The T System is a “mass flow” excavation device that works by using two counter-rotating propellers to channel a powerful high volume/low pressure column of water downwards to clear subsea material, rocks and mud.

With the widespread presence of infrastructure in the Gulf of Mexico, excavation projects can be complex. Traditional excavation systems include plows — common to Europe — and sleds, which are used in the Gulf and other offshore regions. Diving crews equipped with vacuum equipment are also used for shallower projects.

Rotech Subsea has used its alternative “T” system in the Gulf of Mexico and other regions throughout the world. The contact-free design of the tool has shown improvements for major excavation projects both in new developments, such as pipelines, and in decommissioning submerged equipment in storm reclamation projects.

“T” for technology

Rotech’s “T” excavation was conceptualized in 1996 by physicist Hector Susman. The idea was to find viable uses for turbine technology. After several patents, the T System was developed as “mass flow” excavation device. It works by using two counter-rotating propellers to channel a powerful high volume/low pressure column of water downwards to clear subsea material, rocks and mud. It can deliver up to 2,100 gallons per second at 2 to 4 psi.

It operates through a closed-loop hydraulic control system that can be operated at varying rates depending on need. Technicians use live sonar feedback to adjust power levels. As there’s no physical contact with the sea floor or structures, there’s little risk of damage. Depending on soil conditions, the tool is capable of excavating a 5 ft (1.5 m) deep trench — 25 ft (7.6 m) wide at the top — in a single pass at up to 5 ft per minute.

The system was first used in 2001 for an oilfield application on Egypt’s Ha’py pipeline. The 36-in. line also included a 3-in. piggyback and it was live, which called for an excavation plan that was contact-free.

Since then, the system has tackled several types of excavations, including pipeline burial and de-burial for maintenance, wellhead clearance, rock dump removal, decommissioning work, sand wave leveling and hurricane repair work.

Pipelines

Plows are ideal for conditions where soil conditions remain consistent. Where changing soil conditions are a factor, the use of a plow can result in the buckling of a pipeline. Adjustments can be made, but the risk is always present. For this reason, plows are rarely used in the Gulf of Mexico.

Sleds are more common in the Gulf. These operate by attaching to a pipeline and tracking its path with a series of water jets designed to remove sufficient amounts of soil to bury the pipeline on spec. Familiar to many operators, these systems can provide an affordable means of excavation. However, the sled’s contact with the pipeline faces similar problems to the plow and can also cause buckling if not managed properly.

In addition to contact, plows and sleds are more cumbersome. As in the Gulf of Mexico, where infrastructure is extensive, most pipelines will cross others
in its path. Standard practices require the trenching to be aborted a specified distance from the intersection and resumed a similar distance from the
line on the other side. The cross section is then excavated by teams of divers using handheld pumps. Because this process is slow, it is often extended by harsh weather conditions, which can fill in previous excavation during evacuation. While this method remains widely used, the T system has proven to decrease excavation times.

Due to the tool’s contact-free status and its variable capacity, no stutter occurs when excavating through an intersection. The system can either bury the older line further or taper off near the intersection to allow padding to be placed between the lines at the point of contact.

Storm reclamation

The tool has also proven itself in the Gulf’s storm recovery operations. Based on sheer volume, the system can unearth submerged platforms approximately 10 times faster than alternative methods.

Diving is still preferred for lightly submerged decommissioning projects. The company’s T4000 is a compact tool designed for navigating around damaged and deeply submerged platforms to provide access to the wellhead. The tool is also equipped with real-time multibeam sonar equipment that allows the team to monitor and achieve the specific excavation requirements.

The deepest project to date is 3,000 ft (915 m). The system is rated up to 10,000 ft (3,050 m) of water. With the capacity to work at such depths, the tool may eventually help to alleviate integrity problems due to sand waves and assist with thermal insulation of pipes in the deepwater areas of the Gulf of Mexico.

La Zanja

The company’s T8000 tool was hired by a client in Mexico’s Bay of Campeche for a year to trench a 36-in. pipeline, as well as numerous transitions, crossings and box preparations to install mats and sand bags.

The client required 3.3 ft (1 m) cover from natural seabed to the top of a concrete coated pipeline. To get the needed cover, a trench of 7.8 ft (2.4 m) was excavated, with a width varying 9.8 to 16 ft (3 to 5 m).

Due to extremely hard clays encountered on previous projects in the same area, Rotech engineers in Aberdeen developed an ultra high-velocity nozzle. This enabled operators to cut through very firm clays to achieve the required depth. The offshore operators, with the use of the new nozzle, were able to excavate at an average speed of 3.3 ft per minute. Previous excavation speeds had been around 1.6 to 2.3 ft (.5 to .7 m) per minute.

The tool was set at about 3.9 ft (1.2 m) above the pipeline for optimum results. The Seabat 9001 sonar’s ability to profile the trench and provide the top of the pipeline as a reference point enabled precision trenching in even poor weather conditions.

The company also completed 20 different box sections, averaging 33 by 33 ft (10 by 10 m) each, to allow the installation of protective mats and bags between the existing buried pipeline and the new pipeline crossing over the top of it. The company also prepared 32 transitions crossing 16 other pipelines in the area. This was followed by 16 upward and downward transitions of an average length of 591 ft (180 m) to support the new pipeline and reduce stress on it when crossing the existing pipeline.

Covering the Gulf

In 2006, Rotech opened its headquarters in Houston to meet the high demand for excavation services after the 2004 and 2005 hurricane season and to serve the expanding oil and gas market in the region. It has been active not only in the US portion but also in the south. The company completed around 28 major projects in 2007.

The southern portion of the Gulf is heating up as Mexico makes steady progress in developing its deepwater play that could account for 54.8% of the country’s remaining reserves. According to Petroleos Mexicanos (Pemex) figures, the deepwater zone holds up to 29.5 billion boe of the country’s total 53.8 billion boe in prospective reserves.

In 2008 numerous platforms will be installed in the Gulf of Mexico. Existing fields, already in production, have an estimated lifespan of 25 years. New oil fields will extend the lifespan of the Mexico’s oil supplies, ensuring a plentiful supply of subsea pipelines and related infrastructure services and equipment.

The company has had a presence in the region for almost six years and, in 2008, will open a permanent yard equipped with tools and teams of employees in Ciudad del Carmen, Campeche, making it the only dedicated subsea excavation company to be established in the region. Previously, Mexico-based projects were managed from Houston. Equipment spent roughly 10 days each way on trucks to and from the base. Its new facility will now mobilize response to its Mexico-based clients within 24 hours.