GB2349944A

GB2349944A - A pipeline for the transmission of oil

Info

Publication number: GB2349944A
Application number: GB0011586A
Authority: GB
Inventors: Martyn Andrew Witton
Original assignee: Kvaerner Oil and Gas Field Development
Current assignee: Kvaerner Oil and Gas Field Development
Priority date: 1999-05-14
Filing date: 2000-05-12
Publication date: 2000-11-15
Anticipated expiration: 2020-05-12
Also published as: GB2349944B; GB0011586D0; GB9911312D0

Abstract

A pipeline for the transmission oil comprises a pressure containment pipe 20 having oil therein, a shell of insulating material 21 surrounding the pressure containment pipe 20 to reduce the loss of heat from the oil within the pressure containment pipe 20, and a conduit 23 for a fluid heating medium which is located wholly within the pressure containment pipe 20. The conduit 23 has at least one venting arrangement 24 whereby when the fluid heating medium has transferred part of its heat to the oil within the pressure containment pipe 20, the fluid heating medium can be discharged from within the conduit 23 to the environment or internally to within the pressure containment pipe 20. The venting arrangement 24 may be temperature sensitive and may take effect below specified critical temperatures. The conduit 23 may be formed from a flexible material so that it can be deformed by a pig as the pig passes along the interior of the pressure containment pipe 20.

Description

HEATING PIPELINES The invention relates to a pipeline for the transmission

of oil.

in particular, the invention relates to arrangements for heating oil as it flows within and along such a pipeline. While the invention is particularly applicable to subsea pipelines carrying oil from offshore locations, it may also be applicable to overland pipelines in which transmission is improved by heating.

Heavy and waxy oils often need to be heated to a suitable temperature to ensure that flow is possible at reasonable pressures and volumetric quantities.

Where the environment is cold (as it is on the seabed), heat can be lost from the oil at such a rate that viscosity increases and wax forms on the pipe wall. Hydrates also may form under these conditions, particularly as the pipeline back-pressure increases with reduced flow area.

The heat inherent in the oil is normally retained by insulating the pipeline. However, on longer pipelines, insulation alone cannot provide sufficient heat retention. The parameters influencing transmission of the oil are: pipeline length, pipeline diameter, initial pressure, environmental temperature, insulation, and flow rate. When compared to the fluid characteristics such as wax appearance temperature, hydrate formation envelopes, pressure & temperature, each particular set of conditions generates a specific design solution. However, a limit is reached where passive insulation alone cannot assure adequate flow.

Solutions combining pressudsing pumps may also become inefficient if sufficient heat is not imparted to the fluid, and high (natural) pressure systems may form hydrates more readily as temperature fails. For pipelines measured in hundreds of kilometres, the fluid characteristics must be such as to assure flow, and for lines shorter than say 20 kilometres insulation may be economic.

To ensure satisfactory flow within longer pipelines, it has been proposed to employ active heating. The need for active heating exists in a range of pipeline lengths from 10 to 100 kilometres. Active heating uses a medium which adds heat to the oil. Heat is added to the outside of the pipeline (and within the insulaflon) to maintain the viscosity of the oil at a satisfactory value.

The present invention is concerned with a system which allows heat to be added by dissipating heat directly into the oil. Heat passes outward through the oil into a pressure containment pipe which would be insulated externally.

The invention provides a pipeline for the transmission of oil, and comprising a pressure containment pipe to contain the oil, a shell of insulating material surrounding the pressure containment pipe to reduce the loss of heat from oil within the pressure containment pipe, and a conduit for a fluid heating medium and located wholly within the pressure containment pipe, in which the conduit has at least one venting arrangement whereby, when the fluid heating medium has transferred part of its heat to the oil within the pressure containment pipe, the fluid heating medium can be discharged from within the conduit either to the environment or internally to within the pressure containment pipe.

The heating medium conduit may be permanently installed or may be introduced as a temporary means of assisting flow.

It is preferred that there are a plurality of venting arrangements spaced apart along the length of the pipeline.

It is further preferred that the plurality of venting arrangements are operable by thermally activated means which take effect below specified critical temperatures.

In one form of the invention the pressure containment pipe is made of steel.

In another form of the invention the pressure containment pipe is made of composite material.

It is also preferred that the conduit for heating medium is made of flexible material such that it can be deformed by a pig as the pig passes along the interior of the pressure containment pipe.

The conduit for heating medium may comprise a plurality of lines passing along the interior of the pressure containment pipe.

The invention includes a discrete tubular assembly for joining to other similar discrete tubular assemblies to form a pipeline as described above.

A specific embodiment of the invention (together with an explanation of the prior art) will now be described by way of example with reference to the accompanying drawings, in which:

Figure 1 a is a cross section of a (prior art) heated subsea pipeline;

Figure lb is a cross section of a heated subsea pipeline illustrating an embodiment of the invention; Figure 2 is a cutaway perspective view of that pipeline; Figure 3 is a cross section of a heated subsea pipeline according to the invention and illustrating a pigging arrangement; and Figure 4 is a scrap section showing the deformation of a seal in the pigging arrangement.

Heretofore it has been known to heat subsea pipelines with elements surrounding the pressure containment pipe. As shown in Figure la, a pressure containment pipe 10 has insulation 11 surrounding its outer surface. Heating elements 12 are disposed on the outside of the pipe 10 between the pipe 10 and the insulation 11.

As shown in Figure 1b. a subsea pipeline according to the invention comprises a pressure containment pipe 20 having insulation 21 surrounding its outer surface, Following the invention, there is a conduit 23 within the pressure containment pipe, 20. The conduit 23 is suitable for a fluid heating medium.

Conveniently, the heating medium may be steam. Steam is generated at the end of the pipeline nearest to the source of oil, on the oil production platform. Steam may also be generated at an intermediate platform, part way along a long pipeline.

Initially, steam passes down the pipeline (Fig 2), and the steam is cooled as it heats the oil within the pipeline. Temperature sensitive venting arrangements 24 and 25 communicate with the conduit 23, and are spaced apart along length of the pipeline. The venting arrangements are initially set at open, and are designed to close as the steam temperature rises above a preset fixed temperature. Thus at start up, the steam heats the pipeline, and is itself cooled. The cooled steam is vented to make way for hotter steam. As the pipeline heats up, the venting arrangements close progressively along the length of the pipeline. If the temperature drops within the pipeline, the venting arrangements will open to make way for extra heated steam to be pumped down the conduit 23.

Pigging of the pipeline with the conduit 23 within it requires modifications to a conventional pigging arrangement. As shown in Figure 3, a pig 26 has resilient flaps 27 which can deform to fit over the conduit 23.

The conduit 23 may be formed of flexible material, so that the conduit can be deformed by a pig passing along the pipeline. The conduit is not restrained to any particular part of the internal surface of the pressure containment pipe 20, so there is provision to accommodate differential expansion of the conduit 23 and containment pipe 20.

The conduit may be stored on a reel, and inserted into the pressure containment pipe through isolation valves. Steam or other heating medium may be pumped down the conduit and vented internally as a temporary means of assisting the flow within the containment pipe.

Claims

Claims

11 A pipeline for the transmission of oil, and comphsing a pressure containment pipe to contain the oil, a shell of insulating material surrounding the pressure containment pipe to reduce the loss of heat from oil within the pressure containment pipe, and a conduit for a fluid heating medium and located wholly within the pressure containment pipe, in which the conduit has at least one venting arrangement whereby, when the fluid heating medium has transferred part of its heat to the oil within the pressure containment pipe, the fluid heating medium can be discharged from within the conduit either to the environment or internally to within the pressure containment pipe.

21 A pipeline as claimed in claim 1 in which the conduit is permanently installed.

3/ A pipeline as claimed in claim 1 in which the conduit is introduced as a temporary means of assisting flow.

41 A pipeline as claimed in claim 1 or claim 2, in which there are a plurality of venting arrangements spaced apart along the length of the pipeline.

S/ A pipeline as claimed in claim 4 in which the plurality of venting arrangements are operable by thermally activated means which take effect below specified chtical temperatures.

61 A pipeline as claimed in any one of the preceding claims in which the pressure containment pipe is made of steel.

71 A pipeline as claimed in any one of claims 1 to 5, in which the pressure containment pipe is made of composite material.

81 A pipeline as claimed in any one of the preceding claims, in which the conduit for heating medium is made of flexible material such that it can be deformed by a pig as the pig passes along the interior of the pressure containment pipe.

9/ A pipeline as claimed in any one of the preceding claims, in which the conduit for heating medium comprises a plurality of lines passing along the interior of the pressure containment pipe.

101 A discrete tubular assembly for joining to other similar discrete tubular assemblies to form a pipeline as claimed in any one of the preceding claims.

11/ A pipeline substantially as hereinbefore described with reference to and as shown in 5 Figures 1b and 2 to 4 of the accompanying drawings.