FOCUS: PIPELINES Diverless tie-in tool gets first test on deepwater line

David Knott
Senior Editor

The specially designed installation frame that holds the Morgrip mechanical coupling is being deployed for tie-in of an awkward section of Haltenpipe offshore Norway in July 1996 (photo courtesy of Hydra-tight; Fig. 2).

The installation frame sits aboard the MV Regalia construction semisubmersible. The coupling is held in the clamp arms (forward) under the frame, while pipe ends are inserted. One pipe section is then maneuvered by the alignment unit into the coupling (aft) which is then activated (photo courtesy of Hydra-tight; Fig. 4).

A diverless, deepwater tie-in and repair tool has been developed and successfully tested on the Norwegian Sea's Haltenpipe project.

Den norske stats oljeselskap AS (Statoil) and Hydra-tight Ltd., Wolverhampton, U.K., have disclosed details of the technique and installation.

Statoil funded development by Hydra-tight of its Morgrip mechanical connector as a diverless pipeline tie-in and repair system for use in water too deep for existing technologies.

Welding restrictions

Hydra-tight says that Statoil has its own technique and equipment for pipeline tie-ins and repair, known as Pipe Line Repair System (PRS). The operating depth limit for PRS is 360 m, however, because it relies on hyperbaric welding and is therefore constrained to the maximum depth for diving.

But Statoil's recent installation of the Haltenpipe gas export line and Troll oil export pipelines, with water depths up to 350 and 500 m, respectively, and several planned gas trunklines which will pass through even deeper water, forced the operator to look for a new repair technique.

Hydra-tight says the most obvious approach, automation of diving tasks, was not feasible because the ultimate depth limit is set by welding technology.

With the current depth limit for hyperbaric welding technology at about 400 m, Statoil decided a mechanical connector would be the best solution.

Statoil and partners evaluated various welding techniques and a range of mechanical connectors which could potentially be adapted for diverless installation. The evaluation led to the selection of Hydra-tight's Morgrip connector.

Hydra-tight was commissioned in 1994 to carry out a conceptual study. The study involved building a prototype for use on a 6-in. OD pipeline, and developing installation methods.

Following what Hydra-tight calls a successful study, two 16-in. diameter full-scale couplings were built and tested over 9 months by Det Norske Veritas Classification AS, Oslo.

To gauge full installation capability, Statoil targeted Haltenpipe as the first application. This project required a mid-line tie-in at a depth of 160 m.

Metal-to-metal sealing

Statoil's design criteria for the connector included metal-to-metal sealing, external-test capability to enable the joint to be verified, a 50-year installed design life, and joint removability and reusability.

The Morgrip joint (Fig. 1 [62990 bytes]) consists of a center section housing with gripping segments on either side. Each segment contains a series of spring-loaded ball bearings.

To connect two sections of pipe, the gripping segments are slid over the end of each pipe and the center section housing is positioned over the pipe joint, between the gripping segments.

The segments and center housing are then pulled together with stud bolts located around the coupling circumference. As the bolt loads are applied, the ball bearings roll and grip the outer surfaces of the pipe sections.

The Haltenpipe connector specification called for five separate gripping segments on each side of the pipe so that the connector could withstand a load equal to five times the pipeline design pressure of 190 bar. This sprecification later proved to be excessive, says Hydra-tight.

Twin seals are built into the coupling, comprising metal-graphite-metal sandwiches, which are compressed at the same time as the balls grip into the pipe.

To adapt the Morgrip device for diverless applications, Hydra-tight took the diver-installed version and incorporated it into an hydraulically activated body, with a reversible mechanical lock.

The diverless Morpgrip is deployed to the seabed in a specially developed central installation frame, which holds the coupling and aligns the pipe ends (Fig. 2).

The coupling installation frame is 13 m long and 8 m wide, with an hydraulically operated leg at each corner.

This frame is compatible with existing PRS seabed installation equipment, including H-frames, pipe cutting and coating removal tools, and a remotely operated vehicle (ROV) deployed from the surface (Fig. 3 [97298 bytes]).

To install a new section of pipe, one H-frame is deployed to pick up the existing pipeline end, while a second picks up the new section (Fig. 4). The coupling installation frame is set down over the two ends, which it then grips and aligns before joining takes place. This process is repeated for the second joint.

To repair a pipeline, the coupling frame is installed to one side of the fault, with a cutting tool aboard to sever the pipe. This is repeated on the other side of the fault and the damaged section removed.

A new section of pipe is then lowered and inserted, as described.

Hydra-tight says sliding of the coupling onto the pipes and activating the joint takes approximately 40 min, while it can take as little as 12 hr deck-to-deck time to install a joint.

The built-in joint test circuitry allows the operator to move off the site quickly after work is completed. The joint can be pressure tested later, since joint integrity is checked independently.

Haltenpipe tie-in

The 250-km, 16-in. Haltenpipe gas line was installed in 1996 from Heidrun field in Norwegian Sea Block 6507/7 and 6507/8, where water depth is 350 m, to Tjeldbergodden methanol plant in mid-Norway.

In one section, the pipeline rises from 340 m to 140 m deep up a 28° slope. Although hyperbaric welding was feasible at these depths, Statoil decided to test Hydra-tight's technique there.

Statoil has recently announced that the company will phase out diving operations where possible and that Haltenpipe and Troll pipeline operating groups have a diverless-repair policy.

The new tie-in and repair system, says Statoil, has been designed for a maximum depth of 600 m but in principle is independent of depth.

For the Haltenpipe mid-line tie-in, which took place in July 1996, the pipeline section from Heidrun platform was laid to the bottom of the slope, while the shallower section was laid to hang over the top of the slope.

A Z-bend was placed between the two pipe ends, and the installation frame and a large H-frame for handling and alignment were deployed to the seabed. The operation was carried out with MV Regalia construction semisubmersible, operated by Rockwater Ltd., Aberdeen.

Statoil pressure tested the pipeline in October and flowed gas in November. Hydra-tight says this was the first time a completely diverless tie-in had been done, with nothing pre-fitted.

The first two joint fittings went well, says Hydra-tight, but the company found later it could not verify the joint because of a failure of external test circuitry.

The joint was subsequently recovered, and a conventional welded joint was made, says the company. But Statoil nevertheless proved the diverless coupling system to the satisfaction of the Norwegian Petroleum Directorate.

What's next

Development of the installation frame and connector is estimated to have cost $15 million to date. Hydra-tight's development work was carried out under a $3.75 million contract, and the company also was paid about $750,000 for each Haltenpipe connector under a separate contract.

Contracts were let in December 1995 for design and manufacture of the diverless connector installation components.

Subsea Offshore Ltd., Aberdeen, was main engineering, procurement, construction, and installation contractor, with specific responsibility for power and control systems.

The installation frame was made by AMEC plc, London, while Isotek Electronics Ltd., Leeds, designed control equipment for activating the installation frame and the coupling.

Pipe cutting equipment, power modules, and H-frames were already in use by Statoil in hyperbaric welding operations under the PRS scheme.

Hydra-tight said the Morgrip diverless coupling can be used on pipelines up to 24-in. OD. The next development, according to Hydra-tight, will be to scale it up for use on pipelines up to 42 in.

Statoil, anxious to promote shared use of this technology, is speaking to BP Exploration Operating Co. Ltd. about its use in Foinaven field in U.K.'s West of Shetland area.

Statoil has also been discussing the technology with Norske Shell AS, says Hydra-tight, with regard to Draugen field offshore Norway, and also to Petroleo Brasileiro SA (Petrobras) for deep water projects offshore Brazil.