New rock dump system twinned with survey ROV

OE Staff
Friday, April 1, 2011

Subsea equipment specialist Seatools is scheduled to deliver its latest offshore innovation – a remotely operated vehicle system that will combine the functions of a survey ROV with those of a fallpipe rock-dump ROV – this summer.

The pioneering system will be stationed on the new 159m long fallpipe vessel currently being built for Dutch offshore contractor Royal Boskalis Westminster at the Keppel Singmarine yard in Singapore and due to go into service by the end of the year (OE August 2010).

As well as handling the system engineering and construction, Numansdorp-based Seatools has also developed in-house the advanced software that will enable the two ROVs to be easily controlled from onboard the mother vessel. A mathematical model was first made of the complete ROV system as well as of the fallpipe vessel and the umbilicals. This model was later used to test the design of the control system in a virtual environment prior to assembly of the integrated fallpipe ROV.

Seatools knows a thing or two about fallpipe systems, having built eight of the 11 fallpipe ROVs currently operating around the world. This will be its ninth.

Boskalis first approached the company to look into the possibility of integrating the two ROV functions back in 2006. Standard procedure is to use a single fallpipe ROV, using echolocation equipment to deposit the rock in the right places. 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. Therefore, a separate survey run is traditionally made, or a separate freeflying 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 can make it an expensive practice.

Seatools managing director Jan Frumau recalls: ‘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 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 its 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 virtually 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 Boskalis newbuild. Seatools subsequently worked with Boskalis' specialists to further develop the design and work out how best to accommodate the ROV combination in the fallpipe tower.

‘All in all we're dealing with a rather complex and fairly large structure,' explains Frumau. ‘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 moonpool, 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,' notes Frumau. ‘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 a relatively 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 rockdumping job. The components of both ROVs are based on extremely robust mechanical designs 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.' OE

Categories: Subsea Equipment Construction Engineering Products

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