Dry tree semis await their moment
  from: Offshore Engineer
  by: Jennifer Pallanich
  Monday, April 21, 2008

The industry drive for improved economics coupled with a priority for safety has led to the creation of novel concepts as well as hybridization or extension of existing technologies. When operators started asking for less cost-intensive dry trees to be used on quayside integration-friendly semis, the engineering firms jumped to it. US editor Jennifer Pallanich looks at some of the rival solutions now ready for market or close to commercialization.

While operators have deepwater options like spars and TLPs and wet tree semis at their disposal, they seem to like the idea of a dry tree semisubmersible to produce deepwater reserves, if the reservoir is right for a dry tree. For good reason. Using a dry tree means spending less money on maintenance and interventions. The operator has direct vertical access to the well without being beholden to the MODU market. Integrating the semi hull and topsides at quayside means no heavy lifts and yields a rig that can be easily relocated.

Operators and the US government are interested in dry tree semi technology, and engineering firms have risen to the challenge, coming up with different approaches that play to their respective in-house strengths.

Attention on Aker

The Aker Dry Tree Semi (ADTS) approach centers on the Aker Deep Draft Semi design and the in-house Ram tensioner expertise from subsidiary Aker Maritime Hydraulics.

‘The key to get the dry tree semi to work as a concept is the tensioner technology. The rest, being the design of a motion-optimized semi, is fairly trivial,’ says Leiv Wanvik, project manager for front end studies. He says Aker’s strategy is to build on its proven semi without focusing on ‘firsts’ in motion suppression devices. The company is focusing on the technology of developing long stroke tensioners.

About a dozen floating units around the world now use a Ram Rig drilling package that is built around long stroke direct acting tensioners, he says, and the Maritime Hydraulics (MH) team has developed a second-generation long stroke tensioner that is more compact, easier to fit together, and easier to transport, install and change out,Wanvik says. The second generation tensioner accommodates strokes up to 35-40ft, which is the appropriate range for Gulf of Mexico applications, he notes. The second generation tensioner is expected to be ready for the market in 2009, with model testing of the ADTS planned this fall in a US basin.

The ADTS is based on existing technology already in use on high performance drilling rigs. ‘It only differs in its configuration as the ADTS have several risers while the drilling rigs have only one drilling riser,’ explains Wanvik.

The challenge lies in tuning a semisubmersible so it can cope with the effect of the tensioned risers, which act as springs between the vessel and the seafloor. A higher well count, for example, means the whole unit becomes stiffer, he says, which may influence the dynamic behavior of the entire system.

The firm’s semi hull concept for the ADTS will be based on the class of deep draft semi that Aker designed for Chevron’s Blind Faith wet tree semi for the Gulf of Mexico.

‘We’ll use the Blind Faith class of semi and size the tensioners such that they will absorb the required stroke’ and respond favorably to waves,Wanvik says. ‘If you don’t have control over this combined technology, you are behind. These systems have to be very much in harmony.’

Johan Almen , Aker’s project director for the Blind Faith hull and mooring project, says the Blind Faith semi’s deep draft design provides improved motion characteristics over traditional semi designs to accommodate the functionality of the SCRs. Further, he says, by using a rational compartmentalization in the hull and no penetrations in the hull skin, better safety levels result and it provides against unwanted damage conditions that could consume tensioner stroke capacity in the ‘up-stroke’ condition. A refinement in the Aker design is the hull is split in four quadrants that have no communication between themselves.

Aker delivered the Blind Faith platform in July 2007, and the topsides were fabricated at Gulf Island Fabricators in Louisiana. Deck/platform integration took place at Kiewit Offshore Services in Ingleside, Texas, and Aker Marine Contractors handled the platform installation and riser tie-in.

Aker sees its ADTS as being competitive with a TLP in 1000ft of water, especially for low well counts. According to Wanvik, in a benign environment such as Southeast Asia or West Africa, an operator could get a rapid return on its investment by fast tracking a dry tree semi with three or four wells. The concept is designed to work in up to 8000-9000ft of water using current riser material; some changes would need to be made to the riser material to get it to 10,000ft or deeper, he notes.

As the semi doesn’t require offshore commissioning, there’s a shorter time to first oil,Wanvik explains. ‘You simply go out there, hook up, and begin producing.’

So why hasn’t the industry seen dry tree semis before?

‘There’s a psychological threshold in the market with reluctance to be the first to try out a new concept,’ concludes Wanvik. He believes the breakthrough with this concept will likely come with one of the independents.

Aker plans to approach the market with ADTS this fall following completion of model tests.

Exmar’s optimal design

The OPTI series dry tree semi from Exmar is designed around the firm’s existing hull design and current tensioner technology.

‘Our initial analysis verified the feasibility of using this hull design with 16 top tension risers for water depths around 1500-1750m for West Africa,’ says Otto DaSilva , Exmar Offshore’s engineering manager. He says the results ‘show that a design that is simple and reliable is capable of having the 16 top tension risers for this environment. The tensioner maximum stroke for this case is about 5.7m.’

Exmar called on riser specialists 2H Offshore to provide tensioner analysis and information. Their conclusion was that the maximum required stiffness of 190kN/m does not present any technical difficulty. The same response was obtained from tensioner manufacturer VetcoGray, says DaSilva, adding: ‘The question at the end is what type of tensioner do you have to use, and what size and stiffness it has.’

The engineering firm chose its OPTI platform for the dry tree design based on the semi’s motions, he says.

The OPTI design with a dry tree ‘was a natural development’ from a client-paid job, he points out. The concept features a simple design and construction, he adds, and the client and Exmar are in talks about moving forward with it.

Exmar is already building an OPTI unit on spec for early production fields, DaSilva says. The idea of using this kind of unit for early production will likely appeal to operators who face a long wait to bring a production unit onstream due to construction slot restrictions or other delays, he adds. ‘This just gets a jump-start while getting a more permanent production unit started.’

Exmar’s design puts the top tension risers in the center of the hull to reduce motion responses. ‘It’s a well-protected position,’ says DaSilva.

The OPTI design, with its flat single deck layout, allows for greater flexibility for the arrangement of topsides equipment, maintenance and future upgrades, he says.

‘Everything is basically off the shelf. We could do something very sophisticated and (have) a little more improvement, but I’d rather go for a big improvement and little sophistication.’

In some ways, the design is ‘like blurring a line’ because a semi with 16 top tension risers starts behaving like a TLP, notes DaSilva. Deepwater concepts are becoming more uniform, with a ‘smearing’ of concepts, he adds.

The Exmar OPTI design employs conventional 12-point mooring using a taut system of chain-polyester-chain. According to DaSilva, the next step is to verify the design for different sea states. He reckons it will be at least two years before an OPTI design with top tension risers can come onstream.

The design specifications Exmar used on its unit under construction, the OPTIEX, has a 7000t deckload but the design is said to be scaleable between 2000t and 15,000t.With the 7000t design, steel weight is 2000t and equipment accounts for the remaining 5000t. The hull weighs 6000t. DaSilva says this size of hull could support production of 40,000b/d and 50mmcf/d and be deployed in water depths ranging from 250ft to 10,000ft.

FloaTEC floats two

The two dry tree semi solutions in FloaTEC’s design portfolio – the Truss Semi (T-Semi) and Extended Semi (E-Semi) – draw on the strengths of the joint venture company’s parents, Keppel Fels and J Ray McDermott.

‘The difference between them is the installation method of the means to suppress heave,’ explains John Murray , FloaTEC director of technology development, noting both use the same basic deep draft hull design. One version employs a single extendable heave plate, while the other employs a truss section with multiple heave plates.

Both concepts have been through initial model testing, and the company is moving forward with further testing with the objective of commercializing the technology later this year.

‘Note that it is not new technology,’ says Chris Barton , FloaTEC director of business acquisition, of the dry tree semi concept. ‘We’re simply marrying existing technology to add value and create a hybrid to enable dry tree production and direct vertical access into the wellbore.’

Both solutions use versions of FloaTEC’s proprietary DeepDraft Semi hull and rely on hydrodynamic interaction between the heave plate and the hull to reduce motions, Murray points out.With the E-Semi, he adds, the heave plate is installed under the hull as part of the hull construction, so at location the heave plate is lowered. For the T-Semi, the truss would be launched like a conventional jacket and mated with the hull either nearshore or offshore. The E-Semi is said to offer slightly more flexibility in terms of redeployment, while the truss semi is probably more suited for a permanent installation.

Barton sees particular significance in the fact that the motions are limited to commercially available tensioners, currently limited to under 30ft stroke. Longer stroke tensioners beyond 30ft would require more real estate to accommodate and are not commercially available, he notes, adding they would also be prohibitively expensive. FloaTEC is developing dry tree semi base case designs for Gulf of Mexico conditions in two different water depths – 5000ft and 8000ft.

Murray believes the industry will like the flexibility that comes with the well bay design of the dry tree semis as well. ‘There are efficiencies in both well bay and deck layout, thus resulting in more efficient drilling and production operations,’ he says.

FloaTEC sees both designs as options for field developers looking to capture the advantages of dry tree drilling and production and direct vertical access to the wellbore.

The choice of T-Semi or E-Semi, adds the company, will depend on operator preference and the metocean conditions and marine/installation vessel availability in the area.

Modec’s moving spectrum

William ‘Willie’ Rawles , general manager of Modec International’s TLP/semi division, says his company currently sees the dry tree semi as a viable concept for benign environments.

‘We are not yet convinced the Gulf of Mexico environment will be able to use dry tree technology without major changes’ because of significant heave motions, Rawles says. For now, he adds, there are too many problems to be overcome for the GoM environment, including the need for improvements in long stroke tensioner technology.

Modec is targeting its design instead for Brazil, Southeast Asia and possibly West Africa. For the benign environment, explains Rawles, the company designed its platform to be outside the energy spectrum by moving the platform heave response from the wave outside the wave energy. According to Rawles, Modec has designed a semi that achieves the longer response period exceeding 24 seconds, which moves away from the wave energy by optimizing the pontoons to column ratio; the unit has a deeper draft, uses a taut mooring system that reduces the lateral movements of the platform and relies on existing long stroke tensioner technology.

‘We do not want to use what we consider to be extra novel solutions,’ Rawles adds.

Modec recently completed a case study on a dry tree semi application for Brazilian waters that uses existing tensioner technology. The Dry Tree CP Semi uses the company’s central pontoon design and enlarges the platform with no major changes to the existing structure design features, notes Rawles, pointing out that it has larger columns and a deeper draft than the CP Semi used in wet tree applications. The central pontoon design also uses radially oriented columns to move the center water plane area and make the unit more stable, he says.

The firm’s central pontoon concept has been project ready since 2005 but has yet to find application. Rawles says no applicable projects have come up for bid since the concept was put on the market.

The central pontoon concept has been model tested for 6500ft water depth in a wind tunnel and wave basin, he adds. Initial designs considered heavy topsides and fewer risers or light topsides and more risers, with Modec electing to model test the latter option. The central pontoon is ‘not radically different from any of the existing semi designs,’ says Rawles, noting the similar shape and rectangular columns. The difference, he says, comes from moving the columns outside the pontoon, similar to Modec’s TLP design. Moving the columns from atop the pontoons to outside the pontoons creates an easier-tobuild design, adds Rawles, the unit being scaleable without increasing pontoon size. The design places the top tension risers in the center of the platform.

The whole dry tree semi design is ‘also driven more by the mooring hardware,’ Rawles says.

Modec plans to offer its Dry Tree CP Semi to customers on a sale or lease basis, adopting the same business model the company uses with its FPSOs.

SBM stays in tune

‘There’s been a big push to say, “Is there something we can do better?”’ says L Dwayne Breaux , vice president of sales and marketing for SBM Atlantia. The company’s Dry Tree DeepDraft Semi builds on TLP and semi technology.

The big issue with dry trees, according to Neil Williams, SBM Atlantia’s product development manager, is the use of tensioners to maintain acceptable tensions in the production risers. The tensioners can only stroke so much, and it’s critical they don’t overstress the risers or allow them to become slack, he says. Breaux adds that the motion needs to be within 30ft to remain ‘in the known technology range’ for tensioners.

The Dry Tree DeepDraft Semi is designed with quayside integration in mind, with Williams acknowledging that the aim was to capture the spar’s benefits, such as the ability to host a dry tree, without having the disadvantage of offshore integration. ‘If we can do both of those, we can offer a cost-effective solution,’ he declares.

One design factor the company’s engineers took into account was to limit the keel-to-deck height to maximize stability and keep it in a size range that makes quayside integration possible. To achieve acceptable motions, however, the unit needs to have a deep draft, which means a large keel to deck height, explains Williams . ‘So we’ve got these two competing issues,’ he adds.

The industry at large has looked at solutions that allow shallow draft integration and then extending the draft once on site. These solutions either involve jack-down systems to extend draft or wet-mating a deeper hull component. SBM Atlantia looked at yet another solution: tuning the structure for acceptable motions, or modifying the hull geometry.

‘We tune the structures to make them fit for purpose,’ Breaux says of the firm’s decision to push existing technology rather than devise a new solution. ‘The industry keeps telling us they do not want unproven solutions.’

Breaux acknowledges a spar will always show less heave response than a semi will, but the real issue becomes that of achieving acceptable motions. Tuning, he says, will entail altering the column to pontoon displacement ratios. He says the firm has learned volumes from its existing production semis and TLPs for tuning platforms to project specific requirements. Incorporating the vertical riser restraint along with adjustments to pontoon and column geometries, he adds, will facilitate development of a fit-forpurpose design offering both quayside integration and acceptable heave response.

‘We’re looking to take proven technology and tune it within project constraints,’ Breaux says.

Williams agrees: ‘It’s going to be project and payload specific.’

According to Breaux, SBM Atlantia has a design that will work in more benign environments such as West Africa, the Far East or Brazil. The company plans to roll out its dry tree semi for the US Gulf in time for May’s Offshore Technology Conference in Houston.

Technip extends the draft

In the Technip design stable is the Extendable Draft Platform (EDP), the company’s solution to marrying the best features of deep-draft semis and truss spars for dry tree and SCR applications. The design, which has been market ready since 2004, has four- and three-column versions depending on deck load specifications and is available with or without production storage capacity.

The EDP’s heave suppression pontoon allows quayside integration and then lowering, after tow-out to deepwater, to its operating depth of between 50m and 80m to extend the unit’s draft in line with environmental conditions.

Discussing the EDP concept with OE five years ago, Technip’s senior vice president of technology Pierre-Armand Thomas – one of the recipients of this year’s OTC Heritage Awards – said the company had made ‘a lot of progress’ in terms of constructability (OE December 2003).

‘Using a similar technology to that of the rack-and-pinion and locking systems on our TPG 500 units, we have developed a way to use conventional mooring lines to lower the platform, with topsides installed, to the operating extended draft.’

Thomas added that EDPs and spars had almost the same motion characteristics and response curve, with the EDP ‘slightly better on roll value’.

‘And all the technologies developed for spars in terms of riser systems can be applied to EDPs, so this will save a great deal of time in customizing it for future development concepts,’ he said.

Capable of accommodating drilling, production, import and export risers, the EDP can be scaled up to work in 3000m of water and is considered suitable for use as an early production unit or in a marginal field with a short field life. ‘Globally speaking, it is quite an attractive system in that it significantly improves on the motion characteristics of the semis and is measurably cheaper than a TLP and without the water depth limit,’ said Thomas.

The EDP concept – a hybridization of two earlier designs, Technip’s TPG 3300 and Coflexip’s Aker-designed DPS 2001-4 – was brought to commercialization via a series of joint industry projects involving BP, Ocean Energy (now Devon), and Kerr-McGee (now Anadarko). OE

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