44_How to Overcome Challenges With Active Electrical Heating in Deepwater - Offshore

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07/01/2015 How to overcome challenges with active electrical heating in deepwater - Offshore

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How to overcome challenges withactive electrical heating indeepwater

02/01/2009

L. Delebecque, E. Sibaud, M. Scocard, C. Rueda, P.Delbene - Saipem s.a. (France)

Electrical heating of thermal i nsulated pipelines to prevent hydrate formation and waxdeposition in subsea oil prod uction has proven to be technically and eco nomicallyviabl e in shallow water applications.

Based on Saipem experienc e, Direct Electrical Heating (DEH) is the preferred solutioncompared to indirect heating or hot fluid circulation configurations (bundle or pipe-in-pipe).

S aipe m has been working to adapt DEH to deep waters and finds installation is thecritical point and probably the most challenging technical issue. The company hasdeveloped installation procedures and cost-effective solutions to ensure reliable andsafe J-lay installation of DEH systems.

The current trend offshore is to go beyond traditional frontiers to produce smaller fieldsin deeper waters, farther away from main fields, and with challenging fluids. The highpressure and low sea bottom temperatures (around 4 C or 39 F) met in these

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Technology challenges

DEH raises various design challenges, in particular electromagnetic and mechanicalissues, as well as corrosion concerns.

Complex electromagnetic phenomena impact system design and eventually affect

installation. These phenomena include:

The distance between the cable and the pipeline impacts the mutual inductance of the system parts. A larger distance between pipeline and cable increases power requirements. So, the cable must be as close as possible to the pipeline.Metal between the cable conductor and the pipeline reduces system efficiency.Therefore, the piggybacked DEH cable is not armored, making it fragile for installation and operations. The metal reinforcement of any concrete weightcoating to the pipeline must also be considered and metallic ore to increase thedensity of concrete should be discounted.One major issue for DEH systems is the low mechanical resistance of the power cable. It is sensitive to stretching, crushing, and abrasion. Full integral mechanicalprotection of the cable may be required for some projects where the risk of cabledamage is critical.

Olowi field applicationDEH has been selected for use in the Olowi oil and gas field 18 km (11 mi) offshoreGabon. Canadian National Resources International is operator.

Saipem is in charge of design, procurement, and installation of three heated 10-in. (25-cm) pipelines in shallow water. The pipeline with DEH system will be installed in 2009by S-lay installation vessel Castoro 2 .


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Direct Electrical Heating and cable in piggyback arrangement.Click here to enlarge image

This project presented a number of unique characteristics which had not beenencountered on previous projects in the North Sea including;

Shallow water Continuous operation (heating in tail-end production)60 Hz operationReinforced concrete weight coating

Use of anode sledges in Current Transfer Zone.


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Click here to enlarge image

The Olowi DEH system is designed to keep fluid above the WAT (wax appearancetemperature) and for reheating after long shutdowns.

Olowi ProjectWater depth (m): 30 - 40

Flowlines number: 3Pipe length (km): 3.86 - 4.27Pipe ID (in.): 10.75U-value (W/m2K): 4.5Piggyback cable cross-section (sq mm): 1,000Sea water temperature ( C): 13.5 - 27Voltage (kV): 1.7 - 2.4Target temperature ( C): 43

Power demand (MW): 3.0 - 5.0Temperature maintenance:Target temp ( C): 43Power demand (mW): 3.0-3.5Current (A): 1,245Reheating, remediation:Heat generation (W/m): 133Current (A): 1,400

Deepwater issues


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DEH is field-proven to 500 m (1,640 ft) water depth using S-lay since the power cablecan piggyback the pipeline on the laying vessel with limited impact on installation.

With S-lay, sufficient space can be managed to install an additional work station after the tensioners and before the stinger. Furthermore, the gentle V-shaped stinger issuitable for DEH system installation on condition that pipeline position is controlledduring the descent form the laying vessel until touchdown point.

Olowi project location and field layout.Click here to enlarge image

Where DEH will be needed in deeper waters, J-lay will be required. However J-lay isnot as easily adaptable as S-lay method for DEH for the following reasons:

1. There is a unique work station (namely the AST in case of the FDS) on a J-lay


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vessel, so that any additional required operation at this level will immediately impact thelaying rate

2. Deepwater and J-lay installation put additional mechanical constraints on the pipelineand the cable which can be critical given the fragility of the non-armored cable (passagethrough clamping device, passage through stinger).

The field in this offshore West Africa case study has a tie-back of 15 km in water depthsfrom 1,500 m to 2,000 m (4,921 ft to 6,562 ft). The DEH system aims at maintaining thetemperature above the 21 C (70 F) hydrate formation temperature during shutdownsfor a 10-in. ID production flowline and a 4 W/m/K production flowline U-Value.

Based on previous projects, the DEH system is designed to provide about 60W/m to thefluid with a 1,500A supply current and a 3.9kV voltage drop for the 15 km scenario. A

12kV XLPE insulated cable with a conductor cross section of 1,000 sq mm (1.55 sq in)can be used in this case.

J-lay installation

Different installation methods have been developed for DEH installation in deepwater.The typical scenario described in the previous section leading to a preliminary designedDEH system for deepwater applications has been used as a reference for this study.

The three different installation methods are:

1. Piggyback: The attachment of the cable to the pipeline is done on the FDS in the AST. Two possible configurations are where the cable is normally strapped onto thepipeline and where the cable is inserted into a groove made into the coating

2. Simultaneous installation of the pipeline and the cable (namely dual-laying). Theconnection of the cable to the pipe can be done subsea by ROV either just after thepassage through the stinger or after touch down point

3. Installation of the cable and the pipeline in two separate campaigns: The cable isclosed up to the pipe subsea using ROV once the pipe is on the seabed.

Acknowledgements

The authors would like to thank Canadian National Resources International for

permission to publish this paper and Saipem Olowi project team for their support on thissubject.

Editors note: This article is a summary of the paper presented at PennWells Deep


8/12



Gas exports resume from Kvitebjrn

field | Previous Article

Next Article | Nexans to supply Skarv

field DEH system

North Sea Huntington back onlineDec 19, 2014

Noreco says the partner in the Huntington field in the UK central North Sea has been granted

temporary approval to resume production at a restricted rate and the field is back onstream.

Nexans dispatches first Shah Deniz Stage 2 DEH cablesDec 18, 2014

Nexans has delivered its first subsea direct electrical heating (DEH) system consignment to

BP for use in the Shah Deniz Stage 2 project in the Azeri sector of the Caspian Sea.

PSA approves Oseberg life extension offshore Norway

Dec 18, 2014

Petroleum Safety Authority (PSA) Norway has sanctioned continued operations at the

Oseberg Vestflanke in the North Sea.

In-line inspections guide offshore field managementdecisionsDec 12, 2014

Assessing the working condition of aging offshore pipelines is a challenging process made

more complicated by their location on the seafloor and limited access points. An operator off

the coast of ...

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44_How to Overcome Challenges With Active Electrical Heating in Deepwater - Offshore

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