Using pre-set mooring technology allows fourth-generation drilling rigs to compete in deepwater operations while suction anchors offer a competitive alternative.

Suppliers are increasingly relying on polyester rope and syntactic foam offshore. "Pre set mooring systems allow most of the existing semisubmersible drilling rigs to drill in a deeper environment than conventional mooring component design would normally allow," argued Billy Bergeron at Delmar Systems. "A fourth generation rig with mooring components designed for 5,000-ft (1,524-m) water depth (WD) can be utilized in as much as 10,000 ft (3,048 m)." But he notes that the riser and other depth-related rig equipment will also have to be suitably modified to operate at the greater depth.

"The costs of the candidate rig would normally be much lower than [for] a rig which is self-contained with capacity for drilling in the deepwater location."

Bergeron suggested a dynamically positioned rig drilling in 10,000-ft (3,0480-m) WD will be "very expensive" with attendant daily fuel costs and other environmental issues, such as CO2 emissions.
Conversely, an anchor-handling vessel with a well-trained anchor-handling crew can connect a semisubmersible rig to a pre-set mooring spread in a relatively short period - 24 to 30 hours. "Only half of this time is actually on line activity, as the rig can traditionally begin to drive pipe and drill out as the four remaining lines can be connected to the rig with minimal side load exertion," Bergeron said.

This compares with about 50 hours of on line activity for running out and setting conventional mooring for a semisubmersible.

Using a semi-taut configuration combined with embedded suction anchors - where a degree of tension is imposed on the mooring line - enhances rig station-keeping and does not require mooring chain on the grounded section.

Kerr-McGee is using suction anchors to moor a first cell spar at the Gulf of Mexico Red Hawk development. These are 18 ft (5.48 m) in diameter and 78 ft (23.78 m) long, and are the largest suction anchors installed to date in 5,300 ft (1,615 ft) of water by an anchor-handling vessel.

"The advantage is readily apparent in that the rig's excursion area is drastically reduced in comparison to a conventionally-moored rig with catenary mooring with anchors, [with a] chain and wire combination."
Bergeron suggested environmental risks of a spillage are greatly reduced compared with a DP vessel, which, he says, has an inherent risk of drive off. "Drive-offs can cause equipment damage or loss, and environmental damage."

Furthermore rig abandonment is easily accomplished by modifying winch line tension and simply abandoning a rig during severe weather events. "The rig will remain on location until weather has improved and crews are allowed to return," Bergeron said.

With congested subsea field layouts, suction anchors with semi-taut or taut-leg mooring can reduce the mooring footprint, "Many of today's fields have sea floor congestion which does not allow deployment of drag-embedment anchors."

Instead, Bergeron said pre-set mooring with accurately placed suction anchors and a good choice of mooring components can provide a safe and workable solution for a congested field.

"Use of the newly recognized synthetic mooring components on the market will help to alleviate much of the traditional weight problems associated with enhancing the potential of the current rig fleet," he added.
Current mooring depth limits are restricted only by the range of remotely operated vehicles, currently at 10,000 ft to 12,000 ft (3,048 m to 3,658 m). Bergeron said mooring contractors can easily step up to 15,000 ft (4,573 m) if clients desire to go that deep.

"Current mooring design and installation can easily go to 15,000 ft, but each candidate semisubmersible drilling rig has to be evaluated and its drilling capacities may have to be enhanced," he said. "Anchor handler capacities have to be evaluated for the volumes and sizes of components required to accommodate the design."

Because the variable deck load of rigs is a major issue for deployment of fourth and fifth-generation semisubmersibles in deepwater, synthetic products can contribute to reducing the "felt" weight on hulls.

Polyester rope compared to steel wire:

• 96 mm (3.78-in.) steel (710 ton loading design) in water weighs approximately
19 lb/ft 176 mm (6.9-in.);
• 176-mm (800 ton loading design) polyester in water weighs approximately 3.37 lb/ft.

Bergeron said, "The comparison is amazing when each of the eight mooring legs of a traditional semisubmersible drilling rig have approximately 14,000 ft (4,628 m) of mooring components." With eight legs of 14,000 ft (4,628 m) mooring line on a semisubmersible, totaling 112,000 ft, polyester rope offers a possible saving of 15.6 lb/ft over wire rope, thus the total possible weight saving is 1,875,600 lb (937 US tons or 850 metric tonnes).

Furthermore suction anchors offer precise placement and orientation over drag-embedded anchors, and they also offer some comfort over other anchor types. "Skin friction is calculated in relation to the soils data and satisfactory analysis validates the holding power of the design," added Bergeron. Also, he says there is no difficulty installing these in deepwater - as Kerr-McGee's experience with spars in the Gulf of Mexico has shown.

Mooring technology reached new limits October 2003 when Delmar Systems was among the contractors involved in the record depth set by Transocean's Deepwater Nautilus drillship in 8,717 ft (2,657 m) of water in Alaminos Canyon block 857 for drilling on the Great White IV prospect. A combination of polyester and steel wire was used for mooring combined with suction anchors, with the deepest anchor set at 9,391 ft (2,863 m).

The pre-set mooring spread involved eight 9.5-in. 70-ft (21-m) suction anchors attached to 9,000 ft (2,743 m) of 61/4-in. polyester rope, with another 3,500 ft (1,067 m) of 33/4-in. steel wire.

In March 2002 Delmar claimed the previous mooring depth record of 8,009 ft (2,441 m) also set by Shell.

Fiber rope

Kevin Grant, engineering manager at consultants Noble Denton Europe (NDE) pointed out fiber rope is now regarded as "state of the art" for mooring, increasingly used for deepwater, particularly for long-term applications where top tension has to be minimized.

"Depending on a field development, there may also be cost savings involved in using pre-laid fiber rope mooring for deepwater drilling operations," Grant said, who pointed out NDE wrote the industry textbook Engineers Design Guide for Deepwater Fiber Moorings, as well as designing and providing warranties for fiber rope systems. Grant said revisions to the guide are now under consideration because of technological advances with synthetic rope. Quick connectors are now a key part of modern mooring technology, allowing a reduction in the weather window required for connecting floating production systems.

"Pre-laid quick moorings with quick connectors have also been utilized by drilling rigs engaged in field development in the US Gulf of Mexico," said Grant.

Taut leg mooring is also finding more favor with offshore operations. "The length of mooring cables required is typically 50-60% less than that required for a more traditional catenary mooring system," Grant said, while offering less sway, surge and yaw motions, but they have to be pre-laid with either vertically loaded anchors (VLAs), pile or suction pile anchors. Grant notes both VLAs and suction piles are increasingly being used for deepwater taut-leg systems.

Currently single-anchor leg mooring is used where both loading risers and mooring lines are terminated in interconnected swivels allowing relatively simple installation and lower cost, ideal for marginal fields using mobile production facilities.

Alternatively spread-moored systems have been in common use for years - on some North Sea floating production storage and offloading (FPSO) units, floating production units, and floating storage units in benign environments such as the Barracuda and Caratinga FPSOs offshore Brazil in 2,624 ft (800 m) and 3,411 ft (1,040 m) WD respectively. Noble Denton designed mooring systems for both. "The benefit of using a spread moored system is the capital cost reduction, vessel offset reduction and reduced conversion or build time achieved by not having a turret," Grant said.

Alternatively turret moored floating production units are better suited for harsh environments and have been used in the North Sea - such as the Triton, Pierce, and Banff FPSOs, and on the North Atlantic Schiehallion FPSO - NDE provided consultancy for mooring systems on them all.

As the market moves forward, Grant foresees more integrity issues developing for mooring providers as existing systems build up service life. He predicts greater usage of fiber rope too, and of disconnectable FPSOs such as Husky's White Rose project off Canada where Noble Denton is the warranty surveyor for installation of the FPSO. Furthermore, Grant predicts greater use of pre-laid mooring using piles rather than drag-embedment anchors. "This would then allow mobile production facilities to hook up to a field quickly and easily," he said. "It is quite foreseeable that with some of the smaller more marginal fields that drilling and production may take place at several sites in succession without the need to lay complex subsea production facilities."

He also suggested more use of taut-leg mooring in deepwater. "This is a state of the art technology now that it is likely to be used more often in the future as exploration depths increase, such as in the [north Atlantic] Faeroes Gap, where the water depth is in the region of 4,400 ft (1,341 m)," he said. Combined mooring and riser systems may also be seen in future, Grant said, reducing installation complexity and some of the capital cost.

Knowledge

Further knowledge on mooring technology will be gained from joint industry projects and engineering forums, Grant suggested. He said when Noble Denton designed mooring for the Barracuda and Caratinga projects offshore Brazil taut fiber rope was used with underwater connectors between the bottom chain and fiber. "The classification society performed more controls during the manufacturing of products than for well-known products such as chain."

For new mooring types, Grant said regulatory bodies and to a lesser extent operators and classification societies want to see a service record for a mooring type before approving its use. "Therefore it can be difficult to introduce new or novel systems," he said.

Mooring integrity issues raised so far have been with floating production units using "one-off" component designs. Consequently the introduction of new mooring types is bound to lead to more rather than fewer integrity issues, so lessons will have to learnt all concern has previously been raised about the security of mooring for FPSOs.

The UK's Health and Safety Executive and the United Kingdom Offshore Operators Association mounted a joint industry project with Noble Denton in 2001 to investigate turret mooring on FPSOs in the North Sea and concluded, "The potential for multiple line failure is greater than is commonly perceived and should be of concern to operators," the HSE's December 2003 issue Offshore Research Focus newsletter stated.
Grant added, "Overall the study found that integrity was high, but there were still opportunities for multiple line failure due to common degradation issues. The study, therefore, recommended that the operators should take a risk-based view of their mooring systems to establish those areas of concern, so that the risk could be better managed in the future."

Now the mooring study is being extended independently by the consultants to cover international experience on FPSOs, semisubmersibles and spars, "The objective is to analyze FPS mooring failure rates worldwide and to determine a strategy for reducing such failures. The project will look specifically at the particular component failures and when and how often they occur," the certification authority said.
"From this data guidance will be developed on how to detect mooring failures and the project will also look at the health and safety and environmental effects of single and multiple mooring failures, with respect to the overall integrity of floating production systems."

Particular areas to be covered include:

• A review of actual or potential problems and their probable causes;
• Assessment of the health, safety, environmental and commercial consequences of line failure;
• Investigation of the design and selection of mooring connectors and interfaces;
• Mooring line status monitoring and prompt detection of line failure;
• Inspection, refurbishment and maintenance options including in water techniques;
• Sparing provisions.

Grant said lessons learned from this project will have implications for designing large and small mooring systems. Confirmed JIP participants include six operators, four regulatory authorities, two design houses, seven equipment or service providers and an academic institution.

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