Flexible Fallpipe Vessel for Subsea InstallationTue, 01/22/2013 - 13:05
Subsea Rock Installation is a proven, advanced technology that has been used in the Mexican oil and gas industry during the last few years. The purpose of this technique is to protect oil rig foundations, level the seabed prior to the placement of pipelines and cables in coral and reef conditions, and stabilize and protect pipelines and other oil and gas production equipment against miscellaneous threats. This technology is offered by specialized dynamically positioned vessels that can work in water depths surpassing 1000m.
This technology has been applied for many decades – mainly in the North Sea – for the installation, foundation, and protection of pipelines on the seafloor based on a thorough design and planning process, which requires seabed inspection to examine soil consistency, and analyze additional requirements for stabilization. Engineering teams study the seabed surveys in order to establish a suitable route for pipelines to be placed. “If any voids underneath the pipeline would exist, what we call a large free span or rupture could occur,” explains Jos Wellink, Regional Manager of Van Oord Offshore. “As oil fills the pipeline its weight increases, weakening the pipe’s support and possibly risking it to break. Our technology helps to build additional support made out of rock and placed on the sea floor to sustain the pipeline on key locations, starting from the middle.” Also the influence of currents and waves could cause displacement of the pipeline. The uneven conditions in some areas of the sea floor in the Gulf of Mexico mean that this type of protective and stabilizing measures need to be applied before the placement of pipelines.
“Last year, we worked with Pemex on the rock installation over pipelines and cables and we also worked with Mexico’s ports doing the maintenance dredging for the Port of Coatzacoalcos, for instance, to build steadier routes for those pipelines within the Gulf of Mexico,” explains Wellink. “There is more work to be done now in Mexico since Pemex is issuing more projects. The the development of Ayatsil, which will have a large number of platforms and requires extensive pipeline infrastructure, additional work will be required to create the conditions for a stable road there.”
Van Oord Offshore’s vessel Tertnes is suitable for the conditions and infrastructure of Mexican ports, and has already executed several projects for clients working in the country during the past few years. The rocks (that have a diameter of 2.54cm to 12.7cm) used on the project are taken from the rock quarry along the coast into the port, where they are loaded onto one of the flexible fallpipe vessels. Up to 9,600 tonnes of rock are transported in the hull of Tertnes, a vessel that has been modified from a bulk carrier and that is equipped with state-of-the-art rock placement equipment. Once seaborne, the rocks are brought up from the hull to the deck and into flexible fallpipe line. This system consists of a string of retractable buckets that are connected by chains. The fallpipe is lowered all the way down to the sea floor, so that the rock can be accurately placed over the location specified within the design parameters. The ROV (Remotely Operated Vehicle) at the end of the flexible fallpipe has a survey equipment to accurately position the fallpipe and monitor ongoing operations. The rock is placed on the seabed to create a rock berm, on which the pipeline is subsequently laid. This gives the pipeline the necessary stability so that it endures the conditions of the sea, the changing nature of the seabed, and the erosive properties of strong currents pressing against the pipeline infrastructure.
This technology is also suitable for deepwater applications, such as the creation of a stable foundation on the sea floor prior to the installation of deepwater production equipment. However, this application demands a bigger emphasis on analyzing the seabed conditions. The consistency of the soil needs to be studied to see how much weight it can take so that the equipment does not sink. Afterwards, a plan is made on how much rock is needed to create the necessary foundation to hold the equipment’s weight. This application is very common in the North Sea, and could be an operation needed for the development of deep-water fields such as Lakach, Trion, and Supremus in the future.
Flexible fallpipe vessels can also perform maintenance tasks to re-establish the stability of pipelines and cables. To safely and accurately perform this work, Van Oord’s flexible fallpipe vessels carry sophisticated survey and positioning equipment that even allows the company to work on live pipelines and cables. “Companies like Pemex monitor their pipelines continuously since subsea infrastructure is unpredictable by nature,” Wellink describes. “Even properly installed pipelines that have been recently inspected can lose their support on the seabed: strong sea currents might disseminate the sands under pipelines and erode the seabed in which they are standing.” Van Oord Offshore’s Regional Manager explains that there are two ways of facing problems like this. “You can either wait for it to happen and address the problem, or do preventive maintenance by putting rock on top of the pipeline for it to be fixed on the seabed.” This service – which the company calls ‘scour protection’ – has become part of the portfolio that Van Oord offers to support Pemex in its shift towards preventive maintenance.