Ensuring the structural and marine integrity of floating production installations over their lifetime was the overall theme of a technical session at Offshore Europe in Aberdeen this fall. Meg Chesshyre reports.
Floating production storage and offloading (FPSO) vessels have been a part of UK Continental Shelf industry for more than two decades. In that time, there have been incidents with both mooring systems and hull structural integrity that required corrective action.
Advances in turret design
The Quad 204 turret design sets a new standard and base case for the new generation of very large internal turret systems, according to a joint BP/SBM paper presented by Philippe Lavagna, SBM Offshore’s project engineering – mooring EPM manager. The paper compared the main features of the Schiehallion, onstream in 1998, with the turret mooring system of the vessel due to replace it, the Quad 204, being built by Hyundai Heavy Industries in South Korea. Both FPSOs are supplied by SBM Offshore, considered a leader in mooring technology with approxiimately 50 turret mooring systems under its belt.
The Quad 204 turret will be one of the largest in the world, with a mooring force of 2250tonne to meet 100-year environmental conditions. SBM Offshore’s contract, awarded in June 2010, is for the engineering, procurement, construction and transportation of the turret modules including the swivels, mooring lines and suction anchors.
Since the new unit will be moored in the same location as the original Schiehallion FPSO, Block 204/20, west of Shetland, most of the existing subsea infra-structure will continue operations. It will weigh more than 10,000-tonne and will be 94m high – about the same height as London’s Big Ben – making it more than 50% taller than the Schiehallion turret. It will contain significantly more equipment and have a larger throughput. The weathervaning transfer system for fluids (live production, water for injection, gas, various chemicals), power (electrical and hydraulic) and signals (electrical and optical) will be enabled by the world’s largest swivel stack, comprised of 14 swivel units, weighing 265- tonne and measuring 26m. (see Fig. 1).
The Quad 204 turret is based on SBM Offshore’s proprietary bogie-bearing design and will be moored in 450m water depth. The area’s environmental conditions make mooring challenging, with extreme design sea states of up to 100- year significant wave height, about 18m, and high fatigue loading of of one-year significant wave height of about 13m. As a result of experiences on Schiehallion over the last 15 years, the design maturity of the mooring line components, such as steel wire rope and chain, is now quite advanced for the water depth.
Future Atlantic Frontier developments will be further west in even deeper waters, which will present new challenges to the industry. The combination of deepwater (deeper than 1000m) and the extremely harsh environment will represent a first-ever for the mooring industry, requiring tailored modifications. For example, in some countries, the wire rope component will have to be replaced by polyester rope. Although polyester ropes have been used for more than 10 years in the deep waters of Brazil, the sea conditions are significantly less stringent there, compared with the area west of Shetland.
BP awarded the contract for the disconnection of the Schiehallion and the installation of the new FPSO to Technip UK. Technip UK in turn awarded SBM Offshore an additional subcontract in November 2012, which includes phases I-III. Phase I consists of the disconnection and removal of the existing Schiehallion and mooring lines and its subsequent tow to Rotterdam, now currently ongoing. Phase II, occurring in 2014, is the recovery of the old mooring system and installation of the new FPSO anchoring system, including the 20 new piles and chain and spiral strand wire mooring system. Phase III includes Schiehallion’s tow to field and the mooring of the new FPSO, to be carried out in 2015. Schiehallion was decommissioned earlier this year.
The new turret design will provide sufficient space for process sub-systems and utility support for the subsea control system. The turret will comply with a reduced mooring offset envelope for more onerous specified weather conditions, with the existing riser arrangement and new overall subsea production system layout. This includes design requirements for up to 28 riser slots, against 24 for the Schiehallion FPSO, and for the anchor legs to be grouped into four clusters of five mooring lines each, to suit the existing subsea infrastructure.
The turret system’s arrangement will allow anchor lines and risers installation to be diverless – an enhanced safety feature, compared with previous designs. The turret structure has been designed for a minimum 25-year service life. Swivel seals change-out will be possible in-situ, without interruption to production, due to SBM Offshore’s technology. The bogiebearing weathervaning system can also be repaired, and main components, such as wheels, replaced insitu, were this ever to be necessary. This system operated for 15 years on the Schiehallion turret and the bogies performed their function without incident.
In addition to handling the full crude production, water injection and water injection, gas lift, export and import flows, the swivel system will provide for all ancillary services required on the turret fixed part, including electric power and control, chemical injection, water deluge, and air, for turret equipment room pressurization.
“This 10,000-tonne Quad 204 turret is the most critical component of the FPSO,” stated Jeff Mace, BP’s turret delivery manager. The next important milestone, after delivery of the manifold and gantry structures in Korea, will be the mechanical completion of the integrated turret, planned for Q3 2014. BP will manage this process with the assistance of SBM Offshore.
Safety standards based on systematic in-depth analysis and improved reliability for operations are the key enhancements of the Quad 204 turret mooring system over the original Schiehallion. Some key improvements are the riser top-mounted emergency shutdown valves and wind shielding, which could no longer be one complete structure due to the increased size of the Quad 204 turret. Improved operability and reliability aspects are key design drivers to improving technology. The mooring force increased due to the larger size of the vessel and therefore larger safety factors, which contribute largely to a higher reliability.
For the installation of the new mooring system in Q2 2014 and the new FPSO hook-up in 2015, the close interface between SBM Offshore teams optimizes both the mooring system and turret ergonomics. It also facilitates the interfaces for delivery of mooring equipment and readiness of the installation spread. The turret design is tied into to the mooring design and to installation requirements: from the pre-tension of the anchor points up to the hook-up of mooring lines, as well as requirements for maintenance and/or change-out.
“This is the most complex turret SBM has ever supplied to the oil and gas industry. The project combines unique skill sets to the industry; turret mooring supply, field life extension, technology development and offshore contracting” says Laurent Agussol, SBM Offshore major project manager. SBM Offshore says it is the only company in the industry that can complete all stages in-house – engineering, procurement, construction and installation.
At the top end of the market, the requirements of complex mooring systems, such as the Quad 204 turret mooring system, are continually being extended because of deeper water, more severe weather conditions, larger vessels to be moored, higher throughputs, increased pressures and longer design lives. The industry continues to adapt and tailor solutions for safer and more efficient operations offshore, says SBM.
Hotwork repairs
The feasibility of achieving hull structural hotwork repairs on board an operational FPSO was explored in a presentation by Calum MacLean, projects director with Marine Technical Limits. He stressed the importance of forward planning for on-station hotwork repair and demonstrated how major repair work scopes, traditionally requiring completion in a dry dock, can be carried out while the FPSO is producing.
Although, in theory, FPSOs can be taken off station, the reality is that this has huge cost implications and reservoir issues may also make it technically very difficult. Even if the FPSO has a detachable turret, dry docking is likely to result in a minimum of eight weeks off station.
While periodic dry docking can be easily scheduled for sailing ships, FPSO operators are increasingly looking for solutions that will enable them to keep their asset on station and in production for more than 20-25 years. This being the case, it is inevitable that regardless of advances in integrity management strategies, structural hot work repairs will be required on-station.
The relatively small numbers of FPSOs mean that operators may have limited experience with hull structural issues and standard procedures. For example, confined space entry procedure may not adequately address the unique issues presented by working in the enclosed but large space FPSO tanks. Clearly defined procedures are necessary for emergency responses to incidents, IP rescue, fire and muster, isolation standards, boundary hotwork management and tank cleaning.
While a great deal of effort is required to put a safe system of work in place and to organize and execute the repairs, MacLean stressed that the effort required to explain to senior management that often repairing a ship by welding cannot be avoided, should not be underestimated.
In principle, preparation for on-station hotwork repair needs to start before the FPSO arrives on station. Operations management teams should account for in-tank welding in the design, as that is certain to happen in the future, but, more importantly in the development of policies and the safety management system for the installation.
According to MacLean, it was paradoxical to consider that working within the tanks involves work at height and that these very large tank spaces are categorized as “confined spaces.” The key to executing hotwork repairs, while in ,production is fully understanding these hazards and developing an appropriate safe system of work for entry and working in tanks.
MacLean concluded that maintaining the hull structure of an FPSO while on station is not impossible if an appropriate level of planning is carried out. There are solutions available for many of the issues which may arise throughout the life of an ageing installation, however, it is wise to fully understand the work involved long before it is necessary to implement it.
In particular, agreeing on and establishing the parameters for a safe system of work within the operating company, partners, contractors, crew, and any other stakeholder needs careful and detailed consideration and can take a significant amount of time. OE
Image Caption (top):The Quad 204 swivel stack
Image Caption (2nd from top): Scaffold access for extensive steel renewals at height.
Image Caption (3rd from top):The Quad 204 turret mooring system.
Image Caption (4th from top): Tank cleaned and ready for hotwork
Image Caption (bottom):Tank inspection
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