Taking the load

Calibrating the 64 load cells on the Snorre A tension leg platform was a lengthy arduous task, until a new solution, installed late 2014, was found. Elaine Maslin reports.

The Snorre A tension leg platform. Photo by Harald Pettersen, Statoil.

Tension leg platforms (TLP) have become, if not a mainstream platform support design, an established design in the offshore oil and gas industry.

The first TLP was installed in 1984 on the Hutton field in the central North Sea in about 500ft water depth. The Hutton TLP demonstrated the viability of a floating platform tethered to the seabed to drill and produce with surface Xmas trees*.

Since then, TLPs have been used in water depths deeper than 5200ft. But, while they’re established as a engineering option for field developments, one of the requirements for keeping a TLP safe is to have a monitoring system to make sure the tendons that tether the unit to the seabed are kept at the correct tension.

Snorre A has had such a system, but last year the system was given a complete overhaul.

Snorre A

Snorre spans blocks 34/4 and 34/7 in the Tampen area of the Norwegian North Sea, about 140km west of the Sogne Fjord, in the same region as Statfjord and Gullfaks. The field is operated by Statoil and has been producing oil and gas since August 1992 from the Snorre A TLP and from the Snorre B semisubmersible production platform in 2001.

Complete system test set for factory acceptance test.  Photos from from Holmatro.

The Snorre A TLP is an integrated production, drilling and quarters (PDQ) unit, moored to the seabed by steel tethers. Stabilized oil and gas is piped to the nearby Statfjord A platform for final processing. The oil is then loaded into shuttle tankers, while the gas is transported on to continental Europe through the Statpipe/Norpipe system.

To retain, measure and monitor the pre-tensioning of the Snorre A TLP cables, each of the four legs of the platform include constructions fitted with 16 load cells. However, these load cells require regular calibration, which means the weight of the platform must be lifted off the load cell.

Since the platform came on stream, testing and calibration of the TLP’s load cells was carried out using two, 530-tonne capacity hydraulic cylinders, each weighing about 400 kilos, to perform a 21mm lift. The two cylinders would be used to lift one side (i.e. two loads cells) of each set of four load cells, before setting then back down again and moving to the other side. This meant 32 lifts – every three months, which is how often calibration is required.

In 2008, Aker Solutions started a study for Statoil to find alternative ways to carry out the process, which was not only lengthy, but also difficult, due to having to move the two 400 kilo cylinders between each of the four piles — not an easy task in the confined spaces — and due to the location of the 32 sets of loads cells.

Solution

Two 536-tonne capacity hydraulic cylinders, each weighing 400 kilos.

Six years passed before a solution was found, but, the new solution now means the cylinders no longer need to be moved between the columns, with each column having dedicated cylinders, which only have to be placed once within the column to test all of the 16 load cells it contains.

Specialist high pressure cylinder producer Holmatro, founded in 1967 in the Netherlands, was brought into the project in 2014 via its distributor EIVA-SAFEX AS. After discussions concerning the tender documentation and the solution, the firm submitted its bid and was one of two firms in the final running for the project, says Rob Loonen, sales engineer for industrial equipment for Dutch-based international hydraulic equipment supplier, Holmatro, which supplied cylinders, powerpacks, storage lockers and control panels for Snorre A.

By June, after a teleconference meeting with client Aker Solutions, Holmatro was awarded the contract and the project began, with delivery expected in December.

“The initial idea was to use single-acting cylinders,” Loonen says, “but after studying the exact application, we suggested a different system configuration.” Holmatro proposed using double-acting cylinders with a tilting saddle in combination with a special lowering control panel, so potential operation errors would be covered and the load could never come back too quickly on the load cell position. The 64, 536-tonne capacity, cylinders (one each per load cell) are sited next to the load cells. Due to the limited space, a lifting yoke was created by EIVA SAFEX AS to put the cylinders in place. They also produced a sea-fastening system to secure the cylinders underneath the load. For each column, one complete control set (powerpack, control panel and hoses) is kept in the column storage cabins. Each column has its own hydraulic system and a 50-kilo pump unit, so neither have to be moved between columns.

The result is that the load cell calibration work has gone from 32 lifts to eight lifts with no more equipment handling between the platform’s columns. For Holmatro, it was an ideal job. While the company has about 170 standard cylinders available for the offshore sector and other industries, it also specializes in bespoke solutions.

“The nice part of this project is that it’s an industrial solution,” Loonen says. “We always try to help a client get the right solutions. In collaboration with our research and development, production and service departments, we place ourselves in our client’s situation and participate fully in the thinking process to achieve the right solution for their usage requirements. We like to be challenged to come up with new innovative industrial solutions.”

*OTC Brazil, 2013, paper OTC-24512-MS by Rajiv Aggarwal and Richard Souza, Granherne.