Pre-set moorings provide less costly alternative to DP in ultra-deepwater

Earlier generation MODUs can be used in greater depths
with synthetic mooring lines and suction embedment anchors

Peter G. S. Dove, Thomas M. Fulton
Aker Marine Contractors

A suction embedment anchor, part of a preset mooring array for ultra-deepwater, is shown being deployed in the US Gulf of Mexico from the back of an anchor handling vessel. The anchor was tested for holding purposes.

A close-up of a suction embedment anchor on the back deck of an anchor handling vessel.

Part II: This is the second of a two-part series on the mooring of mobile drilling units in ultra-deep water. The first part, which appeared in the April 1997 issue, addressed conventional MODU mooring and installation. This second part addresses the use of preset moorings, how they work and why they are effective.

The definition of a preset mooring is a system where the majority of off-vessel components are installed prior to arrival of a mobile offshore drilling unit (MODU) on location. Generally, the components will consists of a wire/chain combination, but could include submersible buoys (discussed later in this article).

The components are typically buoyed off and sequentially connected to the rig using one or two anchor handling vessels (AHVs). Generally, the existing onboard chain or wire is connected to the preset moorings.

Until recently preset moorings have only been used with permanent mooring applications for floating production systems (FPS). Since 1994, however, experience has been acquired at several locations in up to 3,500 ft in the Gulf of Mexico with a second generation MODU, the Victory Class Ocean Voyager (on term contract to Enserch Exploration), using a specially designed eight-leg preset combination wire/chain system.

The system was designed to survive a 10-year return period hurricane and demonstrated considerable operational improvement over more conventional second or third generation rigs. No mooring equipment or component upgrades were needed on the rig itself, and only one 6,140-brake horsepower (BHP) AHV was required for the preset mooring installation work.

Since only one preset mooring was used (for initial capital cost saving reasons), movement of the rig from one location to another took approximately twice the time expected compared to moving a fourth or fifth generation rig in the same water depth.

Older rig opportunities

A philosophy similar to that used by Enserch for the Ocean Voyager could be applied to a number of older generation rigs, allowing mooring in water depths beyond existing capabilities. A particular rig's depth limit is probably governed by the size of the existing onboard components. The deeper a rig is moored, the higher the required operating tension will be, in turn reducing the extent of available safety factor. For older generation rigs this has been found to be a depth of about 4,500 ft.

Because of the current interest in deepwater exploration drilling, and the demand for deep water capable MODUs, several operators in the Gulf of Mexico and elsewhere are seriously considering the preset mooring option.

Experience gained from the Ocean Voyager program has indicated that two sets of preset moorings should be used. With two moorings, the second mooring can be pre-deployed off the critical path, thus considerably reducing the time for MODU disconnect at one location and connection of it at the next.

For a relatively short term contract (one or two wells), it would be difficult to justify the initial capital cost for two mooring sets unless component leasing is possible. For a contract of one year or more, the overall economics (using newly procured preset mooring components) are competitive compared to a newer generation conventionally moored MODU.

A table accompany this article shows the results of a cost analysis comparing a second generation MODU using preset moorings vs a fifth generation MODU conventionally moored. This analysis is based on a four well program for one year in 4,500 ft of water.

The difference in the day rate between the two MODUs is assumed to be $30,000 per day. It is also assumed that no modifications will be made to the onboard mooring equipment on the second generation MODU, but other necessary drilling upgrades have been completed. Extra time and cost for moving the fifth generation MODU is included in the analysis, but no consideration was given to the extra loss in production days due to the extra moving time.

Single AHV required

An important feature of the preset mooring option is that only one AHV will be required to install the components. At least two of the latest type of AHVs, equipped with large main anchor handling winches and over 12,000 BHP, are required for a conventional deepwater mooring operation.

This is critical considering the current shortage of new generation AHVs (especially in the Gulf of Mexico). The main requirement for the preset mooring installation vessel will be the provision of a large main anchor handling winch and considerable wire and chain storage capacity. The vessel will not necessarily require high horsepower.

A second important feature is for the major part of the mooring operation to be completed off the critical path with minimal MODU downtime. The disconnect and reconnect operations are simple, less likely to have complications, and have less potential for weather downtime. All of these features translate to overall cost savings.

Extending depth limits

Analysis has established that the use of preset moorings (including steel wire rope) with more modern fourth and fifth generation MODUs can considerably extend water depth limits (Part I in April issue). Without using submersible buoys in the system, the limit is probably about 6,000 ft. By adding submersible buoys, this limit can be increased to 7,500 ft.

In order to cope with the size of submersible buoys, it is considered preferable to use several in-line buoys in each leg. An A-frame will be required on the AHV for handling the buoys. The use of taut leg moorings (TLM) will also benefit a preset system by reducing the amount and scope of components and operating condition watch circles.

All these features can principally only be achieved with preset moorings. It should, however, be noted that the use of thrusters to supplement conventional moorings can solve many of the operating problems but cannot extend a rig's survival capability if unmanned in areas like the Gulf of Mexico.

Taut leg moorings

Because of the ability to use a TLM in a preset mooring design, there are several operators worldwide receiving a considerable amount of attention. Obviously, the major issue is how will it be possible to provide an anchoring system capable of withstanding vertical loads.

Piles, either driven or drilled and grouted, have provided the traditional answer to anchors for vertical load applications, but the cost of installation and the inability to reuse them makes piles unattractive for temporary MODU applications.

Driven piles also have a problem in that they are depth limited by the current state-of-the-art in underwater hammer technology (estimated to be about 4,000 ft without considerable development).

Recent joint industry project (JIP) research, conducted by AMC between 1990 and 1995, has established that suction embedment anchors and new types of drag embedment anchors (known as VLAs) offer potential alternatives to piles for TLM applications.

The VLAs, which are essentially large prefabricated flukes embedded at one angle then tripped to allow holding capability up to 90!, have been extensively tested in laboratory and small scale application offshore. Larger scale testing has been executed in the Gulf of Mexico and Brazil with some success. They were recently selected for a floating production application in Brazil. It is the opinion, however, of the authors that further work is required to develop economic and effective ways to install them, especially for removable applications.

Suction embedment anchors

Suction embedment anchors, first introduced in the late 1970s, have recently received considerable attention and have been used for several permanent mooring applications. Essentially, they consist of large diameter piles, enclosed at the top, that are lowered to self-penetrate into the seabed, and are then embedded by evacuating water from the interior with a special ROV-mounted pump skid.

Suction is not used to supplement ultimate holding capacity. A major question has been how they could be used for a MODU application. In 1996, two operators - one in the North Sea and one in the Gulf of Mexico - successfully performed full scale testing of the installation and recoverabilty of suction anchors deployed from large AHVs, proving their feasibility for recoverable MODU moorings. It was further established that large AHVs could be used for the installation operations.

Synthetic ropes

Synthetic rope has been favored by deep water mooring designers for many years. Considerable research and development of aramid rope construction types was conducted in the 1980s, but the downturn in exploration and production in that period stalled interest.

The advent of taut leg mooring, where stretch in the synthetic rope section of each mooring leg is critical, promoted keen interest in polyester rope in the early 1990s. For years, polyester has been used in the marine industry in the form of single point mooring and tow hawsers. It exhibits a good combination of durability and strength and can be fabricated in both parallel-strand and multi-strand rope constructions, with both rope constructions having the ability to be made in up to 1,000-ton break strength capacity within the current state-of-the-art.

As indicated earlier (Part I), polyester can substantially extend the mooring ability of all MODU types, perhaps beyond 10,000-ft water depth limitations, when used in preset moorings. Testing of polyester mooring systems has been performed, and is on-going, in the Gulf of Mexico, North Sea and Brazil.

AMC installed a single leg TLM containing several polyester rope types late in 1996 for the DeepStar Project. The system will remain in place for two years. Brazoil is understood to have made a decision to install a floating production system using polyester later in 1997, and is considering its use for MODU application, also in 1997.

A key issue in the use of polyester in the MODU mooring industry is in the handling for multiple installations. Special handling equipment in the form of rotary or traction winches will have to be developed and more testing envisioned. Durability is also a concern, however, the authors believe the steps already being taken by the Brazilians will shortly be followed by operators in the Gulf of Mexico and the North Sea.

In summary, it is clear that in the near future preset moorings will play a major part in the mooring of both old and new MODUs to be used for deepwater drilling and exploration. This impact could result in considerable overall cost savings for operators.

The application of new anchor and synthetic rope technology, as briefly discussed, will have a significant impact on the depth limits at which moorings can be set and is expected to provide an attractive and economic alternative to the use of dynamically positioned vessels.

REFERENCES: Ettle, R., Bamber, P., Dove, P., Wilson, H., "Mooring an Ocean Victory Class MODU on a Pre-laid Mooring in 3,200 ft of Water at Green Canyon 254", Offshore Technology Conference (OTC 8160), 1996.

Dove, P., Fulton, T., Devlin, P., "Installation of DeepStar's Polyester Taut Leg Mooring," Offshore Technology Conference (OTC 8533), 1997.

AUTHORS: Peter Dove is the president of Aker Marine Contractors, a company specializing in the design and installation of deep water moorings. Over the past ten years, he has been actively involved with all the major mooring projects in the Gulf of Mexico and has been instrumental in advancing the state-of-the-art in this area.

Tom Fulton is a senior staff engineer and ocean engineer with Aker Marine Contractors. He has been instrumental in the development of new anchor designs, taut leg moorings and suction anchor test programs. He has extensive experience with deepwater mooring installations.