PIPELINE SPANNING-1 ROUTE SELECTION KEY FIRST STEP IN AVOIDING OCEAN FLOOR SPANS
Marvin M. Beckmann, James R. Hale, Craig W. Lamison
Brown & Root Inc.
Houston
Pipeline spanning is becoming a greater concern in the U.S. Gulf of Mexico as field development moves into deeper waters.
Two factors combine to create the problem.
First, seafloor topography becomes more irregular as pipelines move from the relatively smooth continental shelf to the steeper, irregular bathymetry of the continental slope. Second, pipeline installation tension increases in deeper water.
This first of two articles looks at how careful route selection may preclude spanning and how that selection process may avoid or reduce costly offshore surveys.
The concluding article analyzes pipeline spans and sets out ways to eliminate spans or correct those that occur despite careful route selection and design strategies.
The discussion is based on recent Brown & Root Inc. experience with pipeline spanning in the Gulf of Mexico.
RECENT EFFORTS
Until recently, most pipelines have been installed in the Gulf of Mexico on the relatively smooth continental shelf.
Designs for these pipelines included an allowable span length. Spans identified in the post-installation survey were few and were simply corrected when found.
Usually, spans have been corrected by placing sand bag supports under the unsupported spans or by jetting out high spots supporting the pipe.
Recently, offshore pipeline projects have moved into water depths greater than 1,000 ft. In these depths, the continental shelf has made full transition into the continental slope.
Here the seabed is steeper and more irregular. The bottom irregularity can be attributed to a number of different phenomena absent on the shallower, flatter seabeds of the continental shelf. These phenomena include mud-slides, gas-hydrate formations, and erosion channels from subsea currents.
Some examples of pipelines affected by the irregular seabeds of the continental slope are:
- Conoco's Jolliet project, where flexible pipe was used to avoid spans (OGJ, Nov. 11 , p. 60)
- AEDC (U.S.A.) Inc.'s pipeline from Mississippi Canyon 486 (MC486) to Grand Isle 94 (G194), where a steel pipeline was used and resulting deepwater spans rectified
- Placid's Green Canyon project which suffered the loss of one section of pipeline during tow-out (OGJ, Feb. 1, 1988, p. 21).
For each of these projects, several preinstallation surveys were necessary to locate acceptable pipeline routes. Even so, the solutions - flexible pipe, deepwater span rectification, and lost pipe - schedule slip - were expensive to implement.
ROUTE CONSTRAINTS
Determining the preliminary pipeline route typically derives from weighing the following constraints:
- Minimization of the pipeline route length.
- Approach terminations (platforms and tie-ins) in the required direction and at the correct location.
Approach direction is especially critical for J-tube risers in which the J-tube orientation determines the pipeline approach angle.
- Maintenance of horizontal route curvature compatible with the proposed installation method. The minimum route curvature is a function of bottom tension, pipe weight, and lateral soil resistance. The relationship is defined in the following:
R = Tb/(Ws x Mu)
where:
R = Horizontal route curvature (without safety factor)
Tb = Tension at seabed
Ws = Submerged weight of pipe
Mu = Lateral soil resistance factor
- Fairway-shipping channel crossings. It is desirable to cross fairways and shipping channels near perpendicularly to reduce exposure to shipping activities and avoid deep-burial requirements.
- Routing in lease blocks not owned by the pipeline company will require permission from the owner of the lease block.
Usually this is not a problem. But in some cases, restrictions may apply to prevent interference with the owner's block-development plans.
After preconstruction survey data are available, the following additional constraints apply:
- Minimization of the pipeline spanning
- Avoidance of undesirable bottom features (mud-slide areas, faults, etc., for example).
ROUTE SELECTION
Shallow-water route selection in the Gulf of Mexico consists of the following steps:
- Selection of a preliminary route based on the most advantageous path between pipeline terminations (usually the most direct route).
- Preconstruction survey of the preliminary route with sufficient survey-corridor width to allow for minor route corrections to avoid obstacles.
- Examination of survey data to identify possible obstacles.
- Determination of final route based on obstacles identified.
In this procedure, it is assumed, based on the data gathered during the preconstruction survey, that any obstacles or irregular seabed areas can be routed around.
Typically, this is the case; it is rare when additional survey data are needed to determine a satisfactory route. Spanning analysis using the survey data is usually unnecessary and thus is omitted.
An interesting point is that the pipeline engineering and survey work are historically conducted independent of each other.
The survey work is not considered integral to the design. Therefore, it is performed under separate contract without interaction with the pipeline designers during the survey.
As more pipelines are planned in irregular seabed areas, consideration must be given to integrating these activities.
Based on shallow-water experience, survey-route selection procedures have been extended into deeper and more irregular seabed conditions. This has not always proved to be cost effective because often several surveys are required before an acceptable route can be found.
This is not cost-effective because several mobilizations/demobilizations of the survey equipment and personnel are required. It may also adversely affect the schedule because it is usually anticipated that one survey will suffice.
Thus, later surveys delay final engineering, permit applications, and ultimately construction.
The problem may be further aggravated by later surveys which fall during bad weather seasons.
Several alternatives have been implemented recently. These include surveys of an extra large area so that many alternate routes can be investigated and analysis made of pipe spans during the survey to ensure that an acceptable route is found.
If the original route is unacceptable, alternative routes are surveyed and analyzed until an acceptable route is found.
SHORTCOMINGS
Both of these methods have drawbacks.
The first is expensive because a very large area must be surveyed to ensure that all feasible routes are covered. It must also be done in sufficient detail to get accurate bathymetry data.
And it must meet requirements for pipeline permit applications established by the U.S. Department of Interior's (DOI) Minerals Management Service (MMS).
The second method requires quick decisions about routing based on the data, analysis, and information available offshore.
This information is often incomplete and the data unchecked.
This method heavily emphasizes decisions made in a nonideal environment; that is, decisions made without the luxury of time and information available in the office.
In addition, areas which may cause spans are not always easy to identify visually.
Many factors contribute to spanning, and without proper analysis it is difficult to predict (on an undulating seabed) where a span is likely.
Contributing to the problem is the variation in scale often used to collect survey data. What may look like a severe problem may not be, and vice-versa.
In Fig. 1, the topography indicated in the top left graph appears to indicate a potential spanning problem, while the graph on the top right does not.
Yet, in reality, the topography indicated on the right will create a long span.
This is illustrated by the lower two graphs of Fig. 1 in which a pipeline is shown superimposed as it would naturally rest on the bottom. The pipeline will span the apparently smoother bottom topography and conform to the apparently more irregular bottom.
The main drawback is that what looks like an acceptable route may in fact turn out to be unacceptable upon later review.
The reason may even be unrelated to spanning, but due to some other constraint.
The result is that additional surveying will be require .
The first method has proven to be the most effective.
Although it may appear to be excessive and wasted survey expense, it will prove to be least disruptive to the project schedule. In fact, it may not be more expensive if two or three additional mobilization/demobilization costs are required to complete the survey work with other methods.
With this method, bathymetry can be generated for a number of different routes, and the most advantageous route selected. In addition, the survey should be done sufficiently ahead of design work so the pipeline route can drive the J-tube and deck piping design (rather than vice-versa).
The cost of span correction and construction delays can easily justify this type of design approach compared with the shallow (smooth-seabed) procedure.
Copyright 1991 Oil & Gas Journal. All Rights Reserved.