Long ROV excursions, high elevation tracking, noisy waters – not a problem
Guest author: Tavis Letherby from CCC
Today’s digital data acquisition capabilities and global positioning systems have made offshore pipeline inspection operations easier and more accurate. With Global Navigation Satellite Systems (GNSS) or Differential GPS (DGPS), we can achieve 20 cm accuracy. The data gathered is also more easily accessible for operators and then easier to analyse.
But, if you don’t have good subsea positioning accuracy, you will not have a good baseline or repeatability, in order to compare new survey data with old. In the past, baseline data could be three metres out – and that was acceptable. Now, we expect less than half a metre and we want repeatability, so we have a solid baseline and know that future surveys will accurately match it.
Underwater positioning is also not always that easy, with some environments being more challenging than others. Recently, we (CCC) were engaged to carry out subsea pipeline external inspections offshore the United Arab Emirates in challenging environments for survey technology.
A 2,000 km pipeline inspection mission
It was a periodic integrity survey of an operator’s pipeline network, some of it dating back to the 1960s and 1970s. The subsea inspection campaign amounts to approximately 2,000 km of pipeline inspection distance, covering 316 pipelines over various offshore fields. The project is being completed in two campaigns. The first ran from last August 2018 to April this year (2019) and covered 1,500 km of pipeline (214 pipelines). The second covers the remaining 102 pipelines and started in August (2019).
The platform used to undertake the subsea external inspection was the CCC owned and operated Saab Seaeye Leopard 1706 remote operated vehicle (ROV). Fitted with three HD cameras, Valeport bathy suite, mini SVP, CP, UT, laser line generator, MCS PRC system and two Kongsberg M3 profiling sonars, it’s tracked using a Sonardyne Mini-Ranger 2 USBL (Ultra-Short BaseLine) system from our CCC Maritime 2 multipurpose dive support vessel.
Shallow water tracking
The scale of the inspection campaign was not a concern. Our main challenge was that these pipelines are situated in waters as shallow as 5.5 m and not more than 30 m. When conducting operations in shallow water you inevitably put a lot of man-made noise into the water – engine noise, ROV noise, etc. There are also surface reflections and high signal to noise, all of which has the potential to reduce the accuracy of subsea positioning when using a USBL system. So we needed to supplement the conventional means for subsea pipeline inspection positioning.
We’re a long-term customer of Sonardyne and we like to standardise, so we have Ranger 2 Pro USBL systems on all our vessels. We’re also very happy with our 6G and 6+ systems. But, because of the shallowness of the water, we decided to use a Mini-Ranger 2, which has a beam plot that suits these needs, and WSM 6+ (Wideband Sub-Mini 6 Plus) beacons, which are proven to be reliable when operated in shallow water.
Dealing with long ROV excursions
But, we still had to deal with noise in the water, which could impact our USBL effectiveness, and an additional challenge – long ROV excursions. For safety reasons, our vessels are not allowed within 50-70 m of platforms in this area, sometimes further, which of course is normal. This means that, sometimes, the ROV has to navigate upwards of 70 to 100 m away from the vessel (and therefore also the USBL transceiver) to the riser base, in order to do the full pipeline inspection.
In the past, when we have done such long excursions, the subsea position of the ROV has become unreliable or erratic. You’re working in 12 m water depth and the vessel has 4 m of draft, so there’s only maybe 8 m between the keel and the seabed. The ROV is 1.2 m high and sitting over a 14 in pipeline, so it’s actually 2 m off the seabed. That means there’s only 6 m between the base of the transceiver head and the top of the beacon. In addition to being at a high elevation, they’re at a long excursion, at 70 m, well outside the optimal boresight of the vessel’s acoustic transceiver.
While Mini-Ranger 2 is great for high elevation tracking – it’s tracked divers in 14 m water depth out to 200 m before – the noise in the water where we’re working is still an issue, which usually means having to do a lot of post processing. Even then, when we’ve done it before, you can still be out by a few metres
For this project, we decided to optimise our USBL by adding an ROV-mounted gyro and motion sensors, to help improve our position with pitch, roll, attitude and heading, when the USBL started to jitter. But, because they’re not fully integrated, it didn’t really do what we wanted.
SPRINT-Nav hybrid navigation changes the game
Then, we found out about SPRINT-Nav. SPRINT-Nav is an all-in-one navigational instrument, which combines a SPRINT INS sensor, Syrinx 600 kHz DVL (Doppler velocity log) and a high accuracy intelligent pressure sensor in a single unit. SPRINT-Nav tightly integrates all the raw sensor data, including individual DVL beams, to provide its high performance.
We’d not tried it before but one of its advantages is that with bottom lock enabled, if you lose your USBL it doesn’t matter. It will preserve the integrity of your positioning solution until your next acoustic fix comes in. This had a great advantage for us in achieving subsea positioning repeatability and reliability.
With SPRINT-Nav 500 fitted to our Leopard, we’ve completely changed the game on this project and indeed all our future shallow water inspections. We have the best of both worlds – USBL and a high-performance INS. You feed SPRINT-Nav the USBL position from Mini-Ranger 2 and it outputs a corrected position, based on the information from its INS and the DVL.
So now, when the ROV goes on an excursion away from the vessel, it doesn’t matter. If the USBL tracking drops out or if it starts jittering, SPRINT-Nav still knows where it is. Our survey and data acquisition team on board the vessel were amazed; straight out of the box, SPRINT-Nav keeps our ROV on course.
So we now have excellent positioning and repeatability, which is critical for quality inspection projects. Our survey accuracy is below half a metre, mainly because of SPRINT-Nav. We have full confidence in the start and end position and, furthermore, we don’t even require any post-processing.
The client is very happy – so we are too. They now have a new baseline survey that all their future inspections can be compared against.
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