Subsea supership

The world's largest deepwater well intervention ship recently set sail from Norway, bound for its first offshore contract. Before it left port, Terry Knott visited the newbuild Skandi Aker to find out what makes this vessel stand out from the pack.

Do not judge a book by its cover', as the old adage goes. At first sight, it would be easy to take the Skandi Aker as another new, bright and shiny offshore service vessel. But take a tour around the decks and you are left in no doubt that this particular bright and shiny vessel fair bristles with purpose designed capability, from the bridge to the bilges.

Hailed by operator Aker Oilfield Services (AOS) as the largest monohull subsea well intervention unit yet built, Skandi Aker is targeted primarily at the deepwater well intervention market and will compete directly with more costly mobile offshore drilling units (MODUs) - semisubmersible drilling rigs and drillships.

'There is a need for more cost-effective subsea well services,' says Johan Reenskaug, AOS vice president for projects. 'Field operators striving for increased oil recovery know that the most important drivers are intervention in existing subsea wells and gaining access to new reservoir drainage points. But these activities are being constrained in today's market by the availability of conventional MODUs. Furthermore, we believe this rig shortage will continue in years to come, particularly as the demand for drilling new deepwater wells is increasing everywhere.

'Hence AOS is focusing on deepwater intervention as a niche market where we compete with deepwater MODUs for subsea well servicing. MODUs currently have day rates in the region of $500,000 and more the plan is to free up scarce and expensive rig time and allow the rigs to do more drilling for clients, while we carry out the intervention operations. To offer this alternative approach, we have brought together in Skandi Aker the complete package of well intervention technologies, supplemented with some new elements, to deliver the most advanced vessel of its kind.'

The move to build the vessel is part of a wider strategy by AOS parent company Aker Solutions to create a comprehensive offshore intervention and oilfield services capability. At the end of last year, AOS took delivery of the Skandi Santos, which is now on a five-year contract with Petrobras installing subsea trees, and later this year the Aker Wayfarer will join the AOS trio, ready to be outfitted for construction work. Meanwhile AOS sister company, Aker Marine Contractors, already has two deepwater installation and construction vessels at work, the BOA Deep Sea and BOA Sub Sea, with a third one, Aker Connector, designed to install long, heavy power cables and subsea umbilicals, being built now and under charter to ABB from 2012.

'The work being done by Skandi Santos offshore Brazil underlines the point about cost effectiveness compared with MODUs,' notes Reenskaug. 'Skandi Santos is engaged on subsea tree installation work - using the vessel to install a tree instead of a rig doing it, can save the client six rig days.'

But it is the subsea well intervention market which is the main driver for the Skandi Aker.

'As an indicator, a platform-based well might typically be worked over once a year at a cost of around $1 million,' Reenskaug explains. 'But subsea wells rarely get intervention due to the high cost of rigs and their availability. A subsea intervention using a MODU may cost $10 million. With Skandi Aker we aim to be somewhere between the two cost extremes.'

Will the market be there to support this ambition? Reensburg points to forecasts for the number of subsea wells worldwide to rise from today's level of around 5000 to almost 8000 over the next four years. And with an operator such as Statoil targeting 55% hydrocarbons recovery from its subsea fields compared to the present 45% or so, he observes that: 'You don't need much well intervention to close this gap'.

Complete solution
Skandi Aker, owned by DOFCON and leased to AOS on a five-year charter with an option for a further five years, was designed and built by leading shipbuilder STX. The OSCV06L design gives the vessel IMO MODU code classification, enabling it to handle hydrocarbons onboard, which is seen as a major advantage over other subsea intervention vessels.

The 16,000t vessel was designed by STX Norway Offshore Design in Ålesund. The 157m long hull was built by STX Europe at Tulcea in Romania, with STX Europe's yard in Søviknes on the northwest coast of Norway responsible for outfitting and completing the vessel. Here, some six months were spent on installing major components such as the six MAN main engines with a total capacity of 19,200kW, six power generators and seven thrusters, and the 140-bed living quarters, followed by a further six months installing the 'offshore equipment'.

Once the vessel was 'delivered' from Søviknes in January, AOS began finishing the topsides at the yard and continued work at Aker Solutions' yard at Hinna near Stavanger. Equipment and systems testing started in April, including putting Skandi Aker through its paces in sea trials to the north of the Shetland Islands. According to AOS, the vessel's low resistance hull shape gives it a transit speed of 18 knots with low fuel consumption, its 27m beam contributes to high stability, and good station keeping is achieved with a Class 3 dynamic positioning system.

'Skandi Aker is equipped to provide a complete solution for regular maintenance of subsea wells at an affordable cost,' says Frank Willumsen, AOS business development manager. 'These include riserless interventions for subsea wireline operations in up to 800m water depth, and riser-based interventions in around 3000m of water for both wireline and coiled tubing. We can also perform well test and clean up operations, subsea installations, and light drilling operations such as through tubing drilling.

'There are not many monohulls that can enter into wells in this way - we believe Skandi Aker is the only monohull vessel with a deepwater riser system for interventions. The next step up is a drillship.'

Willumsen points to just some of the vessel's features which make these and other operations possible.

At the heart of the vessel is a 42m high derrick with active heave compensated (AHC) drawworks with 450t capacity, sited above a 7.2m square moonpool - a 100t load can be installed in up 2000m of water. The derrick is purpose-designed to run risers and has a 29m high V-door, enabling tall packages to be stacked up on deck ready for skidding into the derrick.

The riser is designed for deepwater intervention, having an eight-cylinder, 20m stroke tensioner system to cater for large drive or drift offs, and the capability to support a combined weight of riser and coiled tubing up to 725t. The high pressure riser and lower riser package and have been designed to operate at up to 10,000psi.

'The riser package delivery is out of phase with the vessel delivery so that we can choose the best package to suit a particular deepwater region,' says Reenskaug. 'The first package is designed for the South Atlantic and is currently being fabricated at Aker Solutions' Tranby yard near Oslo for delivery later this year.'

Above and below
Free deck space on the vessel is very apparent, stretching some 65m aft from the midships derrick, and amounting to some 1850m2.

'Unlike a drillship or semi, we don't have to carry drill casing onboard, so we have more free deck space available for stack-ups and other tasks,' notes Willumsen. 'The deck is equipped with a skidding system so we can move loads around on the deck - we can move 100t in the moonpool area and 60t outside, which makes handling in rough weather much safer.'

Deck cargoes above 3500t can be carried with ample space for coiled tubing and wireline rig ups, and the installation of well test equipment at the stern of the vessel if required.

In addition to two deck cranes, there are also two very large AHC marine cranes for subsea construction work, one with 400t lift capacity, the other rated at 50-100t, and four associated passive anti-heeling tanks. Stable operation of the cranes was proven in the sea trials in 6m significant wave heights, along with the simultaneous deployment of both of the vessel's Triton XLS 150 ROVs in 2700m of water. The heavyduty workclass ROVs, housed in a dedicated hangar, are fitted with AHC launch and recovery systems - one ROV is launched through a centre moonpool, the other is deployed over the side, as is a smaller onboard observation ROV.

Below decks, Skandi Aker is fitted with an array of large storage and mixing tanks "around a dozen all told " with associated pumps and agitators to support well intervention operations. The three Calder main pumps can deliver mud, drillwater or brine completion fluids at 3500 litres/minute at 7500psi, while a fourth pump is designed for chemical injection duty, delivering 250l/min at 5000psi.

Although the amount and diversity of below-decks equipment list is impressive, there is no shortage of space to move freely around the kit. 'This is one of the many benefits of the Skandi Aker design - we have been able to plan the layout for more efficient and safer working,' concludes Willumsen.

Despite its readiness for well intervention operations, Skandi Aker's first contract is focused on subsea construction work. At the end of May, the vessel set sail from Hinna for Ghana to install nine suction anchor piles in the Jubilee field operated by Tullow Oil, some 60km offshore in 1100m of water. The 4m diameter piles, 15.5m long and weighing 61t, will be used to moor the field's FPSO. Further work in the field is expected to follow.

Bright and shiny the Skandi Aker most certainly is - and also very capable of fulfilling its niche role in the deepwater well intervention market. OE

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