Seabed, currents posed problems in Gibraltar pipelay

Installation of the twin submarine gas pipelines crossing the Strait of Gibraltar posed significant engineering and pipelay problems for lay contractor Saipem S.p.A. The rocky and highly irregular sea bottom created major pipelay problems and necessitated laying the pipelines with extreme precision and minimum tolerance. Further complicating the pipelay were the high surface and bottom currents which at times could reach up to 5 knots.

Dec. 2, 1996

6 min read

Installation of the twin submarine gas pipelines crossing the Strait of Gibraltar posed significant engineering and pipelay problems for lay contractor Saipem S.p.A.

The rocky and highly irregular sea bottom created major pipelay problems and necessitated laying the pipelines with extreme precision and minimum tolerance.

Further complicating the pipelay were the high surface and bottom currents which at times could reach up to 5 knots.

Specifically, the rocky and irregular sea bed made mooring and anchoring the lay vessel Castoro Sei difficult. In addition, the operation had to resolve problems caused by vortex shedding during and after pipelay.

These difficulties were reported in a presentation by Saipem's S. Bianchi and L. Ferroni in late 1995 to the Deep Offshore Technology Conference in Rio de Janeiro. Saipem modified the Castoro Sei by extending its pipelay ramp to a departure angle of up to 50°.

This modification decreased the pull on tensioners by 60 tons, said Bianchi and Ferroni, when compared to the required pull without extension of the ramp. This resulted in minimization of residual tension, which further resulted in the pipe following the seabed profile, and in a reduction in free-span lengths and height.

In fact, in the deepest part of the pipelay, 400 m of water, free spans were reduced to 700 m.

This and other engineering approaches made possible completion of the pipelay in record time and with minimal seabed interventions, said Bianchi and Ferroni.

Critical areas

Prelay surveys conducted along the pipeline route had identified three sections of the route as "critical areas" because of rugged topography, deep waters, and strong currents. These were situated in the Moroccan sector and amounted to more than 30% of the total route length.

The first critical area in the design route was between approximately KP 4 and KP 5.

The pre-engineering survey campaign had already dedicated to this area most of 5 km of pipe with increased wall thickness of 25.4 mm.

Therefore, it was decided to use mechanical supports in those areas where the operative stress would be exceeded in order to bring the stress back inside allowable limits.

At the same time, however, Saipem did not remove the possibility to still try local rerouting in order to reduce free spans and relevant stresses, said the authors.

The area was extremely uneven and the rerouted pipeline section had to be approached cautiously during laying. At that time, the pipeline touchdown was continuously monitored by ROV from the support vessel Bar Protector.

The pipeline was laid within an accuracy of 1 m without incident.

The second critical area was between approximately KP 12.5 and KP 17. Here, water depths average 380 m with maximum currents. Saipem concentrated on finding a possible reroute outside the official corridor.

One was located and analyzed in some instances maintaining no more than 15-m separation distance between both the pipelines that could avoid the major features (up to 6 m high) in this area.

The main risk was the capability to lay in bends of 2,000-m radius necessary to enter the corridor in high current conditions. This area was therefore subjected to rerouting outside the corridor provided by the client, said the authors.

Unfortunately, despite the numerous analyses and every effort to minimize the length of free spans from the Castoro Sei to the touchdown point, no one could predict with 100% certainty the pipeline behavior in those conditions.

Bianchi and Ferroni said it was therefore decided, because the seabed currents always flow in the direction of east to west, to lay first the line which was on the eastern side (Line No. 2). Another contingent laying corridor was analyzed and studied in case the laying of the first line obstructed the corridor for the second line.

Fortunately, even though Saipem had tremendous problems with currents, it was able to lay both the pipelines in the design corridor, the authors said.

During laying, the pipeline touchdown was continuously monitored by ROV from the Bar Protector.

The barge experienced downtime because of high on bottom currents. In fact, the pipeline was being dragged by the near-bottom currents to approximately 50 m from the touchdown point.

Difficult barge maneuvers had to be carried out to place the pipeline within the intended target area.

Pipeline No. 2 had to be laid so that enough room was left for laying Pipeline No. 1. In some sections, this was impossible. A different route was analyzed.

Critical Area No. 3, approximately KP 22 to KP 24, contained walls up to 30 m high parallel to the design route. The sea bottom was very uneven and again soil configuration required laying the most eastern line first.

In this area, the problem was pipeline stability, especially in bends, with very few contact points. This area averaged water depths of up to 270 m with very strong currents.

But no major problems were encountered during laying, possibly because ROV's continuously monitored touchdown, said Bianchi and Ferroni.

The critical area evinced extremely uneven rocky seabed, the extreme water depth of approximately 270 m, high on-bottom current, as well as surface current velocities, and an almost vertical rock wall on the east side of the route, rising to approximately 10 m.

Pipeline safety dictated the necessity of placing the pipeline in a valley with 11 m accuracy; placement also had to ensure that the pipeline not slide so that the second pipeline had enough room.

During laying, the pipeline touchdown was continuously monitored by ROV from the lay support vessel.

The authors said pipelay progressed smoothly from KP 24 onwards to the Spanish side.

World War II mine

During the prelay surveys, an object was discovered on the pipeline route at approximately KP 25.5. Video surveys of it were shown to Spanish Naval experts who confirmed the object was a World War II mine.

With the barge expected to lay through the area in less than 2 weeks, surveys were immediately carried out to ensure that the new route was clear and safe for construction and to avoid the mine.

The rerouted section involved approximately 3 km of the pipeline and provided a minimum separation of approximately 300 m between the pipelines and the mine. The rerouted section was to the west of the original route.

Prelay survey of the rerouted section was carried out in December 1994 to confirm that the section was clear.

Bianchi and Ferroni said the rerouting design, processing of survey data, preliminary chart production, and initial pipeline stress analysis all ensured that the progress of the pipelay barge was not held up.