AUVs could have role to play in touchdown monitoring

When Technip introduced its pipelay vessel Deep Blue in 2001 it was both the largest and fastest reel pipelay vessel of its type. One of the key design objectives was to have it perform pipeline touchdown monitoring (TDM) without a following survey vessel. It achieved this with its own flying cage remotely operated vehicles (ROVs) and long-range sonar technology.

As Donald Faulds, engineering manager at Technip’s remote systems unit, and Dave Matthews, survey manager at Technip’s offshore engineering division, told the recent International Marine Contractors Association (IMCA) annual seminar, a combination of the right vessel and equipment assets and a specialist team of operators has delivered 10 years of successful operations and laid more than 2,000km of reeled pipe, but the systems will shortly be replaced with a new generation of higher powered ROVs. Looking ahead, they asked, do autonomous underwater vehicles (AUVs) provide the potential to replace or augment TDM capacity?

Although Deep Blue operates in many other modes, including J-lay and flexible, it is the high-speed reel lay of rigid pipes with long layback that provides the most ROV TDM challenges. Deep Blue is a fast reel ship, typically operating at lay speeds of 2km/hr. Higher reel speed is possible but 2km/hr is found to be the limit at which both the lay mechanisms and the ROVs operate effectively. Although the ROVs and cages have high thrust force available they gain significant assistance from the vessel tow and use most of their own power to control lateral position and tether tension.

The touchdown layback limit for the ROVs is currently around 750m. It may be a little further when new systems are tested. There are some situations where the layback distance is too great to be reached by the ROVs, so Technip wondered whether AUVs might provide a potential solution.

Maintaining the correct catenary at all stages of the pipelay is vital to prevent buckling (due to lack of tension) or stretching damage (due to over tension). The dynamic behaviour of rigid/flexible pipelines and umbilicals is very well understood, so if the top tension/ramp angle is known then it is only necessary to add the touchdown point or part of the catenary profile and the full picture can be derived. Lay monitoring therefore tries to identify either touchdown point or catenary, and not necessarily both. However, some clients insist on continuous visual monitoring of touchdown regardless of any other consideration.

TDM is also important to establish that the product is being laid in the correct corridor. Technip typically lays to a +/-5m corridor in North Sea conditions and +/-15m in deepwater (over 300m). Sometimes the corridors are tighter when passing through sensitive or congested areas or to optimise straightness and position for subsequent burial or cover.

The scale of operations is often hard to appreciate. Laying a pipe in a target corridor only a few metres wide in 2500m of water needs skill, position control and accurate monitoring.

‘Standard’ work ROVs have been used for TDM with some success though they have additional risks compared to a purposely configured TDM ROV. Standard top hat and cage tether management systems (TMS) cannot move away from the vessel before releasing the ROV. In very deep water the separation from the line of lay and the ROV umbilical is relatively trivial and, if the prevailing current is adverse, the TMS may be blown directly under the line of the pipe. This forces the ROV to fly along the line of the pipe with high risk of tether damage or, worse still, entrapment.

An issue with a top hat style TMS is that if the tether is pulled hard out of the underside of the TMS it can pull the top of the ROV back causing it to pitch up. Slackening the tether has worse risks for entanglement and, overall, it is not an ideal arrangement for long excursions. The flying cage has a full set of vectored thrusters providing most of the lateral flying capability and instrumentation of the ROV. This deals with the standard work ROV issues with the only penalty being a reduction in flexibility for the fitment of work skids.

The flying capability can be used to increase the ROV excursion but more usually offsets the cage laterally away from the line of the vessel so that the ROV tether route is parallel to the pipe, but at a safe distance. The flying cage maintains the heading of the cage always towards the ROV. The mechanisms line up the tether and deliver it directly to the rear of the ROV. The ROV is pulled backwards by the tether acting on its centre. Typically, the cage is deployed 100m above the seabed and the ROV 10-20m above, with the tether tight between them, not lying on the seabed.

There are two approaches to monitoring the touchdown point (TDP) by ROV. One is to sit above the TDP, looking down with visual and profiling (and/or obstacle avoidance) sonars. This is the normal position for an ROV on a follow ship in TDM mode and can also be adopted by the Deep Blue ROVs on shorter excursions or slower J-lay, providing the cage is sufficiently offset laterally and in altitude so that tether does not contact the product.

The other approach, and the only one when at maximum excursion, is to follow parallel to the pipe during lay, being towed by the lay vessel. When the vessel periodically pauses for anode fitment or other deck work, or simply for TDP confirmation, the ROV flies over to take position fixes at the TDP using the vessel’s acoustic positioning system. During fast lay the ROV and cage are towed by the vessel and the ROV actively thrusts forward to keep the tether tight back to the cage. When the vessel pauses lay, the powered cage controls any tendency to swing back to vertical that a non-flying cage would have.

Long-range sonar looks at the other part of the equation – the catenary. If part of the catenary can be profiled then the touchdown can be interpolated and it is not necessary to visit the actual point of touchdown. The original high-power sonar units were larger and heavier than practical to fit on the ROV. In any case, the underside cage mounting allows maximum separation from the cage thruster acoustic disturbance. A full description of the sonar is not provided here but it is sufficient to say that the Technip configuration is not concerned simply with the pipeline but also survey of the route. The big potential advantage of the sonar is that it keeps the ROV a long way from any risk either to itself or the product.

The original analogue sensors that were used have recently been superseded by more compact digital units. The new equipment is currently on board and undergoing trials. One of the challenges with all sonar systems is that sloping pipes do not give good orthogonal sonar returns. In addition, many flexible products tend to have sonar absorbing sheaths – sometimes dubbed ‘stealth pipe’, although this is not the design intent.

The returns tend to be from the discontinuities: weld joints, anodes, buckle arrestors, pipe features. The longer the range, the larger the discontinuity has to be. Large items can be detected reliably at 750m range. Some work was done on developing simple plastic sonar reflection collars but the economics of slowing the pipelay to attach them did not balance well with the potentially better catenary profiling and so this idea was not pursued.

As a result of its vertical ramp capability, Deep Blue rarely needs a highly extended layback. Few projects exceed the 750m range of its own TDM ROVs. In the case of the lay vessel that cannot bring the lay ramp to the fully vertical position the layback distance can significantly exceed 750m. This is the case with Technip’s Apache II. One project in Brazil required a 1,450m layback – this is beyond the range that an onboard ROV can safely reach. Apache II is not normally fitted with ROVs and operates with a following survey/ROV vessel. This is a situation where it may be appropriate to consider AUV technology. If an AUV is capable of consistently identifying the TDP and remaining at that position, it might well offer a viable alternative to the follow vessel.

Deep Blue is a multipurpose vessel that performs other construction operations apart from pipelay. An analysis of the eight major pipe contracts in 2008 showed that the TDM operations comprised less than 20 per cent of the total ROV dive time. Even in pipelay operations there are many work ROV operations associated with first-end initiation and second-end laydown as well as heavy rigging operations of all types.

Although the ROVs were primarily installed to support pipelay, the benefit of having heavy duty ROVs for construction support has proved very valuable for vessel efficiency (compared to having a second ROV vessel in support). Deep Blue’s ROV systems are due to be changed out during the course of 2012. The original 100hp ROVs with 100hp flying cages will be replaced with 150hp ROVs and 150hp flying cages. The new systems also have greater tether capacity and excursion trials will be arranged when convenient. The new ROVs have an advanced navigation sensor pack combining doppler velocity log, depth sensor, north seeking gyro and full inertial navigation system. This is combined with the acoustic position data so that the ROV has the ability to be ‘parked’ in any position in the water column under automatic software control. It can also be commanded to go to any three-dimensional point in the water column.

This powerful new set of features also applies to the cage (lateral plane only). In vessel equivalent terms this could be described as ROV dynamic positioning (DP). In the old ROV system the pilot had to fly the cage and ROV under manual control at all times. In the new system it will be possible for the surveyors to provide a stream of position co-ordinates to the ROV, placing it at a constant offset to where the pipe should be. Or, the ROV can be precisely commanded to a known point of interest. The cage can also be positioned at a fixed offset to the vessel in ‘vessel-follow’ mode (instead of ‘sub-follow’). There are many possible uses of this new capability which begins service. OSJ