Mudline ESP: Eletrical Submersible Pump Installed in a Subsea Skid
Authors: Costa Bruno Maia Pyramo , Oliveira Pedro da Silva , Roberto Marcello Augustus Ramos 
Published: May 2013

The development of deep and ultra deepwater fields, especially in marginal areas, always presents challenges related to profitability and/or flow assurance related issues for subsea equipment.

Currently, Petrobras has a great number of production wells operating in ultra deep waters, thereby is constantly looking forward to improve its production capacity and reduce its operational expenditures (OPEX) as well.

In order to address the challenge of reducing both OPEX and CAPEX (capital expenditures) when focusing low GOR scenarios, Petrobras has developed a new application for a conventional ESP, the so called Mudline ESP. This innovative technology can be used in large scale to boost oil production from marginal/ remote wells located in deep waters fields.

A prototype Mudline ESP was tested in the Petrobras Atalaia test site to investigate the flow behavior of the system (e.g.: start up with high gas conditions), to determine the maximum allowable GVF and its behavior when powered by a single three phase electrical cable, besides gauging other effects. After its manufacture, the SIT (System Integration Test) was performed in Atalaia test site as well.

After SIT the Prototype Mudline ESP was installed (Dec, 2011) in the Espadarte field (Campos Basin) in a water depth of 1300 m to boost the production during an extended well test. The field performance of the Mudline ESP in Espadarte was above the expectations. These positive results gave confidence in the Mudline ESP, that becomes an extra option for subsea boosting in Petrobras. Soon, this new pumping concept will be operating in both Chinook and Cascade fields in the Gulf of Mexico (GoM) area.

Introduction

The durability of an Electrical Submersible Pump (ESP) system relies upon qualification requirements, subsea installation procedures and - ultimately - how the equipment is operated. As a matter of fact, the main application of the ESP in the subsea area is to increase the oil flowrate and its installation is inside the production well. However, the installation of an ESP in a subsea well can pose a series of risks to the equipment, for instance, shocks on the inner surfaces of completion riser and of well casing. These problems can affect not only the electrical cable, but also the integrity of the ESP itself.

When using the downhole ESP in subsea applications there is the necessity of using a drilling rig to install, retrieve and reinstall the ESP (thus, installation and maintenance costs are very high) and the need to modify the standard Christmas tree design (using a Horizontal Christmas tree concept).

Besides these risks, the environment within a subsea well, where high temperatures and high pressures prevail, may expose the ESP to aggressive conditions, reducing its life expectancy. Because of that, Petrobras usually adopts, whenever possible, the use of gas lift as a back-up system, in case of ESP system failure.

In order to reduce both installation and maintenance costs and to allow an independent gas lift project, Petrobras has developed some alternatives to install the ESP out of production well, in the mudline. It is beneficial to have the booster system installed on the seafloor because there is no need to use costly resources, such as an intervention rig to retrieve and/or install the ESP.

One option for seafloor ESP is to install it inside a dummy well, a solution applied in the Jubarte and Golfinho fields1.

Another alternative developed by Petrobras is the so called ESP on the Skid. The objective has been to conceive a system with an ESP that can be installed on the seabed which has been designed in two modules: the pump module and the flowbase. The pump module comprises two ESP in series with an inclination of 5 degrees from the horizontal. The other part, which is the flowbase, is coupled in a separate unit located below the pump module.