C. Cafaro
JP Kenny (Norway)
Forus, Norway
N. Finotti
Snamprogetti
Milan
Construction continued last year on the Zeepipe system to take Norwegian North Sea gas to the Belgian port of Zeebrugge.
Prominent among the accomplishments of Phase 3 of the project was the installation in 1992 of a subsea connection point (SCP) at the hydraulic mid-point of the system between Sleipner field and the Zeebrugge terminal. (See map of Zeepipe project in OGJ, May 4, 1992, p. 94.)
The SCP provides for new branches to be connected to the original 40-in. trunkline during future phases without the pipeline being shutdown.
ZEEPIPE'S OPTIONS
The 40-in. pipeline installed during 1991 and 1992 will transport gas from the Sleipner A platform (Norwegian sector of North Sea) to the shore terminal at Zeebrugge, Belgium. Operator of the project is Den norske stats oljeselskap AS (Statoil), Norway; engineering contractor is Snamprogetti, Italy.
The pipeline is approximately 800 km (497 miles) long and, depending on possible increase of the demand, the addition of an intermediate recompression station will be necessary in the near future.
The compression unit is supposed to be set-up on the deck of a new platform, referred to hereafter as FCP (future compression platform), to be installed and tied-in to the mid-point of the pipeline route.
Bases for design of the future connection facility are the following:
- The compressor inlet and outlet pipe spool shall be connected to the pipeline without interruption of the gas transportation.
- No flanges are permitted and the pipe spools are required to be tied-in by welding.
- The arrangement of the branch connections shall allow for the passing of pigs to be used for inspecting both the original line and the future pipe spools.
The operator considered and discarded several alternatives before choosing a subsea template.
- Hot tapping. The principle of the hot tapping method can be summarized in two steps:
- Initially, two pipe sections provided with a collar suitable for a future tee connection are installed directly during the laying of the pipeline.
- Finally, tie-in of the lines to and from the FCP occurs with a special tool designed to perform piercing of the pipeline and welding of the new branches without complete depressurization of the system.
This solution significantly limits the initial cost but involves considerable difficulties during the performance of future tie-in operations, with associated unpredictable costs. In addition, piggability requirements are not fulfilled.
- Unmanned riser platform. An unmanned riser platform includes the preinstallation of a platform and the immediate tie-in to the pipeline of its inlet and outlet risers.
The gas flow is deviated to the surface and the layout of the topside piping enables gas transportation to be maintained through a by-pass during the connection of the FCP.
As opposed to the previous case, this solution uses a conventional technique for future tie-in operations but requires substantial initial investment.
- Subsea Tee or Y. Subsea arrangements including forged Tee or Y pieces and valves for the isolation of future tie-in points have already been used in a few North Sea projects.
The requirement of piggability of the future branch fines, however, makes it impossible to adopt a classic Tee solution, whereas the size of the pipe and the effect of the pipeline expansion make the success of a Y piece problematic.
ADOPTED SOLUTION
The solution finally adopted for Zeepipe was the subsea template shown in Fig. 1. The SCP's piping and valves layout is shown in Fig. 2.
The core of the system consists of two couples of 40-in., full-bore valves that provide the possibility of isolating from the main line a section that includes a 180 bend.
This bend, referred as a U-bend, is to be cut during future tie-in operations and replaced by the two pipe spools connecting the SCP to the FCP.
A 20-in. by-pass is provided to maintain the gas transportation while the FCP is being connected. The bypass includes two 20-in., reduced-bore valves which are opened only during future tie-in operations.
Vent and drain connections are setup between each pair of 40-in. valves to enable bleeding of process gas should the valve seals provide insufficient tightness.
Other vent and drain connections are set-up between the two 20-in. valves and near the U-bend to assist precommissioning of the SCP and of the future connection spools.
PIPELINE EXPANSION
Modification of the initial process conditions imposed by connecting the recompression station will create a pipeline free expansion value of approximately 1 m.
Considering the diameter of the pipe, this requires that large expansion loops be set-up to prevent overstressing of the components of the future tie-in facility.
In general, installation of two "conventional" pipe spools, shaped to suit the relative positions of pipeline and template and the expansion requirements, necessitates the following operations:
- Subsea metrology and possible completion of pipe spool assembly on the vessel
- Lowering and subsea handling to achieve the correct fit
- Welding the two ends of each loop, resulting in a total of four hyperbaric tie-ins.
The cost of such offshore and subsea work represents one of the major components of the initial investment.
The arrangement of the SCP piping and structural system has been studied in such a way as to reduce as much as possible the complication and the duration of the offshore and subsea operations.
This goal has been achieved by allocating the pipeline expansion loops inside the template, taking advantage of the room available beneath the sloped walls of the valve's protective structure.
This solution allows for the simultaneous installation of both the template and the pipeline expansion loop and the connection of the SCP directly to the pipeline by two hyperbaric weldings only.
The duration of the operations is significantly reduced with respect to the standard sequence, resulting in an optimum utilization of the most expensive installation spread:
- The heavy lift vessel (HLV), which is demobilized immediately after lowering of the template because no separate installation of pipeline expansion loops are required; and,
- The diving support vessel (DSV) and the on board diving spread, whose activity has been reduced from four to two tie-ins.
SUBFRAME; MAIN UNIT; SUPPORTS
Close alignment between pipeline and template is required to avoid any pipeline shifting. Alignment is obtained by use of a light subframe which is guided to the correct position on the sea bottom by means of wires fixed to the pipeline itself.
Four jacks at the corners allow for compensation of seabottom unevenness and final leveling.
The subframe also accommodates four special sleeves to assist pile stubbing and final locking after completion of driving operations. An hydraulic tool is used to swage the piles into a groove machined inside the sleeve can.
Most of these operations were performed by a DSV.
The structure of the SCP has been designed to allow the installation of all the process pipework by a single lifting operation. The total weight of the main unit (Fig. 3) is 700 tons.
Most of the features of this structure result from special design requirements and environmental constraints. In particular, the design is governed by the presence of large environmental forces due to a design wave height of 21 m with a water depth of 41 m only.
Use of wide flat surfaces has therefore been avoided, and the protection of internal equipment from impact with trawl gears or possible dropped anchors has been achieved by means of a tubular structure.
The entire top area can be opened (by remotely operated vehicle-ROV) to allow access during inspection or operation.
U-BEND COVER; PIPE SUPPORTS
The U-bend protruding from the main unit is protected by a separate cover which is supported by a gravity foundation ballasted with grout. During tie-in to the FCP, the cover will be removed.
The light weight of the structure allows such an operation to be performed by a DSV.
The design of the pipe supporting system has been one of the most difficult technical problems encountered during detail engineering.
Several solutions have been developed in order to comply with the large deformations induced by the pipeline expansion and resist the action of environmental loads applied to the pipe itself.
As shown in Fig. 4, the pipe is either linked to the structure by means of steel-reinforced rubber pads or supported by clamps that allow sliding along the pipe axis and, at some location, along the transverse direction as well. The sliding surfaces have been protected from marine growth by a copper nickel coating.
The seabed consists of a shallow, loose, sand layer 1.5 m thick which rests on stiff clay deposits. The unstable sand layer creates the possibility of significant scouring with possible consequences for the integrity of the template and adjacent pipeline.
To prevent this risk, the sand layer in the SCP area was removed and substituted for by large-grain gravel.
INSTALLATION
The removal of the sand layer was planned to be performed with a hopper-dredger vessel.
In practice, this operation was attempted with a subsea self-tracking vehicle controlled from the DSV but was unsuccessful because of the continuous sticking of tracks into the mud.
The excavation was then carried out by the originally specified vessel.
After completion of dredging, the subframe was installed and a section of pipeline removed to allow the tie-in to the main unit. The excavated area was then filled by gravel dumping.
A few weeks later the main unit was transported to the site by the HLV and lowered to its final position onto the subframe using the piles as guides. This operation was quick and successful, but some unexpected delay was encountered during the grouting of pile-leg annulus, one of the most common offshore operations.
The problem originated from difficulties during stubbing of quick connectors and the operation of grout valves by ROV. No significant vessel stand-by was recorded, however.
The U-bend cover was lowered and guided to its final position. The gravity foundation was ballasted with heavy grout injected from the surface.
In total, the installation required 9 days use of the HLV, including mobilization and demobilization.
TIE-INS
The SCP was tied-in to the pipeline during September and October 1992. Because of the large size of the pipe, the two tie-ins between pipeline and SCP spools took approximately 1 month.
The choice of avoiding the installation of external expansion loops (and the two additional associated tie-ins) has therefore resulted in a significant reduction of costs and has enabled completion of operations before the arrival of severe weather.
Copyright 1993 Oil & Gas Journal. All Rights Reserved.
