Experience with installation of deepwater polyester moorings, new generation anchors

Fleeting knife polyester deployment winch.

Interest in ultra-deepwater stationkeeping for floaters using taut leg moorings (TLMs), polyester rope, and new anchor types capable of withstanding vertical loads, appears to be gaining more focus on both sides of the Atlantic Ocean. Aker Marine Contractors (AMC) has both organized and been involved with numerous research, development, and study projects involving these technologies. These joint industry projects have advanced deepwater mooring and allowed AMC to learn much about the technology.

The work started in the late 1980 as a result of a clear need for synthetic ropes, weight reduction, cost savings, and improved mooring performance. An initial study of deepwater catenary moorings (CM) led to taut leg moorings (TLMs), which in turn lead to the development of new anchor types capable of withstanding vertical loads. The early work focused on aramid fiber (Kevlar), but with recognition of TLM stretch requirements, interest later switched to polyester.

Field experience was gained with the design and installation of a single leg TLM, using various polyester rope types, which was left in place offshore for about two and one-half years. Engineering and testing of conventional anchor types led to a better understanding of anchor design and resulted in development of vertically loaded anchors (VLA), recoverable suction embedment anchors (SEA), and AMC's proprietary suction embedded plate anchor (SEPLA). Deployment and equipment experience was gained in the handling of very large diameter polyester ropes, followed most recently by a full-scale test and installation of a SEPLA anchor. During 1999, AMC installed several test sections of polyester rope in MODU mooring systems in the Gulf of Mexico and the North Sea.

Initial operator interest in mooring systems using synthetic ropes was focused on permanent installations. The Brazilians led the way in this regard with the installation of up to ten offshore facilities using torque-balanced construction polyester rope. Outside Brazilian waters, concern over the durability and longevity of polyester rope in a marine environment hindered industry acceptance. Acceptance of pre-set moorings by the drilling industry, recognition of the high cost of new generation drillships, and the development of TLMs employing polyester rope and anchors capable of withstanding vertical loads have recently sparked great interest in using pre-set polyester TLMs for mooring semisubmersible MODUs in ultra-deepwater.

Many operators feel the drilling industry will provide a useful arena to test deployment procedures for polyester rope and satisfy durability and longevity concerns for the more permanent applications. Additionally, while many operators believe that dynamically positioned (DP) drilling vessels will be preferred for ultra-deepwater exploration, many operators are also stating their preference for using moored vessels for development and, possibly, delineation drilling.

Outside Brazilian waters, polyester TLM technology is expected to be adopted in the short term by operators with ultra-deepwater drilling programs. These operators will most likely be the first to introduce polyester TLM technology for permanent applications.

Rope handling

Elevation view of types of Taut Leg Moorings.

Field experience obtained from Brazil and the Gulf of Mexico has shown that large diameter polyester rope sections can be readily handled, deployed, and recovered without damage. The polyester section lengths are first governed by the manufacturer's ability to handle, load, and transport the shipping reels to the operator's or installer's base port, and second, by the installer's ability to deploy the sections on the deployment equipment.

Generally speaking, with large diameter ropes (150-180 mm diameter), the lengths will currently be restricted to around 1,000 meters. Length restrictions may, however, be relaxed in the future if the rope manufacturers decide to build fabrication facilities with direct access to the sea or waterways where reels can be directly loaded onto barges without the need for road transportation.

A major cause of concern affecting the design and installation of polyester ropes occurs when ropes are allowed to lay on the seafloor (either intentionally or because they have been dropped). Ropes that have lain on the seafloor have been subjected to the ingress of sand and or clay particles into the rope construction. As a result of this ingress, rope samples have demonstrated a strength loss of 20-40%. Laboratory abrasion tests have also demonstrated this phenomenon.

While methods of rope jacketing and improved construction techniques may reduce strength loss resulting from the ingress of particles, it is considered (at least for the moment) that mooring designers must prevent polyester rope sections from coming into contact with the seafloor under all circumstances during installation and operations phases. After anchor system installation, prior to attachment of the floater to the mooring legs, the polyester rope sections must always be suspended off the seafloor with a suitable surface or subsurface buoy. This will require using a length of wire rope or chain as a forerunner and will limit the length of polyester to something less than the water depth.

The alternative solution will be to attach required lengths of polyester rope that exceed the water depth at the time of vessel attachment. While the latter technique might be acceptable for a one-time deployment associated with a permanent mooring, it will require special procedures and equipment for a MODU mooring operation to avoid excessive hookup time.

Experience has shown that large diameter polyester rope sections can be readily deployed without damage off the drum of a typical large modem anchor handling vessel's (AHV) main winch or possibly spooling winch, if so equipped. Experience has shown that the minimum drum diameter (D) for a polyester handling winch should not have less than a 6-10:1 D/d ratio, where d is the diameter of the rope, depending on the tension. It is important to state that the polyester deployment operations should be effected under a relatively light load unless a capstan or traction type winch is used. This is necessary to prevent the polyester rope from digging into lower layers on the drum. It also means the polyester should not be supporting long lengths of chain and/or wire during deployment (lengths which result in tensions exceeding about 50 tons).

Each polyester section will first be spooled off a shipping reel onto the anchor handling winch drum. After deployment of the rope section, the next section will be loaded on the deployment drum. This solution, which has been successfully proven in Brazil, the Gulf of Mexico, and the North Sea, will usually satisfy one-time permanent mooring applications.

With smaller AHVs not equipped with large capacity main anchor handling or spooling winches, a special large drum diameter deployment winch can be added to the AHV deck. The polyester rope section is generally fed on and off the deployment winch with a wire or synthetic rope messenger line. Care must be taken to ensure the polyester mooring rope section is not damaged by the underlying messenger line and is spooled on the winch with a specified minimum tension (usually not less than 5-10 tons).

Where multiple sections of polyester rope must be deployed and recovered, as typically expected with a MODU mooring operation, it is not considered practical or economic to use the above deployment technique, unless the installation AHV is equipped with multiple-drum, very large capacity winches. As an alternative, AMC has developed and employed a portable capstan-type winch, equipped with a special mechanism (known as a fleeting-knife) that allows the polyester rope sections to be deployed directly off shipping reels, via the capstan winch, and over the vessel stem roller.

The fleeting knife mechanism ensures the polyester rope does not ride up over itself and become fouled as it passes over the capstan winch. This equipment has been very successfully used to deploy and recover rope sections of up to 174 mm in diameter. AMC has also used this method and the fleeting knife concept as an add-on to an AHV main anchor winch drum (suitably modified).

In both the above polyester rope deployment techniques, it is necessary to stopper-off the inboard end of the polyester rope prior to making a connection to other mooring jewelry. This procedure is performed using a "Chinese finger" gripping method or a specially developed clamp. Both these methods have been successfully used with loads up to 50 tons.

Torque considerations

The choice of polyester rope construction in a TLM is impacted by the other components in the system. The torque-matched polyester rope should be specified in MODU moorings that contain six-strand wire, since damage to both the wire rope and the polyester line can occur if their torque characteristics are not matched. On the other hand, experience has shown that torque-balanced polyester rope works well when used in combination with chain and/or torque-balanced wire rope (spiral strand or multi-strand types). Torque-balanced components have been extensively used by the Brazilians in their polyester moorings and are ideally suited for permanent systems.

While chain works well in CMs, the authors maintain that MODU TLMs should only include minimum lengths of chain in the pre-set part of the leg, in order to minimize weight while satisfying the design motion requirements of the rig. Possibly no chain, or perhaps only a short length of chain, may be all that is required for a small rig while for a larger rig, a combination of wire and chain making up a fairly long length may be necessary. For cost and weight saving reasons, it does not make sense to add the lengths of chain determined necessary to ensure the polyester rope never comes into contact with the seafloor.

Rather than using chain, the authors believe that six-strand wire rope should be used. It should also be noted that six-strand cannot be manufactured in a torque-balanced construction. Spiral strand wire rope is costly and very difficult to handle and deploy (requiring a D/d on installation winches, sheaves, fairleads, and/or stem rollers of 20-30:1). On the other hand, multi-strand wire, while meeting the torque-balanced requirements, has been found to be far less durable than spiral strand or six-strand wire and, therefore, is not suited for a MODU pre-set mooring. Six-strand is the most commonly used type of wire found in MODU mooring systems today, however, nine and twelve-strand construction wire types have also been used. Either chain or wire is used today as the onboard vessel component for attachment to the pre-set mooring.

There is substantial laboratory test evidence that shows when torque-balanced polyester rope is connected to six-strand wire rope, there is a potential for significant damage to both ropes, resulting in reduction of break strength and long-term cyclic-tension fatigue life. It has been reported that field experience, at least in Brazil, has confirmed this.

Field experience in the Gulf of Mexico has confirmed that a torque-matched polyester rope construction will work effectively when connected to six-strand wire. The design of the torque-matched rope should roughly provide the same torque characteristics as six-strand wire throughout a range of operating and survival tensions. The torque characteristics of most six-strand wire types is similar from one manufacturer to another. The authors, therefore, maintain that for practical (or cost saving reasons), torque-matched polyester rope should be specified in MODU moorings that contain six-strand wire.

TLM anchoring systems

Until the early 1990s, the only proven methods for anchoring in situations with significant vertical anchor loads used driven piles or gravity (deadweight) anchors. Drilled and grouted piles are often rejected due to the uncertainty of grouting integrity. Driven piles using underwater hammers are an accepted technology but are not proven in water depths greater than 4,000 ft and they can be an expensive option due to the high cost of the underwater hammer and the vessels required for installation. Gravity anchors are a very inefficient solution for TLMs high anchor loads.

A JIP managed by AMC and funded by five major oil companies between 1992 and 1994 was conducted to investigate various anchor types capable of withstanding vertical loads for TLM applications. The JIP included model testing in synthetic clay, limited offshore testing, and software development. The JIP introduced new types of drag embedment plate anchors capable of withstanding vertical loads which have become known generically as vertically loaded anchors (VLA). The study also identified suction embedded anchors (SEA) as being viable for TLM applications.

In 1996, as part of the DeepStar project, AMC tested these VLAs (DENLA and Stevmanta) in the Gulf of Mexico with only partial success. Since then, however, Petrobras has proven the designs and has installed at least one permanent mooring system using VLAs, and it is understood another is planned.

During the last 10 years, considerable experience has been gained in the use of SEAs for permanent mooring applications. Over 150 SEAs have been installed worldwide to date, the majority of which were designed as part of permanent mooring systems. In 1997, AMC installed 14 SEAs, each weighing 130 tons, for the Sciehallion FPSO project. In 1998, AMC installed 12 SEAs for the Asgard A project. The installation of these anchors was performed from large AHVs, without A-frames and with minimal modifications. In 1996, AMC, in conjunction with Shell and Amoco, developed a recoverable SEA (patent applied for) specifically for MODU applications.

In September 1996, embedment, load, and recovery tests were carried out in about 1,000 meters of water in the Gulf of Mexico to verify installation procedures and design. Shell adopted this design and procured recoverable SEAs for a MODU (the Marianas) which has now been moored in a maximum water depth of about 2,100 meters using only TLMs for stationkeeping. In 1997, AMC designed three SEAs for the Ad Notam JIP which were intended to anchor a three leg polyester TLM for a test barge in the northern North Sea. Considerable testing was performed, but due to abnormal weather conditions at the time the project was cancelled.

SEAs, while very effective for vertical load anchoring applications, are large and expensive to fabricate. In addition, they take up a lot of space on an AHV. SEAs for FPS or FPSO permanent mooring applications usually weigh in excess of 100 tons and cost from $250,000 to $500,000 each. Even the recoverable SEAs developed for Shell's MODU application weighed about 70 tons.

VLAs sized for MODU applications require high embedment loads and are, therefore, difficult to install in deepwater. Ultimate VLA pull out capacity (UPC) is approximately 2.5 times the maximum drag-in load. Due to this fact, Petrobras had to use two modern 15,000 HP anchor handling tug supply (AHTS) vessels in tandem to embed each of the VLAs recently deployed offshore Brazil. It is also difficult to establish the final embedment depth and overall geographical location of a VLA accurately.

Without accurate information on penetration depth, it is very difficult to accurately establish the anchor's ultimate capacity. Higher safety factors may be required to cope with acceptable depth positioning tolerances and additional mooring line length may be needed to deal with geographic positioning tolerances. In areas with a number of seafloor obstructions inside the mooring pattern, imprecise anchor placement may present severe problems.