UNMANNED DEEPWATER-LINE REPAIR SYSTEM PASSES FULL-SCALE TRIALS
Stefano Venzi, Arrigo Vienna
SNAM SpA
Milan
The first ever full-scale tests of an unmanned, deepwater-pipeline repair system were successfully conducted last year off the coast of Italy.
The Italian gas-transmission company SNAM tested a submersible automatic system (SAS) sealine repair system at a depth of 600 m (approximately 2,000 ft).
The modular SAS allows sealines to be repaired by installation of the Nuovo Pignone mechanical connector.
The system's trials simulated complete repair intervention on the 20-in. TransMediterranean pipeline and provided unprecedented experience to SNAM and to the other ENI companies (Snamprogetti, Milan; Saipem, Milan; and Nuovo Pignone, Florence) involved in this project.
SAS ORIGINS
The idea for the SAS dates from the early 1980s when SNA, L began building the TransMediterranean pipeline, a 2,500 km (1,550-mile) large-diameter pipeline carrying natural gas from Algeria to Italy.
The TransMed crosses both the Sicily Channel and the Straits of Messina with three 20-in. lines whose total length is about 500 km (Fig.1; OGJ, July 7, 1986, p. 54; Nov. 7, 1988, p. 46).
These offshore stretches have established state-of-the-art for deepwater construction and installation. In fact, the pipeline exceeds 600 m (almost 2,000 ft) in the Sicily Channel.
Because of the strategic importance of this sealine, a cost analysis of possible repair interventions was performed. The results showed that in the case of damage to the sealine:
- Depths below 300 m are beyond the practical limit of divers.
- No systems or tools were available at that time for the on-bottom repair of the sealine in deep water.
- Repair cost using traditional techniques (that is, partial relaying and shallow water tie-in) was extremely high, in part because of the time required by the operation and the type of vessel used.
For these reasons, SNAM decided to conduct a research and development program aimed at improving technology for underwater pipeline repair.
THE SAS SYSTEM
The design philosophy was based on the requirements shown in the accompanying box.
The system designed to these requirements resulted in an effectively modular package, each module able to perform one or more functions (Fig. 2).
The launch and recovery system (LARS) module is a dedicated handling system able to deploy all the service modules to the seabed by two cables: a lift cable to support the weight and an umbilical to supply electric power and signals.
The master subsea module is a highly advanced remotely operated vehicle (IZOV) designed and built specifically for the SAS called the "thruster module" (Fig. 3).
Its task is to provide the movement, positioning, power, control, communication, and monitoring of the service modules. It also embodies two small ROV Recons IV to perform visual inspections of the operating area.
The service modules are the specific work packages which perform the repair of the damaged subsea pipeline.
They consist of the following:
- A dredge to uncover the concerned pipe section and prepare the work area
- two H-frames for pipeline lifting to a specific height and for lateral adjustment
- A concrete and polyethylene coating removal and pipe-cutting module 0 A module to recover the damaged pipe section to the surface
- Connector and spool piece installation module--base frame, spool measurement module, end connector installation module, and spool installation module (Fig. 4)
- A mechanical connection system The repair can be performed by cutting a 12-m long section of pipeline and replacing it with an adjustable, telescopic spool piece able to accommodate misalignments up to 10 between the two pipe ends.
SNAM defined the detailed technical specifications and coordinated and supervised the activities of the entire project. Snamprogetti managed both the design and construction.
Nuovo Pignone built and patented the mechanical connection and spool system (the heart of the project), while Saipem was responsible for the sea trials.
From the early stages through the sea trials, the European Community supported the project with a substantial financial contribution.
CONSTRUCTING AND TESTING
Construction philosophy required the hardware to be divided into packages, construction to be subcontracted to qualified and specialized suppliers, and construction and conformity to specified characteristics to be verified with extensive acceptance tests.
This approach also required, as a consequence, an integration activity to ensure that components assembled in different yards fitted together and the installation and verification of all the computer procedures for the different operations.
The construction phase and the integration tests of the modules were long and difficult because of the complexity and size of the project and some unprecedented technical problems.
The most challenging areas of development were the cutting system, the connection system, the connector installation tools, the heave compensator for the handling system, the acoustic navigation system, and the hydraulic system on the thruster module.
FIRST SEA TRIALS
Sea trials were carried out in 1988. The selected site was in the Gulf of Corigliano in the Ionian Sea in the south of Italy at a depth of about 350 M.
The simulation of a total real repair procedure was performed but not fully completed. In fact, during the final phases, a severe problem after insertion of a connector into the cut pipe led to the suspension of the trials.
In spite of these problems the overall evaluation of the system philosophy was positive, and an appropriate modification and improvement program was implemented.
In the following 3 years the system was greatly improved: Many subsystems were added and others tuned up, both major and minor modifications were incorporated, and the system was thoroughly examined to give a better awareness of its capability and limitations, while operating procedures were simplified and detailed,
A specific improvement program was implemented for the connector to ease its insertion and increase its reliability.
FINAL DEEP SEA TRIALS
In 1992, the SAS modification program was completed, and the system was ready for the final test. SNAM decided to perform another repair simulation but this time at a record depth of 600 m.
ln the spring of 1992, in the same place as the previous trials (the Ionian Sea), a 134 m long, 20-in. test pipeline was laid.
The capability of precisely laving a long pipeline section represented one of the aims of the trials, being the first operation in case of "long repair" as envisioned in the SAS procedures.
A "short repair" is the installation of a spool piece and the replacement of about 12 m of pipeline in order to repair localized damage.
For major damage, on the contrary, a longer section of pipe can be replaced by relaying the required length and performing the tie-ins at both ends to the existing line once the damaged section has been cut and displaced.
The laying technique used proved to be efficient, giving a final error of only some meters from the expected position of the pipe ends.
All the SAS modules were embarked on a dynamic positioning vessel that gave unexpectedly high positioning accuracy. An acoustic transponder array was installed on the sea bottom close to the test string to allow safe and precise navigation operations (Fig. 5).
The sea trials operations can be divided into four main phases:
- Pipeline lifting. Pipeline lifting was performed by two 16 metric ton (mt) H-frame modules.
Their four independent legs allow their installation even on rocky seabeds, while the pipe clamp attached to a transversal trolley pen-nits, if necessary, pipe displacement up to 1 M. They also prevent and movement of the pipe during the repair sequence.
After the first H-frame installation, the thruster module was recovered and the second H-frame attached and deployed with the same procedure. A 40-m section of pipe was lifted 1 m off the seabed.
- Pipeline cutting. The cutting module is designed to remove the pipeline weight coating with a milling cutter and the polyethylene coating with a special steel brush and subsequently to cut out the damaged section of pipe.
During the trials the "removal" functions were not used, primarily because the TransMed line from 280 down to 600 m has no weight coating, but also because the Nuovo Pignone connector as modified in the 1991 version can be installed even without removing the polyethylene coating.
During the trials, the cutting module performed two clean and precise cuts with its disk milling cutter. The cut section of pipe was recovered by the cutting module itself.
- Connectors installation. The base frame module was lowered to the seabed and clamped onto the two pipe ends in order to house the following service modules. The measurement module then measured and recorded the relative position of the two ends.
Next the end connector installation module was installed on the base frame module: the first connector inserted and forged on a pipe end.
Until then, all the operations went smoothly. But during recovery of the end connector installation module for the second connector installation, severe structural damage occurred to the base frame module.
The problem was so serious that the base frame module was considered unserviceable and was recovered. A similar structure was built in a few days in order to continue the trials.
The new frame was lowered, and the second connector installation successfully performed.
- Spool piece installation and dry tests. The spool piece was then installed on the two connectors, underwater forged, and tested, The trials ended with the recovery of all the modules and the repaired pipe section. During the autumn, the connection system installed in deep water was tested in a SNAM yard (Fig. 6) to the following conditions:
- Spool piece: compression test up to 285 mt
- Spool piece: pull force test up to 420 mt
- Spool piece: bending moment test with and without internal pressure (320 bar; 4,640 psi) up to 120 mt-m 0 Connectors: internal pressure test of 320 bar
All these tests were successful.
No mechanical tests to the connectors were performed because in the past they have proven absolutely successful and reliable whenever the forging parameters were met and the pressure test passed.
The mechanical connection system tests were witnessed and certified by the DNV Italian agency.
FUTURE DEVELOPMENTS
The positive results of these tests prove the capability of the SAS system to repair a pipeline even in deep water. Considerable experience was gained from testing the single component, the module assembly, and the complete system in different real conditions and environments.
It was also possible to integrate the system and test new and highly advanced components for specific applications, as a result of dedicated studies. Many strictly technological problems on specific pipeline repair operations, however, are still to be definitively resolved.
These include better pipeline cutting and an easier connection system installation, the first for the strict requirements (clear cut, no burrs, squared and round, weld removal), the second because of the weight of the mechanical connection system components that must be precisely handled and inserted with only a few millimeters of clearance available.
Even without these problems, 10 years after the birth of the project, the adopted philosophy today seems to be ready for further improvement.
When the SAS was designed, ROVs available on the market were mainly used to survey and monitor drilling and construction operations.
Now, advanced remotely operated work systems are easily available. If required, they are equipped with high power package and sophisticated manipulative capabilities. And they are absolutely reliable and flexible.
That was clearly demonstrated during the last SAS trials, when a work class ROV assisted the system, demonstrating to be useful above all in managing unexpected or emergency situations.
For those reasons, the concept selected for the SAS, involving only "swimming TV camera" ROVS, could today be improved better to take advantage of current technology.
For example, foresaking the "dedicated system" philosophy, a new repair system could be thought as a "split-system" architecture, that is, rental of one or more commercial work class ROVs and construction of special tools as an interface to the specific repair requirements.
To explore other automation technologies applied to deepwater-pipeline repair, SNAM participated with Saipem and Sonsub in 1992 in another sea trial in which a new repair philosophy, based on "low force" tools and modules was tested.
Copyright 1993 Oil & Gas Journal. All Rights Reserved.