Subseas strategic enabler

Terry Knott
Saturday, September 1, 2012

Purpose-built to match the evolving needs of deepwater subsea developments, the Seven Borealis promises to set a new benchmark in the subsea world. Terry Knott was invited to visit the vessel as it neared completion in the Netherlands.

When Subsea 7s new flagship vessel Seven Borealis arrives offshore Angola later this year to work on Totals CLOV deepwater development, its unique silhouette will send a clear message to the industry at large. For in this one vessel, Subsea 7 has created a combined pipelay and heavy lift construction ship so versatile in its capabilities that it is being likened to a Swiss Army knife and on an unrivalled scale.

That silhouette is dominated by a 5000te mast crane at the vessels stern the largest of its kind in the world a gimballing J-lay tower at midships and a 92m-long S-lay stinger capable of laying pipelines in up to 3000m of water. And beneath the vessels striking external appearance lies an impressive range of tools and equipment, much of it innovative, that the company believes can almost certainly tackle operations on any subsea installation project worldwide.

The 182m-long multipurpose Seven Borealis, capable of pipelaying in up to 3000m of water and lifting up to 5000te, is expected to create a strategic shift in the approach to subsea projects.

Seven Borealis truly is a differentiating asset, says Gael Cailleaux, Subsea 7 VP for offshore resources, who manages the companys fleet of 44 vessels. With this vessel we can lift, lay, construct and survey, all from one platform. It is like sending one ship instead of four to do a full field development project.

His assertion will be put to the test for the CLOV development later this year as Seven Borealis begins work on a $1.3 billion EPIC contract for Total, Subsea 7s largest project award to date.

CLOV, 140km offshore, consists of four reservoirs Cravo, Lirio, Orquidea and Violeta which are to be developed with 34 subsea wells in 1050-1410m of water, tied back to a new FPSO. Under its EPIC contract, Subsea 7 will design, procure, fabricate and install 130km of pipelines, including 40km of pipe-in-pipe production flowlines in J-lay mode, plus 60km of water injection and gas export lines in J-lay and S-lay, all to be handled by Seven Borealis, which will also install a gas export single hybrid riser tower and associated manifolds. Two other vessels in the Subsea 7 fleet the Acergy Eagle and Acergy Legend will be busy installing 80km of umbilicals, spools and jumpers, among other tasks.

On the move

The last port of call for Seven Borealis before it sets sail for Angola is Schiedam in the Netherlands, where the S-lay and J-lay systems have been installed by Huisman Equipment. But the vessels construction history leading up to this has been one with a distinctly international flavour.

In 2007, Nordic Heavy Lift awarded a contract to Singapores Sembawang Shipyard for a newbuild dynamically positioned heavylift crane vessel, designed by Ulstein Sea of Solutions and featuring a 5000t mast crane from Huisman. Keel laying began the next year at Nantong Yahua Shipbuilding in Nantong, China with the hull being launched in October 2009 and then towed to Sembawang for completion. When Nordic Heavy Lift ran into financial difficulties, Acergy acquired the hull at the end of 2009.

At this point Acergy decided to upgrade the 181m-long vessel to convert it into a sophisticated deepwater pipelay and heavy lift construction vessel with the ability to take on a wider range of operations when compared with most of the vessels in the market that carry out SURF (subsea umbilicals, risers and flowlines) installation work.

Large subsea deepwater developments often consist of many different components which frequently demand more than one specialised construction vessel, each dedicated to a single task, vessels which are expensive and limited in availability, and perhaps located in another part of the world at the time of a projects installation phase. Furthermore, in addition to large subsea projects, there are smaller more conventional developments requiring the installation of jackets and topsides along with subsea hardware that do not necessarily need high-end construction vessels but still involve a variety of construction tasks, and often take place in remote regions where suitable construction vessels are not available. Acergys reading of this market situation supported by the companys experience with its successful Polaris subsea construction vessel was that a multipurpose vessel that could install larger infrastructure components, plus pipelines and a variety of subsea equipment in deeper water would indeed be a significant asset in the future offshore marketplace, bridging the gap between SURF installation capabilities and heavy lift vessels. And in the vessel then partly built in Singapore, Acergy could see the opportunity to secure such an asset.

Acergy continued to work with Sembawang and Huisman in its drive to achieve this goal. The vessels original 5000te heavy lift capability would be complemented by a 1000te J-lay system and 600te S-lay system, both to be built by Huisman, and it would also be prepared for the future addition of a 500te flex-lay system for flexible pipelines and risers installation. In 2010, Acergy and Subsea 7 announced their merger the newly formed company would operate under the Subsea 7 name and the conversion work was progressed by the in-house vessel design team of the combined companies.

The mast crane, weighing around 4250te and the largest of its kind, was designed by Huisman and built at its manufacturing base in Zhangzhou in China. The cranes main components were loaded out in February 2011 and transported to Singapore where the mast crane was installed on the deck of the Seven Borealis in a four-day operation. The base of the crane was installed first, then the 11m diameter rotating slew bearing and platform were added onto the base, followed by the 120m long, 1200te crane boom connecting to the slew platform. The final element to go on was the mast head which reaches to 85m above the vessels main deck the boom tip at its maximum elevation will be at 151m above the deck.

With a gimballed suspension, the J-lay tower can be rotated in the horizontal plane and also inclined to the vertical to assist with pipelaying.

The conversion work at Sembawang was very challenging, says Simon Gilbert, offshore manager with Subsea 7. In effect we were converting a newbuild crane vessel that was not yet fully built. We had to carry out the design of the new pipelay systems while also doing the conversion design simultaneously.

Load testing the 5000te capacity mast crane.

Among the many tasks involved in that challenge was the raising by several metres of the 399-bed living quarters and ships control bridge, which sits across the bow of the vessel, to accommodate the installation of the pipelay firing line down the length of the ships centreline. The huge mast crane had also to be modified by creating a strengthened arch in the mast base to allow the firing line to pass beneath and through it.

The mast crane is equipped with a 5000te lift capacity main hook block which is configured as two separate 2500te blocks having two independently operated main blocks means jackets or large suction piles can be lifted horizontally and moved to the vertical with no rigging changes plus a 1200te auxiliary hoist and a 110t whip block. Loads of less than 2500te can be picked up by either of the two main blocks, or the blocks can be configured to be a single swivelling 5000te block.

In Singapore, the fully-reeved crane was load tested multiple times in October 2011, first by individually testing the two main blocks to 2750te, then by lifting 5500te with the combined block, and then a revolving test load of 4400te the load was a ballasted purpose-built 60m-long cargo barge, fabricated in Thailand. A full load of 5000te can be lifted over the stern, while full crane rotation can be achieved with a 4000te load at 40m radius, or 1500te at 78m radius – the slew platform on the crane is driven by 12 electric motors and gearboxes. The cargo barge has now become part of the Subsea 7 support fleet and can be lifted onto the deck of the Seven Borealis, for example laden with stinger sections, if required.

Following sea trials in November last year, the vessel headed to the Netherlands early in 2012, for two important additions in the conversion process.

Preparing for pipelay

First stop in the Netherlands was in Vlissingen where the J-lay tower stood in readiness for Seven Borealis.

We manufactured the J-lay system in 2006 and delivered it to Acergy for its Polaris vessel, explains Anne de Groot, Huisman project director. This system was removed from Polaris earlier this year and brought from Angola to Vlissingen for installation onboard Seven Borealis.

The 1725te J-lay tower was transported to the VDS yard in Vlissingen onboard Jumbo Shippings Fairpartner vessel and offloaded onto the quayside. When Seven Borealis arrived from Singapore the vessels own mast crane was used to lift the tower onboard for installation on the starboard side of the 46.2m wide deck, and from here the 70,000dwt vessel made its way to Huisman's construction yard in Schiedam in late April.

Seven Borealis is the largest vessel we've had in the harbour here, notes de Groot. We had to dredge the River Maas to get it in.

The J-lay systems friction clamps can handle 24m-long double joint coated pipe sections of 4in to 24in in diameter (36in with J-lay collar) and can lay pipelines over the side of the vessel in up to 3000m of water. The system has a 750te static and 937te dynamic tension capacity, and is designed with a 72in clearance to allow components of the pipeline, such as tees, to be integrated into the line and passed through the system it can also handle PLETs (pipeline end terminations) of up to 100te. Two work stations are built into the system, one for welding and NDT, the other for NDT and coating.

A unique feature of the J-lay tower on Seven Borealis is that it is supported by a gimbal suspension, observes Subsea 7s Cailleaux. This allows the tower to rotate in the horizontal plane through 180° over the side of the vessel and it can also incline up to 15° to the vertical. This not only helps with the pipelay angle, but it also gives us the flexibility to lay pipe in one direction while the vessel may be headed in a different direction to match the prevailing sea state. This will be advantageous for installing single hybrid risers and SCRs.

Sections of pipe are fed into the tower using an erector arm nicknamed the magic wand a system of arms and pulleys designed by Huisman that makes the overall system function smoothly.

The other major addition to the Swiss Army knife was the deepwater S-lay capability. Designed and built by Huisman, the S-lay system is able to lay single or double joint pipe sections of 4.5in to 46in diameter, in water depths ranging from 25m to 3000m. The system is equipped with three 200t tensioners and two A&R winches for pipeline abandonment and retrieval, one rated at 600te, the other at 200te.

Up to 2800te of single joint pipe sections can be stored on the port side of Seven Borealis deck. From here, pipe sections are fed into the firing line running along the length of the vessel to be welded into a continuous pipeline the vessel generates its own oxygen and acetylene. The firing line can be configured with eleven stations to make up single joints of pipe, or six double joint stations, offering broad flexibility in the types of pipelines the vessel can install. From the firing line the pipeline passes over the vessels stinger at the stern to be lowered to the seabed in an S-shaped curve.

The stinger is made up in three sections, weighing 1100te in all, with a total length of 92.5m, says Gilbert. The number of sections can be selected to allow us to lay with a stinger radius from 70m to 300m for 3000m of water, with three sections in place, the pipeline leaves the stinger with a near 90° departure angle, pointing almost vertically downwards.

Pipeline touchdowns on the seabed and other subsea operations will be monitored by rovs, one stationed at the stern and one midships, controlled from a dedicated cyber room onboard. The vessel is equipped with two ACV type workclass Schilling rovs fitted out with full survey packages that can operate in up to 3000m of water, and have 1500m long tethers, enabling them to swim out to long-excursion S-lay touchdown points

Versatile lifter

Seven Borealis has three auxiliary cranes on deck; two Dreggen knuckle boom cranes on port and starboard capable of lifting up to 40te, and a Huisman pedestal crane on the port side with 36te lift capacity.

The auxiliary hoist on the mast crane also called the subsea hook will play an important part in installation operations. For subsea work, the hoist will be assisted by active and passive heave compensation with up to 6000m of 109mm low-rotation hoisting wire stored on its traction/storage winch, and can be reeved with one, two or four falls of wire. With four falls it can lift 1200t at 70m radius and deliver 1500m of hook travel, ranging to a single fall installing 300te on the seabed in 6000m of water. The heave compensation system below deck can allow for up to 8m of heave, depending on the number of falls reeved, with the line speed being controlled at up to 3.2m/s.

Left: Pipe joints passing along the firing line for welding and other treatments. The vessels 92.5m-long stinger guides pipelines overboard for S-lay installation in up to 3000m of water.

The heave compensator combined with high lift capacity and large hook travel means equipment can be lifted and lowered straight to the seabed with minimal oscillation and without the need to transfer the load from the crane hook to a special deepwater deck winch.

The onboard cyber room allows two workclass ROVs (inset) to be precisely controlled for a wide range of subsea operations.

Used in combination with the mast cranes two main blocks, the auxiliary hook will give three-point lift capability, which says Subsea 7, takes away the requirement to know the precise centre of gravity of the load at all times during the operation as the balance of the load can be adjusted through the three-point lift arrangement.

When the main hook blocks are not in use the hooks which weigh some 150te can be left in the boom rest on deck, rather than have them hanging from the boom during operations, Gilbert explains. Picking them up again to be ready for use takes only 15 minutes. The boom rest itself is also unique in that the boom does not slide in the rest as it does on most crane vessels to allow for relative movement between the rigid boom and the flexing of the ship structure. On Seven Borealis this movement is allowed for by hinged compensators where the boom rest sits on the deck.

The mast crane is assisted in its operations by nine tugger winches for helping with sling handling, block and load control. The blocks can each be swung out up to 40° to the vertical, giving a spread angle of 80° for the two blocks together, a feature enabling the mast crane to lift dual-point loads without using a spreader beam or specialised slings the tuggers can rotate such a load through a limited angle.

In yet another show of its lifting versatility, the 135mm diameter wire from the 600te S-lay A&R winch can be diverted out of the firing line over a 3m diameter sheave and out over the side of the vessel to a hang off system, effectively providing Seven Borealis with an additional 600te over-the-side lift capacity.

Power plus

Six Rolls-Royce Bergen B32:40 V12A diesel/heavy fuel oil engines, each rated at 5760kW, generate the main power for Seven Borealis, which also has a 1600kW MTU emergency/harbour generator. Main propulsion comes from two 5500kW Rolls-Royce UUC 455 FP underwater demountable four-blade azimuthing thrusters at the stern. The vessel is capable of up to 14 knots maximum 12.5 knots will be the normal transit speed and has an operating draught of 7m to 11.35m.

As an IMO Class 3 DP vessel, Seven Borealis has, in addition to the main propulsion units, four vertically retractable 3200kW Rolls-Royce UL 305 FP azimuthing thrusters, three at the bow and one at the stern. At the bow, the DP capability is boosted by a Rolls-Royce TT 3000 CP tunnel thruster rated at 2500kW. The vessel has no rudder – steering is effected by the fully-azimuthing propulsion units.

The two main engines and bow thrusters give us 35te of thrust, says chief officer Martti Mikkila, standing on the 56m-wide control bridge of Seven Borealis. The retractable thrusters have 3m diameter blades and deliver 50te of thrust. The vessel handles extremely well, not falling off station even at low speeds such as 4 knots. Operating on DP with a full complement of 399 people onboard, we have a very impressive endurance period of 45 days.

Maintaining the correct draught and stability of the vessel during all operations is vital, achieved by seawater ballast held in 20 tanks in the hull containing up to 41,021m3 of ballast water. Ten of the tanks are used to correct vessel heel as heavy loads are moved around by the mast crane by the rapid movement of ballast water from side to side or fore and aft by fifteen pumps, each capable of transferring water at up to 1300m3/h the anti-heel system is designed to allow a 4000te load at 40m radius to be slewed through 90° in about 20 minutes. Rolling motion in heavy seas is dampened by four 2500m3 flume tanks in the hull, reducing roll by about 40%.

Seven Borealis is the pride of our fleet, states Subsea 7 COO John Evans. As we witnessed with the Pazflor development, subsea developments are becoming more complex and require more complex technology, such as that available with Seven Borealis, to match them. Individually, the tools available on this vessel are as powerful as they come, but having all of them available on a single platform is another proposition, one which can add considerable flexibility to the planning and execution of projects. The economics of mobilising a single vessel are at least as important, and for small field developments, where keeping cost down can be a critical factor to a projects viability, Seven Borealis might prove to be the key. For large or small projects, it truly is a strategic enabler. OE

Categories: Equipment Vessels Deepwater Engineering Construction

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