This paper presents hydrate design premises established to reach the finaldesign and operational philosophy for the 3 phase subsea separation system, also known as Marlim SSAO. The main purpose of this pilot station is toseparate the produced water and reinject it into reservoir for pressure supportwhile routing the oil and gas to topside. Since the subsea process station handles multiphase flow where gas and freewater are present, and the system is exposed to low temperatures by the ambientcold sea water, a good strategy to avoid hydrate formation is necessary. Thehydrate strategy must be incorporated as a part of the total system design andshall handle all critical operational scenarios as shut-down and start-ups. Thehydrate strategy is closely linked to the temperature control and theevaluation of need for thermal insulation of the system. Temperature control isalso important in the system because of high sensitivity in fluid properties. General thermal insulation verification analysis and detailed studies of coldspots are required. Real fluid testing was included in order to better evaluatethe hydrate potential. The Marlim SSAO, as an integrated part of a field system from subsea wells totopside, is divided into several parts for the facilitation of the flowassurance and the hydrate prevention strategy: Multiphase lines, water linesand water injection system. The hydrate prevention is very challenging becauseof several open connections between the multiphase lines and the water lines. Hence, usual means as MEG inhibition and thermal insulation have not beenenough to ensure the hydrate prevention strategy and new strategies have beendeveloped. It has been necessary to challenge the strategies in every part ofthe system. The results of the work methodology and the analysis executed indicated thatMarlim SSAO is a safe system to operate from a flow assurance and hydrateprevention point of view. The material presented in here intends to establish akey reference for preservation design of 3 phase subsea separation systems. Itapplies for future generations of these kinds of equipments. Introduction The Marlim SSAO is a subsea processing pilot station which has been installedin the Campos Basin off the coast of Brazil. The objective is to separate mostof the water from the production stream and re-inject it into the reservoir forpressure support while transporting the oil and gas to topside. The SSAO isinstalled at a water depth of 876 m, 341 m from the production wellhead and2100 m from the injection wellhead. The hydrocarbons will be sent to thetopside facilities 2400 m (riser and flowline length) from the SSAO. This paper describes the challenges and innovative solutions on flow assuranceand hydrate prevention strategy for the Marlim SSAO. This includes the mainphilosophy, the preservation of the station in different operational modes, evaluations of identified risks, and calculations and analysis performed tosupport the strategy.

This paper presents an overview of the reservoir requirements regarding thedevelopment of the Marlim 3-Phase Subsea Separation System - SSAO (for theterms in Portuguese). Produced water re-injection (PWRI) in the reservoir insuch scenario is a challenging task, regarding the well injectivity maintenance(SSAO-treated produced water suspended solids and oil content, sub-sea andsub-surface facilities materials compatible with the produced watercorrosiveness), geomechanical effects, and biogenic reservoir souringcontrol. Lessons learned from previous PWRI tests and the implications of somealternatives to the subsea separation and re-injection systems are reported. This work comprehends also the definition of chemicals applyed to preventundesirable effects: a bio-dispersant (to avoid biocorrosion and biofouling inthe re-injection system) and a nitrate salt (to oppose biogenic reservoirsouring potential). Relevant operational aspects are mentioned as well, fromchemical injection to the monitoring of both solid particles and oil dropletscontents in the re-injection water stream. Introduction Marlim field area is 149.4 km2 and the water depth range is 650 - 1050 m. Themain reservoirs of Marlim field are turbidites of Oligocene/Miocene age. All ofthe sandstone facies are poorly-consolidated. Reservoir-rock porosity andpermeability are relatively homogeneous, typically ranging 27 - 30 %, and 1,000- 6,500 mD (4,000 mD average), respectively. The oil API varies from 17° to 24° in the main field area and oil saturationpressure is 265 kgf/cm2, which is 22 kgf/cm2 below the reservoir originalpressure. The Marlim reservoir mechanism is solution gas drive. Water injectionhas been selected as the most feasible reservoir pressure maintenance method, thanks to its simplicity, besides factors like deepwater location, reservoircontinuity, relative scarcity of gas, and favorable characteristics of therelative permeability curves. Initial oil production from the Marlim field was in March 1991 and the peak ofproduction was approximately 630,000 bpd of oil in April, 2002. Seawaterinjection started in 1994. Injection pumps were designed to matrix injectionconditions, but considerable injectivity decline was observed in some of theearlier injection wells. Marlim field was developed with subsea wells, so anyworkover operation to restore the injectivity requires a floating rig to beperformed, which makes them very expensive. Therefore, injectivity preservationis essential to assure that the sustainment of the required seawater injectionvolumes. A comprehensive program was established to control the injected waterquality, imposing strict limits to its suspended solids concentration andparticle size, oil and grease content, as well as corrosion-related parameters(oxygen, CO2 and sulfide content, bacteria counts). Injection rate, injectivityindex, frequency of tubing or flowline substitution, corrosion rates andfilterability are also permanently monitored (Pinto et al., 2001). Some of the Marlim field water treatment facilities are nearly reaching theircapacity. An alternative to reduce the topsides water load is to remove water,as much as possible, on the seafloor. So far, it is not considered feasible, onthe subsea system, to achieve the required water quality to discharge it on thesea. The only alternative is to dispose the separated water in subsurface. InMarlim field, it was decided to re-inject the separated water in the reservoiritself, because there are no proper adjacent geological formations to takeit.