Offshore Survey is a specific discipline of Hydrographic survey primarily concerned with the description of the condition of the seabed and the condition of the subsea oilfield infrastructure that interacts with it.
Hydrographic survey is the science of measurement and description of features which affect maritime navigation, marine construction, dredging, offshore oil exploration/drilling and related disciplines. Strong emphasis is placed on soundings, shorelines, tides, currents, sea floor and submerged obstructions that relate to the previously mentioned activities. The term Hydrography is sometimes used synonomously to describe Maritime Cartography, which in the final stages of the hydrographic process uses the raw data collected through hydrographic survey into information usable by the end user.
Hydrography is collected under rules which vary depending on the acceptance authority. Traditionally conducted by vessels and with Echo sounding, surveys are increasingly conducted with the aid of aircraft and sophisticated electronic sensor systems in shallow waters. United States Navy SEALS and Seabee Underwater Construction Technicians also have the ability to conduct hydrographic surveys. The SEAL/UCT operators are normally called upon before amphibious landings in order to survey the landing beaches.
When surveys are used for the purposes of chart making/distribution or dredging of state controlled waters they are commonly conducted by or under the supervision of national organizations. Coordination of those organizations voluntarily joined with the goal of improving hydrography and safe navigation is conducted by the International Hydrographic Organization. In the United States, hydrographic acceptance is conducted by NOAA for maritime navigation, the Army Corp of Engineers for dredging and marine construction and occasionally the Environmental Protection Agency (on projects such as the GE/Hudson River Super Fund site).
Companies, Universities and investment groups will often fund Hydrographic surveys of public waterways prior to developing areas adjacent those waterways. One example of this would be surveys completed for Riverboat Casinos that ply inland rivers. These large ships require relatively shallow water, but it has to meet minimum depth requirements. Private surveys are also conducted before dredging operations and after these operations are completed. Steel mills and companies that have large private slips and docks have their facilities and open water near their facilities surveyed regularly.
Modern surveying relies as much on software as hardware. Equipment can be installed on inflatable craft, such as Zodiacs, small craft, AUVs (Autonomous Underwater Vehicles), UUVs (Unmanned Underwater Vehicles) or large ships, and can include sidescan, single beam and multibeam equipment.
After data is collected, it has to undergo post-processing. A massive amount of data is collected during the typical Hydrographic survey, often several soundings per square foot. Depending on the final use (navigation charts, Digital Terrain Model, volume calculation for dredging, topography, Bathymetry) this data must be thinned out. It must also be error corrected (bad soundings,) and corrected for the effects of tides, waves/heave, water level and water temperature differences (thermoclines.) Usually the surveyor has additional data collection equipment on site to record the data required for correcting the soundings. Final output of charts can be created in a combination of specialty charting software or a CAD package, usually Autocad.
NOAA maintains a massive database of survey results, charts, and data on the NOAA site.
Offshore Cathodic Protection 101 What it is, and how it works.
Richard Baxter, Jim Britton
How Does Steel Corrode in Water?
To understand cathodic protection one must first understand the corrosion mechanism. For corrosion to occur, three conditions must be present.
1. Two dissimilar metals 2. An electrolyte (water with any type of salt or salts dissolved in it) 3. A metal (conducting) path between the dissimilar metals
The two dissimilar metals may be totally different alloys, such as steel and aluminum, but are more usually microscopic or macroscopic metallurgical differences on the surface of a single piece of steel.
If the above conditions exist, at the more active metal surface (in this case we will consider freely corroding steel which is non uniform), the following reaction takes place at the more active sites: (two iron ions plus four free electrons)
2Fe => 2Fe++ + 4e-
The free electrons travel through the metal path to the less active sites where the following reaction takes place: (oxygen gas converted to oxygen ion - by combining with the four free electrons - which combines with water to form hydroxyl ions)
O2 + 4e- + 2H20 => 4 OH-
Recombinations of these ions at the active surface produce the following reaction, which yields the iron corrosion product ferrous hydroxide: (iron combining with oxygen and water to form ferrous hydroxide)
2Fe + O2 + 2H2O => 2Fe (OH)2
This reaction is more commonly described as 'current flow through the water from the anode (more active site) to the cathode (less active site).
How Does Cathodic Protection Stop Corrosion?
Cathodic protection prevents corrosion by converting all of the anodic (active) sites on the metal surface to cathodic (passive) sites by supplying electrical current (or free electrons) from an alternate source.
Usually this takes the form of galvanic anodes, which are more active than steel. This practice is also referred to as a sacrificial system, since the galvanic anodes sacrifice themselves to protect the structural steel or pipeline from corrosion.
In the case of aluminum anodes, the reaction at the aluminum surface is: (four aluminum ions plus twelve free electrons)
4Al => 4AL+++ + 12 e-
and at the steel surface, (oxygen gas converted to oxygen ions which combine with water to form hydroxyl ions)
3O2 + 12e- + 6H20 => 12OH-
As long as the current (free electrons) is arriving at the cathode (steel) faster than oxygen is arriving, no corrosion will occur.
Figure 1: Sacrificial anode system in seawater
Basic Considerations When Designing Sacrificial Anode Systems
The electrical current which an anode discharges is controlled by Ohm's law; that is:
I=E/R
I= Current flow in amps E= Difference in potential between the anode and cathode in volts R= Total circuit resistance in ohms
Initially current will be high because the difference in potential between the anode and cathode are high, but as the potential difference decreases due to the effect of the current flow onto the cathode, current gradually decreases due to the polarization of the cathode. The circuit resistance includes both the water path and the metal path, including any cable in the circuit. The dominant value here is the resistance of the anode to the seawater.
For most applications the metal resistance is so small compared to the water resistance that it can be ignored. (Not true for sleds, or long pipelines protected from both ends). In general, long thin anodes have lower resistance than short fat anodes. They will discharge more current, but will not last as long.
Therefore a cathodic protection designer must size the anodes so that they have the right shape and surface area to discharge enough current to protect the structure and enough weight to last the desired lifetime when discharging this current. As a general rule of thumb:
Length of the anode determines how much current the anode can produce, and consequently how many square feet of steel can be protected.
Cross Section (Weight) determines how long the anode can sustain this level of protection.
Impressed Current Cathodic Protection Systems
Due to the high currents involved in many seawater systems it is not uncommon to use impressed current systems. Impressed current systems use anodes of a type that are not easily dissolved into metallic ions, but rather sustain an alternative reaction, oxidization of the dissolved chloride ions.
2Cl- => Cl2 + 2e-
Power is supplied by an external DC power unit..
Figure 2: Impressed current cathodic protection system in seawater
How Do We Know When We Have Enough Cathodic Protection?
We know whether or not we have enough current by measuring the potential of the steel against a standard reference electrode, usually silver silver/chloride (Ag/AgCl sw.), but sometimes zinc (sw.).
Current flow onto any metal shifts its normal potential in the negative direction. History has shown that if steel receives enough current to shift the potential to (-) 0.800 V vs. silver / silver chloride (Ag / AgCl), the corrosion is essentially stopped.
Due to the nature of the films which form, the minimum (-0.800 V) potential is rarely the optimum potential, and designers try to achieve a potential between (-) 0.950 V and (-) 1.000 V vs. Ag/AgCl sw.
Figure 3: Protected vs Unprotected structures as verified by cathodic protection potential
Laying Gas Pipeline from Platong field and South Bongkot field to the Third Gas Pipeline Offshore Gas Transmission Project
This project is currently under the Environmental Impact Assessment (EIA) preparation stage, after which, an EIA report will be submitted to the Office of Natural Resources and Environmental Policy and Planning (ONEP) to begin the formal approval process. Project construction is expected to begin around March 2010 to June 2011. During the construction, PTT will ensure zero or minimum impacts in line with the international standard on quality, safety, health, and the environment as clearly stated in its EIA report. (Data as of December 2008)
Platong Field To link the field to the Third Gas Pipeline, a 28-inch gas pipeline is to be laid over a distance of about 50 km, starting from the second production platform, a new platform fabricated by Chevron Thailand Exploration and Production (CTEP).
South Bongkot Field To link the field to the Third Gas Pipeline, a 24-inch gas pipeline is to be laid over a distance of about 40 km, starting from the new production platform of the field, fabricated by PTT Exploration and Production Plc (PTTEP).
Offshore Gas Pipeline Laying
Modes of pipeline laying
As a rule, offshore gas pipeline laying in Thailand requires pipe-laying barges, on which welding of pipes takes place and then from the stern of which the pipes are released to the seafloor. Named after the profiles of pipes as released to the sea, the two main modes are
1. S Lay : This dominates pipe-laying in water depths of up to 100-200 meters, which applies to the Gulf of Thailand, where water depths are only 50-70 meters.
2. J Lay : This is generally found in water depths of 200 meters or more. Naturally, this project follows the first mode.
Pipe welding
Before welding is done, the ends of the pipes need to be prepared, after which they proceed to the firing line. In fitting up the pipes, an internal clamp is used on them. Welding contains several stations, each manned by qualified welders and their assistants. The WPS requirements govern the quality of parameters, including the beveling of pipe flanges, the preheating, the electricity current, and the voltage required for electrode melting, the travel speed, the acetylene gas concentration, and the quality of electrodes (that is, cleanliness and suitable humidity). Non-destructive testing is generally used on completed welding jobs.
Pipe-laying on the Seafloor
Laying pipeline by moving the barge forward by pulling the forward anchors; the pipes gradually leave the stern in the S-Lay pattern. As a rule, pipe strengths are designed to exceed the design pressure. To survey the laid pipes, a remote operated vehicle (ROV) is used.
Survey of Laid Pipelines
Once the laying is completed, another survey proceeds along the route to decide whether the pipes were properly laid on the seafloor. If a section rests on a hole or a depressed terrain, or an area with a changing slope, such that a free span exceeds design values, a corrective action will be taken to ensure that the pipes are duly supported or uneven portions of the seafloor are duly adjusted.
