An impressive example of lean design and fast-track construction of a major offshore structure is now standing in 133m of water in the Norwegian North Sea. Adrian Cottrill looks at the technology-stretching jacket at the heart of Statoil's NKr20.4 billion Valemon field development and the new lift-installation record it has established.
Every unnecessary kilogramme of steel in the jacket for Statoil's Valemon platform has been pared away by design and fabrication contractor Heerema Fabrication Group (HFG). The vital driver for this crusade has been the need to keep the structure's final weight within the capabilities of the world's largest offshore crane vessel and so enable a record-breaking lift into place at location.
The 8800 tonne jacket (plus another 800te of temporary rigging) placed on the seabed at Valemon has also been built in admirably quick time at HFG's yard at Vlissingen in the south of the Netherlands. It took us just 50 weeks of fabrication, from cutting our first steel on 6 June last year to ready for sailaway on 26 May, says Aren Bezuyen, project manager at the yard.
Pre-production drilling is due to start at the platform this month. But the main topside due to weigh about 10,000te and being built by Samsung will not be lifted on until 2014. By that time four production wells will have been pre-drilled through the jacket from a jackup tender rig that will stand alongside for the next two years.
Without the intense design effort put in by in-house design company HFG Engineering at its Zwijndrecht headquarters, a more normal jacket to meet Valemon requirements would likely have consumed a further 1000te of steel reckons Cees Spaans, the company's VP for technology.
The Valemon jacket is the heaviest ever to be lift-installed reckons HFG, and by far the heaviest of the half dozen large jackets so far built by the company at Vlissingen. And it is the first large jacket built by the company for Statoil.
The vessel that lifted it into place on 18 June was Thialf, operated by sister company Heerema Marine Contractors. This semisubmersible shares the title of world's most powerful crane barge with rival S7000, operated by Saipem. Each of these two units sports a pair of main cranes that provide a maximum tandem lift capacity of a little over 14,000te.
In the particular configuration needed for Thialf to lift-install the Valemon jacket, the HFG team needed to keep several thousand tonnes below that for the combined weight of jacket and temporary rigging hanging off the vessel's hooks during the big lift.
Not without considerable effort, and certainly with one or two periods of some pain, they have managed to achieve just that. The company won the 20-month EPC contract rumoured to be worth about $80 million from Statoil in October 2010. This followed the detail design contract won in the preceding month. Earlier competitive FEED studies by HFG and Aker had already established lift-installation as the road ahead for the project.
If a jacket is too heavy for this approach the main alternative is to skid it lengthways into the water off a launchbarge. However, as Cees Spaans points out, if you can lift-install you should do so.
Of course, launching is not only an issue of weight, he continues. You need to have extra strength in the jacket to undergo the launch, skid rails on your barge, along with ballast systems and buoyancy tanks, and favourable weather, all adding to risk.
Client Statoil has a long pedigree in lift-installation of big jackets. Indeed the company's man on the job here, Kjell Arvid Tuen, with the title Statoil representative, Valemon jacket EPC is very familiar with the concept. He has been involved with half a dozen such projects in his 20 years with the oil company.
Ways to lose weight
For Valemon the sights were always set on lift-installation, and therefore on minimum structural weight. Spaans lists some of the measures adopted to achieve that at an offshore location where the 100-year design wave has a peak height of 28m. (Under this extreme condition it is calculated the platform top will displace about 750mm laterally.)
High strength steel is one of the measures to keep weight down, he says. Though not particularly unusual, grades such as 420 and 460 made a helpful contribution.
The weight of anodes required for corrosion protection has been reduced by painting the whole of the jacket, he continues. And the diameter of tubular members is kept as small as possible, only ballooning out at end connection nodes where extra strength is needed to meet fatigue demands.
In addition we have braces that are flooded so they do not have to resist hydrostatic collapse. This is not normal practice because buoyant braces lend themselves to flood detection systems to indicate fatigue damage, but is an acceptable trade-off here. Also, instead of designing in additional strength to meet demands during the temporary transport phase at sea, we have dealt with that in other ways, says Spaans. For example, a system of bracing wires to the inner members dampened any tendency for them to vibrate in high winds.
Once HMC won the installation contract, that phase could be designed specifically around Thialf. One of the differences from S7000 for example is that Thialf's main hook is allowed underwater, avoiding the need to findextra buoyancy to avoid that limitation.
Above all, says Spaans, we worked steadily towards our weight target by doing far more than the normal one or two cycles of engineering to get weight down. We analysed again and again and again.
Both Spaans and Bezuyen speak of the benefits of having so many aspects of the whole project within the Heerema group and the quality of the resulting interaction between the design and construction sides. They also underline the very effective communication with client Statoil and certifying authority DNV right from the beginning.
Statoil's Kjell Arvid Tuen agrees. His own involvement with the project began in the very earliest days when the design basis was being evolved. Then, when the contract was awarded, Tuen went on to spend two years in the Netherlands, starting in HFG's design offices and ending with close to four weeks on board Thialf for the offshore installation.
Clearly he enjoyed the experience. The Dutch and Norwegians have the same cultural attitudes, he says, and communication has been very effective. We started working together from day one of the contract, and that's one of the reasons for the success I think.
Two challenging issues
Certainly the emphasis on lean design, with maximum input as early as possible, paid useful dividends, says Cees Spaans. However, it did not prevent the design process going through a couple of painful periods.
The first of these came during design of the two mighty trunnion nodes in the jacket's lower legs. These huge steel castings, weighing 110te each, formed the all-important pivot points when the jacket was dangling from Thialf's hooks during installation offshore and was rotated from horizontal to vertical.
Built by specialist company Vulcan Sheffield Forgemasters, they are the largest cast nodes ever produced. The challenge of designing these castings was definitely a little bit underestimated, concedes Spaans. In fact in the end it took us nearly three months instead of the two or three weeks expected.
The rotating hub is a very tight fit with just 3mm of clearance, he explains, and the challenge was to prevent it deforming too much under load. There are also another two smaller cast nodes at the lift points in the upper legs. Happily these proved less difficult to design.
Not long after this the designers encountered another unpleasant surprise. In autumn last year they discovered that in their bid to get the thinnest possible steel wall in the main legs, those legs had been left too weak to prevent deformation during the sea journey at two of the main support points on the grillage aboard the transport barge.
The only solution at this stage was to add local strengthening. Just over 40 ring stiffeners were welded inside the 3.5m diameter lower legs at the two affected points. This brought the triple inconveniences of about 60te of extra steel weight, a large amount of planning, and a tremendous lot of hours needed to weld them in. However, that's all part of the EPC risk, observes Spaans philosophically.
A further area of challenge for both designers and fabricators was the grillage and sea-fastening for the journey to the offshore location. The demanding North Sea environment to be designed for (a maximum wave height of nearly 11m during sea transport) meant high loads and large reactions between barge and jacket. It was difficult to find an example from a different project and that was a challenge we solved together with HMC, says Spaans.
What did go very well was the whole certification process, he notes. I thought the main challenge would be to meet fatigue criteria with DNV, not least because when different organisations do the same analysis, they are bound to use different kinds of software.
However, the verification process with DNV went amazingly smoothly, he says. This was a lot to do with them being involved early.
Boat impact and foundations
Designing against boat impact is clearly another area where HFG is proud of its efforts. This generated a very interesting technical discussion, notes Spaans.
The criteria for Valemon are unusually high. Because the platform sits in a shipping lane it could potentially be hit by vessels as big as a large supply boat weighing up to 7500te and moving at up to 5 knots. In addition, a vessel like that can have more complicated aspects to its shape, such as a bulb at the bow, which influence the way it could penetrate a platform.
In more standard locations, the requirements are based on a 5000te vessel moving at up to 4 knots. All this translates into Valemon having to cater for 35MJ of impact energy. This figure is close to three times what is more usually called for.
In general, you want the jacket to be as open as possible to minimise wave load, Spaans points out. But in the area where a boat could impact (between about 12m above and below water level) members need to be more closely packed to prevent a boat penetrating too far into the jacket.
Industry guidelines are based simply on impact energy and point load, he says. They don't look at the physical shape of the boat, nor where you can enter or what you can hit.
So we engaged in a more probabalistic approach to define the real challenge and find the right balance. We went through a question and answer process, asking ourselves just how far a boat could penetrate into the body of the jacket.
To do this, HFG engaged in a very sophisticated study jointly with Dutch research organisation TNO which has done considerable work in this field. At the same time Statoil undertook a similar study through Trondheim's University of Science & Technology.
In the platform's foundations one novelty is the inclusion of cages of 16mm steel reinforcing bar within the annulus of grout that connects each pile to its sleeve once it has been driven. This is part of the latest DNV codes on grouted connections.
The jacket stands on a group of four 84in diameter piles at the base of each of its four main legs. These piles are driven 59m into the medium seabed soil at this site.
The move to add rebar cages to the grout follows a certain amount of trouble with foundations in the offshore wind sector, perhaps too economically designed. There are reports of grout crunching and tube damage. In effect the addition of rebar cages turns the grout layer into straightforward reinforced concrete.
To cater for the pre-drilling operations due to run for two years from July until the platform's main topside arrives, the Valemon jacket carries a wellhead module which will later be partly integrated into the main topsides. This has also been designed and built by HFG in just six months, fabricating at a smaller yard near its main Vlissingen site.
Athough relatively small, at 190te, 25m long and just 3m high, it was also not without its challenges. Rapid action had to be taken last October when Statoil called for the design to be completely changed: from two-deck to single deck. Also, to enable adjustments when the main topside arrives in two years, it is fixed by some 2000 bolts because of the ban on hot work once drilling has started.
In the yard
Summarising the jacket construction work at Vlissingen, HFG project manager Aren Bezuyen describes the rapid pace of early procurement on this fast-track project as especially necessary with certain specialist suppliers like Vulcan Sheffield Forgemasters for the highlydemanding cast nodes. He also outlines the weight uncertainties and puzzles confronted as fabrication proceeded. We did as much work as possible at ground level, building the jacket in four main parts which could be rolled up, says Bezuyen. The heaviest lifts at height were two 400te leg base sections. Late delivery in certain areas meant that we had to do more work at height than we'd hoped. For example we were not able to get all the risers and caissons in place before roll-up.
Another delaying factor, he adds, was the information provided by HMC, themselves wrestling with a jacket that was bringing Thialf to the limits of its capacity. But after less than a year of physical construction work, all was ready by mid-May for the completed jacket to start moving from the yard. In a two-part operation the combination of jacket, support grillage and temporary attachments (a total load of 10,790te) was first moved 35m sideways by load-out subcontractor Mammoet on a massive wheeled array of 400 axles.
Then it was moved 160m forwards onto the barge which would take it to location. Total rolling time was about five hours. Next came ten days of fixing sea-fastenings, guy-lines to braces and suchlike, leading up to sailaway of barge and cargo early on 26 May.
Altogether HFG's Valemon jacket project has notched up half a million manhours. A fifth of that was divided equally between engineering and management. Another fifth was by subcontractors.
The remaining 300,000 manhours went to fabrication in the Vlissingen yard.
Statoil's Tuen sums it up. The big goal right from the start was of course to make the jacket liftable, he says, and in my mind HFG has done a very good job, both technically and commercially.
He has sympathy with HFG over the difficult stages it had to overcome. It's always good to finish as much analysis as possible before you order steel. But it's also important to work as fast as possible. I think HFG did as well as they could. They had to finish by a certain date, so they had to order the steel in the way they did.
As to value for money, Tuen reckons â€˜Valemon is more or less the best we've had. This was helped by our timing in the market, but also by the fact that HFG wanted to show Statoil they were a professional jacket EPC contractor.
It's the first they have ever built for us, and now we have no doubt that they can do it.
Statoil's vice president for Valemon, BjÃrn Laastad, notes: We are very pleased with this delivery from Heerema Fabrication Group, which is a new supplier to Statoil for steel jackets of this size.
As the bright yellow jacket sailed away in the pale dawn light on 26 May, both the designers and the builders in HFG could reflect on a good job done. They had derived all the in-house advantages they could in integrating the engineering and fabrication processes, placed fruitful emphasis on constructability, and been greatly aided by effective communication with their client Statoil and partners.OE