Why mobile drilling units will be able to moor in 10,000 ft depths

Conventional existing and planned mooring arrays, on the left, compare with new mooring arrangements for water depths up to 10,000 ft. on the right. Taut mooring lines on the right allow for smaller spreads, compared with catenary lines on the left. A two-AHV vessel anchor recovery operation is designed to minimize loads during an anchor unseating operation. A wire insert scheme for a MODU mooring using two AHVs is shown. The required engineering includes the analysis of different load cases

Pre-set arrays, suction anchors, synthetic rope are changing conventional mooring concepts

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

  • A two-AHV vessel anchor recovery operation is designed to minimize loads during an anchor unseating operation.
  • A wire insert scheme for a MODU mooring using two AHVs is shown. The required engineering includes the analysis of different load cases involved with the deployment and recovery operation using specially developed software.
PART I: Part one of this two-part series covers the subject of conventional MODU mooring and installation. Part II, which will appear in the May issue, will present details on the use of pre-set moorings and how the use of new technology can considerably extend the water depth capability of moored MODUs.

The recent explosive growth in deep water drilling and production operations in the Gulf of Mexico has posed new technical and commercial challenges to operators. Exploration and delineation drilling is now taking place in water depths up to 6,000 ft, and planned for 7,500 ft depths and beyond within the next two years. Production facilities will shortly follow.

Foremost of these challenges is the availability of suitable mobile offshore drilling units (MODUs) with the ability to remain on station and drill year around, while surviving seasonal tropical storms and hurricanes, often in an abandoned state. While there are a number of factors other than stationkeeping that limit vessel capabilities (riser and BOP control technology), this article addresses only the stationkeeping issues.

While there are several second and third generation MODUs that can moor in water depths up to about 2,000 ft, some with all chain moorings and some with combination wire/chain components, only a handful of the fourth and fifth generation units are able to operate in water depths up to about 5,000 ft and beyond.

Conversion work, currently underway or planned, will stretch the depth limit of some of the older MODUs to an extreme of perhaps 4,500 ft. The addition of wire or chain components to existing mooring systems will also stretch individual unit capabilities (see accompanying figure), but these additions can create operational problems (addressed later in this article) and have led to considerable rig downtime and cost escalation in drilling budgets.

Mooring conceptions

A preconceived notion among many operators and drillers is that once the above-depth limits are approached or exceeded, the only solution in the continuation of ultra-deep drilling is to use dynamic positioning (DP) for stationkeeping. This notion has led several major operators and drilling contractors to invest huge sums in the development of new generation monohull DP drillships. It is understood three new vessels are currently being built and two are being converted.

While the technology for floating DP operations clearly has been proven, the long-term economics may not be so clearly defined, especially until further deepwater field experience has been gained. A number of operating and risk factors must also be addressed in Gulf of Mexico drilling operations, not the least being the overall economic and environmental impact of a potential blowout or riser loss due to a drive-off or emergency riser disconnect.

This factor cannot be over-emphasized in light of the high well pressures and flow rates currently being experienced or anticipated at Gulf of Mexico deepwater tracts. A well loss could lead to an event that dwarfs the environmental impact of the Valdez incident and creates a critical political scenario that could impact all deepwater development.

Pre-set moorings, a key element of permanent mooring systems for floating production platforms, were recently used to successfully moor a second generation rig, normally depth-limited to about 1,500 ft, at several locations in over 3,000 ft in the Gulf of Mexico. The application of preset moorings, coupled with new technology such as taut leg moorings (TLMs), suction anchors, and synthetic rope, can provide a cost effective and safe alternative to DP vessels and considerably extend the stationkeeping capability of all types of MODUs.

An accompanying table summarizes the performance criteria of a fifth generation MODU operating in extreme depths of 6,500 ft, comparing conventional wire/chain mooring with steel TLM (using buoys) and a synthetic rope TLM (without buoys).

Conventional mooring

For the purposes of this article, conventional moorings as related to MODUs are defined as those that can be deployed from the MODU while it is being held on station at site. The components used in these moorings will typically consist of either chain, wire, or a combination thereof, secured to the seafloor with high holding power anchors bolstered in the area where the columns interface with the pontoons during rig transit operations.

Generally, eight mooring legs are used. Chain sections are stowed in lockers, usually in the columns, and during deployment are led over windlasses on the column tops. Wire sections are stowed on rotary winches mounted either on the column tops or, in certain units, in columns or lower pontoons. Traction winches are used to deploy wire sections from rotary storage drums on some of the more modern units. Fairleads for both wire and chain are included at the lower column area, usually above the anchor bolsters.

Factors limiting the size and quantity of chain carried on the units include chain locker capacity and the size and number of pockets included in the chain whelps (wildcats) on the windlasses. The limitations for wire are in the storage capacity on the rotary winches and the size of traction winches (if applicable) or fairleads.

It is important to understand the issues governing the extremes under which MODUs can moor to allow drilling operations to be safely conducted.

Mooring criteria

The first and most important issue is the ability of the unit to remain on station during a design storm. What is a design storm? It has been stated (second reference) that for MODUs a ten-year return period storm should be used if the unit is moored within the vicinity of other structures, and a five-year return period storm if moored away from other structures. Criteria that impact this survivability include limiting acceptable line tensions and line scope during the design storm. It is customary to abandon a MODU at the approach of a hurricane or potential design storm in the Gulf of Mexico. Prior to abandoning the rig, it is customary for operating crews to slack down mooring pretensions, resulting in reduced line tensions.

The second most important criteria is the ability of the MODU to stay within acceptable watch circle limits, while continuing to carry out drilling operations. Typically, this is governed by the extent that the drilling riser can tolerate angle at the lower ball joint, which is influenced by a combination of available riser tension capability and mud weight (as determined by an appropriate riser analysis).

Even with a combination wire-chain systems, this limiting operational watch circle in deepwater cases can prove to be restrictive, especially in loop or eddy current conditions, frequently resulting in the requirement for adjustments in line tensions to maintain the MODU within tolerances. With all chain systems, as used in the older generation rigs (in water depths up to 1,500 ft) the operating watch circle criteria has been critical and in loop or eddy current conditions has in some cases resulted in tugs being used to supplement the moorings for days at a time.

A third criteria is the ability of anchors to hold without slipping. While in the past this has been critical with older style anchor, the introduction of the fixed fluke twin shank anchor types (Stevpris and Bruce) has greatly enhanced anchor-holding capacity, especially in the Gulf of Mexico, and has resulted in fewer anchor holding problems.

Deployment

MODU mooring system deployment at Gulf of Mexico deepwater locations requires considerable engineering and planning to ensure a safe operation. Previously, it was sufficient to deploy mooring systems using older generation anchor handling vessels (AHVs), manned with marine and anchor handling crews with the minimum of preparation and planning.

For the deepwater locations, the high loads imposed on the AHV equipment and rigging, coupled with the complication of adding wire or chain sections to extend MODU stationkeeping capability, necessitates detailed procedure development. The required engineering includes the analysis of different load cases involved with the deployment and recovery operation using specially developed software.

The potential for substantial downtime and cost overruns caused by a dropped mooring line or AHV failure has led many operators to plan and engineer the mooring operations in advance and has served as the basis for introduction of new generation AHVs with high horsepower and large capacity winches.

Ultra-deepwater mooring

A number of problems have to be addressed during ultra-deep water anchor deployment and recovery operations. First, AHV bollard requirements during an anchor run-out have to be considered to ensure the correct amount of mooring scope can be obtained, and the potential for chain or wire chafing on anchor bolsters is avoided or minimized. [More detail on bollard pull in Part II.]

Second, that anchors can be recovered using permanent chain chaser (PCC) wires. Experience has shown that anchor recovery in deepwater using PCCs has led to chain failures close to the anchors where deep burial occurred.

Third, ensure sufficient winch capacity is available on AHVs and, finally, that the change-over operation from a combination wire-chain system is conducted in a safe manner.

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," OTC 8160, 1996.

  • "Recommended Practice for Design and Analysis of Stationkeeping Systems for Floating Structures," API RP 2SK, First Edition, June 1st 1995.

Authors

Peter Dove is the President of Aker Marine Contractors, a company specializing in the design and installation of deepwater 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 senior staff engineer and a degreed 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 deep water mooring installations.

FactorsCase 1Case 2Case 3 -
conventionalsteel TLMpolyester/
wire/chainw/buoyssteel TLM
20-yr working load (kips)350150350
20-yr max. load (kips)743762765
20-yr min. FS (kips)2.122.072.06
20-yr max. offset (%WD)10.116.33.1
20-yr mooring stiffness (kips/ft)1.912.224.15
20-yr max. vertical load (kips)2,5491,1992,474
5-yr working load (kips)350350350
5-yr maximum load (kips)550711593
5-yr min. F.S. (kips)2.862.212.65
5-yr max. offset (% WD)72.91.8
5-yr mooring stiffness (kips/ft)1.213.365.42
5-yr max. vertical load (kips)2,4071,6732,293
TLM = taut leg moorings; FS = factor of safety; WD = water depth.

This table summarizes performance criteria of a fifth generation MODU operating in extreme depths of 6,500 ft. A conventional wire/chain mooring is compared with a steel TLM (using buoys) and a synthetic rope TLM (without buoys).

Copyright 1997 Oil & Gas Journal. All Rights Reserved.

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