LAS OBRAS
5.3 Capacidades de la industria nacional
Pipeline repairs may be required during pipeline installation or during operation. If a pipeline is flooded (water penetrated due to buckling or damage) during pipe laying, the best repair method is to reverse the lay operation and recover the defect point on the vessel for replacement.
Shell’s Mensa project performed a 12-inch repair job at 5,000 ft water depth when the pipe failed at a welding point due to excessive bending stress. Seven miles of pipe from depths between 5,300 ft and 4,700 ft were recovered up the stinger by “reversed lay” and later reinstalled [1]. The use of a repair clamp is another option for repair during installation, if the defect point is small and precisely located.
Abandonment and recovery (A&R) procedures can be used to retrieve the damaged pipeline section during pipelay. The process involves:
1) Identifying the damage by ROV or diver
2) Cutting off the damaged pipe (by cutting saw or shaped charge explosive) 3) Installing a pipeline recovery tool (PRT)
4) Dewatering the pipe, if needed
5) Retrieving the pipe end to the water surface by “reversed lay”
The recovery tool may incorporate a dewatering mechanism with a subsea pig launching apparatus (see Figure 24.1). During operation, there are generally two repair methods available;
• Clamp repair (see Figure 24.2)
• Spool piece repair – on-bottom or surface lift
If the defect is isolated with no significant reduction in pipe diameter, such as a leak or crack due to welding defect or pitting corrosion, a repair clamp method can be used. If the pipe diameter is severely reduced or the damaged section is long, such as a buckling rupture, a spool piece repair method must be used.
The basic tasks and procedures to complete a diverless clamp repair are as follows: 1) Locate the damage
Figure 24.1 Pipeline Recovery Tool (PRT) (Picture taken from TD Williamson factory in Houston)
The on-bottom spool repair method conducts all operations, cuts and connections at sea bottom, without lifting the pipe to the water surface. An expandable horizontal spool or a Z-shaped spool can be used like a horizontal jumper connection method.
The on-bottom spool repair procedures are as the following: 1) Locate the damage section
2) Prepare the work site (lifting the pipe by H-frame or jetting around the pipe) 3) Cut the pipe in two places on either side of the damaged section
4) Put aside the cut section on the sea floor or retrieve to the surface 5) Remove coatings and clean each pipe end
6) Install connectors on each pipe end (test seal integrity)
7) Measure spool piece distance and fabricate spool with connectors 8) Lower, position, and connect the spool piece
9) Pressure test pipeline
The surface lift repair method has been used in shallow water repairs and is expandable to deepwater repairs. This method requires pipe lifting to the surface, so a large vessel to handle the pipe is required. The repair procedures are given below:
1) Locate the damage section
2) Prepare the work site (lifting the pipe by H-frame or jetting around the pipe) 3) Cut the pipe in two places on either side of the damaged section
4) Place a recovery tool (head) at the cut end of the damaged pipeline, dewater if required
5) Lift the damaged pipeline to surface using a single point lifting method 6) Remove (cut off) damaged pipe section at the surface
7) Remove coatings and clean pipe end 8) Install a connector on a sled with a yoke 9) Lower the pipeline back to the sea bottom 10) Repeat for the second end of the pipeline
11) Measure spool piece distance and fabricate spool with connectors 12) Lower, position, and connect the spool
13) Pressure test pipeline
Figures 24.3 through 24.5 show clamp repair, on-bottom spool repair, and surface lift repair sequence, respectively. Figure 24.6 shows shallow water pipeline repair sequence, using a diver, forged stab end connectors, and a misalignment ball flanged
Figure 24.3 Clamp Repair Sequence [3]
Raise pipe and lower repair clamp
ROV opens clamp
ROV closes clamp and tests seals
Figure 24.4 On-Bottom Spool Repair Sequence [3]
Raise the pipe and cut the damaged pipe section. Prepare pipe end for grip & seal coupling installation.
Lower repair sled with a horizontal coupling and a vertical connector hub. ROV installs the coupling to the pipe. Repeat for the other end.
Lower spool piece. ROV connects both connectors and tests seals. Recover rigging.
Figure 24.5 Surface Lift Repair Sequence [3]
Raise the pipe and cut the damaged pipe section. Install pipeline recovery tool and lift the pipe to the surface.
Install repair sled with a horizontal coupling and a vertical connector hub at surface and lower to the seabed. Repeat for the other end.
Lower spool piece. ROV connects both connectors and tests seals. Recover rigging.
References:
[1] OTC paper #8628, “Mensa Project: Flowlines,” 1998
[2] QCS (Quality Connector Systems) Website, www.qualityconnectorsystems.com [3] Oil States Industries Inc. Website, http://oilstates.com
[4] Harvey Mohr, “Deepwater Pipeline Connection and Repair Equipment,” The Deepwater Pipeline Technology Conference, 1998
[5] Alex Alvarado, “Gulf of Mexico Pipeline Failure and Regulatory Issues,” Deepwater Pipeline and Riser Technology Conference, 2000
DEFINITIONS
(Not in alphabetical order. To be updated periodically.)
Hydrogen Induced Cracking (HIC): The mechanism begins with hydrogen atoms
diffusing through the metal. When these hydrogen atoms re-combine in minuscule voids of the metal matrix to hydrogen molecules, they create pressure from inside the cavity they are in. This pressure can increase to levels where the metal has reduced ductility and tensile strength, up to where it can crack open so it is called hydrogen induced cracking (HIC). High-strength and low-alloy steels, aluminium, and titanium alloys are most susceptible.
Hydrogen embrittlement (or hydrogen grooving) is the process by which various metals, most importantly high-strength steel, become brittle and crack following exposure to hydrogen. Hydrogen cracking can pose an engineering problem especially in the context of a hydrogen economy.
Hydrogen embrittlement can happen during various manufacturing operations or operational use, anywhere where the metal comes in contact with atomic or molecular hydrogen. Processes which can lead to this include cathodic protection, phosphating, pickling, and electroplating. A special case is arc welding, in which the hydrogen is released from moisture (for example in the coating of the welding electrodes; to minimize this, special low-hydrogen electrodes are used for welding high-strength steels). Other mechanisms of introduction of hydrogen into metal are galvanic corrosion, chemical reactions of metal with acids, or with other chemicals (notably hydrogen sulfide in
sulphide stress cracking, or SSC, a process of importance for the oil and gas industries). (Source: http://en.wikipedia.org)
Sweet or Sour Crude: The corrosivity of an oil and gas well is increased by the
presence of Cl (chloride) in water solutions, CO2 (carbon dioxide), and H2S (hydrogen sulphide). The crude is considered sweet as long as H2S is not present. However, CO2 alone can cause high corrosion, since it is acidifying the solution and the corrosion is further accelerated if Cl is present.
Sour Crude is defined when the partial pressure of H2S is above 0.05 psi. At higher partial pressures, the corrosion rate on carbon steel is substantially increased by means of making the water phase more acidic and by forming iron sulphide scale. Sulphide stress cracking (SSC) is common in high strength steels.
The impurities (H2S, CO2, Cl, etc.) will need to be removed before the low quality sour crude is refined into gasoline, thereby increasing the cost of processing. This results in a higher-priced gasoline than one made from sweet crude oil. Thus sour crude is usually processed into heavy oil such as diesel rather than gasoline to reduce processing cost.
HIPPS: High Integrity Pressure Protection System is an instrument based over pressure
protective system (OPPS) which is attractive for high pressure/high temperature (HP/HT) developments where it is not possible to design the pipeline and risers to the full
wellhead shut-in pressure. The instrument can include series of fast acting (high sensitivity) pressure relief valve, ESD (emergency shutdown valve), etc. There are less than 6 subsea HIPPS worldwide (mostly in North Sea) and no HIPPS exists in the GOM.
PLEM and PLET: Pipeline end manifold (PLEM) is a sled equipped with multiple
connector hubs. If only one connector hub exists, it is called a pipeline end termination (PLET). Midline sled is commonly called an in-line sled (ILS).
API Degree (gravity): The API (American Petroleum Institute) degree (or gravity), is a
measure of how heavy or light a petroleum liquid compared to water. If its API degree is greater than 10, it is lighter and floats on water. API degree 10 equals to 1.0 specific gravity (SG) of fresh water.
Although mathematically API gravity has no units (see the formula below), it is referred to as being in “degrees”. API degree formula is derived using a hydrometer instrument and designed so that most values would fall between 10 and 70 API gravity degrees. (Source: http://en.wikipedia.org)
131.5
F
60
at
SG
141.5
degree
API
=
o−
Fresh water: 10 oAPI Heavy oil: <22 oAPI
Medium oil: 22 oAPI – 31 oAPI Light oil: 31 oAPI – 45 oAPI
Workover: Maintenance is performed during the service life of the well to ensure the
well produces at optimum levels. In addition to periodic maintenance, producing wells occasionally require major repairs or modification, called "workover." Problems that can result in a workover operation are: equipment failure, wellbore problems, and saltwater disposal.
For problem wells, the remedial workover is performed to increase productivity, to open new producing zones, or to eliminate excessive water or gas production. Examples of these remedial workover operations are deepening, plugging back, pulling and resetting liners, squeeze cementing, etc.
Ovality: Pipe out-of-roundness is the difference between largest diameter and smallest
diameter of a pipe (Dmax – Dmin). Ovality is the ratio between out-of-roundness and average diameter (DNV definition). The ovality defined by API is half of the DNV ovality.
(
)
(
)
(
)
0.8% 0.008 15.90 16.17 15.90 - 16.17 (API) Ovality 1.7% 0.017 15.90 16.17 15.90 - 16.17 2 (DNV) Ovality , 15.90" D , 16.17" D , 16" D If D D D - D (API) Ovality D D D - D 2 /2 D D D - D D D - D (DNV) Ovality nom max nom min max min max min max min max min max min max av min max = = + = = = + × = = = = + = + = + = =RAO: Response amplitude operator (RAO) is used to represent the vessel or floating
structure’s six degree movements due to waves and wind, as below.
Surge Heave Sway Roll Yaw Pitch