North Sea trials verify subsea grout curing

Full-scale tests of a subsea grout mixing and injection skid unit in the North Sea in June verified both the overall functionality of the injection system used for installing a Subsea Grouted Tee and the grout's ability to cure at seabed temperature and pressure.

SSGT technology mechanically connects an off-take fitting to an existing pressurized pipeline, providing an alternative method of hot-tapping into high and low-pressure pipelines without major welding.

Applying the technique to marginal satellite fields could help develop such prospects by dropping the cost of linking to export infrastructure. SSGT also offers a more efficient means of making emergency interventions.

Background

Subsea 7 began a Joint Industry Project in 2006 with UK engineering consultancy, Advantica (now GL Industrial Services UK Ltd.) to develop a grouted tee for subsea applications. The JIP sponsors (BP, Total, and ConocoPhillips) supported conversion of the technology for subsea use with the vision of taking it to ultradeep diverless applications.

Following completion of the JIP, Subsea 7 carried out further testing on the technology in test tanks and laboratories and conducted offshore trials in June 2009, mobilizing a grout mixing and injection skid, complete with test tee assemblies, on its Seven Pelican dive-support vessel. Divers conducted installation and hookup once the skid was deployed to the seabed.

Skid recovery followed grout injection into a test tee assembly and a 24-hr grout curing period. A successful grout quality test supported the technology's readiness state for subsea operations.

Advantages

Key advantages of this method of hot tapping include:

• The absence of hyperbaric welding or a hyperbaric welding habitat.
• Elimination of the need for specialist welder divers and prequalification.
• Shorter lead times (including potential off-the-shelf application in emergencies).
• No requirement for pressure reduction, allowing normal production throughout.
• Reduced expense.
• Potential future diverless application.
The accompanying table compares welded and SSGT options.

The potential exists to use SSGT for acute angle branch, multiple branch, and K-type configured branch applications. Possible current applications include:

• Valve replacement.
• Pipe-branch connection.
• Repair bypass.
• Blockage removal, tooling introduction.

SSGT

SSGT consists of a tee using a top half-shell with a branch either welded or extruded onto the shell during fabrication. The tee-branch consists of a high-pressure double-block-and-bleed hydrogenated nitrile butadiene rubber seal energized through spreader plates activated when the tee is clamped in position around the pipeline (Fig. 1).

This cutaway of the Subsea Grouted Tee technology shows the seals between it and the main pipeline. The lower half of the tee, split into two parts, creates a door for application to the pipe. The tee is sized to create an annular gap into which the grout is injected (Fig. 1).

The lower half of the shell, split into two parts, creates a door arrangement operated hydraulically once the tee is positioned on the pipeline. The tee, deployed in an installation frame, is sized to produce an annular gap between tee and pipeline.

Drying the annular gap and injecting it with epoxy grout create the mechanical bond between pipeline and tee, allowing full transfer of load between the two.

Conversion of the land-based grouted tee to SSGT used a 24-in. stopple branch off a 24-in. pipeline, with an ANSI Class 900 flange connection for shallow-water diver operations (200-m maximum). Conversion involved three stages: design and qualification of SSGT, design and build of a subsea grout injection system, and hyperbaric testing. Divers and subsea equipment installed a 24-in. by 24-in. Class 900 stopple SSGT prototype in shallow-water test tanks (Fig. 2).

Divers install a 24-in. by 24-in. SSGT ANSI Class 900 prototype in a shallow-water test tank at Subsea 7's Greenwell base in Aberdeen (Fig. 2).

Hydraulic pistons and supporting high tensile strength bolts on horizontal flanges allowed the fitting to compress the primary saddle seal onto the main pipeline. O-ring sealing strips integrated into horizontal flanges on the doors and the branch half of the fitting provide a gas-tight environment in the annulus between the main pipe and fitting, allowing water in the annulus to be flushed by flowing nitrogen once the end seals are activated. These O-rings are temporary and only assist with drying and grout injection.

Once the fitting is wrapped around the main pipeline and temporary ends seals are installed, conventional hydraulic bolt-tensioning tools preload bolts on the horizontal flanges. Pressure testing the annulus to 20 psi establishes the seal as more than adequate to inject inert drying agents such as nitrogen. Flushing sea water and potential salt deposits in the annulus between the tee and the pipe with fresh water and then drying the annulus with dry nitrogen to a humidity of less than 20% is sufficient to achieve full structural strength of the grout.

The subsea grout mixing and injection skid unit allows various preinjection testing stages to ensure skid settings match perfectly with ambient subsea conditions. The skid unit has two bladders, one containing resin and the other hardener, the main components of the grout. Each component is pumped independently and mixes at the built-in static mixer. The pumping process takes about 15 min, safely within the potting life of the grout.

After injection, the grout sets while preparation for hot-tap drilling commences. A full-pressure test on the branch precedes hot tap drilling once the grout has cured.

Test guidelines

Completion of the test tank trials helped define parameters for the field tests. Any coating would have to be removed before pipe-surface preparation. Concrete weight coating with reinforcing steel bar and anticorrosion coating must be removed to 500 mm either side of the tee to allow clearance for installation. Shot-blasting the pipe surface to SA2.5 with a profile of 30-70 µm allows for ideal grout injection and bonding of the grout.

Pneumatic (nitrogen) pressure testing of the interspace within the primary saddle seal reached 153 barg, with the main pipeline and branch at atmospheric pressure, without leakage. System tests went as high as 225 barg without leakage.

The grout injection rate on the 24-in. by 24-in. test measured roughly 5 l./min, with 70 l. required to fill the annulus. The grout curing process, commencing once the hardener and resin components are mixed, limits the injection window to 40 min at 4° C.1

Field test

Following the onshore qualifications program at the end of 2008, a subsea mixing and injection skid unit underwent full-scale grout injection trial in the North Sea June 27, 2009. Injection-skid deployment took place on the seabed near Total's Jura gas field at about 113-m water depth and a seabed temperature of 7° C. (Fig. 3).

Field testing of a subsea grout-mixing and injection skid occurred in 113-m water depths at Total's Jura North Sea gas field in June 2009, using the Seven Pelican dive-support vessel (Fig. 3).

The trial sought to verify overall functionality of the injection system and the grout's ability to cure at seabed temperature and pressure. Testing used two 16-in. OD pipe-in-pipe samples with one sleeve fabricated from clear acrylic to allow grout flow path assessment. The trial lasted almost 5 hr and test samples were left at seabed conditions for 24 hr to cure before the whole system was recovered to surface.

Grout flow path inside the test sleeves was similar to onshore qualification injection trials and the results of grout compressive strengths were within the expected parameters.

The deepest hot-tap work being performed by divers is at roughly 250 m. SSGT development is focused on 100% remotely operated applications for use in deep and ultradeep water environments.

SSGT principles can extend to pipelines up to 48-in. OD and cover a pipeline operating temperature range between 2° C. and 70° C.¹

Reference