CONCRETEconcepts

Adam Kopystynski, Shawcor, Pipeline Performance, UK,

explores novel bending methodology for concrete

weight coating.

I t is a common practice to use concrete as a means of providing negative buoyancy for submarine pipelines. While concrete is

typically designed for density, strength, water absorption etc., less is said about the strain that can be developed in such systems during the installation phase and the degree of cracking or spalling (impacting concrete integrity) that can be expected upon flexure.

All pipelines are installed with some radius of curvature and when testing requirements (bending simulations) are driven by the project installation requirements, such testing is relatively rare and can be mechanically difficult.

Shawcor had an opportunity to perform a strain testing programme to test a novel methodology for simulating strain conditions in concrete coated pipes, thus, testing five similar concrete coated pipes in bending to determine the relationship (if any) between plain and slotted concrete to the degree of installation strain generated.

A full scale bending test was performed using a free floating bend test concept. The test was to simulate axial bending during installation in an S-lay configuration. The specified strain to be generated was 0.22% at the mid fibre of the concrete (as referenced in DNV-OS-F101). This strain would be generated by bend radii of the order of 300 m.

A common challenge for this type of testing is to develop a testing methodology that can simulate real world behaviour without introducing artefacts from the test itself. Judgement would be made on installation performance and whether slotting of the concrete would be introduced in order to minimise strain by providing the pipe with discrete points of reticulation.

Test pipesThe pipes to be tested were 44 in. outside diameter (OD) x 24.6 mm wall thickness (WT) x 450 MPa steel, anti-corrosion coated with fusion bonded epoxy (FBE) and an helically applied adhesive bead with concrete (Table 1).

The concrete density was 3040 kg/m3 (190 lbs/ft3) with a nominal strength of 40 MPa (5800 psi).

Testing configurationAs the pipe size was large and high loads were expected during bending, in order not to influence local concrete behaviour and to spread contact load in bearing (of the order of 100 t), a four point, three pipe span arrangement was determined (Heriot-Watt University). Loading would be steel to steel in the area of the welds between pipes, on saddles welded in place to provide flat surfaces to push against. The required loads were generated by two hydraulic rams.

Some of the bending arrangements for these types of tests can have a radius of bend in the vertical orientation, either with the ends anchored to the ground – rams pushing the test string up – or exceptionally, if the test string is long enough to lift one end of the string physically (by crane) to cause the test radius to have a sag in the string. Also, particularly in the case of smaller radii bends simulating reeling installation, the bend can be in the horizontal orientation: typically, against a curved former describing the required radius or by pushing the test string with the ends fixed (and pinned) to the required radius.

To generate the flexure with the present testing methodology, it was determined:

) To have the curvature in the horizontal orientation.

) To have load points that were not fixed. Neither the pinned ends nor the rams were fixed or anchored to the ground or some rig substructure. In effect, this was to be a free floating system with no restraints in the horizontal plane.

The system was geometrically symmetrical (two pipe strings of three pipes bending in opposition to each other). In order to minimise relative movement between the two strings, one had an increase in stiffness by filling

the bore with concrete and was designated as the ‘strongback’.

Monitoring/sensoringIt is typical to calculate the radius of curvature in such a test from local positional information, either simple positional data relative to a fixed datum (e.g. absolute movement of the mid point of the central span) or more complex positional data (e.g. the position of several (fixed) points on the test string measured by surveying level or theodolite from a (fixed) position outside the system).

In the present discussion, the central span of the test string was subdivided symmetrically into Figure 2. Fully welded test string (front) and strongback (rear).

Figure 1. Completed assembly, pinned at the ends of test string and strongback.

Table 1. Summary of tested pipe configurations

Test number

CWC thickness

Type of reinforcement Pain/slotted

Plant

Test 1 75 mm Cage reinforcement Plain A

Test 2 110 mm Cage and wire mesh Plain A

Test 3 110 mm Cage and wire mesh Slotted A

Test 4 75 mm Cage reinforcement Plain B

Test 5 75 mm Cage reinforcement Slotted B

World Pipelines / REPRINTED FROM COATINGS & CORROSION 2016

approximately 1 m sections. The first and last sections were <1 m as the concrete coating was less than exactly 12 m total length. Eyelets were screwed into the concrete at each of these points and an exactly similar pattern was established on a fixed datum, which was located outside of the test and string. Linear voltage displacement transducers (LVDTs) were attached to the datum, and the drawstrings were located on the eyelets on the concrete coating. Before the start of the test, the datum was positioned at the approximate mid height of and aligned parallel to the test string. The drawstrings were lined up to be at 90˚ to the test string.

Although the complete test arrangement – test string and ‘strongback’ – was free to move relative to each element and the datum, motion was generally in the line of the drawstrings. Thus, angular errors were limited and, nonetheless, relative motion (between each of the eyelets) determined the positional data. Plotting test data onto ideal arcs of a circle became easier as radius (and strain) increased, with a very good correlation being established approaching the area of interest (strain of 0.22%).

BehaviourThe ‘DNV-OS-F101 Submarine Pipeline Systems, Section 13 Commentary, H. Installation’ explained: “… concrete crushing may be assumed to occur when the strain in the concrete (at the compressive fibre in the middle of the concrete thickness) reaches 0.2%.”

The compression face of the concrete was video monitored to determine the strain at which concrete spalled. The observations from the tests showed that of two equivalent concrete configurations (75 mm, but from two different coating plants), one did indeed crush at 0.20% strain. The second, however, crushed at 0.17% strain. There can be no easy explanation for the difference, particularly as the slotted configurations did not perform differently to one another. It was observed that the shear capacity was excellent, which limits the freedom of concrete to slip over the anti-corrosion coating relieving stress.

A 110 mm concrete configuration was strained to 0.23% before spalling. The obvious reason for this is that the concrete was reinforced in two layers: a primary cage reinforcement and a secondary wire mesh reinforcement (used as an aid to application), some 20 mm below the surface of the concrete.

In order to allow tighter radii/higher strains, forming slots that interrupt the longitudinal continuity of the reinforcement can reticulate the concrete. Typical slot location would be at 1 - 2 m intervals: at 1 m spacing in the case of the present discussion. This is sufficient to eliminate all spalling in the range of strain being discussed, with slots beginning to close up at around 0.27% strain.

It must be noted that all of the configurations tested showed some circumferential cracking on the tension face of the concrete, but with no associated loss (of mass) of concrete due to cracking.

Discussion

ShearThe axial resistance to shear of the system – concrete over anti-corrosion – was considered to be very good. The anti-corrosion solution was a FBE with an helical bead of Sikadur glue locking the concrete in place. There was no evidence of movement of concrete relative to the FBE. However, visual inspection at the cutback end of the concrete on the compression face while the test was under load/strain, showed a step in the cutback face of the concrete some 5 mm above the surface of the FBE, which was assumed to be evidence of a failure plane. When ‘windows’ were sectioned in the concrete coating on the compression face, the concrete could be removed from the window relatively easily, the failure plane being present some 3 m in from the ends of the coating.

SpallingThe spalling was seen only on plain pipe, always on the compression face of the test string and the area of spalling did not increase after first appearance within the strain range

Figure 3. Central span showing position of LVDTs (Heriot-Watt University).

Figure 4. Typical crack patterns for plain concrete pipe (Heriot-Watt University).

REPRINTED FRO COATINGS & CORROSION 2016 / World Pipelines

of the test. The degree of spalling was limited to <100 kg of concrete per pipe. Notably, the coated pipe weight approximately 17 500 - 23 300 kg.

The general pattern of appearance was symmetrical about the vertical centre of the test string, and there was spalling at the centre of the test string in each case.

CrackingCircumferential cracking was seen on the tension faces of the plain test strings, appearing to be related to the position of the circumferential reinforcing wire.

) Cage reinforcement: the typical positional frequency was around 200 mm, about twice the spacing of the circumferential reinforcing wires.

) Cage and wire mesh reinforcement: the typical positional frequency was around 325 mm, about twice the pattern of the circumferential wire mesh width minus overlap.

The circumferential crack pattern on the slotted pipes was related not to the reinforcing placement as above, but to the position of the slots, specifically there was one crack in the approximate centre of each ring of concrete.

StrainMapping of the curvature of the pipe at any point in the test (any radius) to perfect arcs of a circle was a straightforward, if trigonometric, process. There were minor non-conformities to circular geometry, but these can be largely explained by:

) Relative differences in stiffness of the concrete due to variation in density, compaction strength, thickness, shear performance etc. (seen as a random and changing offset).

) Re-alignment of some of the LVDT drawstring connections to the eyelets in the concrete on

commencement of the test, seen as a permanent offset for that LVDT position.

The key data here was the relative position of the measurement points on the test string (their relationship to each other), not the absolute (global) position. The mathematics was proven on each test occasion.

ConclusionThe challenge of executing the test methodology was met. There were no issues with hold up of the test string or strong asymmetric behaviour, for example. The free floating test arrangement concept was proven, with very good correlation of the bent test string to ideal arc geometry. From this positional data, test strains were calculated, which were then related to the observed instances of spalling. Axial resistance to shear of the concrete over the anti-corrosion was excellent, there was no movement of the concrete coating relative to the pipe and anti-corrosion coating, although there was post-test evidence that a shear plane within the concrete had developed. The spalling was limited to plain pipe only, with weight loss <100 kg per approximately 20 000 kg of coated pipe weight. The degree of variance in recorded effects was within expectation across three randomly selected pipes. Cracking in the concrete was circumferential on the tension face and could be related to reinforcing wire in all cases, and there was evidence that reinforcement closer to the surface of the concrete coating limited the depth of spalling.

The regularity of behaviour (no recorded spalling) in the slotted pipes up to strains of 0.27% in both 75 mm and 110 mm concrete configurations was to be expected.

From the results of the testing, the decision was made to install plain pipe, without slots being cut into the concrete coating, as material loss of the order that was measured in the test would not affect installability or stability on the seabed.”

Figure 5. Central span prepared for testing with LVDTS located on a fixed beam.

World Pipelines / REPRINTED FROM COATINGS & CORROSION 2016