Permanent deepwater moorings beyond 2,000m water depth present a major engineering challenge for naval architects and engineering contractors. Already, the cost of deploying mooring lines exceeds the cost of the lines themselves. 'Lean Installation' mooring technologies and line deployment methodologies offer a solution to the problem.
Evolution of Deepwater Mooring Systems Gathers Pace
By a wide margin, deepwater developments represent the single largest area of growth in oil and gas exploration and production for the foreseeable future in the Gulf of Mexico, West Africa, Asia, Brazil and now the Arctic. Production platforms will be further offshore and, therefore, subject to greater wave and current movements. In the absence of a pipeline infrastructure, they will act as the processing/offloading hub for a number of ultra-deepwater fields and long distance tiebacks.
Deepwater can be defined at water depths between 800 - 2,000m. Beyond this is considered ultra-deepwater, currently the deepest Spar mooring in the Gulf of Mexico is Shell's Perdido Spar at 2,450m, and the deepest moored floating production storage and offloading (FPSO) serving Petrobras' Cascade and Chinook development at 2,500m. Development of ultra-deepwater fields over 3,000m is only a matter of time.
Deepwater mooring lines are made largely from long lengths of synthetic fibre, typically polyester, and short lengths of ground and surface chains. Offshore installation involves multiple mooring lines deployed using large vessels to pre-tension each of the lines. Responding to the increasing cost of deploying mooring lines to greater depths, the mooring industry is looking to advances in materials and connector technologies, together with new mooring methodologies. The concept of 'Lean Installation' is guiding these developments.
'Lean Installation' applies 'Lean' principles to the installation of deepwater/ultra-deepwater mooring systems by reducing waste, adding value and improving processes. Cutting the size and number of installation vessels, and the time to complete the mooring installation, reduces waste. It also involves looking for ways to add value by streamlining deployment and improving the quality of the mooring process: in effect, transforming the deepwater/ultra-deepwater mooring process.
Lean Subsea Mooring Connectors
An early example of 'Lean Installation' was Ballgrab, ball and taper, subsea mooring connector systems. Widely used in deepwater moorings, the two part connector comprises a male mandrel and female tube receptacle, which is self-aligning and self-energising. Its introduction fundamentally changed the deepwater mooring process. For the first time, moorings could be deployed in two stages: the anchor piles and female receptacle attached to the ground chain and installed months ahead of the platform arriving offshore. The platform is then moored by lowering the mooring line and male connector, which is inserted into the female receptacle.
Female connector held in a docking porch on the anchor pile
Over time the mooring methodology has been adapted to accommodate the increasing use of FPSO/submerged turret production buoys. Instead of making the mooring connection on the seabed, the connection is made on the buoy itself.
Female connectors mounted on the side of the STP buoy
This has both practical and economic advantages. It is no longer necessary to attach the mooring lines to the buoy ahead of tow out, making buoy lift out and towing offshore significantly easier. Fewer vessels are needed as there are no mooring lines to support during tow out. It also allows the buoy to be towed through shallower water. Once in position offshore, the mooring lines, installed earlier, together with the anchors and held midwater, are raised up to the buoy and connected.
Deeper moorings will lead to higher loads on mooring connectors. Recent connector developments have focussed on maximising the connector's engineering integrity by optimising forging conditions. Large scale metal forging is complex, and less well understood, than other metal treatment techniques. During forging, the metal is heated and shaped by compressive force, refining the grain structure which, in turn, improves mechanical performance, strength, ductility and toughness.
'Next Generation' Mooring Connectors
Deepwater mooring components need to exhibit high levels of strength and toughness, as well as outstanding fatigue behaviour over many years. While the latter relies heavily on established computer modelling techniques, the factors governing strength and toughness were less clearly understood. Research by First Subsea Ltd and University of Sheffield's Institute for Microstructural and Mechanical Process Engineering (IMMPETUS) into large scale, metal forging processes and resulting mechanical properties has shown the importance of the effect of microstructure in terms of toughness related to different rates of cooling, and the importance of sampling position in achieving representative and consistent toughness values for large diameter billets. In addition, it has allowed First Subsea to determine the optimum steel chemical composition, forging and heat treatment process conditions needed to give more consistent mechanical properties throughout the length of the mooring connectors.
The metals forging research is now being applied to the manufacture of deepwater mooring connectors for the Chevron USA Inc's Jack and St Malo semi-submersible hub production facility in the Jack and St. Malo fields located approximately 280 miles south of New Orleans, Louisiana, in water depths of 2,133 m (7,042 feet) in the Gulf of Mexico. The semi-submersible, floating production unit (FPU) will be moored by 16 Ballgrab, ball and taper, mooring connectors attached to polyester mooring lines in a 4x4 arrangement. The Ballgrab Series III male connectors will be the largest produced so far with an un-corroded 2,599mT (25,491kN) MBL, and compliant with the ABS Mooring Guide 2009.
Quick Release Arctic Moorings
The greater knowledge of metal forging has enabled Ballgrab ball and taper to be applied to the development of quick release connectors for Arctic moorings. Sliding ice sheets can cause lateral toppling loads on FPSO vessels using a submerged turret production buoy. The quick emergency disconnect (QED) mooring connector attached to the buoy are load monitored, allowing automatic breakaway of mooring lines under emergency loads up to 750mT per line. The QED connector is designed to achieve breakaway of the vessel's moorings within 15 minutes, preventing damage to vessel and submerged turret buoys.
Quick emergency disconnect mooring connector
'What If' Deepwater Mooring Scenarios
Naval architects require comprehensive fibre rope test data when designing deepwater mooring systems. The lack of specialised rope test equipment, and the very high cost of testing, has led to a shortage of authoritative data on rope properties for a variety of fibre types and rope constructions. To overcome this, deepwater rope manufacturer, Lankhorst Ropes Offshore Division has developed a rope test machine for deepwater ropes that is providing insights into deepwater mooring rope behaviour, and optimised installation scenarios. The test machine provides extensive testing of rope stiffness and degree of stretch under maximum load for a range of rope yarns and constructions. For the first time, it enables simulation of 'what if' scenarios for storms and hurricanes, and loop current events with associated vortex -induced vibrations on mooring lines.
The new rope test data hassled to rethinking the degree of mooring line pre-loading required to remove bedding-in stretch during installation. Traditional pre-tensioning is based on fixed loads at variable length, loading the rope to 40% MBL (maximum breaking load). Higher MBL ropes in excess of 2,000 tonne will make traditional rope pre-tensioning practices offshore both impractical and uneconomic – the cost of designing and manufacturing higher rated chain tensioning equipment will be extremely high, moreover there simply aren't vessels large enough to handle the loads involved.
Lankhorst Ropes Offshore Division rope test machine for 'What If' scenarios
An alternative, lean installation, approach is to pre-tension ropes with variable loads at fixed length, which is more representative of rope performance during station keeping. Although this will require more rope testing at the project outset, it is more than outweighed by the benefits of lower capital costs for mooring equipment and faster installation times. Pre-loading, where required, can be conducted at lower and safer tensions; with the option of using smaller installation vessels. In some cases, there will be no need to pre-load the mooring ropes ahead of installation. Instead, the ropes are installed at the calculated mooring line pre-tension, thereby reducing the time and cost needed to install the mooring lines.
Deeper Moorings – Stiffer Ropes
As water depth increases, polyester moorings will not have sufficient stiffness by themselves to maintain a vessel on station and prevent over stressing the risers. A 2,000m deepwater polyester mooring line may have 40m elongation, while a 3,000m line would allow 60m elongation under the same environmental conditions, creating greater horizontal offsets which may exceed the limits of risers. For these greater water depths, a stiffer rope is needed. High modulus polyethylene (HMPE) yarn produces a stiffer rope than polyester, and so is more riser-friendly. It is also smaller diameter and lighter than polyester for the same MBL. This has important lean installation benefits.
HMPE rope allows more rope per reel (approximately 900 m HMPE vs. 600 m polyester) and a considerable reduction on the reel dimensions – 4m diameter end flanges with polyester are reduced to 3m with HMPE, allowing more reels per vessel. The weight of the rope-laden reel is also reduced from 19 tonne for polyester to just 7 tonne for HMPE. Fewer, lighter-weight reels can be handled more readily by an anchor-handling vessel, permitting mooring lines and anchors to be installed in one trip, a not insignificant factor where platforms could be up to 200 km offshore.
Compared with polyester, HMPE exhibited poor creep performance in the past, making it unsuitable for permanent, 30 year, moorings and more suitable for MODU moorings. A hybrid mooring line, combining both polyester rope and HMPE segments, provides optimal stiffness; not too stiff or too soft. This is the best solution from both the perspective of mooring system performance and cost of deployment. As the speed of HMPE creep is a function of temperature, mean load, and loading time, the preferred hybrid rope configuration uses stiffer HMPE rope in cooler water, close to the seabed and polyester rope in warmer water closer to the vessel.
The principle benefit of hybrid PE/HMPE ropes is the ability for the mooring system designer to engineer the mooring line's stiffness and use the lengths of polyester and HMPE segments to provide the stiffness needed to handle peak loads during station-keeping while ensuring sufficient elasticity to withstand storms. Recent HMPE yarn developments indicate that an ultra-low creep rope will soon be available for permanent deepwater moorings. However, there will still be a role for hybrid moorings in areas where storms and hurricanes are a recurrent problem.
Advances in Connector Technology
Even with the ability to increase the length of rope per reel, deepwater moorings will still be made up of multiple sections of rope. The time needed to make up these rope-rope and rope-chain connections using traditional H-link, Pear-link and Plate-link connectors can be significant, when you consider that each line may have at least 3 or 4 connectors per mooring line in a 12 line mooring system. They are labour-intensive to assemble offshore and prone to being weakened by the bending moments experienced when deployed over the mooring boat's stern roller. This is less of a problem for short term MODU drilling moorings, but is an area of concern for long-term, 30 year, deepwater moorings. A lean installation alternative is the LankoFirst fibre rope connector which draws on First Subsea's metals forging research project, and developments in synthetic rope splicing technology.
LankoFirst fibre rope connector
The connector is based on improved connector splicing. In place of the single large eye splice used with thimble connectors, for example, the connector uses a more compact eye. This enables the splice to be closer to the 'ideal splice' design with sub-ropes the same length, making it more compact, and thus stronger, than a single, large eye splice. In order to reduce connector make-up times offshore, the rope terminations are spliced into a forged steel, sub-connector during rope manufacture. This is subsequently secured within the connector during connector make-up offshore. The LankoFirst connector has clear practical benefits in terms of higher engineering integrity and is smaller and lighter than current connectors for the same MBL. In addition, it is safer and easier to assemble offshore, whether vertically or horizontally, and easier to run and retrieve across stern rollers and on anchor-handling vessels.
Lean Installation Deepwater Moorings
The 'Lean Installation' approach to deepwater mooring systems is enabling operators, naval architects and engineering contractors to approach deeper moorings safe in the knowledge that mooring technologies, and practices, are in tune with the technical advances and practical demands of deploying ultra-deepwater moorings.
A 'Lean Installation' approach to deepwater line deployment proposed by First Subsea takes as its starting point the ability to be make up connections quicker offshore and thus reduce mooring line deployment time.
Benefits of Lean Installation of fibre rope mooring lines include:
Traditional vs Lean Installation
From a deployment perspective the primary difference between traditional H-link connectors and the new connectors is the approach to its make-up.
H-link connectors involve connecting the large spliced eyes at the rope ends, during rope deployment offshore. In some cases this can be a very time-consuming activity as the connector, thimble, spliced rope eye are firstly aligned together and then held in place whilst the load bearing pin is hammered into position.
For the new LankoFirst fibre rope connector, on the other hand, there are no soft eye splices and no loose thimbles. The end termination is an integral part of the rope rather than an external addition. This is achieved by splicing a sub-connector element into the rope end during rope manufacture. The design of the sub-connector allows for smaller sub rope splices that are closer to 100% splice efficiency than larger splices. Moreover the splice quality is better as the sub-connector is spliced in the factory, without the risk of damaging the fibre.
Lean Installation not only applies to the way the rope is connected offshore. It also applies to deployment of the rope-rope and rope-chain connections offshore. Three connections are possible: clam for rope-rope, link for rope-chain and rope-rope, and snap for rope-rope. In each case the connector elements are integrated within the rope ends during rope manufacture. And each conforms to a 'lean installation' approach to mooring by making the connection simpler, quicker, better and safer offshore.
Continual Innovation in Deepwater Mooring
The LankoFirst fibre connector is the latest development in the continual innovation of deepwater mooring systems. It provides a means of streamlining the make-up and deployment of deepwater mooring lines, but in the bigger picture of global deepwater production its real significance is lies in the opportunity to bring a 'Lean Installation' approach to deepwater mooring.
For more information on 'Lean Installation of Deepwater Mooring Lines contact Brian Green at First Subsea on email@example.com, and download the LankoFirst brochure here