Acergy has steadily been building up experience in using standard flange connectors for the tie-in of deepwater pipelines and risers. To date, more than 100 flanges have been successfully connected using the Modular Advanced Tie In System (Matis) remote connection system, with no leaks reported during pressure testing or subsequent pipeline operation. The majority of these connections have been performed since 2001 in water depths of around 4,600 ft (1,400 m) during the development and subsequent extension of the Girassol field offshore Angola (Figure 1), in addition to a number of connections in Norway in around 985 ft (300 m). What started out in the mid ’90s as a dream to bring simple diver-based pipeline tie-in systems to the deepwater arena has now developed into a field proven reality.

Flange advantages

Bolted flanges are by far the most commonly used connectors for pipe in both topside and subsea applications and have a very good track record for high-pressure oil and gas service. Flanges are relatively simple mechanical connectors where bolt pretension is used to activate a metal gasket providing a metal-to-metal seal. The Matis used for connecting the bolted flanges employs conventional bolt tensioning technology for tightening bolts, which gives a direct relation between bolt tension and axial pipeline product thrust across the joint.

Figure 1. The diverless flange in use at the Girassol field. (Graphics courtesy of Acergy)
These mechanical connectors are well understood, reliable, very cost-effective and available off the shelf from a wide range of suppliers. Extending their use to deepwater applications where remote diverless connection is essential is a major addition to the subsea construction toolbox.

For many deepwater applications the use of flange connectors can bring a level of flexibility to the seabed layout, particularly well jumper design. Well jumper expansion loops can easily be designed and installed in either the horizontal or vertical plane making use of horizontal and/or vertical bolted flange connections. In addition, it is a simple matter to insulate flanges using a thermal insulation cover to prevent the build-up of cold spots along the pipeline to improve flow assurance, which can be vital to the successful operation of deepwater pipelines.

The moment-carrying capacity of flanges is significant compared to other mechanical connectors. This can prove to be a significant advantage for deep water flow lines, which are subjected to forces and displacements from pipe walking and lateral buckling due to pressure and temperature variations and in the accidental case where the pipeline is subjected to snagging loads. Flanges are also available to fit a wide rage of pipeline requirements such as high-pressure/high-temperature and corrosion-resistant alloys. The Matis requires a simple interface with the subsea structure, taking the decision of the diverless connection system selection off the critical path.

Applications

Acergy first started the development of Matis 10 years ago with the engineering and onshore trials of the prototype bolt connection system. This was followed in 1997 by a successful offshore demonstration of the system in 330 ft (100) m in the North Sea. The first commercial application for diverless flange installation was in 280 ft (85 m) of water on the Loke field in Norway in 1999. In 2000 the Matis was developed into two separate systems. The first, a system for larger diameter pipelines, was used for the connection of four 24-in. flanges as part of the installation of a subsea shut-down valve in 720 ft (220 m) of water at the Gulfaks C platform in Norway. The second system was specifically developed for deepwater flowline applications and has since been used to connect more than 100 flanges in around 4,600 ft of water at the Girassol field since the beginning of 2001 up until the present day.

The deepwater Matis equipment has been used at the Girassol field to connect the tie-in spool pieces linking the end of the flowline bundles to the riser pipes at the base of the three hybrid riser towers, involving the connection of horizontal as well as vertically orientated flanges. The equipment has also been employed for the connection of the manifolds to the flow lines. In addition, the equipment has been successfully used to disconnect a flange and a new spool piece installed as part of the installation of the gas export pipeline at the Girassol field.

The rigid tie-in spool pieces are accurately fabricated to link between the two tie-in points based upon the results of the subsea metrology measurements. The spools are then installed and landed in place using a catcher arrangement, which acts as the coarse alignment for the flange faces. If required, this operation can be performed by a separate ship prior to the arrival of the Matis connection equipment and bring some flexibility to the offshore schedule. The Matis equipment, which has been set to be virtually neutrally

Figure 2. An artist's rendering of the flowline tie-in at Girassol.
buoyant, is lowered to the seabed from the support ship in a deployment frame and transferred over to the tie-in structure at the pipeline end. The system has been designed for water depths down to at least 9,850 ft (3,000 m), and as a result it was decided to avoid the need for a control umbilical connecting the system to the surface support ship. All the power and data signals required to control the connection system are supplied by a standard work-class remotely operated vehicle which is docked on to the top of the Matis unit.

Once landed and latched to the tie-in structure (Figure 2), the claw on the Matis unit picks up the free end of the spool piece to bring the flange faces into alignment. At this stage the gasket is installed into the flange and the faces brought into contact. In order to achieve the alignment of the bolt holes it is necessary for one-half of the flange to be a swivel flange. The bolts and gaskets are stored in two carousels located on either side of the Matis unit, which normally carries sufficient bolts to connect three flanges. The bolts are picked up individually from the carrousel and inserted into the flange until half the bolts are in place. Nuts are then run onto the bolts and a set of jacks used to grip the bolts to apply the pretension, the pressure in the jacking system providing a direct link to the pretension in the bolts. This procedure is then repeated for the second half of the bolts to complete the flange connection. If required a test port can be built into the flange for pressure-testing after the connection. However, this is not recommended as these types of leak tests have proven to be unreliable compared to the flowline system hydrotest. In addition, none of the flanges installed by Matis to date have leaked after installation.

Once the flange has been successfully connected the Matis equipment can either be recovered to the surface to load more consumables or transferred to another tie-in location with a through-water transfer. The system has been successfully used for the vertical flange connections at the base of the hybrid riser towers. For this application a rotation module must be fitted to the front of the Matis unit, which is docked horizontally on a frame on the riser tower and then rotated into the vertical position to perform the flange connection.

As well as the ongoing work at the Girassol, field the next challenge for the Matis flange connection system is the tie-in work for BP at the Greater Plutonio field offshore Angola, which is located in around 4,270 ft (1,300 m) of water. For this project flanges have been selected for all of the tie-in connections of the flow lines to the hybrid riser tower.

As well as flowline tie-ins, flanges could also play a useful role in the repair of deepwater pipelines. In this application the options are severely limited, making the use of mechanical connectors the preferred option for diverless pipeline repair. Flanges have the main advantage of being relatively inexpensive, making it possible to hold a set of relevant sizes in stock for when needed. Minor modifications to the Matis would be required to accommodate the requirements of deepwater pipeline repair situations.

The advantages provided by the use of bolted flanges for the connection of deepwater pipelines are many, and we should expect to see their application in many more deepwater fields in the future.

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