The Ormen Lange field are using continuous injection of MEG to prevent hydrate formation. The MEG is regenerated and reused, and corrosion control of the multiphase pipelines are accommodated by a combination of pH stabilised MEG and corrosion inhibitors. In order to avoid scaling if formation water is produced a scale inhibitor will be added to the lean MEG.A comprehensive scale inhibitor qualification program was carried out, and the goal was to qualify one or more scale inhibitors for scale prevention downstream the MEG injection point at subsea wellhead for fully and partial pH stabilisation. 5 suppliers were invited to supply inhibitors to be included in the qualification programme and the suppliers submitted a total of 10 inhibitors. The qualification program included environmental properties, solubility in lean MEG, scale inhibitor screening tests, scale inhibitor dynamic tests, compatibility of chemicals, emulsion and foaming, partitioning, effect on particles formation and settling onshore, consumption/loss of scale inhibitor in the MEG process and stability in MEG regeneration reboiler.The different tests are described and the results are displayed. The results from the qualification program demonstrated that only few inhibitors satisfied all requirements, the chemicals affect the formation and precipitation behaviour of calcium and iron carbonate and that only one combination of scale and corrosion fulfilled all the requirements. If a scale inhibitor has to be used in combination with a corrosion inhibitor compatibility testing is required. Incompatibility between corrosion and scale inhibitor will cause gunking and scaling. A successful combination of corrosion and scale inhibitor will not increase the corrosivity and simultaneously prevent scaling. It has been observed that such combination of chemicals does not prevent formation of particles but prevents the particles to stick to the surface.The scale qualification program demonstrates the complexity in qualifying chemicals and the need for a good dialogue between the involved disciplines.

During qualification of scale inhibitors for the Ormen Lange project, it was discovered that the dynamic tube blocking (DTB) test could not simulate the scaling conditions good enough for gas/condensate subsea fields which will be using pH stabilised MEG for hydrate control. The pH stabilizer in the lean MEG will be present mainly as carbonate and the pH is >10. After mixing with the well fluids the pH will be reduced to ~7. At the subsea mixing point the carbonate will react with CO2 gas and produce bicarbonate. This process could not be simulated with the DTB equipment since it did not allow CO2 gas to be present at the mixing point and the synthetic formation water could not dissolve enough CO2. The result will be too high pH and too high scaling potential. Presence of particles in the lean and rich MEG is also important for the efficiency of scale inhibitors, which in some cases can be difficult to include in tube blocking tests. A new test method has been developed that simulates the mixing point subsea and the pipeline downstream the MEG injection point. Cold lean MEG with particles and warm formation water is mixed and flow down a warm glass wall in a double walled cell purged with CO2 or a mix of CO2 and N2 to achieve the correct CO2 partial pressure and temperature. Then the mix of MEG, formation water and gas is pumped through a glass and/or steel coil. Examination of the glass and steel coils are done visually by the eye and by SEM, respectively. The new test method has been tested and has been used to qualify a scale inhibitor for a subsea development. The use of glass coils showed that the method not only shows presence of scaling if the inhibitors does not work satisfactory, but gunking (in this case separation of components which stics to the surface stemming from scale and/or corrosion inhibitor caused by incompatibility) has also been observed by this method. Introduction The Ormen Lange gas field, which is located in the Norwegian Sea 120 km from the Norwegian coast, is a subsea development. Former Hydro Oil & Energy was operator for the planning and development phase. Shell took over as operator when Ormen Lange came on-stream in October 2007. The subsea production system for the first phase consists of two subsea gas production templates with eight production slots on each template. The subsea production templates are installed at 850 m water depth where the seawater temperature is approximately -1°C. MEG, which is used to avoid freezing and hydrate formation, is transported from an onshore regeneration plant to the subsea production units. The Ormen Lange well fluid contains acetic acid and CO2, in addition to the hydrocarbons, which makes the condensing water from the gas phase quite acidic and thus corrosive. Several methods have been considered for internal corrosion protection of the bottom of the pipelines. Formation water may be produced during the field life. CaCO3 scaling is therefore an important issue. Well intervention at 850 metres sea depth to remove carbonate scale is a very costly operation and efforts to avoid such operations are important. A lot of work has therefore been done to qualify scale inhibitors for Ormen Lange1. Standard tube blocking tests and sophisticated bottle tests were performed to find scale inhibitors for the subsea system without succeeding3. Based on the results from these tests and the fact that none of the test methods used could simulate the mixing point between lean MEG and produced fluids in a satisfactorily manner it was decided that it was necessary to develop a new dynamic test method for testing of scale inhibitors.

Fatigue life calculation of flexible risers is normally based on the assumption that the annulus is dry. Experience has, however, shown that the annulus may become water flooded. Carbon dioxide and hydrogen sulphide permeate from the bore and into the annulus, making the annulus environment corrosive. In such conditions the influence of corrosion fatigue on fatigue life has to be taken into account. A test program was performed to investigate the corrosion fatigue performance of steel armour in water containing carbon dioxide. The results showed that the corrosive environment reduced the fatigue life to about 1/10 to 1/50 of that derived in air. The lifetime was significantly shorter in the environment with 0.1 bar compared to 0.01 bar, indicating that corrosion fatigue becomes more pronounced as the corrosivity increases. The work shown that corrosion fatigue has to be considered for flexible risers with annulus that is or may be water flooded.

A test program has been performed to obtain the permeation coefficients for methane, carbon dioxide and water in PVDF. Small-scale tests showed that water is transported through the PVDF inner sheath of the flexible pipes, and into the annulus. A large-scale test was carried out to verify the small-scale test results. It was performed in a 2″ flexible pipe with length 3 m. The bore temperature and pressure were 100°C and 50 bar, respectively. The pipe was submerged in cold water in order to get a correct temperature gradient in the pipe. The test showed that the annulus of flexible pipe with PVDF inner sheath would become water wet due to permeation, depending upon the bore and annulus conditions.