|The Benchamas Explorer FSO locaed in the Gulf of Thailand. Photo from Chevron.|
In order to improve mooring integrity, Chevron and Technip developed a plan to change out the mooring system at its Benchamas FSO off Thailand. Audrey Leon reports.
Mooring line failures are a major concern for the offshore industry, especially with a rise in the use of floating units expected over the next five years. Douglas- Westwood projects that a total of 139 floating production units will be installed between 2014-2018.
At the SNAME conference in Houston this past February, Kai-Tung Ma, a technical team lead of mooring and geotechnical engineering at Chevron ETC, dis- cussed the company’s mooring line swap out on the turret-moored Benchamas Explorer FSO in the Gulf of Thailand. The FSO began serving the Benchamas field in 1999, which is located 364km south of Bangkok at a water depth of 71m. The vessel’s mooring system was designed to last 10 years, with no winch installed to service the mooring lines.
After an incident where a similar vessel experienced a major failure with its wire ropes in 2009, Ma said that Chevron decided to replace the Benchamas Explorer’s chain-wire-chain system with new sheathed spiral-strand wire ropes.
The goal of the project was to safely change the ropes while not interrupting production. Ma said it was unsafe to replace the mooring lines from the turret because it would have required a large winch, and would have required the field to shut down for hot work, i.e. welding. Chevron worked with contractor Technip to design a surface mooring connection tool (SMCT), which was welded to the back of an anchor handling vessel. This technique was adapted and improved from a previous project Technip carried out on the Alvheim field in 2006.
For the SNAME paper, Life extension of mooring system for Benchamas Explorer FSO, Ma and his co-authors Rachel Price, Dennis Villanueva, Philippe Monti, and Kevin Tan wrote, “The (SMCT) tool allows deck crews to retrieve and hold a mooring line above the water surface for disconnect, reconnect, and repair purposes. The apparatus is designed to allow for easy surface connections when installing or replacing mooring lines and reduces the need for complex underwater mooring line connection processes with ROVs and divers.”
The Benchamas Explorer FSO had nine mooring legs, secured by driven piles in a 3x3 configuration. Ma and his co-authors discovered that there were many existing issues with the mooring system. Leg #5 had been installed slack, which resulted in higher tensions on legs #4 and #6. The use of computer models helped determine that if leg #5 were shortened by 13 links of 117mm chain, legs 4, 5, and 6 could be brought to the same load.
|The microbial influenced corrosion that was encountered on chain links. Photo from Chevron.|
Additionally, the chain-wire- chain arrangement kept the majority of the wire buried in mud. Inspection showed that the wire rope, after 13 years in service, was well-preserved and protected by the soft mud with little corrosion. “The soft clay protects the steel,” Ma said during his presentation. “It stays on the surface of the chain because it’s sticky.”
No broken wires were observed, however the rope did appear greasy near the bend stiffener, Ma and his co-authors wrote. Testing found that no corrosion was found on the segment 1 chain, but an unusual pattern of corrosion was seen on the segment 3 chain near the touchdown point, Ma said. It was later determined through testing that two types of bacteria, which can cause Microbially Influenced Corrosion (MIC), were present. Ma and his co-authors concluded that the metal loss was due to MIC.
Ma said that fortunately for Benchamas project, the mooring system was designed with alternating lighter and heavier sections of chain, with the heavier chain intended to reduce vessel excursions by providing weight. Since only these larger chains were affected by the MIC, there was no adverse impact in the maximum tension or minimum fatigue life of the mooring system, and the MIC did not obstruct the project in any way.
Ma said this project highlights how important design is in the early stages, saying that the field would have benefited from an original design life of 20 years, instead of 10, by choosing sheathed wire ropes over low-cost options.
“Upgrades in the operation phase tend to cost more than initial execution phase,” he told the audience at SNAME. “Don’t be cheap in the beginning.”