Recent successful operations by Stolt Comex Seaway (SCS) involving deep water flowline burial have initiated serious industry inquiry into the burial as an alternate thermal insulation technique.
While flowline burial is not new, much of the industry has been waiting for evidence that deepwater burial is technically and operationally feasible.
To better understand deepwater burial technologies, the questions of why flowlines should be buried and what is the most efficient method of flowline burial must be answered.
Why bury flowlines
In addition to the known benefits of mechanical protection and increased upheaval bucking resistance, flowline burial provides significant thermal benefits. Recent SCS study results appear to show that flowline burial in combination with common external coatings provides sufficient thermal insulation to eliminate the need for pipe-in-pipe (PIP) flowline systems.
A burial and coating (B&C) insulation system is significantly less expensive than equivalent PIP options. In addition, eliminating the need for PIP increases the available installation vessels, reducing installation costs and increasing installation schedule flexibility. Projected savings for typical Gulf of Mexico (GOM) flowlines may exceed 50% of flowline costs.
B&C is an attractive insulation alternative when external coating alone provides insufficient insulation or when required coating costs become prohibitive. B&C provides significant material and installation savings over PIP and appears feasible for relatively long flowlines.
Proven multiphase flow and heat transfer analysis tools exist to determine the benefits of flowline burial. General results and conclusions concerning the thermal benefits of flowline burial may be determined using reasonable assumptions and preliminary project data.
Key input parameters for a flowline burial design are product composition, inlet temperature and pressure, flowline route profile, flowline size, coating thermal properties and thickness, native and backfilled soil thermal properties, trench geometry, and backfill cover thickness. Primary flowline operating parameters are overall heat transfer coefficient (U value), flowline outlet temperature, and time to wax appearance and hydrate formation after shutdown.
The relationships between input and operating parameters depend upon the assumed heat transfer mechanisms. Conduction, convection, radiation, micro-distillation, and macro-distillation can cause heat transfer through a porous medium. For low permeability, low porosity soils such as highly plastics clays, conduction is the primary heat loss mechanism (Winterkorn, 1960).
Heat conduction in soil is largely a function of soil thermal conductivity or K value. In the case of a typical US Gulf of Mexico flowline, for example, the normalized relationship between the overall heat transfer coefficient and the soil thermal conductivity can be determined.
Flowline burial increases allowable transient shutdown times in addition to providing insulation for steady state flow. For many flowlines, B&C can provide longer duration between wax appearance and hydrate formation after shutdown than equivalent PIP systems. Again, in the case of a Gulf of Mexico flowline, a normalized time to reach 70°F after shutdown from steady state for a typical US Gulf flowline can be determined. B&C's transient insulation advantage over PIP increases as the critical formation temperature decreases.
Consequently, B&C's time to a formation temperature of 60°F could be three to five times that for PIP. This advantage allows for greater response time in upset conditions and possibly reduces the need for flowline blowdown, thus reducing expected operating expenses. The same insulation properties that make B&C beneficial for shutdown cause slow temperature uptake during startup. However, there are many operational solutions to address this issue.
In January 1998, SCS was contracted to manage the building of a trenching system capable of burying and backfilling flowlines in water depths to 2,100 meters. The specifications required flowline burial of 1.2 meters as measured from top of pipe to natural bottom for pipe up to 24 in. in diameter. Additionally, the system was required to back-fill over the flowline with a minimum of 1.0 meter of soil.
Following completion of the build program and sea trials, SCS utilized the Talon deepwater burial system to trench and backfill 24 miles of 8 in. flowline in about 600 meters water depth. A Talon system currently is deployed from the Seaway Legend, one of SCS's deepwater dynamically positioned ships.
Post-lay flexible pipeline burial is conducted in a fashion similar to that of cable burial. A slightly stable trench is cut, into which the flexible pipeline enters quickly, touching down at the bottom of the trench before the trench collapses and buries the pipe.
Rigid pipeline requires a more stable trench under the laid pipeline that will remain open for the additional length of time required for the rigid pipeline to touch down. A subsequent back-filling pass over the trench is required to finally insulate the pipe. Efficient flowline burial requires a careful balance between creating a stable trench and providing sufficient flowline backfill.
Modern soil mechanics can be used to predict stable trench profiles if the soil along the pipeline route is accurately characterized. Increased geotechnical survey data allows for less assumption in performing this analysis.
Thanks to experimental work (Rockwell et al., Hurlbert et al., Kolle), coupled with fundamental mechanics, the removal rate and depth of cut in various soils of practical importance are known for various water jet pressures, flow rates, and nozzle diameters. Knowing the jetting horsepower allows accurate prediction of trenching rate. A controlled vehicle speed of advance provides predictable trenching and minimizes plowing by the jetting arms.
Soil removal is required subsequent to the jetting to maintain a desired trench profile. Cutting and eduction rates are compared to select optimal nozzle configurations and jetting hp.
Back filling is accomplished by jetting at strategic soil failure planes, causing trench wall collapse. This technique has been successfully implemented in soft cohesive soils with water jets directed vertically down at some distance outboard of the trench combined with water jets directed horizontally out from inside near the base of the trench.
SCS is continuing to study the thermal characteristics of flowlines buried in deepwater. Additionally, design enhancements are underway to further the Talon's capabilities for other deepwater trenching requirements.
The authors would like to acknowledge design and development contributions from Don Nelson and Mike Serafin of ROV Technologies.