Marine seismic surveying offshore Brazil is always challenging. Accelerated biological growth on in-sea equipment, combined with rough seas, makes it difficult to keep the equipment clean. PGS engineers made a cunning solution to address the problem.

The Problem

Barnacles are the problem. These marine invertebrates thrive in the shallow zone of warm equatorial waters. This is also where we tow our sensitive electronic sensors to accurately image subsurface reservoirs. The equipment, which is tuned to pick up minute echoes from the deep, can be 10 or more kilometers long and is arranged in spreads up to 10 km wide.

Turbulence around the elements in this vast towed seismic spread makes it an ideal habitat for barnacles. Once affixed, the creatures create their own turbulence and as they grow and multiply, it increases. It is a vicious circle that is good for the barnacles but bad for seismic.

Barnacles increase the drag on the towed equipment, increasing costs, risks, and emissions. PGS has devised various approaches to keep the equipment clean, eliminating footholds for the little arthropods to cling to. Before the survey starts, a special coating is applied to the equipment, which is a bit like polishing the hull of a racing yacht to reduce resistance. This wears off after a few weeks. So, during longer projects streamer cleaning is required to scrape and polish the equipment free of biological buildup. PGS has an automated streamer cleaning unit (SCUe) that, once mounted, runs up and down the long streamers.

Complicating Factors

The SCUe works well, provided the weather and sea-state permit the launch of a workboat. However, offshore Brazil sea swells build as they cross the Atlantic and are topped with wind, waves, and challenging currents. These conditions combine to make project planners nervous. Persistent harsh weather can preclude workboat launches for up to weeks at a time. If barnacle growth is too great, the risk of tangled or broken equipment can necessitate bringing all the gear on deck for cleaning. Managing barnacles has become a more pressing problem, as the size and complexity of the equipment configuration have increased, while a stringent health and safety focus has reduced workboat hours.

Scoping a Solution

PGS’ research and development team was tasked with designing a remotely operated vehicle (ROV) to deploy and manage the vital cleaning unit.

It was a knotty challenge, as ROVs are usually designed to work at a sedate 0–2 knots on stationary equipment in relatively calm environments close to the seabed. Furthermore, most are launched and return to approximately the same location, in the lee of the mother ship. PGS designed a vehicle that can operate at 3–6 knots and is launched and returned to a support vessel moving at up to 15 knots – that’s over twice the speed of the seismic vessel and equipment. To ensure deployment and retrieval do not conflict with the seismic operation, ROST can be dropped into the water and towed by the nose, then recaptured in motion. It can maintain steering in rough waters, where the seismic operation determines where it can maneuver, as the support vessel cannot sail over the streamer spread. Finally, it can deploy and recover the SCUe package reliably and smoothly without slowing the seismic operation.

According to Sperre, who manufactured the ROST, it is the fastest moving ROV ever built.

We had to devise mechanisms for advanced control of the vehicle to enable steady flying. Landing the SCUe package on a moving object at speed and with strong side-currents, and then recovering it are not easy maneuvers,” explains principal navigation engineer Trygve Skadeberg.

The PGS solution is equipped with an acoustic node for positioning and has internal controllers for heading, pitch, and depth. Sensors, thrusters and hydrofoils control speed and movement in the spatial degrees of freedom, and to enable 24/7 operations, sonar, cameras and lights make it possible to locate the streamer in the dark.

The launch and recovery system can trigger active heave compensation that ensures that the transporter is relatively unaffected by wave-induced motion. This enables safe launch and recovery in conditions that would make workboat deployment prohibitive from a health and safety perspective.

Hydrodynamic Design

The PGS engineering team had to master several disciplines, including mechanical engineering, electronics, dynamic positioning, hydraulics, hydrodynamics, computer science, control loops, and marine seismic operation.

During the development, the team established collaborations with a variety of internal and external stakeholders. Internal partners helped to define the seismic-operations parameters and had a key role in testing and operating the system. Sperre AS built the ROV unit, while Lidan Marine and Kongsberg Maritime delivered the navigation systems.

Integrated Solution

Designing a successful solution to a real operational problem requires more than creative engineering, there is a strong degree of project management too, explains Rune Tønnessen, R&D Section Manager at PGS. “The solution has to be integrated into various workflows, so the operations and sales teams have to understand it and plan for it, and pilots must be hired and trained to operate it. It also requires the seismic imaging team to adapt its workflow to identify and subtract noise from the seismic records. We have put all of this in place.”
Today, ROST is a system that works and is a game-changer. As we prepare for a new acquisition season offshore Brazil, survey planners, project managers, and the management team onboard the seismic vessel all sleep a little easier. ROST will permit PGS to maintain seismic quality and efficient seismic acquisition.