Large-scale riser field tests involving life size models are not conducted very often due to the scale of the equipment involved. However, they have been essential in understanding certain structural or hydrodynamic behaviours that cannot be evaluated mathematically or investigated in a laboratory.
This type of testing has helped us investigate previously unknown physical effects and in turn predict a riser’s performance during operation much more accurately and effectively.
Here are a few examples of some large-scale field tests of marine risers that we have conducted. These tests were the first of their kind and were planned, engineered and executed by the 2H team.
The success of these trials demonstrated to our clients that we could execute complex hardware-oriented projects, and led the way for a number of subsequent hardware management and supply projects.
SCR VIV Tow Test
PROJECT STRIDE JIP Phase 2
STRIDE (STeel Risers In Deepwater Environments) was a joint industry project (JIP) spanning the years 1997-2002 and involving over 20 participants from operators, installation contractors and manufacturers.
The STRIDE JIP was probably the landmark project that launched 2H on the path to success. The participation of nearly all notable operators and contractors at the time presented a great opportunity for networking and to showcase our capabilities.
Steel catenary risers (SCRs) were pioneered by Shell who installed them to their early TLPs in the Gulf of Mexico. The STRIDE JIP was initiated to understand and develop the technology for wider applications worldwide. One of the major concerns with SCRs was the occurrence of vortex-induced vibrations (VIVs) that could cause fatigue failure of the riser pipe.
We conducted a full-scale tow test offshore to investigate the extent of VIVs caused by currents at different incident angles to an SCR and the effectiveness of suppression strakes in reducing VIV.
We had two 200m long, 10-inch outer diameter pipe strings fabricated. One was left bare and another was fitted with VIV suppression strakes. Each pipe was towed between two tug boats at a constant speed in a Norwegian fjord in Stavanger. The fjord’s sheltered calm water allowed the vessel speed to simulate a ‘slab current’ profile against the curved pipe string. The pipe string was fitted with loggers at 5m intervals to record the pipe’s motions.
The campaign established the use of low-cost, low-power, battery-operated, self-storage data loggers for monitoring dynamic offshore structures. This low-cost monitoring solution was designed and prototyped entirely in-house and made this ambitious tow test economically possible. This project launched 2H’s structural monitoring business, which we later developed into the standalone company, Pulse Structural Monitoring.
This test was a major offshore operation. Extreme care was taken in the planning, design, fabrication, deployment and towing of the pipe strings to prevent fatigue. We worked closely with Rockwater (part of Brown and Root at the time, but part of Subsea7 now, and a member of the JIP) to ensure the tests were carried out safely.
Processing the collected data to make sense of the pipe dynamics after the offshore campaign was a new challenge for our engineering team. Thanks to a number of mathematical experts in the company, we were able to evaluate the vast amount of data, including filtering, spectral analysis, mode matching, etc, and produce meaningful interpretations. The test provided insight into the types of VIVs a curved SCR would generate. The straked pipe recorded substantially reduced vibration compared to the bare pipe string.
SCR Soil Interaction Test
PROJECT STRIDE JIP Phase 3
In a later phase of the STRIDE JIP, 2H addressed another SCR issue. At the time, very little was known about the quantification of additional stresses caused by a pipe’s interaction with seabed soil.
We set out to conduct a full scale soil interaction test in shallow water. For the test, a 4-inch OD pipe string was fabricated. One end was attached to an actuator arm on a harbour wall that could traverse vertically and horizontally in a straight sinusoidal movement. The other end was laid on the seabed and anchored at a prescribed distance away. This catenary pipe arrangement simulated the near seabed portion of an SCR.
Watchet Harbour in the west of England was chosen for the test as it was silted with a thick mud layer with soil characteristics similar to those found in deepwater Gulf of Mexico. The pipe string was fitted with strain gauges at the touch down region to measure the stress response. Watchet Harbour waters emptied twice daily during low tide. This eliminated the need for divers, and the test team was able to walk out to dig trenches and change instrumentation when the harbor was empty. When the water returned, engineers were able to conduct pipe motion experiments during high tide.
There was some concern for public safety, as Watchet Harbour was open to the public every day. To address this, we briefed local fishermen and boat owners to explain the test and enlist their help with marker buoys for the project. Another hazard we faced was working in thick mud, which was often difficult to walk through. We used pierced steel planks to create a safe walkway on the harbour seabed.
The test provided up-close observation and understanding of a pipe’s trench-making mechanism and its interaction with the seabed, vertically and horizontally. Stress amplifications under different loading conditions were also noted that could be used in design analysis.
Pipe-in-Pipe Structural Damping Test
PROJECT BP Shah Deniz Top Tension Risers
The BP Shah Deniz platform’s top tensioned risers were comprised of multiple concentric pipes. In the high currents of the Caspian Sea, the outer pipe was highly fatigue sensitive. It was believed that quantifying the structural damping coefficient of the multi-pipe system would help predict the fatigue damage less conservatively. 2H were contracted to conduct an experiment over a six-month period in a strictly-controlled manner to test this theory.
The damping test was conducted at the Coppe Laboratory in Rio de Janeiro. The laboratory is in an enclosed space with a 20m high ceiling, offering a rare location where this experiment could be conducted indoors without the influence of the weather, particularly disturbance from the wind.
An 18m long, a 1/3 scaled OD concentric multi-pipe string was fabricated and built to stand vertically like a flag pole. The top end was displaced laterally and released to create a freely damped decaying vibration.
The vibration decay time and pattern allowed the damping coefficients to be determined directly. Furthermore, the pipe configuration was altered, e.g. with and without inner pipes, centralizer gap/spacing, etc. to investigate the damping mechanism.
The damping test provided the first ever justification of actual damping coefficients for use in dynamic analysis of a pipe structure.
VIV Tow Test
PROJECT CNOOC Strake Design Sea Trial
The CNOOC Research Institute contacted 2H as they were concerned with the verification of the effectiveness of the VIV suppression strake design they had produced. We were tasked with conducting a sea trial of a large-scale vertically tensioned pipe string to demonstrate the effectiveness of the VIV suppression strake design. The test took place over a nine-month period in the Yellow Sea, China.
To begin with, two 40m long 4-inch OD pipe strings were fabricated. One was left bare and another was fitted with a VIV suppression strake. The two strings were then suspended vertically in parallel from the deployment vessel and towed to simulate current flow conditions. The parallel pipe arrangement subjected both pipes to the same current condition at any time, allowing a direct assessment of the effectiveness of the strake in suppressing VIV in the bare pipe.
The tow test qualified the CNOOC strake design for their future project applications.