for the shortest route, the user must immediately consider at least
two additional factors: seafloor slope and obstacles.
The method for finding an optimal route is to form a mathematical
network approximating the seafloor, its slopes and obstacles. The
network is composed of nodes 328 ft apart on a square grid, each
connected by eight edges representing potential pipeline route seg-
ments. Four edges connect to neighboring nodes to the north, south,
east and west, which are 328 ft long and the other four edges are 464
ft long, leading to the four diagonal neighbors.
A change in slope between two points on a pipeline route affects
both the cost of pipeline construction and the energy required to
move the liquids and gasses transported. Therefore, there is a tradeoff
along any route between the length of the pipe and the slopes it must
travel. The influence of slope is included in the model in two ways.
First, based on a bathymetric grid in GOM3, each edge carries the
seafloor slope between neighboring nodes as an attribute.
Second, using a slider in the interface, the user may input a “slope
importance factor” (β, 1≤β≤99) to weight the relative importance of
slope versus distance in calculating the optimal path. Where the
absolute value of the slope along an edge (measured in percent) is Ɵ
and the length of the edge is d. This equation calculates the weight
(w) assigned to each edge in the network.
Through the data in GOM3, seafloor obstacles are introduced into
the network. They fall into three classes: regulatory no-go areas (e.g.,
marine sanctuaries), quarter-mile buffers around existing platforms
and subsea structures and all known seafloor characteristics that may
constitute hazards (e.g., mud volcanos, hydrates, active seeps). Computationally, edges that transect obstacle polygons are dropped from
the network so they cannot be included in a candidate tieback route.
Given the node spacing, the network for the deepwater, US portion
of the Gulf of Mexico contains 180 million nodes. For computational
efficiency in calculating each route, the network is reduced to a subset along the straight-line path between the origin and destination.
Generally, it is formed by first creating a rectangle with the origin
and destination points at diametric corners. Then a 10-mile buffer is
Four calculated tieback routes with a common origin in Alaminos Canyon (AC) 079 in 4,492 feet of water, currently leased by BHP (pink). There are two
routes to the Nansen Spar: one avoids seafloor obstacles (magenta) and the other does not (light orange). The third route is to the Hoover spar (yellow)
and the fourth is to the Gunnison spar (black). Light solid lines are existing oil (green) and gas (red) pipelines and umbilicals (yellow). The small irregular features of several colors are the boundaries of several classes of seafloor obstacles and the larger polygons are oil (green), oil/gas (green with red
stripes) and gas (red) fields. Current leases are shown in semitransparent yellow.