Pipelayer / Pipelaying Vessels
In step with the ever greater production of oil and gas from offshore fields during recent years, where oil/gas is produced from deposits situated under water, the need of subsea pipelines for transporting the oil/gas has also increased very rapidly. Here it is a question of either transporting the oil/gas from the offshoe field to land or also, and especially with regard to offshore oil fields, connecting a number of oil wells to a common buoy for storing and loading oil, which is then taken from the buoy to land by tanker.
Historically, pipelines have been the primary mode of transporting liquids and gases from subsea well sites to production facilities, between production facilities, and from production facilities to onshore facilities. For many deepwater applications, pipeline systems, called gathering lines, transport production from remote subsea well sites to surface processing facilities. These pipelines are usually small in diameter because they transport production from only one or a few wells. Pipelines may also connect a minimal surface structure at a deepwater well site to its supporting host facility. A minimal surface structure may contain well test equipment or free-water knockout equipment to remove water from the production stream. Removal of the water reduces the possibility of hydrate formation in the pipeline.
Rights-of-way (ROW) are granted for pipelines that are off the operator=s lease or unit. These ROW pipelines are also called transmission lines, trunklines, or sales pipelines. These lines are generally larger diameter pipelines that transport production from several fields after the hydrocarbons have undergone separation and treatment. The destination for ROW pipelines is either another offshore facility for further treatment or refinement of the hydrocarbons or a shore facility for storage.
The operating environment for deepwater pipelines is different from the operating environment of pipelines on the shelf. Deepwater pipelines face higher hydrostatic pressures, colder water and sediment temperatures, different physical stresses during installation, effects from loop currents or eddies during installation or operation, greater amounts or rates of flow assurance chemicals, possibly higher flow rates, greater span (length of pipelines above the seafloor) distances, rugged seafloor topography, and technical challenges to monitoring and repair operations.
Pipeline installation methods for deepwater pipelines may be different from methods used on the shelf. The J-lay and bottom tows installation methods are unique to deep water. Deepwater pipelines may also be installed using dynamically positioned lay barges rather than the traditional anchored systems. While dynamic positioning eliminates the environmental effects from anchoring, air emissions may increase due to combustion of fuel to power positional thrusters.
The term "pipelaying vessel" refers to all possible types of such vessels, e.g. barges, modified bulk carriers, semi-immersible laying vessels etc. The laying vessel can thus either be provided with its own propelling machinery or not. Such a pipeline can either be manufactured at sea, starting with pipe lengths welded together on a pipelaying vessel to form the pipeline as it is laid on the sea bottom, or pipe lengths can also be joined together on land into continuous pipeline units which are then transported out to the site for laying on the sea bed.
Historically, the technique of laying undersea fluid-carrying pipelines had its rudimentary beginnings in England in the 1940's in a War-time project known as "Operation Pluto". In the summer of 1944, 3-inch nominal bore steel tubes, electrically flash-welded together, were coiled around floating drums. One end of the pipe was fixed to a terminal point; as the floating drums were towed across the English Channel, the pipe was pulled off the drum. In this manner, pipeline connections were made between the fuel supply depots in England and distribution points on the European continent to support the Allied invasion of Europe.
No known further development work or commercial use of the technique of laying pipe offshore from reels was carried out after World War II. After a hiatus of about fifteen years, research into the reel pipelaying technique was renewed and was carried on by Gurtler, Herbert & Co, Inc of New Orleans, La. (USA); by 1961, Gurtler, Herbert had sufficiently advanced the reel pipelaying technique to make it a commercially acceptable and viable method of laying pipe in the offshore petroleum industry, able to compete with the traditional stovepiping technique. The first known commercial pipelaying reel barge, called the U-303 was built by Aquatic Contractors and Engineers, Inc, a subsidiary of Gurtler, Herbert, in 1961. The U-303 utilised a large vertical-axis reel, permanently mounted on a barge and having horizontally orientated flanges (generally referred to in the trade as a "horizontal reel"). A combined straightener/level winder was employed for spooling pipe onto the reel and for straightening pipe as it was unspooled. The U-303 first laid pipe commercially in September 1961, in the Gulf of Mexico off the coast of Louisiana and was used successfully during the 1960's to lay several million linear feet of pipe of up to 6 inches diameter.
The "Apache" (now re-named the "Stena Apache") is a self-propelled dynamically-positioned single reel pipelaying ship which has a specially constructed hull comprising a reel support structure for rotatably mounting a vertical reel for unspooling a rigid-walled pipeline. Only a single pipeline is handled by this ship. Other pipe handling equipment includes a pipe bending radius controller; pipe straightening equipment; clamping assemblies; a stern pipe guide assembly and a level wind assembly. A tensioning assembly is also arranged on a support ramp assembly. The pipe exit angle or the water entry angle is from 18.degree. to about 60.degree. since this is the range of angular movement of the support ramp assembly. The upper part of this range of the pipe water entry angles is sufficient to accommodate laying a single pipeline in approximately 3,000 feet water depth. In order to lay pipe at greater depths it is necessary to increase the pipe water entry angle.
The "Stena Apache" vessel is not equipped to lay multiple lines since it has but a single main reel and does not have adequate unused deck space to permit the convenient placement of auxiliary reels. An early suggestion which was made during the vessel's construction phase and mentioned in the above patents, was that portable reels could be placed on the "Apache" deck to permit stern bundling of smaller lines with the pipeline from the main reel. These smaller lines were not required to be passed through the pipe handling equipment with the main reel pipeline according to the suggestion and there were no operative disclosures as to forming a juxtaposed plurality of operational lines by contact with a laying device which is adapted to move all the lines at a common velocity.
The MSV Norlift is a multi-purpose monohull construction vessel which provides a large, stable work platform for subsea construction. Her flexible and rigid reeled pipelay capability, dynamic positioning and an 18 man diving system combine to enable the vessel to undertake a wide range of subsea activities. These include pipelaying using the Reeled Pipelay System, an advanced, technology-led system designed to load and lay large quantities of rigid and flexible pipelines, umbilicals or cables in shallow or deep water. The rigid pipelay system covers pipe sizes from 6" to 12", with the tower angle variable between 30° and vertical, a reel capacity from 12,000m (12" pipe) to 29,000m (6" pipe) at a lay / spooling rate of up to 900m per hour.
Vector drives from Control Techniques were chosen for a rigid steel pipe-laying system on the vessel 'Norlift', a pipe-laying vessel owned by McDermott Subsea Contractors Ltd, which is laying pipelines 190 km West of Shetlands in over 500 metres of water. The main contractor for the drive and spooling system was Baricon Systems Ltd of Aberdeen, Scotland. The drives are fitted on the 1250 tonne product storage reel, undertaking a role which conventionally would have been a hydraulic application, bringing the benefits of the availability of full torque at all speeds and standstill for the control of a massive 18.3 metre reel for the feeding of a 'rigid' steel pipe. The pipe is loaded in 500 metre lengths which are welded together on shore and wound onto the reel. The drive system, originally designed by Control Techniques Birmingham Drive Centre, was reconfigured for this particular vessel by Control Techniques Leeds Drive Centre.
There are increasing requirements in the offshore petroleum industry for laying multiple operational lines in deep water at depths greater than 3,000 feet and in remote areas far from supply bases. To be commercially viable a pipelaying vessel must also be capable of laying either single or multiple operational lines in shallow waters of less than 2,000 feet up to 3,000 feet depth.
In prior-art pipelaying vessels as employed in laying offshore subsea pipelines for such uses as the gathering of oil and/or gas from offshore subsea wells, as, for example, in the Gulf of Mexico, it has been conventional to use one of two main methods to lay the pipe. In the first, or "stovepiping" method, a pipeline is fabricated on the deck of a lay barge by welding together individual lengths of pipe as the pipe is paid out from the barge. Each length of pipe is about 40 feet or 80 feet long. Thus, the pay-out operation must be interrupted periodically to permit new lengths of pipe to be welded to the string.
The nucleus of a "construction spread" is a large derrick barge, pipelaying barge or combination derrick-pipelaying barge capable of offshore operations for an extended period of time in remote locations. These barges, which range in length from 180 feet to 677 feet, are fully equipped with revolving cranes, auxiliary cranes, welding equipment, pile driving hammers, anchor winches and a variety of additional gear. The largest of these vessels are the DB-102, which is one of the world's largest semi-submersible derrick barges in both size and lifting capacity and provides quarters for approximately 750 workers, and a semi-submersible lay barge capable of laying 60-inch diameter pipe (including concrete coating) and operating in water depths of up to 2,000 feet.
The stovepiping method requires that skilled welders and their relatively bulky equipment accompany the pipelaying barge crew during the entire laying operation; all welding must be carried out on site and often under adverse weather conditions. Further, the stovepiping method is relatively slow, with experienced crews being able to lay only one or two miles of pipe a day. This makes the entire operation subject to weather conditions which can cause substantial delays and make working conditions quite harsh.
The other principal conventional method is the reel pipelaying technique. In this method, a pipeline is wound on the hub of a reel mounted on the deck of a lay barge. Pipe is generally spooled onto the reel at a shore base. There, short lengths of pipe can be welded under protected and controlled conditions to form a continuous pipeline which is spooled onto the reel. The lay barge is then towed to an offshore pipelaying location and the pipeline spooled off the reel between completion points. This method has a number of advantages over the stovepiping method, among them, speed (one to two miles per hour); lower operating costs (e.g. smaller welding crews and less welding equipment must be carried on the lay barge); and less weather dependency.
Different types of pipelaying vessels provided with rotatably arranged storage reels are already known, continuous pipe being rolled onto the reel when the vessel is docked at an onshore supply base. The apparatus used for laying underwater pipeline wound on a reel mounted on a floating vessel. In these arrangements, a pipeline comprising a plurality of joined pipe sections is wound on a rotatable reel and the vessel is then moved in a predetermined direction while the pipeline is unwound from the reel and lowered to the bottom of the body of water. While the pipeline leaves the reel, but before it enters the water, it is moved through a plurality of rollers so positioned and arranged as to reverse the bend which had been earlier imparted to the pipeline in order to wind it on the reel.
Heretofore, the physical design of the reel and bending apparatus on vessels has generally been somewhat limited in that it is not readily adaptable to various operational conditions such as the depth of water and dimensional characteristics of the pipe. For example, pipe characterized by large diameter and thick walls is not easily bent toward an acute angle because the pairs of rollers applying forces to the pipeline would have to be disposed closely to one another and great forces applied to them. Also, the reeling and bending apparatuses designed heretofore have generally been able to accommodate reeling and unreeling either from the top or bottom of the reel, but not from both, this generally as a consequence of the physical design and location of the bending apparatuses on the deck. Since it is generally desirable to unreel pipeline from the top of the reel when laying in deep water and from the bottom when laying in shallow water, all being somewhat dependent on the characteristics of the pipe though, it is disadvantageous to not be able to do both from the same vessel and apparatus design.
In all cases, however, the vessel has the drum or reel permanently located in it, which means that after all the pipeline on the reel has been reeled off and laid, the pipelaying vessel must then put into port for "recharging" the reel with a new length of pipelne. This involves breaks in laying work costing both time and money, or it requires several pipelaying vessels.
Another drawback with pipelaying vessels working according to the known reeling technique is that the vessels must be large since the reel is very voluminous (reel diameters of about 24 m are used for a 12-inch pipeline, for example) and have a very large weight when the whole continuous pipeline length is wound up on the reel.
It is commonly known that a straight and rigid body may not be bent intermediate its ends except upon the application of at least three forces, one at each end and one in the middle. This principle has manifest itself in the utilization of three-pair roller systems in the pipe-bending art and although two-roller systems have been devised, none are characterized by a satisfactory flexibility of movement on the deck of the vessel, nor teach a method for enabling the laying of a wide variety of pipeline diameters and thicknesses, optionally from the top or bottom of the reel.
The Swiss-based Allseas Group S.A. is one of the major offshore pipelay and subsea construction companies in the world, operating specialised vessels - which were designed in-house. Allseas is one of the major offshore pipeline installation companies in the world. Founded in 1985, we have gained world-wide experience in all types of offshore and subsea construction projects. Allseas' approach is to support clients already in the conceptual design stage and offer its services for project management, from engineering and procurement up to and including installation and commissioning.
Allseas' shallow water anchored pipelay barge Tog Mor is the third pipelay vessel in the Allseas fleet. She is employed world-wide, primarily in shallow water areas, both in support of Lorelay and Solitaire and independently contracted. The trenching support vessel Calamity Jane supports Lorelay and Solitaire with activities such as pre- and post route survey, crossing preparation and mattress installation, and operates as an independent unit with the mechanical trencher Digging Donald. The dynamically positioned survey vessel Manta supports Solitaire, Lorelay, Tog Mor and Calamity Jane, and also works under independent survey contracts.
Solitaire, the largest pipelay vessel in the world, has set new standards in the pipelay industry. Precise manoeuvring on full dynamic positioning allows the vessel to work safely in congested areas. Her high cruising speed and lay speed make her very competitive world-wide. Lorelay was the world's first pipelay vessel on dynamic positioning, representing a new generation. Being able to manoeuvre precisely and safely - specifically an advantage in congested areas - and having excellent workability, she has made her mark world-wide.
While the Solitaire moves forward, the pipeline slowly enters the water from the ship's stern. Proper calculations, including pipe weight, and water depth and density, determine the optimum speed at which the pipeline is laid. To ensure safe delivery, the pipeline is fed onto a computer-operated device called a "stinger." The stinger, attached to the Solitaire, and resembling an insect stinger with pipeline in tow, feeds the pipeline into the water at a gentle curve and a controlled rate. The biggest pipelay vessel in the world is also the fastest. And depending on the weather, Solitaire can lay from 4 to 7 kilometres (2½ to 4½ miles) of pipeline per day-twice as fast as other pipelay vessels.
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