FOR OVER 40 YEARS, THE SMALL SCOTTISH HIGHLAND TOWN OF WICK HAS WITNESSED HUGE SUBSEA OIL AND GAS STRUCTURES BEING MANOEUVRED THROUGH ITS NARROW STREETS. IT DIDN’T SEEM POSSIBLE TO MOVE EVEN LARGER TOWHEADS ALONG THE ROUTE – UNTIL HEAVY TRANSPORT SPECIALIST MAMMOET AND OFFSHORE ENGINEERING CONTRACTOR SUBSEA 7 BROUGHT IN CUTTING-EDGE VIRTUAL REALITY AND SELF-PROPELLED TRANSPORT TECHNOLOGIES. JOHN MALCOLM TELLS THE STORY.
PHOTOGRAPHY: STEVE MCCANN | PAUL CAMPBELL
Since 1980 Subsea 7, a global leader in the delivery of offshore energy projects and services, has operated a unique site in the far North-east of Scotland at Wester, near Wick, where it fabricates subsea systems called Pipeline Bundles for the North Sea oil and gas market.
These are massive structures, up to 7.7km in length and weighing up to 10,500 tonnes, which are launched at Wester by a fleet of tugs and vessels that haul the completed Pipeline Bundles off the narrow gauge rail lines, down temporary launch ways and out to sea. The Pipeline Bundles are submerged and towed by tugs at a controlled water depth, keeping clear of the seabed to avoid stress fatigue-inducing wave motions and surface shipping traffic. They are among the longest transported engineering structures in the world and are towed in this way to oil and gas installations in UK, Norwegian and Danish North Sea waters, where they are positioned on the seabed for tie-back connection to new and existing facilities.
Towheads – large manifold structures weighing up to 500 tonnes which are located at each end of the Pipeline Bundles – are the only components which are fabricated off-site. As the name suggests, towheads originally served primarily as a mechanical connection for the marine tow from the fabrication site to the subsea oilfield destination, but in recent years they have grown in both complexity and size.
“Towheads are the heart of the Pipeline Bundle,” explains Willie Watt, general manager at Wester and a Subsea 7 stalwart who has supervised over 60 Pipeline Bundle fabrications since 1992 and been involved in their fabrication since 1982. “Our subsea design engineers can be tasked with incorporating valves, pipework, controls, monitoring, chemical injection and other equipment in a unique structure which has very strict physical parameters. Regardless of the complexity of its functionality, every towhead must be able to negotiate the narrow streets of Wick to be hauled along the narrow A99 road to our fabrication site at Wester, 10 kilometres away.”
THE NEW CHALLENGE
In 2018, Subsea 7 reassessed the constraints on road transportation of towheads, and consulted abnormal load contractors Mammoet (which acquired ALE as of January 2020) who proposed using self-propelled modular transporters (SPMTs) with bolster turntables for the first time to negotiate the tight corners. Mammoet’s UK engineers had 30 years’ experience of hauling Subsea 7 towheads through Wick and the company was the first heavy haulage contractor to perform the manoeuvre using conventional tractor/trailer combinations in 1990. “In the early days, we used to have hundreds of curious on-lookers lining the streets to watch the operation,” recalls Watt. “It has now become a familiar sight, especially in
our busy recent years, and although we still get keen towhead ‘spotters’, they’re not in the same numbers.”
But 2020 brought a significant new challenge. “We had been contracted to design a large subsea development for CNOOC International’s Buzzard Phase II (BPII) Complex in the North Sea which included a multi-functional towhead with the flexibility for future expansion,” explains Subsea 7 assistant project manager Craig Brown. “To accommodate the control components and 12 well connection slots, one of the two BPII towheads, the North Terrace Manifold (NTM), would be at least 15 per cent larger than any previous towhead we have moved through Wick.”
Down through the years, Subsea 7 has transported over 170 towheads by conventional heavy haulage tractor/trailer combinations through the streets of the town, including 16 in the last 12 months.
THE SPMT SOLUTION
The BPII NTM towhead, however, called for a different approach. It’s 37m long, 6m wide, 5.5m high and weighs over 290 tonnes – quite a challenge for movement through narrow streets, up an 18 per cent gradient and round four tight corners with only centimetres of clearance. However, Mammoet came up with a proposal based on deploying an unusual SPMT combination using two swivelling bolster turntables to negotiate the difficult route. “SPMTs are most commonly used as ‘platforms on wheels’ for the on-site
transportation of exceptionally heavy loads, built up from different modules, each of which can be fitted with its own engine and steering control system,” explains Mammoet project manager Daisy Partlow.
“However SPMTs also allow exceptional manoeuvrability when used in a double-module bolster turntable combination, although this is a relatively uncommon deployment in the UK. For the NTM towhead, we calculated that with relatively minor route modifications we could deploy two six-axle line modules and use their 360º steering capabilities on each wheel to inch the towhead round the tightest corners. SPMTs also have independent self-levelling and vertical lifting capabilities that allow us to span and oversail low walls and other corner obstacles which would hinder conventional trailers.”
USING VIRTUAL REALITY
At this point, sophisticated hi-tech digital technology was used by Subsea 7 to verify the critical route assessments. The company uses advanced 3D virtual reality (VR) techniques to help design increasingly complex subsea structures and decided to deploy this pioneering technology to generate a highly realistic full-route model for the NTM towhead. The objective was to produce a detailed, highly responsive VR reconstruction of the transportation process to enable project engineers to ‘walk’ the NTM towhead along the route and use the VR headsets and controls to zoom in on any potential clashes.
The first stage of the process involved commissioning an independent, highly detailed 3D survey of the route through Wick’s streets. “Especially on the really tight corners, we spent two days capturing high-resolution point cloud data of the buildings, boundary walls and street architecture,” Brown explains.
“We then imported 3D models of the towheads and SPMTs, along with detailed swept path data provided by Mammoet, and generated a unique fully-animated route model which our engineers could scrutinise and interrogate from flyover and close-up perspectives.”
In particular, the VR facility enabled Subsea 7 to identify essential route modifications which would be required to accommodate the large structure. “Building on Mammoet’s earlier route assessments, we worked with local civil engineering contractor, Gunn’s of Lybster, on wide-ranging route alterations, ranging from dropping kerbs and realigning boundary walls to relocating utilities infrastructure,” explains Watt. “We even arranged for hinged street lighting standards to be installed which can be temporarily demounted during the towhead movement through the town.
“Because of the regularity of our towhead movements, we have maintained good working relations with the local council and utilities providers, all of whom collaborated very effectively on a smooth implementation of the modifications.”
Both towheads were fabricated to Subsea 7’s design in Invergordon, 130 kilometres south of Wick and shipped to Wick harbour on board the specialist heavy transportation barge Terra Marique, together with the SPMT modules. “We decided to use Terra Marique’s RoRo capabilities to drive off both the 290-tonne NTM and 125-tonne SSIV towheads at high water in Wick harbour,” explains Partlow. “For the rough 12-hour sailing up Scotland’s North-east coast, we secured the towheads with rigid sea-fastenings which were subsequently removed to enable the NTM SPMT and SSIV trailer to drive off the ramps and onto the quayside with their towhead cargoes.”
First off was the larger NTM towhead in a smooth 40-minute operation which used the advanced manoeuvrability of the SPMT to self-propel the structure over the barge’s heavy-duty sea ramps and onto the quayside for positioning in readiness for the transit through the town a couple of days later. The following day, the smaller SSIV towhead was hauled off the barge on its 12-axle trailer by the MAN TGX tractor and subsequently moved through the town and out to the fabrication site.
For the twisting 18 per cent gradient up from the quayside, Mammoet hooked up the powerful Trojan 8870 ballast tractor unit to haul both the MAN TGX tractor and the SSIV towhead in tandem, which it executed comfortably at a steady 3kph. The Trojan was then uncoupled and the more manoeuvrable MAN TGX completed a straightforward solo 10km haul along the flat A99 road from the town outskirts to Subsea 7’s fabrication yard at Wester.
“Running the smaller towhead first was a very valuable rehearsal that enabled us to give close physical scrutiny to every metre of the route,” says Partlow. “We had two four-man teams on site, one concentrating on mobilising the SPMTs and the other on configuring the tractor/trailer assemblies. We were subjected to repeated weather delays – even in early spring, Wick experiences wintry storms and freezing temperatures, so you’ve just got to hunker down and stay ready for the weather windows.”
Four days after the SSIV movement, the weather finally eased and the record-breaking NTM towhead started inching up the gradient on the SPMT with a driver/steersman walking alongside each of the two modules using tethered control systems. Subsea 7 chooses to delay towhead movements until after the morning ‘school run’ to minimise disruption to the community, but temperatures were still hovering around freezing as the structure started its ascent. In case of ice or heavy frost, the Trojan had been mobilised at the head of the slope to assist the SPMT if required, but its formidable tow power was not needed.
THE NEW NORMAL?
Once the gradient had been successfully negotiated, the next challenge was the notoriously tight ‘Job Centre’ corner which had challenged many previous, smaller towhead transits. Even seasoned observers were amazed at the ease with which the SPMT negotiated this obstacle. After inching forward, the first bolstered module spun through 90º in a carousel movement and crabbed the load round the bend almost without mounting the specially lowered and strengthened kerb.
“I heard some of the Subsea 7 team say ‘that’s just not normal’ as the SPMT took the bend in its stride – they were expecting the load to sweep out much more,” says Partlow.
“Even though as project engineer I had spent endless hours working on the design on screen, it was still awesome to see such a large structure in real life negotiating a highly challenging route with such ease.”
The movement through the town was accomplished very smoothly in under two hours, and the towhead was parked on the town outskirts in a ‘lift and lock’ system for transfer to a conventional 18-axle trailer for the final leg of the journey along the 10km of open country road to its destination at the Wester site.
The SPMT system by definition moves at walking pace, since it’s controlled by drivers on foot, so the towhead was lifted and locked by a SB1100 hydraulic system while the SPMT was driven out from underneath it and dismantled for the return journey back to base. The 1,100-tonne lift and lock system then lowered the towhead onto the 18-axle trailer which was hauled by the MAN TGX along the final leg of the transit to the fabrication site.
“Achieving this remarkable feat of over-dimensional operation will open up a new generation of larger, more sophisticated Pipeline Bundles,” emphasises Watt. “This enhanced capability will enable our project engineers to design subsea systems which will maximise future oil and gas recovery from marginal and stranded North Sea reservoirs. Wick has always embraced towheads as part of the industrial landscape of Caithness and the spin-off from this remarkable part of the pipeline bundle business has built up a
versatile and highly skilled micro-economy of service companies in plant hire, civil engineering and other support services who respond quickly and efficiently to requests for customised peripheral components.
“Above all, however, delivering this milestone operation so successfully was a triumph for teamwork. We had to meet challenges ranging from extreme weather conditions to Coronavirus restrictions, and I would like to pay tribute to the Project Alliance team who designed the towhead, to Daisy and the Mammoet crew, and above all to our tireless Subsea 7 project and site operations teams who drew on all their vast experience to deliver such a successful outcome,” Watt concludes.
For further information on Mammoet Offshore expertise, please visit https://www.mammoet.com/offshore/