Dutch submersible specialist develops integrated rock-dumping ROV for BoskalisVessel & ROV News // November 12, 2010
Seatools' integrated ROV for Boskalis.
Seatools in Numansdorp in The Netherlands has landed what it describes as the largest order since its establishment in August 1999. Royal Boskalis Westminster has commissioned the company to build a unique underwater robot that will combine the functions of a survey ROV with those of a rock-dumping ROV.
The ROV will be stationed on board a new, 159m long fallpipe vessel that is currently being built for Boskalis at the Keppel Singmarine Pte Ltd shipyard in Singapore.
Seatools will handle the construction and engineering for the project, and develop the advanced software that will enable the ROV to be controlled from on board the mother vessel.
In 2006 Boskalis asked Seatools to conduct a study for a fallpipe ROV with an integrated survey ROV. The standard procedure is to use a single fallpipe ROV, using echolocation equipment to deposit the rock in the correct dumping location.
However, the silt thrown up by the process as well as air bubbles make it impossible to use multi-beam sonar to survey the end result in the same pass. This being the case, a separate survey run is traditionally made, or a separate free-flying survey ROV is carried to perform a survey at a suitable distance behind the rock-dump ROV.
Although these are perfectly workable solutions, the extra time and equipment involved make it an expensive practice.
Jan Frumau, managing director of Seatools, said: “Boskalis asked if we could find out whether a survey ROV could be integrated into a fallpipe ROV in order to enable the rock-dumping and survey procedures to be combined in a single pass."
"An additional wish was that both ROVs preferably would be easily operated. This meant that current operators working the fallpipe ROV – which is also known as the rock-dump ROV – had to be able to operate the survey ROV."
"We therefore had to come up with a concept in which the rock-dump ROV operated as a tether management system. In other words, the rock-dump ROV serves as a launching platform for the survey ROV.”
In the summer of 2007 – after more than a year’s research – Seatools presented the final concept to Boskalis. The proposal comprised a main ROV fitted with two lateral thrusters, with a stainless steel frame in which the survey ROV is carried underwater.
Once it arrives at its operating depth, the survey ROV flies away from the rock-dump ROV to perform its own task practically automatically. The link between the two ROVs comprises a pair of tethers, which are always under tension. This allows the survey ROV in fact only two instead of four freedoms of movement, which simplifies the operator’s job.
Another novel feature is the deflector attached to the underside of the rock-dump ROV: it can be configured to dump the rock to either side of the submersible or the stones can be spread at a larger breadth. Like the ROV, the deflector is controlled from the bridge of the new Boskalis fallpipe vessel.
Boskalis received the innovative concept with great enthusiasm. Seatools was then asked to further develop the design and work on integrating the complete ROV combination in the fallpipe tower.
Said Mr Frumau: “All in all we’re dealing with a rather complex and fairly large structure. The fallpipe ROV can develop a lot of power and carries a substantial amount of survey equipment, but now the survey ROV has to be added to that."
"It had to remain manageable and compact enough to be lowered through the moon pool, so it certainly posed a double challenge to our engineers. Components had to be carefully shoehorned into the available space to make sure the assembly didn’t grow to three times its size. And all the while the configuration needed to be kept simple in order to make it easy to operate. Of course, we also had to consider such aspects as reliability."
"Given the total project scope of building a new fallpipe vessel, the construction cost of the ROV is relatively a small part of this. On the other hand, the ROV is the ship’s single most important piece of equipment; it’s essential for getting the job done. A single failure in a working component of the total system must never result in an uncontrolled termination of a rock-dumping job."
"Both ROVs are based on extremely robust mechanical designs of the components (differently than at standard ROV concepts) to minimise repairs and maintenance. This is why redundant design has been used for all the main components in the integrated ROV, including the power packs."
All the software required to control the two ROVs was developed in-house by Seatools. A mathematical model was first made of the complete ROV as well as of the fallpipe vessel and the umbilicals.
This model is now being used to test the design of the control system. In other words, before the assembly of the integrated fallpipe ROV commences in early 2011, the entire design will have been tested in a virtual environment at the Seatools offices in Numansdorp.
The contract calls for Seatools to deliver the integrated ROV to Boskalis by 31 July 2011. If all goes according to plan, the new fallpipe vessel will be completed by the end of 2011, ready for deployment on offshore oil and gas extraction projects.