SS: Marlim 3 Phase Subsea Separation System: Controls Design Incorporating Dynamic Simulation Work
Authors: Pereira Rafael Merenda , Campos Mario Cesar Mello Massa de , de Oliveira Dennis Azevedo , de Souza Ricardo dos Santos Alves , Filho Marcos Muniz Calor , Orlowski Rene , Duarte Daniel Greco , Raposo Gelmirez Martins , Lillebrekke Christian , Ljungquist Dag , Carvalho Andrea , Fares Martina 
Published: April 2012

This paper describes the control system design for the Marlim three phasesubsea separation system (SSAO) and how the standard subsea control system hasbeen adapted for the new requirements for automated control. This is the mostadvanced subsea process system to date with several " first ever" applicationsof separation equipment subsea: harp, pipeseparator, desanders andhydrocyclones. The SSAO has a total of 7 control loops and a number of complexautomatic sequences.

Further, the paper addresses how dynamic simulation analysis has been used tovalidate the process control strategy and improve the operational proceduresdesigned during the basic engineering phase.

Control and operation of the SSAO has proved to be very challenging for severalreasons:There are strong interactions between different process componentsThe system dynamics are stiff due to small liquid hold-ups and low GOR in thesystemThe pressure drops of inline cyclonic equipment need to be balanced to ensureoptimal performanceConstraints in valve opening/closing speed and the importance of limiting thenumber of valve movements put restrictions on controller performanceInstrumentation is limited compared to topside facilities

The content described above contain several new aspects compared to atraditional subsea control system and this paper will describe systemconsiderations with regards to implemented process control and also theimportance of using dynamic simulations as a design tool.

Introduction

The Marlim three phase subsea separation system (SSAO) was designed to separatethe produced water from the multiphase well stream and re-inject this into theproduction reservoir. Water polishing is required due to the strict OiW contentrequirement of less than 100 ppm. By re-injecting the produced water subsea, the backpressure in the flowline is reduced and the production is increased. Inaddition less water handling capacity is required topside.

The SSAO has been installed in the Marlim Field, Campos Basin, in a water depthof 876 meters, 341 meters from the production well and 2100 meters from theinjection well. Oil, gas, sand and some remaining water will be sent to theP-37, a Floating Production, Storage and Offloading unit (FPSO) with a turretsystem. The SSAO is connected to P-37 with a multiphase line of 2400 meters(riser and flowline). The control of the process system and chemical injectionmanagement system will be located topside on P-37, as well as the power supplyand VSD for the subsea pump.

In Figure 1 it can be seen that the production first goes through a multiphasedesander, which removes most of the sand before the production flow enters thepipeseparator. This is important in order to reduce the amount of sand build upin the downstream equipment and reduced the required frequency of flushingoperations. In the harp, the free gas is removed before the liquid is sentthrough the Pipe Separator to the outlet vessel. The water level in the outletvessel is controlled varying the speed of the water injection pump (WIP). Theoil and gas are re-combined in the outlet vessel and then sent to themultiphase line connected to P-37.

From the outlet vessel, the produced water goes through the water polishingequipment consisting of a desander and a two-stage hydrocyclone module, beforeboosting and re-injection. The requirement for solids and oil content injectedin the injection well is 10 ppm and 100 ppm respectively. For a projectoverview, please see OTC-23230.