Developments advance subsea pipelaying, inspection, repair

This new jet flowhead can cut subsea-pipeline trenches deeply and cleanly at a single pass (Fig. 1).

Recent advances in laying, inspecting, and repairing pipelines are helping to cut both costs and time.

A new dredging system that employs jets to clear a subsea trench for pipelay received trials off Belgium last spring. And within the last year, projects in the Middle East and North Sea employed technologies that promise to make inspecting the surface of a subsea pipeline in difficult terrain easier, less time consuming, and therefore less costly.

And subsea repair of damaged pipelines may take less time with a new "stabbable" pipe connector.

Jet dredging

A new subsea dredging system has shown promise, based on field trials conducted off the Belgian coast near Zeebrugge.

The Flowdredger, developed and patented by Dredging International, Zwijndrecht, Belgium, employs a jet flowhead that can cut a deep trench in one pass. In many situations, a conventional offshore presweep operation will no longer be required, says the company.

The Flowdredger was developed to overcome disadvantages associated with existing, propellerdriven systems which scatter material in all directions including the trench line to front and rear. A large amount of the material removed by these systems is returned to the trench.

Flowdredger avoids the problem by having the head distribute material evenly to each side of the trench line, leaving a very clean, deep trench, says Dredging International.

The jet flowhead was developed and refined over 18 months of research and development and trials. The first trials took place last spring, only weeks after the initial concept had been finalized.

The early jet flowhead was fitted to the hopper suction dredger Antigoon for sea trials. Subsequent engineering improved the jet flowhead's performance.

Flowdredging deploys the suction hopper dredger in an entirely new way, according to the developer. The vessel's hoppers remain empty as its pumps are used to generate a water jet powerful enough to cut a deep trench in sand or clay at each pass with no material taken on board the dredger.

Trials performed so far have set parameters requiring a 10-bar jet; higher pressures may be obtained by uprating the pumping capacity available. Disadvantages of the existing propeller-driven systems are overcome by the jet flowhead's combination of fine nozzles and twin main jets, located at each side of the head (Fig. 1) and distributing material evenly beyond each side of the trench.

The company says that at existing pressures, a 2-m deep trench in sand can be cut at first pass. A bottom width of 10-15 m is achieved easily. In clay, a 1.5-m deep trench can be cut during the initial pass.

The precision of the first pass can be repeated, as the fine positioning of the head is controlled by a specially developed computer system. This system also permits adjustments to the angle of the main jets and, therefore, alterations in the width and profile of the trench produced by the Flowdredger, says the company.

Inspection advance

A tool has been developed and deployed which promises to reduce the expense and difficulty of subsea-pipeline inspection.

AEA Sonomatic, part of AEA Technology, Oxfordshire, U.K., late last year developed the Nautilus II specifically for Abu Dhabi Marine Operating Co. to scan the complete circumference of an underwater pipeline.

Only two Nautilus II instruments currently exist, both in service for Abu Dhabi Marine and operated by AEA Sonomatic personnel.

The company says development of its instrument responds to the reality that subsea inspections are notoriously difficult and expensive to conduct because equipment often must be deployed in dark and confined areas.

Divers usually must manipulate cameras and other inspection instruments in subsea terrain that is difficult, often nearly impossible to reach.

Specifically in the Middle East, oil companies in the past have been hampered by the extremely hard coral bedrock found there, says AEA. They have had to spend thousands of dollars excavating coral to allow conventional scanners to be used.

Such preinspection work requires the costly use of hydrojets and jackhammers and often disrupts other maintenance and inspection vessel activities.

Such expense and preparation are unnecessary with Nautilus II which can scan the complete circumference of an underwater pipeline, requiring less clearance than conventional scanners. Its slim, low profile enables it to scan through a gap of approximately 110 mm.

The instrument has a "clamshell" type of collar which, the developer says, installs quickly and easily, and is available in several fixed diameters to reduce the overall profile of the system.

Nautilus II can perform inspections in various operator-defined resolutions and has line and Rastar scan capabilities in a scan width of up to 500 mm. This width reduces the number of scans per area and the amount of diver intervention required to reposition it.

Scan speeds of up to 125 mm/sec axially and 75 mm/sec circumferentially are achievable. The instrument is controlled from the surface with a 150-m umbilical fitted with a small subsea module containing preamplification and pulser circuitry.

Support software

Ever-improving measurement techniques are necessary so that operators can target maintenance work more precisely. For such equipment as Nautilus II to be effective, therefore, says AEA Technology, requires sophisticated measurement and corrosion-assessment software.

AEA Sonomatic's time-of-flight diffraction ( TOFD ) is one such technique. The company says TOFD, a way of measuring the time taken for signals to travel, is generally accepted as one of the most reliable methods for accurate flaw sizing and flaw-growth monitoring.

If AEA Sonomatic's Microplus real-time imaging system, which can be used with Nautilus II, is set in TOFD-mode inspection, speeds of 2-4 m/min can be achieved. Real-time meaningful "d-scan" data can be presented for easy and quantifiable interpretations. (D-scan is a view along the length of any object viewed from its side.)

AEA Sonomatic believes TOFD is more accurate than such traditional inspection methods as destructive testing and radiography. This measurement technique has a far better detection rate than conventional radiography, the company claims, and it can also quantify defect indications in terms of the length and depth of corrosion cracks.

North Sea site

In 1996, a pipe-laying project in the hostile North Sea tested those claims.

Allseas Engineering used its pipe-lay vessel Lorelay to carry out the Sleipner West 20-in., 13-km project for den norske stats oljeselskap A.S. (Statoil) in the Norwegian sector. Before going offshore, Allseas and AEA Sonomatic's Dutch company compared TOFD with conventional radiography and destructive inspection of a large number of welds.

This program proved the correlation between the defect indications found with TOFD and the actual defects. As a result, Allseas, Statoil, and the Norwegian Petroleum Directorate adopted AEA Sonomatic's inspection technology for the pipe laying.

The actual ultrasonic inspection was performed by AEA Sonomatic personnel who examined the weld root and hot pass immediately after welding. With conventional X-ray inspection, welds are examined for defects only immediately prior to pipe laying.

In the Sleipner West project, any defect inspections identified by the final X-ray station were compared with the results of the ultrasonic inspection carried out by AEA Sonomatic.

If those defect indications were within the acceptance limits of the TOFD technique, then it was considered unnecessary to carry out weld repair. By reducing the number of weld cut-out repairs, each one of which can take 4-5 hr, says AEA Sonomatic, Allseas accelerated pipe laying and realized substantial savings.

In the future, says the company, the popularity of risk-based approaches to operation is likely to lead to a requirement for systematic assessments of the structural integrity of all parts of platforms.

AEA Sonomatic says it is currently conducting research that it expects will lead to Nautilus I and II being equipped with Microplus and TOFD for operation on remotely operated vehicles ( ROVs ). These will be used to inspect subsea welds. They can take the pressure off divers and could help operators to meet the regulatory requirements.

Pipeline repair

In a development related to repairing a pipeline whose damage is revealed by inspection, Oryx U.K. Energy Co. has been the first to use the newest version of the subsea Morgrip 3000 series of pipe connectors from Hydra-Tight Ltd., Darlaston, U.K.

The connectors are equipped with stabbable radial seals that, says Hydra-Tight, have cut diving times by up to 50%.

Oryx installed the first four of the new connectors on an 8-in., 2,500-lb ANSI-rated oil test line that is trenched in the seabed at 130 m water depth. Upheaval buckling had occurred in two places on the line. Diving subcontractor Rockwater reported 4-hr installation time for each connector.

Hydra-Tight says that in such an installation, divers need only to align and stab the connector onto pipe ends being joined before they tighten the stud bolts located around the coupling circumference.

Tightening these bolts causes the ball bearings in the gripping segments to swage into the external surface of the pipe and provide mechanical grip. Tightening also pushes the twin radial seals onto the pipes' surfaces to complete the connection.