Creating a severe-weather riser system

Jeannie Stell

June 1, 2015

Jeannie Stell provides an update on BP’s partnership with Wood Group Kenny to design and manufacture an FPSO riser suitable for the “dark and stormy” weather at BP’s Quad 204 redevelopment.

 

BP and Wood Group Kenny have worked together for the past 15 years, since 2010, to design a robust riser system capable of serving a harsh environment floating production storage and offloading (FPSO) facility with a 25-year life cycle. The robust riser technology will be married to a production platform, the new, purpose-built, turret-moored Glen Lyon FPSO, slated to tackle BP’s massive subsea asset — the Quad 204.

Located west of the Shetland Islands in Blocks 204 and 205 of the UK Continental Shelf at water depths of 300m to 550m, the new Quad 204 is an ambitious project to redevelopment the Schiehallion and Loyal fields. Considered to be one of the largest and deepest developments in the North Sea, the Schiehallion is one of the largest UK North Sea fields discovered to date.

The plan is to construct a new FPSO unit to replace the existing Schiehallion FPSO and the project will include an extension of the existing subsea system with 15 new and replacement flow lines, 21 new and replacement risers and 14 new wells added to the 52 existing wells.

Although the field has been in operation since 1998, recent appraisals show that the field holds more reserves than originally anticipated. This redevelopment will give BP the ability to increase production. The FPSO and its subsea structures will be installed in modules and the total facility is expected to begin producing in 2017 to reach an eventual capacity of 130,000 b/d of oil. The project will extend the life of the two fields to continue production beyond 2035.

The project was approved in July 2011 and is estimated to cost about US$5 billion. Partners in the project include BP (36.3%), Hess (12.90%), Murphy Petroleum (4.84%), OMV (4.84%), Shell (36.3%) and Statoil (4.84%).

Stormy weather

The project is an unusual amalgamation where new equipment will be integrated into an existing brownfield environment. One of the most critical challenges to the project will be the engineering, manufacturing and installation of the FPSO risers, which must be long-lived and able to withstand an extremely severe environment. In fact, the location has been known to experience harsher weather than most other UK marine environments. The topside weather can include 40 mph winds, 18m high waves, and complex, severe currents that can lead to directionally skewed fatigue loading of the platform.

Additionally, the risers and mooring system must be able to withstand brutal winter storms and be sturdy enough to require minimal intervention during normal operations because construction access is only possible from May to September.

BP identified a group of specific, value-added project features it deems necessary for the long-lived redevelopment, including:

New riser system for the Quad204 redevelopment. Images from BP. 

Additionally, the Quad 204 will incorporate some new designs that were formulated as a result of past experience. For example, in the past the Schiehallion used a two-part bend stiffener design. However, the latching mechanism in the design that connects the two parts of the bend stiffener to the bend-stiffener connector experienced several failures over time. The failures resulted in full or partial slippage of the inner bend stiffener of various risers. As a result, some dynamic risers had to be decommissioned and replaced. During the replacement activities, engineers also discovered that the outer sheath contained a 1.1m-long groove, which was believed to be caused by the metal of the inner bend stiffener abrading against the sheath as it slipped along the riser body.

To avoid such future damage, the new Q204 anchoring mechanism design will anchor both parts together without a latching mechanism. This design will be similar to the one used at BP’s Thunder Horse facility. Also, the new design ensures that the outer bend stiffeners have suitable corrosion and abrasion protection to avoid any potential damage.

Another design change will be the use of a new vent-gas monitoring system. The previous system was based on a flow meter, which was unable to detect annulus-venting behavior with enough sensitivity to reduce risk. Therefore, the Q204 will employ an online vent-gas monitoring system to provide continuous readings of the production and gas risers’ conditions. The system will be backed up by a vacuum-testing program during the first two years of operation.

Also, to reduce the probability that embedded debris can occur in the I-tube, the Q204 risers will include a second protective outer sheath than will be thicker than the external sheath. Furthermore, a layer of Kevlar tape will be wrapped between the two outer sheaths to bolster the impact resistance of the outer protective layers. The design includes access hatches below the riser hang-off location so cameras can be inserted as needed into each I-tube to inspect the risers.

As if the harsh weather and strong current weren’t enough, the designers had to plan for damage from marine life as well. In this location, buoyancy modules are vulnerable to hard marine growth such as cold-water coral. If left unchecked, the heavy buildup of coral causes integrity concerns due to the significant loss of buoyancy. To avoid this hazard, the designers of Q204 incorporated specifications for a fit-for-purpose anti-fouling layer to be applied to the buoyancy modules that is expected to retard hard-coral growth attachments. As a result, the Q204 will have the only red buoyancy modules in the North Sea.

Integrity management

The Q204 integrity management system will be a risk-based system that relies on the probability of damage or failure as documented by known flexible pipe failure modes described in the public domain, as opposed to company experience, because BP has never experienced a flexible pipe failure. The integrity management protocol that pertains to flexible risers, bend stiffeners, buoyancy modules, hold-down and hold-back arrangements and cathode protection systems will include:

Together, BP and Wood Group Kenny designed a robust and fit-for service riser system, capable of operating in one of the most severe environmental conditions in the world. The design is based on lessons learned from a history of operating experience in the offshore West Shetland a region. The final riser system will be a combination of state-of-the-art bespoke hardware and integrity-management tools for the harsh weather floating production system.

Based on a paper presented at the 2015 Offshore Technology Conference in Houston, Texas.