2. design considerations for subsea metrology measurements - patrick bruce and chris bath

Design considerations for subsea

metrology measurements

Speakers Patrick Bruce and Chris Bath SUT 18/06/14

Design Considerations for Subsea metrology measurements

Mechanical Design considerations

Measurement requirements

Required measurement and what can be achieved

Examples of metrology measurements

Mechanical Jig Metrology

Bracket Design and offset measurements

Golden rules for Metrology

Engineering Considerations for spools, Risers, pipe work (Patrick Bruce)

Topics to be discussed

Design considerations for Subsea Metrology 2


Material properties (Pipe size, wall thickness, composition of the metal)

Fabrication tolerances Expansion/ Contraction of the pipe work due to heat/cooling. Installation tolerance between each end (heading, pitch, roll and

position)

Seabed friction (if applicable) Accuracy of subsea measurement Optimise costs for all of the above

Design Consideration


Additional Engineering Considerations

Flow rate

Design life

pressure rating


Measurement Definitions


Criterion Definition

Accuracy Is the degree of exactness which the final product

corresponds to the measurement standard.

Precision Refers to the ability of a measurement to be consistently

reproduced.

Reliability Refers to the consistency of accurate results over

consecutive measurements over time.

Traceability

Refers to the ongoing validations that the measurement

of the final product conforms to the original standard of

measurement

Engineers think +/- millimetres

Surveyors think a range of centimetres

Engineers do not understand accuracy's that can be achieved subsea

Surveyors do no understand the design requirements.

Engineers can normally widen the tolerances by making a few modifications. But this usually adds costs.

Surveyors can assist engineering with the choice of equipment, but high accuracy comes at a price.

We need to get together


Engineers and Surveyors


Examples of Metology measurement


Replacement Platform Brace due to vessel damage

Initial inspection with Rope access and Air Divers

Measurements taken with steel tape passed from rope access to air diver.

Considerations Tidal Current Crane access from vessel Access for Diamond wire cutter Control of replacement brace.

Decision to provide like for like brace but provide reduced length of 100mm

each end.

Manufacture Clamps to hold brace in position.

Clamps to take up to - 100m + -100m for end prep.

Examples of Metrology measurement


Tow Head Pipe work replacement

Limited space inside the tow head Diver access also an issue Divers vision is difficult due to parallax in their helmets.

Examples of Metrology measurement


1. Only Design dimensions available

2. Measurement between lower cross

member and hang off point

required.

Replacement riser



Metrology Jig


Level and distance measurement



Metrology Jigs


Pipe Centre tool



Hydrostatic Measurement




Time spent on making good brackets so that offsets are known in XY & Z is a sound investment.

Use of Gyros to obtain heading pitch and roll measurements.

Importance of knowing where the measurements are taken from


Bracket design and offset measurement

Good Bracket easy to fit by ROV, but are the flange offsets known and to what accuracy?

Transponder bracket hung off a tree



Reference point measurements and offsets in X, Y and Z

Flange Flange


Example of transponder bucket to be

inserted into a flange


Example of transponder bucket to be

inserted into a manifold pile guide.


Early discussion on requirements with design engineers is required, surveyors to be aware of the range of metrology tools that could provide the accuracy needed.

If a measurement cannot be competently measured then the design has to change or an alternative measuring system to accommodate is to be considered,

If a project critical measurement has to me made then it should be given the planning and time offshore to provide a successful measurement.

The importance of brackets cannot be underestimated. Consider additional measurements if reliant on ROV placing brackets

Never use just one system to measure anything always use a coarse measurement system to ensure no gross errors are made. This provides confidence in the

measurement.

Always use two methods of calculating the correct measurement, I suggest Pre made spread sheet with the estimated distance and Angles and AutoCAD.

Golden Rules for Metrology



Tie-in Spool Flange Misalignment A Design Engineers Perspective


Pipeline

20

0m

m

Linear

misalignment

200mm

Angular

misalignment

2.5o Typical Z type tie-in

spool.

Structure Tie-in

Plan View

Plan view on

flange

misalignment

HOW MUCH FLANGE MISALIGNMENT IS TOO MUCH?

i.e. BEFORE EITHER THE PIPE OR THE FLANGE IS

OVERSTRESSED?


Example of Pipe stress software output.

Software is used to displace the flange to the required misalignment (linear and angular).

The spool is put into bending as a result of the flange misalignment

The resulting stress in both the pipe and flange is calculated.

The allowable stress for Duplex and Super Duplex materials is heavily de-rated compared to carbon steel. This is due to a

propensity to hydrogen induced cracking. The hydrogen comes

from the anodes.

+90% of production manifolds consist of duplex or super duplex piping / tie-in spools. Therefore, accurate metrology and

fabrication is regularly required to ensure spools are not

rejected offshore.




How much could a flange be misaligned?

The following contribute to flange misalignment and include actual results:

Pipe work fabrication tolerances

Induction bend - tolerance on angle +/-0.5o,

Spool Leg length +/- 33mm,

Flange angular misalignment 0.27o

Onshore as-built tolerances, i.e. The accuracy of the measuring equipment - ?

Seabed bathymetry (topography) i.e. The accuracy of the measuring equipment - ?

Structure installation tolerances +/-1.5o on heading, +/-2m on position, Roll and Pitch? - Function of seabed

Metrology equipment tolerances varies depending on equipment used. (Ref 1)

Reference 1: IMCA Guidance on Subsea Metrology IMCA S 019 Feb 2012

Tie-in Spool Flange Misalignment A Design Engineers Perspective Case Study 1


Allowable flange linear

misalignment for an angular

misalignment of 2.5o

The target linear value was +/-

200mm. The figure illustrates this

wasnt possible for all combinations



Establish maximum allowable flange misalignment at 24 flanges

Case Study 2 24 OD x 25.4mm WT spools


Existing pipeline on

a heading of 83.9o

Manifold to be

installed on heading

353.2o

Tie-in Spool Flange Misalignment A Design Engineers Perspective Case Study 2 Theoretical layout



Example Subsea

Metrology Drawing

Manifold installed to

within 1.32o

Existing pipeline, 0.9o difference from

previous survey = 848mm @54m

0.82o pitch = 772mm

into seabed

Case Study 2 As-Installed Layout


26

As-built Spool

Drgs.

Metrology Spool

Drg.

Note magnitudes of

flange misalignment

just from fabrication /

as-built survey

Note Field Welds for

adjusting leg lengths

in X, Y & Z with the

possibility to mitre

butt welds for angular

corrections

Case Study 2 As-built Spools



Pipe stress analysis performed to establish maximum allowable

angular and linear flange misalignment

Note how little misalignment is

allowed in the plan view. There

was 0.9o difference between the

two surveys of the existing

pipeline!

Case Study 2 Maximum Flange Misalignment



What Can we Do Better?

Better communication between all parties

e.g. metrology reports are rarely forwarded to design engineers.

Consequently, the design engineer has a very poor basis for typical

tolerances.

Understand what magnitude of mitre fabricators can achieve at butt welds.

Induction bends are always manufactured in advance, therefore,

angular misalignment can only be taken out by mitring butt welds or

bending spools. DNV permits mitreing of butt welds up to 3o

2. design considerations for subsea metrology measurements - patrick bruce and chris bath

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