Tension in the deep

Aquatic Engineering & Construction is due to launch a new 120-tonne tensioner in 2015. David Tibbetts, VP Technology, spoke with OE about the methodology behind its design.

Two tensioners from Aquatic’s existing fleet being loaded out for offshore mobilization.  Photo from Aquatic.

When late in 2013, Aquatic was approached by a major oil and gas contractor looking for a tensioner for a future project, it prompted Aquatic to take a look at the market. The result is a new 120-tonne modular tensioner.

The main driver is the move into deeper and deeper waters. According to Douglas Westwood’s deepwater forecast, 2014-2018, deepwater expenditure is expected to increase by 130%, compared to the preceding five-year period, totaling US$260 billion.

The move into deepwater has been driven by a maturation in mature basins onshore and shallow water declines.

There are already established deep water plays, offshore Brazil, the Gulf of Mexico and West Africa – the “Golden Triangle” – and other areas are emerging, such as East Africa.

This means that the flowline handling equipment is having to increase in capacity to meet the higher loads seen in deep water. The contractor who approached Aquatic had asked for a 100-tonne capacity in 1000m-deep water tensioner.

Aquatic decided to research what was already available on the market and what might be required. It found that there was a requirement for tensioners above 100-tonne in more than one region worldwide, including the Golden Triangle.

“Our market research found that tensioner requirements are rising, but the largest portable tensioner available is currently at 75-tonne,” says David Tibbetts, VP Technology at Aquatic, an Acteon company.

Because of the increase in size a larger tensioner would involve, the research suggested a vertically mounded single tensioner would help save desk space, as well as handle the product better. Adding a dual-mode configuration would also enable 2000m water depths to be reached, at least with an umbilical.

But, the research also raised a question, says Tibbetts. This was: “Why this size tensioner and not something even larger, such as perhaps a 550te tensioner for use in waters off Brazil?”

Tibbetts.

“The answer for us currently, is that a tensioner of that size would normally be integrated into the structure of a vessel, and this requirement currently exceeds the Aquatic unique selling point of ease of transportation and mobilization; we send our equipment to boats, rather than requiring customers to send their boats to our equipment.

“As a business, we took a realistic and commercial view on the capability of this tensioner [the 120-tonne tensioner]. It’s a step change from where we are currently and it gives us a guaranteed capacity beyond our dual tensioner solution.”

The tensioner design has a four-track configuration but can also be operated in two-track mode if required i.e., the industry standard. “The co-efficient of friction engineers expect is 0.09 and so we work on the assumption that the industry norms will be used within any new piece of equipment,” says Tibbetts. However, other options were assessed, such as a three-track model, which can offer greater accessibility for the product.

Tibbetts says Aquatic applied whole life design principles to the design – incorporating operational expertise into the design so that you are minimizing the forces necessary to deploy the product; retaining the integrity of the product and enhancing the efficiency of the installation process, says Tibbetts. The result will be on the market this year.