FPSOs and the single point mooring systems that connect them to the reserves they produce are Bluewater’s business. Elaine Maslin speaks to senior project manager Jeroen de Werd about turret design and the Kaombo project.
The Kaombo turret, visualized next to Bluewater’s headquarters, for scale. Photos from Bluewater.
This year, Dutch floating production specialist Bluewater is past mid-way through building its two, largest ever turret systems, the 85m-high internal turret systems for Total’s twin floating production system Kaombo development offshore Angola, for contractor Saipem. With 17m-diameter bearings, they are among the largest turrets in the world.
But, even as the firm, which has been designing single point mooring systems since 1978, breaks its own records, Jeroen de Werd, Bluewater senior project manager on Kaombo, says there’s no standing still. Even bigger systems are being designed and challenges such as arctic and LNG developments receive the necessary attention.
Turret systems are a type of single point mooring system. They can be internal or external, and fixed or disconnectable, and enable oil to flow up from subsea wells onto a vessel. The turret is usually fixed, through a mooring system, while the rolling, pitching and heaving vessel is able to swivel or weathervane around it. Designs are influenced by water depth and the marine environment, which drive the mooring spread and loads the turret has to accommodate, as well as the production system – how many riser slots are required, type of fluids, injection requirements, etc. – and ever more stringent regulations and operator requirements.
The heart of the system is the swivel stack, which enables the transfer of liquids and gases from the risers onto the vessel, via a circular process manifold fixed to the turret and an access structure fixed to the vessel, as well as providing a conduit for power, controls, hydraulics, injection fluids, etc.
Turret size has been steadily growing as fields have been developed in ever deeper, harsher and more remote locations, with greater production capability requirements. Greater controls systems, more remotely controlled switch boards, submerged pumps and greater functionality have also been added, including cross-manifolds, pigging systems, condition monitoring capability and the ability to handle high-pressure high-temperature production. In summary: more robust, reliable and safe turret systems, which are at the same time easier (and therefore most cost effective) to operate and maintain despite their growing functionality.
The lower turret, ready for sailway to Singapore.
Kaombo fits that trend. Total says Kaombo is its largest development today, in deeper waters than Total has been in offshore Angola before, at up to 1950m. The development’s two FPSOs, Kaombo Norte and Kaombo Sul, will produce oil from a cluster of scattered reservoirs covering some 800sq km – or eight times the area of Paris. Covering such an area will mean laying Angola’s largest subsea network for a single project – with more than 300 km of pipelines, according to Total. Rather than a newbuild vessel, Total and Saipem are now converting two crude carriers for the project, at Sembawang Shipyard in Singapore, with 180,000 b/d oil production capacity, 2.9 MMcm/d gas export and 200,000 b/d water injection capacity.
Each turret has to be able to accommodate a nine-point deepwater mooring spread and 18 riser slots, resulting in the 8000-ton a piece systems being built, with 25m-high swivel stacks. Each contains some 10km of piping and 80km of cabling, highlighting some of the complexity involved in these structures. The main bearing, a roller bearing, is 17m in diameter, equivalent to nearly the length of two double decker buses. In comparison, the first turret systems were only a few meters in diameter and weighed just a few hundred tonnes.
“The Kaombo turrets will be among the largest and most complex turret systems ever designed and built to date,” de Werd says. “Each of them will weigh 8000-tonne. It is a massive development in size and in weight as direct consequences of environmental loads, motions, and prescribed requirements.”
The turrets are being built in three locations. The lower turret which includes a pre-fitted main bearing is being built in Abu Dhabi. The upper turret is being built at the Sembawang Shipyard in Singapore, where the vessels are also being converted. The swivel stack and main bearing are being manufactured in Germany.
A 3D model of the turntable and access structure.
Typically, projects like this take about 3-4 years, de Werd says. Design, procurement, and construction take up to 2.5 years, then installation and integration, onshore commissioning and transit to field, hook up and offshore commissioning the balance of the time. The Kaombo project started in April 2014 and first steel was cut in early 2015. The first turret was being integrated in May this year, with integration of the second due later in the summer.
The project has strong Dutch content, with contractors and suppliers including Frames, Drie-D, IHC, Trustlube, Gerritsen, and Trelleborg. Others like Heerema Marine Contractors (HMC) are also involved in the Kaombo development project.
But, while there’s a lot of focus on Kaombo, Bluewater continues to work on other awarded projects, such as Rosebank. The design for the Rosebank FPSO, which would have to contend with huge wave regimes in the rough, deep areas at west of Shetlands (UK), is even larger – at some 30m wave height in 1220m water depth.
The firm is also continuing to develop turret and swivel stack technology for future needs:. “You need to innovate, be smarter, faster, cheaper, deliver sooner and be able to cope with challenging field characteristics like sour and HPHT services, novel requirements and even harsher environments, more remote locations, and 25+ years design lives,” de Werd says. “For example, our current turret designs usually accommodates pressure for water injection and gas export up to 350 bar. Nowadays, exceeding 500 bar is more commonly asked and we are developing 700 to about 1000 bar pressures and temperatures exceeding 130° C. There is quite some development in that respect.”
There is also work ongoing on disconnectable systems with and without DP, for early well tests, where operators might want early cash from their reserves but not investing in a full system for decades. Bluewater is looking at arctic, LNG and heavy oil solutions, as well as systems for marginal or stranded areas.
“Apart from higher safety standards, cost reductions and challenging schedules, those are the greatest opportunities,” he says. “The challenge is to make sure we continue to be able to understand the demands of our clients and locations where they need solutions.” But, while a lot of focus is on technology and system design, having a proper execution model is also crucial, de Werd highlights, and they must go hand in hand on complex systems like this to make sure the projects are a success.
Bluewater also owns and operates a fleet of floating production vessels and floating storage and offloading units, and it is leading a tidal energy project, featured in last year’s Dutch Offshore review.