A subsea bare flowline was plugged with hydrates and waxes causing loss of production. The hydrate plug was successfully dissociated by using the two-sided depressurization method. The wax plug in the subsea flowline was then successfully removed by using a low-cost sustained pressurization method called Micro Annular Pressure Pulse (MAPP). The remediation methods, implementation procedures, and field experiences are described in the paper.

Waxes and hydrates together can make complicated blockages in subsea flowlines. The two-sided depressurization method is a very effective method of clearing hydrate plugs in subsea flowlines. A hydrate melting model and a wax deposition model were useful tools for making critical decisions in hydrate and wax remediations. The MAPP method proved to be a cost-effective method for removal of wax plugs in subsea flowines.

Problem Description

One of the dual subsea flowlines in an offshore oil field was found to be plugged, when production was brought back on line, after an extended shut in. The 8" subsea flowline is composed of a 22,500-ft long bare steel pipe, a 650-ft long flexible tail connecting the main steel pipeline to a subsea manifold, and a 1,800-ft long lazy wave riser which ties the main steel pipeline to a FPSO. The bare flowline was not buried, just laid on the seafloor. The water depths are 900-ft at the subsea manifold and 550-ft at the FPSO, respectively. The seafloor water temperature at the manifold is 50 °F, while the seafloor water temperature near the FPSO is 59 °F, approximately. Four subsea wells are tied into the dual 8" flowlines via the subsea manifold. Parallel to the dual subsea production flowlines is a 6" gas lift line. Only one of eight original wellhead chemical injection lines was available through this project.

Problem Diagnosis

What plugged the subsea flowline? Hydrates, waxes, scales, mechanical damage or sand accumulation? A systematic diagnosis was performed so that appropriate remediation methods could be used to bring the plugged flowline back to service at a minimum cost. Sample analysis of the produced water eliminated the possibility of scale formation in the flowline. No sign of sand production was observed prior to the blockage. A ROV survey of the line found no visible mechanical damage and a comparison to earlier surveys revealed no significant changes occurred.

Hydrate blockage was very likely, since the produced fluids in the subsea flowline fell into the hydrate formation region during shut-in, as shown in Figure 1. The hydrate curve was generated based on the composition of the produced hydrocarbons and the salinity of the produced water. One of the colder subsea jumpers in this field had experienced hydrates blockage and had been remediated successfully with depressurization.

Figure 1: Shut-in condition falls in hydrate region (Available in full paper)

The wax content and wax appearance temperature (WAT) of the crude oil were measured to be 3.9% by weight and 70 °F, respectively.

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