In-line inspection (intelligent pigging) of offshore pipeline. Birger Etterdal

Transcription

1 In-line inspection (intelligent pigging) of offshore pipeline Birger Etterdal

2 Content Type of defects Different inspection tools Objective of any inspection Focus on corrosion mapping for submarine pipelines Selection of inspection technology for corrosion mapping Deliverables from in-line inspections Awareness of defect sizing uncertainties Working with detailed data from UT-based in-line inspections Are there any alternatives to in-line inspections? 2

3 Type of defects that we normally look for in offshore pipelines Metal loss defects (corrosion) Cracks Cross section imperfections (dents, ovalisation, etc.) Pipeline curvature 3

4 Defect types and in-line tools MFL- Pig UT- Pig Caliper pig In some cases these are combined into one assembly Geo-pig 4

5 Objective of any inspection Purpose: establish a basis for condition/integrity evaluations In too many cases, critical anomalies could have been detected at an earlier stage if the inspection results had been properly reviewed (and properly reported) 5

6 Typical failure statistics and use of in-line inspections (ILI) Degradation over time ILI with metal loss detection tool Corrosion 27 % Structural 5 % Impact 24 % Event based External inspection, and in some cases ILI with caliper / geo pigs Anchor 18 % Other 11 % Nat. Hazard 5 % Material 10 % Weld defects? Early phase problem In some cases: ILI w/crack detection More relevant for materials in environment where forms of stress corrosion cracking can occur 6

7 Selecting the right tool for corrosion mapping Gas (MFL) or liquid line (MFL or UT) Stainless steel (UT) Very thick wall (UT); very thick wall is diameter dependent Difficult or inadequate cleaning (MFL) Type of corrosion - Small pits (MFL) - Defect at girth welds (MFL) - Long axial grooves (UT) - Uniform corrosion (UT) - Severe corrosion with respect to pipeline integrity (UT) This pipeline was inspected just some few months before it bursted 7

8 MFL versus UT MFL Tools UT Tools Advantages - Can be used for both gas and liquid pipelines - Can be used for very long pipelines - Large number of vendors Advantages - Direct measurement of the wall thickness - High accuracy - Provides shape of the defects Disadvantages - Can not measure the absolute wall thickness - Known to be less accurate - May not detect long axial grooves - Can not be used in case of very thick pipelines Disadvantages - Can normally only be used in liquid pipeline - Normally more costly - Sensitive to pipeline cleaning slide 8

9 Deliverables from In-Line Inspections Inspection report In most cases in accordance with the POF-specification (POF = Pipeline Operators Forum) Pipe book (or pipetally listing) Feature listing list of e.g. corrosion defects with position information and size information On demand detailed inspection results (e.g. grid of remaining wall thickness from UT-based inspection) 9

10 Pipe book versus system drawing slide 10

11 Typical Metal Loss Feature List (MFL) This value is NOT measured (in most cases not given in the feature list) Note that the defect depth is given as percentage of nominal (reference) wall thickness slide 11

12 Detailed inspection results slide 12

13 Uncertainties in defect sizing Wall thickness For onshore pipeline, verifications digs are usually a part of the in-line inspection scope, i.e. the worst detected defects are excavated and the wall thickness measured by e.g. UT. This is not practical for offshore pipelines (although done in some cases). The sizing accuracy is dependent on the metal loss shape, e.g. depth sizing of a pin-hole defect is more difficult compared to corrosion defects larger extent slide 13

14 Defect Depth - AUT [%] MFL AUT inspection results (onshore pipeline) 100 Comparion between in-line inspection (MFL) and external AUT Defects +/- 20% accuracy Defect Depth - MFL [%] slide 14

15 Defect Depth - AUT [%] MFL AUT inspection results (onshore pipeline) Comparion between in-line inspection (MFL) and external AUT Defects +/- 10% accuracy Defect Depth - MFL [%] slide 15

16 Echo loss the largest problem with UT-based ILI Corrosion Irregular inner (or outer) surface Inadequate cleaning (it is not uncommon to send cleaning pigs prior to an UT in-line-inspection). The cleaning is also affected by corrosion. Presence of hard scale Pig velocity 16

17 Use of stand-off data to fill in lacking WT-data WT = WTSO SO signal 1st reflection (SO) 2nd reflection SO WTSO WT WT = Wall thickness SO = Stand-off (distance between UT-probe and inner wall) SOWT = WT + SO in un-corroded area constant over a moderate axial length 17

18 Example Use of stand-off data to fill in lacking WT-data 18

19 UT based in-line inspections can also be very good and consistent from inspection to inspection!! 2010 inspection 2011 inspection 19

20 Specification for assessment of corroded pipes based on detailed data from UT-based in-line inspections DNV has together with Statoil and DONG Energy developed a specification for how to assess pipelines with axially long corrosion defects (hereunder procedure for cleaning UT-inspection data). NDT Systems & Services, Germany has also participated in this work. The above will be included in next update of DNV s recommended practice DNV-RP-F101 Corroded Pipelines (to be reissued JIP project is launched) Validation (on-going): - A 45m section has been retrieved from a severe corroded flowline - This flowline has been internally inspected several times and the last one just before retrieval - This 45m section will be scanned with AUT (FORCE Technology) - Compare ILI data to AUT data - 4 segments will be burst tested 20

21 Are there any alternatives to In-Line Inspections? No, but - an adequate corrosion management system is the most important tool. In-line inspection should be used to verify that the corrosion management system is working as intended! 14 km long 16 water injection flowline - spot checks topside (or in some cased subsea) give some indications however no guarantee that the pipeline is free for severe corrosion defects. - note that the worst incidences caused by corrosion relate to topside/riser above still water level where other inspection methods are applicable (and in some cases better) No corrosion Severe corrosion 21

22 Safeguarding life, property and the environment 22