DEEPWATER MOORING: Mooring challenges increase with deepwater, rough locations

Nov. 1, 2001
Fiber ropes under study for Ormen Lange
Example of shear-strength profile for deepwater site with very soft clay (Source: Norwegian Geotechnical Institute).
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Norsk Hydro is contemplating use of synthetic fiber ropes to moor the production platform on its deepwater Ormen Lange development off mid-Norway. This is a giant gas-condensate field in water depths ranging from 700-1,000 meters, and prone to fierce winds, waves, and ocean currents. Water temperatures are low at as high as 300-400 meters above the seabed. An additional problem is the uneven seabed caused by the Storegga submarine slide.

Although a development plan is a ways off, steel catenary risers (SCRs) will likely be employed in the gas export solution. But, large diameter SCRs (2 x 24-in. or higher) entail a strict platform offset to accommodate bending stresses at the touchdown point. Norsk Hydro believes that a polyester mooring might be the most economic way of reducing offsets. Also, a taut leg mooring system might be influenced less by the seabed irregularities.

The firm commissioned Noble Denton Europe to review the potential of fiber ropes for the project. Work to date was outlined in a paper by authors from Noble Denton and Norsk Hydro at IBC's recent Risers, Moorings & Anchorings for Deep Waters conference, held in London.

According to the authors Valerie Quiniou-Ramus, Richard Stonor, and Tom Marthinsen, there have been at least 14 research projects or organizations analyzing fiber rope moorings since 1992. Petrobras' Procap 2000/3000 and DeepStar are the best known. Key issues under review include rope behavior and material properties, design, testing, installation, and maintenance.

More tests needed

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Most of the projects have derived data on rope behavior and material properties from laboratory testing of small-scale rope samples - due perhaps to the limited capacities of the available testing machines. Data on the performance of large-scale polyester ropes is therefore limited, although 1,500-ton capacity ropes have been tested satisfactorily in Norsk Hydro projects, conducted by Tension Technology International and the UK's National Engineering Laboratory in Scotland.

The latter, and Norwegian classification group Det Norske Veritas, have machines capable of testing samples up to 2,000 tons capacity. The authors believe that there should be more tests of high strength ropes at full scale during qualification. However, it may prove hard to find machines with sufficient load capacity, bed length, and loading stroke for adequate testing.

So far, most of the operating experience with deep-water fiber moorings has come from warm water locations with mild metocean conditions. Ormen Lange's cold, harsh environment may necessitate significantly higher strength ropes than those currently favored.

Cold water application

The authors view durability as the major issue that needs to be addressed, citing the need to formulate methods of identifying damage caused by minimum tension and soil ingress. Axial compression fatigue could be another concern at Ormen Lange, due to the high loading range in the mooring lines.

The length of bottom chain required to avoid contact between the rope and seabed may also serve to maintain minimum tension in the lines - but this minimum allowable level remains uncertain, and is derived from limited experimental data. They add that the requirement to provide bottom chain lengths for fiber rope moorings also influences the strength of the fiber rope selected.

Current maximum chain strength available is around 2,700 tons. Due to the higher fatigue sensitivity of chain in relation to fiber moorings, there will be a need for chains with a higher nominal break strength than that of the fiber rope component.

In addition to standard class society test requirements, fiber ropes for Ormen Lange should be qualified for:

  • Equipment with a suitable filtering system, with assessment of the latter's capacity to handle particle ingress
  • Ability to withstand a specified minimum level of axial compression cyclic loading.

Other issues

Even if rope/seabed contact has been "designed out" through use of a chain segment at the base of the mooring lines, the ropes can still be accidentally dropped to the seabed during installation. In anticipation, ropes could be provided with soil filters, with tests conducted to identify the likely strength reduction that might be incurred based on a representative Ormen Lange soil composition. These tests should employ full-scale rope specimens, with the same specimen configuration for both break and fatigue tests.

The samples would be immersed in a tank with circulating muddy water under tension-loading conditions. Following particle ingress analysis, laboratory fatigue, and break tests could be conducted to derive the effect of residual abrasion on endurance and strength. Finally, scanning electron microscope magnification of the rope components would show whether particles have passed through the filters, the extent of their ingress, and potential damage that may ensue. Axial compression is a critical consideration in harsh environments, but little data on this subject is currently available.

A further issue for Ormen Lange could be quantification of tension-torsion properties, once the mooring line composition is known. Analysis should take into account the influence of the installation methods as well as the final installed configuration, the authors state. Where possible, torque-balanced components should be chosen to limit the detrimental effects of twist on the ropes' fatigue performance and general integrity.

Wire rope dilemma

John Yeardley of Scanrope, in his paper on mixed moorings at the same IBC event, explained that although chain exhibits no tendency to twist when an axial load is applied, this is not the case with wire rope.

Twisting the rope in the direction of the lay will form a loop, he stated, which, if pulled out under load, will develop into a kink. At first glance, this might not appear severe, but the structural deformation incurred will impair rope strength and fatigue performance significantly. Likewise, a rope allowed to rotate, and then lose turn under load, is likely to form an open kink (or hockle), if the load is suddenly released.

These rotational effects arise from the helical nature of the rope and strand construction commonly applied to wire ropes. An axial load applied to a rope with both ends attached to fixed points will induce an internal torque force. Where one end of the rope is free to rotate, this torque force will cause the rope to rotate until its structural geometry has adjusted to restore zero torque.

This rotational tendency can be reduced in multi-layer wire ropes, Yeardley added, where alternate layers of strands are laid in opposite directions. Although this technique suppresses torque generation, it also limits tolerance to an externally imparted twist, leading to distortions known as "birdcages." These, in turn, lead to foreshortening of the inner strands and simultaneous lengthening of the outer strands, thereby disturbing load distribution and impairing rope strength.

Yeardley added that laboratory tests at the University of Reading, UK, showed that tension/torsion fatigue life for a six-strand wire rope with one end attached to a swivel - and therefore free to rotate - could be less than 5% of the fatigue life of a sample tested with both ends fixed. Swivels are often used with wire ropes to control rotation of suspended loads during handling situations, but could have disastrous effects if used for long-term mooring.

Mixed moorings

In light of this evidence, he said, designers should look more carefully at suitable combinations for mixed moorings. If installed correctly, a spiral strand or parallel core fiber rope attached to a suction anchor at one end and a length of chain at the other should not encounter rotation-induced problems. But a length of spiral strand attached to a six-wire rope could be a recipe for rotational calamity.

Operating depths of mobile drilling rigs can be extended significantly by adding lengths of lightweight fiber rope to the existing mooring system. However, if this involves connecting wire and fiber ropes, there are torsional fatigue implications. Tests at Reading involving constant cyclic rotation of the connection between combined wire and rope systems led to fatigue and premature failure of the wire rope. Problems also came to light during trials of a combined wire/braided rope mooring.

Yeardley claimed that wire/fiber rope combinations could work if the torsional characteristics of the two mooring components were mutually compatible. For example, if the torque generated by each segment at the joint between the two is the same, no damaging rotation should result. While a perfect match is virtually impossible, a close approximation can be achieved.

During tests by ScanRope at Tonsberg, Norway, three-meter-lengths of six-strand wire rope and numerous strength-matched fiber ropes were connected with the outer ends fixed. The rotation of the mutual connection was monitored during loadings to 50% of the rope breaking load and during cycling between 30-50% load. Dramatic differences in rotation were observed when the wire was attached to a torque-matched fiber rope as against a parallel core type. With polyester ropes, the rotation under cyclic conditions fell from over 90° to virtually zero when a torque-matched construction was adopted.

That difference was even more marked when applied to wire rope/HMPE combinations, due to the HMPE ropes' smaller diameter. In an experimental case using three-meter rope lengths, the resultant 90° of rotation would be equivalent to more than 16 full turns on a 600-meter-length of 84 mm anchor wire, which could pose problems during a full-blown storm. But with a torque-matched combination, much less than one turn would occur, Yeardley claimed, minimizing tension/torque fatigue in the wire rope.

Seabed clashes

Jarle Andersen of Structural Engineering and Lars Hilmersen of Aker Marine Contractors spoke of their efforts to avoid suspended chains contacting seabed facilities during maintenance or interventions. In association with one Norwegian sector operator, they have designed polyester fiber rope inserts for use with mobile offshore drilling units (MODUs)on several fields containing templates and other subsea installations.

The ropes used were remnants from another recently discontinued project, with a capacity of 1,000 tons and a length of around 800 meters. This, they claim, provides an adequate safety factor when inserted into a traditional mooring line for an offshore drilling unit, also catering for the effects of seabed exposure and rough handling during installation/retrieval. Both these actions will be investigated in tests planned at Det Norske Veritas' laboratory in Bergen. Currently, three of eight mooring legs on one drilling rig have had fiber rope inserts. Integration from the chain end consists typically of a kenter link (five 3-in. links), an enlarged link, and a wide body shackle or anchor-type shackle/thimble into the rope soft eye.

When the rope is connected to another rope, the team employs either a thimble, anchor shackle, anchor shackle, thimble sequence, or a wide body shackle, large link, wide body shackle combination.

Anchorhandling tugs and supply vessels are used for deployment and retrieval. In the case of anchorhandlers, ropes are spooled from the storage reel to a large capacity drum under tension. When deployed offshore, the fiber ropes are connected to the mooring chain on the deck and eased out over the stern roller. Stopping the rope for disconnecting the forerunner or connection to the mooring chain is effected using a 30-ton strength fiber rope sling or by securing the five-link adapter in the shark jaws.

When the rig moves infield, ropes not used for tow are retrieved and spooled onto the anchorhandler drum while the others are used for station keeping and towing. On arrival at the new location, the rope is re-installed into the mooring line from the winch drum. Each time this happens, the rope is visually inspected for abrasion, damage, or traces of seabed clay.

Transit times from one template to another were not significantly greater than operations using regular mooring chain. Experience suggests inserting fiber ropes in a conventional chain mooring presents a reliable, efficient method for passive protection of subsea structures.

Marlin taut-leg

Another paper by Evan Zimmerman, Matt Smith of Delmar Systems, and John LeBlanc of BP outlined the latter's experience early this year on the Marlin Field - site of the first pre-installed taut-leg mooring system in the Gulf of Mexico.

Marlin has been developed through a tension leg platform in the northern part of the field, with subsea satellites connected to the tension leg platform (TLP) complex through the King and Nile pipelines and umbilicals to the east and west.

These lines were the main constraint to a second set of wells being drilled 4,500 ft southeast of the TLP, necessitating a mooring pattern for the drilling rig within a 4,000 ft radius. Conventional self-contained catenary moorings were not an option given the water depth of over 3,300 ft. The only solution, therefore, was a taut-leg configuration.

Design and engineering for the taut-leg mooring was completed in September 2000. The initial aim was a system suited to most eight-leg MODUs capable of drilling in this depth. Initially, the fourth generation semisubmersible Trans-ocean Sedco Forex Richardson was targeted.

Delmar-designed suction piles have been installed and retrieved more than 130 times in the Gulf of Mexico over the past three years. The original 12-ft-diameter, 60-ft-long piles provided adequate holding capacity for larger fourth/fifth generation semisubmersibles, but the optimum uplift angle of 21°-23° was not suited to Marlin, where 38° was the estimate.

Instead, a 9 1/2-ft-diameter, 70-ft-long design was adapted from a 9 1/2 ft by 60 ft pile used earlier on BP's Mad Dog II development. The extra 10 ft in length, along with the mooring padeye location 20 ft from the bottom of the pile, ensured the required optimum uplift angles would be achieved, depending on soil strength profiles.

An all-steel wire mooring system, as applied for earlier suction pile installations, would have been too stiff, failing to absorb the drilling vessel's dynamic motions in these depths. This was not the case with a taut-leg configuration, which relies on the elasticity of the mooring components to absorb the dynamic tension fluctuations caused by the vessel motions.

In Marlin's case, polyester mooring inserts would have provided the requisite elasticity, but the tight project schedule militated against fabrication of the polyester rope. Also, previous experience suggested discomfort arising from use of polyester ropes in a mooring system stationed close to a major production platform. The chosen solution for absorbing these motions was a catenary shape, using standard 3 1/4-in., R4 chain as the bottom mooring component to provide a dip in the catenary shape. High strength 3 5/8-in. wire rope was also selected as a lighter weight component, which would be suspended by a 50-kip submersible buoy. The latter would provide a small peak in the shape of the line when connected to the MODU's rig wire.

BP's drilling schedule then changed, leading to selection instead of the Noble Jim Thompson semisubmersible to drill the well late-February 2001. Mooring design verification analysis was handled by Noble Drilling.

Two yards were awarded fabrication contracts for the suction piles under Delmar's supervision last September. The first batch was delivered to Delmar's base in Port Fourchon, Louisiana in December. All nine suction pile anchors and mooring legs were to be pre-installed on location in Marlin before the rig's arrival on January 1.

A vertical installation procedure was chosen for the suction pile anchors as this presented minimal risk to the in-place subsea facilities on the sea floor. The purely vertical orientation would also minimize the risk of a pile being installed out of a 5° window of verticality. Use of a single vessel for pile installation also cut costs/coordination requirements. The chosen vessel was an anchorhandler with onboard ROV.

The installation sequence was as follows. The suction pile anchor was hooked from its over-boarding padeye to one of the main winch drums with a gravity release over-boarding hook. Tugger wires were then fairleaded round the vessel and secured to the sides and/or head of the pile - this provided a force to pull the pile toward the stern roller. As the pile approached the roller and started to tilt, load transfer to the overboarding line was effected. The pile began to submerge underwater while venting air inside out through the butterfly valves on top of the pile.

At a position 200-300 ft below the anchorhandler, the pile was suspended in a stable position at an 11° angle. Concurrently, the ROV was jumped with a line from another main winch drum then lowered with the female portion of the subsea connector. The ROV attached itself to this, pushing the open throat to the lowering stern and male subsea connector on top of the pile. Once the female connector had contacted the lowering stern, the winch picked up on the lowering line, and the pile took on a vertical orientation as the lowering line assumed the load. The overboarding line was then slacked off and the gravity hook released.

The next step was to lower the pile to the seafloor, an operation which can take up to three hours in deep water, depending on the winch. In this case, the ROV was deployed simultaneously, diving beyond the pile to the seabed to verify water depth. Once the pile was 50 ft from the sea floor, an ROV identified it visually, following it down to the mudline.

Pile orientation was performed by the vehicle using a stabbing guide, and the pile was then lowered and allowed to self-penetrate under its own weight. Subsequently, the butterfly valves were closed and the ROV pumped the pile into place. At the same time, the ROV monitored verticality, pump differential pressure and insertion rate.

Having been installed, the pile could either be left in situ by disconnecting and retrieving the lower line, or else the mooring line could be connected and pre-set. The ROV managed the latter by disconnecting the female part of the subsea connector from the lowering stern and transferring it over to the mooring stern. Next, the mooring stern was connected to the mooring padeye, now 45 ft below the mudline and extending above the top of the pile with the male part of the subsea connector. Finally, the mooring leg component was installed and buoyed off, awaiting the rig connection.

The size of the suction piles and anchorhandlers allowed three complete mooring legs to be pre-installed during one vessel trip - all nine pre-sets were therefore effected from three trips. However, four trips were required to install all nine legs and two suction pile initiation piles. The anchorhandler was demobilized on January 1, with the Noble Jim Thompson completing its final mooring line connection on February 27.

Other suction applications

In another paper on suction pile technology, Per Sparrevik of the Norwegian Geotechnical Institute commented that suction penetration was a controlled and smooth process, therefore most incidents that do arise relate to subsea/handling operations.

Normally, the suction pile revolves back and forth in a cycle caused by heave motions when suspended above the seabed. The small moment on the anchor may be induced by relative movements of the mooring line or by torque in the lifting wire. Consequently, minimal side friction is needed to dampen the pile's cyclic rotation. Surprisingly, suction piles have been observed to rotate also during deep penetration.

That rotating tendency stops when the suction penetration ceases. It is possible, therefore, that small moments caused, for example, by the mooring line can trigger rotation of the anchor. This may be because all side friction is mobilized as penetration resistance as the pile moves downward, leaving virtually no resistance against rotation.

Sparrevik mentioned skirted foundations for subsea templates. These have been used on numerous installations in the North Sea, and have also been applied on the subsea structures for the deepwater Girassol development off Angola. Another potential application could be the artificial seabed, a concept involving wells extending up from the seabed to a submerged buoy or platform several hundred meters below the sea surface, also housing Christmas trees and wellheads. The buoys could be fixed to the seabed using suction anchors, to create a submerged, mini-TLP.

Drilling operations could also benefit. Pre-installed conductor and wellhead supports can save rig time. In very soft deepwater soils, the conductor, or center pile, could be installed using suction, Sparrevik said. A suction caisson could be mounted around the upper part of the center pile with the dual aim of adding driving force during final penetration and providing lateral support at the seabed during operation. This installation method could provide better sealing and less disturbance than traditional installation methods based on jetting.