question was what could Applus RTD do to meet INEOS’ requirements whilst ensuring a full inspection was done safely and rigorously?
Applus RTD in the UK have been pioneers in the use of rope access for getting to awkward places but this was different. Abseiling on ropes works well on vertical surfaces but how could ropes be used to reach the concave surface forming the top half of the sphere? After discussions between Applus RTD and INEOS it was proposed to use a simple hexagonal frame such as is shown in figure 2. (This shows the prototype “hex” constructed in the Applus RTD rope training school in Falkirk, it is roughly 1/3 of actual size required but still big enough to fill the rope school).
The “hex” was assembled from aluminium sections (rather like a grown-up’s Meccano set) with each being passed through the small hatch. Once assembled inside the sphere itself the hex was suspended from the top of the inside of the sphere and rested on the floor. The addition of outriggers or platforms (see figure 4) allowed access to the interior surface. As soon as one section of the work was completed the whole hexagon could be raised a few inches using a chain hoist and then rotated, just by the pressure of one’s hands on the frame. As soon as the hex was in the correct new position it could be lowered again and allowed to rest on the floor. This meant that every part of the sphere could be reached by the team from the hex. Of course all operatives working on the hex structure were using a fall arrest system, under the direct supervision of a Level 3 rope access supervisor and adhering to the normal rope access safe working practice protocols.
The whole hexagonal structure was assembled inside the sphere in just four days and then the inspection work began in earnest. The Applus RTD crew carried out weld inspections using magnetic particle inspection techniques and using the advanced technique known as TOFD (Time of Flight Diffraction) as pictured in figure 5. Further checks were made by careful visual inspection and the use of conventional ultrasonic testing. Within 36 days the job was complete and the whole “hex” had been dismantled and removed from the sphere. This allowed INEOS to return the sphere to service in a much shorter time and consequently made considerable savings.
The sphere inspection at the INEOS site using the HEX solution was successful in every way and word has spread of our innovative approach. As a direct result another petrochemical company in the same area asked to see what had been done and with INEOS’ permission this was done. They were very impressed and to cut a long story short, Applus RTD have already undertaken a full inspection on one of their spheres and are working on a second. Then a third companyexpressed interest and Applus is now preparing to conduct inspections on two more spheres for that client. News is beginning to travel through the refining and petrochemicals community that if you have a spherical vessel to inspect then Applus RTD is the company to do it promptly, economically and above all else safely.