Kristin – high pressure and temperature
Development of Kristin encompasses 12 subsea production wells divided between four seabed templates, which are tied back to a semi-submersible production platform. The latter also hosts production from Tyrihans, operated by Equinor, and the Maria field where Wintershall is operator. These two developments came on stream in 2009 and 2017 respectively.[REMOVE]Fotnote: https://www.equinor.com/energy/kristin
Pressure and temperature challenges
The definition of high pressure and temperature in a field is a static pressure at the wellhead of at least 690 bar (10 000 pounds per square inch – psi) or a temperature of at least 150°C at the bottom of the well.
Kristin’s reservoir lies well over these limits, with a wellhead pressure of 911 bar (13 200 psi) and a bottom hole temperature of 170°C. These values are higher than for any other field developed so far on the Norwegian continental shelf.[REMOVE]Fotnote: Ibid.
Developing equipment capable of dealing with such forces presented major challenges.[REMOVE]Fotnote: Hundseid, Flaten and Fossum, 2004, “Subsea System Design for the HPHT Kristin Field Development. Thermal and Pressure Loads”, OTC 16688. Wellhead equipment had to be so strong that it created a barrier to prevent the high downhole pressure propagating up the system. Were that to happen, it could damage the flexible hoses and risers which carry the wellstream to the process plant on the platform. These were not designed to cope with such pressures.[REMOVE]Fotnote: This article builds to a great extent on Gjerde, Kristin Øye and Nergaard, Arnfinn, 2019, Getting down to it. 50 years of Norwegian subsea success. Norwegian Petroleum Museum, Wigestrand: 153-154 and 250-251.
Xmas tree – a crucial pressure barrier
One challenge on Kristin was therefore to come up with an Xmas tree for the wellhead which could cope with the high pressure. This assembly of valves is at the very heart of subsea technology – and the guarantor of its safety. On the one hand, it must ensure reliable production. On the other, its valves must be able to shut if and when required. The tree incorporates several barrier elements for controlling the well.
In purely physical terms, an Xmas tree is not particularly impressive. It can be described in the simplest terms as layer upon layer of valves and their actuators, with a wellhead coupling at the base. A tree can be up to eight metres tall and measure roughly five by five metres in length and breadth. Its weight can vary between 10 and 50 tonnes.[REMOVE]Fotnote: Gjerde, Kristin Øye and Nergaard, Arnfinn, op.cit: 153-154.
Kristin - Beyond limits technology. Photo: Equinor
Norway’s Aker Kværner Subsea secured the job of delivering a full subsea package with 10 wells, four templates and a complete control system for Kristin. The high-pressure components were to come from Kværner National in Houston. But testing established that the trees manufactured in Texas were inadequate to the task – they leaked nitrogen. With the aid of Norwegian engineering skills, a solid piece of development work succeeded in producing equipment which could cope with the challenge.[REMOVE]Fotnote: Interview with Raymond Carlsen, 2 November 2017.
Each tree delivered by Aker Kværner was fitted with a choke valve intended to reduce the pressure to a maximum of 260 bar. Should one of these valves fail, three separate safety systems ensured that the high pressure was not propagated further.
First, if a pressure above 260 bar was measured above the choke, the tree’s main and wing valves would close to contain the increase.
Second, should the main and wing valves fail, a high integrity pressure protection system (Hipps) would take over. This comprised two independent shutoff valves positioned in tandem at the start of each flowline, so that the high pressure was confined to the production system in the actual template.
Finally, if choke, wing, master and both Hipps valves were all to fail, a pressure relief valve was positioned on top of each riser.[REMOVE]Fotnote: Gjerde, Kristin Øye and Nergaard, Arnfinn, op.cit: 250-251
Controlling temperature
The other big challenge on Kristin was temperature control. How much heat the flexible risers to the platform could tolerate was limited. The sheath around the riser was manufactured from a material unable to cope with more than 132°C, compared with the estimated maximum at the wellhead of 157°C.
To overcome this issue, Statoil opted for a combination of some cooling effect at the choke valve and heat loss through the flowline before the wellstream reached the critical point in the flexible riser.
Another problem arose if production had to be suspended, when excess cooling in the seabed flowlines could cause the formation of hydrate (hydrocarbon ice). This was combated by installing a direct electric heating system on the pipes which ensured that the temperature remained at least 23°C in a shutdown.
Subsea isolation valves (SSIVs) were installed in the flowlines and gas export pipelines as an extra security measure to prevent their content flowing to the platform under critical conditions.
Such devices became an industry standard after the Piper Alpha disaster on the UKCS in July 1988, when 167 people died in an explosive fire. This was fuelled largely by export gas which kept arriving after the blaze had begun.[REMOVE]Fotnote: Ibid.
All in all, the Kristin project led to a number of components in the underwater factory being further developed and qualified for future subsea projects.
