L. Delebecque, E. Sibaud, M. Scocard, C. Rueda, P. Delbene - Saipem s.a. (France)
Electrical heating of thermal insulated pipelines to prevent hydrate formation and wax deposition in subsea oil production has proven to be technically and economically viable in shallow water applications.
Based on Saipem experience, Direct Electrical Heating (DEH) is the preferred solution compared to indirect heating or hot fluid circulation configurations (bundle or pipe-in-pipe).
Saipem has been working to adapt DEH to deep waters and finds installation is the critical point and probably the most challenging technical issue. The company has developed installation procedures and cost-effective solutions to ensure reliable and safe J-lay installation of DEH systems.
The current trend offshore is to go beyond traditional frontiers to produce smaller fields in deeper waters, farther away from main fields, and with challenging fluids. The high pressure and low sea bottom temperatures (around 4° C or 39º F) met in these scenarios lead to the formation of hydrates and/or the deposit of wax in the case of paraffinic oils. These solids can block flowlines.
Up to now, DEH has been used for flowlines only. It has been identified as a quite mature and robust technology among heating solutions in the case where it is coupled with a wet insulation.
DEH concept
DEH is based on the fact that an electrical alternating current (AC) in a metallic conductor (i.e. cable, pipe) generates heat (Joules effect). In the direct pipe heating system, the pipe to be heated is an active conductor in a single-phase circuit, together with a single core power cable as the forward conductor, located in parallel with and close (“piggyback”) to the heated pipe.
From the platform power supply, two riser cables provide the electric power to the heating system. One of the two single-core riser cables (return cable) is connected to the near end of the pipe, and the other (feeder cable installed in “piggyback”) to the forward conductor which is connected to the other end of the pipe.
The heating system is electrically connected (“earthed”) to the surrounding seawater through several sacrificial anodes for a length of about 50 m (164 ft) at both ends where the cables are connected, called “Current Transfer Zone.” Steel structures must be avoided in these zones, and bracelet anodes are required along the running length at fixed intervals.
Given the loss in seawater and depending on the heating requirements and length of the flowline, DEH system can reach heavy ratings up to current around 1500A and insulation voltage up to 52kV.
DEH systems are on several subsea pipelines in the North Sea including Asgard, Huldra, Kristin, and Norne. These are in shallow water (less than 400 m or 1,312 ft). For these, the U-value range is usually between 3 W/m²K and 8 W/m²K, enabling conventional wet-insulation coating and a power requirement range between 1 mW and 2 mW. These projects are also characterized by high current rate and large power cable cross section.
Tyrihans is characterized by a long flowline and associated a high power demand. The Ormen Lange power cable will be a retrofit. They are also the only projects with water depth exceeding 500 m (1,640 ft). These two projects have highlighted the current technological limitations of DEH system (length, water depth, power demand) and have consequently asked for extensive qualification activities.
Technology challenges
DEH raises various design challenges, in particular electromagnetic and mechanical issues, 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 weight coating to the pipeline must also be considered and metallic ore to increase the density 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 mechanical protection of the cable may be required for some projects where the risk of cable damage is critical.
Olowi field application
DEH has been selected for use in the Olowi oil and gas field 18 km (11 mi) offshore Gabon. 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 2009 by S-lay installation vesselCastoro 2.
This project presented a number of unique characteristics which had not been encountered on previous projects in the North Sea including;
- Shallow water
- Continuous operation (heating in tail-end production)
- 60 Hz operation
- Reinforced concrete weight coating
- Use of anode sledges in Current Transfer Zone.
The Olowi DEH system is designed to keep fluid above the WAT (wax appearance temperature) and for reheating after long shutdowns.
- Olowi Project
- Water depth (m): 30 - 40
- Flowlines number: 3
- Pipe length (km): 3.86 - 4.27
- Pipe ID (in.): 10.75
- U-value (W/m2K): 4.5
- Piggyback cable cross-section (sq mm): 1,000
- Sea water temperature (° C): 13.5 - 27
- Voltage (kV): 1.7 - 2.4
- Target temperature (° C): 43
- Power demand (MW): 3.0 - 5.0
- Temperature maintenance:
- Target temp (° C): 43
- Power demand (mW): 3.0-3.5
- Current (A): 1,245
- Reheating, remediation:
- Heat generation (W/m): 133
- Current (A): 1,400
Deepwater issues
DEH is field-proven to 500 m (1,640 ft) water depth using S-lay since the power cable can 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 is suitable for DEH system installation on condition that pipeline position is controlled during the descent form the laying vessel until touchdown point.
Olowi project location and field layout.
Where DEH will be needed in deeper waters, J-lay will be required. However J-lay is not 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 vessel, so that any additional required operation at this level will immediately impact the laying rate
2. Deepwater and J-lay installation put additional mechanical constraints on the pipeline and the cable which can be critical given the fragility of the non-armored cable (passage through clamping device, passage through stinger).
The field in this offshore West Africa case study has a tie-back of 15 km in water depths from 1,500 m to 2,000 m (4,921 ft to 6,562 ft). The DEH system aims at maintaining the temperature above the 21° C (70º F) hydrate formation temperature during shutdowns for a 10-in. ID production flowline and a 4 W/m²/K production flowline U-Value.
Based on previous projects, the DEH system is designed to provide about 60W/m to the fluid 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 designed DEH 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 the pipeline 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). The connection of the cable to the pipe can be done subsea by ROV either just after the passage through the stinger or after touch down point
3. Installation of the cable and the pipeline in two separate campaigns: The cable is closed 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 this subject.
Editor’s note: This article is a summary of the paper presented at PennWell’s Deep Offshore Technology International Conference & Exhibition 2008 in Perth, Australia. www.offshore-mag.com.
