How to choose a walk to work vessel

In recent years a new development has arisen in the offshore sector. Walk to work vessels. Since the development is new, it may be useful to give some guidelines for potential charterers to make it easier to select the right ship for the project. The article is not intended to be a full and complete analysis. It is merely intended to help you find the most suitable vessel for your project and to explain our vision behind the two vessels.

What are walk to work vessels (W2W) vessels ? 

These ships are intended to give safe access to offshore structures and to increase access during bad weather to platforms both in the wind and oil and gas industry. They are equipped with active heave compensated gangways. The purpose of the active heave compensation in the gangway is to dampen or stabilize it for the motions of the vessel on the waves, so that persons can transfer safely from a vessel to a stable offshore platform. Usually W2W vessels also have accommodation on board. In this way personnel can sleep close to the offshore platform so that time lost in traveling is minimized. Usually there is a work deck for doing hot works and storing materials. And often a crane for transferring light materials is available.

Uses of walk to work vessels 

Walk to work vessels are now being used as offshore access solution in the oil and gas industry during platform upgrades, commissioning and maintenance. Because putting accommodation on a gas platform is expensive to build but also to maintain, the sector has started using more and more platform without these. That leaves an accommodation problem during maintenance and upgrade campaigns. In more shallow waters traditionally the sector used jack ups for this. However with the arrival of accommodation vessels with gangways, this solution has been discovered by the oil and gas companies as a more economical solution for some of these jobs.

In the wind industry W2W vessels have been used a lot as access solution. For instance in combination with cable laying. The vessel with gangway would then be used to accommodate the persons doing the cable pull in, connection, termination and testing of the cables and to transfer the persons to the access deck of the turbine. Also during commissioning of the turbines and during warranty, maintenance and repair works these vessels have become a common solution in te wind industry. Recently the W2W vessels have also proven their use during commissioning and maintenance of converter stations.

Since W2W is relatively new and because of the efficient costings, it is expected that a lot more usages will be discovered in that the amount of jobs involving W2W is set to increase significantly.

Gangway systems available. 

At the moment a lot of new manufacturers have expressed their intention to start delivering a heave compensated gangway solution. However at the present moment there are two manufacturers of active heave compensated gangways that have a significant track record. They are Ampelmann and Uptime Both systems have their own unique features. The main difference is that the Ampelmann philosophy is based on keeping the entire gangway horizontal with heave compensation. The Uptime system is based on keeping the tip of the gangway completely heave compensated and leaves the rest of the gangway to follow the motions needed to keep the tip stable. Both systems have quite an extensive track record. The new systems being developed presently will soon start to demonstrate their workability.

Permanently installed gangway or installed on vessel for the project 

At moment there are just a few vessels sailing around with a permanently installed heave compensated gangway. So both Uptime and Ampelmann offer there systems also for short rental periods. Some new gangway manufacturers will do the same.

Off course the cost for renting such a system for a short period will be much higher than if the gangway is permanently installed on the vessel. In case of chartering a vessel and a gangway separately the charterer will also have to pay for the installation of the gangway. These cost consist of the following:

  • Engineering cost. A plan needs to be made on how the system is fitted on the vessel.
  • Installation cost: There are cost for installing the gangway system on the vessel.
  • Transportation cost: either the gangway needs to be transported to the vessel or the vessel needs to be transported to the gangway. The charterer will pay for this.
  • De-installation cost; after the project the gangway needs to be removed.
  • Return transportation cost: at the end of the project the gangway needs to be returned to the gangway owner and the vessel after that will have to go the port of redelivery.

By renting the gangway separately from the vessel the charterer runs several additional risks.

  • If the vessel is not ready to take the gangway on the delivery date, the charterer will pay charter for the gangway. If the gangway is not ready at the moment of delivery of the vessel, the client will pay charter for the vessel. The same applies if the engineering plan is not perfect and emergency adaptions to vessel or gangway must be made to make the installation fit.
  • But also during the charter, the charterer has increased risk by chartering the vessel and the gangway separately. If the vessel has a break down, the charterer has no recourse to the brake down clause of the gangway. So gangway charter will continue, despite the fact that the charterer cannot use it. Or conversely if the gangway brakes down, the charterer will still have to pay for the vessel. If the gangway is permanently installed and integrated on the vessel than both a breakdown of the vessel and a breakdown of the gangway will lead to activation of the brake down clause. So the charterer will not pay hire for either one. Given the fact that spare parts and repair engineers are not always close by the vessel, all this delay can add up.

Location of the gangway 

As you may imagine the rotating point of the ship (usually close to the middle) is the least effected by pitching and rolling motions of the vessel. The more the heave compensated gangway is moved away from this rotating point, the more the gangway moves when the vessel is pitching and rolling. Therefore the optimum position of the gangway is close to the rotating point of the vessel.

By placing the gangway in the center of the vessel and having a long enough gangway, one can make connection over both sides of the vessel and under a wide angle over both sides. The optimum position for maximum gangway performance is with the vessel head on into the current. Often the number of access points on a platform are limited; sometimes there is only one such point. By being able to make connections over both sides and under an angle, the chances that the vessel can be placed in a position where the current comes in from the front are dramatically increased and thus chances of being able to make a successful gangway connection are also increased. If the vessel can make connection only over one side and the current is coming from the wrong direction, often no gangway connection to the platform can be made.

The height the gangway can reach is important too. When the gangway is installed on the aft deck of a supply vessel, it cannot reach very high. A lot access platforms are located at about 19 m above lowest astronomical tide (LAT). Off course the gangway can make a connection to the access ladder of the platform, but accessing the platform directly is fastest and easiest. Nowadays for increasing the entrance height of the gangway, a pedestal under the gangway can be used.

Special purpose ship or not. 

According to the Solas (safety of lives at sea) conventions a cargo vessel (meaning here ships that are not classed as passenger vessels) are only allowed to take up to 12 passengers on top of the vessel crew. A ship carrying more than 12 passengers by definition must be a passenger ship. This would constitute a problem for offshore jobs that require a lot of personnel. All these project would require passenger classed vessels. To circumvent this problem, the Solas organization has created a new category of ships, the Special Purpose Ships. The offshore personnel working offshore could be considered as Special Personnel. Special personnel is defined as “All persons who are not passengers or members of the crew, carried on board in connection with the special purpose of the ship or because of special work carried out on board that ship.” The difference with a normal passenger on a vessel is that the special purpose personnel has been trained in safety at sea and received a medical check. So these persons require a less high safety precaution than normal passengers for whom it must be assumed that they may not be fully fit and that they have not been trained. How many SPS personnel is allowed varies per ship. However according to Solas, when the vessel has no SPS code, no more than 12 special personnel are allowed on board. Off course this is a matter of local Flag states to act or not. The UK flag state is very keen on this. Other Flag states will most likely follow suit. Also insurance companies can require SPS code to carry special personnel in order to give coverage. Therefore the safest bet for any client is to select a vessel that complies with the SPS code.

More information on SPS code:

Fuel cost and CO2 Emissions 

When choosing an offshore vessel, a big part of the cost is fuel. Generally speaking, the bigger the vessel and the greater the horse power of the engines, the higher the fuel consumption. However recent developments in technology but also the design of the vessel have made vessels more fuel efficient. Some vessels are diesel direct drive, some vessels are diesel electric. Diesel direct vessels, usually have engines with axes for propulsion and generators for the electricity. In case of a diesel electric vessel, all electricity on the ship is created by generators, even for the propulsion of the ship. This means that the vessel will start up as many generators as it needs to produce enough electricity. When it needs less electricity, some generators will switch off. The more accurate this is, the less fuel will be wasted. Fuel consumption between different vessels therefore varies dramatically, depending on age, seize, horse power and technology. Some vessels consume 3 m3 MGO per day, some vessels use more than 20 M3 MGO per day. A difference of thousands of euros per day!

Off course there is also the environmental impact. The more fuel consumption, the more C02 emissions. Especially working in the renewable sector CO2 emissions must be part of the considerations.

DP2 plot and type of propulsion 

DP stands for Dynamic positioning. This is the system that ensures that the vessel can hold its position on an exact location. DP 2 means that the vessel can keep holding position in case of a single failure. This ensures that if something brakes down, the vessel does not bump into the offshore platform. Should this single failure occur, then the vessel must stop the operations and move out of the proximity from the platform as it no longer has the redundancy required to stay close to the platform.

The DP plot is very important for W2W operations. In order for a vessel to work with a gangway, it must be able to hold its position quite well. The reason for this is that the heave compensated gangway has to compensate for every motion the vessel makes. The gangway has a maximum of motions it can compensate for. So every motion the vessel makes, means that the gangway has less spare capacity to compensate for other motions. If the spare capacity is used up to the safety limits, then the gangway needs to be released from the platform and transfer of personnel is no longer possible. Windfarms are usually close to the shore and therefore have strong currents. Because of the nature of them, they are built in areas where there is a lot of wind. This means that the DP plot of the vessel must be very strong to compensate for these factors. The DP Plot determines the limiting environmental conditions within which the vessel will be able to maintain position and heading while using DP.

The DP system will measure and then calculate how much power it has to give to its propellers or thrusters. When these forces change the DP system and vessel must be very fast in responding to the new forces coming onto the vessel. Often offshore platforms have only a few access points. Preferably the vessel would like to be with the vessel head on into the current. If the access point is located in a place where the vessel cannot be head on into the current, it will have to attempt to make the connection while having the current from a different angle. This means again that the vessel has to be strong enough, to hold position even if the current comes in to the vessel at an angle. Therefore the strength of the dp plot is very important.

There are propellers or thrusters or Azimuth thrusters. A propeller is fixed to an ax and it will direct it’s force only into one angel from the ship. If the vessel has controllable pitch propellers it can also adjust the setting of the blades to give more or less power or even go into reverse.

A thruster works in a similar way. It also has a fixed propeller and directs forces into one direction or it can reverse the flow. Thrusters are often used in the bow of vessels as tunnel thrusters or in the aft to support position keeping. Thrusters allow the captain or the DP system to steer the vessel sideways in a certain direction. If the current or wind does not come in straight into the propeller or thruster, it loses part of its effectivity.

The other propulsions system is the azimuth thruster. Here the thruster can turn around 360 degrees. In this way the vessel can direct it’s force directly into the counter force (e.g. current). Azimuth thrusters can be used as propulsion but also as bow thrusters. Azimuth thrusters produce less noise and vibrations then normal thrusters. A normal thruster will force a lot of water through the hull. This makes a lot of noise and vibration. As the azimuth thruster is below the hull, the use of this will hardly cause any noise or vibrations in the vessel. There is also a disadvantage of this. Azimuth thrusters are expensive.

Seize of the vessel 

When choosing the seize of the required vessel several things are important.

The bigger the vessel (length, width, draft), the higher the full consumption. The bigger the vessel, the higher also the charter rate and the operational cost. Off course also age of the vessel matters in this regard. The older the vessel the lower the day rate. These are all off course general rules of thumb and there are always exceptions. Therefore it is important to take all these considerations into account. The more persons you need to accommodate, the more beds you need. Usually the vessel seize grows with the number of beds. The more cargo you need to take, the bigger the storage area has to be and the bigger the vessel. Therefore it pays to make an analysis of what spare parts are really needed to take with you on the project. These parts often come in certain numbers in a package. Sometimes it can be useful to repackage these so you only take the numbers of the parts really needed. Every square meter on the vessel is expensive. So the less square meters you need for storage, the smaller the vessel that you can charter. The parts that you take with you and that are not needed are dead stock. The company has invested in these parts and if these are not used for a long time, warranties on them expire. On the other side, if you bring too few spare parts and you need more parts, getting them from the shore to the vessel may be an expensive undertaking. So it makes sense to make a good analysis of what is needed. When this done, a deck space and number of beds requirement can be made.

Significant wave height performance 

Significant wave height (Hs) is the average of the highest one third of waves (from trough to crest, so top to bottom of the wave) that occur in a given period. So only the 1/3 highest waves of a given period are taken. The reason for this is that waves do not move evenly. If you have an Hs of 2 m, than 1 in 10 of the waves will be 2.14 m; 1 in 100 waves will be 3.02 m and 1 in 1000 waves will be 3.72 m. If your gangway has a limit of 2 m than the gangway may have a lift off when this 3.02 m wave comes along. You don’t want that to happen. Therefore not the average wave height is taken but the significant wave height. This should give reasonable limit to not have the gangway lift off.

First of all it seems like a straight forward case the higher the significant wave height, the higher the workability of the vessel. Unfortunately it is not that simple. The longer period that you are taking to do the transfer the higher factor is needed to multiply the average wave height to get the significant wave height. For a 6 minute period a factor of 1.4 can be applied to the average wave height to get the Hs. For an hour on the North Sea a factor of 1.8 can be applied.

Waves do not come evenly from the same direction. Every wave comes in at a different angle. Nor is every wave of the same length. However the significant wave height performance of the gangway is determined by wind and current coming from the same direction and with a certain wave length.

In theory the vessel is head on into te current, wind and waves. The gangway is exactly on a 90 degree angle on the vessel. Than the vessel lies optimally to have the least movements. Any deviation of these forces and the vessel will also have tendency to roll. The rolling motions of the vessel will limit the performance of the gangway. The length of the waves also has influence. If the waves are very short the vessel will pitch much more violently.

Often forgotten is the human factor. The gangway is placed 12 m to sometimes more than 20 m above the waterline. A person has to walk over this gangway. When the waves below him are very high, and there is a very strong wind, can the person still be asked to cross the gangway?

So the business is of Hs is quite vague and subject to a lot of opinions and interpretations. Why are these data still used. First of all the customers ask for it. Secondly the customer would like to have some indication of what to expect.

So what is realistic. Important is up to which average wave height the gangway can connect to the platform. After that the gangway must be able to stay connected to the platform in much higher waves then the connection wave height. If a wave passes that is higher than the safe transfer height, transfer is not allowed at that moment. The persons to be transferred simply have to wait until this big wave has passed. They can continue the transfer again after it has passed. These moments are controlled and indicated by automatic traffic lights on the gangway.

Fortunately the subject of wave height is also not as important as it looks. Waves are never the same. Wave height and wave direction change by the minute. However normally, waves either build up or they come down, as the wind builds up and comes down. The simplicity of this is that vessel with a higher wave height performance will work longer, than the vessels with a lower wave height performance. But after a while when the waves build up, also the higher wave height vessel has to stop working. And when the weather goes down, the reverse applies. So the difference in weather uptime and down time between higher wave height performance and lesser wave height performance is not a direct ratio in practice. It is not that there is a week of 3 m waves, then a week of 4 m waves etc.

Speed of operations 

Especially in the wind industry where many transfers a day are conducted, speed is of the essence. Before starting operations, the vessel will have to do the DP Checklist and then tune the DP system. On average the bigger the vessel, the more it needs time to tune the engine power on the thrusters and the propellers to find the optimum setting to hold position. Off course a modernly equipped vessel will be faster than an elderly equipped vessel as modern computers and systems are faster. As soon as this is done, the vessel can move to the platform to do the transfers. Depending on the weather and the seize of the vessel, this procedure can take between 30 minutes and 2 hours. The faster the vessel can do this, the faster the job gets started. In order to prevent setting up the DP system before every new transfer at a new location, it is usually preferred that the vessel can travel on DP. As long as the weather criteria don’t change significantly, this can be done safely. This avoid the time to set up the DP system when arriving at the next turbine. However on DP, vessels move much slower. Therefore especially in wind industry operations where many transfers need to be done on different locations, speed on DP is of the essence. Some vessels cannot exceed two knots on DP. Some vessels can travel up to 6 knots on DP. The distance between wind turbines is often around 1000 m, so a lot of time can be lost, using a vessel that is slow on DP.

Passenger comfort 

Many studies have been performed by various institutions about the relation between comfort of accommodation and the effectiveness of the personnel on board of vessels. The first obvious things is that if the persons are well rested, less accidents occur. Therefore a low noise vessel with good beds pays out in safety statistics. Also well fed personnel, produce more. Therefore a healthy and tasty menu will pay out in work performance. It is generally accepted that if personnel is well taken care of, the personnel will take better care of their employer and will be more motivated. And, in the current down turn of the oil and gas industry, personnel seems abundant. However when the industry picks up again, good personnel will be harder to find. So investing in loyalty may be a good thing.

Working at sea is not as popular any more. What are the reasons for that? A large oil company did a survey. Modern youth have been brought up in relatively comfortable homes. Therefore they expect a similar condition in their work environment. They expect a decent, nutritious and tasteful meal. Persons don’t like to be away from home as much as they used to. They have seen the world in their holidays and they miss social contacts when they are away. This can be dealt with quite effectively. A good deal of social contact is nowadays done via the internet. So if people are able to stay in touch with friends and family through the internet, staying some period from home is not as much of a problem anymore. Therefore access to internet on the vessel for personnel is paramount. A lot of persons nowadays want to take care of their body. So a good gym is also a big asset.

Crew Boat landing 

In the wind industry, walk to work vessels often work in combination with crew boats. The crew boats are used to perform crew changes but they also assist in transferring persons to the turbines on nicer weather days. Therefore in the wind industry it is quite critical that the vessel has boat landing for the crew boat, so that persons can transfer from the crew boat to the W2W vessel.

Covered work / storage area 

Sometimes hot works need to be performed on the vessel, because for safety reasons these are often not allowed on the platform. Sometimes a lot of cargo handling needs to be done. Materials need to be taken form containers and containers need to be repackaged afterwards. As weather is not always favorable on the North Sea, it is a great help for personnel if there is covered work and storage area on board of the ship.

Office facilities 

During projects, often project management wants to be on site to supervise operations. Therefore it would be helpful if there are office facilities on board with connection to the internet. In this way the project manager can effectively manage the project from the vessel. Often the charterer using the vessel is a contractor himself. The client of the charterer sometimes also wants to monitor the operations and will want to have his representatives on board. Therefore it is useful to have two separate offices. On top of that it is helpful to have a meeting area so meetings can be conducted.

Working hours. 

As the client pays for the vessel night and day, optimum usage of the vessel will be 24 hours a day. By making as efficient use of the vessel as possible the cost of the work can be controlled. In the wind industry a staggered rotation scheme is advisable. This means that working hours of teams start on a 20 to 30 minutes interval. This avoids persons waiting for the first teams to transfer until the vessel reaches the turbine where they have to work on. It is advisable to have the persons ready with all the equipment on deck and with their briefing completed when it is time for them to transfer. This avoids that the entire vessel is waiting until the persons are ready to transfer. Making sure that turbine has stopped working before the vessel arrives, saves a lot of time. It can often take more than 20 minutes to stop the turbine. If the vessel is waiting all the time for this, a lot of potentially productive hours are lost. Chartering a vessel is a costly operations. Avoiding unnecessary delay can help control these costs.

Project preparation 

A lot of cost saving can be saved by making a good project preparation. The project preparation starts with an overview of the possible landing spots for the heave compensated gangway. Which landing spots are feasible? Do they need any work before they are suited for connection? Sometimes an access point needs to be created. When and how we create this. Also when designing a platform or the turbine, thought needs to be given to this. Installing a landing point when the unit is already at sea is a lot more expensive. Are there alternative connecting points. Sometimes the prevailing weather does not allow connecting on one side, but on another side the circumstances are more favorable. This will help improving the number of workable days. Sometimes the connecting point for the gangway has a locked gate. An important question is then, who has the key for this lock. What is the procedure for opening this. Do we allow climbing over fences or not. In case of a subcontractor working for an asset owner it is useful to get the site owner involved in this. Does he agree with the proposed landing spots and procedures? If the vessel owner is experienced in W2W, it is a good help to involve them in the process as early as possible. The owner will be able to help in project preparation. To help the owner it is useful to supply them with drawings and pictures of the platforms. A good project preparation where a lot of thought is given to the details will save a lot of time and costs when the project has started.

DP Gezina and DP Galyna 

When selecting and converting DP Gezina and DP Galyna we have used these criteria to find a vessel that we believed could be of considerable use for our clients. Both our vessels are equipped with Ampelmann XL Gangway. They are suited for connecting gangways in waves up to 3.20 m. Transfers are monitored with traffic lights. DP Gezina can reach platforms up to 19 m above waterline. DP Galyna can even reach up to 23.5 m above the waterline, thanks to a pedestal under the Ampelmann gangway. The vessels are DP 2 with two Azimuth thrusters aft for propulsion and two retractable azimuth thrusters in the bow. They can travel up to 6 knots while on DP. They are extremely fuel efficient, with a fuel consumption of about 3 M3 MGO per day. They have 50 beds for clients in 38 cabins. The vessels are equipped with a large restaurant, a large recreation room, a smoking room, a fitness room and 2 offices with a total of 7 desks. All cabins have individual TV’s and are connected to bathrooms. The entire vessel is covered by WIfi. A section of the vessels’ restaurant can be segregated to create a separate meeting room. The vessels have a work deck and a covered container storage. Total deck space is 250 M2 and 2*Foot and 5 * 10 Foot containers can be stored. Each vessel has a crane. It is mounted about 10 m above the waterline and from there it can lift 2 tons at 10 m and 20 tons at 10 m. They are heave compensated. Also there is a crew boat landing for crew boats.

As Chevalier Floatels was the first vessel owner with permanently installed gangways on the vessels, Chevalier has a wealth of experience. More than 20 different projects have been completed successfully. Jobs have been done in wind energy projects but also oil and gas.


I hope that you have found this information useful. The information is far from complete or perfect, however I hope the information helps you to understand a little bit more about walk to work. If you have suggestions, or comments please don’t hesitate to write to us. This page may be updated from time to time to include more lessons learned. However the information provided is a service and not a full time job. So please don’t be disappointed if your well-intended comments are not immediately included in the page. In closing I like to thank you for your time in reading this. I hope to meet you some time so you can share your thoughts with me about walk to work.

About Marcel Roelofs 

Marcel Roelofs is the General manager of Chevalier Floatels. Chevalier Floatels has been active in floating accommodation since 2004. Apart from walk to work vessels, it also owns a fleet of accommodation barges.

Legal disclaimer: Although the author and publisher have made every effort to ensure that the information in this letter was correct at publication time, the author and publisher do not assume and hereby disclaim any liability to any party for any loss, damage, or disruption caused by errors or omissions, whether such errors or omissions result from negligence, accident, or any other cause.