Located 110 miles to the west of Shetland, Scotland, the BP operated Schiehallion field, which was discovered in 1993 has been one of the biggest success stories for the UK Continental Shelf (UKCS). Oil was first produced from Schiehallion in 1998, and the field was estimated to hold more than 2 billion barrels of oil.
In 2011 BP took the decision to extend the life of this important reservoir, and the Quad 204 development project was established. The reconditioning of the field will see the existing FPSO replaced, subsea infrastructure renewed and 12 more wells drilled to increase the number from 52 to 64. The aim of the project is to extend the field life to 2036 or beyond, and to see it produce an estimated further 400 million bbls.
Technip in the UK has been tasked to support BP with the recovery and replacement of the field’s subsea system and the hook up to the new purpose built FPSO, the Glen Lyon.
The North Atlantic Ocean can represent a harsh environment and the area West of Shetland is no exception. All planning, development and equipment has to be geared towards withstanding waves as high as 30 m, up to 60 m of FPSO lateral movement and a daily average of just half an hour of sunlight throughout December which can have a significant impact on working practices.
Historically during the summer period there has been as much down time from high current through the water column as from the conditions on the surface (wind and waves). It is common for different current speeds and directions through the water profile, with Atlantic current influences and the movement of water from the Arctic adding to the confusion.
Having been an industry pioneer in flexible pipe technology since the 1970s, Technip has long-standing experience in developing, manufacturing and installing successful subsea systems. However even by those standards, the task in hand was a large one, with 21 risers required for the new FPSO, one of the most extensive riser systems within the UKCS.
Flexible riser testing and manufacture
Harsh environmental conditions combined with the size and scale of the project were the main considerations in the design of the subsea hardware. Technip has worked on projects to the West of Shetland for the past 20 years, during which time the company has garnered much experience of dealing with the testing elements in the region.
The flexible pipes were manufactured safely, and to the highest quality standards, at Technip’s Le Trait plant in France. This required a significant portion of the factory capacity in 2014 due to the technical complexity of the flexible pipes. A total of 21 risers and 78 flexible jumpers, were required. Five different forms of risers and nine varieties of jumpers are included in the system, ranging from 2.5 in. gas lift jumpers to 11 in. water injection risers. Almost 200 end fittings were required, a significant operation considering the installation of an end fitting is predominantly a manual process, with many different intricate elements that need to be installed.
Technip’s scope of work also included the design and supply of the ancillary equipment including clamps, tethers and buoyancy modules. Design analysis was conducted by the Technip engineering team at the outset – proving concepts were suitable as well as providing appropriate dimensions for the flexible structures, end fittings and bend stiffeners. A detailed analysis was also undertaken on the riser fatigue and an extreme and interference analysis conducted to ensure a safe riser configuration, designed to prevent any clashing of the risers once installed.
As part of the project qualification process, the bend stiffener system that would form part of the new Quad 204 riser structure also had to be extensively tested. The company had to prove, before the project started, that it could successfully scale up the existing technology both in terms of application and manufacturing and that the system had the stamina and dexterity to withstand the demands of the surrounding environment for up to 20 years.
The bend stiffener, designed especially for this project, was required to ensure the integrity of the connection between the risers and the FPSO. Given the size of the project, it became evident at an early stage that this would need to be one of the largest of its kind that Technip had ever produced. The riser bend stiffeners required for the Glen Lyon FPSO had to be designed as a two-part assembly to allow for fabricating at this scale and to allow the riser to be spooled onto a standard installation reel. Installation is therefore much more complicated than for a single part as the outer bend stiffener has to be installed offshore.
The integrity of such a large piece of equipment was established by undertaking a full scale qualification program at Technip’s research centre, applying both fatigue and extreme loading to the stiffener. The process was intended to demonstrate the full extent of the stiffener’s capabilities under duress, namely, to simulate the most severe conditions that will be experienced during its service life.
A comprehensive site integration test (SIT) was performed in Le Trait to ensure that the riser system was fully compatible for the installation process. Particular focus was placed on the bend stiffener, bend stiffener connector system (BSC), and the riser installation pull head, which allows for diverless connection of the two-part bend stiffener system to the I-tube of the FPSO.
The scale of the subsea system meant that the fabrication process took two years in total.
Once manufacture and test was complete, the flexible riser pipe was spooled onto twenty one reels and shipped to Lerwick in Shetland (pictures available).
The installation process had its own specific challenges. In order to be able to hook-up all of the new risers (and two new umbilicals and twenty new moorings) in a single offshore season, it has been necessary to pre-install and wet store the 21 risers and shorten down the hook-up critical path, as; generally heavy weather sensitive construction is restricted to the period from May through August.
Over the last three years a wave rider current profiler has been installed just outside of the field and its output is shared with all the vessels in the field. However, the experience of the ROV teams working in these conditions was valuable in maximizing the use of every minute possible in such a short working season.
Technip had been involved in modelling and analysis of the best configuration for wet store and recovery since BP commissioned a study on the subject in 2006. Technip focused the study on efficient pick up and minimising the riser installation schedule once the FPSO has arrived on site. Various seabed wet store routing configurations were looked at, however reverse pliant wave was selected in order to achieve maximum installation flexibility. The hook up method was carefully designed to recover the riser onto the construction vessel and flip the configuration rather than twisting the riser. In short, this means that any riser can be installed first to ensure first oil risers are prioritised and also mitigates any risk of the effect of residual torque in the riser dynamic section (which would be the case where the risers are rotated).
In being pre-installed, risers are connected to the seabed but are then looped back on themselves so that the end that will ultimately be connected to the FPSO is laid on the seabed away from the FPSO. This configuration results in a notable advantage – a significant amount of work is done before the FPSO arrives, which makes the start up more efficient. Riser pre installation on the construction vessel takes between four to five days depending on the size and configuration of the risers.
Each of the risers are individual and while some may have the same services as another, the quantity of buoyancy, riser protection and position of the tether clamps and foundations allow each to develop a unique configuration. The reason behind this specific design is again to avoid subsea clashing; 20 moorings, 21 risers and two dynamic umbilicals creates a congested water column.
More than 7000 individual components made up the riser systems and every component was checked, tested, manifested, boxed, re-checked and loaded on the vessel. The supply project team was responsible of the design and fabrication of the ancillary equipment and had to ensure that the equipment provided was fit for purpose and quality control checked after undergoing FAT and SIT testing.
The components were delivered to Technip’s Lerwick support base where a team of riggers, technicians and engineers re-checked each box and separated all components for each riser. Prior to vessel mobilisation the riser ‘kits’ were moved to a secure staging area on the base where a manifest was again taken. Once the components were loaded onto the vessel the equipment was again checked, all boxes opened and manifested. This process of checking and checking again was key to ensuring no errors were found once the kits had been shipped to the project site.
Installation of the ancillary components onto the risers was time consuming and repetitive. For example the risers each had up to 62 buoyancy modules fitted, a process which involves installing an internal clamp - tensioned to the riser pipe - and two half shells which are in turn attached to the inner clamp.
The decision to pre-install the risers had implications on the bend stiffeners, which are, by their nature, delicate pieces of equipment. Essentially large polyurethane mouldings, they must be handled with great care and at the same time it was of key importance that there was no damage during procedural and handling techniques on the installation vessel. By way of further mitigation, Technip developed a protective jacket to protect the inner bend stiffener as it is laid to the seabed between pre-installation and pick up in 2016. During this time the bend stiffeners were laid on a rubber subsea mat for added protection from any seabed anomalies.
At the outset of the project it was envisaged that there was only space for eight of the risers to be pre-installed, however, as a result of more detailed analysis and due to close collaboration and efficiency gains, by the start of September 2015, all 21 risers had been successfully laid in position awaiting the arrival of the Glen Lyon.
The autumn and winter of 2015/16 saw a continuation of procedure development and analysis in preparation for the pickup and FPSO hook up in summer 2016.
Technip will trial as much of the construction vessel scope as possible over the months ahead and will be responsible for various preparation works in the FPSO turret ahead of the hook-up programme.
It is expected that the Technip construction vessel North Sea Atlantic with its Open VLS (where the tensioners have the capability to open) will be mobilised in the coming months, ready for installation of the first dynamic umbilical. Allowing the FPSO to start communicating with the wells, tress and manifolds. Thereafter the vessel will embark on a four-month programme of riser and umbilical installation for Quad 204.
Adapted by David Bizley
Read the article online at: https://www.oilfieldtechnology.com/drilling-and-production/14032016/rising-to-the-challenge-technip-delivers-ukcs-largest-riser-system-for-bp/
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