Mooring line failures attract industry attention

Mooring lines on floating production systems continue to fail, costing the industry lost time and production— why and what is being done? Elaine Maslin reports.

The use of floating production systems in the offshore oil and gas industry has been predicted to grow at a significant rate between now and 2017.

The number of newbuild units coming onto the market—on top of conversions—is expected to peak in 2016 and 2017 (OE April 2013).

However, the number of mooring related incidents involving floating production units continues to raise concerns in the industry.

Kai-Tung Ma, of Chevron Energy Technology Co., says between 2001 and 2011, there were 21 mooring failures—an average of more than two per year (OTC.13, paper OTC- 24025-MS). Nine were multiple-line failures.

Research by Granherne, a KBR company, (OTC.13, OTC-24181), lists 23 permanent mooring failures since 2000, and four of those were categorized as catastrophic, with riser failure and extended field shut-down.

In addition, at least 20 floating production systems (FPS) had integrity issues requiring intervention and 150 mooring lines were repaired or replaced across 33 FPS, says Sai Majhi, of Granherne.

“We are having too many [repeated] mooring failures,” says Ma. “In 2011, we had four and when preparing this paper, there were another two chain failures and one wire rope failure, so it is ongoing.”

The consequences of multiple mooring line failure can be high, says Ma. It is not just the mooring lines that break—risers and other subsea equipment can be damaged, making reinstatement costly.

The Gryphon Alpha, which saw four of ten lines parted when it came off station in a storm in the UK North Sea in early 2011, cost an estimated US$1.8billion to reinstate, says Ma. Banff, with five of 10 lines parted when it came off station in the same sector later in 2011, will cost an estimated $0.3billion.

“Nan Hai Fa Xian (moored south east of Hong Kong), had four of eight lines broken,” says Ma. “Plant [equipment] was flipped upside down, pipe ruptured and all the risers broken. The message here is that the consequences of failure are high.”

However, when it comes to assessing the cause of mooring line failures, results have not been clear.

Research presented by Ma showed mooring chain accounted for the majority (47%) of all incidents followed by wire rope and then polyester rope. By number of breaks, mooring chain was still the primary cause.

Due to the high variety of root causes, from knotted chain, to fatigue and corrosion, finding trends is “challenging.” Worse, more than half of the root causes were surprises—unexpected issues, hard to pre-empt.

But when failures do occur, Ma says they are often then repeated in a similar area on the same installation.

“Dalia had two chain failures near the anchor; [the second break two years after the first] almost exactly in the same location. There are more examples of multiple breaks in the same place. In your design, you are going to have a weak point and that weak point is going to be in your mooring lines and you are likely going to have similar failures in other lines.”

Another trend is early-life failure. “There is a clear trend of ‘infant mortality’,” says Ma. “So many incidents happen in the first five years (12 of the 23 assessed).”

Majhi’s research showed a similar trend, with the replacement rate of mooring lines peaking in the first two years (about 50 of those assessed since 2000), falling to just under 30 in the fifth year, before slowing increasing towards the 20-year life of the system.

There has been work on improving mooring integrity in the past decade, including the SCORCH (seawater corrosion of rope and chain) joint industry project, says Majhi. However, there is still a lot of room for improvement, from the design phase through procurement, installation and operation and having technically competent oversight. Mooring system inspection, for example, varies widely, from annually to every five years.

“During mooring design, handling and installation, there is very little interface and if it does happen, it is late in the game. If companies try to have a feed-back loop, you find there are gaps in manufacturing, gaps in installation.” Closing these gaps would take the industry in the right direction, he says.

“One of the big challenges is interface, ensuring early and efficient engagement,” says Majhi. It’s also important to ensure that design methodology assumptions are correct, reflecting worse-case scenarios. He says adequate modeling should be carried out, and that once installed, systems should be re-analyzed. “One of the biggest gaps is re-analysis of as-built systems; if information is captured, it is not put back into to mooring analysis,” says Majhi. The procurement and manufacturing phase needs to be closely managed, to make sure design specifications are followedthrough and that strength, fatigue and corrosion protection verification is carried out, he says, as compliance “varies widely” and there are often multi-layered contracts. In addition, materials used should be traceable for future testing.

He said there is a reliance on Class designations for quality assurance and control of hardware manufacture, but a lack of detailed oversight of it. Dedicated, technically competent inspectors are needed to oversee compliance, he says. This includes during testing of materials, such as heat treatment, where control of the process can directly impact test results, and non-destructive testing (NDT) beyond Class requirements.

During installation, mooring hardware, deck equipment, and monitoring equipment needs to be correctly specified and in-situ maintenance and replacement of deck equipment planned in. “Having spare equipment is one of the big savers when offshore, you need to expect there is going to be damage of some critical component,” says Majhi. “You also need to have a good deck-handling protocol so you do not damage sheathing before you put it in the water.”

Once installed, there is a risk the mooring system is “out of sight and out of mind,” instead of being subject to a good inspection protocol and condition assessment with data fed back for future projects and code enhancement, says Majhi. During one failure investigation, it was discovered that the crew on the vessel did not have any kind of monitoring system and they did not realize they had a mooring failure until they saw a fixed platform nearby moving away from them, says Ma.

On an industry level, Ma says that engineering standards should be updated and developed to “raise the bar” industry-wide. Work is being carried out based on API standards to agree on new codes, he says, adding that the move to deeper waters will bring an increased need to understand the “novelty” of going in to deep water.

“The tension is higher and we are going in to high-pressure environments and using longer mooring components,” says Ma. “We should encourage joint industry projects and share lessons learned openly, without identifying facilities.”

Majhi’s conclusion is a little more pragmatic: “It is prudent to anticipate failure and be prepared.” OE