A. Dutta, M. Guinard
McDermott-ETPM West Inc.
Nanterre, France
For switching from piggyback to dual lay for installing two pipelines on the North Morecambe project, Block 110/2a of the Irish Sea, McDermott International Inc.--ETPM Services (U.K.) Ltd. (MET) used its lay vessel DLB 1601 for an unconventional transition operation.
A 3-in. OD methanol pipeline was piggybacked onto a 36-in. gas pipeline for the shore pull and then separated for dual lay for the offshore section.
This concluding article covers the transition operation and reports results and observations of prototype testing onshore, Part 1 (OGJ, Sept. 4, p. 84) discussed the background for the North Morecambe project and engineering for the piggyback and dual lay phases.
"Splitting zone"
The objective of the transition operation was to lift a minimum length of the 3-in. pipeline (hereafter referred as "splitting zone") from its piggybacked configuration and shift it to the dual-lay position.
Therefore all the piggyback saddles in the splitting zone were designed with an easily detachable top cover (with an applied vertical force of about 50 kg) connected to a saddle base strapped to the 36-in. pipeline.
The shore pull (piggyback laying) was completed with the 3-in. (piggyback termination) pipehead positioned 55 m from the barge stern, keeping the 36-in. pipeline on the main laying ramp.
The following procedure was then implemented for the transition operation:
- Lifting of 3-in. pipeline from the top of 36-in. pipeline
- Shifting of 3-in. pipeline away from the 36-in. pipeline
- Partial lowering of 3-in. pipeline
- Recovery of 3-in. pipeline onto the barge.
In fact, the entire operation was designed to work with a sequence-dependent pulling tension provided through the abandonment/recovery (A/R) cable of the 3-in. pipelaying system. Cable alignment was regulated through the positioning of three sheaves, or pullies, S1, S2, and S3 (Fig. 1 (26398 bytes)).
The position of the S2 sheave was important for controlling the line of tension on the 3-in. pipehead. The other two sheaves were employed to guide the A/R cable alignment behind the S2 sheave.
Figs. 1 (26398 bytes) and 2 (32285 bytes) show the typical positions of these three sheaves during the lifting and shifting operations of the 3-in. pipeline, respectively.
Lifting the 3 in.
The 3-in. pipeline was to be lifted in the same vertical plane as the 36-in. pipeline. The lifting operation was designed to be executed in two stages.
The first stage was carried out by initial tensioning of the A/R cable up to 4 metric tons by keeping the S2 sheave about 20 m from the barge stern and 5 m from the sea surface. This was in order to make the 3-in. pipeline to some extent taut and thereby reduce excessive curvature during the final tensioning stage of lifting.
It is important to note that the detachable top covers of the saddles in the splitting zone were removed before the lifting operation by manual pulling of a nylon rope prerigged through the covers during the piggyback laying.
During the initial tensioning stage, approximately 100-114 m of 3-in. pipeline were expected to be lifted, depending upon the water depth.
To avoid interference of A/R cable with the stinger and/or stern ramp structures, the minimum horizontal distance (HD) from S2 sheave to the barge stern and the vertical distance (VD) from the sea surface during final tensioning were kept as 20 m and 10 m, respectively.
The distance to the splitting point (the last secured piggyback attachment) from the barge stern was fixed as 300 m.
To control the vertical and horizontal displacements and thereby the 3-in. pipe stresses around the splitting point, an "inverted-U" bracket support (Fig. 3 (86870 bytes)) was installed atop the 36-in. pipeline during piggyback laying, 14 m from the splitting point. This was also to ensure integrity of the splitting area, particularly for the piggyback attachments.
The main purpose of the final tensioning stage (Fig. 1 (26398 bytes)) was to lift the total length of the 3-in. pipeline up to the splitting point. For variable water depths, the position of S2 sheave from the barge stern was made adjustable.
For the recommended final tensions for lifting, the bracket support was not touched by the pipe (Fig. 4 (33474 bytes)), and the 3-in. pipe stresses around the splitting point were not found to be critical.
It was also revealed that the 3-in. pipe stresses around the bracket support would reach the limit of 75% SMYS if the recommended A/R winch tension were accidentally increased by more than 50%. Such an increase was impossible in the shallow water lifting conditions because the capacity of the A/R winch was limited to 25 metric tons.
The position of the S2 sheave was to be carefully monitored during the transition operation. Some sensitivity analyses were performed, however, to verify the influence of S2 sheave positioning on the 3-in. pipeline-lifting behavior. These studies showed sufficient tolerances in the S2 sheave positioning.
In relation to the recommended positions, 1.0 m and 5 m were found acceptable in the directions parallel to barge stern and perpendicular to the barge stern, respectively, for any water depth.
The position of this sheave with respect to the sea surface was found to depend somewhat on water depth. The tolerances were 3 m 1 m for minimum water depth and 3 m for maximum water depth.
During the lifting operation, the effect of transverse currents was also studied. As expected, pipeline lateral displacement of about 2 m in the freespan was detected. No serious pipe stress due to the current effects at the bracket support location was found, however.
Shifting, lowering; recovering
Following the completion of 3-in. pipeline lifting, the S2 sheave would be moved laterally from the plane of the 36-in. pipeline until all the sheaves came into line with the 3-in. pipelaying ramp (Fig. 2 (32285 bytes)).
During this movement of S2 sheave (controlled by S3 sheave), the A/R winch tension was recommended to be dropped to 10 metric tons as soon as feasible within the 5-m lateral movement limit of S2 sheave.
The purpose was not to exceed the allowable stress limit at the bracket support. The relative positions of the S2 sheave (HD, VD) were kept the same as during the final lifting phase.
As stated previously, the bracket support (Fig. 3 (86870 bytes)) was designed to reduce the 3-in. pipe stresses around the splitting area caused by the shifted configuration of the pipeline. To facilitate its installation and subsequent partial recovery by divers, this support was made with a removable top section.
The fixed base-section design took into account a full contact support for the 3-in. pipeline from the final phase of shifting. During the shifting phase, 3D pipe-stress analyses confirmed the results were well within the allowable limit, even with the current effects.
The purpose of partial lowering was to remove the three sheaves in turn and to reduce the A/R winch tension so that the 3-in. pipeline could be recovered for the dual lay. The operational use of S1 and S3 sheaves was completed with the 3-in. pipeline shifting operation.
Hence these sheaves were removed (S3 sheave first followed by S1 sheave) by moving and holding the S2 sheave closer to S1 sheave location. After the lowering of A/R winch tension from 10 to 4 metric tons (recommended tension of 3-in., 12.7 mm W.T. pipeline during start of dual lay), S2 sheave was lowered and removed.
No pipe-stress problem was found during this reduction of winch tension, resulting in (temporary) partial lowering of certain length of 3-in. pipeline onto the seabed.
The final phase, recovery, resembled any other recovery operation planned for dual lay. With the A/R winch tension set at 4 metric tons and the A/R cable resting on the laying ramp, the 3-in. pipeline was recovered with the conventional step-by-step procedure of barge-reverse movement.
As soon as the pipehead passed through the tensioner, the tension was transferred from the A/R winch to the tensioner. The pipehead was removed and the recovered pipe string welded to a previously fabricated pipe string positioned on the laying ramp.
Dual laying was then commenced by barge movements and conventional feeding of pipes into the two individual parallel laying ramps for the 36 and 3-in. pipelines.
Onshore testing
With the transition operation being carried out for the first time, verification of certain activities by testing was important. In April 1993, onshore prototype testing in Barrow in Furness had the following objectives:
- Load test for the bracket support
- Verifying the 3-in. pipeline configurations around the splitting area before, during, and after transition (lifting, shifting, and lowering)
- Testing the anode as a stopper against temporary saddles in the splitting zone.
It may be noted that no load tests for the saddles and/or their attachments were carried out during this testing. These tests had already been performed separately by the supplier.
The main purpose of the onshore testing was to gain confidence and within a limited time collect practical feedback concerning the 3-in. pipe behavior during the transition.
No sophisticated instrument set-up was therefore necessary for the testing. A simple testing procedure was developed with the readily available equipment.
For the testing setup, four single pipe joints of both 36 and 3-in. pipe were welded to make two 49-m pipe strings 36 in. and 3 in. OD, respectively.
The 3-in. pipe string was fitted on top of the 36-in. string with the temporary piggyback saddles developed for the splitting zone. The bracket support was also installed, 14 m from the splitting point, with four steel straps (1 mm thick, 25 mm wide) and clips.
One anode was installed (by bolting only) on the 3-in. pipe string 19 m from the bracket support (33 m from the splitting point). A crane lifted, shifted, and lowered the 3-in. pipe string, using a spreader bar and a two-point lifting arrangement.
A load cell (Dynaline, 3-metric ton capacity) was used for measuring the applied pull forces.
Some observations
The design test loads on the bracket support determined in the Phase 3 engineering (transition operation) were the following: vertically upward (during lifting) = 1.5 metric tons; lateral (during shifting) = 0.5 metric ton; and vertically downward (from 3-in. pipe resting) = 1.0 metric ton.
The bracket support was pulled vertically and laterally with the loads of 2.5 and 0.6 metric tons, respectively. No displacement of the support appeared. A vertically downward load test (on the support) was carried out by placement of the 3-in. pipe string directly on the support (discussed presently).
All these tests confirmed the adequacy of the bracket support design. The two-part support, fixed by a stabbing and bolting arrangement, was also simple to install and remove.
For installation, it was proposed to position the temporary saddles (with top detachable cover) with a spacing of 12 m in the splitting zone. Testing revealed, however, that this created considerable 3-in. pipe deflection in the mid span, resulting in some risk of fusion-bonded epoxy (FBE) coating damage to the 3-in. pipe.
It was therefore decided to place supporting saddles (without the top detachable cover) between the temporary saddles to reduce the support spacing to 6 m.
From the 3-in. pipe configuration near the splitting point, it was also decided that a protective wrap of cold-applied tape to avoid FBE coating damage would be applied on certain 3-in. pipe lengths from a distance of 3 m from the splitting point.
In the transition operation, during lifting, shifting, and lowering of the 3-in. pipe string, no problem appeared regarding the configuration.
It was decided that the protective wrap for the FBE coating would be applied up to the first temporary saddle, that is, 24 m from the splitting point.
During installation, anodes in the splitting zone were treated as the stopper device against the saddles to avoid any 3-in. pipe sliding through the temporary saddles. During testing, the 3-in. pipe was pulled with an axial force of 0.5 ton.
No anode displacement appeared.
The pulling force was gradually increased and the anode started sliding at 0.8 metric ton load. As this was much greater than the expected maximum sliding load of 0.35 ton, it was concluded that the bolted connection of the anode was adequate for the anode to be used as a stopper device against a temporary saddle.
Special measures
An examination of all possible problems in the transition led to certain special protective measures.
Because the piggybacked 3-in. pipeline was to take a high tension during transition (lifting and shifting), a rigid stopper arrangement for it was installed on the 36-in. pipelines pulling head. This was to prevent any sliding of the 3-in. pipeline through the piggyback saddles.
As Fig. 6 (69636 bytes) shows, the 3-in. pipe end was clamped into two supports welded onto the 36-in. pipe pulling head. In addition, two doubler plates were welded onto the 3-in. pipe adjacent one of the clamps to provide a positive stopper.
The transition, particularly lifting and shifting, depended greatly on A/R cable positioning through the S2 and S3 sheaves. Special measures were therefore taken correctly to monitor the position of these sheaves during operation.
To monitor the S2 sheave distance from the barge stern, a marked-up tugger line (Fig. 1 (26398 bytes)) was connected to the hook of the main crane, which was supporting the sheave.
For monitoring the vertical position of the S2 sheave from the sea surface level, a nylon rope fitted with two marker buoys (to indicate 5 m and 10 m levels) was hung from the supporting hook of the crane.
The S2 sheave lateral position was directly controlled through the S3 sheave and also to some extent by the crane hook positioning.
Control of S3 sheave positioning and its movement during the shifting phase was critical. While its position governed the S2 sheave control, its correct movement was also essential to eliminate any sudden drop in A/R cable tension.
S3 sheave was therefore controlled by a separate cable-sheave combination system (Fig. 2 (32285 bytes)) connected to an auxiliary winch.
As discussed earlier, the temporary saddles in the transition zone were fitted with top detachable covers which were removed one after another before lifting. With well monitored A/R winch tensions, there was actually no problem during lifting, shifting, and lowering.
To ensure a safe operation, however, visual (video) inspection of the 3-in. pipe movement around the bracket support was used during the entire operation. This inspection was performed by ROV from a survey vessel, and the results were transmitted to DLB 1601 through a telemetry link.
In the field
In May-June 1993, the North Morecambe project pipelines were installed with no serious problems.
Removal of top detachable covers of the temporary saddles in the splitting zone was performed by pulling the nylon rope (rigged through the top covers) from an inflatable boat.
Following this operation, strong currents ( 0.6 m/sec) delayed transition, resulting in a 3-hr operational stand-by for ROV deployment. Finally, transition was started with a current of 0.3 m/sec at a water depth of approximately 20 m.
During transition, visual inspection by ROV revealed no unusual pipe movement or position around the bracket support. All activities were consistent with analytical predictions.
The post-lay survey confirmed an irregular as-laid shape of the 3-in. pipeline.
In the straight route section, the variation in separation between the 36 and the 3-in. pipelines was mostly within 3 m from the nominal value of 12 m, as anticipated. It may be noted, however, that the variations increased to 4 m in only four locations.
During laying along the second curve, the 3-in. pipeline was laid with its minimum allowable laying tension to increase the pipelines separation and to ensure a wider safety margin in case of any accidental increase in tension during or after the curve laying, which might result in pipe lateral shifting.
No such event occurred, however. The separation between the pipelines, monitored in real time by an ROV operated from the survey vessel varied 12-20 m. During the dual lay, sea states were well within the limiting values; no emergency abandonment of the pipelines was necessary.
!-- Onshore testing of the shifting and lowering of the 3-in. methanol pipeline verified the design. The support bracket appears in the right foreground; the projected configuration of the 3 and 36-in. pipelines after separation is shown as well (Fig. 5). --
Copyright 1995 Oil & Gas Journal. All Rights Reserved.
