BP’s North West Hutton Platform is undergoing a decommissioning process in the North Sea. The
platform is to have its modules removed and a heavy lift company will move the platform itself into cold storage.
Part of the removal process will involve dropping explosive charges down inside pilings around the legs of the platform to release the suction.
In order to do this it is necessary to find out how far the pilings extend below the seabed. A Subsea 7 ROV crew with a Seaeye Tiger ROV assisted in the decommissioning programme but from its fixed position it was unable to reach the pilings on all of the legs.
It was decided that they needed a highly portable ROV which would be able to reach all of the pilings and also be small enough to fit inside the 60-inch diameter pilings, dive to the bottom and provide accurate depth readings and also a carry out a visual inspection to identify any debris that may be inside.
Oceanscan provided one of their VideoRay mini submersibles and pilot tech. for the duration of the operation. The VideoRay has the benefit of being easily shipped in two pelican cases and as the vehicle weighs only 3kg in air, it is highly portable. Oceanscan also provided a VCR and monitor to record the footage supplied by the VideoRay’s colour camera.
The operation involved deploying the VideoRay from the opposite side of the platform to the Tiger, a relatively simple operation as the VideoRay is lowered over the side using its own umbilical. The vehicle was lowered 40m to the water and then swam just below the surface to locate the leg of the platform at which point it dived down the leg to locate the top of the piles which were 107m down.
One of the drawbacks in using such a small vehicle in a harsh environment such as the North Sea is the wind and tide acting against it. With so much umbilical catching the wind between the platform and the surface and the currents below, the VideoRay made valiant but slow progress. It was then decided to use the Tiger ROV as a “taxi service” to the pilings.
The VideoRay flew down to a level near the pilings on the required leg at which point the Tiger came from the other side of the platform to meet it and gently deposited the vehicle in the required piling. Once the piling had been identified and the videos were rolling the VideoRay made its way down inside the pilings whilst the Tiger monitored and assisted with my umbilical to prevent any snagging on the numerous hazards that adorn the underside of a production platform, not least of which being the numerous anodes on the legs. The VideoRay then flew all the way down the piling until touchdown was achieved some 20m below the seabed at anywhere between 145-160m.
The beauty of the VideoRay is that when it is being recovered from a confined space such as this, it is relatively easy to manually pull the vehicle up by its tether which keeps it taught and avoids it getting caught on any protrusions.
As it arrived at the top of one piling the Tiger guided it to the top of the next and the process was repeated again. Once the legs on the west side were completed it took less than 30 minutes to relocate the whole system to the east side of the platform to reach the other legs.
Despite the North Sea conditions at the time being less than perfect for small ROV operations, the VideoRay battled through, consistently working at depths in excess of 160m.
The VideoRay sustained no breakdowns and with the teamwork between Subsea 7
and Oceanscan the end client was presented with the data they required.
Source: Oceanscan Ltd