Subsea damage and support problems can be quickly solved using injection grouting. James Bell of FoundOcean tells us how its accomplished.
Extending the life of an asset means that the structure can still generate revenue, whereas removing them can be a highly complex operation, often more so than the original installation. There are many options to rehabilitate structures. The state of the asset will have a bearing on the likely success and available restoration options: for example, upgrade works due to regulation changes are likely to be relatively straightforward, whereas vessel damage repairs will be more technically challenging.
Grout is the essential element securing jackets, monopiles, gravity-base structures, and tripods to the seabed via:
Fabric formworks
Fabric formworks are used to prevent or cure pipeline freespans. They can also be used to raise a pipeline or structure by jacking it up. The flexible, high-strength synthetic woven fabric bags are cost effective to make and are easy to deploy by ROV or diver.
Freespan rectification
It is recommended practice for pipeline engineers to adhere to specific design codes like DNV-RP-F105, or similar codes, for freespans. Surveying the seabed topography and local currents can help engineers to plan whether freespans or scour may occur during the installation of a new pipeline. This can influence the design and path of the asset and enable operational teams to plan scour rectification measures into the installation schedule, or incorporate closer inspection regimes during the asset’s operational lifetime.
Grout bags can be installed in both preventative and curative instances. Where the sea floor is particularly undulating, new pipeline installations will require some form of support to ensure freespans fall within the regulatory parameters, preventing unacceptable freespans from occurring.
Current velocity increases locally around a mass that lies on or protrudes from the seabed. This causes localized seabed erosion adjacent to the structure and is known as scour. Curative formworks can be installed when seabed scour has eroded previously supportive sediment, and in some instances, can actually help reinstate the seabed.
FoundOcean recently completed a project in the Bay of Bengal to install formwork supports for a new 32in. export pipeline and 14in. intra-field pipeline. Engineers used computer modelling software to identify tens of potential freespans that would need addressing along the pipelines’ paths. But, it was not possible to confirm whether the spans were acceptable until the pipeline had been laid.
Fifty-one custom grout bags were designed and manufactured to suit the terrain and height of the freespans. The large-base grout bags had anti-scour protection skirt sleeves built into them, which slows down the local current and causes suspended particulate matter to settle. Over time, the skirts become engorged with sediment, reinstating the seabed.
The grout bags required filling in two or more stages due to their volume. This was achieved by injecting grout into the lower section of the bag and allowing it to cure to provide a stable support for the grout injected in the upper sections later. The formworks were installed in water depths up to 186m over a 90-day campaign.
Deepwater pipeline separation
There are other subsea applications for fabric formworks, such as jacking or separating crossing pipelines. This year, the company was contracted to work in the Gulf of Mexico to separate two subsea pipelines that crossed at a depth of 1,370m (4,500ft). The solution chosen protects the lower pipeline and provides support to the upper pipeline.
The two pipelines will be separated by jacking the upper pipeline to 18in. above the top of the lower pipeline. An ROV will then take a 1.2m-high crossover formwork off the deployment sled and maneuver it into position.
Ordinary Portland cement (OPC) grout is mixed on deck. It is pumped to the grout bag via a deepwater grout umbilical, connected to the formwork deployment sled. After sufficient grout has entered the formwork to stabilize the bag, checks are made to ensure that the bag orientation and position is still correct and that it is filling evenly.
The crossover grout bag is designed to be filled in three stages due to its size and volume. The lower compartment is filled and allowed to cure sufficiently to support additional grout in the upper compartments.
Grouted clamps
Another form of remedial work is the grouted clamp. These have been used successfully to restore pipelines, strengthen jacket members, and repair caissons. A clamp is composed of two or more steel segments, which are placed around an object. The segments are bolted together along their edges prior to the injection of grout into the annulus between the clamp and the asset.
The grouted repair clamp allows for greater design and fabrication tolerances, as the grout fills the annulus and evenly distributes the load of the object the clamp is being placed around.
In 2007, the 233 mile, 36in. CATS pipeline supplied 12% of the UK’s gas. IHowever, the pipeline had to be shut down due to damage from a vessel’s dragged anchor. A fast response and first time fix was vital to get supplies back up and running. A repair sleeve was bolted into position and the grout hose was connected to the inlet. Grout was pumped into the annulus. Accurate and complete filling was controlled using carefully positioned overflow ports and valves. The repair was completed ahead of schedule and at short notice, with the client acknowledging the company’s efficiency.
This same technique was used more recently in deeper water. Saipem America awarded a contract to repair a damaged pipeline in the Gulf of Mexico. A cable-laying vessel's anchor had dragged along the seabed and over the 750m-deep pipeline in bad weather. This resulted in the pipeline being operated at a reduced pressure until remedial work was complete.
To enable access, the seabed was dredged around the damaged pipeline section. An ROV transported the two steel half-shells of the clamp to the seabed and maneuvered them into position around the pipeline, and tightened the bolts. Crews delivered grout using a deepwater grouting umbilical system.
Due to the extra load on the pipeline, a fabric formwork was installed under the clamp to provide additional support and prevent buckling. In total, 28 tonnes of cement was used: four tonnes for the clamp and 24 tonnes for the fabric formwork.
Inspection, repair, and maintenance projects demand an experienced supply chain whose skills can be deployed at a moment’s notice. Tried and tested offshore procedures, coupled with planned-for contingencies, and astute offshore personnel, ensure savings in both project costs and time.
James Bell has been managing director of FoundOcean since 2005.He has been involved with numerous challenging projects including the Woodside North Rankin B jacket installation, Thornton Bank Wind Farm, BP CATS pipeline repair, ConocoPhillips Ekofisk strengthening, Exxon Diana Truss Spar riser system, and Umm Shaif Gas Development project. Bell earned a Civil Engineering degree from London’s City University.
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