Hydrate management in deepwater production jumpers connecting production Christmas Trees to a manifold, or directly to a production line, generally requires the injection of large quantities of methanol to flush the line during the preservation sequence that follows a production shut down. The paper describes how thermal insulation, methanol lines and umbilicals are sized to prevent hydrates forming in jumpers. It then goes on to show how a low energy, electrical heating system can improve current operational practices by reducing the requirements for methanol injection and how this simple and efficient device can also be used to dissociate a hydrate plug and eliminate the need for an additional remediation system.
A series of reduced scale tests have been performed at the French Petroleum Institute (IFP) in a 2" insulated pressure cell where several hydrate plugs were dissociated using the electrical heating system. The main results of these tests are presented to establish the thermal and electrical efficiency and to show that a typical production jumper can be preserved with little electrical power.
The paper also provides details and results of a full scale test performed underwater on a prototype section of insulated jumper fitted with the heating and temperature control systems. These tests have qualified this technology for future projects.
These experiments and tests have allowed the efficiency and limitations of the electrical heating system to be determined so that it can be used by Operators seeking environmentally friendly solutions for future deepwater developments that will reduce topsides requirements and simplify the operational hydrate prevention strategy.
Hydrates are crystals in which cages of water molecules are stabilized by host gas molecules such as methane, ethane, propane or carbon dioxide. Hydrates that look like ice can form in production lines and plug them. For hydrates to form, four conditions are necessary: gas, water, low temperatures and high pressures [1]. These conditions are encountered in deepwater developments with subsea Christmas trees where warm production fluids containing gas and water at pressures of more than 100 bars encounter low seabed temperatures of about 4°C around the trees, jumpers and flowlines.
To avoid hydrates, the first line of defence is to provide adequate insulation on all parts the subsea system including the Christmas trees, well jumpers, manifolds and the flowline loops that carry the production fluids back to the topsides on the surface facility. Passive insulation is generally designed to keep the production fluids above the hydrate formation temperature in the flowing condition and for a certain additional period in case of a production shut-down, often referred to as the cool-down time. During an extended production shut-down dead oil can be circulated around the flowline loops to prevent hydrate formation but this does not apply to the production jumpers where gas pockets form in the high spots and water collects at the low points.
