Mooring technology adapting to deeper waters

Extending shallow water mooring equipment and procedures into deepwater is creating some issues, attendees at the 2009 DOT International heard in a presentation by Paul Hughes of First Subsea and Sergio Leite from Lankhorst Ropes' offshore division.

Offshore staff

MONACO -- Extending shallow water mooring equipment and procedures into deepwater is creating some issues, attendees at the 2009 DOT International heard in a presentation by Paul Hughes of First Subsea and Sergio Leite from Lankhorst Ropes' offshore division.

Currently, mooring of spars, FPUs, and FPSOs differs from mooring submerged turret production (STP) or submerged turret loading (STL) buoys in that spars and platforms are held in a fixed position above the water line while buoys are is moored subsea, then raised into the FPSO. The differences in the mooring line installation are as follows:
• Spar, FPU, FPSO – The mooring line is lowered from the surface when the production unit is in position
• STP/STL buoy – The mooring line is pre-installed subsea ahead of buoy tow-out.

The deepwater mooring line in either case will be made of polyester rope and/or strand wire together with a mix of shackles, H-links, Pear / Plate links, Y-links, and ground and tail chains.

The next stage in the evolution of deepwater mooring systems is being driven by developments in materials technology and efforts to reduce the complexity of mooring lines to save time and money. The materials technology is critical becasue the connections are the 'weak links' in the mooring line.

Length measuring of deepwater mooring ropes is important also. The rope length and tolerance must be precise. It is a classification requirement to test the rope interface with either the thimble to be used with the rope, or if used on a pin, with a test pin of the same diameter to represent actual service conditions.

Continued improvements in lightweight mooring rope performance are producing longer, complete lengths of rope. If a rope manufacturer was able to develop a 20g/denier tenacity yarn, the rope diameter and weight per meter would be reduced sufficiently to allow approximately 16,400 ft (5,000 m) of rope onto a single reel using existing production machinery.

The longer lines allow greater surface motions. A 2,000 m (6,562 ft) long line may allow 40 m (131 ft) of vertical movement, whereas a 3,000 m ((,842 ft) line would allow 60 m (197 ft) under the same conditions. This translates to greater horizontal offsets which may exceed riser limits. Stiffer materials or hybrid mooring systems will be required to compensate for the longer line lengths.

Chains will continue to play a vital role in deepwater mooring systems. Here developments in pile and deck chains have focused on offering smaller diameter chain with higher tensile strengths - as they go to deeper water the chain element needs to take more load.

Reducing the amount of mooring line jewelry depends on increasing the length of continuous rope between connections. However, at this point in time, there are not enough vessels capable of handling long lengths of rope.

The mooring industry is working on next generation deepwater connections which will be simpler and quicker to make-up and install. The connections may be integrated with rope manufacture so that all connections are done onshore. Meanwhile, new types of forged H-link have been developed for rope / rope and rope /chain connections that reduce the time needed to make the connections. Further, by modifying the H-link to produce a crossed H-link, the connector can be used as an alternative to mooring line pile shackles.

Looking ahead five – 10 years, the outcome of the rope material and connector developments will be mooring lines designed specifically for deepwater environments with longer rope section, and fewer, simpler connections, greatly reducing the complexity and time needed to make up mooring lines.

11/04/2009

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