Quad 204 takes shape subsea

BP’s Quad 204 redevelopment will be one of the largest subsea developments in the UKCS.

Quad 204 is one of the world’s biggest subsea redevelopment projects, and one of the most complex engineering challenges BP and its project partners have ever undertaken.

It involves redevelopment of the Schiehallion and Loyal fields, to the west of Shetland on the UK Continental Shelf (UKCS). Wood Group Kenny is responsible for engineering the subsea, umbilical, riser and flowline (SURF) system, from the subsea trees to the connections within the new Schiehallion floating production, storage and offloading (FPSO) vessel.

Wood Group Kenny will also support BP through the delivery, testing, installation and commissioning of these systems, which represent approximately US$1 billion worth of subsea production systems, umbilicals, pipeline and risers infrastructure.

As the project reaches a key milestone, with preparations nearing completion for the Schiehallion FPSO tow away, Andrew Train, BP’s project director, offshore program and Wood Group Kenny director, Bob MacDonald look at some of the challenges involved in a subsea project of this scale and discuss how they think it could benefit the subsea industry as a whole.

The Schiehallion FPSO has been moored in 400m of water, 281km (175 miles) west of the Shetland Isles, producing oil from the Schiehallion and Loyal fields since 1998. The reservoir sits 2000m below the seabed, covering an area of approximately 194sq km (75sq mi.) As a result of new exploration activity and the technological developments that have occurred since it was discovered 20 years ago, recoverable reserves are now known to be more than double the original estimates.

Accessing these reserves has required a major expansion of what was already a sizeable subsea development and one of the largest oil producing fields on the UKCS. The original development has already expanded to comprise 51 wells in five subsea drill centers, and the Quad 204 redevelopment project is adding 25 more wells. The addition of the new wells will significantly extend the life of the field, with potential production estimated to reach a billion barrels or more of oil equivalent by the end of its new anticipated design life in 2035.

There were a number of possible approaches to such a major redevelopment and Wood Group Kenny assisted BP in the evaluation of a wide range of concept options. The design being taken forward is replacing the existing vessel with a newbuild state-of-the-art FPSO on the same location and reusing the existing subsea infrastructure, where practicable without compromising safety or quality, replacing it only where required and extending it as required to support additional wells.

While there is a strong emphasis on reuse of existing equipment and infrastructure, the magnitude of the Quad 204 project means that although it is a redevelopment project, it effectively includes as much “new development” as many greenfield projects.

At the same time, the fact that this is a redevelopment creates a number of challenges. For example, BP has not taken an FPSO off-station before, and all the new subsea structures will have to be installed in an already congested seabed that contains a lot of equipment that will be reused. In addition, subsea technology has developed rapidly since the field was first discovered, and although many aspects of the original development were ground-breaking at the time, the existing equipment is not aligned with the state-of-the art technology that is deployed on subsea developments today. These factors all come together to create a highly complex and exciting project.

One of the biggest overall challenges for the Quad 204 subsea project team has been with respect to managing the total size of the development and the quantity of subsea structures involved. For ex-ample, in addition to the 25 new wells, there will be 38km of replacement flowlines, 17km of new flowlines and eight new manifolds; changes to the umbilical system include replacing two dynamic umbilicals, installing three new static umbilicals and 26 associated structures.

Water depth is another key issue. With a depth range of 395m-470m, all the tasks relating to the removal of existing equipment and installation of new equipment, such as valve operations and connection of flowlines, have to be undertaken using diverless techniques in hostile conditions, as will changing parts in the future so this is something that has to be factored into the design and development.

In fact, there are significant issues to be tackled in all aspects of the development, concerning the architecture, the controls system, and the umbilical distribution system, each presenting their own difficulties. Assessment of the existing infrastructure for reuse and the integration of existing and new parts of the development are also complex tasks.

In line with the overall project approach, the original intention was to reuse as many of the existing 15 risers as possible, and there was a significant scope of work in this regard, including an assessment of their general condition which used historic integrity assessments as the starting point. However, there were a number of other factors in addition to the condition of the risers that had to be taken in to account in determining their suitability for reuse- critically whether they were compatible with the new FPSO and the revised metocean conditions.

To reuse the risers it also had to be possible to carry out a lay-down and abandonment procedure that involved leaving the risers on the seabed for about two years before the new FPSO came on-station. As this was not something that the risers were originally designed to do, careful consideration of the procedure was required to ascertain its feasibility.

As part of this process, a trial of the lay-down procedure was undertaken with a single riser, which was left in the abandoned condition for 18 days, prior to dissection and examination of three sections from it.

This examination not only allowed the project team to determine whether any damage had occurred as a result of the procedure but also provided an opportunity for a general inspection of the riser condition. Much was learned from this including the condition of the layers within the flexible riser, and the whole process enabled a very well-informed judgment to be made on the overall suitability of the risers for reuse.

Although the lay-down trial itself did not damage the riser, the data gathered from it did tie in with previous analysis that indicated over-bending of the riser could be expected to occur if it was left on the seabed for two years.

When all the available information was taken into account, it became apparent that only two of the risers were suitable for reuse from all perspectives, and ultimately the decision was made to introduce an all-new riser system.

Another significant element of the QUAD 204 project, and a “technology first,” is the design of the bend stiffener. Bend stiffeners are conical polyurethane moldings designed to control the curvature of the riser under dynamic conditions, and prevent overbending at the interface of the riser with a more rigid structure, in this case the vessel turret.

More onerous conditions lead to larger stiffeners, and as the west of Shetland area is a harsh water environment, bend stiffeners used there are generally larger than they might be in other, less-hostile locations.

The motion characteristics of the new FPSO are more onerous than the outgoing FPSO and these characteristics combined with the harsh water environment have resulted in the need for the design of a new bend stiffener that is around twice the volume of what was used on the original Schiehallion FPSO.

The key challenges here were that the existing design methodologies could not be assumed to be relevant for something so much larger than what had previously been considered. It was therefore necessary to conduct a much more thorough design process than is typical, and a qualification program is underway to ensure that the larger stiffener can be designed, manufactured and installed as well as provide 20 years of service.

Wood Group Kenny has defined the requirements for the qualification program to be consistent with BP practices, and actively involved in ensuring the activities are carried out correctly and that the aims of the program are met. Up to 28 of these large bend stiffeners, which are about 8m long and 1.8m wide at the base, will be required for the redevelopment.

In total, 21 new flexible risers will be installed, which will allow the flow of 320m bbl/d of fluid a day to travel up to the FPSO for processing. Each of the risers will be about 800m long and will be tethered to an anchor, suction-piled to the seabed, giving them enough strength to withstand a 100-year wave as well as the upward force generated by the midline buoyancy modules.

A key area with respect to integrating the existing and new elements of the development is the determination and mitigation of flow-induced pulsations in structures piping. The interaction of a number of factors can cause surges in fluid flow that in turn can cause fatigue damage or cracking in subsea structures.

Addition of the new wells will result in a change of flow regime that may potentially cause changes with respect to flow-induced pulsation in the existing structures and so an important element of the Quad 204 design work is to ensure that structural support is in place to protect against this kind of damage.

A study was undertaken to predict the frequency of flow-induced pulsations within the development and give an understanding of the susceptibility of the existing structures to fatigue failure. Work is now underway to determine how suitable supports can be installed by ROV onto the existing structures while they are in place on the seabed: this will be a challenging process.

With respect to the new equipment it is comparatively straightforward to tackle this—additional bracing can be designed alongside the new equipment, onshore and prior to installation: this is still a complex task but achievable when considered early enough in the design process.

It should be noted, however, that the fact that this is a redevelopment has not only brought challenges but also some benefits in so much as it has been possible to factor in valuable information, lessons learned and experience gained from the existing development into the design of the new facilities.

For example, five new flowlines are being added to the development, and 10 of the existing production flowlines needed to be replaced as a result of corrosion. The experience and assessment of corrosion in the existing flowlines led to the decision to clad all the new flowlines with corrosion resistant alloy in order to mitigate this problem in the future.

When work was first starting on the Schiehallion development, the west of Shetland area was frequently referred to within the oil and gas industry as the Atlantic frontier.

In the early 1990s, Schiehallion was pioneering, not just for BP but for the industry as a whole, and it was instrumental in helping to develop a great deal of deeper water capability, which in turn enabled other deep water developments around the world to be undertaken.

It is hoped that Quad 204 will have a similar impact today, in terms of pushing forward the frontiers of what is possible, and how things are done with respect to field life extension and subsea redevelopment.

The oil and gas industry is being called on to apply increasing levels of ingenuity to tackle increasingly complex developments: Quad 204 has brought together a world-class team in terms of subsea knowledge and expertise and should go some way towards building a capability that can be applied to other projects around the world. OE

Andrew Train is the project director for the offshore program element of the BP Quad 204 project. He has worked with BP for 23 years in various roles, including project engineering, commercial management and project management. He has a degree in Civil Engineering from the University of Strathclyde and an MBA from the University of Warwick.

Bob MacDonald, regional director, Wood Group Kenny (North Sea), started his career in the subsea industry 20 years ago. He holds a degree in engineering technology and a masters in offshore engineering, both from Robert Gordon University, Aberdeen. He has held a wide variety of roles within the subsea sector, working extensively offshore and latterly on overseas assignments in Europe, North and South America and in the Middle East.