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KEY MOORING ISSUES DISCUSSED
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"Our challenge is to help operators find the safest, most effective mooring system to match the range of environmental loads the installation will face."
Ken Huang, Group Head, Global Performance & Mooring System Team, Offshore Engineering, ABS Americas.
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| A suction pile caisson with vertically loaded anchor being launched. |
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| A reel of synthetic mooring rope. |
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n a recent interview with International Offshore News, Ken Huang of ABS discussed a number |
of key issues concerning mooring systems in general and synthetic ropes in particular. Huang is Group Head of the Global Performance & Mooring System Team, Offshore Engineering, ABS Americas. Here are some of his thoughts on a number of the issues raised in the interview.
 Dynamic positioning versus conventional mooring.
It’s not that one is inherently superior to the other. Both have their merits and are equally viable. It mainly depends on the type of operation – drilling or production – and the environment in which you intend to place an offshore structure.
As the industry moves into deeper water, conventional mooring systems are increasingly difficult and costly to deploy. To some degree this is causing a shift to the use of dynamic positioning (DP) for drilling units.
On the production side, DP performs well in fairly benign environments. However, for maintaining station keeping in severe storms, DP systems may prove uneconomic.
With DP, greater human interface for monitoring and automated control is required. Operating costs for DP are always higher than those for passive mooring systems.
Combining the two systems
This seems to be a trend, and it has some advantages, notably:
- Lowering the operating cost.
- Providing for emergencies, in which the mooring system can function when the DP is not able to operate.
ABS has been involved in a recent project in the Gulf of Mexico where the operator is considering – for water depths in excess of 6,000 feet – a DP-equipped MODU with 4 taut-leg mooring lines. These lines can assist the DP and help prevent potential riser damage during severe weather or a power outage.
The challenges of classing the different types of mooring systems designed for different operating environments.
Deeper waters require innovative mooring solutions. At ABS we are seeing solutions for environments such as the North Sea or offshore Canada that combine turret mooring systems with thruster assist for FPSOs.
In more benign environments such as Brazil, we see taut leg mooring combined with vertically loaded anchors. In West Africa, conventional passive spread mooring systems are more common.
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The key considerations in choosing a mooring system are the environmental loads resulting from wind, waves and current. These factors pose increasing risks and technical challenges as activity moves farther offshore and into more hostile environments.
Mooring system design is one element that ABS reviews in the overall classification or certification process. Our challenge is to help operators find the safest, most effective mooring system to match the range of environmental loads the installation will face.
ABS involvement with research on mooring systems.
We’ve been very involved. Our research efforts include active participation in several joint industry projects led by the U.S. Minerals Management Service to enhance understanding of synthetic rope technology. ABS is fully committed to these initiatives.
We have gained a great deal of field experience over the years in the installation of synthetic moorings, including vertically loaded plate anchors. This has given us a competitive edge in these systems.
The strengths and weaknesses of fiber taut leg moorings and suction pile anchors.
These two technologies usually work together, with the suction pile anchor capable of taking the vertical loads from the taut leg moorings. The combination of these two new components for a taut leg mooring system is gaining acceptance in deepwater applications.
However, installation requires special attention to prevent damage to the fiber ropes. Lack of experience in the long-term use of fiber rope is the major unknown in this system. However, this problem is diminishing as new offshore projects increasingly apply these technologies.
ABS reviews the mooring systems to enable clients to assess the strengths and weaknesses of any particular system for the specific environment intended.
We’ve found that one system is not better than the other, per se, but that the key is to find the application of a particular system – or combination of systems – that will work best in that environment. We certainly need innovative mooring design today for challenging applications all over the world.
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