Joint industry programs strive to improve management of hydrates and waxy crudes
Blockages caused by hydrate build-up in production lines are a constant dilemma for deepwater development. A hydrate suppression strategy, factored into the overall project design, can push up capital and operating costs.
An alternative approach could be to transport hydrates through the flowline in a controlled manner as dispersed slurry. According to IFP (Institut Francais du Petrole), this could be achieved in two ways:
1. By exploiting the capacity of certain crude oils to form stable, water-in-oil emulsions
2. Introducing anti-agglomerant additives to condensate or non-dispersive crude oils.
IFP is close to completing a first phase of investigations for the HYSIFLO (HYdrates Slurry Impact on the Flow Properties of Oil Fields) joint industry project (JIP). The purpose of the program to date has been to deliver a prototype Cape-Open predictive software tool known as the Hydrate Flow Module. This is designed to evaluate flow properties and the potential for blockages in pipelines when transporting produced fluids at risk of hydrate build-up.
According to Claude Mabile, deputy director of IFP's Exploration & Production Technology Business Unit, the first-phase objectives have been achieved, including the capability of the software to assess pressure drop. Among the sponsors are one major oil company concerned by insulation issues affecting a planned long-distance subsea tieback.
The Hydrate Flow Module is based on a physical model recently validated for different flow conditions at IFP's Lyre loop in Solaize, south of Lyon. A series of tests were conducted at this facility, using one crude sample provided by one of the sponsors. The aims included widening application of the model to turbulent and multiphase flow conditions. Equipment at Solaize includes a pump designed specifically to re-circulate hydrates, allowing researchers to monitor and analyze hydrates breaking up and re-generating, or shearing into smaller particles.
The first-phase program has focussed on steady-state crude transportation. IFP is discussing options with its sponsors for a second phase. These could extend to examining the impact of stopping and re-starting flow, and the effect of anti-agglomerants on the flow model.
A related two-year JIP which IFP expects to start shortly is AHTo (Anti-Hydrate Top of the Line). "When a deepwater pipeline is transporting a wellstream comprising gas, water, and condensate," Mabile explains, "and conditions are suited to hydrate formation, the majority of the water is in the bottom of the pipe. If you put in additives, it is unclear whether different types of additives are active on the inner walls at the top of the pipe, or whether they prevent formation of hydrates there.
"We are the first organization to work on this issue. The first phase of the program will define a set of experiments, to see how hydrates will stick to pipe walls. We will also seek to mimic the condensation of water without additives at the pipe wall. In a second step, we will study the effect of kinetic inhibitor and anti-agglomerant additives."
A longer-established JIP is Cold Start, currently in its sixth year and its fourth phase. Here the aim is to improve understanding, management, and prediction of re-starting flow of waxy crude in pipelines.
In conditions below the Wax Appearance Temperature (WAT), this type of crude exhibits visco-plastic, thixotropic, temperature-dependent and compressible properties. These are caused by the interlocking gel-like structure formed by a combination of the oil's crystallized paraffin compounds and thermal shrinkage. The main concerns for operators are determining the minimum pressure needed to re-start flow, and the time required to flush the gelled oil out of the pipeline and to re-establish steady-flow conditions.
In previous phases of the program, IFP focussed on understanding the process by means of laboratory rheological characterization, experimental tests on the 150-m (492-ft) long, 2-in. (5-cm) diameter Lyre loop, and development of the StarWaCS-1.5D numerical research code. This resulted in a patented methodology for managing waxy crude flow re-start more efficiently, known as Cold Start.
Phase 4 involves validating this technology and the numerical code via new experiments on the Lyre loop. Other goals are further laboratory measurements to clarify the slip effect and to explain discrepancies in results obtained during previous phases for very low re-start temperatures.
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