Finger on the pulse

The floating production systems fleet has grown steadily over the past two decades, and a total of 216 floating production projects are currently either being bid, designed, or in the planning stage, according to the latest figures from market analysts International Maritime Associates.

With orders for 130-190 production floaters forecast over the next five years, FPSOs are expected to account for up to 75% of future floater orders. With this expected level of market activity, the demand for the monitoring of mooring lines and further advances in related technologies is expected to grow in parallel.

One of the issues with floating installations is that they are moored to the seabed and therefore are subject to the forces exerted during stormy conditions. These exceptional forces can cause deterioration of the mooring lines and in some circumstances result in mooring line breaks. Being able to monitor the tension experienced by the mooring lines enables FPS operators to proactively take the required measures to maintain mooring line integrity.

Pulse Structural Monitoring developed its MoorASSURE system for an FPSO vessel offshore Brazil supplied by SBM Offshore. A new monitoring system was specified for the project, not only to notify the operations team in the event of a line break but also to record the line tension experienced over time so the mooring line performance could be evaluated. The project specification also called for the system to have the capability to identify progressive degradation such as line creep and quantify the need for line retensioning.

Subsea motion data loggers

Pulse decided to use acoustic communication to relay the measured data to the control room to avoid problems associated with cables connecting the system to the surface. As a result, the main components of the MoorASSURE system are Pulse's subsea INTEGRIpod AF motion data loggers, with one being mounted on to the chain hawse of each of the floater's nine anchor legs.

An Inter-M Pulse unit is ready for load out. (Images courtesy of InterMoor)

The real-time system uses inclinometers to measure the angle of the mooring lines and uses those measurements to calculate the tension experienced by the lines. Acoustic communication then connects the system to the vessel, enabling the measuring devices to be positioned outside the load path of the mooring line and eliminating the risk of damage to cables.

Richard Kluth, managing director of Pulse, said in a recent press statement, "The market is demanding systems that will tell the crew where a mooring line is and also determine how it is performing. Mooring line integrity management is extremely important, as the potential production losses from coming off station are very significant."

The success of the project, which has been operating since May 2009, encouraged Pulse and its sister company InterMoor UK to further develop their complementary products so they could be installed as an integral part of the mooring line rather than being mounted onto it.

This involved advancing InterMoor's long-established H-Link long-term connector. H-Links were designed to connect chains and ropes of varying sizes together and have been installed in some of the harshest offshore environments around the world for more than 25 years.

As a standalone connector the H-Link was relatively rudimentary, said Alan Duncan, managing director of InterMoor UK. Speaking at a briefing at the Offshore Technology Conference in Houston, he said it was logical that it could be enhanced in its design and far beyond the scope of "just a chain connector."

"Using the knowledge and experience of H-Link design and manufacture, the H-Link developed into a working component that could be used offshore to allow twist to be taken out of chain/wire systems," said Duncan. "The Inter-M Swivel was developed and has been used offshore in extreme environments as a 'longer-term' connector with all of the design/integrity requirements built in," he added.

Demand for intelligent line monitoring

But throughout the swivel's development process, the market feedback was that clients wanted more. "Throughout the trials and offshore use of the swivel, the customer feedback and requests for an intelligent line monitoring connector led to the development of the Inter-M Pulse," Duncan said. "It fits as a standard part of the chain."

The unit is attached to the mooring chain before deployment on its field test in the UK North Sea on Diamond Drilling’s Ocean Nomad semisubmersible rig.

The new system is the result of close collaboration between Pulse and InterMoor UK, both part of the Acteon group of companies. It is deployable in water depths of up to 1,500 m (5,000 ft) and again features Pulse's subsea motion data loggers as a key component.

The logger uses a motion sensor to record the movement of a structure to its onboard memory over a period of time, with the data then periodically transmitted to the surface via its acoustic link. Topside monitoring software allows for logging program selection, system diagnostics, and data retrieval. Three user-defined logging programs (Infrequent, Frequent, and Emergency) are available to suit different offshore environments.

The logger contains all electronics, batteries, and sensors and is enclosed in a cylindrical corrosion-resistant casing rated for up to 3,000 m (10,000 ft) water depths.

As part of the Inter-M Pulse system, it enables the long-term "intelligent" connection of mooring chains along with the capability for monitoring and tracking the in situ tension of each mooring line. It can then transmit the tension data back acoustically to the topsides control room, according to associated press material released alongside Dun-can's presentation.

North Sea field trial

It was recently the subject of a successful field trial conducted in the UK North Sea. Duncan outlined how a 76-mm, 800-ton minimum break load unit was deployed from the back of an anchor handling vessel over the stern roller for Diamond Drilling's Ocean Nomad semisubmersible rig.

The Inter-M Pulse unit complete with the installed INTEGRIpod AM acoustically-linked motion data logger.

The unit was located on BG Group's Everest 22/10a block in the UK North Sea for a six month drilling campaign. The unit deployed was designed for use on all offshore units, according to the associated press material. The field trial mooring system consisted of a combination chain and fiber makeup with the unit installation located at the fiber and shackle connection.

During a one-month sea trial, sea states of around 12 m (39 ft) were not uncommon. The Inter-M Pulse sent accurate line tensions and inclination readings to the surface control room on board the rig at all times and was immediately operational after its successful installation, according to InterMoor's test findings material. The data was sent back to the topside system via the acoustic data link every minute several hours a day. The sea trial proved that the system will communicate in a range of sea states, and the readings have since been verified and documented.

"It worked great. We are really happy with the trial," said Duncan. "Clients have also now asked for it to be tested in the Gulf of Mexico. They'd like to see a wet trial, and we're hoping to start that in about two months' time there, running it for about six months."

The ability of the system to raise the alarm when a mooring line fails and monitor the performance of the mooring lines and mooring cable creep is a practical advance, and the need for such a monitoring system is real. "Last year there were four vessels that lost a chain," said Duncan during the press conference. "They did not know."