New Technology Enables Development of Norwegian Sea Field
A new enabling technology known as electrically heat-traced flowline (EHTF) will be used to enable system startup and shutdown and to maintain production fluids outside of the hydrate envelope during steady-state operation.
The Ærfugl field is a gas condensate field located in the Norwegian Sea. Heat input into the flowline system is required during shutdown and potentially also during off-plateau production periods. A new enabling technology known as electrically heat-traced flowline (EHTF) will be used to enable system startup and shutdown and to maintain production fluids outside of the hydrate envelope during steady-state operation.
EHTF is a pipe-in-pipe (PIP) system in which heating cables are used to heat the inner pipe. This enables even longer tiebacks and development of fields with challenging hydrate characteristics. The Ærfugl field represents one of the first EHTF projects awarded.
Topside. The Ærfugl field will be tied back to the Skarv floating production, storage, and offloading (FPSO) unit. A container module with a transformer will be installed on the FPSO. From the transformer, the required three-phase voltage can be taken out by a tap exchanger. The three phases are then routed through the swivel paths to the turret, where they are split into multiple triplets in a busbar box. One swivel path is also occupied by fiber-optic (FO) cable, which is used for digital temperature monitoring.
Umbilical. The triplets are routed from the busbar box though the dynamic and static umbilical to the subsea umbilical termination assembly (UTA). The UTA is located close to the inline power-inlet structure (ILPIS), which is the entry point for the power into the flowline. Electrical flying leads are used to connect the UTA to the ILPIS. The FO is also routed through the umbilical and connected to the ILPIS by flying leads.
ILPIS. The ILPIS is shown in Fig. 1. The flying leads from the UTA are connected to wet-mate connectors on the roof panel. In addition to the flying leads for power, two dedicated flying leads for the FO are included. From the wet-mate connectors to the double barrier arrangement, the wires are routed through oil-filled hoses with overpressure. In the double barrier, the wires are routed through two penetrators; the first enters into a chamber with atmospheric pressure, and the second into the annulus, where pressure is lower than atmospheric pressure. Should one barrier begin leaking for any reason, there will always be an active second barrier. The ILPIS is located in the middle of the 20-km flowline.
Flowline Cross Section. From the ILPIS, the wires are routed onto the inner pipe in a helix shape, which ensures that the wires do not experience excessive tension or compression during the reeling operation when the flowline is installed (Fig. 2). The inner pipe is a 10-in. pipe inside a 16-in. carrier pipe. Centralizers are also located on the inner pipe.
The pressure is reduced in the annulus between the inner and outer pipe to improve insulation. Insulation panels are located on top of the heating wires. The insulation panels consist of a microporous material, which means that, when pressure is reduced, the voids in the material become so small that movement of air molecules is restricted, and thus insulation properties are improved.
Vacuum-Seal System (VSS). Vacuum seals are placed at strategic locations along the pipeline, typically at tie-in points. This enables a staged pressure reduction in the PIP annulus. The pressure is reduced in sections up to 3 km in length during spooling onto the reeled pipe-laying vessel.
Steel Bulkhead. To ensure that there is no movement of the inner pipe relative to the outer pipe at the VSS locations, the VSS is always located on a bulkhead, which is a mechanical link between the inner and the outer pipe. The bulkhead features slots for the wires to pass through. The bulkhead itself does not create a seal because air can pass through the holes for the wires.
Centralizers. The centralizers transfer radial loads between the inner and outer pipe. During insertion of the inner pipe into the outer pipe, the centralizers avoid abrasion of the insulation.
Heating Wire. The heating wire is a nickel-plated copper-stranded conductor. Outside the heating wire, each conductor consists of three layers, providing a robust design against partial discharge, which is the main electrical aging mechanism in electrical components subject to high voltage.
Star End. The three wires in each triplet are terminated in the star end. The star end ensures electrical continuity between the three phases. The star ends are located in the annulus of the pipeline end terminations (PLETs) in each end of the flowline.
FO Cable. The FO cable measures the temperature at set intervals, typically every 5 m. In addition, it will be set to measure any cold spots such as inline structures. The fibers are inside a metal tube. The measured temperature is calibrated with temperature sensors on the structures. A short, intense light pulse (10 ns) in the fiber generates Stokes and anti-Stokes scattered light. The interrogator can then calculate the location and the temperature.
Inline Structures. A PLET is located at each end of the flowline. One of the PLETs has a 6-in. branch for connection to a well. There are also two inline tees with 6-in. branches for connection to wells. The inline structures are fabricated with preinstalled heating wires in the annulus. The wires are spliced to the rest of the flowline wires when the structure is being welded into the flowline in the stern ramp of the pipe-laying vessel.
Fabrication and Installation of EHTF
Before installation, the PIP assembly is fabricated at the spool base at Vigra on the west coast of Norway. When the PIP stalks are complete, the annulus is dried and end caps are mounted. Before the arrival of the installation vessel, two stalks will have been welded and spliced together into a 3-km-long section, ready for spooling. There will be a bulkhead with a vacuum seal at each end of the 3-km-long section, and reduced pressure in the annulus. Pressure-monitoring equipment will be placed inside the vacuum seals on both ends to monitor that the vacuum is maintained.
When the vessel arrives at the quayside, spooling of the 3-km-long section begins. When the first end is secured onto the reel, pressure monitoring of this end is performed by the reel hub. Another joint also is welded to the end of the stalk, and vacuum is monitored from the new extremity.
The 20-km flowline is installed in two trips, with 10-km pipe onboard for each trip. When inline structures are welded in during offshore installation, the flowline is cut in the stern ramp before the inline structure is welded in. First, the inner pipe is welded, and then the heating wires and the FO are spliced on both sides of the structure. When all splicing is complete, and insulation panels are put in place, welding of the outer pipe may begin. Once the root weld of the inner pipe is complete (and the annulus is sealed), the vacuum that was lost when the pipe was cut is drawn down again. The pressure reduction is performed in parallel with completion of the outer-pipe weld.
The EHTF system will have three settings corresponding to different heat inputs. One will heat up (high power), one will maintain the heat in the flowline during a shutdown (lower power), and one is assigned to steady state. Only some of the triplets in the cross section are required to supply this power. The remaining triplets will not be connected in the busbar box. These remain redundant and can be used as future spares.
Technology Readiness Level (TRL)
The EHTF technology is being qualified according to API Recommended Practice (RP) 17N and complies with DNV Recommended Practice A203. API RP 17N defines TRL 4 as environment-tested, and TRL 5 as system-tested. All components are fully tested in the correct environmental conditions and are rated at TRL 4. Several system tests also are performed, and the plan is for the whole system to be rated at TRL 5 by mid-2019.
EHTF is an enabling technology that allows the production of more-complex fluids and longer tiebacks while reducing the need for chemical injection significantly. The system also features low power-level requirements (and long no-touch time).
To ensure the robust quality of the system, significant time, costs, and resources have been devoted to developing, testing, improving, trialing, and confirming the suitability of the system to provide the confidence and safeguards that will allow this technology to be brought to market. Because the technology is new, only limited guidance from existing standards could be used to support the development. Thus, the highest requirements from international codes and company specifications have been applied in building the technology.
This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 29523, “New Technology Enables Development of Field in Norwegian Sea,” by Arne Skeie, Subsea 7, prepared for the 2019 Offshore Technology Conference, Houston, 6–9 May. The paper has not been peer reviewed. Copyright 2019 Offshore Technology Conference. Reproduced by permission.