Roger Vielvoye
International Editor
The world's largest and most sophisticated tension leg platform (TLP) was floated out to Snorre oil field in the Norwegian North Sea last month.
The 78,000 ton unit built by Norwegian independent, Saga Petroleum AS, Oslo, was installed in the southern part of block 34/7 and should produce first oil in August, about a month ahead of schedule (Fig. 1).
FIRST NORWEGIAN TLP
Snorre is not only the first tension leg platform in the Norwegian sector, it is also the first deepwater project in Norway. Water on the southern platform site was 1,016 ft deep, compared with 720 ft for the previous deepest conventional unit, the Gullfaks C platform.
Saga's hold on the Norwegian TLP honors will be brief. Conoco is designing a larger concrete hulled platform to be installed in 1,050 ft of water on Heidrun oil and gas field in the Norwegian Sea off mid-Norway in 1995.
Technologically, the Snorre TLP has been an outstanding success. Like all tethered floating platforms, the concept requires strict adherence to topside weight designs.
Snorre went through the design and construction phase without requiring any changes in the topside or hull size to accommodate units that were heavier than scheduled.
In addition, Saga and its engineering designers and contractors introduced a series of weight-saving innovations that allowed Saga to contain the overall weight of the topsides at 40,000 metric tons and the hull at 38,000 metric tons.
These innovations include a completely novel design for the water injection system using vertically mounted pumps and new technology for filtration and deoxygenation, living quarters fabricated from aluminum, a guyed flare stack, the application of program logic controllers (PLCs) to the power distribution system, a state-of-the-art drilling system, and optimized passive fire proofing for structural members.
Snorre TLP will have the capacity to handle 190,000 b/d of oil and inject 380,000 b/d of water. The unit, with 44 well slots plus a 20 well slot subsea production template, will undertake initial separation and processing. Oil and gas are then piped to the Statfjord A platform for final processing and transportation through the Statfjord tanker-loading system.
The hull was built by Kvaerner Rosenburg in Stavanger with Belleli, Italy, as its major subcontractor. Fig. 2 shows the hull under construction.
The integrated deck frame was constructed by Aker Stord. The two units were mated last fall and hooked up at Stord.
The project has also come in under budget. In 1988, Saga estimated total cost, including drilling, at NK21.8 billion ($3.4 billion) but the field is likely to come in at NK20 billion ($3.125 billion).
Saga has an 11.2559% stake in Snorre. The other partners are Statoil 41.4%, Esso Exploration & Production Norway AS 10.3323%, Deminex (Norge) AS 10.0348%, Idemitsu 9.6%, Norsk Hydro Produksjon AS 8.2658%, Elf Aquitaine Norge AS 5.5106%, Amerada Hess Norge AS 1.4559%, Enterprise Oil Norge Ltd. 1.4559%, and Det Norske Oljeselskap AS 0.6888%.
In addition to its shareholding, Esso Exploration & Production has acted as technical assistant to Saga, playing a major role particularly in the early days of the project.
MOORING SYSTEM
The key element in any TLP is the tensioned mooring system. Initially, the Snorre partners investigated the purchase of tether technology from Conoco Inc., the acknowledged world leader in the design, construction, and operation of TLPs. But after failing to agree to terms, Saga sought a Norwegian solution by funding a number of parallel design studies and material qualifications programs. The main contract in this area went to Kvaerner Eureka, Oslo, whose main experience came from precision engineering in the hydroelectric business.
Under an engineering and procurement contract, the company was responsible for system integration and bearing, the anchor latch, torque tool, and top tie-off assembly.
Saga let contract for the tether elements to Aker Stord, and Kongsberg Offshore of Kongsberg, Norway, designed, engineered, and fabricated the tensioner/motion compensator (TMC).
The mooring system consists of four tensioned tethers on each corner, anchored to the seabed through concrete foundation templates, manufactured by Norwegian Contractors (Fig. 3). These cells have concrete skirts that penetrate 40 ft into the seabed. Previous TLPs have used piled steel template foundations.
The 32 in. diameter tubular tethers with a wall thickness of 1.5 in. are secured to the concrete templates through an anchor latch system. The tethers are secured by a screwed pin and box coupling. Each tether is 55.76 ft long, weighing 14.5 metric tons. Eighteen are required to make up a full string.
Horizontal loads on the hull are transferred through a cross load bearing on the base of the hull.
At the top of the tether string, a top tie-off assembly transfers the tension to the hull through the mooring flat.
Kvaerner designed its own flexelement to allow angular movement of the tether at the seabed level. This prevents excessive stress on the string. The tethers are neutrally buoyant to reduce their submerged weight. The Kvaerner flexelement is also included in the cross load bearing.
Saga said each tether string is pretensioned to about 1,600 tons for a total pretension of 26,000 tons. Maximum tether tension during normal operating conditions is 3,500 tons with a maximum abnormal tension of 3,700 tons.
All the tether components have a minimum fatigue life of ten times the 30 year life of the platform. The exception is the coupling welds which have a minimum fatigue life of 90 years and will be inspected regularly by a custom-built non destructive testing (NDT) vehicle.
The tethers also have a leak-before-break capability that provides sufficient residual strength to withstand fatigue and extreme loads after a crack has started to allow water through. This plan enables Saga to charge the string safely.
Saga said that testing the structural behavior and operability, of prototypes or scale models of new mooring system designs was important.
The installation procedure involves making up two tether strings on each corner while the platform is offset from the seabed anchors. Once all four corner tethers have been made up, tensioner/motion compensators (TMCs) will lift the strings to ensure clearance above the anchor units.
The TLP will then be accurately maneuvered over the template with the help of ROVs and an acoustic positioning system, and the anchor latch will be stabbed into place. The acoustic positioning system provides predictions on anchor motions to help the mooring team make the right decision on stabbing the anchor.
At this stage, the TMCs maintain a positive tension of 50 metric tons on the anchors. Using the TMCS, the platform is pulled down 2 m to a pretension of 1,000 tons. The unit is then deballasted to increase the tension to 1,600 metric tons, and the tether is locked off.
The operation is then repeated for the three remaining rounds of tethers. During the positioning of the first two tethers on each comer, a weather window of 72 hr is required.
When the tethers are being assembled, the maximum wave height cannot exceed 3 m, and once the stabbing maneuver is imminent waves must not exceed 2 m.
RISER CONTRACT
Kvaerner Eureka also carried out the rigid riser contract with Cameron Offshore as prime contractor. The contract covered eight production risers, separate oil and gas export risers, and a drilling riser.
Mixed integrated riser joints were used. NKK Japan qualified a threaded connector but because of the load capacity limitations of integral threaded couplings, flanged joints were used in the splash zone and in the lower part of the riser.
As a weight-saving measure, a compact flange was used made up of a tapered flat face and a large number of slender bolts. This gave a strong connection with a small fatigue life on the bolts during service.
Arriving at the welding method and acceptance of the thread connector was really the key to the whole riser manufacturing. Submerged arc welding was chosen after checking out many systems.
The export riser was also designed to contact with sour liquids. To ensure the system was right, Kvaerner Eureka undertook a 720 hr sulfur stress corrosion test.
The company said meeting the fatigue requirements was particularly demanding. Each well has a fatigue life of 15 years, and much of the design work centered on meeting the requirements.
The top end of the riser is supported by a hydro-pneumatic tensioner that can operate in a 100-year storm with only three of the four hydraulic cylinders in operation.
Cylinders are suspended below a structural frame and attached to the tensioner joint, equipped with a threaded adjustment ring for eliminating the riser string stack-up tolerances.
In the event of a wellbay fire, the system has the ability to hold the entire riser. Special fire boots were developed to protect the critical linkages in the system, and extensive tests were undertaken to ensure the structural integrity of the two fire-protected cylinders.
With 44 slots for a likely 36 producers and injectors, space in the wellbays is tight. The Christmas tree is also subject to lateral movement because of the action of the riser. Positioning the 4 in. jumper offtake in this situation presented a major layout challenge.
This was overcome by careful positioning of the tree and location of the header pipe while ensuring that the hose drape and movement did not interfere with other equipment.
HULL CONSTRUCTION
The 24,000 metric ton Snorre hull was built by Kvaerner Rosenberg in Stavanger. The design calls for 1 in. thick steel plate to be used for the four corner columns, each 206 ft high, requiring special heat beds to be installed to bend the plate.
The columns are set on comer nodes linked by pontoons, with the four columns subsequently assembled in three pieces. Fabricating such large volumes of thick steel was beyond the capability of the Norwegian offshore industry and a subcontract for 12 of the 16 corner columns was let to the Italian fabricator, Belleli. The units had to be transported by sea to Norway.
Kvaerner Rosenberg said that apart from handling the thick plate, the critical element in the design and construction was getting enough weight to ballast the hull low enough for the mating with the topsides.
Submersion trials on the hull also suffered from the aftermath of the Sleipner A platform sinking during a submersion test in a nearby fjord in August 1991.
The Snorre hull test was due to take place in deepwater off Stavanger the day after the Sleipner concrete substructure sank. The Snorre test was delayed while additional precautions were taken and all the theoretical calculations were checked.
The site of the test was moved from deep water into shallower water at the edge of the fjord.
WEIGHT-SAVING DECK
Aker Engineering designed an integrated deck as the most weight and cost effective solution for the Snorre topsides. The rectangular deck is 408 ft by 302 ft and 50 ft high, including a 6.5 it double bottom forming the cellar deck. At each end of the deck there is a 38 ft cantilevered section.
Above the shallow cellar deck there are two main decks with intermediate decks fabricated where necessary to support various items of equipment.
The deck was built at the Aker Stord yard north of Stavanger, mated with the hull, and then brought back to Stord for final hookup and inshore commissioning.
To provide maximum safety, the topsides were sequenced with the living quarters at one end of the deck through the utility area, drilling facilities, wellbay, and process area to the flare. The main free-fall lifeboats are located on the cantilevered section by the living quarters, the maximum distance from the hazardous areas.
The design of the drilling system had to take into account the sideways motion of the TLP and the relative motions between the drilling substructure and the riser systems. A full report into the design of the Snorre drilling package appeared in OGJ, Jan. 13, p. 46.
A two-stage single train provides three-phase separation of the well fluids. Partly stabilized crude, above hydrate formation temperature, is then piped to the Statfjord A platform for final processing and transportation.
Passive fire proofing for the structural members was the subject of new computer calculations to determine the thickness and extent. Saga said when compared with estimates based on traditional methods and recommended tables, about 70 metric tons of insulation had been saved in the wellbay area.
The topsides design also applied PLCs to the power distribution system. This resulted in weight and space reductions by eliminating control cabling and produced a high degree of personnel safety by remote control of all breakers in the electrical network.
Saga said the main disadvantage, overdependence on the platform databus, was solved by a combination of redundancy and locally independent PLC units controlling one switchgear unit.
In addition to the main topsides engineering, Aker Engineering was responsible for other design analyses and integration studies. The company said significant cost and time savings resulted from applying computerized design and management techniques, leading to greater integration between the engineering and construction phases of the project.
On Snorre, Aker Engineering included a comprehensive technical information system combined with 2D and 3D computer aided design (CAD). Although the croup has used this tool before, on Snorre it was applied more widely.
It was also the first project where the company modeled the completed integrated deck using 3D CAD. Considerable time savings resulted from greater design confidence and spinoffs from the data base later in the project. Aker was working with Saga on the principles of TLPs long before the Snorre project emerged. The association started in 1973 through a joint TLP development study.
WATER INJECTION SCHEME
The weight-saving exercise for the water injection facilities proved to be one of the most successful on the project. Fig. 4 is a layout of the water injection unit. Saga said it was an example of the good relationship between the operator and Aker Engineering, which was responsible for the design.
Aker brought together three pieces of new technology untried offshore. The unit was contained in a 20 x 20 m prefabricated section capable of supplying 380,000 b/d of treated injection water at 220 bar. The system filters out particles larger than 2 m and removes oxygen to below 10 ppb.
The unit was 550 metric tons lighter than a conventional water-injection package, and the configuration of lightweight equipment allowed Aker to omit one intermediate deck level.
At the heart of the system was the untried deoxygenation system produced by Hydro Minox, Notodden, Norway. The system uses a two-stage stripping and separation process. Seawater and nitrogen flow together in a turbulent state through large serpentine tubes. A palladium catalyst strips oxygen while the nitrogen is reclaimed and recirculated.
Compared with conventional vertical tower deaerators using a vacuum system and an oxygen scavenger, weight savings of about 100 metric tons were achieved.
Heavy traditional filters using sand were also discarded because of weight. Aker Engineering brought in a new system from PuriTech, Oslo, that used two banks of polypropylene filter cartridges which are automatically reconditioned by using a mist of air and sulfuric acid.
This converts organic matter trapped in the filter cartridge to carbon and water which is drained off. The weight saving on sand filters is 250-300 tons while taking up only 50% of the deck space.
Another innovation for the system was the introduction of four large vertical pumps. Conventional water injection units use horizontally mounted pumps with a motor mounted on the deck next to the pump. Although vertical pumps have been used offshore before, they were much smaller than the 4.6 Mw units installed on Snorre. The pumps, supplied by Klein, Schanzlin & Becker (KSB), Frankenthal, Germany, had been developed for the nuclear industry and adapted for the higher pressure requirements of water injection. On the casing of each pump is a 4.6 Mw motor.
A much smaller 0.75 Mw pump and motor has also been installed to handle the injection requirements of the subsea production template. The vertical mounting eliminates the need for alignment between the pump and the motor, making both maintenance and installation much simpler.
The vertical set-up also takes about a sixth of the deck space of a horizontally mounted configuration and generates some weight savings.
One disadvantage of vertical pumps is the need to remove the heavy motor to gain access to the pump mechanism. This drawback has been solved by the installation of an overhead crane. Further weight savings were made by eliminating booster pumps upstream of the main pumps because of higher pressure downstream of the Hydro Minox unit while there is less demand for suction pressure from the main pumps.
With three largely untried elements making up the new system, Saga insisted on a full operational test. The complete unit was loaded onto a barge and shipped to a turbine test unit at Egersund, south of Stavanger. The intensive test program verified the performance and stability of each item and the overall integration of the system. Saga also opted for new advanced membrane technology from Norsk Viftefabrik, Oslo, for a unit to produce 264 cu m of potable water from seawater using advanced membrane technology.
The system operates by reverse osmosis to separate ions and molecules. The Snorre equipment employs pretreatment through filtration and dechlorination. High pressure pumps push water through the membranes and into final treatment through pH adjustment, rehardening, rechlorination, and ultraviolet sterilization.
GUYED FLARE STACK
In designing the flare stack, it was not only the weight but the position of the unit that was important. To meet safety requirements, the unit must be located at the naturally heavy end of the platform.
To compensate, Aker Engineering produced an unconventional design with a 223 ft long, 42 in. flare tube at an angle of 21.6 from the vertical and secured by tensioned cables.
Saga estimates this design produced a weight saving of about 80 metric tons, a 50% reduction on a more conventional design. It was also 80% cheaper to design and build.
As important was the change in the center of gravity resulting from the angle of the flare which cut the requirement for ballast water in this weight sensitive part of the structure.
ALUMINUM LIVING QUARTERS
On any topside, a large volume of steel is used in the construction of the living quarters.
Snorre requires about 180 single cabins for the regular crew and 40 double berths for visiting contractors. Built from steel, this size of unit would weigh about 2,000 metric tons.
The idea of weight saving aluminum quarters, long promoted by aluminum manufacturers and fabricators, was taken up in the conceptual design phase and resulted in a unit weighing only 1,200 metric tons, a saving of some 800 metric tons. Costs for- steel and aluminum fabrication were similar.
The unit was built by Norwegian module fabricators Leirvik Sveis, at Stord, north of Stavanger in a yard close to the Aker Stord facility where the main topsides were assembled.
Engineering and design support, particularly in the area of fire insulation, came from another Norwegian company, Raufoss.
The three modules that make up the living quarters use the stress skin design. Extended deck stiffeners and plate sections are welded together, and the load is transferred through extruded beams to the external and internal walls. These are made of extruded profiles and corrugated sections. Safety is enhanced by the provision of redundant supports at the top of each of the living quarter modules. And in addition to the insulation and fireproofing there is a 1 m-wide gap between the quarters and the rest of the platform. Saga said a jacking system can be introduced for inspection and maintenance of the supports.
SUBSEA SYSTEM
The subsea production template has been installed in 1,098 ft of water, 3.7 miles from the TLP. The unit has 15 wells, but only 10 of them can be used at any one time (Fig. 5). In the first phase of production there will be six subsea producers and four injectors.
Production from the unit is expected to plateau at about 60,000 b/d.
First oil from the subsea system is not scheduled to start until June 1, 1993, although platform production, as mentioned, is anticipated earlier. Providing there are no serious weather hold-ups in the fall and early winter, Saga is hoping, however, that the subsea production target can be advanced by 6 months.
While many offshore operators favor a central manifold to handle the output from distributed subsea wellheads, Saga opted for the large central template to provide flexibility.
With the Lunde formation a largely unknown quantity, Saga wanted a universal manifold of a central template to provide the flexibility for a high or low pressure header or for water injection as required by reservoir conditions.
The 2,000 ton unit, measuring 108 ft x 150 ft, has well slots set in pairs (one in the inner row and one in the outer row) on either side of the central manifold. During the first phase of production lasting 6-8 years, the inner row of well slots will be used. Saga will move to the outer row for subsequent phases of production.
Saga said the system is designed to allow drilling and downhole operations simultaneously with production and water injection.
The template is linked to the TLP by two 8-in. production fines, one 8-in. water injection line, two service lines, and two umbilicals. Ugland Coflexip will be responsible for the flow lines. Kongsberg Offshore has also developed a pull-in and connection tool for the system.
A feature of the system will be the extensive use of through flow line (TFL) techniques for most of the lighter maintenance work, the first time this concept has been used for a major subsea system in deep water.
The template, a truss structure made mainly with large diameter tubulars, was built by Kvaerner Rosenberg in Stavanger. The contract for the electro-hydraulic control system and the cables was completed by Kongsberg Offshore with Kvaerner Subsea Contracting winning the contract for the production trees and well maintenance equipment. Three-meter skirts will sink into the seabed when the template is installed, and the unit will be leveled by pressuring or applying suction to the skirted mud-mat compartments. Final stabilization will come from installing 30-in. conductors in the four corner well slots.
Each set of two wells is protected by upright tubular sections. In addition to protecting equipment during drilling and installation operations in the adjoining wells slots, the design also helps guide equipment into the well slot. A removable funnel guidance structure is installed in the well slot during initial drilling phases. It is removed after setting the 20 in. conductor which has an 18-3/4 in., 10,000 psi wellhead housing.
The TFL system was provided by Otis Engineering and extensively tested at the Rogaland Research Institute's TFL test facility in Stavanger. During these tests an attempt was made to run a 984 ft tool so that instruments can be put below the packer in production wells. Success was limited to running a tool of half the experimental length. To handle TFL servicing, the subsea wells will be equipped with dual 3 in. tubing. Wire line-installed diverters are provided in the tree wye block to deflect the TFL tools into the branch loops. The vertical bore above the diverter is plugged with a special metal seal, a wire line-installed crown plug.
The flow loops connecting the tree to the manifold are standard 5 ft bend through flow line (TFL) loops. A crossover valve is provided near the tree manifold connector hub to facilitate TFL operations in the tree area. The tree's wing valves are located in the manifold for maintenance accessibility,
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
