Dropped objects can damage subsea pipelines, leading to leaks, structural integrity issues, and the potential for environmental damage. Hence, drop object risk is normally assessed during Hazard Identification (HAZID) study.
Subsea pipelines are susceptible to impacts due to dropped objects from vessels, anchors, interference with other pipes, and trawl boards.
Hence, a Dropped Object Study (DOS) is required for any installation or vessel operating in the vicinity of flexible pipes, and ROVs (Remotely Operated Vehicles) should be deployed to determine the location of the dropped object on the seabed and examine the surrounding pipes for any potential damage.
Deck lifting and handling procedures should be in place and intended to prevent dropped object incidents.
Fishing vessels have been documented to also cause damage to the pipe due to impact between trawl boards and pipes, not from the dropped object. Hence, having a vessel exclusion zone extending beyond the touch-down regions of all the risers is usually enforced.
Drop Object Study (DOS)
The Drop Object Study (DOS) refers to a numerical study that assesses the behavior and impact of objects being dropped from a height onto offshore structures, such as oil rigs, pipelines, or platforms. Hence, Drop Object Study is a kind of Quantitative Risk Assessment (QRA).
These studies are conducted to understand how these structures would respond to accidental dropping of heavy objects, evaluate the potential risks, and design measures to minimize any potential hazards. This study provides benefits to an engineer who is responsible for assessing structural integrity.
Hence, the aim of this study is to ensure that the structures can safely absorb or distribute the impact energy, protecting both the structure itself and the personnel working on the offshore facility.
Drop Object Study Workflow
The methodology for a drop object study typically involves several steps.
- Identification of potential drop objects: The first step is to identify the objects that are likely to be dropped on the offshore structures during Hazard Identification (HAZID) study, such as handtools, equipment, or material. This could include items that may accidentally fall from heights, or even potential projectiles in case of extreme weather conditions.
- Selection of drop heights: Different Drop heights are considered to evaluate the potential impact of energy on the structure. These heights are chosen based on the maximum foreseeable height from which objects may fall.
- Develop the impact analysis.
Impact Energy
During Impact Analysis, we have to calculate the Impact Energy which refers to the amount of energy generated during the impact of an object. In the context of a drop of object study, impact energy is the energy transferred to the structure when an object is dropped from a height and collides with it.
The impact energy is calculated by considering the mass of the object and its elevation or velocity at the time of impact.
- E is the potential energy of a dropped object (J);
- m is the mass of the dropped object (kg);
- g is the gravitational acceleration (9.81 m/s2); and
- h is the drop height above the impact surface (m)
Hit Probability
Since, the Drop Object Study (DOS) is considered like the Quantitative Risk Assessment of QRA, the probability of hitting to vulnerable equipment or personal underneath shall also be assessed.
- p(x) = Probability of sinking object hitting the sea bottom at a distance x from the vertical line through the drop point
- x = Horizontal distance at the sea bottom (m)
- Delta = Lateral deviation (m)
Drop Object Protection
Drop Object protection measures are implemented to prevent objects from accidentally falling onto subsea pipelines, structures, or equipment during offshore operations. Some common drop object protection measures such as.
Pipeline Routing: Selecting pipeline routes away from the area with high vessel traffic, fishing activities, or other potential sources of dropped objects.
Safety Nets: Safety nets are installed below work areas, platforms, or scaffolding to catch objects that may accidentally fall. Safety nets are particularly useful in areas where the risk of dropped objects is high, such as during construction or maintenance activities.
Guardrails: Guardrails are used to create physical barriers around the perimeter of work areas, platforms, or equipment to prevent objects shifted from the dedicated area.
Engineering Controls prevent rigging failure: Rigging system, including lifting and hoisting equipment such as cranes, winches, davits, lifting gear, shackles, slings, hooks, etc. The following are engineering control measures to prevent rigging failure.
- Designing rigging systems for strength and stability;
- Using redundancy and safety factor
- Implementing load monitoring systems such as load cells or tension monitoring devices;
- Installing Safety Devices such as limit switches, overload protection, and emergency stop mechanism;
- Regular inspection and maintenance;
- Training and certification.
Clearance Zones: Establishing clear exclusion or clearance zones around subsea pipelines helps prevent vessels, and equipment from approaching too close.
Vessel Traffic Management: Implementing vessel traffic management systems, including navigational aids, restricted areas, and communication protocols, helps prevent vessels from entering prohibited zones near subsea pipelines.
Anchor Drag Analysis: Conducting anchor drag analysis to assess the risk of vessels dragging anchors over subsea pipelines helps identify areas where additional protection measures may be needed, such as anchor handling procedures or installation of protective matting or anchors.
Subsea Pipeline Burial: Burial of subsea pipelines beneath the seabed provides physical protection against dropped objects and reduces the risk of damage from fishing activities, and anchor strikes.
Coating and Corrosion Protection: Applying protective coating and implementing a cathodic protection system on subsea pipelines helps prevent corrosion and mitigate the risk of damage from dropped objects.
Inspections and Monitoring: Regular inspections and audits of working areas, equipment, and procedures shall be conducted to identify potential hazards.
Inspection and Monitoring Techniques
Subsea inspection has the aim of determining any damage or anomalies to flexible pipes such as bucking, kinking, and holes or tears in the external sheath. This provision can be considered as the prevention method since the system’s integrity has existed and it can reduce the likelihood of damage.
Each inspection technique has its advantages and limitations, and the choice of technique depends on factors such as the depth of the pipeline, accessibility, the type of detects being targeted, and budget constraints.
Anyway, there is no single inspection or monitoring technique that can provide a complete picture of the integrity of any flexible pipeline system.
A typical inspection and monitoring system include
- General Visual Inspection (GVI)/ Close Visual Inspection (CVI);
- Polymer monitoring: Online, Offline, topside, and subsea;
- Annulus monitoring;
- Riser dynamic: Tension, angle and curvature;
- Steel armor: Magnetic or radiograph
- Use the process sensors, pressure, and temperature sensors.
Bore Fluid parameters monitoring
- The parameters required for monitoring are temperature, pressure, volume flow rate, and the bore fluid composition;
Visual Inspection by ROV
- The most important and commonly used measures.
- Limit of access. ROV can maneuver around the pipeline with the provision of cameras, and lights but it cannot perform close-up inspection.
- Limit to observation of the small holes in the external sheath.
Coupon Sampling and analysis
- The coupons are placed in a holder in line with the transported bore fluid to observe the rate of degradation.
- Commonly used for high-temperature application
Ultrasonic technique or Ultrasonic Testing (UT)
- UT can detect annulus flooding but it cannot detect wire defects.
- UT can be performed from the surface or using specialized tools deployed by ROVs or divers.
Radiography
- Limited to topside.
- Radiography is used to monitor the condition of a PVDF internal sheath at the end termination.
Vacuum Testing of Riser Annulus
- A most reliable method to determine the presence of water in the riser annulus.
Sonar monitoring (riser and anchor chain monitoring system)
- Limited use to date, but it can detect bend stiffener loss
Proof pressure test
- Short-term assurance only
Effluent factor in the inspection
The dropped object is considered an external impact as the anchor impacts, anchor dragging, trawling, boat impact, fish-bombing, etc. The external impact is an event-based damage reason, and if the annual probability of an impact is constant, the Probability of Failure (PoF) is also close to constant. An inspection will have no or limited change on the PoF, but it is still preferable to inspect the line at regular intervals.
The example PoF category for the dropped object on a pipeline is based on the following information:
- The outcome of the last inspection;
- Frequency of material lifting (high frequency, higher PoF)
- Pipeline diameter and concrete coating thickness (smaller diameter, higher PoF);
- Buried (buried pipeline, low PoF); and
- Marine operation activity (higher marine operation, higher PoF)
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