TUNNEL ALLOWS LANDFALL APPROACH FOR TROLL PIPELINE

Following final assembly at a nearby site, the spools were transported offshore hanging alongside a semisubmersible construction vessel (Fig. 6)before they were lowered to the seabed, along a guide wire to the riser, using a two hook, six-to-eight sling arrangement (Fig. 4)(43707 bytes). 10. Spool to riser tie-in. The air filled spools were aligned to the fixed riser, the habitat was installed on temporary support platforms, and the hyperbaric weld performed. 11. Offshore pipelay. The offshore pipeline was installed along the spool to a pipeline tie-in leg to ensure over length relative to the tie-in position.

A pipe catcher was preinstalled on the spool to protect it and ensure the correct lateral positioning of the pipe relative the spool. 12. Spool to pipeline tie-in. The pipeline was cut, lay-down head and pipe catcher were removed, and the pipe and spool aligned by H-frames before the habitat was set and the hyperbaric weld performed.

SUBGROUND CONSTRUCTION

The main subground structural arrangements for the tie-in construction included installing the riser bundle, piercing shaft outfitting for riser bundle support and annulus concreting, and tunnel plugs.

RISER BUNDLE

The objective of the riser bundle is to provide a structure for securing the riser pipes during installation and operation and also to create an initial sealing of the piercing shaft.

Two riser bundles, one for each of the main tunnel piercings, have been installed. The design is identical except for the risers.

A centralizing structure in the form of two orthogonally oriented, vertical trusses, was also fitted to the lower end of the riser bundle to ease lowering into the piercing shaft.

A 1 m OD titanium water in-take pipe penetrates the riser plug in addition to the risers. This pipe will provide water circulation within the tunnels when they are flooded for pipeline operation.

The lower end of the concrete plug is fitted with a machined steel cone which is to fit into another steel cone premounted in the piercing shaft. These cones, together with a rubber seal and injection arrangements, provided initial sealing of the shaft, before annulus concreting.

A lifting structure and a protection arrangement for the water in-take pipe is fixed to the upper end of the concrete plug. A ring beam with support points for eventual habitat platforms has been included to ease execution of the hyperbaric tie-in.

SHAFT OUTFITTING

The objective of the shaft outfitting is to provide instal- lation guiding and support for the riser bundle and to enable annulus concreting. The premounted installations must also be designed to resist the impact from the final blast.

The "female" steel cone is embedded in a reinforced concrete lining which is anchored by rock bolts (Fig. 5a)(91053 bytes).

Piping and injection hoses for annulus concreting and grout injection are also embedded in the concrete lining. These lines run from the dry side of the tunnel plug and terminate above the steel cone.

TUNNEL PLUGS

There are three plugs with different functions for each piercing.

A safety barrier in the form of a steel gate fixed in concrete lining was established to prevent tunnel flooding in excessive leaks during excavation of the piercing shaft.

A temporary tunnel plug of concrete was constructed to seal the tunnel during the final blast. The plug was fit-ted with piping for flooding and pressurizing the piercing chamber before the final blast and subsequent dewatering.

It also included piping for annulus concreting and cabling for the final blast ignition and some instrumentation.

The safety barrier and temporary plug were removed before tunnel pipeline installation.

A permanent tunnel plug was constructed upon completion of tunnel mechanical outfitting. This creates a second barrier towards the sea in addition to the riser bundle/annulus concreting.

SEABED CONSTRUCTION

Seabed tie-in construction involved tie-in spools, tie-in equipment (hyperbaric weld spread) and temporary arrangements, and rock dumping.

TIE-IN SPOOLS

Tie-in spool dimensions are heavily influenced by the preferred locations of the offshore pipeline corridor and the piercing points.

These locations are again governed by terrain features in the nearshore area and the piercing demand for good rock quality with a minimum of sediment cover.

The spool tie-in areas are characterized by 10-15 m of very soft clay at the offshore pipelines and by inclined seabed with thin layers of moraine and soft clay at the tunnel risers.

The spool layout is furthermore to accommodate seabed bathymetry with 1520 m elevation variation along the spools, expansion-induced stresses within code requirements, fixed riser tie-in stubs with 45 intermediate horizontal angles, and hyperbaric welding equipment requirements.

The resulting spool configuration has an overall length and weight in air in the range of 135-160 m and 130-180 metric ton, respectively.

Despite the size, installing the spools in one piece was preferred. This reduced lifting operations in the vicinity of the risers, eased the loop handling in view of the seabed bathymetry and sediments, and provided the most cost-efficient solution with significant scheduling advantages.

TIE-IN

The spools were tied-in to the risers upon set down prior to offshore pipelay. The offshore line tie-ins were performed sequentially as the lines were laid.

Norwegian-based Pipeline Repair System (PRS) was used for the tie-ins. This was the first job for the upgraded, newly built H-frames and habitat. The welding is fully automatic, while divers are used for equipment set up.

At the riser end, the sloping seabed and narrow riser spacing has introduced the need for habitat-support platforms. A large (20,000 hp) jetprop was used to clean the seabed for boulders and loose sediments prior to piercing, thereby also improving the habitat platform support conditions.

The riser structure is equipped with an interface beam with guidewire connections for precise set down and positioning of the platforms. The contractor preferred, however, to install free standing and more easily retrievable platforms.

Guidewires were used for habitat and spool installation to ensure precise set down and to reduce the risk of accidental impact loads. The spool-to-riser alignment benefitted from an airfilled spool weight of typically 10-20 metric tons.

At the offshore end, a pipe catcher was used to fix the lateral position of the pipeline, and lighter H-frames with greater mud-mats were introduced in order to reduce foundation loads/stresses. The weight of the flooded line and the H-frames, however, creates critical loads on the seabed/rockdump.

ROCKDUMP

The main objectives of rockdump construction are to improve the foundation conditions for the tie-in equipment and to ensure spool functionality during operation.

Virgin seabed bearing capacities for vertical load were typically 0.5-1.0 metric tons/sq m in the area for the spool-to- offshore line tie-in. This could further be reduced to half the value for horizontal loads.

The rockdump design has aimed to provide a minimum vertical load capacity of 2.5 metric tons/sq m for tie-in equipment.

The rockdump configuration is defined by several connected surface "planes," which reflect the straight sections of the spools and the offshore pipeline and the required counterfills.

Rockdump design has been a compromise between the wish to maximize its load-bearing capacity and simplify the surface configuration, which leads to increased thickness, volume, and cost, and the need to limit the thickness to avoid excessive counterfills for stabilization of the main fill and limit settlements of the underlying clay (which will continue to develop for the design life of 50 years).

A minimized thickness furthermore reduces pipe free spans from the rock-dump edge and finally limits volume and costs.

Strict vertical dumping tolerances have been applied. This is to provide the best possible support elevation for the spools relative to the fixed risers and because the tolerances would affect the support capacity, following their relative effect on the limited nominal thickness.

Precise survey data are of extreme importance.

The rockdump has been constructed in two phases: the baselayer is first made by a 1-5 in. grading, then the refined topping is made up by a 1-3 in. grading.

The construction procedures have focused on work sequencing to ease the tolerance fulfillment, while ensuring temporary stability of the rock berms and avoidance of the risk to entrap soft material inside the rockdump.

1996 FIRST GAS

The Troll Phase I piercing area design and construction constituted a complex engineering interface.

Topics range from rock tunneling and subsea blasted piercings with premounted shaft installations to prefabrication of riser bundles with their lightweight concrete plugs to marine operations for the installation of the bundles with subsea concreting for shaft sealing and, finally, to installation and tie-in of pipelines and expansion loops both in the tunnel and on the seabed.

Significant differences exist in the work approach (planning and execution) from tunneling to offshore work.

Traditional Norwegian rock tunneling is characterized by a certain amount of "detail engineering at site" which is in contrast to detailed mechanical design with long procurement and fabrication as well as offshore installation planning schedules.

All subground work and seabed rockdumping was completed in 1994.

The spools were installed and tied-in to the risers in early spring of this year and tied-in to the offshore lines in the autumn.

Commissioning will occur and first gas will flow in April 1996; sales gas will flow in October 1996.

ACKNOWLEDGMENT

The authors wish to thank A/S Norske Shell for the per- mission to publish this article.